| An experimental and analytical study has been conducted to investigate the process of dynamic fracture in graded multifunctional materials (FGMs). FGMs are engineered material in which the composition and properties are varied spatially to optimize the material performance. Crack tip stress, strain and displacement fields for a propagating crack in functionally graded materials are obtained through an asymptotic analysis coupled with displacement potential approach. Using these stress, strain and displacement fields, contours of constant maximum shear stress, constant first stress invariant and constant in-plane displacements are generated and the effect of nonhomogeneity parameter on these contours is discussed. Both opening and mixed mode crack problems are discussed in this context. This is followed by a comprehensive series of experiments to get more insight into the behavior of propagating cracks in FGMs. The full-field stress data around the propagating crack was recorded using dynamic photoelasticity and high-speed digital photography. Due to opaqueness of FGMs, birefringent coatings were employed to obtain the full-field isochromatics around the crack-tip. A relation between normalized crack velocity and normalized dynamic stress intensity factor is established. The transient nature of a fast moving crack-tip in an FGM is also investigated. The analysis is performed for both opening and mixed mode cracks. The higher order terms in the expansions take into account recent past history of the stress intensity factor and crack motion. The re-examination of the crack-tip fields for elastodynamic crack growth in FGMs under transient conditions has potential to significantly alter the crack-tip fields from the commonly assumed steady state crack propagation. To investigate the usefulness of the analysis presented in this work, a sequence of dynamic fracture experiments has been performed. In doing so, the phenomenon of transition from a static crack (K) to dynamic crack is utilized. It was found that during this transition the crack-tip accelerations could go as high as 108 m/sec2. It was found that by not including the transient higher order terms might result into errors as high as 21%. |
