Stress Relaxation Testing

In creep testing, specimens are subjected to a constant stress (or load) at elevated temperature and the strain is measured as a function of time. In some material applications, such as for bolted connections, the application of load at high temperature occurs the other way round. After the bolt/screw has been tightened, the shaft of the bolt/screw is subjected to constant strain, while creep causes the stress (or load) to decrease over time. This “stress relief” is also called relaxation and can gradually lead to the bolt/screw becoming “loose”, demonstrating the importance of stress relaxation testing.

Stress relaxation testing at SMaRT is primarily conducted on electric screw test machines, with some servo-hydraulic availability. Testing can be conducted in strain or displacement control modes. Typically, testing is conducted between ambient and 900°C, although some capacity for higher temperatures is available. SMaRT currently use a range of test machines that provide operation across loads ranging from 0.5kN to 250kN. SMaRT has
the flexibility and resources to accommodate a wide variety of specimen shapes and sizes (bespoke fittings can be manufactured in our workshop).

Stress relaxation testing to assess the behavior of materials involves stretching specimens, by applying a load ramp, to a pre-determined value, using a device at elevated temperature. The device is then switched to strain control (or stroke control, using the position of the machine crosshead) to hold the specimen at a constant strain/elongation. The temporal drop of the stress is then monitored for the required duration fo the test. The characteristic value obtained from the test is called the relaxation stress.

Example of actual stress-relaxation data — Example of Actual Stress Relaxation Data

SMaRT routinely conduct stress relaxation testing in compliance with the following international and industry standards (working to other standards are available upon request):

ASTM E328-13 Standard Test Methods for Stres Relaxation for materials and Structures