Allen T. Hopper

Critical review of the fatigue growth of short cracks

Abstract Retirement for cause (RFC) has become a popular design/analysis philosophy because it facilitates continued use of components which would otherwise be retired based on a safe life philosophy. RFC permits this continued service on the basis that service-induced damage tracked by periodic inspections will not develop to a critical size within a future operating interval. Successful implementation of RFC requires fracture mechanics technology to predict in-service growth behavior. Recent observations suggest that nonconservative estimates of crack growth rate (and therefore inspection interval) and critical flaw size arise when conventional linear elastic fracture mechanics (LEFM) is applied to predict the growth of physically small cracks. This paper summarizes the results of an extensive limited-circulation critical review of the phenomenology and mechanics of short crack growth with a view to identifying when this anomalous growth makes RFC analyses untenable in terms of LEFM. Factors which control the growth of short cracks are enumerated. It is shown for unnotched samples that the apparent effect may be traced to microcrack closure and the metallurgical, mechanical, and environmental transients which develop in the transition from initiation to steady-state growth. For notch samples the anomalous growth of cracks is traced to the inelastic action that develops a displacement-controlled notch field, which, contrary to LEFM analysis, dominates crack extension. Mechanics analyses relevant to characterizing the growth of short cracks are discussed. A crack tip opening displacement criterion is indicated to be appropriate.

Elastic-plastic analysis of edge-notched panels subjected to fixed grip loading

Abstract An expression for the mode I stress intensity factor for single edge-notch panels under fixed-grip loading is derived using known solutions for uniform tension and pure bending cases. An expression for the stress distribution along the fixed boundary is also derived. Then, using this stress distribution, an approximate expression for the J -integral is derived. The stress intensity factor and the J -integral solutions are compared with finite-element analysis results.

aci-Reductones: Drug design, enantioselective syntheses and biological activities within lipid membranes

Atherosclerosis is a chronic inflammatory disease which correlates with both elevated levels of lipids and depleted levels of endogenous antioxidants, a measure of oxidative stress. Oxidatively modified LDL (LDL-ox), monocytes, macrophages, smooth muscle cells, adhesion cell molecules, cytokines, foam cells and reactive oxygen species (ROS) are intrinsically linked to the inflammatory response element, nuclear factor-κB (NF-κB), in a biochemical cascade that leads to arterial plaque formation and progression.1 Dual lipophilic chain breaking antioxidant/arachidonic acid (AA) metabolic inhibitors are expected to retard atherosclerotic plaque formation through simultaneous blockade of multiple mechanisms of atherogenesis, protect against ROS induced tissue damage and restore natural antioxidant defenses. Several series of racemic and enantiomerically pure 4-substituted 2-hydroxytetronic acids, which represent both substrates and inhibitors of cyclooxygenase have been prepared and screened in a multitude of in vitro biological assays. These aci-reductones inhibit NF-κB activation (50% @ 30nM), protect against Cu++ catalyzed LDL oxidation in vitro and prevent monocyte/HUVEC binding significantly better than the antilipidemic drug probucol, and enantioselectively inhibit AA induced platelet aggregation (AAIPA) of human platelet rich plasma (PRP). This data indicates that some of these experimental antioxidants may be particularly useful for the treatment of atherosclerosis and associated vascular diseases.

An Approach to Life Prediction of Domestic Gas Furnace Clam Shell Type Heat Exchangers

During normal operations, the heat exchanger shells of domestic gas furnaces experience alternately rising and falling temperatures. These cyclic temperatures in turn induce cyclic strain patterns in the shell. The fatigue life of the shell is primarily dependent upon the temperature range of the heating cycle, rate of heating and cooling, the cyclic strain amplitudes, the cyclic straining rates, the material of which the shell is made, and the ambient environment.