Treatment of Strain Data: Measurements on Plastics and Composites

Treatment of Strain Data
When strain measurements are completed, it is often necessary to adjust the data for known and correctable errors, if present, prior to data reduction for principal strains and stresses. Typical corrections, irrespective of the test material, may include those for transverse sensitivity, thermal output, gage factor variation with temperature, and, for large strains, Wheatstone bridge nonlinearity and gage factor variation with strain. Of these, correction for thermal output is potentially the most important since the other errors are normally small compared to the intrinsic uncertainties involved in strain measurement on plastics and composites.

Isothermal creep properties of polypropylene (homopolymer) at 20 deg C (68 deg F). (Ref. 16 )
If the test material is a plastic with a low elastic modulus, there can also be a sizable error due to reinforcement by the strain gage. But post-correction for this error obviously requires previous calibration of the reinforcement effect for the subject combination of test material and type of strain gage. An alternate approximate method is to measure the mechanical properties of the material with the same type of strain gage, and employ the resulting "apparent" properties in the data reduction process ( Ref. 15 ). Problems with a low-modulus thermoplastic can become particularly severe if the material creeps perceptibly during strain measurement. This condition can sometimes be alleviated by simply scaling down the system of applied loads until the creep rate is negligible. In other cases when long-term static loading is involved, it is usually necessary to obtain creep data over the full ranges of stress, time, and temperature relevant to the product application. An example, shown above, illustrates the creep properties of a low-modulus thermoplastic at a single temperature ( Ref. 16 ). This type of information, repeated at other temperatures as appropriate, is obviously required before any rational interpretation can be made of the strains (or strain rates) observed on real test objects. It should also be noted that creep measurements on low-modulus plastics can be significantly affected by strain gage reinforcement effects ( Ref. 17 ).

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