Seat members 31 are held in a state of being movable with respect to first slide members 21 or second slide members 22 correspondingly. By rotating screws 39 in directions to increase the distances d between surfaces A of the first slide members 21 or the second slide members 22 and surfaces B of the seat members 31, chucks 25 are moved in directions to increase the distance between the chucks to apply pretension to a test piece correspondingly. When backlash in a force transmission system from a support part to the respective chucks 25 is eliminated, a biaxial tensile test is started.

A test piece includes a tensile testing part and load applying pieces that are respectively connected to sides of the tensile testing part. Grooves are formed on bottom surfaces of the load applying pieces. Grooves are formed on upper surfaces of the load applying pieces. These grooves respectively partition the upper and lower surfaces of the tensile testing part and the load applying pieces.

A wrinkle generation determination method for a press formed part includes acquiring, for each load ratio in in-plane biaxial directions, a relationship between an in-plane biaxial strain generated in a process of inducing out-of-plane buckling in a measurement part and a load, acquiring an in-plane biaxial strain at a point where a primary differential coefficient of an in-plane uniaxial strain on which a compressive load acts becomes local maximum as a stable behavior limit strain, and acquiring a stable behavior limit line that will be an index for wrinkle generation under a biaxial stress condition by plotting, on two-dimensional coordinates, the stable behavior limit strain acquired for each load ratio in a biaxial stress test in which a load in the in-plane biaxial directions is applied to a cruciform test piece for biaxial test so as to induce out-of-plane buckling in the measurement part.

Seat members 31 are held in a state of being movable with respect to first slide members 21 or second slide members 22 correspondingly. By rotating screws 39 in directions to increase the distances d between surfaces A of the first slide members 21 or the second slide members 22 and surfaces B of the seat members 31, chucks 25 are moved in directions to increase the distance between the chucks to apply pretension to a test piece correspondingly. When backlash in a force transmission system from a support part to the respective chucks 25 is eliminated, a biaxial tensile test is started.

A rig for testing thermomechanical loads comprises grip portions for gripping a test specimen, and a frame that join the grip portions to each other, the thermal expansion coefficient of the grip portions being greater than the thermal expansion coefficient of the frame. The frame may be made from metal.