6 - Brittle materials

Summary

Introduction

To obtain high strength often suggests inorganic and non-metallic materials where hardness provides resistance against high thermal or mechanical loads. In a book concerned with dynamic extremes, these stimuli include materials propelled to impact at high velocity. The design paradigm requires a material to deliver an easily formed structural component capable of resisting impulsive loads of high amplitude and arbitrary duration. At the present time the reality is that these aspirations are only partially met. The response of these materials to idealised one-dimensional loading under shock is not yet understood in full detail and fully three-dimensional loading is only described empirically. Nevertheless the response of glasses and ceramics to dynamic loading has been investigated by the impact community over the past 30 years so that at least a library of data exists. In that time much has been learnt but vital questions remain unresolved, particularly understanding contact, penetration, fragmentation, inelastic behaviour and failure that are encountered in the response of a brittle material to impulsive loading.

Even the qualitative understanding of the response of brittle materials to dynamic loading has not been reflected in advances in constitutive models for them. This results from an incomplete knowledge of operating mechanisms that are consequently not reflected in global models. Further, there is a wide range of microstructures represented in this grouping, ranging from amorphous silicate glasses to polycrystalline ceramics containing both crystalline and amorphous phases. Clearly to construct adequate models for such heterogeneous materials to work on numerical platforms requires a macroscale description of behaviour, yet at present even subscale approaches have not described the processes operating in these heterogeneous media where the phases interact at the mesoscale. It is scale that remains the key frontier that bridges the continuum to microscale behaviour, and it is the mesoscale where the defects that control failure in the bulk are found.