Cracked Lap Shear 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.

Development and mechanical evaluation of induction low Shear Friction Stir Spot rivets (ILSFSR) as crack arrest features in co-consolidated and welded thermoplastic laminates

The technique of Induction Low Shear Friction Stir Riveting (ILSFSR) stands as an alternative riveting process, which utilizes the melting and the resolidifying nature of thermoplastic resins to install metallic rivets into thermoplastic laminates without the need for pre-drilling. In this work, the aspects of the method’s implementation are presented, including its theoretical basis, the technical background, the apparatus, and the rivets’ installation procedure. Moreover, a mechanical evaluation of the resulting joints was held to determine their crack-arresting efficiency along carbon fiber reinforced co-consolidated low-melt PolyArylEtherKetone (LM-PAEK) laminates. A coupon scale testing campaign was employed, including mixed-mode Crack Lap Shear specimens containing the ILSFSR features, which were quasi-statically and fatigue-loaded. The interfacial crack growth results, which were compared to reference specimens, indicated the method's potential to provide significant damage retardation, resulting in an increase of 26% of quasi-static interfacial strength and complete confinement of fatigue crack growth, thus aiding in the damage tolerance design for critical engineering components.

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.

Quasi-static and fatigue crack growth simulation in co-consolidated thermoplastic joints containing crack arrest features

In the present article, quasi-static and fatigue crack growth in co-consolidated thermoplastic Crack-Lap Shear (CLS) specimens containing two different Crack Arrest Features (CAFs) have been numerically simulated. The substrates have been manufactured from unidirectional T700 carbon fiber reinforced Low-Melt PAEK thermoplastic material. The Friction Stir Spot Welding (RFSSW) and the Induction Low Stir Friction Riveting (ILSFSR) CAFs were studied. Crack growth simulation has been based on the cohesive zone modeling method and has been implemented via the LS-Dyna FE software. Both static and fatigue crack growth models have been successfully validated against results from respective mechanical tests. Both CAFs have been proven capable of delaying crack growth, especially for fatigue loading. An evaluation of various installation parameters of the two features, such as the diameter, the distance to the crack tip, and the installation depth, was performed for the quasi-static loading cases. All parameters have been found to affect the crack stopping efficiency, but the most important effect comes from the diameter of the CAF. Furthermore, a series of quasi-static and fatigue simulations have been run on a mono-stiffener element, representative of an aeronautical structural assembly, in order to evaluate numerically the efficiency of the CAFs at a higher scale. The results showed no crack stopping for the ILSFSR and a low crack stopping for the RFSSW for fatigue loading.

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.

The mix-mode fracture behaviour of aerospace composite joints co-cured by different forms of advanced thermoplastics

The co-cure joining of thermoset composites by thermoplastics is an advanced approach to develop robust composite joints. Herein, Polyethylenimine (PEI) and Polyetheretherketone (PEEK) polymers in a form of film, woven mesh, hollow mesh and carbon fibre reinforced plastic were utilised as interlayers for the co-cure joining of an aerospace composite. A cracked-lap shear test was used to investigate the fracture toughness and mechanisms of the co-cured composite joints under a mixed mode-I/II loading condition. The experimental results revealed that the insert of PEI and PEEK interlayers significantly enhanced the mix-mode crack resistance of the composite joints by different levels. The most remarkable increase in the crack initiation energy and crack propagation energy was 161% and 613%, respectively, that were obtained by adding the PEEK film interlayers. The main toughening mechanisms of the PEI film joined composites were crack-pining and matrix toughening of the PEI particles, which were formed by the diffusion of the PEI resins into the composite epoxy matrix during the curing process. Plastic deformation and fracture of the PEEK resins were the main toughening mechanisms of the composites that were co-cure joined by the PEEK films, woven meshes, hollow meshes and carbon fibre reinforced tapes. The crack resistance of the composite joints correlated well with the level of the PEEK fracture phenomena during the mix-mode fracture process.

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.

Monitoring the fatigue crack growth behavior of composite joints using in situ 2D-digital image correlation

ABSTRACTTo predict lifetime and damage tolerance of adhesively bonded joints, crack growth behavior need to be understood, as it oftentimes serves as a basis or proof of, e.g., numerical models. Within this work, a method is presented to measure crack length in dynamically loaded adhesively bonded CFRP-joints by means of 2D digital image correlation. Automated picture acquisition at the maximum force level within a cycle allows measurements during the fatigue testing. The strain level in front of the crack tip was defined as maximum possible strain within the bond line. Higher strains were denoted as crack. Marking the crack path automatically led to the associated crack length and could afterward be derived to a crack propagation rate. Different destructive tests were carried out on composite single, double and crack lap shear specimens.

Numerical evaluation of crack stopping mechanisms in composite bonded joints due to corrugation and bolts

In this paper, the crack stopping mechanisms in corrugated composite bonded joints and hybrid bonded/bolted joints were evaluated numerically using the cohesive zone modeling approach. For the study, the DCB (double-cantilever beam) and the CLS (crack-lap shear) specimens were modelled. The first two specimens were subjected to static loads and the latter both to static and fatigue loads. The analysis was performed using the LS-DYNA explicit FE code. Fatigue crack growth simulation was performed using an in-house developed user-defined subroutine (UMAT). The numerical results reveal a crack stopping in the corrugated DCB, no crack stopping in the corrugated CLS and a reduction of crack growth rate in the bonded/bolted CLS for both static and fatigue loads. The methods and the findings of the present study can be used for the design of crack stopping features in adhesively bonded primary composite aircraft structures.

Analytical Study of Elastic-Plastic Fracture in the Crack-Lap Shear Multilayered Beam Configuration

Abstract This paper reports an analytical study of delamination fracture in the Crack-Lap Shear (CLS) multilayered beam configuration with taking into account the material non-linearity. A delamination crack was located arbitrary along the beam height. It was assumed that the CLS mechanical response can be described by using a power-law stress-strain relation. It should be mentioned that each layer may have different material constants in the stress-strain relation. Besides, the thickness of each layer may be different. The classical beam theory was applied in the present study. The non-linear fracture behaviour was analyzed by the J-integral. Analytical solutions of the J-integral were obtained for homogeneous as well as for multilayered CLS beams. In order to verify the solutions obtained, analyses of the strain energy release rate were developed with considering material non-linearity. Material properties and crack location effects on the non-linear fracture behaviour were investigated. The analysis revealed that the J-integral value increases when the material non-linearity is taken into account. It was found also that the J-integral value decreases with increasing the lower crack arm thickness. The approach developed here is very convenient for parametric fracture analyses. The solutions derived can be used for optimization of the CLS multilayered beams with respect to their fracture performance.

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 Three-Dimensional Functionally Graded Multilayered Beam

Delamination fracture behavior of the three-dimensional functionally graded multilayered crack lap shear (CLS) beam configuration was studied analytically in terms of the strain energy release rate with linear-elastic fracture mechanics methods. The CLS under consideration can consist of an arbitrary number of layers, each having different thickness and material properties. The material of each layer is functionally graded along its width, thickness, and length. A delamination crack is arbitrarily located along the beam height. The strain energy release rate was derived by analyzing its density in the beam cross section ahead and behind the crack front. It was additionally analyzed using the strain energy release rate was performed by using the beam strain energy for verification. The effect of the material gradient and crack location along the beam height on the delamination fracture behavior was evaluated.

Delamination properity of 2060 aluminium lithium alloy laminate

Delamination is an significant property of aluminium lithium alloy laminate for damage tolerance design. Double Crack Lap Shear specimens of "2/1" structure are used for testing delamination properity. Experiments of 2 stress ratio conditions R=0.06, 0.5 are conducted, and 5 stress levels for each stress ratio. Delamination growth data b-N are obtained from 4 crack tips’ locations. The energy release rate, Gd, of the "2/1" structure laminate are calculated. Alderliesten modelis used for describing delamination growth db/dN and energy release rate Gd. Parameters Cd and nd in Alderliesten model are determined for R=0.06 and R=0.5 by linear fitting. An analysis is made by comparing the results of different stress ratio conditons.

Delamination in a Two-Dimensional Functionally Graded Beam

An analytical study of delamination in the crack lap shear beam is performed. It is assumed that the material is functionally graded along the width and height of the beam. Delamination is studied in terms of the total strain energy release rate by applying methods of linear-elastic fracture mechanics. An additional analysis of the total strain energy release rate is performed by considering the strain energies in the beam cross sections ahead of and behind the crack front for verification. The effects of the crack location and material gradient on delamination are evaluated.

Delamination analysis of a layered elastic-plastic beam

PurposeThe purpose of this paper is to perform a theoretical analysis of delamination fracture behaviour of the Crack Lap Shear layered beam configuration taking into account the material non-linearity. A delamination crack located arbitrarily along the beam height was considered in this study.Design/methodology/approachThe beam mechanical behaviour was described by using the Ramberg-Osgood stress-strain relation. Fracture was analysed by applying the J-integral approach. Besides by using symmetric Ramberg-Osgood stress-strain curve, fracture was investigated also by Ramberg-Osgood stress-strain curve that is not symmetric with respect to tension and compression. The J-integral solutions were verified by performing elastic-plastic analyses of the strain energy release rate.FindingsThe effects of crack location and material properties on the non-linear fracture behaviour were evaluated. It was found that the material non-linearity leads to increase of the J-integral values. Therefore, the material non-linearity has to be taken into account in fracture mechanics based safety design of structural members composed by layered materials. The analytical solutions derived are very useful for parametric investigations of delamination fracture with considering the material non-linearity. The results obtained can be applied for optimisation of the beam structure with respect to fracture performance.Originality/valueThe present study contributes for the understanding of delamination fracture in layered beams that exhibit non-linear material behaviour.

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.