Self-adaptive stress analysis based on stress convergence

A new algorithm for self-adaptive stress analysis is presented. The algorithm is based on h-convergence and uses the measured stress convergence rate as an error indicator. A theoretical relation between stress convergence rate and pointwise error of stresses is derived for both the 1-D and 2-D case. Some examples are presented to illustrate the behaviour of the algorithm.     

Effective stress intensities in stress corrosion cracking

The phenomena of branching and blunting of stress corrosion cracks are reviewed and their effects demonstrated for a martensitic steel. The stress intensity that a crack can sustain is proportional to the square root of its tip radius, so that blunt cracks require a higher apparent stress intensity. A simple procedure is outlined for converting apparent stress intensities to effective stress intensities, so eliminating anomalous effects due to crack branching and blunting.

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Résumé On passe en revue les phénomènes de ramification et d'arrondissement de l'extrémité des fissures de corrosion sous tension, et on examine leurs effets dans le cas d'un acier martensitique.L'intensité de contrainte qu'une fissure peut supporter est proportionnelle à la racine carrée de son rayon d'entaille, en sorte qu'à des fissures émoussées correspondent une intensité apparente de contrainte plus élevée. On propose une procédure simple permettant de convertir les intensités apparentes de contraintes ou intensité effective, ce qui permet d'éliminer les effets parasites associés à la ramification et à l'arrondissement des fissures.

     

Weibull stress distribution for static mechanical stress and its stress/strength analysis

The method's steps to estimate the Weibull shape (β) and scale (η) parameters, based only on the ratio of the maximal and minimal principal stresses (σ₁/σ₂) and on the designed reliability (R(t)) are given in Section 4.1 . The method's efficiency is based on the following facts: (1) The square root of σ₁/σ₂ represents the base life on which the Weibull lifetimes are estimated (see Equation  61 ). (2) The mean of the logarithms of the expected lifetimes (g(x)) is completely determined by the determinant of the analyzed stress matrix (see Equation  13 ). (3) The Weibull distribution is a circle centered on the arithmetic mean (μ), and it covers the whole principal stresses' span (see Figure  5 ). (4) σ₁/σ₂ and g(x) completely determine the σ_1i and σ_2i values, which correspond to any lifetime in the Weibull analysis (see Equation  54 ). And (5) σ₁/σ₂ and η completely determine the minimal and maximal lifetime, which corresponds to any σ_1i and σ_2i values (see Equation  57 ). Additionally, by using the addressed stress β and η parameters, when the stress is either constant or variable, the formulation to estimate the designed R(t) index is given. The steps to determine both the material's strength average (μ_M) for a desired R(t) index and the R(t) index, which corresponds to a used μ_M value, are given.     

Failure in laminates in tension under increasing stress,constant stress,and cyclic stress

Both monotonic and cyclic tension experiments have been carried out to fracture on transparent laminates made from flat ribbon glass and polyethylene sheets by heat bonding in a vacuum. The distribution of the measured tensile fracture stresses in monotonic loading correlates very well with the distribution of fracture stresses calculated from the measured distribution of element fracture stresses and the yield strength in shear of the polyethylene, according to a detailed statistical theory worked out earlier by Scop and Argon. Although the expected mode of fatigue damage by propagation of delamination cracks emanating from isolated stable fractures in reinforcing elements was observed, actual fatigue failure was a result of a more rapid mechanism of continued fracturing in reinforcing elements by a humidity-induced time-integrated static fatigue process. While laminates subjected to static stresses of the same magnitude as those in the dynamic experiments failed by the development of an identical form of damage during the same length of time under stress in laboratory air, other laminates tested in the same manner in dry air had 5 to 10 fold increased lives. In addition re-testing of individual elements of delaminated composites showed that elements can often be damaged during lamination, which must be taken into account in any quantitative study.     

Calculation of stress intensity factor from stress concentration factor

Using the general formulas of stress concentration factor, methods for calculating stress intensity factor are mentioned.These methods make use of the several known values of stress concentration and radius of curvature at the point of stress concentration to form expression of stress concentration factor. Values of stress concentration from handbooks or experiments and others can be used.This paper deals with plane elastic, longitudinal shearing and thin plate bending problem.     

The relative impact of work-related stress, life stress and driving environment stress on driving outcomes

Previous research has shown the association between stress and crash involvement. The impact of stress on road safety may also be mediated by behaviours including cognitive lapses, errors, and intentional traffic violations. This study aimed to provide a further understanding of the impact that stress from different sources may have upon driving behaviour and road safety. It is asserted that both stress extraneous to the driving environment and stress directly elicited by driving must be considered part of a dynamic system that may have a negative impact on driving behaviours. Two hundred and forty-seven public sector employees from Queensland, Australia, completed self-report measures examining demographics, subjective work-related stress, daily hassles, and aspects of general mental health. Additionally, the Driver Behaviour Questionnaire (DBQ) and the Driver Stress Inventory (DSI) were administered. All participants drove for work purposes regularly, however the study did not specifically focus on full-time professional drivers. Confirmatory factor analysis of the predictor variables revealed three factors: DSI negative affect; DSI risk taking; and extraneous influences (daily hassles, work-related stress, and general mental health). Moderate intercorrelations were found between each of these factors confirming the ‘spillover’ effect. That is, driver stress is reciprocally related to stress in other domains including work and domestic life. Structural equation modelling (SEM) showed that the DSI negative affect factor influenced both lapses and errors, whereas the DSI risk-taking factor was the strongest influence on violations. The SEMs also confirmed that daily hassles extraneous to the driving environment may influence DBQ lapses and violations independently. Accordingly, interventions may be developed to increase driver awareness of the dangers of excessive emotional responses to both driving events and daily hassles (e.g. driving fast to ‘blow off steam’ after an argument). They may also train more effective strategies for self-regulation of emotion and coping when encountering stressful situations on the road.     

Thermal stress hysteresis and stress relaxation in an epoxy film

Thermal cycling of an epoxy coating on silicon through the glass transition temperature (T _g) revealed a large stress hysteresis on the first thermal cycle through T _g and a change in the stress–temperature slope at T _g resulting from the change in the epoxy elastic properties due to the glass transition. This stress hysteresis was not observed on subsequent thermal cycles through T _g. However, after the coating was annealed (aged) below T _g (for hours or longer)—during which the stress relaxed exponentially with time—the stress hysteresis returned. The magnitude of stress hysteresis, on cycling through T _g, was found to correlate to the magnitude of long-time relaxation that occurred during annealing at temperatures below T _g.     

A superconvergent stress recovery technique for accurate boundary stress extraction

An element‐base superconvergent stress recovery technique is developed for accurate boundary stress extraction. In the present method, higher‐order stress fields are assumed for all stress components and higher order elements are used for the construction of necessary matrices. Unknown coefficients for the assumed stress field are obtained by minimizing the sum of the stress errors and two equilibrium residuals in the least squares sense. The two residuals are derived based on the principle of virtual work. Numerical examples including a three‐dimensional problem have demonstrated that the present method can stably extract very accurate boundary stresses even for coarse meshes. Copyright © 1999 John Wiley & Sons, Ltd.     

24-DOF quadrilateral hybrid stress element for couple stress theory

A 24-DOF quadrilateral hybrid stress element for couple stress theory is proposed in this study. In order to satisfy the equilibrium equation in the domain of the element, the $21\beta $ Airy stress functions are chosen a assumed stress interpolation functions, and beam functions are adopted as the displacement interpolation functions on the boundary. This element can satisfy weak C $^{0}$ continuity with second-order accuracy and weak $\hbox {C}^1$ continuity simultaneously. So the element can pass the enhanced patch test of a convergence condition. Moreover, the reduced integration and a stresses smooth technique are introduced to improve the element accuracy. Numerical examples presented show that the proposed model can pass the $\hbox {C}^{0-1}$ enhanced patch test and indeed possesses higher accuracy. Besides, it does not exhibit extra zero energy modes and can capture the scale effects of microstructure.     

Viscoelastic stress analysis

This paper formulates the viscoelastic problem so as to allow the use of higher-order differential equation solution techniques. Viscoelastic models describing the functional relationships of stress and strain for the material are used. However, the procedure outline is valid for more general relationships. The advantages of using Runge–Kutta integration formulae are indicated and an example demonstrates the excellent results obtainable. The computational efficiency of the method is emphasized and error prediction is discussed.     

Determination of stress intensity factors from stress concentrations for V-notched beams

The calculation of stress intensity factor for a sharp V-notch is done from the limiting value of stress concentration factors as the root radius of the same V-notch rounded at the tip approaches zero. The stress analysis is carried out using the complex variable formulation of Muskhelishvili and the conformal mapping technique. The notch tip is rounded by a modified Schwartz-Christoffel transformation.     

Effect of mean stress on residual stress relaxation in steel specimens

Abstract

Residual stress relaxation was investigated by subjecting specimens with various mean stress levels to strain controlled cyclic loading. The material studied was mild steel in three different conditions. The mean stress levels ranged from 100 to 200 MPa, and two strain amplitudes, 0·05 and 0·06%, were studied in detail. The residual stresses in the specimens were measured before and after mean stress relaxation experiments. It was found that experimental factors such as temperature variations and crack growth have a significant influence on the results. Based on the experimental results, it is proposed that the mean stress relaxation exponent should be divided into two parts: mean stress dependent and mean stress independent. The first includes the contribution of quasi-static relaxation, i.e. mean stress dependent plastic deformation. The second part includes the contribution of cycle dependent mean stress relaxation, which does not depend on the mean stress.