Strain offset

The effect of cold working by cold rolling of sheet on the yield, at 0.2% offset strain, and tensile strengths of copper sheet is shown in Figure 1. 

Elements that can dissolve in copper, such as zinc, tin, and nickel for example, increase annealed strength by varying amounts depending on the element and the quantity in solution. The effect of selected solution hardening elements on tensile properties of annealed copper aUoys is iUustrated by the data in Table 4, where the yield strength is the stress at 0.2% offset strain in a tensile test. 

Fig. 2-11 Example of determining yield point and offset strain.

There are a number of indicators of fatigue damage that have attracted interest in the literature. During the life of a component subjected to fatigue, the material can exhibit changes in modulus, permanent offset strain, shape of the hysteresis loops, and temperature rise of the specimen surface. Direct evidence of matrix crack density can be obtained by surface replication, while a more detailed analysis of microstructural damage requires scanning electron microscopy (SEM). 

Fig. 6.14 Experimental (coarse lines) and predicted hysteresis loops (fine lines) for unidirectional SiQ/CAS. Tlie predictions, which assume partial slip along the fiber-matrix interfaces, were based upon a residual compressive stress in the fibers of 50 MPa and values of r of 8, 2 and 1 MPa. The loop width and the permanent offset strain increase as t decreases. After Pryce and Smith.54...

In this case, the permanent offset strain is independent of the maximum stress level,7... 

Equation (8) predicts that the permanent offset strain will decrease as r decreases. 

Given the yield strength from step 6 and the elastic modulus from step 7, calculated with Eq 2 the v ue of the SED offset strain, 0.06 % (Table 3). 

All but four of the fifty specimens have yield strengths and offset strains calculated with the SED method that are equal to or lower than the values from the 0.2 % offset method. In the three environments (dry/cold, moderate, and humid/hot) the average yield strengths are ... 

Table 6 shows the tensile properties of six adhesives in the diy/cold and humid/hot environments. All adhesives were tested at a 10" /s strain rate. On average, the time to rupture was 6 min in the diy/cold environment and 9 min in the humid/hot environment. The tensile properties of adhesive FM 300K are averages for replicate specimens (Table 5). For the other adhesives, only one specimen was tested per environment. Nine of the twelve specimens have yield strengths and offset strains calculated with the SED method that are equal to or lower than the values from the 0.2 % offset method.

On a stress-strain curve there can be a location at which an increase in strain occurs without any increase in stress. This represents the yield point that is also called yield strength or tensile strength at yield. Some materials may not have a yield point. Yield strength can in such cases be established by choosing a stress level beyond the material s elastic limit. The yield strength is generally established by constructing a line to the curve where stress and strain is proportional at a specific offset strain. 

Figure 2.54. Illustration of a true stress vs. strain curve and comparison of stress-strain curves for various materials. UTS = ultimate strength, and YS = yield strength. The tensile strength is the point of rupture, and the offset strain is typically 0.2% - used to determine the yield strength for metals without a well-defined yield point.Reproduced with permission from Cardarelli, F. Materials Handbook, 2nd ed.. Springer New York, 2008. Copyright 2008 Springer Science Business Media.

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