A general method for determining mixed-mode stress intensity factors from isochromatic fringe patterns

Dynamic characterization of brittle fracture is possible by relating the instantaneous stress-intensity factorK(t) to the velocity of propagation of the crack. High-speed photographic systems are employed with photoelastic methods to obtain a sequence of isochromatic-fringe patterns representing the state of stress associated with the propagating crack. Methods for determiningK(t) from these isochromatic patterns are reviewed.

Dynamic photoelastic studies of fracture

Two-parameter methods of fracture analysis for determining the stress-intensity factor from photoelastic isochromatic-fringe data were critically reviewed. The methods of Irwin, Bradley and Kobayashi, and Smith were developed in detail and differences in the three approaches were noted. Theoretical fringe loops were generated for a crack of length 2a in a semi-infinite plate with biaxial loading. These fringe loops were used to compare the three analysis methods and to determine the accuracy of each method.

A critical review of methods for determining stress-intensity factors from isochromatic fringes

Controlled semi-elliptical surface flaws were produced in hot-pressed SiC by Knoop microhardness indentation. Flawed specimens were placed in 4-point bending in order to determine their critical stress intensity factor, KIC, at both room and high temperatures. Room-temperature fracture and KIC values after annealing were sensitive to the annealing environment; this behavior correlated with the active/passive nature of the oxidation process. Flaw healing was observed for annealing exposures in air. Room-temperature KIC values increased with increasing annealing temperature. High-temperature KICvalues decreased with increasing temperature as a result of a decrease in the fracture surface energy.

Controlled Surface Flaws in Hot‐Pressed SiC

Stress intensity factors for corner cracks emanating from fastener holes under tension

A series of tests was performed using the stress freezing and slicing technique of photoelasticity in order to obtain the stress fields near points of maximum flaw penetration and where the crack border intersects the tensile surface of the plate for naturally grown surface flaws under fields of remote cylindrical bending. Geometrics investigated ranged over crack depth to crack length ratios of approximately 0.13 to 0.55 and over crack depth to plate thickness ratios of approximately 0.27 to 0.79. Results were compared with several approximate theories on the basis of graphs of normalized maximum in-plane shear stress versus normalized distance from the crack tip and stress intensity factor in order to assess the validity of the several field theories. A numerical technique for data conditioning was employed from which the stress intensity factors were computed and compared with theoretical values.

Local stresses near deep surface flaws under cylindrical bending fields

A series of stress freezing photoelastic experiments were conducted on large plates containing central holes with cracks emanating from the edge formed by the intersection of the hole with the plate surface. Loads were applied through rigid pins with neat fits in the holes. Stress-intensity factors (SIF) were estimated by a computer assisted least squares analysis of the photoelastic data taken from slices near the points of intersection of the flaw border with the hole boundary and the plate surface. Results indicate that the local mode of loading changes from Mode 1 near the hole boundary to mixed mode near the plate surface. The analysis is extended to include mixed mode loading, and results are compared with an existing approximate theory.

Stress Intensities for Cracks Emanating from Pin-Loaded Holes

The stress intensity factors (SIFs) at the end points of flaws emanating from the corner formed by the intersection of a plate with a hole were determined using stress freezing photoelasticity and a numerical technique known as the Taylor series correction method to extract the SIF values from the photoelastic data. The geometries studied were crack depth to thickness ratios of about 0.2, 0.5, and 0.75; crack depth to crack length ratios of about 1.0 to 2.0; and crack length to hole radius ratios of about 0.5 to 2.0. The SIFs were determined at the intersection of the flaw border with the plate surface (KS) and with the edge of the hole (KH). It is shown that extension of a crack emanating from a corner of intersection of a hole with a plate under monotonically increasing load is not self-similar and that as the flaw depth increases, KH decreases and KS increases. Existing theories and design criteria significantly overestimate the SIF at both the hole and the surface except for shallow flaws at the hole and deep flaws at the surface.

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C. W. Smith

Papers

Local stresses near deep surface flaws under cylindrical bending fields

Stress Intensities for Cracks Emanating from Pin-Loaded Holes

Stress intensity factors for deep cracks emanating from the corner formed by a hole intersecting a plate surface