STP185

    The Effects of Speed in the Mechanical Testing of Plastics Testing

    Published: Jan 1956


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    Abstract

    The mechanical test behavior of most plastics materials is so complex that it is necessary to divide the subject into reasonably natural categories to discuss it effectively. For this purpose, a primary division between destructive and nondestructive tests may be made, and each of these divisions, in turn, may be further divided on the basis of the time scale of the test concerned. In both primary divisions there exist tests which may be termed long-time, short-time, and dynamic. Obviously this second categorization into three sub-groups is not so absolute as the first. The concept is felt to be qualitatively valid, however, and will be pursued here. The reason why plastics are time-dependent or sensitive in their mechanical behavior can be found in their molecular structure (1). Unlike crystalline materials, plastics rarely possess any repetitive, ordered geometrical pattern of structure. The long chain molecules describe tortuous, three-dimensional paths throughout the bulk of the material, resulting in a thoroughly tangled and entwined structure. When such an affair is loaded, it can deform in three ways: (1) primary valence bond deformation, (2) valence bond rotation, and (3) displacement of a chain segment with respect to its neighbors. All of these three mechanisms can be initiated simultaneously, but the latter two require a much longer time, comparatively, for their effects to be evident, despite the fact that their magnitudes greatly overshadow the valence bond deformation effect if the load is allowed to act for an appreciable interval. As a consequence, the externally measured deformation of a plastic specimen consists of the summation of the three effects, the individual parts of which are not easily dissociated and identified. The existence of a distribution of relaxation mechanisms with distinctive time scales further complicates the situation. Add to this the fact that the fracture strengths of many plastics depend upon the rate of deformation or load application employed, and it becomes obvious that the only practical way to assess the time influence on mechanical properties is to observe those properties under the rate conditions of interest. A review and discussion of the various methods for so doing, and the results which they reveal, constitute the balance of this paper.


    Author Information:

    Dietz, A. G. H.
    Professor of Building Engineering and Construction and Chairman of Plastics Committee, and Assistant Professor of Materials, Massachusetts Institute of Technology, Cambridge, Mass.

    McGarry, F. J.
    Professor of Building Engineering and Construction and Chairman of Plastics Committee, and Assistant Professor of Materials, Massachusetts Institute of Technology, Cambridge, Mass.


    Paper ID: STP46841S

    Committee/Subcommittee: E01.02

    DOI: 10.1520/STP46841S


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