A rig with two orthogonal, servo-hydraulic actuators based around a universal testing machine provides a flexible, low-cost biaxial testing facility and has been used to examine the influence of direct biaxial stress on deformation and crack propagation, particularly in high cycle fatigue. In principle, a balanced, horizontal loading axis is supported independently of the specimen, coincident with its axis, on low-stiffness springs to accommodate the vertical movements of the horizontal loading train; vertical and horizontal force variations, including inertial effects, are negligibly small. For 0 to 50 kN equibiaxial fatigue loading on 6-mm steel plate specimens containing center cracks up to 35 mm in length, a frequency response in excess of 20 Hz was obtained from a single 45 litre/min hydraulic pump. The paper discusses problems encountered in design and operation and recommends further improvements. Finite element stress analysis was used to help derive a cruciform geometry specimen adaptable to compressive and through-zero loading with a satisfactory biaxial stress field over the center section. Fatigue tests on mild steel plate indicated the significant role of specimen geometry in biaxial crack growth studies and showed a decrease in crack growth rate in equibiaxial tension compared with uniaxial tests but a substantial increase during Mode II loading (pure shear, or equibiaxial tension-compression). For angled crack studies tensile crack opening displacements during biaxial crack growth result in rotational relative movement of the two loading axes, which obviate the use of fixed axis systems because of imposed constraints.
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