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The stress intensity factor at the crack tip of a compact tension specimen is [4] = [() / / + / / + /] where is the applied load, is the thickness of the specimen, is the crack length, and is the effective width of the specimen being the distance between the centreline of the holes and the backface of the coupon.
The crack tip opening can be calculated from the length of the crack and opening at the mouth of the notch. According to the material used, the fracture can be brittle or ductile which can be concluded from the graph. Standards for CTOD testing can be found in the ASTM E1820 - 20a code. [9]
Rainflow counting identifies the closed cycles in a stress-strain curve. The rainflow-counting algorithm is used in calculating the fatigue life of a component in order to convert a loading sequence of varying stress into a set of constant amplitude stress reversals with equivalent fatigue damage.
The important characteristic of these tests is that the strain rate is low, for example extension rates selected in the range from 10 −8 to 10 −3 s −1.The selection of the strain rate is very important because the susceptibility to cracking may not be evident from result of tests at too low or too high strain rate.
Notches are commonly used in material impact tests where a morphological crack of a controlled origin is necessary to achieve standardized characterization of fracture resistance of the material. The most common is the Charpy impact test , which uses a pendulum hammer (striker) to strike a horizontal notched specimen.
According to ASTM D883, stress cracking is defined as "an external or internal crack in a plastic caused by tensile stresses less than its short-term mechanical strength". This type of cracking typically involves brittle cracking, with little or no ductile drawing of the material from its adjacent failure surfaces. [ 1 ]
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The specimen is then loaded monotonically. A plot of the load versus the crack opening displacement is used to determine the load at which the crack starts growing. This load is substituted into the above formula to find the fracture toughness . The ASTM D5045-14 [4] and E1290-08 [5] Standards suggests the relation