Self-aligning four-point bend testing rig and sample geometry effect in four-point bend fatigue

A self-aligning four-point bend testing rig was designed and made which can minimize the possible misalignment associated with a four-point bend test and be used to study the fatigue of materials both at room and elevated temperatures. The stress distribution between the inner-rollers in a specimen under four-point bend, that is the nominal pure-bending section length, was analyzed with respect to various load-span/specimen-thickness ratios (t/h) and support-span/load-span ratios (L/t) using a finite element method. It was found that the stress distribution could vary with both t/h and L/t. It was found that values of t/h and L/t between 1.2 and 1.5 and between 4 and 5, respectively, were the optimum testing geometry which led to a relatively uniform stress distribution consistent with the value calculated by beam theory. Fatigue tests (R = 0.1 and frequency = 20 Hz) were carried out on samples with different thickness in a peak-aged 8090 Al-Li alloy using the rig. The results appear to support the finite element results. The S-N curve of the 8090 Al-Li alloy was measured using the optimum testing geometry in the four-point bend, and it was found to be consistent with that reported in the literature.