Abstract: In the illustrated embodiment, the amplitude of sinusoidal pressure varians in the test piece is adjustable even during operation by summing the sinusoidal volume variations in two piston assemblies whose relative phase is continuously adjustable. The eccentric drives for the piston assemblies may be coupled via a bevel gear arrangement whereby angular shifting of the intermediate gear about the common axis of the outer gears shifts the phase relation between the eccentric drives. A desired minimum resultant pressure may be maintained in the test piece by introducing a bias pressure value equal to or exceeding the amplitude of the pressure variation, and such bias pressure may be adjusted in response to any permanent deformation of the test piece to maintain the desired minimum resultant pressure.