Simulation of Fatigue Crack Propagation in Ductile Metals by Blunting and Re-sharpening

Laird and Smith [(1962). Philosophical Magazine 8, 847–857] proposed a plastic sliding-off mechanism for the stage II fatigue crack growth via striation formation. In their view, the fatigue crack extension results solely from the changing character of deformation at the crack tip during loading and unloading. In particular, the crack tip blunts during the loading stage and folds into a double notch during the unloading stage, resulting in striation formation. In order to verify Laird’s plastic blunting mechanism for ductile polycrystals as well as for ductile fcc single crystals, FE calculations were performed for a rectangular plate with an initially sharp crack under plane strain conditions. The plate was subjected to a fully reversed tension-to-pressure cyclic load perpendicular to the crack plane (Mode 1). In the single crystal case the crack propagation simulations were carried out for cracks with crack plane (001) for two different crack growth orientations [110] and [100]. No initial radius for the crack tip was assumed. The actual shape of the crack tip followed from an initially sharp crack by repeated remeshing. To model the constitutive behavior typical for polycrystalline ductile metals, J₂ hypo-elasto-plasticity model with Armstrong–Frederick kinematic hardening was used. To model the constitutive behavior typical for ductile fcc single crystals, a geometrically nonlinear version of Cailletaud’s model based on the multiplicative elasto-plastic decomposition of the deformation gradient was implemented into the FE program ABAQUS. For simplicity, only octahedral slip systems were considered. Using repeated remeshing for severely distorted elements at the advancing crack tip, deformation patterns in the sense of Laird’s mechanism for fatigue crack propagation with striation formation were obtained in the case of the polycrystal simulation as well as in the case of the single crystal simulation for [110] crack growth direction. The simulation for [100] crack growth direction with the same stress level as for [110] direction also yielded crack extension by progressive large deformations but without striation formation. The dependence of the fatigue striation formation on the crack growth direction as predicted by the simulation of crack propagation in single crystals is verified by the experimental results of Neumann [(1974). Acta Metallurgica 22, 1155–1165] on pure copper single crystals.     

The importance of pial blood supply to the development of peritumoral brain edema in meningiomas

In a retrospective analysis, the authors studied the pial and dural blood supplies in 74 intracranial meningiomas and quantified their associated peritumoral brain edema (PTBE). The extent and localization of pial blush in relation to the total tumor volume were determined angiographically. The amount of edema and tumor size were calculated using computerized tomography. The edema-tumor volume ratio was defined as Edema Index (EI). There were 49 meningiomas with PTBE; of those tumors, 46 were supplied by pial vessels, and three were supplied exclusively by dural vessels. Tumors without PTBE showed no pial blush. The mean EI in meningiomas with pial blush was significantly larger (EI = 3) than in meningiomas without pial supply (EI = 1.1; p < 0.0001). Meningiomas with a smaller pial supply than dural supply had a significantly smaller mean EI than tumors with a pial supply equal to or greater than the dural supply (EI = 2.9 vs. EI = 3.7; p < 0.015). In 69.9% of cases with pial blood supply, major portions of the edema were located adjacent to the tumor region supplied by pial vessels. Edema index differences among tumors of different subgroups, as defined by size or histology, were significantly related to the pial supply in each subset. Thus, pial blood supply may be associated with the development of PTBE in meningiomas.     

Diurnal rhythm of HMG CoA reductase activity in canine intestine is independent of luminal contents

Activity of the enzyme 3-hydroxy-3-methylglutaryl coenzyme A reductase (EC 1.1.1.34) measured in isolated segments of canine intestinal mucosa showed a distinct diurnal rhythm. Total activity changed over a twofold range with a peak occurring during midday, shortly after feeding. Since the isolated segments had no contact with luminal contents, the rhythm was not directly related to food components or bile salts. Humoral or neural influences must mediate the rhythm. The diurnal rhythm persisted for at least 3–5 mo, but was lost by 10 mo following formation of the isolated segment, possibly because of mucosal involution.     

The magnetic metrology of materials—A review

This paper gives a survey of the present state and new aspects of the measurement techniques for the characteristics of magnetic materials (except for recording materials) with several examples. Particular attention is paid to the aspects of reproducibility and traceability to SI units, and to the current trend of quality assurance and standardization.     

Computer-assisted instruction: MR imaging of the knee

Computer-assisted education offers a unique method of learning that is complementary to conventional learning techniques. The regional anatomy of musculoskeletal magnetic resonance (MR) imaging lends itself well to discrete learning modules on the computer. This article describes a computer-assisted educational program for MR imaging of the knee. The program is divided into four sections: MR imaging physics, anatomy, pathology, and a quiz. The program has cross-references between the sections that allow comparison of normal versus pathologic findings, review of specified pathologic entities, and self-testing of comprehension. The student can progress through the program in a linear or nonlinear fashion at his or her own pace. All images in the program were digitally acquired with an MR imager and transferred directly to the computer used to develop the program. The program is available throughout the radiology department on a local area network and in the medical school library on stand-alone units. The program is currently being used at a medical school as a supplement to the recently implemented problem-based learning curriculum and for resident education.