William F. Brown

Bio: William F. Brown is an academic researcher from Glenn Research Center. The author has contributed to research in topics: Plane stress & Compact tension specimen. The author has an hindex of 4, co-authored 4 publications receiving 1612 citations.

Fracture toughness testing

Abstract: : A comprehensive survey is presented of current methods of fracture toughness testing that are based on linear elastic fracture mechanics. General principles are discussed in relation to the basic two-dimensional crack stress field model and in relation to real three-dimensional specimens. The designs and necessary dimensions of specimens for mixed mode and opening mode (plane strain) crack toughness measurement are considered in detail. Methods of test instrumentation and procedure are described. Expressions for the calculation of crack toughness values are given for the common types of specimens.

Elastic analysis of a mode II fatigue crack test specimen

Abstract: A mode II specimen for fatigue crack growth measurements was described by Buzzard, Gross and Srawley [1]. Subsequently Buzzard [2] published results of displacement measurements at the crack mouth and at a selected point on the crackline for this specimen. A preliminary analysis of crack mouth displacements and stress intensity factors using boundary value collocation was also given [1]. The analysis while giving qualitatively useful results could not accurately represent all boundary conditions. The present paper presents a finite element analysis of the mode II specimen which gives more accurate stress intensity factors and displacement results than previously presented. In addition, load point displacements were measured in order to allow an experimental determination of the stress intensity factors.

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.

Crack growth and development of fracture zones in plain concrete and similar materials

Abstract: A calculation model (the Fictitious Crack Model), based on fracture mechanics and the finite element method, 1s presented. In the model the fracture zone in front of a crack is represented by a fictitious crack that 1s able to transfer stress. The stress transferring capability of the fictitious crack normally decreases when the crack width increases. The applicability of linear elastic fracture mechanics to concrete and similar materials is analysed by use of the Fictitious Crack Model. It" Is found that linear elastic fracture mechanics 1s too dependent on, among other things, specimen dimensions to be useful as a fracture approach, unless the dimensions, for concrete structures, are in order of meters. The usefulness of the J-integral, the COD-approach and the R-curve analysis is also found to be very limited where cementitious materials are concerned. The complete tensile stress-strain curve is introduced as a fracture mechanical parameter. The curve can be approximatively determined if the tensile strength, the Young's modius and the fracture energy are known. Su1table~lest methods Tor determining" TRise~~/'Voerties are presented and test results are reported for a number of concrete qua! » a. A new tyr * very stiff tensile testing machine Is presented by which it is possible to carry r . itable tensile tests on concrete. The complete tensile stress-strain curves h •«• feen determined for a number of concrete qualities. The thesi' overs a complete system for analysing crack propagation in concrete as it Include* -ealistic material model, a functional calculation model and methods for determiv" «/ the material properties necessary for the calculations. Therefore this work oi r t to be of use as a base for further studies of the fracture process of concrete < i similar materials.