Authors: Masakazu Kobayashi, Hiroyuki Toda, Tomomi Ohgaki, Kentaro Uesugi, David S. Wilkinson, Toshiro Kobayashi, Yoshimitsu Aoki
Abstract: The local elastic and plastic strain during deformation are very complicated and different
form the macroscopic strain, because most materials have inhomogeneous microstructure. In this
study, local strain distribution in three dimensions has been measured using the new developed
method based on image analysis in high-resolution synchrotron radiation computed tomography
(SR-CT). Model and practical specimens, which made of cupper alloy and aluminum alloy,
respectively, were prepared for a development procedure and application of local strain
measurements. Gauging intervals of microstructural features before and after deformation gave us
information of inhomogeneous local strain distribution in three dimensions. High strain was observed
in a necking region appeared after tensile deformation in the model sample. A combination of
non-destructive measurements by SR-CT and three-dimensional analysis revealed inhomogeneous
strain distributions in practical aluminum samples.
1153
Authors: Masakazu Kobayashi, Hiroyuki Toda, Tomomi Ohgaki, Kentaro Uesugi, David S. Wilkinson, Toshiro Kobayashi, Yuji Kawai, Yoshimitsu Aoki
Abstract: A tracking procedure for the high-resolution X-ray computed tomography (CT) has been
developed in order to measure 3-D local strain within a deforming material in high-density. A
dispersion-strengthened copper alloy model sample with alumina particles, which contains micropores,
was visualized by the synchrotron radiation CT. The pores observed in reconstructed CT
volumes were used as tracking markers. The developed tracking method using a set of matching
parameters, which classifies matched, pended and rejected markers, exhibited high ratio of success
tracking. Furthermore, the ratio was improved by applying the spring model method, which is one
of the particle image velocity (PIV) methods utilized in the field of the fluid mechanics, to the
pended markers. The method based on the image analysis of CT imaging volumes provides us 3-D
high-density strain mapping.
2377
Authors: Ji Dong Kang, David S. Wilkinson, J. David Embury, Khalid Hussain
Abstract: A number of mechanical tests and metallographic techniques have been used to
investigate the mechanism of ductile fracture of AA5754 sheet. The sequence of events in the
development of shear localization is clarified using in situ strain mapping on both the sample
surface and through thickness direction during tensile tests. It is observed that the failure mode
changes from cup-cone type to shearing with increasing Fe content in both continuous cast (CC)
and direct-chill cast (DC) AA5754 sheets. However, this transition happens in CC with much lower
Fe content than DC. As very little damage is found near the fracture surface, this suggests that
damage may be a consequence of the shear process rather than a trigger that determines material
ductility. For both CC and DC with same Fe content of 0.21%, fracture strain of CC is much lower
than DC. It is postulated that this is due to the differences of particle distribution in these two
materials, especially the increased fraction of stringer type structures which exist in CC material.
985
Authors: David S. Wilkinson, Xin Jian Duan, Ji Dong Kang, Mukesh K. Jain, J. David Embury
Abstract: This paper addresses the effect of microstructure on the formability of aluminium alloys
of interest for automotive sheet applications. The bulk of this work has been on the alloy AA5754 –
both conventional DC cast alloys and continuous cast alloys made by twin belt casting. It is known
that alloys such as these contain Fe as a tramp impurity which results in Fe-based intermetallic
particles distributed through microstructure as isolated particles and in stringers aligned along the
rolling direction. It is thought that these particles are the cause, both of the reduced ductility that is
observed as the Fe level rises, and the relatively poor formability of strip cast alloys, as compared
with those made by DC cast. Conventional wisdom suggests that the reduction of ductility is due to
the effect of particles as nucleating sites for damage. However, most studies show that these
materials are resistant to damage until just before fracture. We now believe that effect is actually
related to the development of shear bands in these materials. We present experimental data which
supports this conclusion. We then show how the FE models we have developed demonstrate the
role of shear instability on fracture and the role played by hard particles. We show how a unit cell
approach can be used to incorporate the effect of particle density and morphology on shear
localization in a way that includes statistical variability due to microstructural heterogeneity. This
leads to a set of constitutive equations in which the parameters are distributed from one region to
another. These are then fed into a macroscopic FE model at the level of the specimen or the
component in order to determine the effect of microstructural variability on shear instability and
ductility.
183
Authors: J.C. Mach, Ji Dong Kang, A.J. Beaudoin, David S. Wilkinson
85
Authors: Xin Jian Duan, Mukesh K. Jain, M. Bruhis, David S. Wilkinson
Abstract: The occurrence of intense shear band is a prelude to failure in many Al-sheet materials. In the present study, a full field optical system measurement technique (digital image correlation) and the finite element method are used to characterize the sequence of deformation in uniaxial tension before and after the intense shear band formation in AA6111-T4. The results indicate good agreement between the measurement and the predictions in terms of shear band width, strain distribution along the gauge length and the failure mode.
737
Authors: J. Sarkar, T.R.G. Kutty, David S. Wilkinson, J. David Embury, David J. Lloyd
583
Authors: O. Flacher, Jean Jacques Blandin, K.P. Plucknett, David S. Wilkinson, C.H. Caceres
411
Authors: Éric Maire, J. David Embury, David S. Wilkinson
1167
Authors: R. Ham-Su, David S. Wilkinson
885