Cracking Models; Broken Parts

In critical conditions of stress-state, fatigue, or hostile environment, where the objective is to design components and structures for longevity, durability, integrity, and reliability, then the balance between empirical engineering design and physical modelling based on mechanisms of deformation and fracture in the material under attack is shifted in favour of physical modelling. Physical modelling provides the means to assess the relative severity of different loading regimes, as well as load/environment interactions. Furthermore, the physical model points to something of the greatest value; it suggests the proper form that constitutive equations should take in the design process. When combined with experimental evidence, physical modelling has the economic advantage of reducing the high cost of vast experimental programmes of duration of many thousands of hours. Existing design methodologies at the higher structural size scales can be supported and justified by fundamental understanding at lower size scales through the physical model. A word of caution: physical modelling has to be underpinned by the actual mechanisms of failure best observed by direct means such as in situ scanning electron microscopy.