A testing apparatus having four-bar linkages pivotable to sleeves on opposite vertices with the sleeves of each vertex rotationally attached to each other. Links of each linkage are pivotally attached to loading plate assemblies securing a test specimen. During loading, the assemblies move toward or away from each other; thereby, applying compression or tension to the specimen. A pressure system fluidly impacts opposite faces of a piston of the assembly such that one of the faces is pressurized and impacts arms of the assembly for a sliding motion to move toward or away from the longitudinal axis of the apparatus thereby, applying a compression or tensile load on the specimen or augmenting the loads applied by the movement of the loading plate assemblies. The pressure system includes a controller connected to a reservoir, a pressurized source, a plurality of shutoff valves and pressure-adjustable check valves.