The design data required for the fracture mechanics approach are the model parameters that define failure in terms of strain energy release rate. Most commonly, test pieces are required to obtain data for Mode I loading, Mode II loading and Mixed-Mode I/Mode II loading. Various test pieces have been developed for measurement of joint properties, generally using beam type geometries. Those specimens recommended for fatigue and durability testing are illustrated in Figure 5.
The tapered double cantilever beam (TDCB) was developed to give constant G with crack length for constant load. The parallel sided double cantilever beam can also be used for mode I loading. Parallel beams are easier to manufacture and can be loaded in a variety of ways to achieve various modes of loading. It is also often necessary to use thin sheet material so that the actual surface properties of the material to be used in an application are included. In this case, the thin substrates must be reinforced by bonding a thicker layer of material to prevent the thin substrate from yielding in the test.
Testing should be performed primarily under ambient laboratory conditions to measure the baseline fatigue resistance. However, If the service environment is likely to be at a different temperature or humidity, then fatigue testing will be required at the service specific conditions.
Figure 5: Various test piece geometries for fracture mechanics testing
The reinforced double cantilever beam (RDCB) test piece has been developed for thin substrate materials used in many structures. The test piece provides a universal specimen design that can be used for fatigue testing under a variety of different modes of loading. This geometry is a development of the standard DCB test piece, whereby both substrate beams have been reinforced in order to allow cyclic fatigue testing of thin metal substrates (typically less than 3mm), that would normally deform plastically without the reinforcement. If the metal substrates can be supplied in thicker sections, while maintaining the surface properties of the thinner gauge material, reinforcement of the substrates may not be necessary. The basic test piece design is shown in Figure 6.
Figure 6: Typical RDCB Test piece configuration for 2 mm substrate thickness
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