Cracked Lap Shear Specimen Research Articles

Monitoring Crack Tip Position in Adhesively Bonded Joints Under Mixed I+II Mode Loading

The use of adhesive joints has gained favour in recent years, due to their capacity for weight reduction, improved stress distribution, and enhanced fatigue resistance. However, the widespread use of adhesive joints in structural applications is hindered, because reliable and mature inspection and defect detection techniques are not yet available. Adhesive joints in structural applications are subject to fatigue loading, during which debonds can initiate and propagate, ultimately leading to potential failure. In the majority of cases, joints operate under mixed-mode loading conditions. In this work, steel Cracked Lap Shear specimens were tested to investigate the crack growth behaviour of adhesive joints under mixed I+II mode loading. To address the challenge of monitoring these joints, a range of experimental techniques was employed. These techniques included Visual testing, Digital Image Correlation, Optical Backscatter Reflectometry, and localization by Acoustic Emission. The different methods were evaluated and compared to determine their effectiveness in locating the crack tip within the joints during fatigue propagation.

Investigation of the influence of design parameters onto the cracked lap shear specimen

The Cracked Lap Shear (CLS) specimen allows a mixed-mode loading condition to be generated without a special test fixture using a simple tensile test. The mixed-mode ratio generated is nearly constant over a wide range of crack growth and is representative of the loading conditions in many structural applications of bonded joints. However, this specimen is not standardised, leading to very dissimilar specimen designs in the literature. In this study, the primary design parameter is the sample width in terms of its application for adhesive characterisation, with a typical layup having a 0° layer in contact with the bond line. The results of quasi-static and fatigue tests using varying specimen widths between 5 mm and 40 mm emphasise the specimen width’s influence. Moreover, CLS specimens with the same layup but without an adhesive layer between the adherends are investigated to study the effects of width scaling without a toughened adhesive bond. While the static tests show a linear relationship between specimen width and joint strength, it is different in fatigue tests, where the crack growth rate increases with decreasing specimen width, both for specimens with and without adhesive bond between the joining partners.

Experimental analysis on the mixed-mode fracture behavior of structural adhesives

The wide use of adhesive joints supposes that accurate strength predictions methods exist for design. Cohesive zone models (CZM) consist of the most accepted method, although the fracture toughness (GC) of the adhesive in the different loading modes should be firstly characterized, and the mixed-mode behavior or fracture envelope known. This work experimentally evaluates the cracked-lap shear (CLS) test for mixed-mode GC assessment of two epoxy adhesives. Different theoretical reduction methods were evaluated. The work consisted of testing CLS specimens with the different adhesives, and carrying out the respective data processing. Results showed that the model of Brussat et al. led to offset results compared to the other methods.

Analysis of peel and shear strains in cracked lap shear specimens subjected to fatigue loading using digital image correlation

Adhesive bonding presents many advantages, such as efficient manufacturing and improved structural performance [1]. However, in structures subjected to fatigue, cracks might initiate and propagate in joints, leading to in-service failure [2]. Most adhesively bonded joints are subjected to combination of peel and shear loads, so mixed I+II mode loading conditions are present [3]. In this work, Cracked Lap Shear specimens, which feature mixed I+II mode loading conditions, were tested under fatigue loading. During tests, crack growth was monitored using Visual Testing and Digital Image Correlation. With Digital Image Correlation, opening and sliding displacements in the bondline were extracted from the substrates’ displacement fields and compared against a Finite Element Model, revealing a highly strained process zone ahead of the crack tip. Results highlight the usefulness of DIC in capturing the deformation behaviour of adhesive joints under mixed mode loading conditions.

Monitoring crack tip position in Cracked Lap Shear specimens subjected to fatigue loading

In recent years, interest in adhesively bonded joints has significantly grown, as it offers numerous advantages with respect to other joining techniques. Bonded joints are being increasingly adopted in structures subjected to fatigue loading, which might initiate and propagate crack-like debonding defects. The ability to detect and locate these defects is crucial for increasing overall safety. This study aims to investigate the capability of two commonly used Non-Destructive techniques, namely Digital Image Correlation and Visual Testing, to correctly locate the crack tip of a debonding damage. For this purpose, a specific Cracked Lap Shear specimen, which features mixed mode I-II loading conditions, was designed, manufactured, and tested under fatigue loading. Two different adhesives were used. The results showed that Digital Image Correlation was able to easily identify the crack tip, while visual inspection proved to have some difficulties due to the prevalence of mode II, which makes crack identification more troublesome.

Evaluation of the cracked lap shear test for mixed-mode fracture toughness estimation of adhesive joints

Cohesive zone models are a widely used method in adhesive joint strength predictions, but it requires the knowledge of fracture toughness of the adhesive in the different loading modes, together with the mixed-mode behaviour. The cracked lap shear test is a simple but seldom studied mixed-mode test and, as such, a deeper knowledge about its accuracy and of the available data reduction methods is required. This work consists of the experimental and numerical analysis of the cracked lap shear test to estimate the mixed-mode fracture toughness of two structural adhesives. The experimental work consisted of testing cracked lap shear specimens with the different adhesives. Numerically, cohesive zone model were used to predict the behaviour of bonded joints under mixed mode, while assessing the validity of the available data reduction methods to estimate fracture toughness. While the experimental analysis showed that the Brussat’s method led to offset results compared to the other methods, the cohesive zone model analysis confirmed the inadequacy of the Brussat's method, and the suitability of the Grady and Kinloch's models for mixed-mode fracture analysis.

Cluster-Based Joint Distribution Adaptation Method for Debonding Quantification in Composite Structures

Adhesive bonding is widely adopted in aeronautic structures to join composite materials or to repair damaged substrates. However, one of the most common failure modes for this type of joint is debonding under fatigue loading. In the past years, it has been proven that deboning quantification is feasible, given that abundant experimental data are available. In this context, using domain adaptation to assist diagnostic tasks based on labeled data from similar structures or simulations would be thoroughly beneficial. However, most domain adaptation methods are designed for classifications and cannot efficiently address regressions. A fuzzy-set-based joint distribution adaptation for regression method has been developed by the authors, tackling regression problems but being limited to single outputs. The novelty presented in this paper exploits clustering techniques to approach multi-output problems, adopting a modified multikernel maximum mean discrepancy to improve the domain discrepancy metric. The proposed method is applied to cracked lap shear specimens to assist debonding quantification. Several domain adaptations are investigated: from simulations to experiments, and from one specimen to another, proving that the accuracy of damage quantification can be improved significantly in realistic environments. It is envisioned that the proposed approach could be integrated into fleet-level digital twins for nominally identical but heterogeneous systems.

Numerical simulation of quasi-static and fatigue debonding growth in adhesively bonded composite joints containing bolts as crack stoppers

ABSTRACT In this paper, the cohesive zone modeling (CZM) method has been employed to simulate static and fatigue debonding growth in adhesively bonded composite joints containing bolts as crack stoppers. The cracked-lap shear specimen (CLS) for static and fatigue loads and the wide single-lap shear (WSLS) structural element for static load have been modeled. In both cases, mixed-mode I+ II loading conditions are present in the bondline. Bonded and bonded/bolted configurations of the specimens have been analysed in order to evaluate the efficiency of bolts as crack stoppers. Numerical simulations have been performed using the LS-DYNA FE code. For the static debonding simulation, a standard bilinear traction-separation law has been applied while for the simulation of fatigue debonding growth a modified traction-separation law has been implemented using experimental data derived from fatigue tests. The modification has been realized by means of a user-defined subroutine (UMAT) having the capacity to account for crack divergence by making use of a crack front detection algorithm based on effective element’s length. A parametric study on the effect of distance between bolts and crack-tip has been performed. Additionally, the WSLS structural element has been subjected to impact load aiming to create a realistic initial disbond scenario. The residual strength of the element has been predicted by applying a quasi-static load after impact. The numerical results reveal that the presence of bolts under certain circumstances can cause crack growth retardation, under both static and fatigue loading conditions. This study emphasizes on the importance of fatigue crack growth simulation and aims to contribute to the numerical design of crack stopping features as a means of complying towards certification of adhesive bonding in primary composite aircraft structures.

Fatigue crack growth characterization in adhesive CFRP joints

Adhesive joints find an increasing use in lightweight structures, which is proportional to the evolution of carbon fiber-reinforced plastics (CFRPs). Understanding of fatigue crack growth behavior in adhesive CFRP joints is essential for the efficient maintenance and repair of existing joints and the design of new joints. Here, the fatigue crack growth behavior of adhesive CFRP joints under Mode-I, Mode-II and Mixed-Mode I + II loading conditions is characterized experimentally by means of Mode-I fatigue fracture toughness tests, Mode-II fatigue fracture toughness tests and the Mixed-Mode fatigue lap shear test. For the three different tests, the Double Cantilever Beam (DCB), the End-Notch Flexure (ENF) and the Cracked Lap Shear (CLS) specimens are used, respectively. Crack growth versus number of cycles is reported and modified Paris-laws are derived. The DCB specimens failed in cohesive failure mode while the ENF and CLS specimens in adhesive. The crack growth in the DCB specimens was more stable and showed a smaller scatter among the different specimens than the ENF specimens. Crack propagation with number of cycles in CLS specimen was almost linear. The results reported herein suggest a full experimental characterization of fatigue crack growth behavior of the considered aerospace CFRP/adhesive material system and can be proved very useful in the development and validation of fatigue crack growth simulation models.

Fracture Analysis of a Special Cracked Lap Shear (CLS) Specimen with Utilization of Virtual Crack Closure Technique (VCCT) by Finite Element Methods

Some of the most important characteristics due to a fracture investigation of a special specimen are taken into account. Debonding considerations for a composite/steel cracked lap shear (CLS) specimen by utilization of finite element methods (FEM) as well as a virtual crack closure technique (VCCT) approach have been investigated. Strain energy release rate, delamination load case and direct cycle fatigue analysis have taken into consideration in this study, and the corresponding simulations have been done by ABAQUS/Standard. Linear elastic fracture criteria are used for validation of numerical results from the simulation. For comparison of three different categories of analysis, some special characteristics such as effective energy release rate ratio, bond state, time at bond failure and opening behind crack tip at bond failure have been illustrated. In this work, a detailed analysis of a special CLS specimen debonding by using VCCT and FEM is presented and varied results for validation of this kind of combination are obtained and have been discussed.

Analysis of cracked lap shear testing of tungsten-CFRP hybrid laminates

In this work, we studied the effects of residual stresses on the testing of a cracked lap shear specimen (CLS). The analysis focused on tungsten-CFRP laminates for radiation protection applications. A 3-D finite element model was generated and the virtual crack closure technique (VCCT) was applied to determine fracture toughness. Crack propagation was analyzed using cohesive zone modelling (CZM) to combine multiple strain gauge data from experiments and the information from the VCCT. Our analysis indicated that residual stresses cannot be ignored in a valid analysis of tungsten-CFRP hybrid laminates—the experimentation with hybrid CLS specimens requires numerical crack simulation.

The relationship between pure delamination modes I and II on the crack growth rate process in cracked lap shear specimen (CLS) of 5 harness satin composites

Carbon fiber reinforced polymers (CFRP) structure can include dropping-off plies in order to comply with design requirements aiming at significant weight savings. However this type of discontinuity represents a potential source of delamination initiation and propagation which requires assessment of the mechanisms acting at the crack tip. This research investigates the influence of delamination modes I and II on the overall damage process observed in CLS specimen subjected to cyclic loads. The main contribution of this work focuses on the identification and physical interpretation of complex failure mechanisms in harness satin fabric. For this purpose a detailed fractographic analysis was carried out to qualitatively assess the surface fractures in these type of laminates. Results obtained for cyclic loaded CLS specimens were compared to analytical closed form solutions available in the literature. Results indicated that delamination front exhibited distinguishable delamination modes I and II propagating at constant mixed mode ratio (GI/GT).

Modified DCB and CLS specimens for mixed-mode fatigue testing of adhesively bonded thin sheets

The fatigue behavior of adhesive joints made with commercially coil-coated thin aluminum sheets has been measured using modified double cantilever beam (DCB) and cracked lap shear (CLS) specimens that avoided yielding. The thin sheets were bonded to a stiffener using a reinforcing adhesive layer. Experimental and finite element analyses demonstrated that the reinforcing adhesive layer had an insignificant effect on the stress state at the crack tip and the resulting fatigue behavior. The modified DCB test geometry was then used to measure the effect of the loading phase angle and test environment on the fatigue performance of the pretreated aluminum sheet. It was observed that the phase angle affected the threshold strain energy release rate, G th , and the crack growth rates. The presence of moisture in the test environment greatly reduced the fatigue threshold. It was also observed that the G th was sensitive to the rolling line orientation on the surface of the sheet.

Interfacial Fracture Toughness of Adhesively Bonded Joints

The aim of the present paper is to evaluate the interfacial fracture toughness of an Al/Epoxy adhesive system with a crack lying at the interface. A cracked lap shear specimen loaded in four point bending is adopted and the fracture toughness is pointed out in terms of the critical complex stress intensity factor. To this aim numerical analyses of the fracture specimen have been carried out using a commercial finite element code. In addition, fracture surfaces are analyzed and the locus of failure is discussed.

Treatment of CFRP by IAR method and its effect on the fracture behavior of adhesive bonded CFRP/aluminum composites

It was shown in the previous studies that adhesive shear strength of carbon fiber reinforced plastics (CFRP) to aluminum composites could be improved by the surface treatment of CFRP using Ar + ion irradiation. In the present work, the effect of CFRP treatment by Ar + ion irradiation on the fracture behavior of CFRP/aluminum joint was studied. The aluminum used was 7075-T6 and the CFRP used was multi-directional graphite/epoxy composites whose stacking sequence was [0°/±45°/0°] 3s. The surface of CFRP was treated using Ar + ion irradiation in an oxygen environment. The Ar + ion dose used was 1×10 16 ions cm −2. Fracture toughness of CFRP/aluminum joint was determined from cracked lap shear specimens using work factor approach. Then, the fracture toughness of ion beam-treated CFRP/aluminum joint was compared with that of untreated CFRP/aluminum joint. The results showed that the fracture toughness of ion beam-treated CFRP/aluminum case was about 72% higher than that of untreated CFRP/aluminum case. X-ray photoelectron spectrometer analysis showed that intensity of hydrophilic bonds, CO (carbonyl group) and OCO (carboxyl group) was increased by the Ar + ion-irradiation in an oxygen environment. Scanning electron microscope examination showed that cohesive failure occurred for ion beam-treated CFRP/aluminum joint while adhesive failure occurred for untreated CFRP/aluminum joint.

표면처리된 CFRP와 알루미늄 복합재료의 파괴인성 향상에 대한 연구

In this study, the effect of surface treatment of CFRP and aluminum on the fracture toughness of CFRP/aluminum composites was investigated. CFRP was surface-treated by Ar^+ ion beam under oxygen environment, and the aluminum was surface-treated by DC plasma. CFRP was adhesively bonded to aluminum using the secondary bonding procedure. Cracked lap shear specimens were used to determine fracture toughness. Three cases of cracked lap shear specimens were made depending on the surface treatment. The values of fracture toughness of three cases were compared to each other. It was found that the fracture toughness of ion beam-treated CFRP/aluminum composites was almost 72 % higher than that of untreated CFRP/aluminum composites. The fracture toughness of CFRP/plasma-treated aluminum composites was 50 % higher than that of untreated CFRP/aluminum composites.

Investigation on surface treatments of CFRP and aluminum to improve fracture toughness of adhesively-bonded CFRP–aluminum joints

In this study, the role of surface treatments of CFRP (graphite/epoxy composite) and aluminum (7075-T6) on the adhesively-bonded CFRP-aluminum joints has been investigated. The CFRP was surface-treated by Ar+ ion irradiation in an oxygen environment and the aluminum was surface-treated using a DC plasma. Ar+ ion irradiation treatment was carried out at Ar+ ion dose of 1016 ions/cm2. Plasma treatment was carried out at a volume ratio of acetylene gas to nitrogen gas of 5:5 and the treatment time was 30 s. The effect of surface treatments on the fracture behavior CFRP-aluminum joints was determined from fracture tests using three different CLS (cracked lap shear) specimens: (1) untreated CFRP/untreated aluminum, (2) ion-irradiated CFRP/untreated aluminum and (3) untreated CFRP/plasma-treated aluminum. Fracture behaviors (fracture load, fracture toughness, fracture surfaces) of these three different specimens were compared. The results showed that both fracture load and fracture toughness of CFRP-aluminum joints were in the following order: ion-irradiated CFRP/untreated aluminum specimen > untreated CFRP/plasmatreated aluminum specimen > untreated CFRP/untreated aluminum specimen. SEM examination of fracture surfaces showed that fracture occurred as an interfacial failure for untreated specimens. On the other hand, a cohesive failure in the adhesive was the primary fracture mode for specimens surface-treated by ion irradiation or plasma.

Fatigue behaviour and life prediction of fibre reinforced metal laminates under constant and variable amplitude loading

Fatigue crack growth of fibre reinforced metal laminates (FRMLs) under constant and variable amplitude loading was studied through analysis and experiments. The distribution of the bridging stress along the crackline in centre-cracked tension (CCT) specimen of FRMLs was modelled numerically, and the main factors affecting the bridging stress were identified. A test method for determining the delamination growth rates in a modified double cracked lap shear (DCLS) specimen was presented. Two models, one being fatigue-mechanism-based and the other phenomenological, were developed for predicting the fatigue life under constant amplitude loading. The fatigue behaviour, including crack growth and delamination growth, of glass fibre reinforced aluminium laminates (GLARE) under constant amplitude loading following a single overload was investigated experimentally, and the mechanisms for the effect of a single overload on the crack growth rates and the delamination growth rates were identified. An equivalent closure model for predicting crack-growth in FRMLs under variable amplitude loading and spectrum loading was presented. All the models presented in this paper were verified by applying to GLARE under constant amplitude loading and Mini-transport aircraft wing structures (TWIST) load sequence. The predicted crack growth rates are in good agreement with test results.

Determination of Fracture Toughness, GC of Graphite/Epoxy Composites from a Cracked Lap Shear (CLS) Specimen

Previously, the work factor approach was applied to determine fracture toughness, Gc of unidirectional graphite/epoxy CLS specimen, and it is required to apply the approach to determine Gc of multi-directional CLS specimen. In this work, the elastic work factor, η el form of multi-directional CLS specimen was calculated numerically as a function of delamination length in order to investigate stacking sequence effect on η el. The calculated η el was applied experimentally to determine fracture toughness, G c of multi-directional ([0/±45/0] s/[02/±452/02] s, [0/±45/0] s/[02±452/902] s, and [902/02] s/[04/904] s for the lap and the strap) graphite/epoxy composites. For each case, the Gc values determined from the η el approach were compared with those determined from the compliance method. Numerical results show that η el is not affected by the fiber angles of plies, and η el is linearly related to delamination length. It was found from the experimental results that Gc determined from the η el approach is in a maximum 15% range of difference with that determined from the compliance method for each case. It was also found from the SEM examination that delamination propagated irregularly through interfaces of lap and strap (8th and 9th plies) for [902/02] s/[04/904] s case while no delamination transfer was observed for [0/±45/0] s/[02/±452/02] s and [0/±45/0] s/[02/±452/902] s cases.

Determination of Fracture Toughness, G c of Graphite/Epoxy Composites from a Cracked Lap Shear (CLS) Specimen

Determination of Fracture Toughness, G c of Graphite/Epoxy Composites from a Cracked Lap Shear (CLS) Specimen