John E. Srawley

Bio: John E. Srawley is an academic researcher from Glenn Research Center. The author has contributed to research in topics: Fracture mechanics & Stress intensity factor. The author has an hindex of 13, co-authored 15 publications receiving 2612 citations.

Wide range stress intensity factor expressions for ASTM E 399 standard fracture toughness specimens

Abstract: For each of the two types of specimens, bend and compact, described previously for plane strain fracture toughness of materials, E 399, a polynominal expression is given for calculation of the stress intensity factor, K, from the applied force, P, and the specimen dimensions. It is explicitly stated, however, that these expressions should not be used outside the range of relative crack length, a/W, from 0.45 to 0.55. While this range is sufficient for the purpose of E 399, the same specimen types are often used for other purposes over a much wider range of a/W; for example, in the study of fatigue crack growth. Expressions are presented which are at least as accurate as those in E 399-74, and which cover much wider ranges of a/W: for the three-point bend specimen from 0 to 1; and for the compact specimen from 0.2 to 1. The range has to be restricted for the compact specimen because of the proximity of the loading pin holes to the crackline, which causes the stress intensity factor to be sensitive to small variations in dimensions when a/W is small. This is a penalty inherently associated with the compactness of the specimen.

Stress-Intensity Factors for Single-Edge-Notch Specimens in Bending or Combined Bending and Tension by Boundary Collocation of a Stress Function

Abstract: : A boundary-value-collocation procedure was used in conjunction with the Williams stress function to determine values of the stress-intensity factor K for single edge cracks of various depths in specimens subjected to pure bending. The results are of use in connection with K(sub Ic) fracture toughness tests, which utilize rectangular-section crack-notch beam specimens loaded in four-point bending, and are in good agreement with published results derived from experimental compliance measurements. The results are expressed in convenient, compact form in terms of the dimensionless quantity Y(exp 2)=K(exp 2)B(exp 2)W(exp 3)/M(exp 2), which is a function of relative crack depth a/W only, where B and W are the specimen width and thickness and M is the applied bending moment. On the assumption that the condition for a valid K(sub Ic) test is that the maximum nominal stress at the crack tip should not exceed the yield strength of the material, the K(sub Ic) measurement capacity of bend specimens was estimate as a function of a/W. The measurement capacity is proportional to the yield strength and to the square root of the specimen depth, and it is greatest for a/W in the range 0.2 to 0.3. Values of K for single-edge-notch specimens subjected to combined bending and tension were obtained by superposition of the present results and those of earlier work for specimens loaded in uniform tension. These values are of interest in connection with the use of single-edge-notch specimens that are off-center pin-loaded in tension. It is shown that the K(sub Ic) measurement capacity of such specimens is not very sensitive to the eccentricity of loading.

Compliance and stress intensity coefficients for short bar specimens with chevron notches

Abstract: For the determination of fracture toughness especially with brittle materials, a short bar specimen with rectangular cross section and chevron notch can be used. As the crack propagates from the tip of the triangular notch, the load increases to a maximum then decreases. To obtain the relation between the fracture toughness Kic and maximum load Pmax, calculations of Srawley and Gross for specimens with a straight-through crack were applied to the specimens with chevron notches. For the specimens with a straight-through crack, an analytical expression was obtained. This expression was used for the calculation of the Kic−Pmax relation under the assumption that the change of the compliance with crack length for the specimen with a chevron notch is the same as for a specimen with a straight-through crack.

Mechanical properties of solid polymers

Abstract: A concise, self-contained introduction to solid polymers, the mechanics of their behavior and molecular and structural interpretations. This updated edition provides extended coverage of recent developments in rubber elasticity, relaxation transitions, non-linear viscoelastic behavior, anisotropic mechanical behavior, yield behavior of polymers, breaking phenomena, and other fields.

The Mechanical Design of Nacre

Abstract: Mother-of-pearl (nacre) is a platelet-reinforced composite, highly filled with calcium carbonate (aragonite). The Young modulus, determined from beams of a span-to-depth ratio of no less than 15 (a necessary precaution), is of the order of 70 GPa (dry) and 60 GPa (wet), much higher than previously recorded values. These values can be derived from ‘shear-lag’ models developed for platey composites, suggesting that nacre is a near-ideal material. The tensile strength of nacre is of the order of 170 MPa (dry) and 140 MPa (wet), values which are best modelled assuming that pull-out of the platelets is the main mode of failure. In three-point bending, depending on the span-to-depth ratio and degree of hydration, the work to fracture across the platelets varies from 350 to 1240 J m -2 . In general, the effect of water is to increase the ductility of nacre and increase the toughness almost tenfold by the associated introduction of plastic work. The pull-out model is sufficient to account for the toughness of dry nacre, but accounts for only a third of the toughness of wet nacre. The additional contribution probably comes from debonding within the thin layer of matrix material. Electron microscopy reveals that the ductility of wet nacre is caused by cohesive fracture along platelet lamellae at right angles to the main crack. The matrix appears to be well bonded to the lamellae, enabling the matrix to be stretched across the delamination cracks without breaking, thereby sustaining a force across a wider crack. Such a mechanism also explains why toughness is dependent on the span-to-depth ratio of the test piece. With this last observation as a possible exception, nacre does not employ any really novel mechanisms to achieve its mechanical properties. It is simply ‘well made’. The importance of nacre to the mollusc depends both on the material and the size of the shell. Catastrophic failure will be very likely in whole, undamaged shells which behave like unnotched beams at large span-to-depth ratios. This tendency is increased by the fact that predators act as ‘soft’ machines and store strain energy which can be fed into the material very quickly once the fracture stress has been reached. It may therefore be advantageous to have a shell made of an intrinsically less tough material which is better at stopping cracks (e. g. crossed lamellar). However, nacre may still be preferred for the short, thick shells of young molluscs, as these have a low span-to-depth ratio and can make better use of ductility mechanisms.