Fatigue Dynamics - Fracture Toughness Testing

printable version Crack initiation data is vital to the component designer, but the behavior of a material after a crack has initiated is also a major concern in many applications. Selecting a material that exhibits significant resistance to crack growth can frequently be a critical decision. The determination of these crack propagation effects is often called "fracture toughness" testing.

	One type of crack propagation test involves the determination of crack growth per load cycle as a function of stress-intensity factor range. This test procedure requires the measurement of the cyclic load applied to the specimen while simultaneously measuring the change in crack length for a given number of cycles. These measurements are made at multiple load ranges, usually with several specimens; test data is normally plotted on a log-log scale.

Another type of test used to determine crack propagation resistance is usually referred to as a 'plane strain' fracture toughness test. This test evaluates the ability of a material to prevent crack growth while being subjected to an increasing load rather than a cyclic load. The applied load is measured simultaneously with the crack 'opening' displacement, rather than the crack length. A linear plot of this data is made and values from the plot are used in the plane strain fracture formula to calculate a fracture toughness number.

These and other fracture toughness tests are valuable tools in designing components that avoid sudden catastrophic failures.

WOULDN'T CRACK PROPAGATION TESTING TIE UP OUR UNIVERSAL TEST MACHINE FOR LONG PERIODS OF TIME? Fracture toughness tests usually require notched specimens that have a crack at the root of the notch induced by cyclic loading. This 'pre-cracking' phase of the test procedure can consume the largest portion of the total test time and frequently is accomplished with the same test machine that performs the more complex 'fracture' phase of certain tests. A more efficient approach to this procedure is to use a simpler Fatigue Dynamics 'precracker' machine to prepare the cracked specimens so the more complex test machine is free to perform other tests. (A precracker machine also may allow labs with only a static tensile tester, which may be incapable of providing fatigue precracking, to conduct fracture toughness tests.) printable version