A mechanical model of fatigue crack propagation. Report 2

Conclusions The proposed model of fatigue crack propagation based on the solution of the cyclic elastoplastic problem of the stress-strain state [1] makes it possible to take into account the effect of the triaxial stress state on the deformation of the material at the crack tip. The proposed algorithm of calculations of the state of damage on the basis of the principle of linear damage summation and also the agreement between the calculated and experimental data confirm the assumption on the controlling role of low-cycle damage in the mechanics of crack propagation in cyclic loading described from phenomenological positions. The main advantages of the proposed model are:The possibilities of calculating endurance in crack propagation or calculating the crack propagation rate for cases in which the variation of the range of the stress intensity factor along the crack length in structural members takes place at a variable loading asymmetry;the possibilities of describing the effect of loading asymmetry on the fatigue crack-propagation rate using only the strain criterion (Coffin's equation) since the range of the plastic strain intensity at the crack tip is, as shown in [1], a function of not only the range of the stress intensity factor K but also of its maximum value K_max;the possibilities of describing the dependence of K_th on loading asymmetry based on the assumption on the constancy of the size of the structural element for the given material;the possibilities of describing the crack propagation rate in all the three sections of the dL/dN=f(K) diagram, starting with the values K similar to K_th and ending with the value of K at which monotonic quasistatic fracture becomes the controlling process.Translated from Problemy Prochnosti, No. 8, pp. 14–18, August, 1985.     

Beta brass bicrystal and tricrystal stress strain behavior

Beta brass single crystal, bicrystal and tricrystal stress strain behavior were determined for a series of isoaxial specimens in which the bicrystals and tricrystals were incompatible in shear along the grain boundary. Volume fractions of grain boundary deformation were also determined to ascertain the effect of increased constraint on grain boundary strengthening. It was found that bicrystal flow stress was considerably higher than single crystal flow stress and that tricrystal strain hardening was only slightly larger than that of the bicrystal. For the tricrystals the grain boundary deformation zone is larger than that of corresponding bicrystals because of both configurational and constraint effects. This larger zone indicates that secondary stresses extend further from the boundary in tricrystals than in bicrystals. It was concluded that despite increased constraint, , the average grain boundary stress for the tricrystals, is smaller than that for corresponding bicrystals and that because of the increased extent of the secondary stress,σ _{b i}, the stress in the center component away from the grain boundary, is higher than that of the single crystal at corresponding strains. The relationship of these observations to corresponding stresses in polycrystalline material is examined. where Taek Dong Lee was formerly a Graduate Student. This paper is based on a thesis to be submitted by Taek Dong Lee in partial fulfillment of the requirements for the degree of Doctor of Philosophy at Polytechnic Institute of New York.     

Elastic moduli of a cracked body

Statistical solutions for the internal stress and displacement fields are constructed for a body in an external applied stress field, and containing a prescribed distribution of penny shaped cracks. From the fields, a stress-strain relationship is calculated for the body, and effective elastic moduli are derived. The fields are constructed as expansions in the parameter \(\overline {Na^3 }\) wich is assumed small (N is the crack density, anda is the crack radius). Angular integrals over the distribution function are performed for two special cases that represent a bedded crack distribution, and an isotropic crack distribution. A criterion for the validity of the use of these quasistatic results in dynamical calculations is presented.     

The envelope glycoprotein ectodomains determine the efficiency of CD4+ T lymphocyte depletion in simian-human immunodeficiency virus-infected macaques

CD4+ T lymphocyte depletion in human immunodeficiency virus type 1 (HIV-1)-infected humans underlies the development of acquired immune deficiency syndrome. Using a model in which rhesus macaques were infected with chimeric simian-human immunodeficiency viruses (SHIVs), we show that both the level of viremia and the structure of the HIV-1 envelope glycoprotein ectodomains individually contributed to the efficiency with which CD4(+) T lymphocytes were depleted. The envelope glycoproteins of recombinant SHIVs that efficiently caused loss of CD4(+) T lymphocytes exhibited increased chemokine receptor binding and membrane-fusing capacity compared with those of less pathogenic viruses. These studies identify the HIV-1 envelope glycoprotein ectodomains as determinants of CD4(+) T lymphocyte loss in vivo and provide a foundation for studying pathogenic mechanisms.     

Immunogenicity of a Salmonella typhi CVD 908 candidate vaccine strain expressing the major surface protein gp63 of Leishmania mexicana mexicana

Attenuated Salmonella typhi are attractive for use as live vector vaccines to express protozoal antigens and deliver them to the human immune system. The gene encoding the mature form of Leishmania mexicana mexicana gp63 under control of tac promoter was integrated into the delta aroC locus of the chromosome of attenuated delta aroC, delta aroD S. typhi strain CVD 908. After oral immunization of BALB/c mice with two 1 x 10(9) colony forming unit doses given 21 days apart, CVD 908 omega (delta aroC::Ptac-gp63) elicited a broad T cell-mediated immune response against L. m. mexicana gp63 as demonstrated by: (1) lymphoproliferative response to fixed whole L. m. mexicana promastigotes; (2) activation of IL-2 (but not IL-4)-producing lymphocytes; (3) appearance of cytotoxic T cells against mouse mastocytoma cells expressing gp63. This T-cell mediated immune response was associated with significant protection in F1 (BALB/cXC57Bl/6) mice challenged in their footpads with a wild type strain of L. m. mexicana.     

Thioglycollate-elicited murine macrophages are cytotoxic to Mycoplasma arginini-infected YAC-1 tumor cells

We describe a collective episode of psychogenic illness in an indigenous group (Embera) of Colombia, geographically isolated from its native homeland and surrounded by non-indigenous settlers. The condition, which affected three young adult men and six adolescent women, was attributed by them to a spell (maleficio). It was designated as ataques de locura (madness attacks) according to their traditional medical system; and as a conversive disorder with dissociative features by psychiatrists. Different therapeutic approaches, including antipsychotic medication, religious healers and traditional herbal remedies were unsuccessful. Contact with shamans of the same ethnic origin, on the other hand, proved to be an effective way of dealing with the symptoms. We interpret the situation as an expression of psychosocial stress secondary to cultural change. This medical problem bears close resemblance to other specific culture-bound syndromes such as ataques de nervios or possession syndromes and gives clues to ways of dealing with psychogenic expressions of cultural stress.     

The continuance and spillover of everyday tensions in distressed and nondistressed families.

This study compared 38 distressed and 35 nondistressed families on 2 interaction processes: the continuance of tensions involving the same family members and the spillover of tensions from one family subsystem to another. One parent reported perceptions of family tensions and satisfaction ratings each day for 2 wks. Sequential analyses revealed that distressed, compared with nondistressed, families experienced (1) greater continuance of tensions, particularly tensions that occurred the following day during the same time period, and (2) more spillover, particularly between marital and child-related tensions. Correlations with daily satisfaction ratings indicated that marital dissatisfaction in distressed families is highly related to the occurrence of marital tensions. These data support the importance of examining daily events and multiple family subsystems in order to understand family distress. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Detection of coronary stenoses on source and projection images using three-dimensional MR angiography with retrospective respiratory gating: preliminary experience

OBJECTIVE: Our objective was to study the ability of three-dimensional MR angiography with retrospective respiratory gating to reveal stenoses in proximal coronary arteries on source and projection images. CONCLUSION: Proximal coronary artery stenoses can be identified using three-dimensional MR angiography with retrospective respiratory gating, both with projection images and on source images alone. Reasons for missed lesions included collateral vessels and retrograde flow distal to complete occlusion and volume averaging of vessels with adjacent structures. Causes of false-positive interpretations included small foci of decreased signal intensity distal to complete occlusion, partial volume effects on individual partitions, and regions of distal vessels leaving the imaging plane.