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Stress and displacement fields for an unsteadily propagating crack under mode I and II loading are developed through an asymptotic analysis. Dynamic equilibrium equations for the unsteady state are developed and the solution to the displacement fields and the stress fields for a crack propagating with high crack tip acceleration, deceleration and rapidly varying stress intensity factor. The influence of transients on the higher order terms of the stress fields are explicitly revealed. Using these stress components, isochromatic fringes around the propagating crack are generated for different crack speeds, crack tip accelerations and the time rate of change of stress intensity factor, and the effects of the transients on these fringes are discussed. The effects of the transients on the dynamic stress intensity factor are discussed when a crack propagates with high acceleration and deceleration. The effect of transient on the time rate of change of dynamic stress intensity factor below Rayleigh wave speed in an infinite body is also studied.