Development, Testing and FE-Simulation of Adhesively Bonded Overlapping Tube Specimen

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University of Pisa

School of Engineering – Department of Civil and Industrial Engineering Specialistic Course of Mechanical Engineering

Master Thesis

Development, Testing and FE-Simulation of

Adhesively Bonded Overlapping Tube Specimen

Candidate: Andrea Pippucci

Supervisor:

Prof. Francesco Frendo Prof. Leonardo Bertini Dott. Jörg Baumgartner

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CONTENTS CONTENTS

1 Introduction 4 1.1 Definitions . . . 5 1.2 Background . . . 7 1.2.1 Adhesion . . . 7 1.2.2 Linear Viscoelasticity . . . 8 1.2.3 Cure Process . . . 10 1.2.4 Kissing Bond . . . 12 1.3 Aim of thesis . . . 12 2 Analysis 14 2.1 Materials . . . 14 2.2 Setting Analysis . . . 14 2.2.1 Concentricity . . . 15 2.2.2 Desired Thickness . . . 17 2.3 Geometry of joint . . . 18 2.3.1 FEM Analysis . . . 19 2.3.1.1 Modeling . . . 19 2.3.1.2 Scarf Angle α . . . 26

2.3.1.3 Adherent Tip Thickness t0 . . . 27

2.3.1.4 Adhesive Thickness . . . 28

2.3.1.5 Indented Scarf Joint . . . 29

2.3.1.6 Lap Joint Outside Taper . . . 32

2.4 Choices . . . 34

2.5 Strength Analysis . . . 35

3 Manufacturing 35 3.1 Setting and Jointing . . . 36

3.2 Curing Process . . . 38

4 Testing 39 4.1 Concentricity . . . 39

4.2 Static Tension Testing . . . 40

5 Results 41 5.1 Concentricity . . . 41

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CONTENTS CONTENTS 5.2.1 Long Lap-Joint . . . 41 5.2.2 Long Lap-Joint 1 . . . 41 5.2.2.1 Long Lap-Joint 2 . . . 42 5.2.3 Little Lap-Joint . . . 44 5.2.3.1 Little Lap-Joint 1 . . . 44 5.2.3.2 Little Lap-Joint 2 . . . 45 5.2.4 Indented Scarf-Joint . . . 46 5.2.4.1 Indented Scarf-Joint 1 . . . 46 5.2.4.2 Indented Scarf-Joint 2 . . . 47 5.2.5 Ideal Scarf-Joint . . . 48 5.2.5.1 Ideal Scarf-Joint 1 . . . 48 5.2.5.2 Ideal Scarf-Joint 2 . . . 49 5.2.5.3 Ideal Scarf-Joint 3 . . . 50 5.2.5.4 Ideal Scarf-Joint 4 . . . 51 5.2.6 General Considerations . . . 52

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1 INTRODUCTION

1 Introduction

The attention to CO2 emissions and the increasing of vehicles´ security and com-fort lead to the need to limit their structural weight. For these reasons in auto-motive industry the use of non-metallic materials, light metals and steels which are not weldable or difficult to weld, is increasing. The tendency towards a larger material diversity drives to the adhesive bonded joints. The important advantages over other connection methods, such as spot-welding or mechanical fastening, are a higher stiffness due to a more uniform stress distribution by a continuous joint (Figure 1) and the ability to join dissimilar materials; in addition, it has a higher fatigue and corrosion resistance. Bolts and rivets are often points of high stress concentration that can lead to structures having a lower static and fatigue strength than an adhesive bonded system. However, bonding needs an extreme surface preparation and it degrades due to environmental effects [1].

Figure 1: Improved stiffness (left) and stress distribution (right) of adhesive bonded joints in relation to riveted joints

Both issues are solved choosing particular materials and taking some precau-tions during manufacturing. Furthermore, in vehicle body production there is a

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1 INTRODUCTION 1.1 Definitions

hot-curing step yet: the bonding cure process can be integrated well. In the am-ple field of bonding, this thesis is included in the project about the analysis of the glue´s behavior during fatigue tests made on a tubular specimen with different kind of applied stresses. This goal is obtained using various joint´s geometries and various loads.

1.1 Definitions

A general bonded joint can have a particular shape and material but there are some things that don´t change. It is constituted from two fundamental parts: adherent and adhesive. The adherent (or substrate) is an object that has to be joined with another object: they can have different material properties but they are named in the same way. The adhesive is the part of joint that bonds the adherents together and resists separation. Adhesion is the phenomena for which these parts stay to-gether and it is constituted by whole forces (covalent, van der walls, acid-base, etc.) involved between them. Instead, cohesion involves the forces inside one ob-ject. The area between the adhesive and an adherent is named interphase. It has different properties from those of the bulk adhesive or adherent and the mechani-cal behavior of joint depends from it. The interface is the plane of contact between the surfaces of adhesive and adherents. When the joint is bonded, there are two important measurements: the overlap area and length. The overlap (or bonded) area is the contact area between the adhesive and an adherent. The overlap (or bonded) length is one of the two dimensions of the area: it´s often the smaller of them. The ideal joint would be loaded in shear and the overlap area must be larger possible. However, it changes with the geometry of the joint. There are several configurations, Figure 2 presents the most important ones.

There are two principal kinds of stress in the adhesive layer: peel and shear stress. The peel stresses are which perpendicular to the overlap length; instead, the shear stresses are which perpendicular to the peel ones.

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1.1 Definitions 1 INTRODUCTION

Figure 2: Adhesive joint configurations[4]

There are three ways in which a bonded joint can failure: cohesive failure in the adhesive or in the adherent and interfacial failure. So, while the adherent and the adhesive break in the cohesive way, the interphase breaks into interfacial one. Figure 3 illustrates these differences.

Figure 3: Difference between kinds of failure

Cohesive failure means that the adhesion is not the weakest link in the load bearing chain. In the automotive industry, the cohesive failure of the adhesive

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1 INTRODUCTION 1.2 Background

layer is a prerequisite of the joint-specific life expectancy. Otherwise, there are some problems such as unlimited results and security of the adherents.

1.2 Background

1.2.1 Adhesion

A lot of theories try to explain the adhesion but its understanding needs of all of them because they emphasize a different aspect of the same problem. The most important are mechanical, adsorption and diffusion theories. In the first case, it focuses on interlocking between adhesive and a rough adherent surface: it leads to increase plastic energy dissipation during fracture in the bulk adhesive. The adsorption theory is based on primary, such as covalent, and secondary forces, as van der Waals. Primary bonding may be necessary to achieve bond durabil-ity. Finally, the theory of polymer chain dynamics is discussed on the diffusion one. Another aspect of the adhesion is the quality of surfaces. In general, it needs of good wetting, great roughness and no contaminant. Wetting is the capacity of a liquid to maintain contact with a solid surface and it depends from cohesive forces of the adhesive. Wetting is also responsible for capillarity. It is another fundamental property for adhesion: the ability of the liquid to penetrate in the solid. For this a good bond needs great roughness even if significant roughness can lead to brittle behavior and can seriously reduce the extent of wetting. These aspects need processing which imply the presence of contaminants, such as oil. The removal of contamination is a fundamental requirement in establishing good adhesion. Once a time made the best and considered the final state, it proceed to choose the correct adhesive. The adhesive materials are classified using various categories: polymer base (i.e., natural or synthetic), functionality in the polymer “backbone” (i.e., thermoplastic or thermoset), physical forms (i.e., one or multiple components), chemical families (i.e., epoxy, silicon), functional type (i.e., struc-tural, hot melt, pressure sensitive, water-base, etc.) and methods of application

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1.2 Background 1 INTRODUCTION

[1].

1.2.2 Linear Viscoelasticity

These materials can often, when subjected to small strain and small strain rates, described by the theory of linear viscoelasticity [3].

The polymer is usually classified as viscoelastic material: a behavior between elastic solid and viscous liquid. It depends from factors as temperature, mea-surement frequencies, environmental factors (i.e. moisture) and load. The most important difference between an ideal-elastic and a viscoelastic material is shown in the time course of the elastic reaction of the material to a load. A consequence of this is that, for example, the modulus of elasticity of a viscoelastic material is not a constant, but changes in the course of the stress time. The reason for this behavior is relaxation-related cooperative restoring processes in and between the long chain molecules of the plastics .

There are one important phenomena regarding to this materials: the stress relaxation. It is realized applying a constant deformation and measuring the stress relaxation (Figure 4).

Figure 4: Stress relaxation

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strain function jumps from zero to ε0 at t = 0. The simplest mechanic model

that represent this behavior is Maxwell´s one. If a system is constituted from M Maxwell´s elements in series, it´s able to demonstrate that its compliance is the sum of compliance of each elements. This is called generalized Maxwell model which describes this behavior with a system of spring and dash pots, see Figure 5.

Figure 5: Generalized Maxwell model

The relaxation function is often written as a Prony series:

E(t) = E_∞+ M

∑

m=1 E_m· exp(− t a_T · τm )

with the long-term or equilibrium modulus E∞,(Em, τm) is the discrete relaxation

spectrum and aT is the shift factor. The viscoelastic properties depend often on

temperature and aT is the factor that moves the values calculated in a temperature

T0to others calculated in the reference temperature T0. The dependency is inside

formula if all relaxation times are affected by temperature in the same way, the material can be defined as thermorheologically simple. In this case it´s allowed the application of the Time-Temperature Superposition Principle (TTSP). This implies that the same variation of a mechanical quantity (compliance, modulus or damping), that is gotten changing the temperature in a fixed frequency, can be obtained varying the frequency in a fixed temperature. With a DMA (Dynamic Mechanical Analysis) it obtains necessary data to make a complete viscoelastic analysis.

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1.2 Background 1 INTRODUCTION

1.2.3 Cure Process

During the application, the adhesive is a liquid, which can be simply managed and which can wet better the adherents. Then to get a bonding joint, it has to convert into a solid: this process is known as hardening or curing. In particular it transforms from a viscous fluid to a viscoelastic solid. This process can be done in different ways and it depends from the kind of glue. Here, a short extract is given based on [2].

The cure cycle can be described by a single variable: the degree of cure

q(t) =H(t) H_∞

where H(t) is the accumulated released heat and H∞ is the ultimate heat after

completion of the chemical reaction. This parameter can be determined directly by monitoring the heat flow to and from a material sample. A DSC (Differential Scanning Calorimetry) is often used for it.

The transition from a viscous liquid to viscoelastic solid can also be seen as a sol-gel transition. The moment the polymer turns into a solid is the gel point, Figure 6.

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Before the gel point, the polymer consists of finite clusters and is called a sol as it is soluble in a solvent. From the gel point on, it is called a gel and is not soluble anymore since it consists of a macromolecule of infinite molecular weight.

Non-crystalline polymers can show a rubbery behaviour with low stiffness at high temperatures and a glassy behaviour with high stiffness at lower tempera-tures. The temperature at which the polymer transfers between these two states is called glass transition temperature Tg. Glass transition affects not only mechanical

properties. The heat capacity shows a peak at Tgand the slope of the CTE

(Coef-ficient of Thermal Expansion) has a discontinuity at Tg. Therefore, it is common

to determine Tgby measuring the specific volume as a function of temperature or

via a DSC.

The glass transition temperature can affect the cure rate. When the tempera-ture falls below the glass transition temperatempera-ture, the mobility of polymer chains becomes severely restricted. The reaction becomes diffusion-controlled. The cure process slows down significantly, even to a point where complete cure is not reached anymore. To avoid incomplete cure in bonding processes, adhesive producers usually recommend a cure temperature well above the final glass tran-sition temperature. Structural adhesives require moduli in the order of magnitude of 109Pa. To meet these requirements, they cannot operate in a rubbery state, but need to be cooled down below their glass transition temperature after cure .

During the cure process the volume of the adhesive changes. Temperature in-duces expansion and shrinkage over a non-isothermal cure cycle, the cross-linking of polymer chains reduces the specific volume of the adhesive. This phenomenon is called chemical shrinkage. It can vary for different types of adhesives, Figure 7 shows it.

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1.3 Aim of thesis 1 INTRODUCTION

Figure 7: Chemical shrinkage for different adhesives[2]

1.2.4 Kissing Bond

Kissing bonds refer to the situation where two surfaces are only partially bonded or are debonded but touching or in very close proximity: it´s mean a poor adhesion. Figure 8 illustrates the most important kinds of defects it could be at the end of the cure. The unbond and disbond or zero-volume bond are two examples of kissing bond.

Figure 8: Possibile defects in a bonded joint [1]

1.3 Aim of thesis

Structural stress concepts are applied to estimate the life of glued joints. The durability for a bonded system can be tested by applying cyclic loads on speci-mens with different geometries. The idea is to obtain the glue´s behavior on dif-ferent stress distribution with the same load using difdif-ferent geometry of joint. In

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1 INTRODUCTION 1.3 Aim of thesis

the butt-joint we can get only normal stresses with a traction load and only shear stresses with a torsion one; in the lap-joint, instead, in the first case we have shear as most important stresses and peel stresses due to equilibrium, in the second case only shear stresses as before. Finally, in the scarf-joint there are always a various mix of both kinds of stresses: the ratio between them depends to the scarf angle. There are some scientific articles about fatigue life estimation of tubular bonded joint under multiaxial loading but not any regarding the material of interest of this work. The project has the goal to characterize the dynamic behavior of this par-ticular adhesive using a tubular geometry. It started with the simple butt-joint and it is developing with the single lap-joint. The goal was to get shear stresses as main ones also in the tension testing. Unfortunately during the manufacturing of the tubular single lap-joint, it found the phenomena of kissing bond.

The scope of this thesis is to analyze and to test possible solutions to this problem with the same geometry (Lap-Joint) or others.

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2 ANALYSIS

2 Analysis

The idea was studying the phenomena of kissing bond on the lap-joint and, at the same time, designing a new geometry of joint. In the first case, fixed the thick-ness of adhesive and the cure process, it thought to improve the manufacturing processes and to change the length of overlap area. With the new geometry it would like to get the same goal of lap-joint without the problem of kissing bond. It started as “white paper” except for the fixed adhesive’s thickness. In both case, the most important aim was to guarantee the concentricity and the desired thick-ness between the parts of specimen during the manufacturing process. Thus the analysis is divided in two main parts: the first about setting process and the second about the new geometry.

2.1 Materials

The materials which were used in this work are the structural alloy S355 for both adherents and BETAMATE 1496V as adhesive. BETAMATE 1496V is an epoxy resin, its hardening proceeds by chemical reaction that requires a curing agent or a catalyst to initiate the process. Metal ions of the substrate or moisture can act as catalysts as well. As epoxy, it is a thermosetting polymer: generally stronger than thermoplastic materials due to the three-dimensional cross-linked networks of infinite or immeasurably high weight molecules. After the cure process, they become infusible, insoluble materials. Thus they have high mechanical strength, a brittle behavior and they are used as structural body adhesives. The mechanical property of the materials of composed are shown in Table 1:

E[MPa] ν σy[MPa]

Betamate 1496V 1600 0.4 40

S355 210000 0.3 355

Table 1: Materials properties[9]

2.2 Setting Analysis

A reliable specimen is when the two parts, of which it’s composed, are coaxial and the adhesive’s thickness is the desired one. These can be obtained in different ways which should have some common properties: low cost, as simple as possible,

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2 ANALYSIS 2.2 Setting Analysis

considering that it should make a lot of specimens at the same time, and it would be useful for all joint’s geometries.

2.2.1 Concentricity

This property is important for all types of specimens but mostly for lap-joint. In all geometries the concentricity allows to get a good joint and an uniform stresses, in the lap-joint it guarantees also the desired thickness.

This goal can be obtained in two way: with a fixing device or with a particular surface in the geometry of the specimen. In this paragraph it is tackling the first topic, the other solution is presented in 2.3.1.5. The various solutions that can be found should use the surfaces of specimen as references: these surfaces can be divided in external and internal ones.

Internal This solution allows to get a good internal surface of adhesive be-cause in this way it isn’t able to spread inside during curing process. A spreading outside can be eliminated by machining after cure with low cost and low time. In addition, in this way, it gets a better control of adhesive’s shape.

The idea was using a Teflon Body inside (Figure 9) that expands 15 times more than steel and so, during the curing process, it should be in touch with the internal surfaces of specimen and it should make them concentric. If internal surfaces of specimen’s parts have a good tolerance they will be coaxial after manufacturing. The problem is that it isn’t so reliable as solution: there were same examples where, using Teflon Body, the parts aren’t coaxial after manufacturing.

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2.2 Setting Analysis 2 ANALYSIS

In addition, a body inside the specimens leads to thermal stresses due its ex-pansion: it should not be a problem considering the small thickness of adhesive and the big difference between material properties (teflon-steel). Finally, while we can think about some problems during the extraction of this body after curing, it was not verified; anyway, handling the specimens after curing process isn’t the best choice.

External The advantages of this solution are that, with good tolerance of external surfaces, the concentricity is ensured by the more reliable contact steel-steel in contrast to the other solution (teflon-steel-steel); in addition it isn’t able to lead to thermal stresses and problems of extraction. The disadvantages are the internal spreading of adhesive and the impossibility to make coaxial parts with different external geometries. The last aim can be solved using a expensive and complex fixing device.

Considering that the specimens are cylindrical, the idea was to use a V-guide (Figure 10a). To make some specimens in the same time, a plate with 5 V-guides was drawn (Figure 10b). In the horizontal solution there is the problem of loss of adhesive in the upper side of joint due to gravity during the curing process. Thus it needs to make the specimens vertically. For this reason the final solution (Figure 10c) have two horizontal locking bars.

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2 ANALYSIS 2.2 Setting Analysis

(c) Final Solution

Figure 10: Fixing Device

2.2.2 Desired Thickness

Getting thickness less as 1 mm isn’t simple using the fixing device and surely it is not cheap. The solution used in other geometry was to put something, of the dimension of desired thickness, inside the adhesive before jointing process. In this work it was used glass balls. This was done also following the DIN EN 14869-1[11]. Another solution, in the new geometry, was found but it is explained in 2.3.1.5.

Once gets the adhesive thickness, it has to be maintained during the curing process. The glue expands due to increasing of temperature and it moves the parts each other modifying its thickness. Thus it has to constrain the vertical relative po-sition of the specimen’s parts. It was done by a screw, inside the specimen, linked with the two plane lateral surface of specimen and closed by a nut to eliminate the possibility of vertical movements. Figure 11 illustrates the case of butt-joint.

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2.3 Geometry of joint 2 ANALYSIS

Figure 11: Glass balls and Platelet-Nut to lock vertical movement

2.3 Geometry of joint

The general dimensions of tubular specimens are about 150 mm as length, 47.7 mm as external diameter and 5 mm as thickness. These were limited by fixing devise used in the tensile machine and by which of butt-joint to be able to compare results.

The shape of joint together to the kind of load determines the type and the amount of stresses inside the adhesive. The perfect joint must be loaded only in shear and the load-bearing area must be as large as possible [1]. As wrote before, the necessity getting shear stresses also in the traction test and the problem of kissing bonds lead to the scarf-joint (Figure 12): the overlap surface is conical and not more cylindrical.

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2 ANALYSIS 2.3 Geometry of joint

Where α is the scarf angle, t is the adhesive thickness and t0 is the adherent tip thickness.

Below it are going to describe the influence of these factors on the stress dis-tribution in the overlap.

2.3.1 FEM Analysis

FEM (Finite Element Method) compared to FDM (Finite Differential Method) ob-tains more detailed solution, it’s more “flexible” considering the possible changes in the model and it could need less time to be implemented. Thus it’s the most appropriate method for this analysis. Stresses and strain in the joints were gotten with FEM using Abaqus® as program.

2.3.1.1 Modeling The parts of specimen were modeled as 2D axisymmetric using the option “Include twist”: it gives to each element 3 DOF instead of 2 in the plane. So it has less computational time and it can measure all interested output also with torsion as load.

Element The elements which were used are CGAX4R (Figure 13): a 4-node generalized bilinear axisymmetric quadrilateral element with reduced integration, twist and hourglass control. With z as coordinate in the symmetric axes, r as radial direction and φ as third angular coordinate (twist angle), each node has got 3 degrees of freedom (uz, ur, φ ).

Figure 13: Element type

Constraints To create the assembly, the adhesive and the adherent were con-nected with “Tie Constraints”: “surface-to-surface” one. The hypothesis that cur-ing process has success was assumed. In this case the different materials would

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be related, it gets just one object. This constraint makes the translational and ro-tational motion as well as all other active degrees of freedom equal for a pair of surface. (Figure 14).

Figure 14: Surfaces Constraints

Mesh The area of interest is the joint in the center of the specimen: there is a gradual growing on the size of the mesh going to the opposite sides (Figure 15).

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Load and Boundary Conditions To simulate the tension testing, the bound-ary condition was modeled as a “Encastre” constraint (Figure 16a) of a lateral face of the specimen and the load as a negative pressure on the other face (Figure 16b). This is not the best choice to model the tension testing: an axysimmetric constraint and a displacement as load should be used. This model was implemented also and the results were the same as the simplest one. Thus the choice to model in this way. Considering that torsion gives just constant (or linear in the scarf geometry) shear stresses in the overlap, it wasn’t considered. The reference load for all next analysis is 20 kN.

(a) BCs (b) Load

Figure 16: Model of BCs and Load

Path In a bonded joint there are three possibilities to choose the nodes where doing analysis (Figure 17a): in the external interface (blue line), in the internal one

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(green line) and in the middle of adhesive (red line). The differences between the choices are shown in Figure 17. The reference system n-s is in agreement with the path: s-coordinate is along the path instead n-coordinate is perpendicular to it. It gets peel stress in the plane which has n-coordinate as outgoing verse in the n-direction; shear stress is obtained in the same plane but in the s-direction. P-coordinate is the path. The shear stress in this paragraph is not considered.

(a) Different paths: external interface (blue line), internal interface (green line) and middle of ad-hesive (red line)

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(c) Peel stress in the internal interface

(d) Peel stress in the middle of adhesive

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(f) Peel Stress in all paths

Figure 17: Peel stress in interfaces (external and internal) and in the middle of adhesive

In the external interface (Figure 17b), at the begin of path, there is a compres-sion instead in the end of it there are positive stresses. The same trend of peel stresses but in the contrary way there is in the internal interface (Figure 17c). In the middle of adhesive (Figure 17d) there is compression in both edges and the same trend in the rest of the path. In Figure 17e are highlighted the most loaded areas of the joint in traction.

Figure 17f illustrates the peel stress in all possibilities. There is the same trend in all of them except the values in the edges. In addition the thickness of adhesive is less than 1 mm so the hypothesis of no-stresses changes in the thickness direction is consistent. For these reasons the middle of adhesive was chosen as path for all next analysis.

Convergence The most important problem in these analysis is the stress gularity. Due to the presence of perfectly sharp corners in the model, stress sin-gularities occur in these points. This means that the value of the stresses at these

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points is completely dependent on the mesh density. The “h-convergence”, where h is the element size, was used to verify the convergence of the results (Figure 18). As wrote before, the path used is the middle of the adhesive. Instead Figure 18a illustrates the convergence of all path with just three different element sizes, in the Figure 18b there is the h-convergence analysis of the maximum stable value in the path at about 0.25 mm from the edges with more element sizes.

The values in the middle of path are stable until the edges where it founds a convergence of the results close to the end of them. So only the convergent results were used, the values in the end of edges are not reliable.

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(b) H-convergence analysis of the maximum stable value in the overlap lenght

Figure 18: H-convergence analysis with three different element size

2.3.1.2 Scarf Angle α The definition of scarf angle is the smaller angle be-tween the axes of tubular part and the conical surface (Figure 12).

Considering the others parameters constant, the increase of α implies much more peel stresses and less shear ones. There is a reason to get it smallest possible and another reason to get it close to butt-joint. In the first case, we would like to have a reasonable “lap effect”: the main stresses are the shear ones and in the trend of peel stresses there are peaks in the edges with a good ratio between them and the average. This to compare the scarf-joint with lap-joint. In the other case, we would not get kissing bond.

Figure 19 shows that peel stresses on the overlap are about constant until 30° and we have a good ratio (about 2) between the maximum and minimum one at about 20°. In addition, α = 20° gives the same shear stress as Lap-Joint in the middle of overlap.

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Figure 19: Comparison of stress distribution between Lap-Joint, Scarf-Joint with different scarf angle

2.3.1.3 Adherent Tip Thickness t0 In the edge of the conical surfaces there could be a smooth with so called “adherent tip thickness” (Figure 12). It is likely that the adherent tip will undergo large vertical displacements under load, due to the adherents have almost zero stiffness in the edges. Considering the others parameters constant, as it’s illustrated in Figure 20, this modification leads to load eccentricity so to more peel stresses and it complicates the manufacturing process of the specimen. At the end, it was chosen to have a t0= 0 in all specimens. From now the Scarf-Joint with t0= 0 will be named Ideal Scarf-Joint and that with t06= 0 will be named Regular Scarf-Joint.

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Figure 20: Comparison of stress distribution between Scarf-Joint with different adhesive tip thickness

2.3.1.4 Adhesive Thickness There are some scientific research where the ef-fect of this parameter is illustrated. Objois et al. (1999), in the case of simple scarf joint (not tubular) and static load, demonstrate that the maximum strength of adhesive is at 0.1 mm and it decrease slowly with the increase of thickness, instead it drops with the decrease. About fatigue test with tubular single-lap joint, Lee et al. (1991) show that the endurance limit increase as the adhesive thickness decreased; however, it was found also that the adhesive bonding operation without introducing eccentricity was very difficult if the adhesive thickness was less than 0.15 mm. Finally, if the adherent tip thickness is present the increase of adhesive thickness leads to load eccentricity and so, as before, to more peel stresses (Figure 21). The final choice was 0.3 mm also to compare results with other geometries (es. butt-joint).

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Figure 21: Comparison of stress distribution between Ideal Scarf-Joint with dif-ferent adhesive thickness

2.3.1.5 Indented Scarf Joint As wrote before, it was found another solution to get concentricity and at the same time also the desired adhesive thickness: a particular configuration of the geometry of scarf-joint. Figure 22 illustrates this shape. In practice it is composed from a cylindrical surface (Figure 22a) that guar-antees the concentricity and a plane surface (Figure 22b) that leads to the desired adhesive thickness. These need good tolerance of the parts but these constraints are more reliable than others. In addition, this solution allows to get a only adhe-sive joint in the scarf-joint: no glass balls inside.

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(a) Cylindrical surface

(b) Plane surface

Figure 22: Indented Scarf-Joint

The FEM analysis of this joint (Figure 23) shows the same trend of others con-figuration except in the edges. Concerning the peel stresses, at the beginning of the path it gets the same trend of the Ideal Scarf-Joint considering the absence of adherent tip thickness; at the end there is a more smooth trend due to the stiffness around the adhesive and there are unstable results caused by the decrease of adhe-sive thickness until zero. The rounded sharp was chosen as 0.3 mm: a smaller one would get more peak of stress but, at the same time, it would not have adhesive in the cylindrical surfaces 22a.

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2.3.1.6 Lap Joint Outside Taper It was found also a solution about lap-joint that leads to less stiffness in the overlap area, less peaks in the edges and flat trend of stresses in the middle of overlap. Figure 24 illustrates the difference between this solution and the standard lap-joint.

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Methods to increase joint strength Rounded joint is a modification of ad-herent’s geometry that reduce singularity in the sharp corners (Figure 25).

Figure 25: Rounded Joint[1]

Fillets allows a more uniform load transfer in the joint and so less peaks of stresses in the edge. Figure illustrates the case with and without fillet.

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2.4 Choices 2 ANALYSIS

2.4 Choices

The final specimens chosen to be produced and than tested are shown in Figure 27. Attached there are the technical drawings of them. The adherent tip thickness was taken null considering that it would lead to more complicated geometries of joint without a real advantages in this work. For the same reason, the lap-joint outside taper wasn’t produced even if it is interesting concerning the trend of stresses. Finally, it was chosen to produce two versions of the lap-joint: “Long Lap-Joint” with an overlap length of 12.5 mm and “Little Lap-Joint” with an overlap length of 5 mm. The general dimensions of each part are: Dext= 47.7mm , Dint= 37.7mm,

L= 80mm. Two specimens for each Lap-Joint version, two Indented Scarf-Joint and four Scarf-Ideal Joint were produced for a total of 10 specimens. In Appendix A 6 there are the technical drawings.

(a) Ideal Scarf-Joint (b) Indented Scarf-Joint (c) Lap-Joint

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3 MANUFACTURING 2.5 Strength Analysis

2.5 Strength Analysis

Even if the interesting test is a fatigue one, to get a reasonable indication of the joint’s strength, it did the static elastic analysis until the failure. With this analysis it got the load that would break the joint. The model is completely as before. The maximum principal stress got from FEM was compared to σyof the adhesive. The

breaking load was gotten when the ratio between these data was 1. The maximum principal stress was taken just in a node: in the same path as before (in the middle of adhesive thickness), in the middle of the overlap length. In this way it gets more reliable results even if increased.

The results of this analysis are shown in Table 2.

Joint Breaking Load [kN]

Long Lap-Joint 67

Little Lap-Joint 38

Indented Scarf-Joint 53

Ideal Scarf-Joint 53

Table 2: Breaking load of each kind of joints got from FEM

3 Manufacturing

Once the parts are done, the next steps were to manufacture the specimens. The differences with the starting manufacturing were just in the setting, using the new fixing device. To make this process more reliable it was done a Check List (Figure 28) where there are the needed materials and the tasks that it would be fol-lowed during the manufacturing. Each task is numbered and it has the possibility to be commented in the space next to it (Comments) to improve the process.

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3.1 Setting and Jointing 3 MANUFACTURING

Check List

Materials: Teflon Body and Ring, N-Heptan, glass balls, fixing device (with all components and screws), staff to deposite and to expand adhesive.

n. Task Comments Check 1 Clean the surfaces to be bond with

N-Heptan

2 Put into the oven the adherents of specimen at 60°C and take out them when the temperature is reached 3Lap Inser the Teflon Ring in the right

adherent

4 Deposite the adhesive and expand it on the interested surfaces of both adherents 5Scarf Put glass balls in the adhesive

6 With the fixing device in horizontal position, make the joint (Scarf: attention to which adherent is on the top and which on the bottom) 7 Mantaining coaxiality with fixing

device, remove the adhesive spread due to excess of it

8 Fix the specimen in the fixing device, than move it in the vertical position 9 Fix the vertical position with platelet and

nut

10 Put the specimens in the oven at 180°C 11 After 30 min that temperature reached

170°C, turning off the oven and open a bit its door

12 When the oven reachs RT, taking out the specimens from the oven

13 Put the specimens in a room with 10% of moisture and take them into for 10 days 14 Make test

Figure 28: Check List

3.1 Setting and Jointing

The machined parts have some oil in their surfaces. Thus, first of all, the surfaces to be bond were cleaned with N-Heptan.

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3 MANUFACTURING 3.1 Setting and Jointing

Betamate 1496V is so viscous that at RT (Room Temperature = 23°C) it’s too stiff to use. It was warmed up until 60°C in the oven. Also the parts were put into the oven.

After this, the specimens were created. A Teflon Ring is used for Lap-Joint to make the desired overlap length (Figure 29). Then other inserts were used, as Glass Balls, the adhesive was deposited and expanded on the interested surfaces of adherents and, finally, the joint was done with the fixing device.

Figure 29: Teflon Ring in the Lap-Joint

At the end, it got three specimens in the fixing device (Figure 30). So it needed of two times to make all specimens.

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3.2 Curing Process 3 MANUFACTURING

3.2 Curing Process

The curing process was not changed and it is illustrated in Figure 31. The speci-mens have to stay for 30 minutes at about 180°C.

Figure 31: Curing Process

The heating and cooling rate are fixed just because the oven is not programmable. There was just a sensor to know the temperature inside so that it could turn off when it was necessary. Oven and sensor are illustrated in Figure 32. During the cooling time the door of the oven was a bit open.

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4 TESTING

4 Testing

Before the tension testing it was measured the concentricity of the parts of each specimens.

4.1 Concentricity

To make a comparison between the systems that get concentricity to the parts of the specimens, it measured the angle β between the external surfaces of the two parts (Figure 33) with this formula:

β = tan−1 (Lmax− Lmin) Dint

Figure 33: Measurement of concentricity

L was misured for each specimen four times in four different points of the surfaces. To solve the equation, Lmax and Lminare taken. This for all specimens.

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4.2 Static Tension Testing 4 TESTING

4.2 Static Tension Testing

The static tension testing was done in the tension machine illustrated in Figure 34. In all tests it was used the same displacement rate of 0.1 mm/min.

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5 RESULTS

5 Results

5.1 Concentricity

As result of the measurement explained before, Table 3 was obtained. Both Fixing Device as Indented Geometry lead to more concentricity then Teflon Body; only with second one it gets α < 0.1° .

Setting System α [°] Teflon Body 0.5 Fixing Device 0.1 Indented Geometry 0.05

Table 3: Concentricity in different setting systems

5.2 Bonding

Once the tension testing was done, it checked the bonding of each specimen and it analyzed the Load-Displacement chart. This was considered more significant than the classic σ − ε chart.

5.2.1 Long Lap-Joint

5.2.2 Long Lap-Joint 1

Figure 35 illustrates the Load-Displacement chart: the maximum load is of 37.7 kN. The results of bonding is illustrated in Figure 36. As it is indicated, there is the problem of kissing bond.

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5.2 Bonding 5 RESULTS

Figure 35: Load-Displacement chart

Figure 36: Photos of the bonded surfaces: internal adherent on the left and exter-nal one on the right

5.2.2.1 Long Lap-Joint 2 In this specimen DELO-SACO® PLUS was used: a blasting material with organo-functional adhesive agent [10]. This should give more strength to the interface areas, so the adhesive should work better.

Figure 37 illustrates the Load-Displacement chart: the maximum load is of 49.7 kN. The results of bonding is illustrated in Figure 38.

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5 RESULTS 5.2 Bonding

As it is indicated, there is the problem of kissing bond yet but less than before. For this reason the strength of the joint is increased. Furthermore, a little flat behavior is visible at the end of the chart. It could be a plastic behavior of adhesive due to the blasting material that leads to a more cohesive failure in the adhesive because the interface areas are more strong with it. Finally the decreasing of kissing bond implies that this phenomena is caused by a poor adhesion between the adhesive and adherent, taking constant the curing process.

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5.2.3 Little Lap-Joint

5.2.3.1 Little Lap-Joint 1 Figure 39 illustrates the Load-Displacement chart: the maximum load is of 18.2 kN. The results of bonding is illustrated in Figure 40. As it is indicated, there is the problem of kissing bond. There is a flat behavior at the end of the chart. It could be a plastic behavior but in the failure there aren’t so clear plastic breaking.

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5.2.3.2 Little Lap-Joint 2 Figure 39 illustrates the Load-Displacement chart: the maximum load is of 20.7 kN. The results of bonding is illustrated in Figure 40. As it is indicated, there is the problem of kissing bond. Also in this chart there is a flat behavior at the end but at the same time there aren’t so clear marks of plastic breaking.

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5.2.4 Indented Scarf-Joint

5.2.4.1 Indented Scarf-Joint 1 Figure 43 illustrates the Load-Displacement chart: the maximum load is of 58.2 kN. The results of bonding is illustrated in Figure 44. In this joint there isn’t the problem of kissing bond. As is indicated in the figure, there is adhesive in the cylindrical and vertical surfaces.

There is a flat part at the beginning of the chart: it could be a plastic behavior and than a breaking of some little areas inside the joint, for example the bonding in the cylindrical and vertical surfaces.

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5.2.4.2 Indented Scarf-Joint 2 Figure 45 illustrates the Load-Displacement chart: the maximum load is of 55.2 kN. The results of bonding is illustrated in Figure 46. In this joint there isn’t the problem of kissing bond. This specimen has less concentricity than the other Indented one: the less strength could be due to it. Once again there is adhesive in the cylindrical and vertical surfaces but there is the flat part in the chart.

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5.2.5 Ideal Scarf-Joint

5.2.5.1 Ideal Scarf-Joint 1 Figure 47 illustrates the Load-Displacement chart: the maximum load is of 28.4 kN. The results of bonding is illustrated in Figure 46. This specimen had β ∼= 1°: this result was due to a human mistake during the setting process. In this joint there isn’t the problem of kissing bond but, as it is indicated in the figure, there are same area where the bonding isn’t a good one.

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5.2.5.2 Ideal Scarf-Joint 2 Figure 49 illustrates the Load-Displacement chart: the maximum load is of 26.5 kN. The results of bonding is illustrated in Figure 50. Also this specimen had β ∼= 1° due to the same reason. As well, there isn’t the problem of kissing bond but, as it is indicated in the figure, there are same area where the bonding isn’t a good one.

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5.2.5.3 Ideal Scarf-Joint 3 Figure 51 illustrates the Load-Displacement chart: the maximum load is of 53 kN. The results of bonding is illustrated in Figure 52. In this joint there isn’t the problem of kissing bond.

The only difference with the specimens before is the concentricity: this spec-imen had β ∼= 0.15°. So one more order of magnitude of concentricity leads to more than 20% of strength. Furthermore, the start of the chart is now as others.

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There is a flat part after the beginning: it could be due to the vertical surfaces of breaking that might be break before the main overlap area. Finally, the bonding is great so the concentricity leads also to a better joint.

5.2.5.4 Ideal Scarf-Joint 4 Figure 53 illustrates the Load-Displacement chart: the maximum load is of 48 kN. The results of bonding is illustrated in Figure 54. In this joint there isn’t the problem of kissing bond. Also this specimen had β ∼= 0.15°. So it is worth that one more order of magnitude of concentricity leads to more than 20% of strength. Furthermore, the start of the chart is now as others. Finally, also here the bonding is great.

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Concentricity is so important because a decrease of it leads to a more possi-bility of bending load and a inconstant thickness of the adhesive. An increase of β implies a non-collinearity between the axes of the parts of the specimen: if the traction load is collinear with one of them, it won’t be as well with the other part. This leads to a bending load. Than an increase of β implies also an increase of adhesive thickness in a edge of the overlap length and a decrease in the other edge.

5.2.6 General Considerations

In all of the specimens, it can see different kind of breaking: cohesive failure in the adhesive and interfacial one. In any of them, cohesive failure in the adherent occurs.

Figure 55 illustrates the Load-Displacement charts of all specimens. All of them have the same E-module at the beginning except the two specimens with low concentricity.

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6 DISCUSSION AND CONCLUSION

The brittle behavior, which was modeled, appears in most of the charts of all specimens and where there is a different behavior it is not clear if it’s a plastic one. Table 4 illustrates the comparison between the breaking load provided by FEM simulation and that got from testing. Considering the presence of kissing bond in all Lap-Joints, the presence of others bonding surfaces (vertical and cylindrical) in all Indented Scarf specimens and the unconcentricity of the first two Ideal Scarf ones, it can be considered a good first prediction.

Joint FEM [kN] Testing [kN]

Long Lap 67 (1) 37.7 , (2) 49.7

Little Lap 38 (1) 18.2 , (2) 20.7

Indented Scarf 53 (1) 58.2 , (2) 55.2

Ideal Scarf 53 (1) 28.4 , (2) 26.5 , (3) 53 , (4) 48 Table 4: Comparison between FEM and Testing breaking load

In the beginning of same charts there is a flat part and it should be a testing error. It has to be demonstrated yet.

6 Discussion and Conclusion

All Lap-Joint Specimens show the problem of kissing bond, also with the overlap length of 5 mm. Instead the Scarf-Joint specimens don’t show the problem in any case. The idea is that to not get the kissing bond there should be a minimum perpendicular component of bonding direction during the joint construction. In fact, Lap-Joints have not this component: the bonding surfaces overlap each other with parallel directions. During the Scarf-Joints construction, instead, the bonding surfaces move each other with a perpendicular component. It should be right because in this way there is a component of pressure perpendicular to the these surfaces that might lead to a better adherence between adhesive and adherents. Thus a future development might be to inject the adhesive inside the joint: in this way it gets some perpendicular pressure of the adhesive against the interested surfaces.

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6 DISCUSSION AND CONCLUSION

Concentricity has a main rule in bonding as demonstrated. Thus the fixing device could be development to be less customized so more flexible towards the specimen’s dimensions.

As reliability, the bonding depends too much from the person who makes the joint. The possibility to make it with injection mode could be also for this a better solution. The Indented Scarf-Joint got good results as concentricity, as strength and as bonding but there is adhesive in the cylindrical and vertical surfaces. Its influence in the joint’s behavior is difficult to calculate and it is not repeatable.

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REFERENCES REFERENCES

References

[1] Lucas F. M. da Silva, Andreas Ã–chsner, Robert D. Adams (auth.), Lucas F. M. da Silva, Andreas Ã–chsner, Robert D. Adams (eds.), Handbook of Adhesion Technology

[2] Konstantin Priesnitz, On local panel distortions due to hot-curing adhesives [3] L. Andreozzi, Analisi dinamico-meccanica in sistemi viscoelastici

[4] Dan Gleich, Stress analysis of structural bonded joints

[5] J. Baumgartner, H. Schmidt, G. Rybar, T. Melz, L. Ernstberger, D. Teuten-berg, O. Hahn, G. Meschut, B. Schneider, C. Nagel, Auslegung von geklebten Stahlblechstrukturen im Automobilbau für schwingende Last bei wechsel-nden Temperaturen unter Berücksichtigung des Versagensverhaltens

[6] Choothum Jeenjitkaew, Kissing Bonds in Adhesive Joints: A Holistic Ap-proach for Surface Chemistry and Joint Mechanics

[7] Stefan Paulke, Thermomechanical Simulations & Measurements for Adhe-sives

[8] M. Beghini, Course of Mechanical Behavior of materials [9] BETAMATE 1496V (DOW), Technical Datasheet [10] DELO-SACO PLUS (DELO), Technical Datasheet [11] DIN EN 14869-1

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REFERENCES REFERENCES

Appendix

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47,70 g6 37,72 H7 A A 39,72 H7 80 16 + + 0,03 0,02 19 64,91 +-0,050,05 A SECTION A-A SCALE 1 : 1 0.01 A B A B 4,07 +-0,05_{0,05 R0,30} 20° - 0°_0,10° DETAIL A SCALE 2 : 1 2.3 2 0.01

20°_ind_ext

GEWICHT: A4 BLATT 1 VON 1 MASSSTAB:1:2 ZEICHNUNGSNR. STÜCKZAHL: SIGNATUR NAME ENTGRATEN UND SCHARFE KANTEN BRECHEN WENN NICHT ANDERS DEFINIERT:

BEMASSUNGEN SIND IN MILLIMETER OBERFLÄCHENBESCHAFFENHEIT: TOLERANZEN: LINEAR: WINKEL: QUALITÄT PRODUKTION GENEHMIGT GEPRÜFT GEZEICHNET SKALA: MATERIAL.:

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ÄNDERUNG ZEICHNUNG NICHT SKALIEREN

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47,70 g6 37,72 H7 A A 80 19 16 + + 0,03 0,02 39,72 g6 A SECTION A-A SCALE 1 : 1 0.01 A B A B 20° -0° 0,10° 0,50 X 45° R0,30 4,95 +-0,050,05 DETAIL A SCALE 2 : 1 2.32 0.01

ZEICHNUNG NICHT SKALIEREN ÄNDERUNG

SKALA: GEZEICHNET GEPRÜFT GENEHMIGT PRODUKTION QUALITÄT

WENN NICHT ANDERS DEFINIERT: BEMASSUNGEN SIND IN MILLIMETER OBERFLÄCHENBESCHAFFENHEIT: TOLERANZEN: LINEAR: WINKEL: MENGE: ENTGRATEN UND SCHARFE KANTEN BRECHEN

NAME SIGNATUR DATUM

ZEICHNUNGSNR.

MASSSTAB:1:2 BLATT 1 VON 1

A4 GEWICHT:

20°_ind_int

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STÜCKZAHL: MATERIAL.:

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47,70 g6 37,72 H7 A A 80 16 + -0,03 0,02 19 45 46,60 20° - 0°_0,10° A SECTION A-A SCALE 1 : 1 2.3 2 0.01 A B 0.01 C A B C R23 DETAIL A SCALE 2 : 1

2 20°_ideal_ext

GEWICHT: A4 BLATT 1 VON 1 MASSSTAB:1:2 ZEICHNUNGSNR. BENENNUNG: ÄNDERUNG ZEICHNUNG NICHT SKALIEREN

WERKSTOFF: DATUM SIGNATUR NAME ENTGRATEN UND SCHARFE KANTEN BRECHEN OBERFLÄCHENGÜTE:

WENN NICHT ANDERS DEFINIERT: BEMASSUNGEN SIND IN MILLIMETER OBERFLÄCHENBESCHAFFENHEIT: TOLERANZEN: LINEAR: WINKEL: QUALITÄT PRODUKTION GENEHMIGT GEPRÜFT GEZEICHNET

S355

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37,72 H7 47,70 g6 A A 80 19 16 + + 0,03 0,02 45 20° - 0°_0,10° 46,60 A SECTION A-A SCALE 1 : 1 2.3 2 0.01 A B 0.01 C A B C R23 DETAIL A SCALE 2 : 1

20°_ideal_int

GEWICHT: A4 BLATT 1 VON 1 MASSSTAB:1:2 ZEICHNUNGSNR. ÄNDERUNG ZEICHNUNG NICHT SKALIEREN

2 S355

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47,70 g6 37,72 H7 A A 80 16 + + 0,03 0,02 19 43 17,90 SECTION A-A SCALE 1 : 1 2.3 2 0.01 A B 0.01 A _B

Raccordo non quotato R0.1

Lap1_ext

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47,70 g6 37,72 H7 A A 80 16 + + 0,03 0,02 19 42,40 17,90 SECTION A-A SCALE 1 : 1 2.3 2 0.01 A B 0.01 A B

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47,70 g6 37,72 H7 A A 80 16 + + 0,03 0,02 19 43 11,50 SECTION A-A SCALE 1 : 1 2.3 2 0.01 A B 0.01 A B

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47,70 g6 37,72 H7 A A 80 16 + + 0,03 0,02 19 42,40 11,50 SECTION A-A SCALE 1 : 1 2.3 2 0.01 A B 0.01 A B

2 GEWICHT:

Lap2_int

A4 BLATT 1 VON 1 MASSSTAB:1:2 ZEICHNUNGSNR. ÄNDERUNG ZEICHNUNG NICHT SKALIEREN

Development, Testing and FE-Simulation of Adhesively Bonded Overlapping Tube Specimen

Master Thesis

Development, Testing and FE-Simulation of

Adhesively Bonded Overlapping Tube Specimen

Contents

1

Introduction

1.1

Definitions

1.2

Background

∑

1.3

Aim of thesis

2

Analysis

2.1

Materials

2.2

Setting Analysis

2.3

Geometry of joint

2.4

Choices

2.5

Strength Analysis

3

Manufacturing

Check List

3.1

Setting and Jointing

3.2

Curing Process

4

Testing

4.1

Concentricity

4.2

Static Tension Testing

5

Results

5.1

Concentricity

5.2

Bonding

6

Discussion and Conclusion

References

Appendix

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