Toughness of short fiber composites : an approach based on crack-bridging

Sammanfattning: The presented work considers how to properly characterize fracture properties of short fiber composites (SFC). Associated with fracture of SFC is the creation of a comparably widespread fracture process zone. This zone develops since a number of inelastic failure mechanisms (e.g. debonding, microcracking, fiber failure and fiber pull-out) take place in the vicinity of an advancing crack. In the present approach, a bridging law (or cohesive zone law) approach is adopted in order to characterize the fracture toughness of the material. Conventional fracture toughness measures, such as KIC were in most cases found not to be applicable. This was because fundamental small-scale yielding geometry requirements could not be fulfilled in experiments. The bridging law approach captures previously mentioned mechanisms in terms of a closure stress (bridging stress). This stress acts between two fictitious crack planes. The relation between crack opening displacement and bridging stress is governed by the bridging law. Parts of the presented work consider determination of bridging laws from experiments (Paper I and Paper III). Different experimental configurations, double cantilever beam (DCB) specimens loaded with pure moments and double edge notched tension (DENT) specimens, were used in the two studies. A main conclusion from Paper I is that the large differences in fracture characteristics between two sheet molding compound (SMC) composites could be explained on the basis of bridging laws and their influence on fracture energy. Similar observations were made in Paper III. In Paper III, it also was evident that the intrinsic non-linearity of bulk SMC material has to be considered separately in the data reduction of experimental results, in order to capture the bridging law. Bilinear approximations of decreasing bridging laws were obtained as a result from the study. A closer investigation on the mechanical behavior of SMC with varied composition was performed in Paper II. Various mechanical tests, including tension, compression, in situ studies, DCB and stiffness degradation measurements through quasi-static cyclic loading-unloading experiments, were employed. The purpose was to characterize and understand observed differences between conventional and toughened SMC with low density additives. The applicability of the proposed bridging law approach is confirmed by the work presented in Paper IV and Paper V. In these papers, the previously measured (Paper I and Paper III) bridging relations are used as a constitutive property in predictions of structural behavior of specimens with varied geometry. Paper IV considers that bridging law parameters can be used to predict and explain the change in notch-sensitivity observed on SMC DENT-specimens with varied geometry. A comparably simple analytical route (neglecting non-linear bulk behavior and shape of bridging law) is employed with satisfactory results. In Paper V, the use of the finite element method (FEM) in conjunction with measured bilinear bridging laws, allows reconstruction of experimentally measured compact tension (CT) specimen load vs. displacement curves with good accuracy. Three different CT specimen geometries are considered. Modeling and experimental results from Paper V also shows that compression failure often is of equal importance as tensile, in real structures and loading conditions.