Possibility to Determine Aluminum Wheels Fatigue Life by Local Strain Concept

As part of an extensive investigation on automotive aluminum wheels, a study was carried out regarding the validity of theoretical fatigue life calculations by means of the local strain approach. Tests to substantiate the theoretical prediction were carried out on a test facility which allows for service-like variable amplitude biaxial loading in addition to rotation of the wheel. Under these simulated service conditions the strain-time histories from strain gages in critical areas of wheels were recorded on magnetic tape. These stress sequences were used twofold, first as test-control-signal for testing of unnotched test coupons taken from production wheels and secondly as input for theoretical fatigue life predictions. The specimen on which the material data for the theoretical prediction were determined were also taken from wheels; the data consisted of mono-tonic and cyclic stress-strain curves as well as strain controlled ε-N curves. A comparison was made between the results of specimen tests, theoretical fatigue life prediction and actual wheel tests under simulated service conditions. It was found, in all cases (plain alloy cast material, age hardened materials, forged material) that the fatigue life to initial crack could be predicted with sufficient accuracy if a cumulative damage sum of 0.5 rather than 1.0 was assumed. For the durability of the total wheel, however, it was found that the stress distribution in critical areas is also of decisive importance.