Delamination Growth In Composite Laminates Research Articles

In-situ damage mechanism investigation and a prediction model for delamination with fibre bridging in composites

Carbon-fibre reinforced composites are susceptible to delamination. Fibre bridging is an important shielding mechanism frequently observed in delamination. The presence of these bridging fibres can significantly increase interlaminar resistance, making it critical to represent this phenomenon for delamination characterization in composite laminates. To this end, in-situ SEM examinations were carried out to thoroughly explore damage mechanisms around delamination front as well as in bridging fibres. It was found that micro-cracks initiated at fibre–matrix interface can gradually develop and coalesce into micro-delaminations ahead of the main crack. The accumulation of these micro-delaminations can finally cause macro delamination propagation. The performance of bridging fibres can be summarized as three typical stages, i.e. bending, fibre–matrix peeling and final breakage with crack opening. Subsequently, theoretical discussions on bridging stress distribution were conducted in accordance with these bridging mechanism examinations, contributing to a new traction-separation constitutive to represent fibre bridging performance. A FEA prediction model was finally developed to characterize delamination behavior with fibre bridging. The simulation results can agree well with the experimental data in the entire delamination, demonstrating its effectiveness in fibre-bridged delamination representation. This study also demonstrated the importance of having in-depth understanding on fibre bridging mechanisms to appropriately represent bridging performance during delamination growth in composite laminates.

A criterion for predicting delamination growth in composite laminates

To develop a simplified and physically sound delamination-failure criterion, this paper reports a joint experimental and numerical study in which out-of-plane quasi-static indentations with different indentation forces were applied to the centre of fully clamped rectangular carbon fibre-reinforced polymer (CFRP) laminates. The results show that two basic micro-delamination growth modes, which are separately dominated by opening and sliding microcracks, can be distinguished according to the fracture morphologies of the delaminations along the 0° fibre direction. Both are driven by the out-of-plane shear stress σ13, and the corresponding delamination failure threshold is the out-of-plane shear strength S13. Based on these results, a concise Hashin-type delamination-propagation criterion containing only a single stress component and single strength parameter is proposed, which, by incorporating it into the finite element model, the delamination growth along the fibre directions of multidirectional CFRP laminates can be precisely predicted; the prediction error is less than 10 %.

Influence of interface ply orientation on delamination growth in composite laminates

The standard experimental procedures for determining the interlaminar fracture toughness are designed for delamination propagation in unidirectional specimens. However, in aerospace structural components, delamination usually occurs between plies at different orientations resulting in different damage mechanisms which can increase the value of the fracture toughness as the delamination propagates. Generally, numerical analyses employ the value measured at the delamination onset, leading to conservative results since the increase resistance of the delamination is neglected. In this paper, the fracture toughness and the R-curves of carbon/epoxy IM7/8552 are experimentally evaluated in coupons with delamination positioned at 0°/0° and 45°/−45° interfaces using Double Cantilever Beam (DCB) and Mixed-Mode Bending (MMB) tests. A simplified numerical approach based on the Virtual Crack Closure Technique (VCCT) is developed to simulate variable fracture toughness with the delamination length within a Finite Element code using a predefined field variable. The results of the numerical analyses compared with the experimental data in terms of load-displacement curves demonstrate the effectiveness of the proposed technique in simulating the increase resistance in delamination positioned between plies at 45°/−45° interface.

Numerical modeling of two-dimensional delamination growth in composite laminates with in-plane isotropy

The two-dimensional (2D) delamination growth in fiber-reinforced polymer (FRP) laminates with in-plane isotropy under Mode I loading condition was numerically investigated using finite element analyses. Two sizes of plate models were developed, focusing on different fracture stages. Cohesive elements were employed to simulate the fracture behavior in the presence of large-scale bridging (LSB). The influences of the pre-crack shape/area, loading zone shape/area and fracture resistance were parametrically studied. It was found that either a flatter pre-crack shape or a flatter loading zone shape could result in higher initial structural stiffness and less uniform distribution of the strain energy release rate (SERR) along the pre-crack perimeter during crack initiation and early propagation. However, they had only a minor effect on the stiffness after full fiber bridging development in all directions. The plates finally achieved constant stiffness, which increased linearly with the fracture resistance. The final crack shape was dependent on the loading zone shape and area, but the effects were relatively weak.

A novel total fatigue life model for delamination growth in composite laminates under generic loading

It is really important to have a reliable model to represent fatigue delamination growth (FDG) behavior for composite structural design and life evaluation with the wide applications of composites in aerospace industry. Therefore, a new total fatigue life model for delamination growth in composites has been proposed for engineering in present study, according to introducing a new crack similitude parameter and using the total life strategy. It clearly demonstrates that the FDG behavior under various fatigue loading conditions in the entire strain energy release rate (SERR) range can be appropriately characterized with this model. Particularly, a master resistance curve can be obtained to determine the FDG behavior under various R-ratios at a given mode mixity. And these resistance curves can shift downwards with mode mixity. This model will be important for material characterization, comparison and development, which is prerequisite for the applications of the damage tolerance philosophy in composite structural design and life evaluation of a composite structure.

Optimization of thickness and delamination growth in composite laminates under multi-axial fatigue loading using NSGA-II

Employing NSGA-II, this paper aims to achieve a damage tolerant structure enjoying maximum delamination resistance and thinness; therefore, we simultaneously define minimization of laminate thickness and delamination growth as objective functions. Fiber orientation angle, ply thickness, and stacking sequence are chosen as design variables. The authors consider symmetric glass/epoxy laminates with middle layers containing a single matrix crack.By applying multi-axial fatigue loading, the initiation and growth of local delamination from the tip of the matrix crack in the damaged ply interface became possible. Finally, it is indicated that NSGA-II has good convergence with damage optimization in cracked glass/epoxy composite laminates.

An energy based approach for reliability analysis of delamination growth under mode I, mode II and mixed mode I/II loading in composite laminates

Since there are many uncertainty sources in composite laminates such as fiber and matrix material properties, reliability analysis of laminates under specific loading modes has been of significant importance in many applications. This paper represents a method for reliability analysis of E-Glass/Epoxy composite laminates in the presence of the most common and catastrophic failure mode in macro-scale, i.e. delamination under mode I, mode II, and mixed mode I/II using an energy based approach for the first time. Limit state functions for the reliability analysis of delamination growth under mode I, mode II, and mixed mode I/II loading are formulated based on energy release rate along with the interlaminar fracture toughness in each of the mentioned modes. Having random variables and limit state functions determined based on energy, the reliability assessment is carried out using first order reliability method (FORM) and second order reliability method (SORM) and the obtained results are verified by Monte Carlo Simulation (MCS). Finally, the effects of different modes of delamination including mode I, mode II, and mixed mode I/II on the reliability of delamination growth in composite laminates are obtained. It is indicated that the reliability of E-Glass/Epoxy composite laminates in mode I delamination has its lowest value, while in mode II has the highest. Amongst the considered random variables in mode I, mode II, and mixed mode I/II, thickness of the laminate is the most effective one in the reliability of mentioned composite laminates under different delamination modes.

Experimental Investigation of Delamination Growth in Composite Laminates under a Compressive Load

This paper focuses on the use of no-contact experimental techniques for monitoring the interlaminar damage evolution in composite laminates. Indeed, Infrared Thermography and Digital Image Correlation are adopted to investigate, in composite plates with artificial delamination, the influence of the delamination initial position on the delamination growth. The paper also investigates the feasibility of using a no-contact experimental technique for the measurement of displacement and strain during mechanical tests, such as the Digital Image Correlation, to evaluate, by means of indirect measurements, the delamination growth as a function of the applied load.

Delamination growth in composite laminates of variable stiffness

SummaryThe production of new composite laminates with variable stiffness within the surface of plies was enabled by tow‐placement machines. Because of the variation of stiffness, these materials are called variable stiffness composite laminates (VSCL). Recently, many attempts were made to investigate their structural behaviour. In this contribution, a first‐order shear deformation theory is selected to model the multilayered composite laminates. The adopted theory is enhanced by the extended finite element method (XFEM) to describe discontinuities at element level of any interface of interest. To predict the location of the delamination onset, a traction–separation law is developed that is consistent with the XFEM topology. An exponential softening behaviour is implemented within the interface to model the delamination growth in a mixed‐mode direction. In order to solve the non‐linear equations of the delamination propagation, an arc‐length method is applied. The effect of the curvilinear fibre orientation on the location of the delamination onset is investigated. Subsequently, the structural response of the laminates is computed. According to the simplicity of the new approach using the XFEM; and based on the computational cost for calculating the stiffness of VSCL, the method is able to determine structural response of VSCL with less computational effort. Copyright © 2016 John Wiley & Sons, Ltd.

Simulation of fatigue delamination growth in composite laminates under mode I loading

An effective approach based on a cohesive zone model is proposed to simulate the propagation of delamination in the laminated composite materials subjected to high-cycle fatigue and pure mode I loading. It was implemented in the finite element software ANSYS®. Shell elements connected by bonded contact with the cohesive material properties were used. The main advantage of this approach is the algorithm for introducing fatigue damages by applying additional displacements. A double cantilever beam test was analyzed to validate the proposed approach.

Scaling parameter for fatigue delamination growth in composites under varying load ratios

Fatigue delamination growth in composite laminates is strongly influenced by mean loads or load ratios. Description of this behaviour currently relies on empirical curve fitting, which renders it difficult to predict fatigue lives of composite structures subjected to variable amplitude fatigue loading. This paper presents a new scaling parameter that is consistent with the similitude concept and incorporates the crack-tip shielding effects of fibre bridging under fatigue loading. Static and fatigue experiments were carried out on IM7/977-3 composite laminates under mode I and mode II. Large-scale fibre bridging was observed as a major toughening mechanism under both static and fatigue loading. To correctly account for the effect of fibre bridging, an inverse method was developed to determine the traction stresses acting in the crack wake. The new scaling parameter, accounting for the effect of bridging by cross-over fibres, is shown to unify the fatigue growth rates under different load ratios obtained in this study.

Interpreting the stress ratio effect on delamination growth in composite laminates using the concept of fatigue fracture toughness

This paper provides a study on fatigue delamination growth in composite laminates using energy principles. Experimental data has been obtained from fatigue tests conducted on Double Cantilever Beam (DCB) specimens at various stress ratios. A concept of fatigue fracture toughness is proposed to interpret the stress ratio effect in crack growth. The fatigue fracture toughness is demonstrated to be interface configuration independent but significantly stress ratio dependent. An explanation for this phenomenon is given using SEM fractography. Fracture surface roughness is observed to be similar in different interfaces at the same stress ratio. But it is obviously more rough for high stress ratio in comparison with that for low stress ratio, causing the fatigue resistance increase. Therefore, the stress ratio effect in fatigue crack growth can be physically explained by a difference in resistance to crack growth.

Peridynamic modeling of delamination growth in composite laminates

Delamination growth predictions in previous peridynamic models were based on the assumption of constant critical stretch for each interlayer bond interaction. This study presents a new approach to terminate the interlayer peridynamic bonds. The critical stretch of a bond is implicitly determined by using the measured critical energy release rate values for different modes of deformation, and can vary depending on the degree of deformation. The bond failure occurs when the amount of energy required to remove the bonds across a unit surface equals the critical energy release rate of the interface between the two layers. This approach is applied to model delamination growth in double cantilever beam (DCB) (Mode I) and transverse crack tension (TCT) (Mode II dominated) specimens. The peridynamic predictions correlate well with the numerical and experimental results available in the literature.

Intra and damage analysis of laminated composites using coupled continuum damage mechanics with cohesive interface layer

In the present article a model based on coupling of continuum damage mechanics with cohesive interface layer is proposed in order to predict the progressive damages including the large delamination growth in composite laminates. A new interface cohesive constitutive law is developed which is compatible with 3D continuum damage mechanics (CDM). To avoid the difficulties of 3D mesh generation and 3D interface modeling between the layers, the cohesive interface layer is implemented in the Reddy’s full layer-wise plate theory. An angle-ply laminate is analyzed to evaluate the developed CDM+cohesive layer in edge delamination initiation and evolution during uniaxial tension loading. The proposed approach is demonstrated to predict progressive damage and final failure load of angle-ply laminated composites both accurately and effectively.

A damage model for the prediction of static and fatigue-driven delamination in composite laminates

This paper presents a continuum damage mechanics failure model to predict mixed-mode delamination growth in composite laminates subjected to static and high-cycle fatigue loading. The proposed formulation has been developed for robust nonlinear finite element formulations based on explicit direct time integration schemes, particularly the central difference method. The failure model has been implemented as a user-defined material model into ABAQUS/Explicit finite element code within C3D8 hexahedron solid elements. Numerical simulations were performed at coupon level for double cantilever beam, end-notched-flexure, mixed-mode bending and mixed-mode flexure specimens. Predictions obtained using the proposed failure model were compared with experimental results available in the open literature. Good agreement between numerical and experimental results was found.

A mesh-free simulation of mode I delamination of composite structures

Abstract A numerical and experimental investigation for the analysis of delamination problem under mode I loading in composite material is presented. Firstly, the simulation of the delamination under mode I loading and failure of composite materials based on the cohesive segments model is investigated by using the meshfree method. With the partition of unity of moving least-squares shape functions, the discontinuities at the cohesive segments are approximated with additional degrees of freedom at the nodes. An iterative solution scheme between the continuous and discontinuous fields is presented to solve mode I delamination growth. Secondly, to verify the meshfree method’s results, an experimental investigation and the finite element method were used for the simulation of delamination. The experimental study used a double-cantilever beam made of carbon/epoxy laminate (AS4/3501-6) which consists of 10 plies in [0]10 and [0/90/0/90/0]s layup with delamination inserted in the middle of the laminate. The critical fracture force, which can be experimentally measured, was used to calculate the mode I delamination fracture toughness of the carbon/epoxy laminate. Results obtained from the meshfree method showed very good agreement with experimental data for single-mode delamination under mode I loading. The meshfree method could also be used effectively to produce delamination growth in composite laminates and is especially suitable for the simulation of complex delamination patterns that are difficult to model using traditional numerical methods.

Coupling of Continuum Damage Mechanics with De-Cohesive Element for Delamination Analysis in Laminated Composites

To predict the progressive damages including the large delamination growth in composite laminates, a new interface de-cohesive constitutive law is developed which is compatible with 3D continuum damage mechanics (CDM). To avoid the difficulties of 3D mesh generation and 3D interface modeling between the layers, the interface element is implemented in the Reddy’s full layer-wise plate theory. An angle-ply laminate is analyzed to evaluate the developed CDM+Interface procedure in edge delamination initiation and evolution at final stage of CDM damage progress.

Modelling of delamination growth in composite laminates by the virtual internal bond method

The paper examines the potential of the virtual internal bond (VIB) model to predict delamination initiation and growth in composite laminates. Finite elements implementing a formulation of the VIB model based on a two-parameter bond potential were used to simulate the resin-rich interface between adjacent layers. The performance of VIB elements was assessed by modelling the fracture response of unidirectional laminates with different cohesive strengths and fracture energy values in DCB, ENF and MMB tests. Even though some issues that restrict the applicability range of the model remain to be addressed, the good agreement achieved between finite element results and both analytical solutions and experimental results demonstrates the capability of VIB-based finite element models to correctly simulate the mechanics of delamination under different loading modes.

Buckling-driven delamination growth in composite laminates: Guidelines for assessing the threat posed by interlaminar matrix delamination

This paper is concerned with development of a simple procedure to assess the threat posed by interlaminar matrix delaminations to the integrity of composite laminates when they are situated in a compressive stress field. Depending upon the size of the delamination, its location below the surface and on the level of the applied stress, it may or may not buckle away from the rest of the laminate. If it buckles, the post-buckling response will depend upon the applied compressive stress level, the thickness of the delaminated ply and the interlaminar critical energy release rate of the composite material for the mode mixity present at the delamination front. The pre- and post-buckling response is examined in its simplest form and guidelines provided for assessing whether or not a delamination detected during quality control or inspection poses a threat to the safe operation of the composite laminate.

Finite Element Interface Technology for Modeling Delamination Growth in Composite Structures

An effective and robust interface element technology is presented for connecting and simulating crack growth between independently modeled finite element subdomains, for example, composite plies. This method has been developed using penalty constraints and allows coupling of finite element models whose nodes do not necessarily coincide along their common interface. Additionally, the present formulation leads to a computational approach that is very efficient and completely compatible with existing commercial software. The present interface element has been implemented in the commercial finite element code ABAQUS as a user element subroutine, making it easy to test the approach for a wide range of problems. Additionally, the interface element formulation has been modified to simulate delamination growth in composite laminates. Thanks to its special features, the interface element approach makes it possible to release portions of the interface surface whose length is smaller than that of the finite elements. In addition, the penalty parameter can vary within the interface element, allowing the damage model to be applied to a desired fraction of the interface between the two meshes. Results for double cantilever beam and end-loaded split specimens are presented. These results are compared to measured data to assess the ability of the present damage model to simulate delamination growth.