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Previous work established that tensile stress cycling of fibrous glass reinforced resin composites degrades their macroscopic mechanical properties by inducing microcracks in the cross-linked glassy polymer matrix. While the greatest damage occurs in the first stress cycle, it continues to accumulate during subsequent loadings, and composite modulus reductions of 20 to 25 percent after 100 to 200 cycles are common. Detailed studies of unrein-forced matrices indicate that much of the work required to drive a crack is consumed by molecular cold drawing and partial orientation in the glassy material at the crack tip. Further, this work can be increased substantially by suspending small particles of certain elastomers throughout the cross-linked matrix material; increases in the fracture surface work term of an order of magnitude, or more, are possible. When such toughened epoxies or polyesters are reinforced with fibrous glass and then stress cycled, the internal microcracking previously observed in the composite material is eliminated. The degree of toughening conferred by the elastomer particles is related to their average size, the distribution of sizes, their composition, the composition of the matrix, the volume fraction of elastomer present, and the adhesion between particles and matrix. Recent work has revealed that other, additional factors are important. These latter include the initial molecular weight of the elastomer, the nature of elastomeric copolymers which can be used, the conditions of plane stress or plane strain under which toughness measurements are made, the detailed nature of the catalyst used to harden the glassy matrix, and the morphological features of the particles. All of these factors will be described and their significance discussed.
composites, cracking (fracturing), glass fibers, fiber composites, mechanical properties, microcracks, reinforced plastics
McGarry, F. J.
Professormember ASTM, Massachusetts Institute of Technology, Cambridgs, Mass.
Sultan, J. N.
Research assistant, Massachusetts Institute of Technology, Cambridgs, Mass.