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The Bueche-Halpin theory for the fracture of viscoelastic bodies is extended to predict the statistical variability of rupture data for both uniform and nonuniform excitation histories. The concept of cumulative damage is examined in light of some critical experimentation. It is shown that the geometry of the distribution is a sensitive functional of the excitation history and that the solution of this problem is the key step in the development of a general theory for fatigue. (Author).
Technical reports published by the Air Force Materials Laboratory during the period 1 January 1967-31 December 1967 are abstracted herein and indexed by branches of the laboratory, technical subject matter, investigator, project monitor and contractor. Reports on research conducted by the Air Force Materials Laboratory personnel as well as that conducted on contract are included.
A theory is developed for predicting the time-dependent size and shpae of cracks in linearly viscoelastic, isotropic media, and its validity is demonstrated by applying the theory to crack growth and failure of unfilled and particulate-filled polymers. Starting with a bounded solution for the stress distribution near a crack tip in an elastic body and the extended correspondence principle for viscoelastic media with moving boundaries, a simple equation is derived for predicting instantaneous crack tip velocity in terms of the opening-mode stress intensity factor; although the undamaged portion of the continuum is assumed linear, no significant restrictions are placed on the nature of the disintegrating material near the crack tip and, therefore, this material may be highly nonlinear, rate- dependent, and even discontinuous. A further analysis is made to predict the time at which a crack starts to grow, and then some explicit solutions are given for this so- called fracture initiation time, the time- dependent crack growth, and the time at which gross failure occurs under time- varying applied forces and environmental parameters. Following a derivation of the linear cumulative damage rule, an examination of its theoretical range of validity, and a discussion of the experimental determination of fracture properties, the theory is applied to monolithic and composite materials under constant and varying loads. Some concluding remarks deal with extensions of the theory to include finite strain effects, crack growth in the two shearing modes and in combined opening and shearing modes, and adhesive fracture. (Author-PL).
Equations are developed for predicting opening and sliding modes of crack growth along planes of geometric symmetry in viscoelastic orthotropic media with and without large prestrains. Except for the small zone of failing material at the crack tip, the body is assumed to be linearly viscoelastic with respect to the changes in stress and strain which occur during crack propagation. Results obtained previously for isotropic media are first reviewed, and then a relation is derived for predicting crack tip velocity in an orthotropic body in plane strain; included is a numerical example using constitutive properties of a fiber-reinforced plastic. Extension to fracture of prestrained media and crack growth between certain types of dissimilar media is then made. Finally, it is argued that these crack velocity relations can be employed in plane stress problems and in many three-dimensional cases, and, with a small change, can be used to predict the time of fracture initiation. (Author).