Experimental Procedure Chapter 4:

Experimental Procedure

Chapter 4:

Introduction

4.1.

The parameters involved in a FSW based technology are various and can influence in different ways the mechanical properties of the joins. In order to explain in more accurate and proper way the path that has been followed to obtain the final result, is convenient to take as reference the figure below.

As explained in the introduction the main idea is the comparison of the well-known technology of FSW with the new process developed recently of Stationary Shoulder FSW with the final aim of an active application in aeronautical structures.

In order to simplify the analysis of the two processes the parameters have been classified in: • Material Parameters

• Process Parameters

Once that best set of parameters has been defined, according to microscope investigation, bending test, hardness test and tensile test, new weld has been done in order to proceed to further investigations. Particular attention was used in the study of the local deformations of the specimens under monotonic load with the intention of detecting irregular behaviour or weak point in the welds and the crack resistance of the joints.

Figure 4-1: Schematic representation of the experimental procedure followed in this work.

4.1.1. Process Parameters

The process parameters are strictly dependent on the type of technology used. The main point is that in order to obtain good quality welds the rotational speed for the SSFSW needs to be higher than the one for the conventional FSW. This is related, as explained before, to the different heat generation in the two processes and the different type of mechanical actions applied by the probe to the material.

In the case of SSFSW a tilt angle of 1° of the spindle with respect to the Z-axes of the tool in the direction of the welding path was applied, following a criteria based on the experience and previous studies [1]. In this way is possible to increase the pressure on the back part of the shoulder and decrease the one in front, obtaining a better surface finishing. This procedure was possible just for the SSFSW due to the concave shape of the non-rotating shoulder while was not possible for the conventional process performed with a flat shoulder with threads, as described in the next paragraph.

In this study the parameters used, have been selected from previous internal works and thanks to the great experience accumulated in years of research by the WMP department. In the table below is shown the set of parameters, strictly related to the machine, chosen for this research. It is important to remark that the FSW specimens were welded in force control, while the SSFSW specimens in position control.

Welding Speed (mm/s) Rotational Speed (rpm)

FSW 3 5 8 10 600

SSFSW 1 2 3 4 1200

Table 4-1: Process Parameters.

In order to make a comparison these factors have been related together in a formula (equation 3-1) developed from the work of Khandkar [2]; in this way is sufficient to vary just one of the two main parameter involved to change the energy input transferred into the material.

Equation 4-1 𝑞𝑠= 𝜂2𝜋𝜔𝑇_60𝑓 • 𝑞𝑠: Energy input per unit length, or heat input (kJ/m) • 𝜔: Probe rotational speed (rpm)

• 𝑇: Applied torque (Nm) • 𝑓: Welding Speed (m/s)

• 𝜂: Efficiency of the heat transfer into the weld

Differently from the approach of Khandkar, that was considering as torque the one directly caused by the shear forces generated by the probe, with an approximation of a uniform distribution of these along the tool, with this formula, more practical to use, the torque is the one measured directly by the machine. In the case of the formula above the main approximation is due to the fact that are not considered the internal frictions in the machine during the process. In any case this rough calculation provides an upper bound of the heat transferred into the material and is possible to take in consideration of this effect with the term of efficiency.

The efficiency is also an important term when we compare the two different technologies. It is important to notice that not all the heat produced by the shoulder in the conventional FSW is necessary to obtain a good quality weld. Not only the friction between the workpiece and the tool causes the self-generated heat involved in the process, but also by the deformation heating associated with shearing within the workpiece. In the case of SSFSW the heat produced during the process is much lower (a difference of around 70°C has been measured with thermocouples between FSW and SSFSW) and the most of it is due to the action between the probe and the materials, resulting in a process more efficient and localizing the heat where is more necessary. Comparing the results obtained is possible to determine a value of 𝜂 around 0.65 in the case of FSW respect to SSFSW.

In the continue of this work the comparison between the two processes will be realised just equating the four different welding speed in order to avoid confusion and have a term of comparison directly measured by the machine and not derived with uncertainty by the previous equation.

The probe length, which is the other parameter mentioned in scheme 3-1, is fixed in the case of standard FSW and a specific tool for the specific thickness that is necessary to weld needs to be realised. In the case of SSFSW, being the probe free to rotate in the stationary shoulder, it is possible to adjust this length in order to have a full penetration and increase the pressure of the shoulder on the surface of the materials. A more detailed description of the tools used for the two processes is given in the following paragraphs.

4.1.2. Material Parameters

The problem connected to the materials is due to the fact that in this study the joining process is between two dissimilar aluminium alloys, AA 2024-T3 and AA 7050-T7651, that have different chemical compositions and mechanical properties and they have experience different heat treatments. As explained in the Chapter 2 FSW and SSFSW are joining processes with an asymmetric behaviour perpendicularly to the welding line, due to the difference of heat generated in the AS and RS of the weld. This is the reason why the first part of this work has been concentrated on the research of the best configuration for the position of the two materials in order to obtain the best mechanical properties for both the technologies investigated.

The other problem of interest that has been studied is related to the position of the materials in relation with the rolling direction. The sheets of aluminium used for this work have a thickness of 2 mm and have been obtained by cold rolling, causing an elongation of the grains in longitudinal direction and consequently different mechanical properties in different directions. The scheme here below can be useful to understand better the matter.

The elongated grains are represented in this figure as dashed lines. In the first case, the rolling direction of the two materials is parallel to the welding line while in the second case this one is perpendicular to the joint.

Materials

4.2.

Our project partner EMBRAER purposed the two different aluminium alloys, AA 2024-T3 and AA 7050-T7651, used in this study. EMBRAER is using these alloys to produce on their airplanes the skin panels and the stringers respectively. The dissimilar welds were realised using plates of

Figure 4-2: Schematic representation of the material disposition.

the base material cold rolled with dimensions: 300mm x 150mm x 2mm. The welds were performed along the 300 mm side in order to have enough space to cut out all the necessary specimens.

4.2.1. AA 2024-T3

The 2000 series of aluminium alloys has been widely used in the construction of aircraft component that are designed in order to have good mechanical strength, good resistance to fatigue crack growth and high level of damage tolerance compared to the other aluminium alloys [3]. The main alloying elements of the AA 2024 are copper and magnesium and the high strength is due to the precipitation of Al2Cu and Al2CuMg. However, the presence of copper in

high amounts is detrimental for the corrosion resistance.

The aluminium 2024-T3 has been one of the most used materials in fuselage construction, the chemical physical and mechanical properties of this alloy can be found in the tables below.

Chemical composition of AA 2024 (%)

Cu Zn Mg Mn Fe Si Cr Zr Ti Al

4.4 - 1.5 0.6 ≤0.5 ≤0.5 0.1 - 0.15 Remainder

Table 4-2: Literature data for chemical composition of the aluminium alloy AA 2024 [3].

Mechanical properties of AA 2024-T3

(MPa)** UTS (MPa)**

Elongatio n at break (%)** Modulu s of Elasticit y (GPa)** Fracture Toughness, KIC (MPa∙m1/2₎ * 2.78 134 L T D L T D 20.15 72.5 37 37 9 319 325 487 474 468 *Literature [4].

** Tested by M. Eng Anne Groth (WMF, HZG) and evaluated by the author of this report. Table 4-3: Mechanical properties of the aluminium alloy AA 2024-T3.

The term T3 that follows the aluminium alloy identification code is necessary to recognise the type of heat treatment that was applied to the material. In this case T3 represent an aluminium that was “Solution heat treated, cold worked, and naturally aged to a substantially stable condition”[4].

4.2.2. AA 7050-T7651

The 7000 series of aluminium alloys has been used for aeronautical components that need to have high strength and good fatigue resistance; typical applications are the upper wing skins and horizontal and vertical stabilizer. The 7000 series is the one that show the higher strength of all the other classes of aluminium alloys thanks to the combination of elements as zinc, magnesium and copper. One of the main problem of these alloys is the high susceptibility to corrosion in hostile environment like the one that can be found in the life of an aircraft [3]. The AA 7050 is produce in thick plates or extrudes and this is the reason why it was hard to find this material in the desired thickness used in this study. The chemical and mechanical properties of AA 7050-T7651 are listed in the tables below.

Chemical composition of AA 7050 (%)

Cu Zn Mg Mn Fe Si Cr Zr Ti Al

2.3 6.2 2.25 - ≤0.15 ≤0.12 - 0.1 - Remainder

Table 4-4: Literature data for chemical composition of the aluminium alloy AA 2024 [3].

Mechanical properties of AA 7050-T7651

Density

(g/cm3₎ Hardness, _Vickers Yield Strength _(MPa) UTS (MPa)

Elongation at break (%) Modulus of Elasticity (GPa) Fracture Toughness, KIC (MPa∙m1/2₎ 2.83 171 490 552 11 71.7 L T 30.8 26.4 Table 4-5: Mechanical properties of the aluminium alloy AA 7050-T7651 [4].

The T7651 after the alloy code intend that material has been “Solution heat-treated, stress relieved by stretching a controlled amount and then artificially over aged in order to achieve a good exfoliation corrosion resistance. The aluminium receives no further straightening after stretching” [5].

FSW and SSFSW Systems

4.3.

The conventional FSW and SSFSW joints have been realized directly at the WMP department of the HZG using the FSW Gantry System. This machine has three degrees of freedom: referring to the figure 4-3, the Z and Y-axes movement are realised via a transverse motion of the welding head whereas the X-axis movement is realized via a movable table. The working angle of the installed welding system can be set up prior to and is fixed during the welding procedure.

The FSW Gantry System, shown in the figure 4-3, is conceived as a rugged frame structure that has the scope of support for the welding head absorbing the force involved with the processes. The cross beam has the possibility to adjust the work angle of the tool holder platform. There is the possibility to mount different clamping system on the movable table.

The main technical data for the machine are the following: • Axial Force (Z): 80 kN

• Side Force (Y): 15 kN

Z

Y X

Figure 4-3: HZG FSW Gantry System.

• Force in Welding Direction (X): 20 kN • Rotational speed of the Spindle: 3500 rpm • Continuous Spindle Torque: 190 Nm • Maximum Spindle Torque: 340 Nm • Tilt Angle: Between -3° and +3°

• Welding Speed: Between 1 and 160 mm/s • Maximum Continuous Weld Length: 2390 mm

4.3.1. Clamping System

The clamping system is playing an important role during the welding process. A movement of the plates during the weld can cause not only a bad superficial condition but also many internal defects, as lack of penetration and high porosity, or a complete failure of the joint itself. In the present work the same clamping system was used for both the processes. The typical clamping configuration is shown in the picture below.

The clamping system consists of an aluminium plate with a steel inlay as backing bar. The prevention of horizontal movement of plates, perpendicular to the welding direction, is due to two adjustable steel blocks per each side. One aluminium bar is also used to avoid horizontal movement of the plates in the welding direction.

Vertically, as it is possible to see from the figure 4-4, the plates were clamped with two steel bars along all the length of the weld blocked with the devices pointed with an arrow in the

Figure 4-4: Standard FSW and SSFSW clamping system on the Gantry System.

picture. During the process it is possible to have a protrusion of the plates due to the heat and the forces developed by the weld and the clamping system.

4.3.2. FSW Tool

The tool and its geometry play a critical role in the process. They two main objectives of the tool are to generate the heat necessary for the process and at the same time produce the material stirring with consequent plasticized flow [6].

Since the patent of this technology [7], many different FSW tools have been invented with the aim of providing a more uniform heat generation and an improvement in the mechanical performances of the joints. In general the standard conventional FSW tool has two main components:

• The shoulder is the element that generate the major part of the heat by the friction with the surface of the material, keep the heated volume of the softened material in the plasticized zone, in order to avoid loss that can generate porosity and cavity imperfections, and contribute little to the mixing of the material in the crown side of the weld.

• The probe result fundamental in the phase of plunging in the material, is just this that is generating the necessary heat to start softening the material to be welded. The probe during the welding phase is giving the major contribution to the stirring and mixing of the material. The geometry of the probe can strongly influence the material flow and this is the reason why is covering a fundamental role during the process. A well distributed material flow induces a more uniform microstructure with better mechanical properties.

The tool used in this study is an evolution of the basic tool used at the beginning and can be seen in the figure below where are indicated the two main component previously described.

The main characteristic of this tool are:

• Flat shoulder of 13 mm of external diameter with a spiral profile;

• Conical probe with a diameter of 5 mm at the base and 3 mm at the top. The probe has a left-handed thread with three flat surfaces on at the sides (TriflatTM_).

• Shoulder and probe are made with the same material: HOTVAR® developed by

Bohler-Uddeholm Corporation. This material is a high performance molybdenum-vanadium

alloyed hot-work tool steel, necessary for applications where hot wear and plastic deformations are the dominating failure mechanism.

4.3.3. SSFSW Tool

In case of SSFSW joints the shoulder gives a low contribution to the heat generation since is simply sliding on the surface and is just keeping the softened material close to the plasticised zone. The heat in this case is more localised around the probe and it is uniformly distributed through the thickness. The component of heat generated by the friction is decreased and in this way becomes more importance the one due to the deformation of the material.

The probe use in the two processes is the same and the only different consist in the different clamping with the spindle since, in order to have a non-rotating shoulder, the probe used for SSFSW needs to be directly connected with this, by-passing completely the shoulder that is bolted with a static flange.

Figure 4-5: 3D illustration of the FSW tool used for this study.

The shoulder, as previously said, gives a small contribution to the process and for this reason is not completely flat, but concave in order to reduce the friction, as can be seen in scheme of figure 4-6. The shoulder has a curvature with a radius of 48 mm and is realised in HOTVAR® like in the previous case.

Characterisation

4.4.

The mechanical performance of welded joints, especially for aeronautical components, is a complex problem that needs to be investigated under different point of view.

The first step is related to the material science and involves the macrograph investigation of the samples and the measure of elements that can influence the welding behaviour in relation with the parameters utilised for the process and Energy Dispersive X-Ray Spectroscopy (EDX) evaluation.

Static Mechanical performances of the weld have been discovered by means of the basic tests with particular attention to the local properties in the different zones affected by the weld. Following this approach in this work transverse tensile test, three point root bending test and microhardness test have been used.

The last, but probably the most important characterisation for an aeronautical application, is related to the answer of the joint in the presence of a crack or a defect under an applied load. In order to define the damage tolerance of the weld toughness test has been applied.

Figure 4-6: Schematic view and section of the SSFSW tool.

In the following figure is presented an example of the cutting plans used to obtain the specimens necessary for the different typology of tests, all the specimens for all the type of test were obtained perpendicularly to the welding direction and more specimens per each weld were taken in order to understand the constancy of the mechanical properties along the weld.

It is important to notice that in order to follow the standard ISO 25239-1 [8], a length of at least 50 mm should be left from the beginning and the end of the welding line.

In the following paragraphs is shown a detailed description of the procedure used for the different type of test and the type of specimen used.

Figure 4-7: Cutting plan for tensile specimens SSFS welded with 3 different WS.

Macrograph Investigation

4.5.

The samples used for the microscope analysis have been cut directly in the department using the Axiotom cutting machine from Struers, mounting the cut off wheels 30A35 apposite for the materials with HV<300 with dimension 350x2.5 mm. In order to apply high forces to the sample during the cutting process the feed speed was regulated at 0.8 mm/s.

After cutting, the samples were prepared following the procedure shown in table 1. It is important to remark that the mounting of the samples was done in order to show always the face in the welding direction.

Steps

Description

Embedding Clarocit-Polymeric Resin – Colour Green Grinding 1 SiC Grinding Paper – Grade: 320 – Time: ≈ 1 min Grinding 2 SiC Grinding Paper – Grade: 600 – Time: ≈ 1 min Grinding 3 SiC Grinding Paper – Grade: 1200 – Time: ≈ 1 min Polishing 1 Diamond Suspension – 0.3 µm – Time: ≈ 3 min Polishing 2 OP-S Suspension – 0.5 µm – Time: ≈ 3 min

Figure 4-8: Sketch of the sample for macrograph and EDX analysis of SSFSW joint.

Etching Dix-Keller Reagent – Immersion Time: 15 s Table 4-6: Sample Preparation

The grinding and polishing procedures have been realized using the automatic machine “Struers Tegramin 30”; in this way is possible to obtain a better surface quality and a major uniformity in the treatment of the samples.

The macrographs were realised using the optical microscope “LEICA DM IRM”, equipped with a “LEICA DFC295” camera for the acquisition of the pictures.

The microscope present five different objective lenses (1.6x; 5x; 10x; 20x; 50x) and an optical zoom of 10x. Due to the size of the specimens was impossible to fit the entire sample in just one picture, especially in the case of the higher magnitudes, for this reason a minimum number of four pictures has been take and then these have been merged together with the use of the software Adobe Photoshop CS and Microsoft ICE.

During the evaluation of the FSW and SSFSW joints there are some main aspects that are important to be considered:

• The mixture between the two different materials; • Internal imperfection;

• Sufficient probe penetration.

On the picture obtained from the merged images has been also measured the area of the Stir Zone in order to correlate this measure with the mechanical properties of the specimens. To realize this was used the software “ImageJ – Image processing and analysis in Java”.

EDX Analysis

4.6.

Energy Dispersive X-Ray Spectroscopy (EDX or EDS) is a microanalysis technique used for the analysis of the chemical elements present in a sample. During EDX analysis the characteristic x-ray spectrum emitted from a sample during irradiation of a focused electron beam is used to characterize the chemical elements of the analysed volume.

When the sample is irradiated by an electron beam, electrons are ejected from the atoms comprising the sample’s surface. The resulting electron vacancies are filled by electrons from a higher state, and an x-ray is emitted to balance the energy difference between the two electrons states. The x-ray energy emitted by each element has a distinctive characteristic that allows the identification of the element considered.

For the EDX analyses during this work a Scanning Electron Microscope (SEM) JEOL JSM-6490LV, with an EDAX X-ray detector and the EDAX Genesis analysis software was used.

The evaluated chemical element was Zinc, since that this is the only element with significant difference between the two aluminium alloys. The AA 7050 has a percentage of Zinc between 5.7 and 6.7%, while in the AA 2024 this percentage is minor than the 0.25% [4].

Bending Test

4.7.

The three point bending test is a destructive test used historically to assess the quality of the welds, even prior of the advent of Friction Stir welding processes. The reason for the use of this test, more qualitative than quantitative, is related to the simplicity and the low time consuming related to this test. This test can be performed immediately after having welded the plates and can give an immediate feedback about the quality of the joining. The bending test can be considered an “in or out” test, fixing a minimum acceptable angle, considering the one that can be reached by the base material, and then decide if the specimen has passed or not the test.

The specimen used in this study for this test is shown in the picture below and has been cut using the same machine used for the samples of the macrograph analysis.

The test has been performed following the standard ASTM E 190-92 [9]. The equipment used for the test is represented in a schematic way in the figure 4-10.

All the parameters necessary for the machine set up can be related with the thickness of the specimen, in our case we had:

• a = 2 mm; • l = 15.2 mm; • D = 8 mm.

Figure 4-9: Sketch of the geometry of the FSW and SSFSW root bending specimens.

Figure 4-10: Bending Test set up.

In order to have more comparable results the specimens has been tested in the Zwick/Roell universal testing machine with a load capacity of 100 kN. The test has been stopped when the drop in the load was reaching the 75% of the maximum force applied, in this way all the specimens has been tested in the same condition. After the end of the test the angle has been measured, as shown in the previous picture, and considerations about the quality of the welds has been done.

Microhardness Test

4.8.

The micro indentation hardness test is a certified method for measuring the hardness of the material on a microscopic scale. Depending on the type of indenter used to test the material there are many different type of microhardness tests: Rockwell, Vickers, Knoop, etc. In the present work the type of test used is the Vickers microhardness test method that is performed using “a square-based pyramidal-shaped diamond indenter with face angles of 136°”[10]. This type of test is used to measure the ability of the material to withstand a permanent, plastic, deformation and give a qualitative idea of the variation of the strength across the weld. It has been proved that the hardness value measured in the test is related in a proportional way to the strength and to the yield stress. The relation with the yield stress was found to be [11, 12]:

Equation 4-2 𝜎𝑦= 3.55𝐻𝑉

In this equation the yield stress is expressed in MPa and the Vickers’ hardness in Kg/mm2_.

The microhardness tests were performed on a Zwick/Roell ZHV machine, applying a load of 0.2 kgf for 10 seconds, always in accord with [10]. Depending on the purpose of the study, have been done one or three lines of indentation. When just one line of indentations was done this was positioned in the middle of the thickness of the specimen, so one millimetre from the top or bottom surface; on the other hand with the three lines of indentations, a distance of 0.5 mm from the top surface and between each line was taken. The distance between the indentations was 0.5 mm and the total length of the horizontal profile was 30 mm, symmetrical respect the weld centre. An example of the test configuration can be seen in the figure below.

The microhardness test is also important to obtain additional information about the SZ, TMAZ and HAZ sizes in the case that these are not clearly visible during the macrograph analysis. This problem is particularly accentuated concerning the HAZ where the grain distortion is much smaller respect to the other characteristic areas of the weld.

Figure 4-11: Exemplary microhardness measurement indentations across a FSW joint.

Tensile Test

4.9.

The tensile tests were performed with the help of a DIC system, whose functionality is explained in the next chapter, on a Zwick/Roell universal testing machine with a load capacity of 100 kN. In order to have a comparison between the data obtained from the DIC, the elongation was measured using a mechanical extensometer MTS with a gage length of 50 mm, similar to the one in the picture.

The test was executed following [13] at room temperature, around 22°C, and with a constant transverse speed of 1mm/min. In order to combine this test with the DIC system the specimens have been designed with an extended central area for an easier application of the paint pattern. The specimens sketch can be seen in figure 4-13.

Figure 4-12: Standard extensometer for tensile test.

Figure 4-13: Sketch of the specimen used for the tensile test.

R-curve determination Test

4.10.

This test was applied just to the specimens obtained by SSFSW, being the one that have showed the best results in all the previous test performed. The R-curve is defined in the standard ASTM E561 as “a continuous record of toughness development plotted against crack extension in the material as a crack is driven under a continuously increased stress intensity factor, K” [14]. As explained in the previous chapter, dedicated entirely to the fracture mechanics concepts, the advancing of the crack in the material is happening when the driving force overcome the resistance against crack growth of the material. In this study, the main parameter used to characterise the resistance of the material against crack growth is the δ5.

This parameter has been discovered to have a higher independence, respect to the classical CTOD that is not measured directly in front of the crack tip, of size and geometry of the specimens [15]. The δ5 is measured directly on the surface of the specimen at the side of the

pre-crack tip and the gage has a length of 2.5 mm at either side of the crack. In this way is possible to measure in the center-cracked tension panel, M(T), two different values of the crack opening displacement for the two advancing crack, differently from the classical CMOD method. In elastic-plastic fracture mechanics field has been demonstrated that the relation between δ5 and the J-integral can be expressed as [16]:

Equation 4-3 𝛿5= 𝐾𝑝𝑙 2 𝐸𝜎𝑦= 𝐽 𝜎𝑦

In this equation 𝐾𝑝𝑙 is the stress intensity factor corrected in order to consider the effects of plasticity.

For each specimen was not measured just the δ5 at the two crack tips but also the classical

CMOD in case of failure of one of two parameters.

The design of the specimen was decided according to the standards ASTM E561 [14] and GKSS EFAM GTP-02 [17]. The main problem in the decision of the specimen size regards the width of the specimen. In order to have a crack growth is important that the stress in the in the net ligament, determined as the difference between the width of the specimen and the effective crack, is below to the yield stress of the material. In order to satisfy this requirement the standard gives a recommended width in function of the ratio between the maximum stress intensity factor and the yield stress of the material. In this case, missing the data for the real

𝐾𝑚𝑎𝑥 of the welded materials, the specimen was dimensioned just considering the values for the AA 2024-T3 that is the material with highest value of stress intensity factor and lowest yield stress and deciding a possible value from the literature [18]. The representation of the specimen can be seen in figure 4-14.

The notch, which is not represented in the sketch, was positioned in three different zones of the weld: AS, RS and SZ. The notch length was 50 mm and was symmetric respect the longitudinal axes of the specimen.

Other important parameters that need to be established before starting the test programme are related to the fatigue pre-cracking of the specimen. The constraint to follow is that the machined notch plus the fatigue pre-crack should be between the 25% and the 40% of the specimen width. All the information about the pre-cracking phase can be found in the table 4-7. Notch Length (mm) Pre-crack Length (mm) Maximum

Load (kN) R Load (kN) Medium

Load Amplitude (kN) Load Frequency (Hz) 50 3 16 0.1 8.8 7.2 20

Table 4-7: Fatigue Pre-Cracking parameters.

The approach used to develop the δ5-R-curvewas the multi-specimen method in which the test

was stopped at different levels of load for each specimen and after the ductile propagation of the crack was measured post mortem. The first test of each series of specimens was brought

Figure 4-14: Drawing of the center-cracked tensile specimen M(T).

until the end in order to have a full force-displacement curve and decide when to stop the following tests. The following tests, as is possible to see from the table 3-8, were stopped respectively at the maximum of the load-displacement curve and around the 80% of the maximum. The test transverse speed was constant and fixed at 0.25 mm/min and.

Welding Speed (mm/s) Crack Position Load Level (kN) 1 Advancing Size 97.5 90 80 Stir Zone 101 95 80 Retreating Size 99.5 90 75 2 Advancing Size 98.2 95 80 Stir Zone 105.7 100 87.5 Retreating Size 98.1 90 75 3 Advancing Size 101.2 100 80 Stir Zone 102.9 103 85 Retreating Size 101 100 80

Table 4-8: Test Parameters.

Due to small amount of specimen per each condition, the multi-specimen method was cross-referenced with the electrical potential drop method. Combining the two methods it was possible to interpolate the curve obtained from the potential drop method for the broken specimen and fit through the three experimental points obtained with the multi-specimen method.

The electrical potential drop method is based on the principle that the potential distribution around a crack varies with the extension of it. For this reason is necessary to create an electric circuit, with the specimen that is a part of it, and measure the changing of the Volts during the test. The measurement obtained can be related to the crack extension using the formula [17]:

Equation 4-4

𝑎 =2𝑊_{𝜋 cos}−1_� cosh(𝜋𝑦/2𝑊)

cosh �(𝜙/𝜙0) cosh−1( cosh( 𝜋𝑦_{2𝑊) / cos(𝜋𝑎}0/2𝑊))� � Where y is the distance between the electrodes close to the machined notch.

The use of all these instruments caused many problems for the installation on the specimens, considering also the necessary use of the anti-buckling guides as suggested by the standard ASTM E561 [14]. In addition to this, in order to perform the tests using the DIC system, there was the necessity to leave some free surfaces where apply the stochastic pattern and focus the camera. The final solution for the test set up can be seen in the picture below for the two sides of the specimen.

The specimen not broken after the test, in order to measure the crack length, have been cracked by fatigue until the final failure with R=0.7 and an Fmax equal to half of the maximum force reached during the test. The use of a fatigue test to break the specimen is for the simple reason that a fatigue crack has easily recognisable characteristics, impossible to be mistaken with the one of the original crack. The high value of R was with the purpose of avoiding contacts between the surfaces of the crack that would make impossible the recognition of the stable crack growth.

Figure 4-15: Final set-up for the R-curve determination test.

The measurements of the initial crack and final crack have been done on the colour 3D laser microscope VK-9700 Keyence at a magnification of 200x . With this powerful tool was not only much easier the recognition of the cracks, not simple in the case of an aluminium alloy with a normal optical microscope, but also have the 3D map of the fracture surface, with the possibility to implement it in a FEM software for the simulation of the crack path. An example of this is presented in the figure below.

Figure 4-16: Example of the 3D map of the fracture surface.

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