Plastics & Composites as Mechanical Test Materials: Measurements on Plastics and Composites

Plastics and Composites as Mechanical Test Materials

In terms of experimental stress analysis, one of the more significant differences between plastics and metals is certainly in the basic mechanical properties such as the modulus of elasticity. As illustrated below, elastic moduli for plastics are often two or more orders of magnitude lower than those for metals. A practical consequence of this difference is that measured strains on plastic test objects tend to be considerably larger than on metals, and can readily exceed 1 percent. The relatively high-elongation strain measurements in such cases can place special demands on strain gage bonding and wiring procedures.

Order-of-magnitude comparison of room-temperature and elastic modulus -- selected materials versus 1018 steel. [ (1)T300/5208 graphite/epoxy; (2) Scotchply1002 glass/epoxy; (Subscripts 1, 2 denote fiber and transverse directions, respectively). (Ref. 3 ) ].
Another potentially significant effect of the lower elastic modulus -- and one that is less amenable to user alleviation -- is that the presence of the installed strain gage may, at least locally, reinforce the test material. Perhaps indicative of the state of the art in strain measurement on plastics is the dearth of published data on reinforcement effects. There are indications, nevertheless, that reinforcement by the gage can lead to very large measurement errors, particularly for the lower-modulus plastics. (Ref. 1 and 2 )

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