Residual stresses removing

Clark [28] for A 533 Gr B Cll carbon steel plate 15 cm thick containing a butt weld. After welding, the plate was given a PWHT at 620 °C for 40 h, followed by furnace cooling. Therefore, it must be assumed that the HAZ properties were restored and residual stresses removed almost entirely. In fact, it can be seen how the FCGR in base metal and HAZ are much the same. The weld metal, instead, behaves much better. FCGR obtained with CT specimens 2 inches (2T-CT) and... 

Weldments subjected to corrosive attack over a period of time may crack adjacent to the weld seams if the residual stresses are not removed. Gas—tungsten arc welding and gas—metal arc welding ate recommended for joining magnesium, the former for thinner materials and the latter for thicker materials. Maintaining a protective atmosphere is a critical issue in welding these alloys. 

Stresses from welding result principally from the effects of differential thermal expansion and contraction arising from the large temperature difference between the weld bead and the relatively cold adjacent base metal. Shrinkage of the weld metal during solidification can also induce high residual stresses. Unless these residual stresses are removed, they remain an intrinsic condition of the weldment apart from any applied stresses imposed as a result of equipment operation. 

Post-processing, e.g. machining (for dimensional restoration) or shot peening (remove residual stresses), can be an added source of variability. 

The methods used to measure residual stresses in a eomponent are performed after the manufaeturing proeess, and are broadly elassed into two types meehanieal (layer removal, eutting) and physieal (X-ray diffraetion, aeoustie, magnetie). Further referenee to the methods used ean be found in Chandra (1997), Juvinall (1967), and Timoshenko (1983). 

In tests on a particular plastic it is found that when a stress of 10 MN/m is applied for 100 seconds and then completely removed, the strain at the instant of stress removal is 0.8% and 100 seconds later it is 0.058%. In a subsequent tests on the same material the stress of 10 MN/m is applied for 2400 seconds and completely removed for 7200 seconds and this sequence is repeated 10 times. Assuming that the creep curves for this material may be represented by an equation of the form (r) = At" where A and n are constants then determine the total accumulated residual strain in the material at the end of the 10th cycle. 

In contrast chemical and electrolytic polishing enables a smooth level surface to be produced without any residual stress being developed in the surface because the surface is removed by dissolution at relatively low chemical potential and at relatively low rates is such a way that metallic surface asperities are preferentially removed. For this to be most effective the solution properties must be optimised and the pretreatment must leave an essentially bare metal surface for attack by the electrolyte. 

When a viscoelastic material is subjected to a constant strain, the stress initially induced within it decays in a time-dependent manner. This behavior is called stress relaxation. The viscoelastic stress relaxation behavior is typical of many TPs. The material specimen is a system to which a strain-versus-time profile is applied as input and from which a stress-versus-time profile is obtained as an output. Initially the material is subjected to a constant strain that is maintained for a long period of time. An immediate initial stress gradually approaches zero as time passes. The material responds with an immediate initial stress that decreases with time. When the applied strain is removed, the material responds with an immediate decrease in stress that may result in a change from tensile to compressive stress. The residual stress then gradually approaches zero. 

Generally, a variety of mechanical deformation processes cause the nonuniform deformation that results in the formation of residual stresses. This nonhomogeneous deformation in a material is produced by the material s parameters, largely its process parameters such as the tool geometry and frictional characteristics. For example, the rolling of a strip can be accomplished by using relatively cold squeeze rolls. In the rolling process, parameters with a small roll diameter and little reduction produce deformation penetration that is shallow and close to the surface, whereas the interior of the strip remains almost undeformed. After the removal of the deformation forces and a complete... 

Though secondary stresses do not affect the bursting strength of the vessel, they are an important consideration when the vessel is subject to repeated pressure loading. If local yielding has occurred, residual stress will remain when the pressure load is removed, and repeated pressure cycling can lead to fatigue failure. 

The difference between the stress at the epoxy-metal interface (0.062 in. of material removed) and the overall measured stress Increases with Increasing coating thickness, up to a thickness of approximately 13 mils. For thickness >13 mils, the residual stress changes only slightly. 

After the removal of the autofrettage pressure the part shows the compressive residual stresses required to reduce the operational stresses effectively (see Chapter 1 Introduction Fig. 1.4.10). Successful autofrettage treatment needs tough and sufficiently strong steels which should offer a certain potential for strain-hardening without unacceptable embrittlement. 

After preforming, there are still residual stresses and entrapped air in the preformed part, which invariably causes cracking mainly during the initial stage of sintering. The removal of entrapped air or degassing requires some time called resting time, which depends mainly on the wall thickness. 

The development of residual stresses in poly(amic acid) coatings (precursors to polyimide films) was investigated using force-temperature experiments. Results indicate that the residual solvent content is dependent on the stress level. Experiments were performed using poly(N,N -bis(phenoxyphenyl)pyromellitamic acid) at temperatures between 60° and 80° C to insure no cycloimidization. By altering the balance between internal stress and the driving force for solvent evaporation further solvent removal may occur. 

The solvent removal process in coatings has been addressed as a transport problem (8,9) and from a mechanics viewpoint. Several methods have been developed to measure residual stresses in coatings. For organic coatings many of these methods utilize plate or beam deflection (6, 10-13). Only recently have they been applied to PI coatings. 

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