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General Description of Tensile Test Rig
A tensile test rig consist of a universal testing machine and the material specimen. Universal testing machines are usually hydraulically operated. The specimen is held by clamps. These clamps are attached to two horizontal crossheads. The height of one of the lower crosshead can be adjusted via a screw column and hydraulic ram. The other crosshead is held in place. As the lower crosshead is pulled down by the hydraulic ram the material specimen is subjected to tensile force. The tensile strength of the material is determined by measuring the relationship between the tensile force the material is subjected to and the extension of the material.
Test Specimen Geometry
The size and shape of test specimens is determined by international standards. This ensures that different materials are compared on a like by like basis. One standard test of specimen geometry consists of a cylindrical rod. The rod is thicker at each end and has a narrow section in the middle. This ensures that when the specimen fractures it will fracture in the middle section rather than at either of the two ends. Sometimes the thick end sections are screw threaded to aid the tensile test grip clamps. Another standard geometry consists of a rectangular plate, again with thick ends and a narrow middle section.
Grip Materials
Grips consist of the clamps on the test rig and any intermediary material between the clamp and the specimen. Intermediary materials consist of collars or pins that are separate from both the clamp and the specimen. Intermediary materials can reduce friction and tension around the grip, or increase the strength of the grip. Intermediary materials are usually made of steel. The clamps on the test rig are usually made from steel. Because the clamp will be subjected to high stresses during the test procedure the clamp must be very strong. Clamps are usually made from high-strength alloy steel.
Grip Geometry
There are five basic types of grips. These are the threaded grip, the cylindrical serrated wedge grip, the split collar grip, the pin grip and the flat serrated wedge grip. In the threaded grip the cylindrical end of the material specimen has a screw thread that attaches to a corresponding thread in the clamp. In the cylindrical serrated wedge grip the cylindrical specimen end is held by a grip with serrated wedges to increase the strength of the grip. The split collar grip consists of a pair of metal pieces that wrap around the thick ends of the specimen. The pin grip is used with flat specimens: a hole is drilled through the flat ends of the specimen and a pin is inserted into this hole. The clamp then holds the pin. The flat serrated wedge grip is used to hold flat specimens, and it has a pair of serrated surfaces to hold the specimen.