Matthew Hudspeth

High speed synchrotron x-ray phase contrast imaging of dynamic material response to split Hopkinson bar loading

The successful process of amalgamating both the time-resolved imaging capabilities present at the Advanced Photon Source beamline 32ID-B and the proficiency of high-rate loading offered by the split Hopkinson or Kolsky compression/tension bar apparatus is discussed and verification of system effectiveness is expressed via dynamic experiments on various material systems. Single particle sand interaction along with glass cracking during dynamic compression, and fiber-epoxy interfacial failure, ligament-bone debonding, and single-crystal silicon fragmentation due to dynamic tension, were imaged with 0.5 μs temporal resolution and μm-level spatial resolution. Synchrotron x-ray phase contrast imaging of said material systems being loaded with the Kolsky bar apparatus demonstratively depicts the effectiveness of the novel union between these two powerful techniques, thereby allowing for in situ analysis of the interior of the material system during high-rate loading for a variety of applications.

Note: Dynamic strain field mapping with synchrotron X-ray digital image correlation.

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.

In situ damage assessment using synchrotron X-rays in materials loaded by a Hopkinson bar

Split Hopkinson or Kolsky bars are common high-rate characterization tools for dynamic mechanical behaviour of materials. Stress–strain responses averaged over specimen volume are obtained as a function of strain rate. Specimen deformation histories can be monitored by high-speed imaging on the surface. It has not been possible to track the damage initiation and evolution during the dynamic deformation inside specimens except for a few transparent materials. In this study, we integrated Hopkinson compression/tension bars with high-speed X-ray imaging capabilities. The damage history in a dynamically deforming specimen was monitored in situ using synchrotron radiation via X-ray phase contrast imaging. The effectiveness of the novel union between these two powerful techniques, which opens a new angle for data acquisition in dynamic experiments, is demonstrated by a series of dynamic experiments on a variety of material systems, including particle interaction in granular materials, glass impact cracking, single crystal silicon tensile failure and ligament–bone junction damage.

Carbon nanotube fibers as torsion sensors

Carbon nanotube fibers possess the ability to respond electrically to tensile loading. This research explores their electrical response to torsional loading; results demonstrate that applied twist compacts the fiber, resulting in increased electrical contact between carbon nanotubes. Shear strains in excess of 24% do not result in permanent changes in electrical resistance along uninfused fibers, while irreversible changes in electrical resistance arise from applied shear strains of 12.9% in epoxy infused fibers. Bulk shear modulus is approximated to be 0.40 ± 0.02 GPa for unreinforced and 2.79 ± 0.64 GPa for infused fibers.

The effects of off-axis transverse deflection loading on the failure strain of various high-performance fibers

Single filaments are subjected to a transverse deflection loading environment in efforts to gain insight into the failure strain of soft-body armor systems experiencing transverse impact. The fiber types utilized for all such experiments are Kevlar® KM2, Spectra® 130d, Dyneema® SK62, Dyneema® SK76, and Zylon® 555. In order to understand the effect of indenter shape, three different indenter geometries are utilized, namely a 0.30 caliber rounded head, a 0.30 caliber fragment simulation projectile (FSP), and high-carbon steel razor blades. The angle at failure is also varied in order to evaluate the presence of a stress concentration developed around such indenters through angles that would be produced during the transverse impact of single fibers/yarns. Loading with the rounded indenter yields failure strain values similar to pure longitudinal tensile experiments. Fibers loaded via a razor blade show a drastic reduction in failure strain, although the demonstrated failure strains are reasonably similar for all tested angles. Most interestingly, fibers loaded with the FSP show a reduction in failure strain with increasing loading angles, with low angle and high angle failure strains being similar to failure strains of fibers loaded with the rounded indenter and razor blade, respectively. In efforts to gain further insight into the method of fiber failure due to different loading configurations, post-mortem fracture surfaces are imaged for Kevlar® KM2 and Dyneema® SK76.

Effect of loading rate on mechanical properties and fracture morphology of spider silk.

Spider silks have been shown to have impressive mechanical properties. In order to assess the effect of extension rate, both quasi-static and high-rate tensile properties were determined for single fibers of major (MA) and minor (MI) ampullate single silk from the orb weaving spider Nephila clavipes . Low rate tests have been performed using a DMA Q800 at 10(-3) s(-1), while high rate analysis was done at 1700 s(-1) utilizing a miniature Kolsky bar apparatus. Rate effects exhibited by both respective silk types are addressed, and direct comparison of the tensile response between the two fibers is made. The fibers showed major increases in toughness at the high extension rate. Mechanical properties of these organic silks are contrasted to currently employed ballistic fibers and examination of fiber fracture mechanisms are probed via scanning electron microscope, revealing a globular rupture surface topography for both rate extremums.

In Situ Visual Observation of Fracture Processes in Several High-Performance Fibers

Three different high-performance fibers have been imaged in situ during Kolsky bar tensile loading using two different techniques, namely optical microscopy and phase contrast imaging (PCI). Kevlar® KM2, Dyneema® SK76, and S-2 Glass® fibers have been pulled using an instrumented Kolsky bar, thereby shedding light on the failure process of each fiber type. Both the Kevlar® KM2 fiber and Dyneema® SK76 fiber exhibit rupture defined by varying degrees of fibrillation, with the former typically showing longer fibrillated ends than the latter. S-2 Glass® failure was found to exhibit a brittle fracture mode at a single point, although post-mortem analysis commonly yielded disintegration of the fiber gauge length, which is concluded to occur post the initial break due to fiber snap back or bending. Finally the efficacy of utilizing the PCI technique to achieve higher levels of spatial and temporal resolution is discussed.

Simultaneous X-ray diffraction and phase-contrast imaging for investigating material deformation mechanisms during high-rate loading

A simultaneous X-ray imaging and diffraction technique has been developed for studying dynamic material behaviors during high-rate tensile loading provided by a miniature Kolsky bar.

Deformation field heterogeneity in punch indentation

Plastic heterogeneity in indentation is fundamental for understanding mechanics of hardness testing and impression-based deformation processing methods. The heterogeneous deformation underlying plane-strain indentation was investigated in plastic loading of copper by a flat punch. Deformation parameters were measured, in situ, by tracking the motion of asperities in high-speed optical imaging. These measurements were coupled with multi-scale analyses of strength, microstructure and crystallographic texture in the vicinity of the indentation. Self-consistency is demonstrated in description of the deformation field using the in situ mechanics-based measurements and post-mortem materials characterization. Salient features of the punch indentation process elucidated include, among others, the presence of a dead-metal zone underneath the indenter, regions of intense strain rate (e.g. slip lines) and extent of the plastic flow field. Perhaps more intriguing are the transitions between shear-type and compression-type deformation modes over the indentation region that were quantified by the high-resolution crystallographic texture measurements. The evolution of the field concomitant to the progress of indentation is discussed and primary differences between the mechanics of indentation for a rigid perfectly plastic material and a strain-hardening material are described.

Why the Smith theory over-predicts instant rupture velocities during fiber transverse impact

The effect of multi-axial loading on single Kevlar® KM2 fibers is explored with an emphasis on correlating the results to fiber/yarn transverse impact. A 0.30 caliber fragment simulation projectile (FSP) is slightly modified to act as a transverse loading indenter. Fiber failure angles are forced between ∼0° and 50° in efforts to deduce the deleterious effect caused by such angles. Said angles are also enforced in order to create the geometry that would be produced at an impact velocity causing immediate fiber/yarn rupture in transverse impact experiments. The effect of fiber angle around the FSP indenter is experimentally studied along with an analysis of the specific angle causing immediate fiber failure. It is shown that there exists a demonstrative reduction in fiber longitudinal failure strain of KM2 filaments due to this multi-axial stress state, thereby questioning the recurrent assumption that fiber performance within a body armor system is dominated by failure in pure tension.