Consequence Of Residual Stresses Research Articles

Thermodynamic Conditions for Consolidation of Dissimilar Materials in Bimetal and Functional Graded Structures.

Functional Graded Structures and Functional Graded parts, made using dissimilar materials, are designed to provide specific properties to the final product. One of the most promising methods for manufacturing 3D Functional Graded objects is 3D laser cladding and/or direct energy deposition. However, the construction of graded and especially layered graded structures in the process of joining materials with different thermophysical properties under certain conditions is accompanied by the formation of cracks along the phase boundaries, which are a consequence of residual stresses and/or chemical segregations. The conditions for phase consolidation are macroscopic balancing of residual stresses in the region of the interface. In a broader sense, in the field of the interface, it is necessary to consider the thermodynamic equilibrium of the phases in connection with mechanical equilibrium. In this regard, the article proposed criteria for the thermodynamic affinity of phases in the area of the Functional Graded Structures interface, including the coefficients of thermal expansion and isobaric and isochoric heat capacities of the phases. Examples of cracking and the use of the obtained criteria are provided.

A finite element approach for gastrointestinal tissue mechanics.

The biomechanical properties of gastrointestinal (GI) tissue play a significant role in the normal functioning of the organ. GI soft tissues exhibit a highly nonlinear rate- and time-dependent stress-strain behaviour. In recent years, many constitutive relations have been proposed to characterize these properties. However, a constitutive relation is not sufficient to analyse the biomechanics at the organ level with complex loading and boundary conditions. Hence, for a refined mechanical analysis, a finite element (FE) implementation of the constitutive relation is needed. Here, we propose an FE implementation of a finite nonlinear hyperviscoelastic model suitable for soft biological tissues. The FE model has been validated at first by comparing its results with the analytical solutions of a standard linear solid, and then it has been used to recreate experimental observations performed on tissue strips obtained from different animals. We have also proposed a method, in this work, to construct a residually stressed FE model so that the consequences of residual stresses on GI mechanics can be examined. Our FE formulation was able to capture the nonlinear soft tissue properties and also demonstrated that the addition of residual stresses reduces stress concentrations and the stress gradient in the GI wall.

Variation of adhesive force at the interface of Pd and SrTiO3 as a consequence of residual stresses

Initially, Pd thin films were deposited over a hard substrate using electron beam physical vapour deposition. The growth and the surface roughness of the films were analysed and their effects upon the conventional indentation test were discussed. Afterwards, an experimental method is described which can measure the critical fracture force in thin films using oscillating indentation. Initially, repetitive contacts at a single point with the purpose of identifying the fracture time provide the fracture force versus fracture time plot. Non-linear curve fitting of the data reveals the theoretical fracture force by a single indentation, which is called the critical fracture force. Arguments are put forward to show the relation between piling up height and applied force. Discrepancies were observed in the plot of the ratio between total indentation depth and piling up height versus applied force when higher loads than a critical fracture force were applied. Discrepancies appear as a result of indenting the substrate. A nanoscratch test facilitated the possibility of measuring adhesion strength and adhesion energy of the films considering the measured critical fracture force as the maximum applied force. The relation between residual compressive stresses, adhesion strength, plastic deformation and piling up area was discussed using dislocation theories. Indentation with high applied loads leaves behind large plastic deformation and reduces the accuracy and reliability of the test results. Hence, lower loads (in the order of nanonewtons) were applied using atomic force microscopy in the friction mode. A pulling off force was mapped in each thickness of Pd films. The results confirm that the area around a hillock exhibits a higher pulling off force due to the local stress relaxation as a consequence of hillock formation. By repeating the mapping process over different areas with various applying forces, the plot of the pulling off force versus applied load was drawn representing discrepancies in the results at higher loads. This phenomenon is associated with the plastic deformation in the films.

Microstructural characterization of laser nitrided titanium

Laser gas alloying of Ti in a nitrogen atmosphere results in a modified surface layer consisting of hard ceramic TiN dendrites embedded in a ductile substrate with on top a thin TiN layer. Hardness as high as 1,800 Vickers has been measured. However after laser processing, as a consequence of residual stresses, cracking occurs. These cracks can be prevented by preheating the sample or by using diluted nitrogen.

Bewertung von Eigenspannungen bei quasistatischer und schwingender Werkstoffbeanspruchung

AbstractAssessment of Residual Stresses in Quasi‐statically and Cyclically Loaded MaterialsThe actual knowledge of residual stresses is extremely extensive (see e.g. [1–8]). Nevertheless, in many cases, the consequences of residual stresses on the mechanical behaviour of materials are still qualitatively assessed by experience. This fact is mainly due to the great expense which is necessary to obtain sufficient quantitative information concerning residual stresses in technical parts. Without such data, however, it is not possible to assess residual stress states objectively.In modern materials technology, residual stresses are usually applied in a twofold manner. On the one hand, they are used as a mostly uncontrollable argument in the interpretation of unexpected failure. On the other hand, they are utilized as a valuable and cheap tool in improving the strength of components.It is well established the principally no material states of practical importance exist which are free of residual stresses. Consequently, the evaluation and assessment of residual stresses in technical parts is of great importance. In this respect, special interest is focussed on such residual stress states which improve the materials behaviour under particular loading conditions.This paper deals with the fundamental aspects of the assessment of statically and cyclically loaded components bearing 1st kind residual stresses. First, some basic principles of the theory of elasticity and the theory of equivalent stresses are outlined. Then, well‐accepted definitions of residual stresses are presented. Afterwards, characteristics of the mechanical resistance of materials are described, which can be determined from simple loading tests. Subsequently, the principles of elementary failure considerations are treated followed by a discussion of the influence of residual stresses on the behaviour of statically loaded components. Typical examples are dealt with. Finally, some basic facts concerning the fatigue behaviour of metallic materials are described, and the consequences of residual stresses on cyclically loaded components are discussed. Presenting characteristic examples, the progress is documented which has recently been achieved in the understanding of the effects of residual stresses on the fatigue behaviour of differently treated materials.

Stress Measurement at Fatigue Crack Tip using PSPC

The growth of fatigue crack builds up the compressive residual stress with the plastic zone in front of the crack tip. The presence of this residual stress is one of the main causes for the crack closure. The consequence of residual stress is a reduction of the crack tip stress intensity variation during a load cycle. Although this effect is well understood in a qualitative manner, it has not been completely quantified because of a lack of information about the load and residual stress distributions at the crack tip.

The Effect of Tensile and Compressive Deformations on Fatigue Strength of Carbon Steel

The phenomenon of fatigue strength being increased by preliminary working has often been the subject of studies, and the increase of fatigue strength due to plastic deformation is thought to be a consequence of residual stress and micro structural change caused by plastic deformation, so that the fundamental relation between fatigue strength and residual stress or micro structural change is to be studied.In this paper, the effect of preliminary working on the fatigue strength of S45C steel is studied from the influences of residual stress and micro structural change caused by tensile and compressive deformations. As the result of the study, it is found that micro structural change is considered to be more responsible than residual stress for the increase of fatigue strength.