Crack Deflection Near a Ply Interface in a Composite Laminate

Initiation and growth criteria for transverse matrix cracks in composite laminates

Development a refined numerical model for evaluating the matrix cracking and induced delamination formation in cross-ply composite laminates

This work proposes a mathematical model for analysis of damage in multi-layered composite structures in different service conditions. The growth of damage in the form of delamination and matrix crack has been predicted. This study includes propagation of damage in square plate and spherical shell with existing matrix crack under application of static uniformly distributed load. Virtual crack closure technique and extended layerwise method are incorporated for this simulation. The extended layerwise method is used for simulation of damage like delamination and matrix crack in multi-layered composite structures while Virtual crack closure technique is used for determination of strain energy release rates in different service conditions. An algorithm is also proposed for propagation of arbitrary and changing shape type damages.

An investigation was performed to study the delamination growth in two different cross-ply laminated three-point bending specimens. A two dimensional (2D) finite element (FE) analysis was firstly carried out to determine the strain energy release rates during the delamination of the beams. Contact elements were used to prevent the material interpenetration on the crack surfaces. To study three dimensional (3D) effects on the crack growth in the composite beams, 3D FE analysis was developed. The 3D results showed that the distribution of the energy release rates along the delamination front are not constant. At the free surface a coupling between mode II and mode III energy release rates was also observed, and it was further noticed that the dominating deformation mode is different for distinct laminate lay-ups even under a pure mode I loading condition. Comparison of the 2D and 3D FE analyses suggested that the critical delamination toughness obtained from 2D computations be conservative. INTRODUCTION Laminated composites have demonstrated their usefulness and potential increases in many structural applications. These materials must be designed to meet various engineering requirements and to withstand the service loads. Delamination is the major failure mode of laminated composites and is now widely investigated by static and/or low velocity impact loading tests. Delamination is initiated by two types of cracks: shear cracks in matrix and normal tension cracks perpendicular to fibres. These interlaminar cracks immediately propagate into ply interface and the associated stress concentration may then initiate delamination cracks. The initiation and growth of delamination is governed by the interlaminar fracture toughness of the material. For most of the specimen geometries used in laminate testings it is possible to obtain analytical solutions using beam theory [1,2,3,4], A general method for calculating the total energy release rates Gj from the local values of forces and * Dept. of Matr. Sci and Eng., Oregon Grad. Inst, P.O. Box 91000, Portland, OR 97291-1000, USA. * Dept. of Strength of Materials, Riga Tech. University, Kalku St. 1, LV 1047, Riga, LATVIA. Transactions on Engineering Sciences vol 6, © 1994 WIT Press, www.witpress.com, ISSN 1743-3533 252 Localized Damage bending moments in a cracked laminates has been suggested by Williams [5]. These formulas [5] are the same as well known formulas for Double Cantilever Beam (DCB), End Notched Flexure (ENF) and other specimens developed previously by other methods. Formulas are simple only for the case of isotropic layers. Fig. la shows a laminated cross ply beam with a central edge notch or edge crack in the bottom layer. In this case delamination crack with length a runs into the ply interface normal to the notch. This case of delamination growth due to bending in graphite/epoxy laminates was investigated by Sun [6]. A formula to calculate the total energy release rate was also given as Here D = /4,, •/),, -B^, where AH, EH and D%% are beam stiffnesses, and subscripts 1 and 2 denote values in section 1 and 2 respectively (see Fig. Ib). |2P |2P

Higher reliability and miniaturization for automotive sensor applications are more and more demanded. These sensors are exposed to harsh environments like extreme temperatures, fast temperature change and humidity. Flip chip interconnections using the Stud Bump Bonding (SBB) Technology provide a solution to fulfill these demands and requirements. For SBB interconnections, the failure modes during reliability testing are shifted from bump fatigue to crack initiation and propagation in the fillet, the die as well as cracks within the underfill. The present work focuses on chip side fillet cracking. The crack initiation and propagation and therefore the reliability of a flip chip interconnection depends strongly on the geometry of the components, the used assembly technology and the materials. Therefore, different chip sizes were used. Moreover, the SBB technology using underfill as well as Non Conductive Adhesive (NCA) were utilized. In order to identify the influence of board technology, different materials properties for the board were studied. In the FE analysis, a fracture mechanics approach was used to quantify the impact of these parameters. The locations of crack initiation in the underfill fillet were determined by conventional stress analysis. A bulk crack in the upper part of the fillet was modeled and the impact of the described parameters on the stress intensity factor was obtained. Once crack propagation occurred, the bulk cracks turns into a delamination of the vertical chip edge and the underfill. Therefore, interfacial fracture mechanics approach was utilized. The energy release rate and the bimaterial phase angle were used to characterize interfacial cracking. The Virtual Crack Closure Technique (VCCT) was used to obtain stress intensity factors for the bulk material as well as the energy release rate and phase angle for the delamination.

Stress and energy based prediction of crack distribution pattern in general cross-ply laminates

A bi-directional damage model for matrix cracking evolution in composite laminates under fatigue loadings

A damage model for matrix cracks in composite laminates under fatigue loading based on continuum damage mechanics

This paper presents electrical resistance change due to moisture in a CFRP cross-ply laminate with and without matrix cracking. First, the electrical resistance change was measured for wet unidirectional and cross-ply laminates at room temperature and below the freezing point. Next, an equivalent circuit model was proposed for the cross-ply laminate, taking the resistance change ascribed to matrix cracking into account. It was found that resistance characteristics of the intact cross-ply laminate were similar to those of the longitudinal ply. Resistance properties of the wet cross-ply laminate with matrix cracking were dominated by those of the longitudinal ply, while the resistance change of the dry cracked cross-ply laminate was affected by the matrix crack resistance. The resistance of the dry cracked laminate decreases below the freezing point. Contrarily, the resistance of the wet cracked laminate increases below the freezing point.

Mixed-mode fracture of adhesively-bonded pultruded composite lap joints

Microscopic damage progress in high temperature CFRP cross-ply laminates under thermal cycling was investigated. Two types of laminate configuration were tested to study the effect of ply thickness. Matrix cracks initiated in both 0° and 90° plies and the number of cracks increased as the number of cycles increased. The saturated number of cracks in 90° plies was twice as large as that in 0° plies. To discuss these experimental results, shear-lag analysis was used to evaluate both the energy release rate associated with matrix cracking and the thermal residual stress in cracked plies. It was implied that the effect of the maximum thermal residual stress as well as the energy release rate range were significant in the quantitative evaluation of the matrix cracking induced by the thermal cycling.

Weak and strong bi-material interfaces and their influence on propagating cracks in plane elastic structures

A pragmatic modelis proposed to predict interlaminar and intralaminar damage, i.e., delamination and matrix cracking, in filament wound pipes. The pipe is modelled as an assembly of sublaminates connected along their interfaces. The initiation of delamination and transverse matrix cracking is predicted based on stress-based failure criterion. Delamination propagation is governed by the energy release rates. After the occurrence of damage, constraint between sublaminates is removed to model the delamination and a ply discount model is used to account for the materialdegradation effects of matrix cracking. The establishment of a leakage path is comprised by jointed matrix cracking in each layer and delamination at each interface. Finite element analysis has been carried out to simulate the quasi-static indentation of filament-wound composite pipes. Despite the simplification of treating each damage mechanism independently (i.e., no direct interactions), good agreement has been achieved between experimentalresul ts and the predictions of the model for the load–indentation relationship, the evolution of multiple delaminations and the formation of a leakage path.

Degradation of SiC/BN/SiC minicomposites in heat–stress–moisture–oxygen coupling environments, I: Static fatigue test study

Interpreting acoustic energy emission in SiC/SiC minicomposites through modeling of fracture surface areas