3D Studies of Indentation by Combined X-Ray Tomography and Digital Volume Correlation

Abstract

Hardness testing obtains material properties from small specimens via measurement of load-displacement response to an imposed indentation; it is a surface characterisation technique so, except in optically transparent materials, there is no direct observation of the assumed damage and deformation processes within the material. Three-dimensional digital image correlation (digital volume correlation) is applied to study deformation beneath indentations, mapping the relative displacements between high-resolution synchrotron X-ray computed tomographs (0.9 μm voxel size). Two classes of material are examined: ductile aluminium-silicon carbide composite (Al-SiC) and brittle alumina (Al2O3). The measured displacements for Hertzian indentation in Al-SiC are in good agreement with an elastic-plastic finite element simulation. In alumina, radial cracking is observed beneath a Vickers indentation and the crack opening displacements are measured, in situ under load, for the first time. Potential applications are discussed of this characterization technique, which does not require resolution of microstructural features.

Yield behavior beneath hardness indentations in ductile metals, measured by three-dimensional computed X-ray tomography and digital volume correlation

“…The Al-SiC deformation was simulated with the same mesh that was used for the Mg specimens, with the following nonlinear elastic (Ramberg-Osgood) material properties: elastic Young's modulus 102 GPa, yield stress (0.2% proof stress) 230 MPa and hardening exponent 9.7. These were obtained by curve-fitting to tensile data [39], and agree with published results for a similar material [40]. The Poisson's ratio used was 0.27, which is the literature value for aluminium alloys such as Al6061 [40].…”

Synchrotron X-ray characterization of crack strain fields in polygranular graphite

“…The combined dataset was then corrected for small rigid body displacements and rigid body rotations, measured relative to the reference, based on the algorithm described in Ref. [59]. The final data have a coordinate system defined such that: the lowest point of the notch tip is at y ¼ 0 and the y axis is positive towards the bottom of the specimen; the x axis is positive towards the right, with the notch tip at x ¼ 4.5 mm; and the z axis is in the X-ray beam propagation direction, with centre of the notch at z ¼ 1.5 mm.…”

“…In high quality tomographs of suitable microstructures, for which the internal features provide the necessary "speckle" contrast, the displacement measurement precision is sub-voxel, 1 and both elastic and plastic deformations may be studied. Example applications of combined tomography and image correlation include observations of granular compaction of powders [57], the measurement of elastic crack fields [58] and analysis of the deformations induced by indentation [59,60] to determine material's elastic and plastic properties by retro-engineering. Polygranular graphite is very well suited to this type of analysis, as it has significant porosity that is distributed over a wide range of length scales [2,10,15].…”

Correlation of Materials Property and Performance with Internal Structures Evolvement Revealed by Laboratory X-ray Tomography

“…Research works are still very active and versatile in the experimental mechanics field. Deformation and fracture are still common interest for metals and alloys [ 28 , 29 , 30 , 31 , 32 , 33 , 34 , 35 , 36 , 37 , 38 , 39 , 40 , 41 , 42 ], foams and porous materials [ 11 , 14 , 43 , 44 , 45 , 46 ], and composites [ 13 , 47 , 48 , 49 ]. XRT has also been considerably extended with other in situ measurements including materials processing [ 50 , 51 , 52 , 53 , 54 , 55 ], materials interaction with specified conditions involving extreme temperature [ 56 , 57 ], corrosion [ 58 , 59 , 60 ], electrochemical environments, as in batteries [ 61 , 62 , 63 ], etc.…”