Environmentally Influenced Mixed Mode Fatigue Crack Propagation of Titanium Metal Matrix Composites
- PDF / 1,496,746 Bytes
- 7 Pages / 594 x 774 pts Page_size
- 54 Downloads / 234 Views
I.
INTRODUCTION
UNIDIRECTIONALLY reinforced titanium metal matrix composites have long been considered for applications in dynamic structures. ~'2 However, little work is reported on some important properties such as mixed mode fracture and fatigue crack propagation (FCP) behavior of these composites. Because of the inherent anisotropy and the presence of fiber-matrix interfaces, understanding mixed mode fracture behavior of such composites is essential. Studies were, therefore, aimed at the mixed mode behavior of the asreceived composites and the results have been reported elsewhere.3 It was observed that the mixed mode fatigue crack propagation of the as-received BnC-B/Ti-6A1-4V composites was self-similar, i.e., along the initial notch and parallel to the fiber orientation at all the fiber orientations.3 The fiber orientation was the weakest path for the crack growth. It was shown that the mixed mode FCP for the composite can be expressed by da/dN~(AG) m
m ~ 3
for a particular fiber orientation, and can be expressed by da/dN~[AG/GF
(0)] '~
for all the fiber orientations where GF(O) represents the failure strain energy release rate for the composite which was analytically estimated with the help of a simple linear elastic fracture mechanics model. ~'4 The influence of gaseous environments on the FCP of the longitudinal BaC-B/Ti-6A1-4V specimens was also studied.3'4 It was shown that the humid air enhanced the FCP rates of the composites considerably? It has been documented that a Ti-6A1-4V alloy showing similar microstructure as the matrix alloy in this investigation does not show appreciable difference in the stage II FCP rate for dry and wet air at R = 0.1.5 The enhancement of the FCP rates in humid air for the longitudinal B4C-B/Ti-6A1-4V composite was thereDEEPAK MAHULIKAR is a Research Scientist with Olin Metals Research Laboratories, New Haven, CT 06511. H.L. MARCUS is Harry L. Kent, Jr. Professor of Mechanical Engineering, Department of Materials Science and Mechanical Engineering, The University of Texas, Austin, TX 78712. Manuscript submitted April 5, 1982. METALLURGICALTRANSACTIONS A
fore an interesting observation. The composite system has brittle boron filaments which contribute to the FCP behavior. It has been suggested 6 that humid air weakens the boron filaments in aluminum-boron composites similar to other ceramics. Boron is very much susceptible to humidity embrittlement. Considerable increase in FCP rates was observed for the A1/B composites when humid air was introduced in the vacuum chamber.6 Since the fiber matrix interfaces and the boron fibers can provide embrittlement sites for the humidity, it is important to study the effect of humidity on the mixed mode FCP behavior of the titanium metal matrix composites. The titanium metal matrix composite system consists of brittle boron filaments and ductile Ti-6A1-4V matrix. This presents a unique situation for the fatigue crack closure effect. Fatigue crack closure has been studied extensively for isotropic materials. 7-~2 During the lower
Data Loading...
