Papers by Keyword: Relaxation

Abstract: Shot peening process is a cold performed function to enhance the mechanical properties which is widely used in many industries. This process introduces compressive residual stress which was proven to increase the fatigue life, geometry stability and corrosion resistance. However, the benefit of the residual stress is still unstable due to the relaxation during the operation. This paper will study on the trend of the relaxation of residual stress against cyclic loading as well as the change in the hardness. The material used in this study is carbon steel ASTM A516/ SA 516 Grade 70. Shot peening process with steel shots was applied to the samples to introduce compressive residual stress in the samples. Cyclic load was applied to samples after shot peening process with low load of 52Mpa (20% of Yield Strength) and high load of 208Mpa (80% of Yield Strength). The measurement of residual stress using X-Ray diffraction and hardness test was done on the samples to study the trend of the relaxation of residual stress and the change in hardness values. The result shows that more relaxation of residual stress occurs if the applied cyclic load is higher. The change of hardness trend is found non-sequenced in this study due to random coverage of shot peening.

Abstract: This paper focuses on the calculation of residual stresses due to shrinkage with a tensile creep effect. Whereas the shrinkage of concrete causes stresses in the material, the tensile creep counteracts the shrinkage as a stress relaxation mechanism. The main objective of this paper is to evaluate the ageing coefficient c (referred to as Trost-Bazant Coefficient) reflecting the load history. The coefficient is used for the residual stress analysis by means of a simplified method called Age-adjusted Effective Modulus Method. The tensile creep effect was evaluated according to the rheological model provided by Eurocode 2. Although the Eurocode predicts the creep for the structural members subjected to compressive stresses, this study proves that it can be used for the tensile creep prediction as well. We tested three types of concrete: reference concrete, high-performance concrete with reduced shrinkage magnitude by means of special admixtures, and fibre concrete with the content of polypropylene fibres. From the obtained results, it can be stated, that the ageing coefficient can be considered to be the value of 0.45 for any shrinkage development. It was also proved, that the tensile creep value essentially affects the magnitude of residual stresses, even in the “early age” concrete. The correctness of the calculated residual stresses was verified by means of a Ring-test.

Abstract: Design and analysis of critical components in energy (nuclear, solar and fossil power), aerospace, automobile and chemical industries based on detailed inelastic analysis can enhance structural integrity and thereby economy. Especially for the components exposed to very high temperature thermomechanical fatigue loading, unified inelastic analysis based life prediction may enhance accuracy. A unified constitutive model (UCM) with features of strain rate-dependence, static recovery, mean-stress evolution, strain range-dependence, and finally creep damage is developed. The modified UCM is validated against simulating a broad set of strain-controlled isothermal and anisothermal fatigue and fatigue-creep responses, and stress-controlled creep responses of Haynes 230. Some of these results are presented to demonstrate improved simulations by the modified UCM. Importance of damage parameters in improving simulations in the tertiary creep regime is observed.

Abstract: In the case of exceptional accidents, nuclear containment structures may be submitted to an internal temperature increase. This may have an influence on the prestressed concrete structures behavior regarding both its mechanical performance and its porosity. The presented study got interested on the impact that a temperature increase may have on the mechanical behavior of the steel reinforcement for both prestressing strands and rebars. In order to remain in realistic situations, it was chosen to study temperatures between 20°C and 140°C. Some experimental investigations regarding the tensile behaviour of steel rebars and their adherence within concrete will first be presented. Then, some investigations on steel strands will be described: some tensile tests at different temperatures, and some relaxation tests to check how the level of prestress loss may be affected by the temperature. This experimental study is part of a national French project (MACENA) aiming at assessing the impact of an accident on the behavior of nuclear containment structures. The gathered experimental data will be used for their damage assessment.

Abstract: Effects of long-term concrete volume variations (creep, shrinkage). The value of the variation of stress in the tendons due to creep, shrinkage and relaxation at time t, the value of the variation of stress in the tendons at time t, due to the relaxation of the prestressing steel.

Abstract: This article presents results of research aimed at identifying the phenomenon of stress relaxation in compressed concrete elements. The study used a special test stand, designed and constructed by Dr.Eng. Krzysztof Kaminski which allows permanent strain of cylindrical concrete sample by adjusting force. Cylindrical samples with a diameter of 15 cm and 10.5 cm and a height of 30 cm for the study were used. The samples were made of different classes of concrete. There was also defined the influence of applied tensile stress and the age of the sample on the relaxation phenomenon. The expected result of the research was to obtain relaxation curves of concrete samples with different compressive strength and age. Relaxation of concrete is important for the analysis of bending elements sections, which behavior during the operational phase of the concrete can significantly deviate from the assumed one, causing other than triangular or parabolic distribution of compressive stresses.

Abstract: The elastic properties of cortical bone tissue and other types of bone have been determined by the classical methods such as tensile stress and shearing stress. In recent years, by nanoindentation method, it has developed techniques for measuring the viscoelastic properties of bone tissues. In the same time, they show effects the dependent on time due to loading. The time dependent behavior of such viscoelastic materials may be described by constitutive equations whose variables are stress, deformation and time. These equations may be expressed by means of rheological models. Furthermore, bone tissues present both the phenomenon of creep and relaxation, indicating that they have a rheological behavior. In this paper viscoelastic behavior of bone is simulated numerically, and analyzed in Simulink, using Burgers rheological model.

Abstract: We consider a variational formulation of gradient elasto-plasticity, as they arise in the incremental formulation of the plastic evolution problem, subject to a class of single-slip side conditions. Such side conditions typically render the associated boundary-value problems non-convex. We first show that, for a large class of plastic deformations, a given single-slip condition (specification of Burgers' vectors and slip planes) can be relaxed by introducing a lamination microstructure. This yields a relaxed side condition which allows for arbitrary slip in a prescribed family of slip planes. This relaxed model can be thought of as an aid to simulating macroscopic plastic behavior without the need to resolve arbitrarily fine spatial scales. We also discuss issues of existence of solutions for the relaxed model.

Abstract: The presented paper delas with constitutive equations of the linear theory of viscoelasticity. The integral representations for homogeneous isotropic medium located in isothermal conditions are introduced. Relations are derived for the relaxation and creep processes, as well as relations between these functions in the Laplace images. In the solution we assume quasi-static external force loading under which the analyzed viscoelastic body in each time t shows infinitesimal deformations. General isotropic tensor of fourth degree G_ijkl (t) we use to derive the relationship between deviator stress and deformation, as well as the relations between stresses and distortions that produce volumetric changes. It is shown that for the linear viscoelastic problem can be used Stieltjes convolution and its algebraic properties. Derived relations and procedures followed to the results of differential-operator's representation of constitutive equations of linear viscoelasticity theory that have been published in the previous work of Authors.

Abstract: This paper deals with the use of special asphalt-rubber mixture, the Stress Absorbing Layer (SAL). Description of SAL and test methods is given in theoretical part of this paper. Several different mixtures were designed and selected ones subsequently tested. Low-temperature properties, rutting test, bending tensile relaxation, stiffness modulus and fatigue properties were determined. These parameters are stated for asphalt-rubber mixtures with aggregate sizes up to 5 mm, 8 mm or 16 mm.