We present a dynamic strain field mapping method based on synchrotron X-ray digital image correlation (XDIC). Synchrotron X-ray sources are advantageous for imaging with exceptional spatial and temporal resolutions, and X-ray speckles can be produced either from surface roughness or internal inhomogeneities. Combining speckled X-ray imaging with DIC allows one to map strain fields with high resolutions. Based on experiments on void growth in Al and deformation of a granular material during Kolsky bar/gas gun loading at the Advanced Photon Source beamline 32ID, we demonstrate the feasibility of dynamic XDIC. XDIC is particularly useful for dynamic, in-volume, measurements on opaque materials under high strain-rate, large, deformation.

Deformation twinning in pure aluminum has been considered to be a unique property of nanostructured aluminum. A lingering mystery is whether deformation twinning occurs in coarse-grained or single-crystal aluminum, at scales beyond nanotwins. Here, we present the first experimental demonstration of macro deformation twins in single-crystal aluminum formed under ultrahigh strain-rate (∼10 6 s −1 ), large shear strain (200%) via dynamic equal channel angular pressing. Deformation twinning is rooted in the rate dependences of dislocation motion and twinning, which are coupled, complementary processes during severe plastic deformation under ultrahigh strain rates.When we talk about crystal deformation, what do we actually talk about? Crystal defects [1]. Crystal defects such as dislocations (line defects) and twins (planar defects) play a critical role in plastic deformation and ultimately govern the multifarious mechanical behaviors of many crystalline materials [2,3]. While both dislocation slip and deformation twinning are dependent on an intrinsic material property -stacking fault energy [4,5] (SFE), their sensitivities to SFE differ considerably. A notable example is pure aluminum, a typical face-centered cubic (fcc) metal with high SFE (104-142 mJ m −2 ) [6], in which deformation twinning rarely occurs even deformed at low temperatures and/or at high strain rates [7,8]. This rareness of deformation twinning in such materials is normally attributed to the following two reasons: (i) a large number of slip systems in fcc metals render dislocation slip a very efficient deformation mode [9,10], and (ii) the nucleation of twinning partial dislocations require much higher shear stresses than trailing partial dislocations due to the high unstable twin fault energy [11]. Searching for macro deformation twins in pure aluminum and revealing the underlying mechanisms have been of sustained interest in the past decade.Molecular dynamics (MD) simulations first predicted that nanoscale deformation twins can nucleate under high tensile stress (2.5 GPa) and high strain rate (10 7 s −1 ) in nanograined aluminum [6,12], and subsequent experiments confirmed this prediction in nanograined aluminum films under different kinds of severe plastic deformation (SPD) [13][14][15]. One explanation was proposed based on classical dislocation theory [16]: when grain size decreases to tens of nanometers, normal dislocation activities are greatly suppressed by the high fraction of grain boundaries (GBs); as a result, deformation twinning takes over as the dominant deformation mechanism [13]. Besides nanograin size effect, many simulation and experimental studies suggest that deformation twinning prefers to occur at high strain rates in fcc metals [17,18]. This rate-dependent twinning mechanism has been corrobarated by a very recent experiment on pure aluminum with comparatively large nanograins (50-100 nm) [19]. However, there has been no solid evidence for deformation twinning in single-crystal or coarse-grained pure aluminum. It is natural ...

Real time, in situ, multiframe, diffraction, and imaging measurements on bulk samples under high and ultrahigh strain-rate loading are highly desirable for micro- and mesoscale sciences. We present an experimental demonstration of multiframe transient x-ray diffraction (TXD) along with simultaneous imaging under high strain-rate loading at the Advanced Photon Source beamline 32ID. The feasibility study utilizes high strain-rate Hopkinson bar loading on a Mg alloy. The exposure time in TXD is 2-3 μs, and the frame interval is 26.7-62.5 μs. Various dynamic deformation mechanisms are revealed by TXD, including lattice expansion or compression, crystal plasticity, grain or lattice rotation, and likely grain refinement, as well as considerable anisotropy in deformation. Dynamic strain fields are mapped via x-ray digital image correlation, and are consistent with the diffraction measurements and loading histories.

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a b s t r a c tDynamic compression experiments are conducted on micron-sized SiC powders of different initial densities with a split Hopkinson pressure bar. Digital image correlation is applied to images from high-speed X-ray phase contrast imaging to map dynamic strain fields. The X-ray imaging and strain field mapping demonstrate the degree of heterogeneity in deformation depends on the initial powder density; mesoscale strain field evolution is consistent with softening or hardening manifested by bulk-scale loading curves. Statistical analysis of the strain probability distributions exhibits exponential decay tail similar to those of contact forces, which are supposed to lead to the grain-scale heterogeneity of granular materials. Impact-induced compaction and/or sintering of powders is an important approach for synthesizing bulk ceramics, metals, alloys and composites with improved mechanical properties [1,2], and widely used in, for example, powder metallurgy [1,3,4]. The formation of a high quality compact depends on a number of factors, and identifying the important ones can help reduce undesired microstructures, including large density variations and internal cracks [5]. High strain-rate (10 2 -10 3 s À1 ) dynamic compression of granular materials, such as sand and soil, is also of interest in civil engineering [6,7]. Obtaining spatially and temporally resolved compaction dynamics in highly heterogeneous granular materials, is critical to understanding deformation mechanisms and developing constitutive models for powder compaction [5], but has been an experimental challenge.For dynamic compression or high strain-rate loading in general, split Hopkinson pressure bar (SHPB) has been widely used for various materials including granular materials [7,8]. Strain gauges are effective for obtaining bulk, rather than meso-scale, responses. Local deformation dynamics can be characterized with twodimensional (2D) strain field mapping, using optical digital image correlation [9] or X-ray digital image correlation (XDIC) [10,11].XDIC is advantageous for the penetration capabilities of X-rays, and relies on images acquired with such techniques as X-ray phase contrast imaging (XPCI) [12][13][14]. XPCI is particularly useful to image low-Z powders including SiC, and the particles naturally serve as speckles. While density or particle displacement distributions under quasi-static loading have been examined to certain detail [15,16], measurements on dynamic strain distributions in ceramic powders with high-speed XDIC are extremely rare. Shock compaction experiments on powders usually yield bulk-scale stress-density relations [4,17,18] and grain-scale deformation dynamics is largely untouched [10]. Heterogeneous force distribution in granular materials (force chains) has been widely observed [19,20]. Nonetheless, heterogeneity in deformation has not been fully investigated from a quantitative point of view.In the present study, high strain-rate compression experiments are conducted on micron-sized SiC powders of different i...

We investigate shock-induced deformation of columnar nanocrystalline Al with large-scale molecular dynamics simulations and implement orientation mapping (OM) and selected area electron diffraction (SAED) for microstructural analysis. Deformation mechanisms include stacking fault formation, pronounced twinning, dislocation slip, grain boundary (GB) sliding and migration, and lattice or partial grain rotation. GBs and GB triple junctions serve as the nucleation sites for crystal plasticity including twinning and dislocations, due to GB weakening, and stress concentrations. Grains with different orientations exhibit different densities of twins or stacking faults nucleated from GBs. GB migration occurs as a result of differential deformation between two grains across the GB. High strain rates, appropriate grain orientation and GBs contribute to deformation twinning. Upon shock compression, intra-grain dislocation and twinning nucleated from GBs lead to partial grain rotation and the formation of subgrains, while whole grain rotation is not observed. During tension, stress gradients associated with the tensile pulse give rise to intra-grain plasticity and then partial grain rotation. The simulated OM and SAED are useful to describe lattice/grain rotation, the formation of subgrains, GB migration and other microstructures.

We investigate dynamic fracture of C/SiC composites under high strain-rate compression or tension with split Hopkinson pressure bar (SHPB) and gas gun loading. Components of the as-fabricated composites are mapped and quantified with X-ray computed tomography, including C fibers and fiber bundles, SiC matrix, and inter-and intrabundle voids. Compression loading is applied along the out-of-and in-plane directions by SPHB at strain rates of 10 2 − 10 3 s −1 along with in situ X-ray phase contrast imaging. Out-of-plane direction compression and tension are examined with gas gun impact at strain rates 10 4 − 10 5 s −1. For the out-of-plane loading, compression induces fracture via void collapse and shear damage banding, while delamination dominates fracture for the in-plane direction compression. With increasing strain rates, the compression failure modes transit from interbundle to intrabundle fracture of SiC, and then to fiber and bundle breaking. Tensile failure involves delamina