Cracked Lap Shear Research Articles

CHARACTERIZATION OF DELAMINATION GROWTH BEHAVIOUR OF HYBRID BONDED LAMINATES

Abstract— The objective of this work is to study the delamination growth behaviour of hybrid composite‐aluminium bonded laminates. A modified Double Crack Lap Shear (DCLS) specimen was chosen for this study. An expression relating the delamination size and the compliance of a DCLS specimen was derived, and a test method for the delamination growth rate in DCLS specimens developed. The delamination sizes and the delamination growth rates of DCLS specimens were determined by monitoring the compliances of specimens during fatigue. Delamination growth rates at different stress ratios (R= 0.1, 0.3,0.5) were measured. A Walker‐type equation for the delamination growth rate was obtained by a multiple linear regression analysis.It was shown that the compliance method for determining the delamination growth rate of DCLS specimens is not only convenient and practical, but also accurate. The delamination size in DCLS specimens increases linearly with cycles during fatigue, i.e. delamination growth rate is constant, independent of delamination size. The energy release rate was adopted to characterize delamination growth behaviour. Good agreement between the Walker equation and test results of the delamination growth rate was found.

Observations of fatigue crack initiation and propagation in an epoxy adhesive

Fatigue crack initiation and propagation were investigated in structural adhesive joints consisting of 7075T6 aluminium adherends bonded with a mineral filled structural epoxy (Cybond 4523GB, American Cyanamid). Three types of joints were tested to achieve mode I (double-cantilever beam specimen, DCB), mixed mode I–II (cracked lap shear specimen, CLS), and mode II (end notch flexure specimen, ENF). All tests were conducted under ambient conditions with load ratio of 0.1 at a frequency of 30 Hz. Fatigue loading significantly reduced the strain energy release rate (G) required to initiate a crack compared with static and quasi-static loading. For the load ranges tested, fatigue precracks doubled the time to cause a resumption of crack growth under mode I loading. Negligible differences in crack initiation times (time to generate a crack from a fillet or resume extension of an existing crack) were observed for mixed-mode I–II and mode II specimens with cracks starting from fast mode I precracks, intact fillets and fatigue precracks. For the adhesive system tested, the relative influence of the mode ratio depended on whether the rate of crack propagation was plotted versus G max or % G c (percentage of the quasi-static critical energy release rate at the particular mode ratio). When expressed as a function of % G c, debonding rates were greatest under mixed-mode conditions at a given % G c, and were indistinguishable under mode I and mode II loading. However, when expressed as a function of G max, the propagation rates at a given G max were the same under mixed-mode and mode I loading, and smaller under mode II loading. This means that the allowable loads for joints in fatigue will depend on the mode ratio; for mixed-mode joints it will be a smaller fraction of the quasi-static allowable load than for mode I or mode II joints. Threshold energy release rates ( G max) under mode I and mixed mode I–II loading were essentially the same, and were obtained equally from extrapolated crack propagation rates or crack initiation times. For this adhesive system, it is recommended that adhesive joint design be based on threshold values for zero crack growth, because crack propagation rates show too much scatter to be relied upon for the prediction of in-service subcritical crack growth, particularly under mode I and mode II loading.

The cracked lap shear specimen revisited—a closed form solution

This paper revisits the cracked lap shear specimen and reports a closed form solution to determine three fracture parameters, the energy release rate, the fracture mode mixity, and the fracture efficiency parameter. The solution is based on a beam-column approach pioneered by Goland and Reissner. Because of the geometrically nonlinear nature of this specimen, it is found that the fracture parameters are functions of five independent nondimensional parameters. The closed form solution reported in this paper provides a simple and useful tool to design the cracked lap shear specimen and to understand the behavior of this test geometry. The results in this paper suggest that to design a specimen with an energy release rate less dependent on the crack length, the adherend thickness should be small compared to the specimen length, and the thicker or stiffer adherend should be used as the strap. For a specimen of unequal adherend thickness, using the thicker adherend as the strap would significantly reduce the likelihood of yielding in the adherend. Compared with the finite element analyses found in the literature, the closed form solution shows good agreement in energy release rates, but less satisfactory agreement in fracture mode mixities. Finally, the closed form solution is used to give a reasonable explanation of anomalous debond behavior in a series of fatigue experiment using the cracked lap shear specimen.

Delamination Fracture Criteria for Composite Laminates

Due to the singularity nature of delamination, the fundamental concept of fracture mechanics was applied. However, the oscillatory characteristics near the delamination tip prohibited the use of conventional definition assuming the crack embedded into a homogeneous solid. A proper definition for the stress intensity factors and energy release rates of bimaterial interface cracks was introduced in this paper to study the fracture criterion for delamination. To measure the delamination fracture toughness, the test specimen usually used for the unidirectional composites was redesigned to suit for the cases that the delamination lies between two laminae with different fiber orientations. The test results show that the double cantilever beam (DCB) test is near to pure Mode I and the end-notched flexural (ENF) test is near to pure Mode II. Therefore, like the cracks in homogeneous materials, the test methods DCB and ENF are useful for the measurement of delamination fracture toughness GC and G11,. Based upon this conclusion, a series of cracked-lap shear (CLS) and modified end-notched flexural (MENF) tests and finite element simulation are conducted in this paper to establish the mixed-mode delamination fracture criterion. To validate the proposed fracture criterion, two commonly encountered problems are predicted and tested. One is the prediction of the most probable interface for the onset of delamination and its possible minimum load, in which no delamination was embedded prior to loading. The other is the prediction for the test with delamination existing prior to loading. The results show that the prediction by using this proposed criterion is well matched with the experiment.

Finite element analysis of crack growth in adhesively bonded joints

In this paper, adhesive-bonded joints are analysed using the finite element method by computing the strain energy release rate for debonding. Paired nodes are generated on the interfaces and released one after another in a proper sequence, modelling gradual crack growth. Strain energy release rates in modes I and II are evaluated. The method is validated for a sample cracked lap shear (CLS) specimen. Bonded joints are studied for various ratios of elastic moduli, thicknesses of adherend and adhesive, and total length to overlap length, for eight different crack growth locations along the interfaces. Theoretical failure loads are predicted in terms of a critical strain energy release, and material properties and geometries are suggested that may result in extending the load-carrying capacity of these joints.

Characterization of delamination behavior of unidirectional graphite/PEEK laminates using cracked lap shear (CLS) specimens

Delamination failure criterion is an important tool for characterizing the fracture behavior of laminated composites under mixed loading. In this paper, a fracture envelope was built based on the energy release rate as a fracture criterion of graphite/PEEK laminates. Unidirectional cracked lap shear (CLS) specimens were employed to calculate mode I and mode II energy release rates ( G I, G II. Static fracture tests were conducted using the specimens with two different lap to strap thickness ratios in order to obtain a wide range of G I/ G II values. The G I/ G II values for each thickness ratio were calculated numerically using finite element analysis. The results showed that as a delamination length changes, the G I/ G II varies from 0.13 to 0.48 depending on the lap to strap thickness ratio. It was also found that a linear fracture envelope may be appropriate for a CLS composite specimen.

The effect of geometry on the fracture of adhesive joints

In this paper, a recently suggested method for fracture load prediction of adhesive joints is demonstrated to compare well with experimental data for aluminium joints bonded with a rubber-toughened structural epoxy (Permabond ESP 310). The method is also used to investigate the effect of varying geometric parameters such as adherend lengths and thicknesses on the strength of adhesive joints such as the single lap shear, cracked lap shear and double strap joints.

Fracture load predictions for adhesive joints

An engineering approach to fracture load predictions for adhesive joints is presented. The approach is based on the premise that the in-situ strength of the bondline can be characterized by the fracture envelope (critical energy release rate as a function of the mode of loading), for a specific adhesive system. By using the J integral for large deformations together with large-deformation beam theory, a simple closed-form expression is obtained for the energy release rate per unit area extension when a crack propagates in the bondline of a generalized adhesive joint (adhesive sandwich). This technique, together with a published method for mode partitioning, enables fracture load prediction by comparing the calculated fracture parameters with the critical ones from the fracture envelope. The approach is shown to predict fracture loads accurately for a variety of joints including the cracked lap shear (CLS), the single lap shear (SLS) and the double strap (DS) joint.

A Study on the Application of Work Factor Approach to Composite Laminates

Static fracture tests using the cracked lap shear (CLS) specimens made of thermoplastic composite APC-2 were conducted to verify if the elastic work factor devel oped previously is a valid parameter to calculate energy release rate from a single speci men. Also, the elastic work factor was derived theoretically based on a beam analysis. The experimentally determined elastic work factor, (η el)exp was compared with those calculated from a beam analysis and a finite element analysis. The results showed that the elastic work factors determined from these three ways are comparable to each other. In ad dition, the elastic work factor was applied to determine a critical energy release rate, and it was found that the elastic work factor approach can be used to determine energy release rate from a single specimen.

Computation of energy release rate components by the use of near-tip stress and displacement distributions

A simple method, called the two-term parameter technique, is introduced for the computation of the energy release rate in specimens made of composite materials. A cracked lap shear specimen with a zero degree stacking sequence was employed. The mode II energy release rate, calculated by the two-term parameter technique, was compared with that determined by using the crack closure method. The results show that the two-term parameter technique is very comparable to the crack closure method and does not require exact information of the stress and displacement distributions at the crack tip to get the energy release rate. Moreover, it is shown that while the crack closure method depends on the crack extension size, the two-term parameter technique is less affected by it.

An application of the single specimen technique to a double cantilever beam specimen to determine GIC from a single test record

The single specimen technique is effectively used in classical fracture mechanics for evaluating the J-integral from a single test record. It was previously shown using a cracked lap shear specimen that the technique can be applied to composite structures and the energy release rate of composite laminates can be determined from a single test specimen. The objective of this investigation is to determine the η el form of a double cantilever beam (DCB) specimen, and to compare it with that of a compact tension (CT) specimen. The result showed that while η el is linearly related to crack length for the CT specimen, it is proportional to the reciprocal of the crack length for the DCB composite specimen. Also, G Ic determined from the compliance method was compared with that determined from the η el approach. The result showed that the η el form determined from a single beam analysis can be used to determine G Ic from a single test record.

Method for Computing Energy Release Rate Using the Elastic Work Factor Approach

The elastic work factor ηel concept was applied to composite structures for the calculation of total energy release rate by using a single specimen. Cracked lap shear specimens with four different unidirectional fiber orientation were used to examine the dependence of nel on the material properties. Also, three different thickness ratios (lap/strap) were used to determine how geometric conditions affect ηel. The ηel values were calculated in two different ways: compliance method and crack closure method. The results show that the two methods produce comparable ηel values and while ηel is affected significantly by geometric conditions, it is reasonably independent of material properties for the given geometry. The results also showed that the elastic work factor can be used to calculate total energy release rate using a single specimen.

An Analysis of Curvature and Layup Effects on Delamination in Notched Composite Beams

Two-dimensional finite element modeling has been used to determine strain energy release rates for delamination cracks in straight and curved notched tensile beams. The notched tensile beam (NTB) is a low-order analog of a cylindrical pressure vessel with an external part-through hole. The material was an S-glass/epoxy fiber rein forced composite material. Specimens were 8 plies thick with notches 2 and 4 plies deep. Uni-directional [0°]8 and multi-directional [0°,0°, +77°, -77°]2 layups have been in vestigated. Strain energy release rates were calculated using the virtual crack closure tech nique. The finite element results reveal a mode II dominance similar to the cracked lap shear (CLS) specimen. The effect of curvature was to lower the strain energy release rates slightly (5-13%). Total strain energy release rates were generally higher for the multi- directional layup (by 1½ to 3 times), but the GII/G I ratio may increase or decrease depend ing on the notch depth and the orientation of the plies bounding the delamination.

Influence of fiber/matrix interfacial adhesion on composite fracture behavior

A systematic study has been conducted to identify the effect of fiber/matrix interface strength on various composite properties. A new fiber treatment technique was developed to allow fibers to be treated and then made into prepregs and composites of acceptable quality. T500 carbon fibers were treated with release agent to establish the extreme case of poor fiber/matrix interface. Composite systems made of toughened epoxy R6376 and T500 fibers with and without such a treatment were subjected to a number of fracture and impact tests. For tests involving propagating pre-existing delamination cracks, such as double cantilever beam (DCB), end notched flexural (ENF) and crack lap shear (CLS) methods, the material properties were not appreciably affected by the release agent-treated fiber surfaces. For tests that had to initiate cracks in specimens without pre-introduced cracks, such as impact and edge delamination, the material variables and failure modes were highly sensitive to the fiber/matrix interface. The critical role of the fiber/matrix interface in crack initiation was demonstrated in this study.

On the fracture parameters in a clamped cracked lap shear adhesive joint

A stress analysis using a beam theory approach similar to that suggested by Goland and Reissner is undertaken for a clamped crack lap shear ( cls) adhesively bonded joint. Analytical solutions of the normal and shear stresses in the adhesive are derived for the simple case where the lap and the strap have equal thickness and moduli. In a subsequent step, these expressions are used in the framework of a crack-closure technique to obtain the Mode I, Mode II and total energy release rates for a crack propagating along the bondline. For an illustrative example, a comparison with a finite element analysis showed reasonably good agreement. This closed-form approximation of both the Mode I and Mode II components of the energy release rate allows the use of fracture mechanics to describe debond growth in mixed-mode conditions representative of actual structures.

A Fracture Mechanics Approach to Characterizing Cyclic Debonding of Varied Thickness Adhesive Joints to Electroprimed Steel Surfaces

Abstract Applications of adhesive bonding for automotive structures have been increasing in recent years due to improvements in the types of adhesives available and in improved knowledge of bonding procedures. Consequently, there exists a demand for design techniques to assess the influence of bondline thickness on adhesive joint strength. One design approach currently being used is based on limiting shear stresses in the adhesive while designing to eliminate peel stresses. Another design approach is based on fracture mechanics and accounts for shear and peel stresses and both static and fatigue modes of failure. The present study applies fracture mechanics to investigate the mixed-mode response of cracked-lap-shear (CLS) joints bonded with unprimed and electroprimed steel surfaces. Three bondline thicknesses equal to 0.254, 0.813, and 1.27 mm were evaluated for unprimed and primed bondlines. For the experimental portion of the study, debond growth rates (da/dN) were measured using a remote imaging system...

Failure load prediction of structural adhesive joints

The objective of this study was to determine how the fracture of adhesive joints depends on elastic beam parameters describing the adherends and the applied loads. The basic specimen geometry was the cracked lap shear joint constructed of aluminium alloy with various adherend and bondline thicknesses. Loads were applied in different combinations of bending, tension and shear to generate a failure envelope for each adhesive and specimen geometry. It was found that crack propagation for precracked specimens occured at a critical strain energy release rate but was also a function of the GI/GII ratio and the bondline thickness. The experiments also showed that the loads required to propagate a crack in a precracked specimen were always lower than the loads required to break the fillet. Hence, by treating uncracked joints as being cracked, where the fictitious crack tip is assumed to coincide with the location of the fillet, a conservative estimate of the failure load is obtained.

Effect of Bondline Thickness on Mixed-Mode Debonding of Adhesive Joints to Electroprimed Steel Surfaces

Abstract Structural applications of adhesive bonding have been increasing in recent years due to improvements in the types of adhesives available and in improved knowledge of bonding procedures. Consequently, there exists a demand for techniques to assess adhesive joint strength, particularly along bondline interfaces where compliant adhesives contact more rigid metallic surfaces. The present study investigates the mixed-mode response of cracked-lap-shear (CLS) joints bonded with unprimed and electroprimed steel adherend surfaces. Three bondline thicknesses, representative of structural automotive joints, were evaluated for unprimed and primed bondlines. Experimental results for static load versus debond extension were input to finite element analyses for computing debond parameters (strain energy release rates). The debonds always initiated at a through-the-thickness location that had the greatest peel component of strain energy release rate. The total strain energy release rate values correlated well with trends in joint strength as a function of bondline thickness.

A Nonlinear Analysis of the CLS Specimen

A nonlinear analysis of a cracked lap shear (CLS) specimen was carried out based on the theory for tie rod problems. The specimen in this case was an unidirectional Ciba Geigy T300/914 laminate with orientation and layup of (0°)9 at the thick section and (0°)5 at the thin section. In the tie rod analysis the CLS specimen was modelled as two beams joined eccen trically with the joint at the crack tip. The analysis gave the distribution of bending moments and shear forces along the CLS specimen, the deflection of the CLS specimen and the total energy release rate ( G) associated with delamination growth. The energy release rate was found to be independent of the crack length except for long crack lengths. In order to separate the mode I and mode II contributions of the energy release rate, bend ing moments and shear forces were calculated at a distance from the crack tip. The cross- sectional forces and bending moments were thereafter used as prescribed loads in a linear finite element analysis of a short section containing the crack tip. Using a crack closure scheme, the mode I ( GI) and mode II ( GII) energy release rates were calculated. The agreement in total energy release rate calculated from the tie rod analysis and the finite element analysis was good.

Stress Analysis of the Cracked-Lap-Shear Specimen: An ASTM Round-Robin

Abstract An ASTM round-robin was conducted to evaluate the state of the art in stress analysis of adhesively bonded joint specimens. Specifically, the participants were asked to calculate the strain-energy-release rate for two different geometry cracked-lap-shear (CLS) specimens at four different debond lengths. The various analytical techniques consisted of two- and three-dimensional finite element analysis, beam theory, plate theory, and a combination of beam theory and finite element analysis. The results were examined in terms of the total strain-energy-release rate and the Mode I to Mode II ratio as a function of debond length for each specimen geometry. These results basically clustered into two groups: geometric linear or geometric nonlinear analysis. The geometric nonlinear analysis is required to properly analyze the CLS specimens. The three-dimensional finite element analysis gave indications of edge closure plus some Mode III loading. Each participant described their analytical technique and results. Nine laboratories participated.