CHAPTER 2

CHAPTER 2

Literature Review

This chapter is a review of the previous work done which relates to this project. It will present the general background knowledge required and previous similar studies.

2.1 Delamination

Delamination is a major problem in composite laminates because it reduces mechanical properties and limits the safe life of a component. The susceptibility to delamination cracking in composites either compromises the exploitation level of their use in engineering applications, or limits their application envelope to non-primary structural components.

Delamination is an interlaminar damage and can be defined as a crack which runs in the resin rich area between plies of different fibre orientation and not between lamina in the same ply group [7]. Interlaminar damage can occur either at the fabrication stage (eg. problems occurring during cure or lay-up process) or at the stage of transport, storage and service (eg. local forces, thermal action or low velocity impact). Sound design methodology and fabrication techniques are

required to avoid delamination initiation. Figure 2.1 represents the three types of fracture path in continuos fibre reinforced composite materials.

Figure 2.1: Fracture path in composite materials [8]

Delamination might occur either within the bulk of the material or near the surface. With surface delamination, stability of crack growth may produce global instability of a structural component under compression. Multiple cracking without separation of layers is also typical for composite structures.

Delamination in composite laminates can grow either under monotonic, quasi-static or under cyclic loading.

Delamination resistance can be partitioned into two main categories:

• resistance to initiation, which represents the critical value of the interlaminar fracture toughness of a composite material to resist the onset of growth of a defect.

• resistance to propagation, which involves the capability of the material to arrest an existing crack.

Thus to understand delamination initiation and propagation, an understanding of interlaminar stresses is required. Several approaches exist to predict delamination

initiation and propagation. Two approaches are mainly used, one based on the strength of the material and the other one based on fracture mechanics [8].

2.1.1

Strength of Material Approach

This approach is based on the comparison of local states of stress with relevant strength. It is attractive for finite element analysis because strength can be easily determined. This method is based on the Quadratic Stress Criterion proposed by Brewer and Lagace [9].

2.1.2

Fracture Mechanics Approach

Interlaminar delamination is often treated as a crack in metallic structural material in terms of conventional fracture mechanics.

Fracture mechanics is a macro theory that defines parameters which characterise failure over regimes of known size, following the assumption that all bodies contain flaws or cracks. Fracture mechanics studies the response of a material containing such a flaw or crack of a known length a. According to the response of the material, a fundamental division of fracture mechanics into linear elastic (LEFM) and elastic-plastic (EPFM) is made. EPFM is used when the material exhibits a high degree of plasticity at the crack tip zone, invalidating description by linear elastic fracture mechanics. The more general J-integral method is then applied to characterise fracture. If the behaviour of the material is linear, elastic and homogeneous, even when large deflections produce a non-linear load-deflection curve then LEFM can be applied [10].

Fibre reinforced composite materials comply with LEFM conditions, being brittle in character, with a small crack tip damage zone in comparison with other specimen dimensions.

There are two LEFM approaches. The first is the elastic stress intensity field approach originally developed by Irwin, which uses the stress intensity factor K to describe fracture. Developed initially for isotropic materials, the solution of stress-field problems in anisotropic media involves the use of complex analytic function theory and can be very difficult. The second is the global energy balance approach based on work by Griffith, which uses the Strain Energy Release Rate (SERR) G to describe fracture, assuming that crack growth can be characterised in terms of energy per unit area necessary to create a new surface area. This approach results in surprisingly simple solutions and can be a very useful tool for design purposes. Delamination growth occurs when the SERR exceeds a certain critical value, GC, usually referred to as delamination toughness of the material.

GC can be related to KC, the critical value of the stress intensity factor for

delamination growth, depending on the assumed stress conditions [10].

Considering the material in Figure 2.2, with a crack length a, a uniform thickness b, undergoing self-similar propagation with a crack speed of å, under an applied load P, the delamination toughness GC is defined as the energy required for the

creation of the fracture area dA = b · da. By adopting the static LEFM assumptions (brittle fracture, small damage zones in comparison with other dimensions, all energy dissipation is embodied in G) and solving the energy balance equation, the fundamental expression for the strain energy release rate can be obtained:

GC = P 2

/2b · dC/da (Eq. 2.1)

Where C is the compliance defined as:

Figure 2.2: General loading on a cracked body [6]

GC is considered to be a material property, independent of specimen geometry,

which includes the effects of the matrix toughness, the fibre properties, the fibre to matrix adhesion as well as the interface strength between plies. However delamination toughness GC is dependent on the mode of fracture, which in turn is

defined by the type of loading the crack tip is subjected to. It is therefore necessary to define carefully the conditions under which the crack is initiated and propagated. Figure 2.3 depicts the three fundamental modes of fracture.

Figure 2.3: The three fundamental modes of fracture [6]

Mode I is the crack opening mode, where a tensile stress is applied in the direction normal to the plane of crack propagation. Mode II is the crack shear

normal to the crack tip. Mode III is the tearing mode, where a shear stress is applied parallel to the crack tip. There are subsequently three critical SERRs: GIC, GIIC, GIIIC.

Fracture in structures occurs under a triaxial state of stress and the mode of fracture is seldom a pure one. However, it can be decomposed into the three fundamental modes and the behaviour under the actual stress state can be characterised by the combination of the performance in the three modes. Therefore GC can be expressed in terms of GIC, GIIC and GIIIC by:

GIC + GIIC + GIIIC = GC (Eq. 2.3)

Delamination initiation is, in most cases, a mixed mode phenomenon (mode I mixed with mode II) in which the contribution to each individual mode depends upon several factors [8]:

• Ply thickness

For thin plies, mode I delamination tends to be absent due to the negative interlaminar normal stress. By increasing the ply thickness, interlaminar normal stresses become positive and thus mode I delamination appears. In thin specimens mode II delamination occurs due to the high magnitude of longitudinal shear stress. By increasing the thickness of the laminates, the energy release rate GII decreases while GI increases leading to mode I delamination.

• Fibre orientation of the adjacent continuous plies

The energy release rate in mode I, II or III changes with the fibre orientation of the adjacent layer as presented in Figure 2.4.

0 30 60 90 Fibre orientation E n e rg y r e le a s e r a te

Figure 2.4: Schematic of the influence of fibre orientation on delamination initiation [8]

There has been a significant amount of effort over the years from material manufacturers and users to improve delamination resistance. Having realised that delamination resistance mainly depends on the matrix and the fibre-matrix interface, several techniques were introduced:

• Changing the matrix with new tougher thermoset or even thermoplastic matrices;

• Modifying the fibre-matrix interface region by fibre sizing thus improving the fibre-matrix bonding strength;

• Toughening of the matrix by the addition of another phase like rubber or by interleaving where thin layers of a tough material are inserted in between the prepreg layers;

• Through-the thickness reinforcement.

GII

GI

Although the above techniques showed some improvement in the delamination resistance of the material, additional problems were also introduced:

• Significant reduction in hot/wet performance;

• Reduction of in plane compression strength because of the reduced lateral support provided by the matrix to the fibres;

• Each material produced with one of these techniques had to be tested and pass an expensive qualification process.

These problems limited their use. However, they remain under attractive investigation. The most effective and promising way to improve the delamination resistance is by through-the-thickness reinforcement [11], and is discussed later in paragraph 2.3.

2.2

Impact

In aircraft structures, impacts are produced by bird strikes, stones, impact on the runway, hail or dropped tool during maintenance. Tests using a falling impactor with defined mass are intended to represent such events.

In composite structures, impacts are very damaging because they reduce in plane properties, particularly in compression. Barely visible damage on the surface of the component may cause severe internal damage. Composites do not react as metallic structures after an impact event: while metallic structures generate plastic deformation to absorb the impact, composite materials accommodate the incident energy and impose deformation by fracture of both fibre and matrix through damage mechanisms [8].

Understanding the failure mechanisms responsible for this internal damage and investigating the parameters that affect it, is very important in modelling and predicting the damage growth under various loading and environmental conditions. This leads to the next important step which is the prediction of the residual strength of the material. This is a key issue for the design of damage tolerant structures [11].

In a composite the first failure is by out of plane cracks initiated in the top ply, on the non-impacted face, parallel to the fibre direction. Then the failure propagates ply by ply through the laminate, but this propagation is reduced by the different orientation of the plies. Nevertheless, the impact forces generate shear stresses parallel to the plies so the delamination cracks induced are initiated and propagate parallel to the ply interface under shear stresses. The first stage of the impact damage is a function of the mode II critical energy release rate, GIIC.

However to initiate damage a critical load (PC) is required. This critical load to

initiate damage is related to the critical energy release rate GIIC by:

PC 2

= [8π2 E t3 / 9 (1-ν2)] GIIC (Eq. 2.4)

where E is the Young’s modulus, t is the thickness of the laminate and ν is the Poisson’s ratio [12].

This critical load PC is a function of the impact energy, the laminate thickness

and the stacking sequence. It was found that the PC can be identified on

load–time history curves for impact events resulting in impact loads slightly above the PC, and for impact loads considerably greater than the PC. Regardless

of the impact energy, if significant damage occurs, then PC can be identified on

the load–time history curve [13]. Figure 2.5 represents a load–time history curve of an impact; the first drop on the curve represents this critical load PC.

Figure 2.5: Force vs. time curve during impact

Below this threshold value only barely visible impact damage (BVID) is produced. There is very little evidence of the impact on the surface but sufficient subsurface damage to degrade in-plane tension, compression and fatigue performance. Above this value, on the surface directly in contact with the impactor a small dent is produced and larger damage occurs on the back face.

2.2.1

Definition of Low / High Velocity Impact

Two types of impact can be defined, categorised into low or high velocity impact, but no clear transition exists between these categories.

Some authors define low velocity impacts by quasi-static events; their upper limit may vary from 1 to 10 m/s depending on the target stiffness, material properties and impactor characteristics. High velocity impacts instead are dominated by stress wave propagation through the material, leading to very localised damage; the upper limit is set from 10 to 100 m/s.

Other authors classified impacts depending on the damage produced. They

0 2000 4000 6000 8000 10000 0 0.001 0.002 0.003 0.004 Time (s) L o a d ( N ) PC

breakage, whereas low velocity impact is characterised by delamination and matrix cracking [8].

2.2.2

Failure Modes in Low Velocity Impact

In composite material and particularly in fibre reinforced plastic laminates, four modes of failure exist [7]:

• Matrix damage or cracking occurs parallel to the fibres due to tension, compression or shear. This type of failure is induced by transverse low velocity impact and takes the form of matrix cracking and/or debonding between fibre and matrix. Matrix cracking can be divided into two categories: shear cracks induced by transverse shear stress through the material, which are inclined at 45°, and bending cracks induced by tensile bending.

• Delamination can be seen as a result of the bending stiffness mismatch between adjacent layers. Delamination is dependent on fibre orientation, material properties, stacking sequence and laminate thickness. It is more likely to occur for short spans and thick laminates with low interlaminar shear strength. Delamination caused mainly by transverse impact, only occurs after a threshold energy has been reached and in the presence of a matrix crack. It grows by interlaminar longitudinal shear stress and transverse in-plane stress in the layer below the delaminated interface and by interlaminar transverse shear stress in the layer above the interface.

• Fibre failure occurs under the impactor due to locally high stresses and indentation effects on the impacted face and high bending stresses on the non-impacted face. It is a precursor to penetration.

• Penetration is a macroscopic mode of failure and occurs when the fibre failure reaches a critical extent, enabling the impactor to completely penetrate the material.

2.2.3

Post Impact Residual Strength

As stated previously, impact damage reduces strength, even BVID can cause a strength reduction of up to 50%. Residual strength in tension, compression, bending and fatigue will be reduced by varying degrees depending on the dominant damage mode.

The residual tensile strength follows the typical curve shown in Figure 2.6. In region I no damage occurs because the impact energy is below the threshold value. Once the critical energy has been reached (region II) the residual tensile strength quickly decreases to a minimum as the extent of the damage increases. In the last region the residual strength reaches a constant value because the impact energy is too high, resulting in penetration, and leaves a hole in the specimen.

Figure 2.6: Residual tensile strength vs. impact energy [7]

Delamination is well known to cause a large reduction in residual compressive strength because the ability of the matrix to resist fibre buckling is much reduced

For residual flexural strength, it has been noticed that by increasing low velocity impact energy, both flexural modulus and strength decrease for ductile specimens whereas brittle specimens exhibit no losses until complete failure occurs.

2.2.4

Influence of Different Parameters on the Impact Damage

Studies reveal that both intrinsic and extrinsic parameters affect the damage resistance of a composite laminate. Intrinsic parameters, which are related to the material properties include laminate thickness, fibre and matrix type, material form, fibre volume content, stiffener spacing, stiffener type and stacking sequence. Extrinsic parameters related to the experimental conditions include impact energy, impactor diameter, impactor shape, temperature at impact, impactor mass and impactor stiffness. A program run by the FAA [14] to study the most important parameters affecting the damage resistance of a composite material concluded that there are 14 parameters that affect the residual compressive strength. These are presented in Figure 2.7 in order of significance. The number in parentheses indicates the overall ranking of the parameter.

Figure 2.7: Parameters that affect the impact resistance of CFRP [14]

IMPACT RESISTANCE Intrinsic Extrinsic Laminate thickness (3) Fibre type (7) Material form (8) Fibre volume (9) Stiffener spacing (10) Stiffener type (11) Lay-up sequence (12) Matrix type (14) Impact energy (1) Impactor diameter (2) Impactor shape (4) Temperature at impact (5) Impactor mass (6) Impactor stiffness (13)

However, Figure 2.8 shows that not all of them are of the same importance. The parameter ranking was produced by taking into account the damage area and the fibre failure length.

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2 1 2 3 4 5 6 7 8 9 10 11 12 13 14

Figure 2.8: Relative ranking of the importance of the impact resistance parameters [14]

The most important parameters are discussed below.

• Impact energy: The delaminated area after an impact is a function of the impact energy. Depending on the impact characteristics of the laminate there is an energy threshold up to which no internal damage or minimum damage will be generated which does not affect the laminate strength. When this energy threshold is exceeded the damage increases dramatically. Subsequently the reduction in compressive strength is significant due to reduced lateral support offered by the matrix to the fibres. As the energy keeps increasing, the internal delamination damage is reduced but the fibre breakage increases due to highly localised energy input rate. Depending on the support offered to the laminate, fibre breakage will either be on the

mode is that the structure cannot become globally ‘aware’ of the impact event. Finally if the energy is further increased penetration will occur.

• Impactor characteristics: The impactor’s physical characteristics are very important for the damage generated during impact. For low velocity impact the Hertz contact law can be used since the duration of contact is comparatively high. It says that the contact force is proportional to a modified Hertz constant which depends on the striker radius, the Poisson’s ratio and the Young’s modulus of the impactor. This relationship shows that the impactor characteristics have a significant effect on the resulting force [15].

• Laminate thickness: According to Classical Plate Theory, the definition of thin and thick laminates is given by the ratio of in-plane length to thickness, l/h. A ratio of 20 is considered as the minimum requirement for a thin laminate. Equation 2.4 shows that the energy threshold depends on the thickness of the laminate. With increasing thickness the energy threshold at which damage is initiated increases. Thin materials present a larger damage zone because they show a lower moment of inertia and global bending occurs during the penetration process as a result of the lower stiffness. As a consequence, the energy absorbed during penetration is reduced and extended damage along the 0° direction is obvious for thin laminates. In thicker specimens the damage area is limited in the local zone because of the high bending stiffness.

• Stacking sequence: The stacking sequence or lay-up defines the construction of a composite laminate in terms of the sequence of the constituent plies of which it is composed. The number represents the angle in degrees between the fibre orientation of each ply and an external reference. Each ply is considered to have the same thickness, fibre/matrix type and fibre volume

isotropic stacking sequence. Stacking sequence has an effect on the position, the size, the orientation and the shape of the delaminated area through the specimen thickness. Increasing the number of dissimilar plies increases the damage resistance. To reduce the damage area, ply grouping should be avoided, because stacking plies of the same fibre orientation together will increase the stress concentration at the adjacent interfaces due to the increased bending stiffness within that ply group. This increase in stress concentration creates layer delamination [8].

• Matrix effect: The purpose of the matrix is to contain the fibres and hold them as a structural unit, protect them from external damage, transfer and distribute the applied loads to each fibre and contribute some additional properties such as ductility or toughness [16]. The role of the matrix in the impact damage resistance is fundamental. A tougher matrix can absorb more energy through deflection hence reducing the energy available for the initiation and propagation of damage. Impact response of composite laminates are strongly influenced by the toughness of the matrix but it is more effective in increasing the damage initiation load PC than in increasing the resistance to delamination

growth [17].

2.3

Through-the-Thickness Reinforcement (TTR)

In the search for methods to arrest an existing crack in laminated composites, TTR techniques have been developed. The importance of resistance to crack initiation lies in the fact that composites tend to fail catastrophically thereafter, particularly under fatigue loading conditions. However, the newer concept of designing components under the presumption of pre-existing cracks or flaws, and the difficulty of arresting an existing crack, renders resistance to propagation more significant compared to resistance to initiation.

TTR consists of the insertion of some kind of reinforcement in the thickness direction of laminated composites. Stitching, tufting and Z-Pinning are the current existing methods. TTR bridges delaminations and shields their crack tips from the applied load so reducing the crack driving force. Crack growth can be rendered stable, which is a critical feature in damage tolerant design, ultimate strength can be increased and notch and impact sensitivity reduced. TTR techniques can contain damage by completely arresting a propagating crack. TTR is currently the most promising method of confronting the problem of delamination in laminated composites [2].

2.3.1

Stitching

Stitching involves the insertion of a dry fibre yarn (carbon, kevlar or glass) into the laminate and the formation of a loop or an interlocking pattern with another thread (Figure 2.9).

Stitching requires two-sided access and special tooling. The manufacturing advance most beneficial to stitching has been the introduction of resin transfer processes, which allow dry stitched preforms to be infused with resin. This enhances process speed and allows stitching through thicker material. It also reduces damage to the laminate fibres, considering the fact that a large diameter needle (2mm) is continuously driven through the laminate with considerable force. The use of stitching with prepreg materials is limited, as the viscous resin will foul up the needle making further insertion difficult [18].

2.3.2

Tufting

Tufting uses a similar insertion technique to stitching , but it is a one-side access process. It still requires special tooling. A needle is used to push the yarn through the laminate as well as through a form tool described as an ‘elastic foam’. The frictional forces, generated in the laminate and in the foam, keep the yarn in the laminate, while the needle is withdrawn [2] (Figure 2.10).

Tufting schematic

Needle

Loops

Thread

Laid-up

fabric

2.3.3

Z-Pinning

Z-Pinning is the insertion of short Z-fibres (pins) orthogonally to the plane of the composite plies during the manufacturing process, before the resin matrix is cured, effectively pinning the individual layers together [4].

Z-fibres, also called Z-pins, represent a new technology which has been developed in the early 1990’s, to enhance the performance of composite structures, mainly the through-thickness properties. Z-fibres are inserted in the z-axis in composite laminates as a reinforcement to prevent delamination and further failure. Foster-Miller Inc. was the first to develop the Z-pins. Following growing interests in this technology, Aztex Inc. was created to manufacture and market this technology.

Z-fibres can be employed as a global or a local reinforcement for increased the-thickness strength, improved damage tolerance and through-thickness thermal conductivity. Z-pins can also be used as a secondary structure attachment method for greatly improved joint performance at a reduced weight. This kind of reinforcement might be used principally in the aerospace industry for example in the development of damage tolerant skins, survivable composite wing structures and unitised aircraft structures [8]. Z-pinning was originally intended as a mechanical fastener replacement, attributing at the same time improved damage tolerance properties. It currently does so successfully for the F/A 18 E/F US Navy aircraft, joining the duct and aft structure to the main fuselage, since June 2000, with excellent weight and cost savings as a consequence. This property depicts the capability of Z-pinning to be used locally and selectively where an increase in the delamination resistance is required. There is further potential in extending the use of Z-pinning to help the integration of more structural components, while lowering manufacturing timescale and cost by selectively reinforcing structures.

2.3.3.1 Z-Fibres

The most commonly used fibres are pultruded from bismaleimide resin impregnated carbon tows (T300/BMI), although a choice of materials is available (glass, quartz, boron, silicon carbide, steel, titanium and aluminium). Common Z-pin diameters are 0.28mm, manufactured by 1k tows (1000 tex, or 1kg per km of fibre bundles) and 0.51mm made from 3k tows, but diameters from 0.15mm to 1mm are possible. They are chamfered to an angle of about 45° at both ends to aid insertion and offer excellent adhesion to thermoset resin. They are characterised by the following parameters:

• Pin diameter (from 0.15 mm to 1 mm);

• Pin length (from 5 mm to 19 mm);

• Material (carbon, glass, quartz, boron, silicon carbide, steel, titanium and aluminium);

• Areal density (from 0.75% to 10%).

2.3.3.2 Preform

The Z-fibres are supplied inserted in a two-density two-layer foam at the required areal density (the areal density is the percentage of the total cross section area of pins over the reinforced area); the foam filled with pins is called a ‘preform’ (Figure 2.11). In the case of composite Z-fibres, Aztex Inc. has developed a two-stage process to manufacture the Z-fibre preform [19]:

In the pultrusion process, the carbon fibres are pulled off a bobbin, into a small bath where the resin is held at an elevated temperature. The material exits the bath through a die and immediately enters a long oven. The cured material, exiting the oven, is wound up on a spool, then post-cured if required. The rodstock is then ready to be inserted into the sandwich foam. The number of carbon fibre tows used during the pultrusion determines the Z-fiber diameter.

Figure 2.11: Z-fibre preform

The preform’s low-density foam, called ‘support foam’, is usually made from polystyrene and is located at the top. At the bottom, the medium-density foam, called ‘base foam’, is usually Rohacell LastaFoam material. The support foam is used to hold the pins prior to use, but is designed to collapse easily. In addition to locating the pins accurately, the base foam offers better stability to the lower part of the pins and prevents them from buckling during the insertion process. Automated machinery cuts the long rodstock of pin material and inserts it directly into the foam. A cutting device shears the rodstock, leaving an acute angle at the ends of the pins. This angle assists the pin penetration into the part to be reinforced and minimises carbon fibres filament breakage and distortion.

The preforms are characterised by the following parameters:

• Z-fibre material;

• Z-fibre diameter;

• Insertion angle;

• Areal density of the pins;

• Type and thickness of support foam;

• Type and density of base foam.

2.3.3.3 Insertion of the Pins in the Laminate

The principle of the insertion appears to be relatively simple. The preform containing the pins is placed on the top of the composite in the uncured state. The support foam is then compressed by applying heat and pressure that force the pins out of the foam and into the laminate. Then the composite is cured. Two ways of pin insertion have been developed over the years:

• Autoclave insertion process, also called heat and pressure insertion process, was introduced at the same time as the Z-fibres and was initially developed by Aztex Inc. to drive the Z-fibres into laminates. In that process, outlined in Figure 2.12, the preform is placed on the top of the uncured laminate, using a release film between the laminate and the preform and a backing plate on the top. Combination of heat and pressure compacts the preform and pushes the Z-pins in. The residual foam is then removed and discarded at the end of the

On the other hand, because of the high pressure required, only relatively low pinning density can be used (< 1%). The method is now rarely used, the UAZ insertion becoming the preferred option.

Figure 2.12: Heat and pressure insertion process [6]

• Ultrasonically Assisted Z-fibre (UAZ) insertion requires some hardware that can be simplistically described as an ultrasonic hammer. The ultrasonic device is composed of a standard power supply (Branson 900BCA) and a sequence of transducer, signal booster and insertion horn. Normally 20kHZ transducers, with maximum amplitude of 20µm at 100% out put, are used. The horn, supplied by Aztex Inc., can be of different sizes, depending on the insertion work to be done. The most commonly used horn has a footprint of 25mm x 25mm. More complicated horn shape can be designed for areas which are partially obstructed or difficult to access. Aztex Inc. has developed several types of UAZ devices, hand held units, semi-automated gantries and fully automated 6 axes robots. Completely automated insertion processes are under development. In the case of Z-pins insertion using a hand held unit, the power supply is connected to the transducer, which is held in a ‘gun casing’. The insertion horn is directly screwed on the transducer (Figure 2.13). Like a

gun, the ultrasound is activated by pulling the trigger of the casing. The amplitude of the vibration can be modified by adjusting the settings on the power supply.

Figure 2.13: Hand held UAZ unit

The set-up of a gantry is more complicated (Figure 2.14). Compared to a hand held system, the ultrasonic stack of the gantry includes a ‘booster’, which allows the amplitude of the vibration of the horn to be changed. The use of the booster is made possible by the fact that the transducer and the booster are fixed on a rigid platform. The booster is mounted on the gantry at a node point of the signal (point of the wave signal where the amplitude is zero). The insertion horn is connected to the booster. The change of the amplitude (or gain of the booster) can be calculated by applying the energy conservation law to the booster, the transducer and the horn; the masses on each side of the fixed node being different but known. The total amplitude yield by the ultrasonic stack is calculated by a simple multiplication of the amplitude of the transducer by the gain of the booster and the gain of the horn. Usually amplitudes from 25µm to 50µm are used for the insertion of composite

Z-Figure 2.14: New gantry currently in use at Cranfield University

A foot pedal activates the ultrasonic device and the compressed air system, which drives the whole ultrasonic assembly down and presses against the laminate stack for insertion. The pressure to be applied, as well as the speed of the travel of the head can be tuned to optimise the insertion of the pins. Using a gantry offers the advantage of being sure that the horn is accurately perpendicular to the laminate. It also allows controllable and reproducible insertion pressure and speed to be achieved. A digital gauge, mounted on the ultrasonic carriage, allows the pinning depth to be checked.

2.4

Previous Work on Z-Pinned Laminates

Structural designers use laminated fiber reinforced materials because they have high in-plane strength to weight ratios. A preferred form of laminated construction is integrally stiffened cocured structure because this method of fabrication reduces fabrication and assembly costs. The drawback to the use of

thickness strength. Moderate out-of-plane loads lead to delaminations, skin-stiffener separations and failed joints.

The presence of the Z-pins in composite laminates is shown to result in dramatic increase in the resistance to crack propagation under both forms of loading, mode I and mode II, in comparison with the control specimens. In particular, the catastrophic failure associated with the mode II shear loading configuration is suppressed in the pinned sample [21]. It is generally agreed that this mode of loading represents a significant proportion of loading under realistic in-use conditions of composite; thus the apparent ability of the Z-pins to stabilise this fracture process offers a potential for the control of delamination damage under impact or fatigue loadings. The role played by the Z-fibers in the overall energy absorption associated with the creation of the fracture surface appears to be dependent on the extent of the shear displacement reached. Pins may either in shear as the crack passes through or they absorb energy by deforming and eventually pulling out under a mixed loading mode [22].

Stress analyses showed that a Z-fibre within a region of intact laminate resists through-the-thickness load only when those loads are applied in the vicinity of the Z-fibre. When a Z-fibre resists through-the-thickness stresses, a shear lag type of stress is transferred between the pins and the surrounding material. Also Z-fibres do not have an effect on crack initiation but they limit the crack propagation. However, once an in-plane crack has propagated throughout a region that contains an intact Z-pin, this one will pick up the through-the-thickness load. Thus the stress at the crack tip will be greatly reduced and the crack propagation will be limited [23].

It is also suggested from study on lap shear carbon/epoxy specimens that rods inserted the through-the-thickness of a laminate both increase and decrease the

near a rod which increases the critical stress whereas they decrease because the load is carried before the crack occurs, thus the stress near the surrounding laminate is reduced as well. It is also shown that the behaviour of the rods changes with their orientation. Fibre pull out and debond occur when the fibres are oriented in the direction of the applied load. When fibres are oriented to resist the applied shear displacement, the rods accommodate crack sliding displacement by a combination of internal plasticity and laminate damage, bending through large angles at the delamination plane before eventually pulling out. To avoid the pull out mechanism which reduces the strength of the laminate, the rods might present a rough surface to limit the waviness of the plies. The presence of the rods permits both to increase the ultimate strength compared with an unreinforced specimen and to lead to a stable crack growth [24].

Previous work on z-pinned composite lap joints show that z-pinning is highly effective in increasing the ultimate strength, elongation limit and fatigue life. Improvements to the monotonic and fatigue properties are attributed to transitions in the failure mechanisms, from unstable joint debonding in the absence of pins to stable debonding in the presence of pins followed by ultimate failure by pin pull-out or shear fracture or tensile laminate rupture. Which mechanism induces ultimate failure in the presence of pins depends on their volume content and diameter [25].