Effects of Temperature on the Evolution of Yield Surface and Stress Asymmetry in A356–T7 Cast Aluminium Alloy

As the electrification of vehicle powertrains takes prominence to meet stringent emission norms, parts of internal combustion engines like cylinder heads are subjected to an increased number of thermal load cycles. The cost-effective design of such structures subjected to cyclic thermo-mechanical loads relies on the development of accurate material models capable of describing the continuum deformation behaviour of the material. This study investigates the effect of temperature on the evolution of flow stress under cyclic loading in A356-T7 + 0.5% Cu cast aluminium alloy commonly used in modern internal combustion engine cylinder heads. The material exhibits peak stress and flow stress asymmetry with the stress response and flow stress of the material under compressive loading higher than under tension. This peak and flow stress asymmetry decrease with an increase in temperature. To compare this stress asymmetry against conventional steel, cyclic strain-controlled fatigue tests are run on fully pearlitic R260 railway steel material. To study the effect of mean strain on the cyclic mean stress evolution and fatigue behaviour of the alloy, tests with tensile and compressive mean strains of +0.2% and −0.2% are compared against fully reversed (Rε = −1) strain-controlled tests. The material exhibits greater stress asymmetry between the peak tensile and peak compressive stresses for the strain-controlled tests with a compressive mean strain than the tests with an identical magnitude tensile mean strain. The material exhibits mean stress relaxation at all temperatures. Reduced durability of the material is observed for the tests with tensile mean strains at lower test temperatures of up to 150 °C. The tensile mean strains at elevated temperatures do not exhibit such a detrimental effect on the endurance limit of the material.

Settlement and Bearing Capacity of Rectangular Footing in Reliance on the Pre-Overburden Pressure of Soil Foundation

This article presents a solution for the quantitative evaluation of the stress–strain state (SSS) and the bearing capacity of rectangular foundations, factoring in the unit weight of the soil mass and different values of pre-overburden pressure (POP). In order to assess the SSS of the soil subgrade below a rigid rectangular footing under a uniformly distributed load, the authors applied the Boussinesq basic solution for an elastic half-space subjected to a vertical point load on its surface. As a result, the formulas for vertical stress, mean stress, shear strain, and volumetric strain for any point in Cartesian coordinates (x, y, z) and foundation settlement were determined. Additionally, the application of Hencky’s system of physical equations, with non-linear dependencies between mean stress and volumetric strain as well as deviator stress and shear strain, along with the experimental curves, depicts the relationships between bulk modulus and volume stress, and shear modulus and shear stress. The authors point out the non-linear behavior of the subgrade soil and propose a method for estimating the bearing capacity of a rigid rectangular foundation.

Fatigue Life Prediction of Rolling Bearings Based on Modified SWT Mean Stress Correction

The existing engineering empirical life analysis models are not capable of considering the constitutive behavior of materials under contact loads; as a consequence, these methods may not be accurate to predict fatigue lives of rolling bearings. In addition, the contact stress of bearing in operation is cyclically pulsating, it also means that the bearing undergo non-symmetrical fatigue loadings. Since the mean stress has great effects on fatigue life, in this work, a novel fatigue life prediction model based on the modified SWT mean stress correction is proposed as a basis of which to estimate the fatigue life of rolling bearings, in which, takes sensitivity of materials and mean stress into account. A compensation factor is introduced to overcome the inaccurate predictions resulted from the Smith, Watson, and Topper (SWT) model that considers the mean stress effect and sensitivity while assuming the sensitivity coefficient of all materials to be 0.5. Moreover, the validation of the model is finalized by several practical experimental data and the comparison to the conventional SWT model. The results show the better performance of the proposed model, especially in the accuracy than the existing SWT model. This research will shed light on a new direction for predicting the fatigue life of rolling bearings.

Influence of Second-Order Effects on Thermoelastic Behaviour in the Proximity of Crack Tips on Titanium

Background The Stress Intensity Factor (SIF) is used to describe the stress state and the mechanical behaviour of a material in the presence of cracks. SIF can be experimentally assessed using contactless techniques such as Thermoelastic Stress Analysis (TSA). The classic TSA theory concerns the relationship between temperature and stress variations and was successfully applied to fracture mechanics for SIF evaluation and crack tip location. This theory is no longer valid for some materials, such as titanium and aluminium, where the temperature variations also depend on the mean stress. Objective The objective of this work was to present a new thermoelastic equation that includes the mean stress dependence to investigate the thermoelastic effect in the proximity of crack tips on titanium. Methods Westergaard’s equations and Williams’s series expansion were employed in order to express the thermoelastic signal, including the second-order effect. Tests have been carried out to investigate the differences in SIF evaluation between the proposed approach and the classical one. Results A first qualitative evaluation of the importance of considering second-order effects in the thermoelastic signal in proximity of the crack tip in two loading conditions at two different loading ratios, R = 0.1 and R = 0.5, consisted of comparing the experimental signal and synthetic TSA maps. Moreover, the SIF, evaluated with the proposed and classical approaches, was compared with values from the ASTM standard formulas. Conclusions The new formulation demonstrates its improved capability for describing the stress distribution in the proximity of the crack tip. The effect of the correction cannot be neglected in either Williams’s or Westergaard’s model.

A Study to Assess the Effectiveness of Laughter Therapy on Reducing Level of Stress Among Primary School Teachers of Selected School in Hoshangabad MP India

The purpose of the study was to assess the amount of stress among primary school teachers in Hoshangabad, evaluate the efficiency of laughing therapy in lowering their level of stress, and discover a relationship between their pre-test level of stress and chosen demographic characteristics. A pre-experimental research was carried out among primary school teachers at Servite Convent English Medium School in Hoshangabd, near Housing Board Road. The data collecting period was extended from January 21 to February 21. Thirty elementary school teachers were chosen using purposive sampling. There was a pre-test, an intervention, and a post-test. Descriptive and inferential statistics were used to analyse the data. The study found that most instructors' stress levels were moderate in the pre-test and mild in the post-test. It was severe in 10% of the instructors in the pre-test, but no one showed significant stress in the post-test. The mean stress score after the exam was lower than the mean stress score before the test. A substantial relationship was discovered between the amount of stress and certain demographic characteristics such as age and stress-reduction strategies. The majority of primary school instructors had mild to moderate stress, which was alleviated by laughing therapy. As a result, laughing therapy should be employed to relieve stress among elementary school instructors.

Creep behavior of cemented sand investigated under cyclic loading

The influence of stress state on the creep behavior of an artificial cemented sand resembling soft rocks was evaluated. The stress state was characterized by a mean stress and a stress ratio. The cyclic stress-induced creep test was adopted in this study, where the cyclic loading involved a constant deviator stress and a cyclic mean stress (or confining pressure) of the same amplitude and period; the test indicated similar trends as the conventional creep test with a shorter time to creep failure at less creep strain. Results showed that when the creep strain is large enough, the greater the creep strain accumulates, the smaller is the post-creep strength. Under the same number of cyclic loads, with the same stress ratio, the creep strain and the steady-state strain rate in the secondary creep stage increase with increasing mean stress; with the same mean stress, the two said parameters also increase with increasing stress ratio. It was also found that the time to reach creep failure decreases with decreasing mean stress and increasing stress ratio. The stress ratio is proposed to account for the tendency of a stress state to cause failure, and the cyclic variation of mean stress, which is equivalent to the effective mean stress with pore water pressure being zero in the tests conducted, reflects the effective stress state of a geomaterial under fluctuations of groundwater table. Under a fixed deviator stress, a soft porous geomaterial subjected to cyclic variation of effective mean stress may yield contraction and could lead to failure if the stress ratio is high. The findings can help explain the mechanism of ground subsidence or slope failure subjected to cyclic fluctuations of groundwater table.