The role of pre-existing heterogeneities in materials under shock and spall

Abstract

There has been a challenge for many decades to understand how heterogeneities influence the behavior of materials under shock loading, eventually leading to spall formation and failure. Experimental, analytical, and computational techniques have matured to the point where systematic studies of materials with complex microstructures under shock loading and the associated failure mechanisms are feasible. This is enabled by more accurate diagnostics as well as characterization methods. As interest in complex materials grows, understanding and predicting the role of heterogeneities in determining the dynamic behavior becomes crucial. Early computational studies, hydrocodes, in particular, historically preclude any irregularities in the form of defects and impurities in the material microstructure for the sake of simplification and to retain the hydrodynamic conservation equations. Contemporary computational methods, notably molecular dynamics simulations, can overcome this limitation by incorporating inhomogeneities albeit at a much lower length and time scale. This review discusses literature that has focused on investigating the role of various imperfections in the shock and spall behavior, emphasizing mainly heterogeneities such as second-phase particles, inclusions, and voids under both shock compression and release. Pre-existing defects are found in most engineering materials, ranging from thermodynamically necessary vacancies, to interstitial and dislocation, to microstructural features such as inclusions, second phase particles, voids, grain boundaries, and triple junctions. This literature review explores the interaction of these heterogeneities under shock loading during compression and release. Systematic characterization of material heterogeneities before and after shock loading, along with direct measurements of Hugoniot elastic limit and spall strength, allows for more generalized theories to be formulated. Continuous improvement toward time-resolved, in situ experimental data strengthens the ability to elucidate upon results gathered from simulations and analytical models, thus improving the overall ability to understand and predict how materials behave under dynamic loading.

Dislocation generation in diamond under extreme loading

“…From this emission it is concluded that the presence of a void or other defect-generation site can significantly lower the threshold for dislocation activity and the beginning of plasticity within diamond, depending on its orientation. This effect has been previously analyzed and quantified by Traiviratana et al, 22 Bringa et al, 23 and Flanagan et al 18,19 for other materials and is a direct consequence of the increased maximum shear stress generated by the presence of a void. The threshold stress is also dependent on void size, as has been demonstrated.…”

“…The normal stress at the void surface is zero (traction free), generating maximal shear stresses at 45 to it. This was quantified by Traiviratana et al, 22 Bringa et al, 23 and Flanagan et al, 18,19 among others. [24][25][26][27][28][29] Thus, a 4-nm (diameter) void was introduced in our simulations in order to increase the local stresses within our simulation and stimulate the generation of defects in diamond.…”

“…12 The compression of diamond nanospheres also gives evidence of the effects of nanostructure morphology on defect generation. 16 It is well known that voids can act as stress concentrators, lowering the critical stress necessary for the development of dislocations and defects in materials, [17][18][19] and that small-grained industrial diamond is commonly under-dense.…”

In situ measurement of damage evolution in shocked magnesium as a function of microstructure

“…Predicting nucleation and growth of void damage in ductile metals under dynamic loading remains a challenging problem to which much effort has been devoted (1)(2)(3)(4)(5)(6). Under shock and subsequent release loading, it is well known that the interaction of the resulting rarefaction waves can lead to the creation of a region of transient tensile stress, eventually inducing nucleation of voids at defects in the microstructure.…”

“…Department of Mechanical and Aerospace Engineering, University of California San Diego, 9500 Gilman Drive, La Jolla, CA 92093, USA 2. Los Alamos National Laboratory, Los Alamos, NM 87545, USA 3. Program in Materials Science and Engineering, University of California San Diego, La Jolla, CA 92093, USA.…”