Unrecovered strain

Example curves of nominal stress versus nominal strain for three load/unload cycles between zero stress and a nominal strain of 3 are given in Fig. 4.9 and 4.10 below for two polymers based on DBDI and MDI. As seen, notable features were the pronounced hysteresis, and unrecovered strain. 

Notable features were the pronounced hysteresis, unrecovered strain and Mullins effect (whereby re-loading follows a stress-strain path closer to the unloading path than the original loading path). From curves such as these we calculated several quantifiers of the inelasticity. Consider the first cycle for material PU1, shown above. It defines three zones A, B and C in the stress-strain diagram. 

A second clear result from the present work is the influence of phase separation on the constitutive response. Evidence for some phase separation was seen in SAXS for all the materials studied. But in the PTHF-based materials it was particularly pronounced (as evidenced by SAXS peak intensity). Inelasticity as measured by hysteresis and unrecovered strain was reduced by increased phase separation. 

First cycle work input, unrecovered strain and hysteresis... 

The results presented above reveal a dramatic difference in response between first loading to a given deformation, and second and subsequent loadings. The model thermoplastic PUs studied in this work showed large, systematic variations in first loading response— as quantified by tensile modulus, work input, hysteresis and unrecovered strain—depending on their chemical structure and molecular packing. 

Recovery curves from creep under constant stress are also illustrated in Figure 4.2, which indicates that at any selected time the extent of recovery (and so the unrecovered strain) is directly proportional to the stress that had been applied formerly. The relation between recovery and creep will be derived in Section 4.2.1. 

Constant-strain tests are more popular and simpler to use. In this case, sample discs of rubber are compressed between two polished steel plates to a predetermined strain by using spacers and bolts. The permanent set is usually quoted as the unrecovered percentage of the applied deformation, and is not related to the original sample height. The BS 903 rubber test specifies a cylindrical test piece 13 mm in diameter and 6.3 mm high. The sample dimensions are especially important in compression due to the shape factor effect. Recovery time before measurement of final set value is usually 30 min., and must be specified as this creep recovery is time-dependent. 

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