Fracture of Polymers Under Repetitive Loading
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FRACTURE OF POLYMERS UNDER REPETITIVE LOADING J.A. SAUER AND C.C. CHEN* Rutgers University, Department of Mechanics and Materials Science, P.O. Box 909, Piscataway, N.J. 08854 *Now at Ethicon Inc., Somerville, N.J. ABSTRACT The fracture surface morphology of various polymer specimens, fractured under alternating loading, has been examined by scanning electron microscopy. Illustrations are given of both typical and unusual fracture surfaces and the influence of such variables as composition, molecular weight, presence of a dispersed second phase and induced thermal effects is discussed. INTRODUCTION Under repetitive loading at a sufficiently high stress amplitude, but at a maximum stress that is well below the yield stress, all polymers, whether ductile or brittle under simple tension, develop a microcrack after some number of cycles and subsequently fail in a macroscopically brittle manner by increasingly rapid crack propagation. However, examination of both external surfaces and fracture surfaces show that crack initiation and crack propagation is preceded and accompanied by localized plastic deformation. Frequently this is craze deformation, induced by the stress concentrating effect of surface flaws or defects. Depending on the material and the particular test conditions, localized shear yielding may also be involved. In recent years, as polymers have seen increasing, use as structural materials, the study of fatigue in polymers has received considerable attention and reviews of recent research results are available [1-5]. In the present paper we are concerned primarily with the morphology and micromorphology of fatigue fracture surfaces. Various examples of fatigue fracture morphology in both homogeneous polymers and in 2-phase polymers are presented and discussed. MATERIALS AND PROCEDURE Fatigue tests have been carried out on many different polymers, including polystyrene (PS), medium and high impact polystyrenes (MIPS and HIPS), styrene acrylonitrile polymers of varying molecular wieght (SAN), acrylonitrile-butadiene-styrene (ABS), extruded and cast polymethylmethacrylate (E-PMMA and C-PMMA), and PMMA's containing various amounts of sorbed water. The fatigue tests, unless otherwise noted, were carried out at a controlled stress amplitude using an Instron servohydraulic testing machine. Tests were made at various frequencies ranging from 0.2 to 21 Hz. The specimen temperature rise due to hysteresis effects was monitored by means of a Barnes infrared sensor. The fatigue fracture surfaces were examined by an Etec scanning Detailed data concerning cycles to craze initiaelectron microscope (SEM). tion, cycles to fracture and induced thermal effects have been presented elsewhere 15-91. RESULTS
AND DISCUSSION
Unmodified polymers The fatigue fracture surface of many polymers tends to have certain Mat- Res. Soc. Symp. Proc. Vol. 79. 1987 Material, Research Society
372
Fig. 1
SAN
31 MPa, 21Hz
Fig. 2
E-PMMA
41.4 MPa, 2 Hz
characteristic features. Generally, there are two distinct regions on the fracture surfa
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