Constant-Rate-of-Strain Tests

Splash cover Hand wheel for adjustment Stirring motor 

FIGURE 10.2 Mechanical failure tests (a) tensile test on dumbbell specimen (b) tensile test on ring specimen (c) notched crescent and trousers tear specimens (d) Izod impact test machine (Data from HaUinan, A. J., Jr., J. Qual. Tech., 25, 85, 1993) (e) motor-driven brittleness temperature tester. (Data from Book of Standards, ASTM, Baltimore, MD, 1958, 286,356.) 

Molded, die-cut, or lathe-cut rings can be used for rubbery samples. The rate of strain is uniform, so that the stress-time record is also a stress-strain record. With care the strength measured on ring samples is the same as that on dumbbells. However, stress concentration at the holder is difficult to avoid even when the holder is lubricated or rotated. Also, the initial portion of the stress-strain curve may be distorted because the stress needed to deform the ring into an oval may mask the stress due to uniform elongation. 

Tests with fibers offer another complication. With the uniform cross section, any local yielding or necking is likely to start at the holders and the effective length of the sample may be increased by jaw penetration. Long samples or multiple samples wound around spools at either end will minimize the uncertainty in length. 

We have pointed out that the extension at break for a glassy sample often is so small that it becomes hard to measure. In a flexural test, elongation occurs on one side of the sample and compression on the other. A small elongation at break corresponds to a large flexural deflection, which is easily measured. The simple beam with sliding supports is used most often. 

---

Instead of constant rate of traverse, it would be perfectly feasible to use a constant rate of strain testing machine for rubbers, but this complication and expense has never been considered worthwhile. 

Consolidation Oedometer test, Constant Rate of Strain test 1... 

---