Brittle materials at high-loading rates: an open area of research

Glass Stability Changes the Nature of Yielding under Oscillatory Shear

We perform molecular dynamics simulations to investigate the effect of a glass preparation on its yielding transition under oscillatory shear. We use swap Monte Carlo to investigate a broad range of glass stabilities from poorly annealed to highly stable systems. We observe a qualitative change in the nature of yielding, which evolves from ductile to brittle as glass stability increases. Our results disentangle the relative role of mechanical and thermal annealing on the mechanical properties of amorphous solids, which is relevant for various experimental situations from the rheology of soft materials to fatigue failure in metallic glasses.

Strain Rate Dependencies and Competitive Effects of Dynamic Strength of Some Engineering Materials

Comparison of strength characteristics of engineering materials is usually based on comparison of data from quasi-static tests. However, under dynamic loads, strength characteristics of materials are unstable and the strength ratio of two materials can change dramatically. This paper presents a comparative study of the strain/stress rate dependencies of the critical stresses of various quasi-brittle and ductile engineering materials, as well as their various internal states. A case of continuously increasing load in compression and splitting tests is considered. Experimental results were obtained for bricks, mortar and limestone in dry, saturated and frozen states; copper and titanium in different structural states; and road bitumens with different modifiers. The results show that such dependencies can intersect, thereby realizing a strength competition effect. This means that one material can withstand higher stresses under a quasi-static load and lower stresses under a dynamic load in comparison with another material. Thus, such an intersection suggests that the choice of a material based on quasi-static parameters may be erroneous for structures intended for dynamic modes. The analysis of strain/stress rate dependencies of ultimate or yield strength is carried out on the basis of the incubation time approach. The introduction of dynamic strength characteristics with the dimension of time is considered. This parameter characterizes well the response of the materials to dynamic loading. The calculation results show the possibility of estimating the critical stresses and the strength competition effect at high rate loads using no more than three parameters, which can be easily used in engineering practice.

Numerical analysis of a testing technique to investigate the dynamic crack propagation in armour ceramic

Ceramic materials are numerically studied to understand their fracturing behaviour upon dynamic conditions and impact loadings. During a ballistic impact of a projectile against a ceramic armour system, an intense fragmentation composed of numerous oriented cracks, develops in the target. It is the reason why the conditions of crack initiation, propagation and arrest in these materials need to be investigated. In the present work, a dynamic testing configuration has been developed in order to characterise the dynamic fracture toughness (K1,d), considering a single crack that propagates from the specimen notch tip. The “Rockspall” testing technique, which employs a two-notch specimen loaded in a spalling experiment, was used. Thanks to the reflection of a compression wave into a tensile load from the sample free-end, a single dynamic crack is triggered. The sample geometry is optimised by means of a series of FE numerical simulations involving an anisotropic damage model.