The design of new grips for multiaxial fatigue tests

Most of the planar biaxial machine grips currently used for cross-shaped specimens have several advantages: stiffness to radial loads, fast and easy sample mounting. Generally, they lack a sufficient alignment precision (inducing a static bending during clamping) and might exhibit flexion during load. These shortcomings are obviously of major importance when it comes to fatigue tests. Other types of grips avoid some of these problems, but usually at the cost of a significantly larger size. Moreover, measurement techniques used with biaxial testing machines have evolved considerably: Digital Image Correlation, Stereo Correlation, Thermography, X-Ray Diffraction. It is thus necessary to take into account these new requirements along with the defects of the existing grips. The considered solution offers a higher bending stiffness without considerable increase in size, permitting significant additional instrumentation. Also, it allows the measurement and the adjustment of the alignment of the grip with respect to its hydrostatic bearing, thus reducing considerably the out of plane motions. Auxiliary devices can be attached to the grips, particularly LASER displacement sensors that provide direct measurement of the distance between the grips and of their relative tilt.