The microdeformation and damage mechanisms in glass fiber reinforced reaction injection molded (RRIM) polyurethane composites have been studied. Two surface conditions represented the extremes in good and poor fiber-matrix adhesion. Deformation mechanisms observed directly by an SEM technique with the specimen deformed in three-point bend were correlated with the number and amplitude distribution of acoustic emission (AE) events detected during tensile testing. Yielding of the unreinforced resin was accompanied by microcracking initiated at holes that produced AE with a peak at 43 db. A similar AE pattern observed in the untreated fiber composites was also identified with microcracking at holes in the resin and at rod-shaped holes produced by debonded fibers. The amine-treated fiber composite exhibited considerably more AE activity in the yield and post-yield regions with additional peaks in the amplitude distribution at 59 db and 65 db. These were identified with microcracking at the well-bonded interface and fiber fracture respectively.

SYNOPSISMicrocrazing in the struts of flexible polyurethane foams was discovered during compressive deformation and observed directly in the scanning electron microscope. Attributed to this phenomena was the decrease in stress at maximum compression and the intensity of acoustic emission during compressive cycling. The higher content of styrene-acrylonitrile ( SAN ) copolymer in these foams resulted in higher modulus, more severe microcrazing, an increase in acoustic emission activity, and a decrease in the stress at maximum compression as cycling progressed.

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SYNOPSISHard elastic behavior is characterized by high porosity and high recoverability from large strain, and initial Hookean elasticity was discovered in polyurethane foams containing styrene-acrylonitrile ( SAN) copolymer particles. The presence of SAN particles introduces a heterogeneous morphology, and when the foam was strained in the SEM, it was observed that the struts became highly porous with profuse voiding nucleated by the SAN particles. It was found that these flexible polyurethane foams had a similar morphological structure in the strained struts as did the typical hard elastic materials. The phenomenon of stress depression, when foam specimens under stress were subjected to nonswelling liquids, was utilized to probe the role of surface stress in these hard elastic foams. An analytic methodology established for other highly porous hard elastic materials based on stress depression was utilized to obtain the average distance between voids in the struts. The calculated values were in good agreement with direct scanning electron microscopy observations, confirming that voiding initiated at the boundaries of SAN particles.