Bolt-load Retention Testing of Magnesium Alloys for Automotive Applications

For automotive applications at elevated temperatures, the need for sufficient creep resistance of Mg alloys is often associated with retaining appropriate percentages of initial clamp loads in bolt joints. This engineering property is often referred to as bolt-load retention (BLR); BLR testing is a practical method to quantify the bolt load with time for engineering purposes. Therefore, standard BLR test procedures for automotive applications are desired. This report summarizes the effort in the Structural Cast Magnesium Development (SCMD) project under the United States Automotive Materials Partnership (USAMP), to provide a technical basis for recommending a general-purpose and a design-purpose BLR test procedures for BLR testing of Mg alloys for automotive applications. The summary includes results of factors influencing BLR and related test techniques from open literature, automotive industry and research carried out in this laboratory project. Areas considered to be critical to BLR testing are reviewed, and recommendations are outlined including: (1) BLR sample and test fixture; (2) washer and nut; (3) load measurement method; (4) instrumentation of strain gages and calibration; (5) data acquisition; (6) heating device; (7) initial loading; and (8) baseline test. The results from this research may be incorporated into an SAE/USCAR standard.