In Situ Biaxial Deformation at the Diamond Light Source

In sheet metal forming a range of in-plane strain states may be needed in producing a single component. Much lab based mechanical testing is focused on simlple uniaxial loading producing a stretch along the load axis and an unconstrained contraction on the transverse axes. When deformation on the in-plane transverse axis is constrained towards a plane strain condition the material ductility generally drops considerably. As tensile stresses are applied along two orthogonal in-plane axes biaxial deformation is the result and rather larger strains to failure can be achieved compared to the plane-strain case. There is a clear a need to understand the deformation, hardening and texture evolution under a range of biaxial strains so as to understand and improve sheet forming processes.
forming limit diagram
Forming Limit Diagram: dividing combinations of strains along orthogonal in-plane axes that are that can be achieved for a given material from those which lead to failure.
biaxial rig
Biaxial deformation rig. The angle of the inclined rods can be varied to alter the ratio of displacements for vertical and horizontal grips.

To this end Dave Collins have designed and costructed a mechanism converting simple uniaxial actuation in a standard load frame into biaxial loading of a cruciform specimen. We took the rig and loading frame down to the Diamond Light Source and conducted the first in situ biaxial straining experiments there on beamline I12 (JEEP: Joint Engineering, Environmental, and Processing) in Experimental Hutch 2. We conducted experiments using a ~90keV incident X-ray beam and a transimission geometry so that Debye-Sherrer rings could be collected on a 2D area detector (Thales Pixium RF4343). The positions and intensity variations of the diffraction rings provide information concerning the lattice strain and texture evolution during deformation. The examples below show that there are obvious changes in texture for the unixaial loading case that alter the intensity variations around the the diffraction rings as deformation proceeds. While for the biaxial tensile straining case the intensities remain relatively unaffected by the deformation. The texture and lattice strain variation is also being modelled using crystal plasticity FEA by colleagues Dr Tomi Erinosho and Prof Fionn Dunne.

Uniaxial Loading
Biaxial Deformation

Related References

A synchrotron X-ray diffraction study of in situ biaxial deformation
DM Collins, M Mostafavi, RI Todd, T Connolley and AJ Wilkinson
Acta Materialia (2015) vol. 90, 46-58

Assessment of X-ray diffraction and crystal plasticity lattice strain evolutions under biaxial loading
TO Erinosho, DM Collins, AJ Wilkinson, RI Todd, FPE Dunne
International Journal of Plasticity (2016) available on-line1

Contact: Angus Wilkinson
Dave Collins