Past exam of the MPhil in micro- and nanotechnology enterprise course of the University of Cambridge / 2018 / Paper 2 / Section B / 21B / ii / Solution

A strain increment $\Delta {\gamma }_j$ applied at time $t_j$ contributesLinearity lets the contributions add, and time-translation invariance makes the kernel depend only on elapsed time:In the continuous limit this is Boltzmann superposition,The material's prehistory must be included, or it must start from a specified relaxed state. Aging, yielding and large-strain changes can violate this simple superposition. An instantaneous Newtonian contribution can be written separately as ${\eta }_s\dot{\gamma }(t)$; for an ideal step it is an impulse, not an ordinary finite relaxation tail.