Fatigue tests tension-compression

Several criticisms of these parameters have recently been pointed out. First, they have no specific association with a material plane (i.e., they are scalar parameters), despite the fact that cracks are known to nucleate on specific material planes. With traditional parameters it is difficult to account for the effects of crack closure under compressive loading. Traditional parameters have not been successful at unifying experimental results for simple tension and equibiaxial tension fatigue tests. Finally, a nonproportional loading history can always be constmcted for a given scalar equivalence parameter that holds constant the value of the scalar parameter, but which results in cyclic loading of material planes. For such histories, scalar parameters incorrectly predict infinite fatigue life. 

Fig. 35. Variation of and N, . with for SAN samples containing 30% AN tested in tension-compression fatigue at 27.6 MPa and at 21 Hz...

For the rubber modified copolymer, ABS, we have carried out fatigue studies over a range of stress amplitudes and frequencies under reversed tension-compression cycling in addition, some tests have been made under cycUc tension. Results will be presented first for tests carried out under different applied amplitudes and comparisons will be made between ABS and SAN, between ABS and HIPS, and between our data on ABS and that of others... 

Fatigues tests have been run on a series of ABS samples under reversed tension-compression at a stress amplitude of 27.6 MPa, at four different frequencies, viz. 0.02, 0.2, 2, and 21 Hz. The average fatigue lifetime as a function of frequency is shown in Fig. 46 and, for comparison purposes, the data for HIPS obtained at 17.2 MPa is also shown. The average fatigue life-time of ABS increases in a linear manner with frequency on this log-log plot. The rate of increase, however, is reduced compared to that of HIPS, or of PS (Fig. 15). The reduced frequency sensitivity is perhaps a result of a reduced magnitude of the p-transition in the SAN copolymer compared to that in PS. A reduced frequency sensitivity of FCP rate in ABS vs. a HIPS-modified PPO has also been noted... 

For example, Melin et al. [20] showed that fatigue tests of impact-damaged laminates in tension—compression fatigue at R values of — 1 and —5 have almost identical stress-life (S-N) curves when the lives are plotted in terms of maximum compression stress. Tests at an J value of -1 have a mean stress of zero and equal excursions into tension and compression, whereas tests atR= -5 will have stress cycles with tension... 

Fig. 4.67 Test equipment for uniaxial tension/compression fatigue loading.

Sta Stadlbauer, F., Koch, T., Planitzer, F., Fidi, W., Archodoulaki, V.-M. Setup for evaluation of fatigue crack growth in rubber Pure shear sample geometries tested in tension-compression mode. Polym. Test. 32 (2013) 1045-1051. 

---