The energy release rate (ERR) of a fiber-matrix debond crack in a unidirectional composite subjected to transverse tension is studied numerically. The focus of the study is the effect of the proximity of the neighboring fibers on the ERR. For this, a hexagonal pattern of fibers in the composite cross-section is considered. Assuming one fiber to be debonded at certain initial debond arc-length, the effect of the closeness of the surrounding six fibers on the ERR of the crack is studied with the inter-fiber distance as a parameter. Using an embedded cell consisting of discrete fibers in a matrix surrounded by the homogenized composite, a finite element model and the virtual crack closure technique are used to calculate the ERR. Results show that the presence of the local fiber cluster accelerates the crack growth up to a certain initial crack angle, beyond which the opposite effect occurs. It is also found that the residual stress due to thermal cooldown reduces the ERR. However, the thermal cooldown is found to enhance the debond growth in plies within a cross-ply laminate.

Despite efforts to improve the overall quality of composite materials, the presence of fiber/matrix interfacial defects seems inevitable. In composites with high fiber volume fraction, the small inter-fiber spacing can lead to development of high tensile radial stress at the fiber/matrix interfaces on cooldown from a high cure temperature. This stress can cause failure from defects at the interfaces earlier before any mechanical loads are applied. In the present paper, we study further progression of cracking from a preexisting disbond (debonding crack) that has been formed by thermal cooldown on remotely applying transverse tension to the composite. In the finite element model, a local region of hexagonally packed fibers embedded in a homogenized composite is analyzed. The cooldown induced disbond is assumed to initiate at the location where tensile radial stress resulting from cooldown is the highest. Energy release rate of the debonding crack is calculated by the Virtual Crack Closure Technique (VCCT). Upon loading, it is found that the debonding crack tends to grow towards the symmetry plane normal to the loading direction. Furthermore, this crack is found not to kink out of the interface until it has fully propagated past the symmetry plane. As a result, further growth of the cooldown induced disbond as well as the potential kinking process are found to be the same as when the disbond initiates due to applied transverse tension. 

Damage in composite materials initiates at the length scale of one or a few fiber diameters, governed by the local stress fields. Further progression of the failure events is governed by conditions existing in a material volume representative of geometrical aspects such as fiber orientation and thickness of the plies, as well as the extent of stress field perturbations caused by damage entities. Failure of a composite structure occurs at attainment of a critical state in its response related to the designed functionality. Assessment of failure must therefore involve analyzing failure events from initiation until the relevant criticality state, with proper account of the length scales at which the respective failure mechanisms occur. Approaches for this purpose are necessarily of a multiscale nature. This chapter discusses a particular approach that incorporates the micro-, meso-, and macroscales in one single framework and is aimed at describing the deformational response of multidirectional composite laminates with multiple cracking in different orientations.

This study examines the effect of manufacturing induced voids on failure of adhesive joints. A single lap joint with preexisting crack between the adherend and the adhesive is considered and the crack growth behavior is studied in the presence of a void in the adhesive. The analysis conducted is numerical using finite elements and a revised virtual crack closure technique for calculating the energy release rate of the interface crack. After verifying the numerical model for a case where analytical solution exists, it is used to gain insight into the failure of the adhesive joint by conducting a parametric study where the size, shape and location of the void with respect to the crack tip are varied. The case of two preexisting cracks on opposite interfaces in the presence of a void is also examined.

Unidirectional (UD) composites are building blocks in most load bearing structural components for lightweight applications in aerospace, automotive and wind energy industries. The loss of the structural load bearing capacity is governed by the instability of the fiber breakage process in the UD composites. When subjected to increasing or repeated tensile loading along fiber direction, the first failure event within these composites occurs as discrete fibers break at weak points followed by fiber/matrix debonding due to high stress concentration caused by fiber breaks. Upon further loading, or on repeated loading, more fiber breaks occur along with other accumulated damage events such as debond growth and matrix cracking. Final failure of a UD composite occurs when a critical fracture plane is formed by interconnecting individual broken fibers and associated debonding through matrix cracking. This failure process has emerged from numerous experimental studies, which also suggest that the critical fracture plane contains only a small number of broken fibers for commonly used composites such as glass/epoxy and carbon/epoxy. However, the mechanisms underlying the critical fracture plane formation are not clear. As the first step to clarify the creation of a critical fracture plane, the conditions for connectivity of a broken fiber end with neighboring broken fibers is studied in this work. In order to investigate the local stress field surrounding the broken fiber, a finite element (FE) model is constructed in which six neighboring fibers are placed as a ring of concentric axisymmetric cylinder embedded in the matrix. The discrete fiber region is surrounded by a concentric outer cylinder ring of homogenized composite. The entire FE model is subjected to axial tensile loading. To account for the consequence of the stress enhancement at the broken fiber end, a debond crack at the fiber/matrix interface extending a short distance from the fiber end is included in the analysis. Realizing that the debond crack by itself would not connect with other fiber failures, focus of the stress and failure analysis is placed on deviation of the debond crack laterally into the matrix. For this purpose, matrix cracking in two possible modes ductile and brittle is considered, Energy based criteria are used to study the competition between the cracking modes and the crack path into the matrix from the end of debond to the neighboring fibers is determined. Next the failure of the neighboring fibers caused by the intense stress field accompanying the matrix cracks is studied. The conditions for generating a plane connecting the initially broken fiber end to subsequent fiber failures are finally determined. Further ongoing studies are aimed at clarifying the limiting conditions for avoiding the fiber failure criticality, and thereby improving the load bearing capacity of UD composites. The statistical considerations regarding fiber failure will also be incorporated in these studies.

Fiber breakages are commonly found during composites manufacturing process. In the current study, the effect of manufacturing induced fiber break on local tensile failure in unidirectional (UD) composites is investigated numerically. In the finite element (FE) model, a broken fiber is placed centrally in a hexagonally packed UD composite and is assumed to be perfectly bonded to the matrix. Since the stress perturbation caused by a single fiber breakage is local, only the six most affected nearest-neighbor fibers are modeled and are placed as a ring of concentric axisymmetric cylinder embedded in the matrix. The discrete fiber region is surrounded by a concentric outer cylinder ring of homogenized composite. The entire FE model is subjected to axial tensile loading. Upon loading, it is found that matrix crack would most likely initiate perpendicular to fiber axis by cavitation due to tri-axial stress state near fiber break, and the thermal residual stress is found to promote the cavitation process. Once the matrix crack initiates from fiber break, fracture mechanics methodology is adopted by using extended finite element method (XFEM) to simulate the matrix crack propagation. The stress concentration factors (SCF) along the neighboring fibers are calculated during matrix crack propagation and obtained results show that the maximum SCF is the highest when matrix crack reaches a neighboring fiber. Finally, the statistical consideration regarding neighboring fiber failure is incorporated and it is found that the initial fiber breakage, together with the matrix cracking that follows, greatly enhance the probability of neighboring fiber failing at the local region close to the original fiber-break plane, which indicates that a planar fracture plane is expected if final tensile failure of UD composite starts from a manufacturing induced fiber break.

This chapter reviews the damage mechanisms operating in composite materials under repeated application of loads. Microscopic observations of the failure events are presented and the progression of damage is deduced. The effects of fiber stiffness, matrix inelasticity, and the fiber–matrix interface on the damage mechanisms are discussed using a conceptual framework called fatigue life diagrams. Emphasis is placed on fatigue of unidirectional continuous-fiber composites with polymeric matrices.

Traditional damage and failure analysis of composite materials has been on homogenized solids without defects. In recent years, it has been possible to observe and characterize defects induced by manufacturing. This fact, and the need to understand the effects of defects for possible cost reduction, has given impetus to studies of damage and failure with account of defects. This chapter will review some of these studies, giving illustrative results. A comprehensive multiscale failure analysis scheme incorporating manufacturing defects will then be described.

This chapter focuses on modeling of failure mechanisms in polymer matrix unidirectional (UD) composites that do not involve failure of fibers. The failure initiation resulting in crack formation in matrix and at fiber–matrix interfaces in UD composites under in-plane loading is examined. Recognizing that the local triaxial stress fields govern the failure initiation, modeling approaches to different possible crack formation processes are discussed.

Failure in composite materials consists of a series of events that initiate at a scale that is in the range of a few fiber diameters and is commonly described as a microscale. A larger scale, called a mesoscale, governs further occurrence of the failure events. This scale is given by a representative volume element, which represents geometrical features such as layer thickness and fiber orientation and distances over which stress field perturbations by the failure events interact. Chapter 14 in this book described a multiscale approach to the deformational response of composites at the structural (macro) scale. This chapter will address depletion of load-bearing capacity (strength) at the macroscale, also by a multiscale approach. The role of virtual testing in the multiscale approach will also be discussed.