Papers by Keyword: Stress Relaxation

Abstract: Compressive residual stresses have been found to affect fatigue crack growth behavior by delaying the crack initiation and by decelerating the crack propagation rate. Therefore, various mechanical surface treatment techniques have been developed to produce the compressive residual stresses on the surface of components. However, the residual stresses will relax due to cyclic loading. Hence, the stability of residual stress during fatigue process is a great importance aspect for design of components. In this paper, the ultrasonic surface deep rolling was used to generate the compressive residual stress near the surface of Ti-6Al-4V. The stress relaxation behavior was identified during the low cycle fatigue process. The X-ray diffraction method was used to determine the magnitude and sign of residual stress. Results showed that under cyclic loading, the residual stress relaxation occurred fast in the first few cycles then became stable. Furthermore, it was found that relaxation rates of residual stress were depended on the applied stress.

Abstract: Chemical strengthening is a method of generating compressive stress on the glass surfaces by ion-exchange to improve their strengths since old ages. As its targets have been expanding to larger and thinner glasses recently, different strength profiles are required depending on the applications and selecting appropriate ion-exchange parameters becomes even more important. Therefore, we developed a simulation model of chemical strengthening which enables predicting the stress profiles by ion-exchange in order to help optimizing the ion-exchanging parameters. Parameter study using the simulation model with change of temperature results in precise stress prediction, the compressive stress on the surface with a margin of error of ± 3% and the depth of layer with that of ± 10%. Furthermore, the parameter study introduced following two technical findings; (1) both inter-diffusion coefficient and mass-transfer coefficient could obey the Arrhenius equation, (2) both actual temperature and fictive temperature could influence on the stress relaxation. These findings are of great importance in comprehending the phenomena associated with ion-exchange. It has possibilities for beneficial feedback to the composition development and optimum stress design in accordance with each application.

Abstract: The aim of the presented work is to establish relationship of behavior of stress relaxation process in 5056 alloy in vacuum conditions for stress levels of 116 MPa, 83 MPa and 50 MPa and temperatures of 293 K and 353 K. Method of circular specimens with equal flexural strength was used as the main method for stress relaxation testing. Results of the studies allowed to develop the advanced methodology for evaluation of stress relaxation, which increases measurement accuracy up to 0.04 MPa. Increase of accuracy was achieved using the proposed new shape of a specimen of equal flexural strength, its manufacturing technology and developed design of accessories for precision marking. It was established that for studied temperatures stress relaxation in vacuum is occurring more intensively independently from level of initial stress. That fact can be explained by increase of influence of dislocation mechanism on stress relaxation. The results of the presented studies serve as a basis for increase of reliability and consistency of dimensions of elements of aircraft's structure and their parts and components, which are operating in vacuum conditions.

Abstract: The objective of the study was to design a simulation model that approximates the real viscoelastic behavior of spinal units in stress relaxation tests. The spinal units were sampled from porcine and used in fresh condition, without frizzing or drying. The mechanical tests were conducted on Mecmesin 5i testing machine using the appropriate devices for compression tests. On the other hand, using Maxwell constitutive equation for relaxation, a Simulink model was created. At the end, the relaxation curve derived from the simulation model was draw next to experimental obtained data in order to represent the fitting.

Abstract: In constitutive models of polymers, there has been a long history of the use of strain-rate dependent viscous processes, such as the Eyring and Argon models. These are combined with elastic elements to generate viscoplastic models that exhibit typical phenomena such as rate dependent yield, creep and stress relaxation. The Eyring process is one of the most frequently used such mechanisms. It has two significant drawbacks: it implies a temperature dependence of mechanical behaviour that is in an opposite sense to that observed; and it predicts a strain rate dependence of yield stress that is less complex than that observed, leading to the requirement for two or more Eyring processes. In recent years, new ideas for amorphous polymers have been developed that lead to an alternative plastic mechanism that addresses these concerns. In this paper a constitutive model that incorporates this mechanism is developed, and its effectiveness in modelling macroscopic mechanical behaviour of polymers is explored with respect to published data.

Abstract: TiN films are widely used as strengthening coatings for cutting tools. In a previous study, the crystal structure of TiN films deposited by arc ion plating was found to be strongly influenced by the bias voltage during deposition. The TiN films deposited under a high bias voltage were found to have a high compressive residual stress of-9.5 GPa. The residual stresses of the TiN films relaxed to thermal stress levels upon annealing. In the present study, the authors investigate the relaxation of the residual stress during heat treatment in atmosphere for TiN films with different initial residual stress values. The surface layer of the TiN films is oxidized by heat treatment in an air atmosphere. However, the crystalline state of the TiN films is hardly affected. So far, the residual stress of the TiN film hardly changes in a temperature state lower than the heating temperature when it is relaxed by heat treatment.

Abstract: This paper presents the results of a study of stress relaxation in Ni₃Ge single crystals with the L1₂ structure oriented along the [001] axis. The anomalous temperature dependence of the strain rate at relaxation has been determined.

Abstract: Several stress relaxation and creep tests of high temperature material are performed. According to the characteristics of stress relaxations and the superposition equation of diffusion and Maxwell equations of two stages, equivalent relaxation time and equivalent relaxation rate are proposed. Considering equivalent relaxation rate as the creep rate under constant stress, the relaxation-creep conversion model is built up and presented. Then the steady-state creep curve and creep rate are calculated. The results show that the numerical results are in good agreement with the experimental data. It indicates that equivalent relaxation rate can be employed for the analysis of steady-state creep rate. The conversion model and method can be used to design the creep strength and predict the life of the component at high temperature.

Abstract: Articular cartilage is a complicated material to model for a variety of reasons: its biphasic/triphasic properties, heterogeneous structure, compressibility, unique geometry, and variance between samples. However, the applications for a biomimetic, cartilage-like material are numerous and include: porous bearings, viscous dampers, robotic linkages, artificial joints, etc. This work reports experimental results on the stress-relaxation of equine articular cartilage in unconfined compression. The response is consistent with simple spring and damper systems, and gives a storage and loss moduli. This model is proposed for use in evaluating biomimetic materials, and can be incorporated into large-scale dynamic analyses to account for motion or impact. The proposed characterization is suited for high-level analysis of multi-phase materials, where separating the contribution of each phase is not desired.

Abstract: This paper investigates experimentally and numerically the degree to which the stress relaxation in shot-peened simple geometric features of steel turbine materials can be modelled by using an understanding of simple eigenstrain distributions. The residual stress is determined as the elastic response of whole component when the plastic strains caused by shot peening is incorporated as an eigenstrain in an appropriate finite element (FE) model. The application of a subsequent live load is then modelled as an additional load step in the FE model which will superpose the effect of this loading on the residual stress field. The results show that the eigenstrain approach is particularly useful in predicting residual stress relaxation in shot-peened components.