Fracture toughness tests measure a material’s ability to resist the growth or propagation of a pre-existing flaw. The flaw or defect may be in the form of a fatigue crack, void, or any other inconsistency in the test material. Fracture toughness tests are performed by machining a test sample with a pre-existing crack and then cyclically applying a load to each side of the crack so that it experiences forces that cause it to grow. The cyclic load is applied until the sample’s crack grows. The number of cycles to fracture is recorded and used to determine the material’s fracture growth characteristics.
Purpose of fracture toughness testing:
Fracture toughness is the stress that causes a pre-existing crack or flaw to grow or propagate. It is an important material property in the manufacturing industry, since the presence of flaws is not completely avoidable. The stress intensity factor, which is a function of the flaw size, geometry, and loading, is used to determine a material’s fracture toughness. A material’s stress intensity factor and fracture toughness are related to one another in the same manner that stress and tensile stress are related to each other.
Types of fracture toughness tests:
For the majority of fracture toughness tests there are three different modes of fracture for crack propagation. Mode 1 requires that the applied load be in the normal direction of the crack plane. In mode 2 the load is applied along the length of the crack plane. Finally, for mode 3 the load is applied across the width of the crack plane. Generally there are two different configurations for the test sample: single edge notch bend (three point bending) and compact tension. A three point bending specimen has the initial crack located at the midsection on the opposite side from the point where the midsection load is applied with opposing points of force located at each end on the same side as the crack. A compact tension specimen is oriented so the load is applied on each side of the crack in a way that extends the width of the crack.
Types of materials tested with fracture toughness tests:
Nearly all manufactured materials will contain a defects, flaws or cracks to some magnitude and may experience fracture due to these inconsistencies and when or if this does occur the fracture toughness of that material will be exhibited. The materials that are most commonly tested for fracture toughness are similar to those tested in fatigue tests but have slightly different orientations. These materials include metals, plastics, ceramics and composites among others as well as many other rigid substrates that may contain defects.