Test Pieces: Design, Preparation, Measurement and Protection
Fatigues testing in test pieces are classified into either specimen or components. Specimen is arbitrary when it comes to the shape and dimension and is small and simple in dimensional characteristics. Whereas a component shape and size are generally given but in order to simulate fatigue testing on components a simplified version of the actual component is used. Based on the description, classification of specimen and components are: unnotched specimen, notched specimen, simulated components, scale models and actual components.
A test piece is usually composed of the grip portion, test portion and a transition portion. The transition portion is the bridge joining the grip portion and test portion. It should be noted that when using a tool maker microscope when looking for fractures in the test portion which may not have an effect on the material, that it may have an effect on the grip portion. Grip portions as well as the transition portion of the material should be studied under a tool maker microscope looking for fissure or effect on the fatigue strength. It is suggested in order to minimize the effect on the grip portion it may be reduced considerably. This is depending on the test portion and the proportion of the grip portion. Since failure in the grip portion affect the result and therefore should be discarded. Note that the test section should be in relation to the grip section, where a large reduction is allowed when the grip is provided with screw threads.
Fatigue testing on unnotched specimen is to determine the fatigue properties under varying conditions. This also includes the effect of various factors on the fatigue life of the specimen. To ensure a uniform stress is subjected to various test portions it should be designed in a manner wherein the grip portion does not develop premature failure or stress to both the grip and transition portion.
Tension compression specimen has a smoother transition from the test portion to the grip portion of the specimen. Earlier design with a cylindrical test portion used by Haigh in 1912 viewed under a tool maker microscope show a cross section from the enlarged thread ends with ½ inches to the test portion of 0.2 to 0.3 inches in diameter in a gradual taper manner.
Repeated bending specimen on circular specimen is seldom subjected to this type of test but on a rotating bending specimen test instead. In a four point loading machine where the bending is constant over the test portion length wherein earlier design was composed of reduced cylindrical test portions which were connected to the larger end by fillets.
Because of cost some testing are done through simulated components and on a scaled down version model. This is true to complicated components and wherein cost is primary. Models although scaled may be subjected to repeated loading.
Although a scaled model may be used the only guaranteed method of determining fatigue characteristics of components is by using a full scale specimen on a variety of chosen stress levels. More on this topic