Frank W. Zok

The physics and mechanics of fibre-reinforced brittle matrix composites

This review compiles knowledge about the mechanical and structural performance of brittle matrix composites. The overall philosophy recognizes the need for models that allow efficient interpolation between experimental results, as the constituents and the fibre architecture are varied. This approach is necessary because empirical methods are prohibitively expensive. Moreover, the field is not yet mature, though evolving rapidly. Consequently, an attempt is made to provide a framework into which models could be inserted, and then validated by means of an efficient experimental matrix. The most comprehensive available models and the status of experimental assessments are reviewed. The phenomena given emphasis include: the stress/strain behaviour in tension and shear, the ultimate tensile strength and notch sensitivity, fatigue, stress corrosion and creep.

The Transition from Stiff to Compliant Materials in Squid Beaks

The beak of the Humboldt squid Dosidicus gigas represents one of the hardest and stiffest wholly organic materials known. As it is deeply embedded within the soft buccal envelope, the manner in which impact forces are transmitted between beak and envelope is a matter of considerable scientific interest. Here, we show that the hydrated beak exhibits a large stiffness gradient, spanning two orders of magnitude from the tip to the base. This gradient is correlated with a chemical gradient involving mixtures of chitin, water, and His-rich proteins that contain 3,4-dihydroxyphenyl-l-alanine (dopa) and undergo extensive stabilization by histidyl-dopa cross-link formation. These findings may serve as a foundation for identifying design principles for attaching mechanically mismatched materials in engineering and biological applications.

Fatigue of ceramic matrix composites

Fatigue in ceramic matrix composites typically occurs when matrix cracks are present. It proceeds by cyclic degradation of the sliding resistance of the interface. The basic mechanisms are discussed and a methodology is developed that enables fatigue life predictions to be made, based on a minimum number of experimental measurements. The methodology relies on analysis of hysteresis loops. Changes in modulus upon cyclic loading as well as the permanent strains are predicted, as well as the fatigue threshold and the S-N curve.

The role of interfaces in fiber-reinforced brittle matrix composites

Abstract The dominant influence of fiber coatings on the mechanical performance of brittle matrix composites is addressed. These effects are described in terms of two independent mechanical parameters, the debond energy, Γ i , and the sliding resistance along the debond, τ. Complications associated with mode mixity, roughness and coating microstructure are emphasized. A mechanics of composite behavior based on these parameters is described, together with measurement approaches that allow Γ i and τ to be evaluated either in situ or in model composites. Key problems associated with fiber coatings are identified and discussed.

Plastic flow and fracture of a particulate metal matrix composite

The effects of particle volume fraction and matrix temper on the flow and fracture characteristics of a series of particle-reinforced metal matrix composites under tensile and compressive loadings have been examined. Under compressive loading, a steady-state regime is attained in which the composite flow stress is proportional to the matrix flow stress at the same level of strain. The strength enhancement associated with the particles increases with increasing particle content and the matrix hardening exponent. The trends are consistent with predictions of finite element calculations of unit cell models, treating the particles as either spheres or cylinders with unit aspect ratio. Under tensile loading, the particles crack at a rate dependent on the intrinsic strength characteristics of the particles as well as the flow characteristics of the matrix. Particle cracking causes local softening, which reduces the work hardening rate as compared with compression deformation. This lowers both the strength and the ductility. Experimental measurements have been combined with finite element calculations to develop a damage law, incorporating the effects of the matrix strength on the particle stress. The damage law has been used to simulate the tensile flow response of the composites, using appropriate cell models under either isostrain or isostress conditions. Though the trends obtained from the simulations are in qualitative agreement with the experimental results, they tend to underestimate the flow stress. In all cases, tensile fracture is preceded by the formation of a neck. The condition at the onset of necking is consistent with the Considere criterion. Differences in necking strains between the composites and the monolithic matrix alloy have been rationalized on the basis of the rate of damage accumulation.

Metals and the Integrity of a Biological Coating: The Cuticle of Mussel Byssus

The cuticle of mussel byssal threads is a robust natural coating that combines high extensibility with high stiffness and hardness. In this study, fluorescence microscopy and elemental analysis were exploited to show that the 3,4-dihydroxyphenyl-L-alanine (dopa) residues of mussel foot protein-1 colocalize with Fe and Ca distributions in the cuticle of Mytilus galloprovincialis mussel byssal threads. Chelated removal of Fe and Ca from the cuticle of intact threads resulted in a 50% reduction in cuticle hardness, and thin sections subjected to the same treatment showed a disruption of cuticle integrity. Dopa-metal complexes may provide significant interactions for the integrity of composite cuticles deformed under tension.

Large scale bridging in brittle matrix composites

The influence of the bridging zone length on the resistance curve behavior of three brittle-matrix composites is examined. The experimental measurements are correlated with models of crack bridging (taking into account the finite specimen dimensions) and compared with the resistance curves expected when small-scale bridging conditions prevail. The results demonstrate that the resistance curves of composite materials strongly depend on both the absolute length of the bridging zone and the length of the bridging zone relative to the total crack length and the specimen width. The latter effects are due to large-scale bridging. The results suggest that the resistance curves of toughness measurements obtained from small test specimens may overestimate the true behavior and thus, caution must be exercised in interpreting some of the recently published data. The implications for future resistance curve measurements are discussed.

Design of metallic textile core sandwich panels

Abstract Metallic sandwich panels with textile cores have been analyzed subject to combined bending and shear and then designed for minimum weight. Basic results for the weight benefits relative to solid plates are presented, with emphasis on restricted optimizations that assure robustness (non-catastrophic failure) and acceptable thinness. Select numerical simulations are used to check the analytical results and to explore the role of strain hardening beyond failure initiation. Comparisons are made with competing concepts, especially honeycomb and truss core systems. It is demonstrated that all three systems have essentially equivalent performance. The influence on the design of a concentrated compressive stress that might crush the core has been explored and found to produce relatively small effect over the stress range of practical interest. “Angle ply” cores with members in the ±45° orientation are found to be near optimal for all combinations of bending, shear and compression.

Deformation consolidation of metal powders containing steel inclusions

Abstract Uniaxial consolidation experiments have been conducted at room temperature for two deformable metal powders (1100 Al and Pb5%Sb) containing various amounts of spherical steel inclusions. The experiments illustrate that the inclusion phase offers little constraint to matrix deformation at volume fractions

Fracture mechanism maps in stress space

Abstract Diagrams can be constructed in stress space which show, for metals and alloys, the competition between the processes which lead to fracture. These include yield, necking, void nucleation, ductile fracture, brittle grain-boundary fracture, cleavage, shear fracture, and plastic rupture. Simplified diagrams are constructed for E.T.P. copper, α-brass, two steels and an aluminium alloy. The diagrams show how the fracture mechanism changes with stress state and help rationalize a number of apparently conflicting observations. They have application in predicting the behaviour of metals under complex stress states.