Brian N. Cox

Bio: Brian N. Cox is an academic researcher from Rockwell Automation. The author has contributed to research in topics: Fracture mechanics & Crack closure. The author has an hindex of 49, co-authored 178 publications receiving 8781 citations. Previous affiliations of Brian N. Cox include Rockwell International.

Cohesive models for damage evolution in laminated composites

Abstract: A trend in the last decade towards models in which nonlinear crack tip processes are represented explicitly, rather than being assigned to a point process at the crack tip (as in linear elastic fracture mechanics), is reviewed by a survey of the literature. A good compromise between computational efficiency and physical reality seems to be the cohesive zone formulation, which collapses the effect of the nonlinear crack process zone onto a surface of displacement discontinuity (generalized crack). Damage mechanisms that can be represented by cohesive models include delamination of plies, large splitting (shear) cracks within plies, multiple matrix cracking within plies, fiber rupture or microbuckling (kink band formation), friction acting between delaminated plies, process zones at crack tips representing crazing or other nonlinearity, and large scale bridging by through-thickness reinforcement or oblique crack-bridging fibers. The power of the technique is illustrated here for delamination and splitting cracks in laminates. A cohesive element is presented for simulating three-dimensional, mode-dependent process zones. An essential feature of the formulation is that the delamination crack shape can follow its natural evolution, according to the evolving mode conditions calculated within the simulation. But in numerical work, care must be taken that element sizes are defined consistently with the characteristic lengths of cohesive zones that are implied by the chosen cohesive laws. Qualitatively successful applications are reported to some practical problems in composite engineering, which cannot be adequately analyzed by conventional tools such as linear elastic fracture mechanics and the virtual crack closure technique. The simulations successfully reproduce experimentally measured crack shapes that have been reported in the literature over a decade ago, but have not been reproduced by prior models.

A mechanistic approach to the properties of stitched laminates

Abstract: New insights are presented into the mechanisms responsible for changes to the in-plane mechanical properties of polymer matrix laminates as a result of through-the-thickness stitching. A critical appraisal of a large amount of published mechanical property data reveals that stitching usually reduces the stiffness, strength and fatigue resistance of a laminate by not more than 10–20%, although in a few cases the properties remain unchanged or increase slightly. This range of changes is observed for loading in compression, tension, bending or shear. Softening and strengthening mechanisms are proposed to account for the changes in properties due to stitching. Areas of research that are needed to further the understanding of the relationships between mechanisms and properties are identified. These include more detailed reporting of the physical properties of the stitched and unstitched laminates (e.g. fiber content, fiber distortions), the stitching conditions (e.g. yarn tension) and more thorough examination of observed mechanisms.

Failure mechanisms of 3D woven composites in tension, compression, and bending

Abstract: Observations of failure mechanisms in monotonic loading are reported for graphite/epoxy composites containing three-dimensional (3D) interlock weave reinforcement. The key phenomena are delamination and kink band formation in compression, tow rupture and pullout in tension, and combinations of these in bending. The materials exhibit great potential for damage tolerance and notch insensitivity. This is partly due to the presence of geometrical flaws that are broadly distributed in strength and space; and partly to the coarseness of the reinforcing tows, which leads to extensive debonding and reduced stress intensification around sites of failure. Rules of mixture corrected for the effects of tow irregularity suffice to estimate elastic moduli. Rough estimates of the stress at which the first failure events occur in compression or tension can be made from existing micromechanical models. Ultimate tensile failure might be modeled by regarding failed tows that are being pulled out of the composite as a cohesive zone. The characteristic length estimated for this zone, which is a direct measure of damage tolerance and notch insensitivity, has very large values of order of magnitude 0.1–0.5 m.

A mechanistic interpretation of the comparative in-plane mechanical properties of 3D woven, stitched and pinned composites

Abstract: A comparison of substantial published data for 3D woven, stitched and pinned composites quantifies the advantages and disadvantages of these different types of through-thickness reinforcement for in-plane mechanical properties. Stitching or 3D weaving can either improve or degrade the tension, compression, flexure and interlaminar shear properties, usually by less than 20%. Furthermore, the property changes are not strongly influenced by the volume content or diameter of the through-thickness reinforcement for these two processes. One implication of this result is that high levels of through-thickness reinforcement can be incorporated where needed to achieve high impact damage resistance. In contrast, pinning always degrades in-plane properties and fatigue performance, to a degree that increases monotonically with the volume content and diameter of the pins. Property trends are interpreted where possible in terms of known failure mechanisms and expectations from modelling. Some major gaps in data and mechanistic understanding are identified, with specific suggestions for new standards for recording data and new types of experiments.

Bioinspired structural materials

Abstract: Natural structural materials are built at ambient temperature from a fairly limited selection of components. They usually comprise hard and soft phases arranged in complex hierarchical architectures, with characteristic dimensions spanning from the nanoscale to the macroscale. The resulting materials are lightweight and often display unique combinations of strength and toughness, but have proven difficult to mimic synthetically. Here, we review the common design motifs of a range of natural structural materials, and discuss the difficulties associated with the design and fabrication of synthetic structures that mimic the structural and mechanical characteristics of their natural counterparts.

Particulate reinforced metal matrix composites — a review

Abstract: The physical and mechanical properties that can be obtained with metal matrix composites (MMCs) have made them attractive candidate materials for aerospace, automotive and numerous other applications. More recently, particulate reinforced MMCs have attracted considerable attention as a result of their relatively low costs and characteristic isotropic properties. Reinforcement materials include carbides, nitrides and oxides. In an effort to optimize the structure and properties of particulate reinforced MMCs various processing techniques have evolved over the last 20 years. The processing methods utilized to manufacture particulate reinforced MMCs can be grouped depending on the temperature of the metallic matrix during processing. Accordingly, the processes can be classified into three categories: (a) liquid phase processes, (b) solid state processes, and (c) two phase (solid-liquid) processes. Regarding physical properties, strengthening in metal matrix composites has been related to dislocations of a very high density in the matrix originating from differential thermal contraction, geometrical constraints and plastic deformation during processing.

Perspective on the Development of High‐Toughness Ceramics

Abstract: The science governing the strength and fracture of structural ceramics has developed from a mostly empirical topic in 1965 into a mature discipline that now sets the standards in the field of mechanical behavior. The intent of this review is to provide a perspective regarding this evolution, followed by succinct descriptions of current understanding. The rapid developments in the field are considered to have commenced upon the first concerted attempt to apply fracture mechanics concepts to ceramics, beginning in the middle 1960s. This allowed a distinction between the separate contributions to strength from the flaws in the material and from the microstructure, as manifest in the fracture toughness. Another contribution that accelerated the learning process was the development of indentation techniques, which allowed trends in the damage resistance of new ceramics to be assessed on a routine basis. However, the most important development, which originated at about the same time, was the discovery of toughened zirconia alloys. The ensuing research on these alloys established two vital precedents.