Measurement of Flow Stress by the Ring Compression Test

To measure the flow stress of metals under forming conditions, i. e., at large strain rates and elevated temperatures, a method based on the ring compression test is proposed. In this method, ring-shaped specimens are compressed between two flat tools with or without lubrication. The coefficient of friction between the specimen and the tool is determined from the change in inner diameter, and then the load and reduction in height measured in the test are converted into average flow stress are derived from the rigid-plastic finite element calculation. This method is simple to carry out and does not require control of the friction during compression. Ring compression tests for some kinds of lubricants, billet materials an strain rates are carried out. The method is confirmed to provide flow stress-strain curves within an error of 5%, and is effective especially for measuring the flow stress of heat-resistant materials such as Ti alloys and Ni base superalloys which are formed at elevated temperatures. As an example, flow stress-strain curves of Ti-6 Al-4V alloy measured by the present method are demonstrated.