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A study has been made of the strength characteristics of machined glass‐ceramic surfaces using an indentation flaw technique. The strength is found to decline or increase with progressive removal of the machining damage layer by a polishing procedure, depending on whether the indentation is made after or before the polishing. The results are interpreted in terms of a residual compressive stress in the initial machined surface. It is argued that failure to allow for the contribution of this stress in the fracture mechanics description of flaw growth could lead to significant errors in the evaluation of crack propagation parameters.

A two-part study has been made of the fatigue characteristics of brittle solids using controlled indentation flaws. In this part a general theory is developed, with explicit consideration being given to the role played by residual contact stresses in the fracture mechanics to failure. The distinctive feature of the formulation is a stress intensity factor for well-defined indentation cracks, suitably modified to incorporate the residual component. Taken in conjunction with a standard power-law crack velocity function, this leads to a differential equation for the dynamic fatigue response of a given material/ environment system. Reduced variables are then introduced to facilitate generation of "universal" fatigue curves, determined uniquely by the crack velocity e'xponent, n. A scheme for using these curves to evaluate basic fracture parameters from strength data is outlined. In this way the foundation is laid for lifetime predictions of prospective brittle components, as well as for reconstruction of the crack velocity function. One of the major advantages of the analysis is the manner in which the residual stress parameters are accommodated in the normalized fracture mechanics equations: whereas it is understood that all strength data are to be taken from test pieces in their as-indented state, so making it unnecessary to have to resort to inconvenient stress-removal procedures between the contact and failure stages of testing, a priori knowledge of the residual stress level is not required. The method is proposed as an economical route to materials evaluation and offers physical insight into the behaviour of natural flaws.

The inert strength and dynamic fatigue properties of fusedsilica optical fibers are studied using subthreshold indentation flaws, i.e., flaws without radial cracks. These subthreshold properties differ from those obtained in comparative tests on silica rods containing postthreshold indentation flaws in three major respects: (I) the inert strengths are significantly higher than predicted by extrapolation of the postthreshold data;(2) the slopes of the dynamic fatigue plots are likewise greater, indicating a greater susceptibility of the subthreshold flaws to chemical kinetic effects; and (3) the scatter in strengths is wider. These trends reflect the change in mechanical response reported for optical fibers with "natural" flaw populations in going from ordinary to ultra-high-strength regions. Direct observations of the indentation sites up to the point of failure indicate that the property differences can be interpreted in terms of a transition from propagation-controlled to initiationcontrolled fracture instabilities at reduced contact loads. The subthreshold instability condition is modeled qualitatively as a two-step, deformation-fracture process, with strong emphasis on the importance of residual stress fields in parametric evalnations. The relevance of the results to the practical issue of fiber reliability, most notably in connection with the potential dangers of using macroscopic crack velocity data to predict long-lifetime characteristics, is addressed.