Performance of Two Distinct Cohesive Layer Models for Tracking Composite Delamination

Two distinct cohesive layer models are developed for numerical simulation of delamination growth in composite layered specimens tested under static loading. One of these designated as the UMAT (user supplied material) model has a small, but finite thickness and the other designated as the UEL (user element) model has zero initial thickness. Crack growth in double cantilever beam specimens as well as in two test configurations of a composite plate carrying some discontinuities subjected to lateral load are studied using the models. It turns out that UEL model, though slightly more involved, is able to predict both crack initiation and large crack growth with sufficient accuracy. The UMAT model too consistently predicts crack initiation, but is unable to predict the crack growth accurately. It gives consistently higher loads for given crack extensions and predicts that the crack growth shuts off prematurely. Careful examination of the stresses in the cohesive layer of the UMAT model, in the upstream of the crack tip indicates that a ‘neck’ develops due to compressive stresses at some distance from the crack tip. Apparently it is the formation of this neck that ‘locks’ the crack from growing and is the cause of the inaccurate results given by the model.     

A Rate Independent Cohesive Zone Model for Modeling Failure in Quasi-Brittle Materials

Computational modeling of failure in quasi-brittle materials at various length scales is important. In this work we present a rate independent cohesive zone model for modeling failure in quasi-brittle materials. The proposed model can simulate cracking, slipping, and crushing of planes through a traction-separation law. A single surface hyperbolic failure criterion, which naturally comes as a direct extension of Coulomb friction criterion with cut-off in tension and cap-off in compression, has been developed. A Euler backward integration scheme together with a global-local Newton solver compatible with a substepping strategy has been used in numerical computations. The proposed model is then used for modeling of shear wall panels. The numerical results obtained are validated by comparing them with experimental results available in literatures.     

An Experimental Method for Dynamic Delamination Analysis of Composite Materials by Impact Bending

An improved experimental method for characterizing dynamic delamination growth in composite structures has been developed and verified using high speed photography and explicit finite element simulation. The method is based on a three-point bending device. End notch flexure carbon fiber composite beam specimens were subjected to both quasi-static and impact rates of Mode II loading. The experimental results showed no significant strain rate dependency of the delamination fracture toughness. This important result complements the scarce and conflicting data available in the literature, and serves as a reference for calibration of numerical modeling strategies.     

Failure Analysis of Bolted Joints in Cross-ply Composite Laminates Using Cohesive Zone Elements

A strength prediction method is presented for double-lap single fastener bolted joints of cross-ply carbon fibre reinforced plastic (CFRP) composite laminates using cohesive zone elements (CZEs). Three-dimensional finite element models were developed and CZEs were inserted into subcritical damage planes identified from X-ray radiographs. The method makes a compromise between the experimental correlation factors (dependant on lay-up, stacking sequence and joint geometry) and three material properties (fracture energy, interlaminar strength and nonlinear shear stress-strain response). Strength of the joints was determined from the predicted load-displacement curves considering sub-laminate and plylevel scaling effects. The predictions are in a reasonable agreement with the experimental data.     

The Problem of the Cohesive Zone in Numerically Simulating Delamination/Debonding Failure Modes

This paper addresses the task of using a commercial non-linear Finite Element code as a design tool to simulate (and design) the behaviour and performance of laminated composite structures. It is shown that the need to model the stress field ahead of a crack tip, in a tiny ‘cohesive zone’, does not necessarily mean having to use tiny finite elements.     

Numerical Modeling of Combined Matrix Cracking and Delamination in Composite Laminates Using Cohesive Elements

Sub-laminate damage in the form of matrix cracking and delamination was simulated by using interface cohesive elements in the finite element (FE) software ABAQUS. Interface cohesive elements were inserted parallel to the fiber orientation in the transverse ply with equal spacing (matrix cracking) and between the interfaces (delamination). Matrix cracking initiation in the cohesive elements was based on stress traction separation laws and propagated under mixed-mode loading. We expanded the work of Shi et al. (Appl. Compos. Mater. 21, 57–70 2014) to include delamination and simulated additional [45/?45/0/90]_s and [0₂/90_n]_s {n?=?1,2,3} CFRP laminates and a [0/90₃]_s GFRP laminate. Delamination damage was quantified numerically in terms of damage dissipative energy. We observed that transverse matrix cracks can propagate to the ply interface and initiate delamination. We also observed for [0/90_n/0] laminates that as the number of 90° ply increases past n?=?2, the crack density decreases. The predicted crack density evolution compared well with experimental results and the equivalent constraint model (ECM) theory. Empirical relationships were established between crack density and applied stress by linear curve fitting. The reduction of laminate elastic modulus due to cracking was also computed numerically and it is in accordance with reported experimental measurements.     

Interaction of Matrix Cracking and Delamination in Cross-ply Laminates: Simulations with Stochastic Cohesive Zone Elements

Two main damage mechanisms of laminates—matrix cracking and inter-ply delaminationare closely linked together (Joshi and Sun 1). This paper is focussed on interaction between matrix cracking and delamination failure mechanisms in CFRP cross-ply laminates under quasi-static tensile loading. In the first part of the work, a transverse crack is introduced in 90o layers of the cross-ply laminate [01/904/01], and the stresses and strains that arise due to tensile loading are analyzed. In the second part, the cohesive zone modelling approach where the constitutive behaviour of the cohesive elements is governed by traction-displacement relationship is employed to deal with the problem of delamination initiation from the matrix crack introduced in the 90o layers of the laminate specimen. Additionally, the effect of microstructural randomness, exhibited by CFRP laminates on the damage behaviour of these laminates is also accounted for in simulations. This effect is studied in numerical finite-element simulations by introducing stochastic cohesive zone elements. The proposed damage modelling effectively simulated the interaction between the matrix crack and delamination and the variations in the stresses, damage and crack lengths of the laminate specimen due to the microstructural randomness.     

Standard Test Methods for Delamination Resistance of Composite Materials: Current Status

This paper presents a survey of the current status of test methods for the measurement of delamination resistance of composite materials, with particular emphasis on the work performed in this area by ESIS, the European Structural Integrity Society. First, existing mode I fracture test standards are described. We then present work currently underway, both to extend the range of application of these mode I tests and to standardise mode II, mixed mode (I/II) and mode III tests. Finally, we discuss tests to characterise fatigue crack propagation.     

Delamination Controlled Ballistic Resistance of Polyethylene/Polyethylene Composite Materials

We have investigated ballistic response of polyethylene/polyethylene (PE/PE) composites to impact by Uzi bullets. For comparison, limited work was carried out on PE/aramid fiber hybrids and PE fiber/Polycarbonate plate laminates. The plates exhibited an average ballistic resistance, V ₅₀, of approximately 90 m/s per 1 kg/m² area density. In term of the protection level per thickness, the ballistic resistance was 76 m/s per 1 mm. Visual and microscopic examinations identified indentation and delamination as the prevailing failure mechanisms. The delamination, energy calculated on the basis of a simple delamination model considering the fracture surface energy of the matrix, was shown to fully balance the dissipated kinetic energy of the bullet, while the contribution of the fiber fracture process was negligible. This is taken as strong circumstantial evidence for the significant role of this failure process in the ballistic resistance of these composites.     

混凝土裂缝端部粘聚力的计算

混凝土裂缝端部断裂过程区的粘聚力分布是导致混凝土断裂呈现非线性特性的重要原因。基于混凝土的断裂特性和虚拟裂缝端部存在粘聚力的分析模型，并通过分布函数的特性分析，提出了粘聚力分布函数的两种简化表达式：一为单参数待定式，另一为双参数待定式。由变形体叠加原理，推导出计算单参数待定函数公式和计算双参数待定函数代数方程组。进而通过裂缝张开位移实测数据即可求得粘聚力分布，并且给出了适当的算例分析和讨论。     

Improvement of an Exponential Cohesive Zone Model for Fatigue Analysis

Journal of Failure Analysis and Prevention - Cohesive zone model is an important tool for fatigue analysis, especially for fatigue crack growth along with an interface. A pioneering model is the...     

具任意脱层复合材料梁的模态分析

本文对具任意脱层的复合材料梁进行了模态分析。基于弹性理论建立了考虑剪切变形时复合材料脱层梁的基本方程式。对脱层梁进行了分区处理，方便地描述了脱层长度，脱层位置。利用边界条件，区间位移连续性条件和内力平衡条件建立了梁模态分析的特征方程式。通过实例计算，得出了不同脱层位置和不同脱层长度对脱层梁模态分析的影响。     

A Computational Model for Interfacial Damage Through Microstructural Cohesive Zone Relaxation

A model introducing cohesive zones around material interfaces to simulate interfacial damage in microheterogeneous materials is developed. The material behavior within the cohesive zones is unknown a-priori, and is weakened, or "relaxed", on the continuum level from an initially undamaged state, by a reduction of the spatially variable elasticity tensor's eigenvalues. This reduction is initiated if constraints placed on the microstress fields, for example critical levels of pressure or deviatoric stresses, are violated. Outside of the cohesive zones the material is unaltered. Numerical computations are performed, employing the finite element method, to illustrate the approach in three dimensional applications.     

Signal Quality Improvement Using a New TMSSE Algorithm: Application in Delamination Detection in Composite Materials

This paper introduces a novel method to improve the quality of ultrasonic phased array signals for localizing with accuracy delamination defects. The improvement is achieved by the introduction of a new threshold for the Shannon energy. In first, we have applied the threshold modified S-transform algorithm (TMST) in the case of ultrasound B-scan. Thereafter, we have adapted and applied the S-transform Shannon energy (SSE) algorithm in the field of ultrasonic testing. At last, we have proposed a novel algorithm based on threshold modified S-transform and Shannon energy (TMSSE) to increase the improvement of the ultrasound B-scan. A simulation study has been carried out simulating a composite material containing three defects in different positions in order to highlight the phenomenon of delamination. Experimental tests were performed on a sample of carbon fiber reinforced polymer composite material (CFRP) with a delamination defect close to the front face. Both experimental and simulated results show that the proposed method can improve the quality of ultrasound B-scan which enhances the localization of delamination defects.     

On the Method of Determination of Strain Energy Release Rate During Fatigue Delamination in Composite Materials

In this paper, the problem of calculation of the energy release rate for a fatigue test on composite material has been investigated. The application of the Linear Elastic Failure Mechanics (LEFM) leads to the use of varation of the energy release rate ( G). As the energy release rate is a function of the load squared, the variation of G becomes either a function of variation of the load squared ( G = f((P²))) or a function of the square of the load variation ( G = f(( P)²)).In this paper, we determine, by different fatigue tests, which of the two theoretical results is the best to describe the experiments. These fatigue tests have been made on DCB test-specimen in mode I with different R ratios (R = P_max / P_min) and different maximum loads. The material was a unidirectionnal glass-epoxy.The results show that considering G as a function of ( P)² seems more appropriated to describe a cracking test in fatigue.     

On the Cohesive Zone Model for a Finite Crack

The paper concentrates on the development of the crack tip model with the cohesive zone in an infinite plate with a finite crack of mode I. The estimation of the length of the cohesive zone and the crack tip opening displacement is based on the comparison of the local stress concentration according to Westergaard's theory with the cohesive stress. To calculate the cohesive stress, von Mises yield condition at the boundary of the cohesive zone is employed for plane strain and plane stress. The model of the stress distribution with the maximum stress within the cohesive zone is discussed. The calculation results of the crack tip opening displacement are compared with the Dugdale solution for the plane stress. This revised version was published online in July 2006 with corrections to the Cover Date.