Creep & Stress Rupture Testing

Creep is a slow failure mechanism that occurs in materials exposed to a constant load or stress, below the elastic limit of the material, for a protracted period of time. Temperature can significantly affect the rate of creep deformation in materials; the greater the temperature the greater the rate of deformation. It is important to quantify and understand this type of deformation to ensure the safety of components designed to operate at elevated temperatures.

Stress rupture testing can be considered to be a creep test without the use of extensometry, which is used on a creep test to monitor the deformation of the specimen throughout the duration of the test. As the name suggests, stress rupture testing is focused on measurement of the time taken for the specimen to rupture. Permanent plastic elongation can also be determined from the test.

SMaRT incorporates a dedicated creep testing laboratory primarily focussed on testing of metals and alloys at elevated temperatures. The facility includes both constant-load (30kN max capacity) and constant-stress (15kN max capacity) tensile creep machines that can operate at temperatures up to 1100°C. In addition, SMaRT has the capability to test ceramic matrix composites (CMCs) under constant tensile stress (10kN max capacity) at temperatures up to 1400°C. All frames are linked to an electronic data
logger system which records and monitors temperature
and extension of the specimens throughout the test.

Creep & stress rupture testing can be conducted under constant load or consatant stress. The differences are described as follows:

1) Constant Load

Constant load testing is performed on a machine with a straight lever arm. As the specimen elongates, the capstan on the frame automatically adjusts to maintain the lever arm in the horizontal position, keeping the load application at a fixed length along the lever. If the auto-levelling facility wasn’t present, the test would not be under a constant load or constant stress condition.

2) Constant Stress

Constant stress machines have a curved lever or cam. Unlike the constant load machines, the frames do not have any automatic levelling to the cam. Instead, as the cam lowers, the distance of the applied load from the pivot is reduced and therefore the force acting on the specimen decreases. The design of the cam (i.e. curvature) is such that the drop off in load is proportional to the reduction in cross section of the specimen and hence maintains a constant stress on the test piece. The machines at SMaRT are designed to work on a specific specimen geometry with a 1” gauge length. If you require constant stress creep testing, please enquire about the specimen design.

SMaRT routinely conducts creep or stress-rupture testing in compliance with the following international and industry standards (working to other standards are available upon request):

  • BS EN 2002-005 Metallic materials – Aerospace series – Test methods for metallic materials – Part 005: Uninterrupted creep and stress-rupture testing
  • BS EN ISO 204 Metallic Materials – Uniaxial creep testing in tension – Method of test
  • ASTM E139 Standard test methods for conducting creep, creep-rupture and stress-rupture tests of metallic materials