Long Term Creep and Stress Relaxation Tests

Introduction

Experiments used to characterize the long term creep or stress relaxation properties of materials are technically challenging in that our objective is to generate a constant stress or strain over a long time without using costly universal test instruments so that the testing can be cost effective. To do this, simplified test stands are constructed and typically, short time data is compromised. A couple of the test systems used at Axel are shown below.

Experiments:

  • Long Term Creep
  • Long Term Stress Relaxation

  • Long Term Creep

    When a material sample is subjected to a constant stress, an increase in the deformation takes place with time, this behavior is called creep. The processes causing creep may be physical or chemical in nature, and under all normal conditions both types of process will occur simultaneously. However, at normal or low temperatures, creep is dominated by physical processes. At high temperatures or long times, chemical processes are dominant. One type of creep instrument measures the straining in the test specimen continuously and consists of a small rig with a chamber and a clip on extensometer. Dead weights are placed on a loading pan with a lever arm to increase the effect of the weight by a factor of 10. The rigs are connected to a data acquisition box connected to a computer, storing the strain data over time.

    Image of plastic tensile specimen in a load term creep frame.
    Schematic Drawing of a Creep Test Stand

     

    Image of long term creep loading frames.
    Some of the Dead Weight Based Mechanical Creep Frames Fitted with Environmental Chambers

    Image of long term creep loading frames.
    Tensile Test in a Thermal Test Chamber on a Creep Test Stand

    Long Term Stress Relaxation

    When a constant strain is applied to a material sample, the force necessary to maintain that strain is not constant but decreases with time. This behavior is called stress relaxation. The processes causing stress relaxation may be physical or chemical in nature, and under all normal conditions both types of process will occur simultaneously. However, at normal or low temperatures, stress relaxation is dominated by physical processes. At high temperatures or long times, chemical processes are dominant. One type of relaxation instrument measures the reaction stress continuously and consists of a small rig with a load cell. When testing at elevated temperature the rig is placed in a cell oven. The rigs are connected to a data acquisition box connected to a computer, storing the force and temperature data over time. The Elastocon system used at Axel Products is a continuous measurement system.

     

    Image of an Elastocon long term stress relaxation system.
    Elastocon Oven System with Compression Fixtures Inside of the Oven