The inability of polymer chains to deform the required amount within the time allowed by a stress‐to‐failure event results in brittle failure. Factors related to brittle behavior of films were investigated. By both conventional and novel testing techniques, stress–time–temperature relationships were studied to determine the brittle point. From high‐speed tensile tests at strain rates up to 106%/min., the critical strain rate (incipient brittleness as indicated by failure accompanied by a marked decrease in ductility) was determined for representative films. High‐speed motion pictures (6,000 frames/sec.) of film failure in rice bag drop tests, were used to ascertain the time of deformation to be related to brittleness. Deformation rates were calculated to be of the order of 130 to 230 × 103%/min.
A system is described for automatic collection of stress–strain data for polymeric films on magnetic tape for subsequent automatic data reduction and computation of a variety of stress–strain parameters. Two shaft analog‐to‐digital encoders attached to a conventional tensile tester feed digital information to an incremental pulse recorder which stores all stress and strain data in two channels on 0.625‐in. magnetic tape. Specimen information and test constants are inserted by the operator through a keyboard located at the tensile tester. The tape is subsequently fed into a standard digital computer through a special incremental pulse magnetic tape reader. The data pulses are internally stored and converted by means of a special program into eight significant stress–strain parameters for polymeric film samples, plus averages and standard deviations for groups of replicate determinations. These parameters include: corrected length of specimen, initial maximum tensile modulus, offset yield stress, strain at break, strain at maximum stress, maximum tensile stress, tensile stress at break, and work to break. While the program described applies to polymer film testing, both it and the digital recording equipment accessories are applicable with suitable modification, to any analog experimental data. Significant savings in man‐hours and increased information output result from use of this system.
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