•The optimal fatigue parameters for the small-number samples were acquired by using the search path searching to quantify the lifetime with stress levels.•The equivalent lives corresponding to ...different stress levels could be well correlated with the fatigue data under the highest stress level.•The predicted fatigue P-S-N curves were more accurate with small-number data at the low stress level when considering the variable standard deviation.•For small sample fatigue data, improved backward statistical inference approach can achieve more conservative fatigue life curves than the traditional group method.
To increase the prediction accuracy of small-number sample life data, improved backward statistical inference approach (ISIA) was formulated using the modified distribution coefficients while fitting the fatigue probabilistic S-N (P-S-N) curve. Based on the search path searching for fatigue parameters, the novel scheme captures the optimal coefficients that can efficiently correlate the life distribution and stress levels. The equivalent lives can be obtained through the conversion of fatigue life to the highest stress level. To acquire a realistic P-S-N curve of full-scale components, the life distribution effect of small-number samples can be considered when fitting curves. Results show that for a set of data with an accurate standard deviation at the first stress level, the predicted lifetime using the ISIA is approximately 8.7% of the conventional group method and is only 10.5% of the original backwards statistical inference method. Meanwhile, by comparing the predicted results with the fatigue life testing data of full-scale alloy steel EA4T axles, the accuracy of fatigue P-S-N curves by using ISIA is further confirmed. For the modern axle materials and welded structures widely used in railway vehicles, the newly-proposed ISIA can not only achieve a reliable fatigue life, particularly in the high cycle fatigue regime, but also provide a more conservative fatigue P-S-N curve than traditional fitting methods.
Full text
Available for:
GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
Biomarkers for chronic fatigue Klimas, Nancy G; Broderick, Gordon; Fletcher, Mary Ann
Brain, behavior, and immunity,
11/2012, Volume:
26, Issue:
8
Journal Article
Peer reviewed
Open access
Abstract Fatigue that persists for 6 months or more is termed chronic fatigue. Chronic fatigue (CF) in combination with a minimum of 4 of 8 symptoms and the absence of diseases that could explain ...these symptoms, constitute the case definition for chronic fatigue syndrome/myalgic encephalomyelitis (CFS/ME). Inflammation, immune system activation, autonomic dysfunction, impaired functioning in the hypothalamic-pituitary-adrenal axis, and neuroendocrine dysregulation have all been suggested as root causes of fatigue. The identification of objective markers consistently associated with CFS/ME is an important goal in relation to diagnosis and treatment, as the current case definitions are based entirely on physical signs and symptoms. This review is focused on the recent literature related to biomarkers for fatigue associated with CFS/ME and, for comparison, those associated with other diseases. These markers are distributed across several of the body’s core regulatory systems. A complex construct of symptoms emerges from alterations and/or dysfunctions in the nervous, endocrine and immune systems. We propose that new insight will depend on our ability to develop and deploy an integrative profiling of CFS/ME pathogenesis at the molecular level. Until such a molecular signature is obtained efforts to develop effective treatments will continue to be severely limited.
Full text
Available for:
GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
•Fatigue tests using AM Ti-6Al-4V were conducted at elevated temperature.•Maximum defect size due to AM process was statistically investigated.•The applicability of Murakami’s model for AM-Ti at ET ...was experimentally shown.
Rotating bending fatigue tests were conducted at room temperature (RT) and at elevated temperature (ET) of 250 °C using a Ti-6Al-4V alloy fabricated by an additive manufacturing (AM) process, and the effect of defects on fatigue strength was investigated. Particularly, the applicability of Murakami’s model for predicting the fatigue limit at ET was examined. Microstructure observations revealed that the AM Ti-6Al-4V alloy had acicular α + β microstructures while the conventionally manufactured (CM) Ti-6Al-4V alloy had equiaxed α phases in β matrix. Round-shaped defects and crevice-liked sharp defects with the size of up to 50 μm were observed in the AM Ti-6Al-4V, which were not observed in the CM Ti-6Al-4V. The fatigue strengths at 107 cycles of the CM Ti-6Al-4V were 625 MPa at RT and 475 MPa at ET, and those of the AM Ti-6Al-4V were 300 MPa at RT and 250 MPa at ET. Scanning electron microscopy (SEM) on the fracture surfaces revealed that fatigue cracks in the AM Ti-6Al-4V specimens initiated from defects at ET as well as they did at RT. The fatigue strengths of the AM Ti-6Al-4V at RT and at ET were evaluated by Murakami’s model and the predicted value agreed well with the experimental value.
Full text
Available for:
GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
Fatigue symptoms are reported by a majority of patients with multiple sclerosis (MS). Reliable assessment, however, is a demanding issue as the symptoms are experienced subjectively and as objective ...assessment strategies are missing. The objective of this study was to develop and validate a new tool, the Fatigue Scale for Motor and Cognitive Functions (FSMC), for the assessment of MS-related cognitive and motor fatigue. A total of 309 MS patients and 147 healthy controls were included into the validation study. The FSMC was tested against several external criteria (e.g. cognition, motivation, personality and other fatigue scales). The item-analysis and validation procedure showed that the FSMC is highly sensitive and specific in detecting fatigued MS patients, that both subscales significantly differentiated between patients and controls (p < 0.01), and that internal consistency (Cronbach’s alpha α > 0.91) as well as test—retest reliability (r > 0.80) were high. Cut-off values were determined to classify patients as mildly, moderately or severely fatigued. In conclusion, the FSMC is a new scale that has undergone validation based on a large sample of patients and that provides differential quantification and graduation of cognitive and motor fatigue.
Full text
Available for:
NUK, OILJ, SAZU, UKNU, UL, UM, UPUK
•The √area parameter model is applicable to defects with a maximum size of ∼100 µm.•The long-crack threshold, ΔKth,lc, is applicable to defects larger than ∼100 µm.•Models based on the relative ...stress gradient can be used for blunt defects.•An equation to predict the critical size of detrimental defects is introduced.•The critical defect size increases with increasing load ratio.
The defect tolerance of three different martensitic stainless steels (17-4PH, X20Cr13 and AISI403) under cyclic loading was investigated. Fatigue tests were performed with specimens containing diverse artificial defects (corrosion pits, single and multiple drilled holes, sharp circumferential notches and pre-cracked holes). The measured cyclic strengths are compared to predictions based on the area parameter model proposed by Murakami and Endo. This model is well applicable to small sharp defects if the threshold condition for crack propagation determines the fatigue limit. The transition size from small to large defects, areatrans, – above which the threshold stress intensity factor range, ΔKth, becomes a constant value – is between 80 and 166 µm for the investigated steels. For larger defects, the threshold for long cracks, ΔKth,lc, serves well to predict the fatigue strength under fully reversed loading, i.e. at a load ratio of R = −1. Comprehensive tests were performed to study the influence of mean stress, and threshold conditions were determined that enable to predict the fatigue limit at different load ratios.
A simple equation is introduced to estimate the critical defect size, areacritical, which is the size above which defects become detrimental under cyclic loading. It has been found that areacritical increases with increasing load ratio while areatrans is independent of R. Furthermore, higher values of the threshold stress intensity factor range were determined for large defects (with sizes larger than areatrans) compared to long cracks if the load ratio is higher than R = −1. This could be explained by the plane stress condition at the material surface which is prevalent for surface defects, while the condition for long cracks can be assumed as plane strain.
It is further demonstrated that the effect of small defects on the fatigue strength of stainless steels is highly sensitive to the notch root radius, ρ. Small holes with diameters of 100 µm (ρ = 50 µm) or larger are less detrimental for the fatigue strength than defects with high stress concentrations – such as corrosion pits.
Full text
Available for:
GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
•The very-high cycle fatigue properties of honeycomb sandwich structure are examined.•The core direction of the specimen affects the fatigue properties.•The main failure modes are core wrinkles, core ...cracks, and face delamination.•The fatigue stiffness degradation curve exhibits three different trends.•The core has multi-point failure damage characteristics.
In this study, the high-cycle and very-high-cycle fatigue properties of carbon fiber reinforced plastic (CFRP)-aramid honeycomb sandwich structure, which is a commonly used structure in aviation industry, are examined under two different core directions using the ultrasonic three-point bending fatigue test. Further, the fatigue failure modes and damage mechanism of the structure are revealed. The results show that the fatigue stiffness degradation curve of the honeycomb sandwich structure exhibits three different trends: continuous decline, two horizontal regions and two reduction regions, and two horizontal regions and one reduction region. The shape of the stiffness degradation curve is affected by the direction of core. The S-N curve presents a double-linear decline feature, and there is no traditional fatigue limit. The fatigue life of the specimen is related to the type of stiffness degradation. The optical microscopy images show that the main failure modes of specimens are core wrinkles, core cracks, and face delamination. Overall, based on the stiffness degradation curve, failure modes, and real-time images of the test specimens, the damage evolution process and the corresponding mechanism of the honeycomb sandwich structure are analyzed to conclude that the core has multi-point failure damage characteristics.
Full text
Available for:
GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
•Multiaxial models can be successfully adapted to estimate fretting fatigue life.•New data for Ti-6Al-4V cylinder/plane contact to evaluate wear-based life analysis.•Critical distance/plane method ...yields accurate estimates of fretting fatigue life.•Inclusion of wear in the modeling did not significantly improved the life estimates.•Elastic-plastic analysis of AA6201-T81 contacting wires improves life estimates.
The aim of this work is to show that multiaxial fatigue modeling can be successfully adapted to estimate crack initiation life of mechanical assemblies under fretting conditions. The paper is divided into two parts. In Part I, the focus is on the study of size and gradient effects but considering the influence of incorporating fretting wear on the life estimation procedure. New data for a Ti-6Al-4V cylindrical on plane contact configuration were produced to assess the analyses. It is shown that critical plane criteria, coupled with a critical distance which varies with life, are capable to provide accurate estimates of fretting fatigue life in the medium high cycle regime. The inclusion of wear in the modeling did not significantly improved the life estimates but increased a lot the computational cost. In Part II, the paper extended this life estimation approach to the contact of wires taken from overhead conductors. Fretting fatigue data of AA1350-H19 wires and new tests on the more resistant AA6201-T81 corroborated the successful use of the life methodology to other materials and applications.
Full text
Available for:
GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
PDF
