High Mean Stress Research Articles

Safety assessment of interaction behavior of high cycle and very high cycle fatigue with creep at high temperature

Interaction of fatigue and creep behavior is the dominant failure mode for high cycle or very high cycle fatigue under high mean stress at high temperature. As for large rotating component working as high-speed at high temperature, such interaction drives the need to update strength assessment and design method. In this work, a constant life diagram incorporating dual-criterion for evaluating the interaction behavior between high cycle/very high cycle fatigue and creep at high temperature was proposed, and then verified using six types of materials data. This dual-criterion method was proved to be more precise for assessing the serious fatigue-creep interaction behavior in long-life regime, that was promising for design and safety assessment of high-temperature rotating components.

Effects of mean stress on the fatigue properties of core‐in‐sheath‐type carbon/glass hybrid thermoplastic composite rods: Experimental investigation and practical predictive method

AbstractA systematic acquisition of fatigue test data was conducted under different stress ratio conditions to investigate the effect of mean stress on the fatigue properties of carbon/glass hybrid thermoplastic composite rods. Our findings revealed that as the stress ratio increased with a higher mean stress, the stress dependency of the fatigue life also increased, and the slope of the S‐N curve became gentler. Under identical stress ratio conditions, hybrid rods with a greater volume fraction of carbon fiber exhibited the highest strength and a more gradual slope of the S‐N curve. To account for mean stress effects, an equivalent stress amplitude was introduced, resulting in data collapse at around 107 cycles, rendering this approach an industrially useful method to predict fatigue behavior when working with limited experimental datasets. This study presents a practical technique for estimating fatigue strength under mean stress through a weighted average efficiency and equivalent stress amplitude.

Interaction and superposition of creep damage and high-cycle fatigue of coarse-grained nickel-base alloy 247

ABSTRACT The cast coarse-grained polycrystalline Alloy 247 is investigated, which is typically used in the rear turbine blades of land-based gas turbines. High rotational speeds up to 10,000 rpm in combination with high temperatures induce creep deformation. Additionally, gas turbine blades are subjected to high-frequency cyclic stresses. This complex loading state requires the investigation of the interaction of creep damage and high-cycle fatigue (HCF). Therefore, HCF tests with a stress ratio of Rσ = −1 were carried out on as-received and pre-crept specimens. In comparison, an HCF test series with high mean stresses (Rσ = 0.5) was conducted to consider the superposition of creep and fatigue. Since nickel-base alloys exhibit a pronounced elastic anisotropy, the grain orientations were considered in the damage analysis. The results show that creep-induced grain boundary damage is a dominant structural parameter for the HCF behaviour of Alloy 247 in both, sequential and superpositioned creep-fatigue loading.

Evaluation of the stress generated during biomechanical preparation with nickel–titanium rotary endodontic file using finite element analysis

Introduction: Adequate biomechanical preparation of root canals is the essence of successful endodontic therapy. Roots with curved canal framework led to endodontic complexity. The hassles in the preparation of such complex canals have prompted the inauguration of nickel–titanium (Ni-Ti) instruments with properties of shape memory and superelasticity. They benefit root canal preparation with causing less aberration, yet intracanal separation due to fatigue remains a concern. The study was intended to evaluate and compare the stresses developed during biomechanical preparation with Ni-Ti rotary file in straight, moderate, and severely curved root canals using finite element analysis (FEA). Materials and Methods: Six real sizes, three-dimensional root models were designed in SolidWorks software with varying angles of curvature (2°–45°), and using the same software, ProTaper Gold F2 rotary file was developed. All the designed root models with the rotary file inserted into each model were subjected to FEA. ANSYS Software (version 17.2 Company, Commonwealth of Pennsylvania, USA) 2015 was used to simulating root canal shaping, and the stresses generated in all the root canal models were observed. Results: Highest mean stress was observed with 45° models which was 235.95 MPa, followed by a 30° simulated model showed 220.36 MPa and least stress of 125.22 MPa was observed with 2° models (P > 0.05). Conclusions: Increased canal curvature has definitely resulted in generation of more amount of stress.

High-temperature ratcheting and low-cycle fatigue failure of a friction stir welding Al-Zn-Mg-Cu alloy

High-temperature ratcheting fatigue behavior of a friction stir welding (FSW) Al-Zn-Mg-Cu alloy was investigated. The substructured grains in the weld nugget zone (WNZ) undergo continuous dynamic recrystallization (CDRX) during ratcheting fatigue. The WNZ re-precipitates intragranular η′ and η particles and forms the precipitate free zone (PFZ) in the vicinity of the grain boundaries. At high mean stress or stress amplitude, ratcheting failure with obvious necking takes place due to large ratcheting strain. At low stress levels, intergranular fatigue microcracks initiate from grain boundaries of the WNZ due to the local strain incompatibility between the PFZ and grain interiors. The fatigue microcracks coalesce along grain boundaries and propagate via typical striation formation mechanisms. Low-cycle fatigue failure occurs at the WNZ instead of the low hardness zone (LHZ).

Anisotropic constitutive model of frozen silty clay capturing ice cementation degradation under high mean stresses

Research results show that comparing to unfrozen soil, frozen soil has more complicated deformation behavior owing to the presence of ice. On the basis of triaxial tests results of frozen silty clay (FSC) at ˗6 °C, due to the ice cementation, the critical state line (CSL) of FSC does not cross the origin of the deviatoric plane. The critical deviatoric stress of FSC increases nonlinearly as the mean stress growing; however, further increases of the mean stress give rise to the decreases in the critical deviatoric stress. Because under high mean stresses, the ice-melting and-crushing (IMAC) cause ice cementation degradation. To predict the deformation of FSC, a new anisotropic constitutive model that considers the degradation of ice cementation is proposed. In the proposed model, first, the elastic parameters of FSC are obtained by conducting triaxial shear loading-unloading (TSLU) tests and isotropic compression loading-unloading (ICLU) tests. Second, yield and plastic potential functions are proposed, the two involve the modified mean stress and indicate the degradation of ice cementation. Third, a double hardening law pertaining to isotropic hardening and rotational hardening is developed by considering initial anisotropy and stress-induced particle rotation under shear loading. Finally, the comparison results confirm that the developed constitutive model is valid and suitable for describing the stress–strain relations of FSC.

Further development of detail categories for bolt‐assemblies subjected to normal stress in steel constructions

AbstractThe fatigue strength of bolts is determined not only by the geometric design but also by production‐related influences such as rolled before heat treatment, rolled after heat treatment or even hot‐dip galvanising. For the bolts of steel constructions considered in this paper, these production‐related influencing factors are considered very differently in the evaluation of the fatigue resistance, depending on the used respective standards. This is for example done according to the still valid version of EN 1993‐1‐9 in the unified form of detail category 50, taking into account the size effect, but independent of the other influencing factors mentioned before. This is not least due to the very limited available experimental database and test documentation. The authors address this topic in this paper, especially for large‐size bolts used in large quantities in wind turbines. This article presents an evaluation of results from 22 fatigue test series (n = 411 tests) on 3 different high‐strength bolt assembly systems with varying manufacturing conditions. The data base is covering a diameter range between M12 and M72 and focusses on test series with high mean stress in a representative order of magnitude for practical application of the bolts. For their further evaluation, the findings described in the technical literature are considered in relation to the different tested manufacturing conditions. The additional comparison of the test results with the current design codes serves to a better classification of the results. The authors give a proposal for the adjustment of detail categories of bolt assemblies subjected to normal stress.

Effect of Nature-Themed Recharge Room on Healthcare Workers' Level of Stress and Anxiety.

To determine if there is a difference in stress and anxiety before and after the use of the nature-themed recharge room. Psychological support measures have been noted to be relevant to nurses; however, the effect of the pandemic on the healthcare workers' emotional and psychological well-being led to urgent calls to implement psychological support measures more widely. The study utilized a 1-group pretest and posttest design. One hundred sixty hospital employees utilized the recharge room, with 131 completed questionnaires counted in the data analysis. Data were collected using the demographic form, Perceived Stress Scale, and State-Trait Anxiety Inventory. Most participants were between 25 and 35 years old, female, worked the morning shift, had 3 to 5 years of experience, preferred a water feature theme, and used the room for 10 minutes. The mean stress preintervention score was 7.44, and postintervention score was 7.17, with the difference not statistically significant. The mean anxiety preintervention score was 14.17, and postintervention score was 8.48, with the difference statistically significant at a P < 0.05. Females working in the hospital for 1 to 5 years were physicians/residents, nursing support staff, and leaders with the highest mean stress (4-item Perceived Stress Scale) and anxiety (6-item State-Trait Anxiety Inventory) preintervention scores. Comparing the preintervention and postintervention anxiety levels, the highest reduction was noted among females working in the hospital for 1 to 3 years and nursing support staff who have used the room for 5 to 15 minutes with 2 or fewer people. Organizational leaders should offer psychological support programs, such as the nature-themed recharge room, to help reduce the healthcare workers' stress and anxiety.

Unveiling the Fatigue Behavior of 2D Hybrid Organic-Inorganic Perovskites: Insights for Long-Term Durability.

2D hybrid organic-inorganic perovskites (HOIPs) are commonly found under subcritical cyclic stresses and suffer from fatigue issues during device operation. However, their fatigue properties remain unknown. Here, the fatigue behavior of (C4 H9 -NH3 )2 (CH3 NH3 )2 Pb3 I10 , the archetype 2D HOIP, is systematically investigated by atomic force microscopy (AFM). It is found that 2D HOIPs are much more fatigue resilient than polymers and can survive over 1 billion cycles. 2D HOIPs tend to exhibit brittle failure at high mean stress levels, but behave as ductile materials at low mean stress levels. These results suggest the presence of a plastic deformation mechanism in these ionic 2D HOIPs at low mean stress levels, which may contribute to the long fatigue lifetime, but is inhibited at higher mean stresses. The stiffness and strength of 2D HOIPs are gradually weakened under subcritical loading, potentially as a result of stress-induced defect nucleation and accumulation. The cyclic loading component can further accelerate this process. The fatigue lifetime of 2D HOIPs can be extended by reducing the mean stress, stress amplitude, or increasing the thickness. These results can provide indispensable insights into designing and engineering 2D HOIPs and other hybrid organic-inorganic materials for long-term mechanical durability.

Triaxial stress and failure modes in hydrothermal mineral systems

Rock mechanics experiments show that the magnitude of the intermediate principal stress significantly affects rock failure. Since triaxial stress states (no principal stress is zero) are ubiquitous in the crust, and polyaxial axial states (all three principal stresses are different) are general, the magnitude of the intermediate principal stress should have an important effect on hydrothermal mineralisation. For example, extensional veins or dykes in vein-hosted gold or porphyry deposits may have multiple orientations when the intermediate and least principal stresses have similar magnitudes, or single orientations when the intermediate and maximum principal stresses are similar. The Griffith-Murrell triaxial fracture criterion with a tensile cutoff can be used to illustrate the effects of the intermediate principal stress on failure. At the lowest values of mean stress, the criterion suggests that only extensional failure can occur. At low–intermediate values of mean stress, either extensional or shear failure may occur: extensional failure is favoured when the intermediate and maximum principal stresses have similar magnitudes. At higher mean stresses, shear failure will occur at lower values of pore fluid pressure and differential stress when the magnitudes of the intermediate and minimum principal stresses are similar.

Fatigue performance of ultra-high-strength steel laser cut notches under variable amplitude loading

Experimental fatigue tests with variable amplitude (VA) load were conducted for notched uncoated and hot-dip galvanized (HDG) notched laser cut components made of S960 ultra-high-strength steel (UHSS). Tests were conducted with low (constant minimum) and high (constant maximum) mean stresses and major differences between fatigue lives with the tests on identical equivalent stress were found. A multiparametric 4R method application, in conjunction with the theory of critical distances (TCD) to obtain fatigue-effective stresses at notch, was utilized on the mean stress correction and fatigue strength assessment. The introduced 4R method was proven to be highly efficient to consider different mean stress levels based on the experimental results and different loading and specimen types could be evaluated with single FAT class.

Very high cycle fatigue of high carbon chromium bearing steel at high compressive mean stresses

Very high cycle fatigue (VHCF) of high carbon chromium bearing steel 100Cr6 (SAE52100; JISSUJ2) is investigated with ultrasonic fatigue testing for compressive mean stresses with minimum stresses σmin at compressive yield stress σCY, for σmin = 0.59σCY, and for load ratio R = −1. Fatigue limits are found for σmin = σCY and for σmin = 0.59σCY, whereas no fatigue limit below 109 cycles exists at R = −1. Cracks leading to VHCF failures exclusively start at interior inclusions. Considering inclusions as cracks, the maximum stress intensity factor of a load cycle being below the effective threshold stress intensity factor determines the fatigue limit.

Creep-fatigue voids and sub-grain boundaries assisted crack initiation for titanium alloy in VHCF regime with high mean stress at 400°C

Very high cycle fatigue failure of bimodal titanium alloy at 400 °C and high mean stress has been investigated. Many circular creep-fatigue voids (CFVs) were found underneath the crack initiation zone, which were distributed in the β transformed matrix and near the grain boundaries (GBs) of primary α grains. By the energy dispersive spectroscopy (EDS) element mapping, the oxygen element was found to be enriched around the CFVs. In addition, many sub-GBs of small-angle GBs could be observed inside the primary α grains. Based on these, a new mechanism of CFVs and sub-GBs assisted crack initiation and early propagation in bimodal titanium alloys was proposed in very high cycle fatigue regime.

Experimental study on uniaxial ratchetting of VHB 4910 dielectric elastomer

In this work, a series of uniaxial cyclic tests of VHB 4910 dielectric elastomer were carried out to investigate the uniaxial ratchetting and its dependence on the stress level, stress rate, peak/valley stress hold and loading history. The results show that in the uniaxial stress-controlled cyclic tests, VHB 4910 produces obvious ratchetting, and the higher the applied stress level and the lower the stress rate, the more remarkable the ratchetting is. Compared with the experimental results of pure-shear cyclic deformation, the apparent modulus of VHB 4910 during uniaxial cyclic deformation is lower and the dissipation is smaller. In addition, the uniaxial ratchetting of VHB 4910 is also affected by the peak/valley stress hold and loading history. The peak stress hold will promote the generation of ratchetting, but the valley stress hold will hinder the increase of ratchetting strain. The cyclic loading history with a higher mean stress will inhibit the development of ratchetting in the subsequent cyclic loading case with lower mean stress.

Identifying vulnerable children's stress levels and coping measures during COVID-19 pandemic in Japan: a mixed method study.

BackgroundThe COVID-19 pandemic has disproportionately affected vulnerable children and youth. In Japan, despite evidence that the paediatric age group holds a lower risk of infection than the older population, there was a nationwide closure of schools as an early public health measure. Acknowledging that school closures brought heightened psychological and physical stress among Japanese children, we aimed to explore vulnerable children’s experiences of the COVID-19 pandemic in Japan, focusing on socially disadvantaged subset of the population.MethodsWe used an adapted version of the ‘Perceived Stress Scale for Children’, with additional free-text space, delivered online to children attending three non-profit organisations which provide support for this group of vulnerable persons and families experiencing social disadvantage. Simple descriptive analysis was undertaken on the quantitative data; we used thematic and content analysis for the qualitative data.ResultsThirty-six children participated in the online survey, mean age was 11.3 years, majority (61%) were male. The mean overall stress score (score distribution width: 0–39) was 14.8, with no difference in score distribution by age or gender. Free-text responses obtained revealed a range of stressors and protective factors. Schooling, COVID-19 fears, family tension and pandemic measures were sources of stress; family—in particular, the support of the mother—food, friendship and recreation were sources of comfort. While most responses indicated positive coping mechanisms, some displayed maladaptive behaviours.ConclusionsThe children in this cohort had high mean stress scores overall. Responses indicated that they were acutely reactive to COVID-19 as well as pandemic public health measures, and that missing schooling and contact with friends exacerbated their stress. Family was a source of strength as well as stress. A tailored public health response to COVID-19 needs to take into account the concerns voiced by vulnerable child populations be based on equity and child rights.

Analiza zamora prednapregnutih vijčanih spojeva

This article presents Wöhler plots, or S-N curves, for use in the analysis of bolt fatigue of preloaded bolted joints. Preloaded bolts under cyclic loading have a high mean stress with a small alternating stress. This is combined with a large stress concentration at the thread root. The method of fatigue analysis presented uses S-N curves with a zero mean stress. The high mean stress experienced by the bolts is accounted for using a function to calculate a damage-equivalent stress. Notch sensitivity was considered to modify S-N curves for materials with chemical compositions encompassed within the material specifications for high strength bolts. This produced S-N curves for stress concentrations relevant to bolt threads. Curve fitting techniques were used to express these curves as a function of the equivalent stress and stress ratio. An estimate of residual stresses in bolts produced by thread rolling was made. The work provides a practical method of calculating fatigue life.

Mean Versus Variability: Disentangling Stress Effects on Alcohol Lapses Among Individuals in the First Year of Alcohol Use Disorder Recovery

Although stress is a well-known predictor of alcohol use lapses among individuals seeking recovery from alcohol use disorder (AUD), most research has relied on retrospective self-report using conventional questionnaires that explore stress effects at the level of the mean. Ecological momentary assessment (EMA) overcomes many of the shortcomings of questionnaire-based, retrospective self-report by using real-time, in-the-environment evaluations for the acquisition of ecologically valid data that can also capture stress variability. The present investigation used EMA to disentangle stress effects on alcohol lapses among individuals in the first year of an AUD recovery attempt by exploring associations between mean-level stress, stress variability, and subsequent alcohol use. Participants (N = 42) completed 6 days of EMA monitoring and were then followed up 90 days later to assess alcohol use. Putative associations were explored using hierarchical regression controlling for demographic factors and pre-baseline alcohol use, with percentage days abstinent from alcohol at follow-up as the outcome variable. An interaction effect was observed such that the combination of high mean stress level and high stress variability was associated with the lowest percentage of days abstinent. For those with high mean stress levels, this relationship was attenuated as stress variability decreased. The findings support previous research linking stress to alcohol use lapses; however, these results indicate that the stress/alcohol use relationship is more nuanced than previously described. Our findings suggest that stress variability should also be considered in clinical contexts when assessing risk conferred by mean-level stress.

Viscoplastic Rheology of α‐Quartz Investigated by Nanoindentation

AbstractQuartz is an abundant mineral in Earth's crust whose mechanical behavior plays a significant role in the deformation of the continental lithosphere. However, the viscoplastic rheology of quartz is difficult to measure experimentally at low temperatures without high confining pressures due to the tendency of quartz (and other geologic materials) to fracture under these conditions. Instrumented nanoindentation experiments inhibit cracking even at ambient conditions, by imposing locally high mean stress, allowing for the measurement of the viscoplastic rheology of hard materials over a wide range of temperatures. Here we measure the indentation hardness of four synthetic quartz specimens and one natural quartz specimen with varying water contents over a temperature range of 23°C to 500°C. Yield stress, which is calculated from hardness but is model dependent, is fit to a constitutive flow law for low‐temperature plasticity to estimate the athermal Peierls stress of quartz. Below 500°C, the yield stresses presented here are lower than those obtained by extrapolating a flow law constrained by experiments at higher temperatures irrespective of the applied model. Indentation hardness and yield stress depend weakly on crystallographic orientation but show no dependence on water content.

Mechanism of HCF-creep interaction in a type 316LN stainless steel

HCF-creep interaction in type 316LN stainless steel is investigated at 923 K by conducting HCF tests well below the endurance limit with high mean stress (high R-ratio). High mean stress inflicts creep damage at 923 K, which when imposed on a small alternating stress, leads to significant HCF-creep interaction. Depending on the magnitude of mean stress (R-ratio), three distinct regimes are identified, viz. (a) HCF regime (b) HCF-creep interaction regime and (c) creep regime. Detailed fractographic investigation revealed distinct fracture characteristics pertaining to each of the three regimes. HCF-creep interaction regime is marked by significant intergranularity at low values of mean stress which changes to predominantly dimpled fracture with increasing mean stress. The above observations are corroborated through detailed EBSD investigations which bring out the underlying deformation mechanisms responsible for failure under HCF-creep interaction.

Ratcheting-Fatigue Behavior of Harmonic-Structure-Designed SUS316L Stainless Steel

Stainless steels with harmonic-structure design have a great balance of high strength and high ductility. Therefore, it is imperative to investigate their fatigue properties for engineering applications. In the present work, the harmonic-structured SUS316L stainless steels were fabricated by mechanical milling (MM) and subsequent hot isostatic pressing (HIP) process. A series of ratcheting-fatigue tests were performed on the harmonic-structured SUS316L steels under stress-control mode at room temperature. Effects of grain structure and stress-loading conditions on ratcheting behavior and fatigue life were investigated. Results showed that grain size and applied mean stress had a significant influence on ratcheting-strain accumulation and fatigue life. Owing to the ultrafine grained structure, tensile strength of the harmonic-structured SUS316L steels could be enhanced, which restrained the ratcheting-strain accumulation, resulting in a prolonged fatigue life. A higher mean stress caused a faster ratcheting-strain accumulation, which led to the deterioration of fatigue life. Moreover, a modified model based on Smith–Watson–Topper (SWT) criterion predicted the ratcheting-fatigue life of the harmonic-structured SUS316L steels well. Most of the fatigue-life points were located in the 5 times error band.