Vibratory Stress Relief In Manufacturing Processes

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Many manufacturing processes use large amounts of energy in order to make useful products. One such process is thermal stress relief (TSR) process that is widely used to reduce/modify the internal and residual stresses in parts that are introduced by other manufacturing procedure such as fabrication, machining ,or assembly. The TSR  process is  typically done in large furnaces and is heated by combustion of fossile fuels.The large consumption of energy, thermal influence and air pollution affects on environment are the biggest concerns of this process. As an alternative to the TSR , vibratory stress relief (VSR) has the advantages of  low energy consumption and dramatic reduction in pollution to the environment. This paper contains analytical model using finite element techiniques to show how vibration alters the residual stresses in part in a beneficial way ,mathematical analysis of VSR and applications of VSR.

Welding is widely used for construction of many structures. Since welding is a process using locally given heat, residual stress is generated near the bead, tensile residual stress degrades fatigue strength. Some reduction methods of residual stresses are,  heat  treatment and shot peening are practically used. However, those methods need special equipment and consume much time.

The residual stresses in parts will cause them to distort either during manufacturing  process or later during useful life of the part. The widely used traditional process of thermal stress relief involves raising the temperature of the part under carefully controlled conditions several hundred degrees , holding at the high temperature until the bulk of the stresses have been removed,and then slowly cooling the part inorder to prevent introducing new residual stresses .For some parts this may have to be done several times during manufacturing process.Thermal heat treatment of stress relief always results in some loss of strength in the part,some distortion during the process, and oxidation of the surface of the part.Vibratory stress modification is an alternative method for altering or reducing residual stresses in parts so that  they do not  distort during subsequent manufacturing process and during the life of the part.

The basic theory is that cyclic vibratory stresses added to the residual stresses exceed the yield strength of the material and causes local plastic deformation and substantial reduction of residual stresses. Vibratory stress relief is accomplished by vibrating the part at the particular frequency and amplitude for short period of time thereby using a minute amount of energy and minute amount of pollution compare to the traditional heat treatment process. VSR system can be regarded as damped forced vibration system. As mentioned above the VSR  process is applied during/after welding process.

Analytical Model

Experimental setup for butt-welding

Yield stress of metal immediately after welding is very low. It is considered that permanent deformation can be generated by very low external load. In this analysis, reduction of tensile residual stress on the bead of thin plate is dealt with using two-dimensional model. Since specimens are thin plates, plane stress state is considered. Hence, an analytical model shown in Figure 2 (Z e=0) is used considering actual stresses in plane. x-axis is longitudinal direction of the specimen, direction of surface vibration, and y-axis is transverse direction of the specimen, direction of the bead. As shown in Figure 2 (Z e>0), springs in transverse direction are extended by  from the equilibrium position. In this case,  is initial residual stress. It is assumed that restoring force-deformation relation of the springs is represented by the perfectly elasto-plastic model. Accordingly, when stresses in x-axis and y-axis are considered to be principal stresses, it is assumed that springs are yielding according to Tresca yield criterion as shown in Figure 3. This model is excited in x-axis.

Vibratory stress relief method is used which avoids side effects TSR (distortion, oxidation, and degradation of mechanical properties). The geometrical form of components is controlled in tolerance that is permitted. VSR method permits to modification in metals used for components to conserve resources and prevent pollution. The procedure of VSR can be used in manufacturing the rails of Maglev system effectively.
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