The effect of low cycle fatigue, ratcheting and mean stress relaxation on stress-strain response and microstructural development in a dual phase steel

This study examines the cyclic plastic deformation behavior and microstructural development of a dual phase steel in both symmetric and asymmetric cycling in strain and stress control modes. The low-cycle fatigue (LCF) and mean stress relaxation (MSR) tests show very similar fatigue lifetimes. However, fatigue lifetimes reduce and prominent accumulation of directional strain was observed in ratcheting. A microstructural analysis has revealed that the type of cyclic test carried out has a noticeable impact on the substructural development, and this has been correlated with differences in accumulated tensile strain. Electron backscatter diffraction investigation has shown larger in-grain misorientation for ratcheting specimen in comparison with LCF and MSR specimens. The orientation of ferrite grains was found to have very little effect on their substructural development, and strain localization commonly occurred in the ferrite at the ferrite/martensite interface.

Effect of martensite morphology on low cycle fatigue behaviour of dual phase steels: Experimental and microstructural investigation

The low cycle fatigue (LCF) behaviour of a dual phase (DP) steel with different martensite morphologies has been investigated in the present work. DP steels with coarse martensite morphologies show inferior LCF life in comparison with fine martensite morphologies for all martensite volume fractions examined. It is suggested that this is be due to the development of larger local plastic strain concentrations in the ferrite with a coarser microstructure, compared to the finer microstructural morphology. Fatigue cracks were observed to initiate inside ferrite grains, and to preferentially propagate through the softer ferrite phase. The average sub-cell size was finer in samples with higher martensite volume fractions, but the sub-cell size was almost unaffected by the martensite morphology.

Influence of backpressure during ECAP on the monotonic and cyclic deformation behavior of AA5754 and Cu99.5

Ultrafine-grained (UFG) metals produced by equal channel angular pressing (ECAP) exhibit outstanding mechanical properties. They show high strength under monotonic loading as well as strongly enhanced fatigue lives in the Wöhler S-N-plot compared to their coarse grained (CG) counterparts. It could be shown that the fatigue lives can be significantly enhanced further by applying backpressure during ECAP. Besides the positive effect of backpressure on the processability of hard to deform materials via ECAP, the hydrostatic stress induced by backpressure also influences the mechanical properties under monotonic and cyclic loading. Therefore the influence of backpressure on ECAPed Cu99.5 and on the ECAPed aluminum alloy AA5754 was investigated. It is shown that backpressure has no effect on the hardness and grain size in Cu99.5 but changes the grain boundary misorientation to higher fractions of low angle grain boundaries. Also the temperature dependency of the yield strength as well as the hardening behavior under monotonic compression is affected. The cyclic deformation behavior of Cu99.5 is not strongly influenced by backpressure, but the mean stress level changes drastically. The fatigue life increases with the application of backpressure at low plastic amplitudes due to a change in the crack initiation and propagation. Aim of this work is the investigation of the influence of backpressure during equal channel angular pressing (ECAP) on the mechanical properties under monotonic and cyclic loading. Therefore we performed hardness measurements, compression, and fatigue tests on ECAPed Cu99.5 and AA5754. The results are discussed in terms of microstructure and relevant deformation and damage mechanisms.

Bimodal grain size distributions in UFG materials produced by SPD - their evolution and effect on the fatigue and monotonic strength properties

In ultrafine-grained (UFG) materials produced by severe plastic deformation (SPD) techniques such as ECAP (equal channel angular pressing), bimodal grain size distributions have been observed under different circumstances, for example shortly after ECAP, after rest or anneal and/or after mild cyclic deformation at rather low homologous temperature. It has been shown that the mechanical monotonic and fatigue properties of some UFG materials can be modified (sometimes enhanced) by introducing a bimodal grain size distribution by a mild annealing treatment which leads, in some cases, to a good combination of strength and ductility. Here, the conditions under which bimodal grain size distributions evolve by (adiabatic) heating during ECAP and during subsequent annealing or cyclic deformation will be explored, and the effects on the mechanical properties, as studied by the authors and as reported so far in the literature, will be reviewed and discussed. In particular, the role of temperature rise during ECAP will be considered in some detail.

Bending fatigue testing of commercial purity titanium for dental implants

High fatigue strength is one of the major requirements for dental implant materials. It was previously shown that the fatigue strength under conventional stress-control tension–compression testing can be doubled for commercially pure (CP) titanium processed by equal channel angular pressing. However, the fatigue endurance of an implant exposed to cyclic loading in corrosive media (bodily fluids) may potentially be compromised. In this work, non-conventional bending fatigue testing in air and in simulated body fluid (SBF) has been carried out for coarse-grained and ultrafine-grained CP titanium.

Material properties under drawing and extrusion with cyclic torsion

A copper bar was drawn while a lead bar was extruded through a cyclically twisting die in a specifically designed experimental rig. The drawing/extrusion load fluctuated at the same frequency as that of die twisting. The load tended to be at a level of monotonic deformation when the die was changing direction. The degree of the reduction in load for both the drawing and extrusion processes depended on the deformation conditions and requires optimisation for industrial application.

Cognitive functioning in chronic fatigue syndrome and the role of depression, anxiety and fatigue

Objective: This study was designed to investigate the role of depression, anxiety, and fatigue in Chronic Fatigue Syndrome (CFS) sufferers' objective and subjective cognitive performance. Methods: Twenty-three CFS sufferers and 23 healthy control participants were compared on objective and subjective assessments of cognitive performance. Depression, anxiety, and fatigue were also evaluated. Results: CFS sufferers did not demonstrate any impairment in objective cognitive functioning compared to the control group, and objective performance was not related to their higher levels of depression or their level of fatigue. Depression scores only accounted for a small amount of the variance in CFS sufferers' lower subjective assessment of their cognitive performance compared to control participants. There were no differences between the groups on anxiety scores. Conclusion: The results are discussed in terms of the heterogeneity of the CFS population and the complex interaction of symptomatological factors that characterise CFS.

A contested disease state : chronic fatigue syndrome

This paper aims firstly to look at characteristics of the condition, and current issues related to the symptomatology and presentation of ME-CFS. Secondly, it identifies methodological problems in researching ME/CFS and demonstrates that the experimental approach to researching the condition can be informed by more qualitative, constructivist approaches. Thirdly, it argues that ME-CFS is a contested state whose acceptance by the medical profession requires the legitimation of a demonstrated biomedical basis

Physical activity, emotional stress, sleep disturbances, and daily fluctuations in chronic fatigue symptomatology

The current study explored the relationships between physical and emotional stress and the symptomatology of chronic fatigue syndrome (CFS). Fifty-four CFS patients were studied using a longitudinal design. A self-report format was used to collect daily measures of major physical (sleep disturbance and physical activity) and emotional (subjective emotional stress level) stressors, as well as measures of levels of fatigue and secondary symptoms. The variables accounted for a moderate variance at the individual and occasion levels. Sleep disturbance and emotional stress were found to be positively associated with levels of fatigue and symptomatology, whereas physical activity was found to have a negative relationship with fatigue only. The severity of fatigue and symptoms were found to fluctuate daily in relation with the variables, indicating the complex nature of the associations.

Understanding ring strain and ring flexibility in six - and eight-membered cyclic organometallic group 14 oxides

A simple model was developed for the approximation of ring strain energies of homo- and heterometallic, six- and eight-membered cyclic organometallic group 14 oxides and the degree of puckering of their ring conformations. The conformational energy of a ring is modelled as the sum of its angular strain components. The bending potential energy functions for the various endocyclic M–O–M′ and O–M–O linkages (M, M′=Si, Ge, Sn) were calculated at the B3LYP/(v)TZ level of theory using H3MOM′H3 and H2M(OH)2 as model compounds. For the six-membered rings, the minimum total angular contribution to ring strain, ERSGmin was calculated to decrease in the order: cyclo-(H2SiO)3 (13.0 kJ mol−1)>cyclo-H2Sn(OSiH2)2O (7.0 kJ mol−1)>cyclo-H2Ge(OSiH2)2O (4.9 kJ mol−1)>cyclo-H2Si(OSnH2)2O (3.4 kJ mol−1)>cyclo-(H2SnO)3 (1.7 kJ mol−1)>cyclo-H2Si(OGeH2)2O (0.8 kJ mol−1)≈cyclo-H2Ge(OSnH2)2O (0.7 kJ mol−1)>cyclo-H2Sn(OGeH2)2O (0.1 kJ mol−1)≈cyclo-(H2GeO)3 (0 kJ mol−1). All of the six-membered rings were predicted to adopt (nearly) planar conformations (a=0.996<a<1). By contrast, all eight-membered rings were predicted to adopt strainless, but puckered conformations. The degree of puckering was predicted to increase in the order: cyclo-(H2SiO)4 (a=0.983)<cyclo-H2Sn(OSiH2O)2SiH2 (a=0.959)<cyclo-(H2SiO)2(H2SnO)2 (a=0.942)< cyclo-H2Si(OSnH2O)2SiH2 (a=0.935)<cyclo-(H2SnO)4 (a=0.916)<cyclo-(H2GeO)4 (a=0.885). The differences in ring strain and the degree of puckering were linked to the different electronegativities of Si, Ge and Sn. The results obtained are consistent with experimental ring strain energies; reactivities towards ring opening polymerizations or ring expansion reactions and observed ring conformations of cyclic organometallic group 14 oxides.

The limits of medicine and the social consequences for sufferers of chronic fatigue syndrome

Chronic fatigue syndrome (CFS) appears to be made up of several clusters of illness categories acting alone or in tandem to cause the decline of health through; fatigue/exhaustion, sensitivity/allergies, pain, general muscle and joint pains, cognitive impairment and gastrointestinal problems. This study investigated how patients interpret, evaluate and respond to the complex and varied symptoms of chronic fatigue syndrome. Data were collected from persons with CFS using a survey (n=90) and an interview (n=45). The researchers investigated how chronic fatigue syndrome is diagnosed by medical practitioners, how the label of CFS is determined and the social consequences for the patient. The results confirm the limited ability of the biomedical paradigm to diagnose adequately and treat effectively 'socially constructed' and medically ambiguous illnesses like CFS. In the absence of a legitimated regime of medical treatment for CFS, a range of often expensive treatments are employed by CFS sufferers, from formal use of pharmaceutical drugs through to 'alternative' therapies, including herbal, vitamin, homeopathic, esoteric meditative techniques, spiritual healing and general counselling are taken in no particular order.

The effect of laser power density on the fatigue life of laser-shock-peened 7050 aluminium alloy

Laser shock peening (LSP) is an innovative surface treatment method that can result in significant improvement in the fatigue life of many metallic components. The process produces very little or no surface profile modification while producing a considerably deeper compressive residual stress layer than traditional shot peening operations. The work discussed here was designed to: (a) quantify the fatigue life improvement achieved by LSP in a typical high strength aircraft aluminium alloy and (b) identify any technological risks associated with its use. It is shown that when LSP conditions are optimal for the material and specimen configuration, a —three to four times increase in fatigue life over the as-machined specimens could be achieved for a representative fighter aircraft loading spectrum when applied at a representative load level. However, if the process parameters are not optimal for the material investigated here, fatigue lives of LSP treated specimens may be reduced instead of increased due to the occurrence of internal cracking. This paper details the effect of laser power density on fatigue life of 7050-T7451 aluminium alloy by experimental and numerical analysis.

Pituitary adenylate cyclase-activating polypeptide induces translocation of its G-protein-coupled receptor into caveolin-enriched membrane microdomains, leading to enhanced cyclic AMP generation and neurite outgrowth in PC12 cells

Pituitary adenylate cyclase-activating polypeptide (PACAP), a member of the secretin/glucagon/vasoactive intestinal peptide family expressed throughout the nervous system, binds to the PACAP-specific G-protein-coupled receptor family members to promote both neuronal differentiation and survival. Although the PACAP receptor is known to activate its effector protein, adenylate cyclase (AC), and thus enhance cAMP generation, the molecular mechanism utilized by the receptor to activate AC is lacking. Here, we show that PACAP induces neurite outgrowth in PC12 cells by induction of translocation of the PACAP type 1 receptor (PAC1R) into caveolin-enriched Triton X-100-insoluble microdomains, leading to stronger PAC1R-AC interaction and elevated cAMP production. Moreover, we demonstrate that translocation of PAC1R is blocked by various treatments that selectively disrupt caveolae. As a result, intracellular cAMP level is decreased and consequently the PACAP-induced neurite outgrowth retarded. In contrast, addition of exogenous ganglioside GM1 to the cells shows the opposite effects. These results therefore identify the PACAP-induced translocation of its G-protein-coupled receptor into caveolae, where both AC and the regulating G-proteins reside, as the key molecular event in activating AC and inducing cAMP-mediated differentiation of PC12 cells.