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6.LII 2012 Residual Stress

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6.LII 2012 Residual Stress RESIDUAL STRESSES (FUNDAMENTALS ) REFERENCES : 1. MASUBUCHI, K. 1959. NEW APPROACH TO THE PROBLEM ON RESIDUAL STRESS AND DEFORMATION DUE TO WELDING. TRANSPORTATION TECHNICAL RESEARCH INSTITUTE REPORT 8(12) 2. SATOH, K., AND TERASAKI, T. 1976. EFFECT OF WELDING CONDITIONS ON RESIDUAL STRESS DISTRIBUTIONS AND WELDING DEFORMATION IN WELDED STRUCTURES MATERIALS. J. JAPAN WELD. SOC. 45(1): 42 –53 3. OPTIMISING PLATE GIRDER DESIGN - R. ABSPOEL DIVISION OF STRUCTURAL ENGINEERING, DELFT UNIVERSITY OF TECHNOLOGY, DELFT, THE NETHERLANDS 4. WELDING DISTORTION OF A THIN -PLATE PANEL STRUCTURE BY C. L. TSAI, S. C. PARK AND W. T. CHENG 5. DESIGN OF STEEL STRUCTURES BY PROF. S.R SATISH KUMAR, PROF A.R SANTHA KUMAR, IIT CHENNAI 6. RESIDUAL STRESSES BY T. HÖGLUND, ROYAL INSTITUTE O F TECHNOLOGY, STOCKHOLM 7. NEW FATIGUE PROVISIONS FOR THE DESIGN OF CRANE RUNWAY GIRDERS BY JAMES M. FISHER AND JULIUS P. VAN DE PAS 8. ENHANCING FATIGUE STRENGTH BY ULTRASONIC IMPACT TREATMENT SOUGATA ROY* AND JOHN W. FISHER 9. LIMITATIONS OF AVAILABLE INDIAN HOT -ROLLED I -SECTIONS FOR USE IN SEISMIC STEEL MRFS BY RUPEN GOSWAMI, JASWANT N. ARLEKAR AND C.V.R. MURTY 10. STRESS CORROSION CRACKING BY NATIONAL PHYSICAL LABORATORY(NPL) BY SWARUP DAS LII INDIA, GURGAON ¡ INTRODUCTION It is generally assumed that the distribution of stresses in section of members subjected to axial tensile force is uniform. However, there are some parameters like residual stresses and connection which result in a non-uniform distribution of stresses. Residual stress developed when the member is formed and are due to the production process. Their origin can be thermal, either developed during the solidification of the steel or during welding parts of the member; or they can be mechanically induced when trying to produce counter deflection or when straightening the member. The induced stresses are self equilibrated and although they do not affect the ultimate resistance of member they induce non-linearities in the strain-stress behavior and greater deformabilities. The ultimate limit state is reached when the entire section has yielded. Although the behavior of the section is non-linear, the ultimate limit state is identical for both the cases with and without residual stress. If a part of a member undergoes non-uniform, plastic deformation stresses arise within the elastic area. The sum of negative and positive stresses is always zero, if there are no external forces. The inhomogeneous deformation field which generates residual stress is caused by thermal processes such as cooling after extrusion and welding, mechanical processes such as cold rolling and straightening by means of traction. For a welded T-profile the residual stresses may be formed as follows: The weld is very warm in the beginning. The zone next to the weld is also very warm. When the material cools down, the weld shrinks because of differences in density between the hard and the soft material. Further, the weld will shrink because of the thermal diffusion factor. The surrounding cold and stiff metal prevent this shrinking. This part of the cross section is subject to compressive stresses while the area closest to the weld string is loaded with tensile stresses. 2. OPTIMISING PLATE GIRDER DESIGN In the design of steel plate girders a high degree of optimisation is possible. In the sight of rising steel demands from booming economies and environmental aspects of material production, optimisation in terms of material use is becoming more and more beneficial. Optimising a girder for bending action is achieved by moving material away from the neutral axis of the beam, in other words, by making the web of the plate girder more slender. When lateral supports are used to prevent lateral torsion buckling, then flange induced buckling, torsion buckling of the flange or yielding of the flange will become the critical failure mechanism. The high slenderness causes that the deflection of the beam is not governing. In this strength driven design it is possible to take advantage of higher steel grades and thus to achieve even further reduction of the section. During the fabrication of plate girders undesirable stresses and deformations develop mainly as a result of uneven temperature distributions. These stresses and deformations (imperfections) may significantly affect the performance (e.g. ultimate strength or fatigue life) of the structure. In the research described in this paper an effective numerical method to predict these imperfections is developed with the objective of incorporating this knowledge into the design procedures. 3. WELDING DISTORTION OF A THIN -PLATE PANEL STRUCTURE Welding thin-plate panel structures often results in warping of the panels. Several mitigation methods, including preheating and prestressing the plates during assembly, have been investigated and used by some fabricators. Distortion behaviors, including local plate bending and buckling as well as global girder bending, It is found that buckling doesn’t occur in structures with a skin-plate thickness of more than 1.6 mm unless the stiffening girder bends excessively. Warping is primarily caused by angular bending of the plate itself. The joint rigidity method (JRM) is found to be effective in determining the optimum welding sequence for minimum panel warping. Warping is a common problem experienced in the welding fabrication of thin walled panel structures. Several factors that influence distortion control strategy may be categorized into design-related and process- related variables. Significant design-related variables include weld joint details, plate thickness, thickness transition if the joint consists of plates of different thickness, stiffener spacing, number of attachments, corrugated construction, mechanical restraint conditions, assembly sequence and overall construction planning. Important variables are welding process, heat input, travel speed and welding sequence. These design practices include choosing plates with appropriate thickness, reducing stiffener spacing, using a bevel T-stiffener web, optimizing assembly sequencing, properly applying jigs and fixtures and using the egg-crate construction technique. Better control of certain welding variables will eliminate the conditions that promote distortion. This includes reducing fillet weld size and length, including tack welds; using high-speed welding; using a low heat input welding process; using intermittent welds; using a back-step technique; and balancing heat about the plate’s neutral axis in butt joint welding. The implementation of distortion mitigation techniques during welding counteracts the effects of shrinkage during cooling, which distorts the fabricated structure. These mitigation techniques include controlled preheating, mechanical tensioning, thermal tensioning, pre-bending fillet joints, presetting butt joints and using appropriate heat sinking arrangements. All these mitigation techniques are to balance weld shrinkage forces. Heat sinking also balances welding heat about the neutral axis of the joint. Some of the aforementioned distortion control methods may increase fabrication costs due to requirements for more energy, increased labor and potentially high-cost capital equipment. Some methods may not be suitable for automated welding or may reduce the assembly speed due to interruption from fixtures or stiffener arrangements. Depending on circumstances of the fabrication environment and type of structures, different distortion control methods may provide more adequate solutions to certain problems than others. Understanding their capability and limitation of all these distortion control methods is critical to a successful welding fabrication project. ror 4. DESIGN OF STEEL STRUCTURES One of the various factors affecting the lateral-torsional buckling strength is Magnitude and distribution of residual stresses. The effect of residual stresses is to reduce the lateral buckling capacity. If the compression flange is wider than tension flange lateral buckling strength increases and if the tension flange is wider than compression flange, lateral buckling strength decreases. The residual stresses and hence its effect is more in welded beams as compared to that of rolled beams. 5. HOW TO MEASURE RESIDUAL STRESS The most common method is the destructive method, which is based upon the technique of cutting the specimen in a number of strips. The residual stresses are calculated from measurements on each strip. There are two methods of measuring. The first is to measure the length of the strip before and after the cutting it from the section. If Young's modulus is known, it is easy to apply Hooke's law and determine the residual stress. The second method is to mount electrical resistance strain gauges on the strips and determine the residual stresses by applying Hooke's law. The last method is that which is most commonly used today. Note that Hooke's law can be applied since residual stress is essentially an elastic process. With the methods stated here only longitudinal residual stresses are determined. However, these are of most interest from a structural point of view. 6. RESIDUAL STRESS IN EXTRUDED PROFILES A number of experiments where residual stresses are determined for different types of profiles have been made. These consist of different alloys and were manufactured by various processes. Here, the results from experiments on I-profiles are reviewed. Experiments conducted on I- profiles consisting of different alloys show that the residual stresses are randomly distributed
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