Section 3 Small specimen testing

3.1.1 In this connection, a small specimen is considered to be one of the specimens taken from a crank throw. Since the specimens shall be representative for the fillet fatigue strength, they should be taken out close to the fillets, as shown in Figure 2.3.1 Specimen locations in a crank throw.

3.1.3 The (static) mechanical properties are to be determined as stipulated by the quality control procedures.

3.2.1 It is advisable to use un-notched specimens in order to avoid uncertainties related to the stress gradient influence. The push-pull testing method (stress ratio R = -1) is preferred, but especially for the purpose of critical plane criteria, other stress ratios and methods may be added.

3.2.2 In order to ensure principal stress direction in push-pull testing to represent the full size crank throw principal stress direction, and when no further information is available, the specimen shall be taken in 45 degrees angle as shown in Figure 2.3.1 Specimen locations in a crank throw.

3.2.3 If the objective of the testing is to document the influence of high cleanliness, test samples taken from positions approximately 120 degrees in a circumferential direction may be used. See Figure 2.3.1 Specimen locations in a crank throw.

3.2.4 If the objective of the testing is to document the influence of continuous grain flow (cgf) forging, the specimens should be restricted to the vicinity of the crank plane.

3.3.1 If the specimens are subjected to torsional testing, the selection of samples should follow the same guidelines as for bending above. The stress gradient influence must be considered in the evaluation.

3.3.2 If the specimens are tested in push-pull and no further information is available, the samples should be taken out at an angle of 45 degrees to the crank plane in order to ensure collinearity of the principal stress direction between the specimen and the full size crank throw. When taking the specimen at a distance from the (crank) middle plane of the crankshaft along the fillet, this plane rotates around the pin centre point, making it possible to resample the fracture direction due to torsion (the results are to be converted into the pertinent torsional values).

3.4.1 If the test purpose is to find fatigue properties and the crankshaft is forged in a manner likely to lead to continuous grain flow (cgf), the specimens may also be taken longitudinally from a prolonged shaft piece where specimens for mechanical testing are usually taken. The condition is that this prolonged shaft piece is heat treated as a part of the crankshaft, and that the size is so as to result in a similar quenching rate as the crank throw.

3.4.2 When using test results from a prolonged shaft piece, it must be considered how well the grain flow in that shaft piece is representative for the crank fillets.

3.5.1 The fatigue strength achieved by specimen testing shall be converted to correspond to the full size crankshaft fatigue strength with an appropriate method (size effect).

3.5.2 When using the bending fatigue properties from tests mentioned in this section, it should be kept in mind that successful continuous grain flow (cgf) forging leading to elevated values compared to other (non-cgf) forging will not normally lead to a torsional fatigue strength improvement of the same magnitude. In such cases, it is advised to either carry out also torsional testing or to make a conservative assessment of the torsional fatigue strength, e.g. by using no credit for cgf. This approach is applicable when using the Gough Pollard criterion. However, this approach is not recognised when using the von Mises or a multi-axial criterion such as Findley.

3.5.3 If the found ratio between bending and torsion fatigue differs significantly from √3, one should consider replacing the use of the von Mises criterion with the Gough Pollard criterion. Also, if critical plane criteria are used, it must be kept in mind that cgf makes the material inhomogeneous in terms of fatigue strength, meaning that the material parameters differ with the directions of the planes.

3.5.4 Any addition of influence factors must be made with caution. If for example a certain addition for clean steel is documented, it may not necessarily be fully combined with a K-factor for cgf. Direct testing of samples from a clean and cgf forged crank is preferred.