Effects of Clamp Force on Fatigue Strength of Aluminum Alloy Bolts

In order to achieve recent weight reduction for transport equipment, it is necessary to replace steel with nonferrous materials not only for structural parts but also for joining elements such as bolts and nuts. Fatigue strength of high strength steel bolts has rarely been influenced by clamp force which acts as mean stress to the bolts because of shakedown caused by local plastic deformation at the first thread root. However the effects of clamp force on fatigue strengths for aluminum alloy bolts have not been sufficiently revealed yet. In this study, fatigue tests and elasto-plastic analysis for aluminum alloy A5056 and A6056 bolts have been conducted to reveal the influence of clamp force on fatigue strength. Hexagon head bolts made of aluminum alloy A5056 and A6056 were used. For the internal thread parts for the A5056 bolts, A5056 nuts were used. For the internal screw parts for the A6056 bolts, machined nuts made of die cast material ADC12 considering actual usage were used. Results showed that the fatigue strength for A5056 bolts decreased with an increase in the mean load. Although the fatigue strength for A6056 bolts decreased with an increase in the mean load if the mean load was lower than 6 kN, the fatigue strength for A6056 bolts did not decrease if the mean load was greater than 6 kN. Results of elasto-plastic analyses showed that the causes, that the behaviors of these fatigue strengths were different, were reduction of mean stress due to local plastic deformation at the root of the first thread so-called shakedown. This result indicates that we can tighten A6056 bolts with high clamp force although cannot tighten A5056 bolts with high clamp force.