straining-head

from The Century Dictionary.

• noun In a testing-machine for metals and other materials, the part which is operated by power to produce stress in the test-piece and cause the strain of which its deformation is the measure. This head may be actuated by hydraulic pressure or by screws, one end of the test-piece being fastened to the head by a holder or wedges, while the other is held in a similar holder in a part connected to the weighing-levers and scale-beam.