Failure and Damage Analysis of Advanced Materials

In material science or structural mechanics, failure is generally the loss of load carrying capacity of a material unit or structural element. This definition introduces the fact that failure can be examined in different scales (microscopic, mesoscopic, macroscopic). In addition, one has to distinguish among brittle, ductile, and intermediate material behavior. In structural mechanics, if the structural response is beyond the initiation of nonlinear material behavior, failure is related to the determination of the integrity of the structure.

In principle, failure criteria correspond to phenomenological material behavior modeling. They describe the occurrence of failure at different loading conditions. Although there are no physical principles on which failure criteria can be based on, there are still a lot of suggestions available in the literature. Similarly due to the lack of generally accepted failure criteria, the formulation is up to now under research.

The criteria based on the introduction of some empirical assumptions for critical values defined by the stress or strain state are denoted as the engineering one. In addition, characteristics of the stored strain energy or power can also be used. Based on some of these hypotheses and their consequences failure criteria will be discussed here.

Modeling of the plastic behavior for isotropic and anisotropic metals is the topic of this article. The motivation for such work is briefly introduced. Then, a description of the main features of plastic deformation in metals at different scales is summarized to prepare the subsequent choices and assumptions made in the next sections. The main properties of the Cauchy stress tensor are reviewed because it is the main variable for plastic yielding. The description of plasticity for isotropic metals is discussed, which includes the yield condition, the flow rule and strain hardening. Then, the generalization of the concepts to plastic anisotropy, which is particularly important to the case of metal sheets and plates, is outlined. Finally, the influence of the constitutive description for plasticity on failure is briefly discussed using two examples.

This chapter presents the main aspects on failure and damage of cellular materials. Tensile, compression and fracture mechanics properties of plastic foams are presented and the main influence factors are investigated: density, temperature, loading speed and loading direction. Particularly for fracture toughness the mixed mode loading and size effects are discussed. The potential of digital image correlation as a tool to observe the damage of polyurethane foams is also highlighted.

This paper is a collection of various analytical methods for predicting some of the properties of laminated fibre reinforced composites that can be used when designing composite laminates, or when validating numerical methods of estimating these properties. To begin, convenient methods are given to estimate the properties of undamaged single plies and undamaged symmetric laminates. Methods of predicting fracture in homogenized anisotropic materials are then described, which exploit some very useful properties of orthogonal polynomials. Example solutions are given which are compared with known accurate solutions. The problem is then considered of quantifying, using analytical methods, the dependence of the effective thermoelastic properties of a damaged laminate on the density of ply cracks in the 90⁰ ply of a cross-ply laminate. Many very useful inter-relationships are given showing how most of the effective properties of damaged laminates depend on a single damage function. Some example predictions are given for a typical carbon fibre reinforced laminate. Finally, a model is described for predicting the progressive degradation of a unidirectional fibre reinforced composite that is degraded by an aggressive environment causing defect growth in the fibres and eventually the catastrophic failure of the composite. It is also shown how the time dependence of residual strength may be estimated. An example is given of a normalised failure/time curve, and some associated residual strength curves that can be the basis of design methods to avoid the failure of composites that will be exposed to aggressive environments.

Fiber reinforced composite materials provide a high level of structural safety against failure by tolerating damage (failure at microstructure level) while still sustaining significant loads. Using full load-bearing capacity of composite structures requires, however, reliable analysis of failure at micro and macro levels. The current failure theories, which are for homogenized composites, are unable to meet this requirement. This exposition reviews some of the commonly used theories to examine reasons for this limitation. Physical mechanisms underlying failure in composite materials are then discussed, motivating development of failure analysis with multi-scale approaches. Such approaches can also include effect of defects on failure. A comprehensive scheme is put forward for future development of physically based failure analysis of composite structures.