Open-hole Composite Laminates Research Articles

Simulating Initial and Progressive Failure of Open-Hole Composite Laminates under Tension

A finite element (FE) model is developed for the progressive failure analysis of fiber reinforced polymer laminates. The failure criterion for fiber and matrix failure is implemented in the FE code Abaqus using user-defined material subroutine UMAT. The gradual degradation of the material properties is controlled by the individual fracture energies of fiber and matrix. The failure and damage in composite laminates containing a central hole subjected to uniaxial tension are simulated. The numerical results show that the damage model can be used to accurately predicte the progressive failure behaviour both qualitatively and quantitatively.

Static strength prediction in laminated composites by using discrete damage modeling

Discrete Damage Modeling of complex local failure patterns in laminated composites including matrix cracking, delamination, and fiber failure was performed. Discrete Damage Modeling uses the Regularized eXtended Finite Element Method for the simulation of matrix cracking at initially unknown locations and directions independent of the mesh orientation. Cohesive interface model is used both for Mesh Independent Cracking as well as delamination propagation. The fiber failure mode is modeled by two different methods in tension and compression. Tensile failure is predicted by Critical Failure Volume criterion, which takes into account volumetric scaling of tensile strength. Compression fiber failure is simulated with a single parameter continuum damage mechanics model with non-compressibility condition in the failed region. Ply level characterization input data were used for prediction of notched and unnotched laminate strength. All input data required for model application is directly measured by ASTM tests except tensile fiber scaling parameter and compression fiber failure fracture toughness, which were taken from literature sources. The model contains no internal calibration parameters. Tensile and compressive strength of unnotched and open hole composite laminates IM7/977-3 has been predicted and compared with experimental data. Three different layups, [0/45/90/−45]2S, [30/60/90/−60/−30]2S, and the [60/0/−60]3S, were modeled and tested and showed good agreement with experiment in the case of tensile loading, whereas the compressive strength was generally under predicted for unnotched laminates and overpredicted for open hole laminates.

Influence of ply cluster thickness and location on matrix cracking evolution in open-hole composite laminates

Abstract The influence of cluster thickness and its position on the damage evolution of open-hole composite laminates, subjected to uniaxial in-plane tensile loads, is studied in this work. The Discrete Damage Mechanics model of Barbero-Cortes augmented by a fiber damage criterion is employed. Several stacking sequences with clusters in different positions and thicknesses inside the laminate are analyzed. The influence of cluster thickness and its location on: the crack-density evolution, applied load, longitudinal stress and its contour plots is studied for all the stacking sequences selected.

Recent Developments in Modelling of Progressive Damage in Fibre-Reinforced Composites

This paper provides an overview of recent developments in the modeling of progressive damage in fiber-reinforced composite laminates. Some insights into modeling the size effects of open-hole composite laminates under in-plane tension and compression, the significance of ply-blocking and delamination are discussed. Recent interest in the interaction and migration of matrix cracks and delamination, resulting in development of integrated XFEM-CE and floating node methods will also be presented.

Progressive damage analysis of open-hole composite laminates under compression load

Firstly,compressional experiments were conducted on open-hole composite laminates. The progressive damage during the loading process and the final damage mode of the specimen were detected with a digital microscope,scanning electron miroscope and an X-scan equipment. Damage initiation and delamination between layups of 45° and 90° were observed during the experiments. Secondly,the failure modes of composite laminates are divided into intra-laminar failure and inter-laminar failure. A multiscale model based on the micro-mechanics of failure MMF3 theory and the interface cohesive element method for the damage analysis of open-hole compression laminates was developed. At last,the initiation and propagation of the damage and the failure modes of the laminates were predicted with the model. Numerical results such as the damage initiation position of fiber and matrix,the delamination propagation process and the final failure modes are in good agreement with experimental results,which indicates that the model is applicable to analysis of progressive damage of open-hole composite laminates under compression load.

Progressive damage modeling of open-hole composite laminates under compression

Despite the recent success of modeling progressive damage of open-hole fiber reinforced composites subjected to tension (OHT), it is still a challenging task to predict the strengths and the damage progression of open-hole composite laminates under compressive loading (OHC). Herein, we propose a progressive damage model for OHC based on our early model for OHT and apply it to study the size effects of OHC. In the proposed model, continuum shell elements are used to account for both in-plane and out-of-plane deformation and delamination is modeled using cohesive elements. A smeared crack model is used to model the progressive failure of composite plies. It is found that the proposed model can predict accurately the experimental strengths and damage patterns with the assumption that the translaminar fracture toughness for blocked plies increases. The different failure mechanisms of the sublaminate scaled laminates of the stacking sequence 45/90/-45/0ms and the ply-level scaled laminates of stacking sequence 45n/90n/-45n/0ns are found to be closely related to the in-plane shear stress of the central 0° ply block and the initiation of interface delamination.

An investigation into the damage development and residual strengths of open-hole specimens in fatigue

An extensive experimental program was carried out to investigate and understand the sequence of damage development throughout the life of open-hole composite laminates loaded in tension–tension fatigue. Quasi-isotropic carbon/epoxy laminates, with stacking sequence [452/902/−452/02]S, [45/90/−45/0]2S and [45/90/−45/0]4S were examined. These were selected on the basis that under quasi-static loading the [452/902/−452/02]S configuration exhibited a delamination dominated mode of failure whilst the [45/90/−45/0]2S and [45/90/−45/0]4S configurations showed a fibre dominated failure mode, previously described as “pull-out” and “brittle” respectively. Specimens were fatigue loaded to 1×106 cycles or catastrophic failure, which ever occurred first. A number of tests were interrupted at various points as the stiffness dropped with increasing cycles, which were inspected using X-ray computed tomography (CT) scanning. A static residual strength program was carried out for run-out specimens of each configuration.

Non-linear progressive failure analysis of open-hole composite laminates under combined loading

This article presents a finite element-based study of geometrically non-linear progressive failure analysis of open-hole composite laminates, subjected to combined in-plane and out-of-plane loading. Different ply- and constituent-based failure criteria and material property degradation schemes have been coded in Patran command language and a progressive failure analysis code is developed. Progressive failure analyses of the sample composite laminate, with an open-hole, subject to different combined loading are performed to study the effect of the geometric non-linearity on the first ply failure and progression of failure. For mode independent failure criteria, a method is proposed for the determination of separate degradation factors for fiber and matrix failures which are assumed to occur simultaneously. The results show that under combined in-plane and out-of-plane loading, depending on the load case linear analysis can significantly underestimate or overestimate the failure progression compared to the geometrically non-linear analysis even at low levels of out-of-plane loading.

Strength Prediction in Open Hole Composite Laminates by Using Discrete Damage Modeling

The present paper addresses the issue of direct simulation of complex local failure patterns in laminated composites. A model capable of the discrete modeling of matrix cracking, delamination, and the interaction of these two damage modes is proposed. The analytical technique develops the regularized extended finite element method for the simulation of matrix crack initiation and propagation at initially unknown locations, as well as a cohesive interface model for delamination. The regularized extended finite element method preserves the Gaussian integration schema in each element regardless of the enrichment required to model cracking and is capable of representing the complex kinematics of crack networks in composite laminates. The technique uses independently measured standard ply-level mechanical properties of the unidirectional composite (stiffness, strength, fracture toughness). Failure simulations of composites containing open holes are presented. Although the process of crack initiation is impossible to capture precisely due to local material variations the proposed method exhibits excellent agreement with experimental data for matrix and delamination crack growth in quasi-isotropic open hole graphite-epoxy composites with thick plies, where the composite fails in the delamination failure mode.

멀티스케일 모델링 기법을 이용한 섬유강화 복합재료의 미시역학적 파손예측 및 검증

In this paper, a micro-mechanical failure prediction program is developed based on SIFT (Strain Invariant Failure Theory) by using the multi-scale modeling method for fiber-reinforced composite materials. And the failure analysis are performed for open-hole composite laminate specimen in order to verify the developed program. First of all, the critical strain invariants are obtained through the tensile tests for three types of specimens. Also, the matrices of strain amplification factors are determined through the finite element analysis for micro-mechanical model, RVE (Representative Volume Element). Finally, the microscopic failure analysis is performed for the open-hole composite laminate specimen model by applying a failure load obtained from tensile test, and the predicted failure indices are evaluated for verification of the developed program.

Numerical analysis of size effects on open-hole tensile composite laminates

The tensile strength of open-hole fibre reinforced composite laminates depends on in-plane, thickness and ply lay-up scaling. Translaminar (fibre direction) mode I fracture toughness has recently been experimentally determined to be thickness dependent. This paper presents a computational study of the tensile strength prediction of open-hole laminates using a cohesive zone model. To the authors’ knowledge, it is for the first time in the literature that the thickness-dependence of translaminar fracture toughness is accounted for in the numerical modelling of composites. The thickness size effect in the strength of open-hole composite laminates failed by pull-out is accurately predicted for the first time by a deterministic model. It is found that neglecting delamination in the numerical models will lead to mesh-dependency and over-estimation on the predicted strength. Smeared crack model with cohesive elements to model delamination is able to predict the correct failure mode; but it is found not suitable for accurate strength predictions for laminates failed by delamination.

Theoretical and experimental investigation of stress redistribution in open hole composite laminates due to damage accumulation

Two methods were examined for the prediction of stress redistribution due to subcritical damage accumulation near open holes in composite laminates. A finite element analysis in conjunction with the method of material property degradation was performed to assess the accuracy of the fiber direction stress redistribution prediction due to formation of longitudinal splitting. A simple case of a unidirectional composite with an open hole was considered. The method was unable to accurately predict the fiber stress relaxation due to longitudinal splitting, which in the case of laminates with holes is of paramount importance for the accurate prediction of ultimate strength. Three-dimensional ply level modeling of discrete damage near an open hole in a quasi-isotropic composite laminate was subsequently considered. Normally the mesh configuration is dictated by the boundaries of the specimen, such as the presence of a hole, creating formidable difficulties to modeling matrix cracking, which is aligned with the fiber direction in a given ply. A mesh independent displacement discontinuity modeling method based on higher order shape functions was constructed for this purpose. The surface of the displacement jump associated with matrix cracking was defined in terms of the domain Heaviside function approximated by using higher order polynomial B-splines. Moiré interferometry was used to determine the strain and displacement fields in the surface layers of a quasi-isotropic composite, previously prestressed beyond the damage initiation load. Good correlation between the experimental data and the stress redistribution predicted by the mesh independent damage modeling technique was observed.

Theoretical and experimental investigation of stress redistribution in open hole composite laminates due to damage accumulation

Abstract Two methods were examined for the prediction of stress redistribution due to subcritical damage accumulation near open holes in composite laminates. A finite element analysis in conjunction with the method of material property degradation was performed to assess the accuracy of the fiber direction stress redistribution prediction due to formation of longitudinal splitting. A simple case of a unidirectional composite with an open hole was considered. The method was unable to accurately predict the fiber stress relaxation due to longitudinal splitting, which in the case of laminates with holes is of paramount importance for the accurate prediction of ultimate strength. Three-dimensional ply level modeling of discrete damage near an open hole in a quasi-isotropic composite laminate was subsequently considered. Normally the mesh configuration is dictated by the boundaries of the specimen, such as the presence of a hole, creating formidable difficulties to modeling matrix cracking, which is aligned with the fiber direction in a given ply. A mesh independent displacement discontinuity modeling method based on higher order shape functions was constructed for this purpose. The surface of the displacement jump associated with matrix cracking was defined in terms of the domain Heaviside function approximated by using higher order polynomial B-splines. Moire interferometry was used to determine the strain and displacement fields in the surface layers of a quasi-isotropic composite, previously prestressed beyond the damage initiation load. Good correlation between the experimental data and the stress redistribution predicted by the mesh independent damage modeling technique was observed.

A Detailed Investigation of the Micromechanisms of Compressive Failure in Open Hole Composite Laminates

Compression failures in composite laminates containing circular holes are often preceded by the development of a damage zone which grows with increasing com pressive load. This damage zone which appears similar to a flat, tension fatigue crack in metals, initiates at the edges of the hole and propagates across the width of the laminates leading to final failure. The damage zone is initiated by local fiber buckling and/or shear crippling into the edges of the hole. The length or size of the damage zone increases with increasing load, propagating stably until it reaches a critical size. Then, unstable growth begins, and the zone completely traverses the specimen. Subsequently, a more cata strophic failure occurs giving brooming and/or delamination. Several tests of 2.54 cm wide AS4/PEEK containing a 0.16 cm diameter hole were inter rupted prior to catastrophic damage growth to allow for a careful examination of the fiber shear crippling and/or microbuckling. These observations were accomplished using scan ning electron microscopy. Direct observation of the specimen revealed the surface size of the shear crippling zone. To determine the extent of this damage through the thickness of the laminate, systematic sectioning techniques were employed. From these sectioning studies, the extent and mode of failure were determined, and a three-dimensional schematic of the damage zone was developed.

Measurements of the Stress Supported by the Crush Zone in Open Hole Composite Laminates Loaded in Compression

Compression failures in composite laminates containing circular holes are often preceded by the development of a damage zone which grows with increasing compressive load. This damage zone which appears similar to a flat, tension fatigue crack in metals, initiates at the edges of the hole and propagates across the width of the laminates leading to final failure. The damage zone is initiated by local fiber buckling and/or shear crippling in the edges of the hole. The length or size of the damage zone increases with increasing load, propagating until the zone completely traverses the specimen. Subsequently, a more catastrophic failure occurs giving brooming and/or delamination. Measurements of the load supported by the damage zone were made, allowing a more realistic constitutive rela tionship to be defined for the composite material in the crush zone.