Plastic Fracture Testing Setup

Plastics pervade our entire lives, used in products from our computers and cell phones to bullet proof personal and vehicle armor. We don't often consider plastics as structural materials, however we use plastic chairs and tables, ride in cars whose interiors are mostly constructed of plastic, and wear bicycle helmets with protective plastic shells. The fracture behavior of plastics is just as important as it is for metals and composite materials. As their name implies, plastics undergo plastic deformation and primarily exhibit elastic-plastic behavior. However, some polymeric materials can be very brittle, so fracture toughness testing of plastics is conducted for both plane-strain (KIc) and plane-stress (JIc) methods, depending on specimen size and crack growth behavior. Common test procedures for fracture toughness of plastics attempt to create a natural crack in the plastic test specimen by tapping a razor blade into the notch root. If the plastic is too ductile, starter cracks are typically created by pressing or sawing a razor blade into the notch root. Some methods do allow fatigue precracking of the plastic specimen in the same manner as metal fracture specimens. Plane strain (KIc) precracked specimens are loaded at constant rate until the material fractures, measuring force and crack displacement. J-R curves can be developed thorough multiple specimen or single specimen test methods. Multiple specimen J-R curves use a different specimen for each point on the J-integral vs. crack growth curve, by loading each specimen to a predetermined crack extension. Single specimen J-R curves employ load/unload cycles on a single specimen, increasing the crack extension with each cycle, and calculating the J-integral for each load cycle. TestResources offers complete software and controls packages equipped for both plane-strain (KIc) and plane-stress (JIc) fracture testing of plastics. These packages simplify the process of programming test methods and collecting data and perform test validation and data post-processing. ASTM and ISO have developed standard test methods for testing the fracture toughness of advanced ceramic materials. These standards provide methods that can be replicated by independent labs, ensuring specimens are tested in the same manner and conditions, allowing test results validation between manufacturers and customers. Popular fracture toughness test methods for plastics are ASTM D5045 for plane-strain fracture toughness and strain energy release of plastics, ASTM D5528 for mode I interlaminar fracture toughness of fiber-reinforced polymer composites, ASTM D6671 for mixed mode interlaminar fracture toughness of unidirectional fiber reinforced polymer matrix composites, ISO 17281 for fracture toughness of plastics at moderately high loading rates, and ISO 13586 for linear-elastic fracture toughness of plastics. TestResources recommends the use of servo-hydraulic or electro dynamic test machines for fracture toughness testing. These test machines can be used for every stage of fracture toughness tests, capable of the high frequencies needed for specimen fatigue precracking and the high forces needed for fracture toughness measurement. Clevis grips are used for fracture testing of compact tension C(T), disc shaped compact tension DC(T), and arc shaped tension A(T) specimens, while side edge bend SE(B) and arc shaped bend A(B) specimens are tested using special bend fixtures that have rollers capable of rotation and slight translation.