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ASTM Selected Technical Papers
Fatigue Mechanisms
By
JT Fong
JT Fong
1
Editor, and Chairman of Symposium Organizing Committee
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ISBN-10:
0-8031-0345-X
ISBN:
978-0-8031-0345-0
No. of Pages:
923
Publisher:
ASTM International
Publication date:
1979

This paper deals with an examination of fatigue micromechanisms present in polymers of various chemistry and microstructure. At low stress-intensity (ΔΚ) levels in several amorphous homopolymers, poly (methymethacrylate) (PMMA), polycarbonate (PC), poly (vinyl chloride) (PVC), polystyrene (PS), and polysulfone (PSF), each with viscosity-average molecular weight (M¯v) in the range 1 × 105 < Mv < 4 × 105, the macroscopic and microscopic appearances of the fracture surface were observed. Observations were also made in both rubber-modified amorphous polymers such as an acrylonitrile-butadiene-styrene (ABS) copolymer and a rubber-toughened PS-poly (phenylene oxide) blend, and the crystalline polyacetal (PA). Regardless of the degree of surface roughness, the crack front in all these materials advanced discontinuously in increments equal to the bandwidth after remaining stationary for as many as 105 fatigue cycles (in PA). In single-phase amorphous polymers, the discontinuous growth process was identified with the failure of a single craze ahead of the crack tip. At high ΔΚ levels the fracture surfaces of most polymers were striated, with one striation representing the increment of crack advance during one load excursion. It is concluded that large errors can be made in estimating the overall fatigue life of engineering plastics if the significance of the microscopic bands observed at low ΔΚ and the striations observed at high ΔΚ as fatigue fracture markings is misinterpreted.

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