About 40 years ago I inherited project engineering responsibility for a fiberglass jet engine seal housing and was immeadiately informed by the quality and materials liasons that the specified material had failed the required tensile test. Reading the report confirmed that the yield stress of all the "dogbone" samples was way below the minimum requirement. Upon opening the packet of tensile specimens, I observed that all of them broke off at one end, not in the rectangular reduced-area section. The lab technician had dutifully measured the width and thickness of this section, calculated the area, and divided this value into the force value the tensile-test machine provided, giving a totally meaningless PSI value. The cause of the "yielding" of the specimens was obvious as the holders were two vice-like jaws with sharp teeth that litterally crushed and fractured the large ends of the "dogbones." No wonder they broke there! The whole test had to be repeated by another lab that had the proper holding fixtures, and the material easily passed.