Stress Relaxation in Bolted Joints of Die Cast Magnesium Components

As the use of magnesium continues to grow in the automotive industry, new developments for lightweight driveline components are gaining more interest. Therefore, the need to develop alloys with greater creep resistance and better knowledge of the bolt load behaviour is necessary. Through continuing efforts in research and development, appropriate technical solutions can be found. As a tool in this development work, laboratory test equipment for direct measurements of the bolt load behaviour has been developed based on the use of strain gauged bolts.

Bolt load retention behaviour was investigated using Grade 8.8 M8 x 1.25 and M10 x 1.50 standard fasteners which were modified and equipped with strain gauges. The modified fasteners recorded the bolt load during thermal cycling of bolted test specimens of die cast AZ91, AS21 and AE42 magnesium alloys. The test specimens were exposed to various thermal cycles for 100 hours and 500 hours at 125° C and 150° C. The effect of washer size, bolt diameter and coupon thickness are presented along with the effects of disassembly and reassembly.