Creep during static loading

Creep induced elongation - it is well known for PE-composites that during long term static loading a temperature-dependent creep behaviour can be observed, which is similar to that of neat PE-fibres [35]. 

Significant in this figure is the narrower range of usable static loadings at the bottoms of the curves that resulted when the density is reduced. Important consideration in comparing foams of different densities is their compressive creep resistance, and their ability to resist undergoing a permanent thickness loss during their time under load. As the density decreases, so does the creep resistance. 

Chose an equally-spaced increasing sequence of stress Oj = /Ao (i = 1, 2, 3,...) and denote the associated static and creep time to failure fs(f) = fs(Oi) and t (i) = 4(0i), respectively. During static tests, the strain rates are kept constant and the force deflection curves are considered linear until failure is reached. Therefore, the stress increase is assumed to be linear during the static test. The linear stress gradient is approximated by a staircase function with the steps o O3,05,..., shown in Figure 6.15. This means that the overall degradation in a CSR test can be considered as series of creep loads. From Eq. 6.9 it follows ... 

Liquefaction flow sUdes are one of the most catastrophic forms of ground failure. Flow failures often result in massive lateral and vertical movements of soil, occasionally for hundreds of meters laterally. While most commonly observed during earthquakes as a result of seismically induced liquefaction, flow failures also have occurred as a result of non-seismic (static) loading, displacements (i.e., shear strains) induced by global instability or creep, and dynamic loading. These failures are driven by static shear stresses that exceed the available shear resistance in the soil after the soil liquefies. The available shear... 

Attempts to project results from short-term creep tests to a longer time scale often fail because the increase of shear strain during a static load creep experiment will lead to a change of the state of stress in the adhesive joint and will eventually cause an excessive amount of superimposed tensile stress near the end of the overlap area. Therefore, the dotted lines in O Fig. 34.13 merely represent a guide to the eye projection rather than a mathematically confirmed extrapolation. 

Experimental creep data for ceramics have been obtained using mainly flexural or uniaxial compression loading modes. Both approaches can present some important difficulties in the interpretation of the data. For example, in uniaxial compression it is very difficult to perform a test without the presence of friction between the sample and the loading rams. This effect causes specimens to barrel and leads to the presence of a non-uniform stress field. As mentioned in Section 4.3, the bend test is statically indeterminate. Thus, the actual stress distribution depends on the (unknown) deformation behavior of the material. Some experimental approaches have been suggested for dealing with this problem. Unfortunately, the situation can become even more intractable if asymmetric creep occurs. This effect will lead to a shift in the neutral axis during deformation. It is now recommended that creep data be obtained in uniaxial tension and more workers are taking this approach. 

During creep, a loaded polymer component will gradually increase in length until fracture or failure occurs. This phenomenon is usually referred to as creep rupture or, sometimes, as static fatigue. 

---