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 OF 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 over a cross section. It seems there is no simple rule for stress distribution as there is for rolled steel sections. Residual stresses are low, the compressive stresses almost never exceed 20 MPa and tensile stresses are much lower. These values are measured on the surface of the profiles. At the centre of the material the values are probably lower since residual stresses usually change sign from one side to the other. Different alloys do not affect the intensity and distribution of residual stresses. The residual stresses in extruded profiles have a negligible effect on the load-bearing capacity. 7. RESIDUAL STRESSES IN WELDED PROFILES In contrast to what has been said for extruded profiles, residual stresses cannot be neglected in welded profiles. The welding produces a concentrated heat input, which causes the remaining stresses. Large tensile stresses in connection to the web and balancing compression stresses in other parts are characteristic.

8. NEW FATIGUE PROVISIONS FOR THE DESIGN OF CRANE RUNWAY GIRDERS Abrupt changes in cross section, geometrical discontinuities such as toes of welds, unintentional discontinuities from lack of perfection in fabrication, effects of corrosion and residual stresses all have a bearing on the localized range of tensile stress at details that lead to crack initiation. These facts make it convenient and desirable to structure fatigue design provisions on the basis of categories, which reflect the increase in tensile stress range due to the severity of the discontinuities introduced by typical details. Application of stress concentration factors to stresses determined by usual analysis is not appropriate. However, fluctuating compressive stresses in a region of tensile residual stress may cause a net fluctuating tensile stress or reversal of stress, which may cause cracks to initiate.

9. ENHANCING FATIGUE STRENGTH The existence of localized tensile residual stresses to the order of yield stress in the weldments and in the adjoining base metal produced by shrinkage during the cooling process after welding. In the initial stages of fatigue crack growth from inherent crack-like defects in an as-welded detail, most of the fatigue life occurs in this region of high tensile residual stress. Under cyclic loading the material around the internal flaws in this area is always subjected to a fully effective tensile stress cycle even in some cases of stress reversal. Residual stresses can become tensile or compressive depending on their relative magnitude. Particularly at the low stress range or in the high fatigue cycle regime the treated details became unfavorably sensitive to factors that were less significant to crack growth than to crack initiation, It is anticipated that the benefits from improved joints will generally increase with yield strength due to introduction of higher beneficial compressive residual stress. It also introduced beneficial compressive residual stress to the order of yield stress of the material at the treated surface. Cumulatively these improvements enhanced fatigue strength of the treated details by increasing the fatigue crack initiation life and the fatigue crack growth threshold. 10. LIMITATIONS OF AVAILABLE INDIAN HOT-ROLLED I-SECTIONS FOR USE IN

SEISMIC STEEL MRFS local buckling can occur in Indian hot-rolled I-sections at low post yield strains due to presence of residual stresses. Material non-linearity was shown

to begin at about 70 to 43 percent of the plastic moment capacity for residual stresses of 70MPa and 140MPa respectively. Consequently, flexural plastic capacity is reached at extreme fibre strain of about 2.4 to 2.8 times the yield strain. This high strain can cause local buckling [Paul et al., 1999]. All these aspects raise the concern on the stability of structures built using the available Indian hot-rolled I-sections with tapered flanges for resisting earthquake effects. 11. SEISMIC DESIGN OF STRONG-AXIS WELDED CONNECTIONS IN STEEL MOMENT It’s well known fact that due to the presence of residual stresses in rolled section, it yields at an early stage i.e., half of its yield tress or so and also reaches to its full capacity when the extreme fibres strain is much higher than the yield strain. 12. STRESS CORROSION CRACKING Stress corrosion cracking is cracking due to a process involving conjoint corrosion and straining of a metal due to residual or applied stresses. The cracking continues at low stresses and commonly occurs as a result of residual stresses from welding or fabrication. The cracking is normally transgranular, although it may switch to an intergranular path as a result of sensitisation of the steel. Real components will typically contain defects and design details, such as notches, sharp changes in section, welds, corrosion pits etc, that will produce a stress concentration, hence allowing the threshold stress to be exceeded locally even though the nominal stress may be well below the threshold. Furthermore, residual stresses produced by welding or deformation will frequently be close to the yield stress. As one of the requirements for stress corrosion cracking is the presence of stress in the components, one method of control is to eliminate that stress, or at least reduce it below the threshold stress for SCC. This is not usually feasible for working stresses (the stress that the component is intended to support), but it may be possible where the stress causing cracking is a residual stress introduced during welding or forming. Residual stresses can be relieved by stress-relief annealing, and this is widely used for carbon steels. These have the advantage of a relatively high threshold stress for most environments, consequently it is relatively easy to reduce the residual stresses to a low enough level. In contrast austenitic stainless steels have a very low threshold stress for chloride SCC. This, combined with the high annealing temperatures that are necessary to avoid other problems, such

as sensitisation and sigma phase embrittlement, means that stress relief is rarely successful as a method of controlling SCC for this system. For large structures, for which full stress-relief annealing is difficult or impossible, partial stress relief around welds and other critical areas may be of value. However, this must be done in a controlled way to avoid creating new regions of high residual stress, and expert advice is advisable if this approach is adopted. We can’t easily change the material or the temperature, and it is virtually impossible to eliminate the residual stresses associated with welding and forming of the stainless steel.

13. ULTRA HIGH-STRENGTH SEAMLESS, HOT ROLLED HOLLOW SECTIONS FROM

ISMT UHS hollow sections do not contain any weld and are hot formed. As a result, these have a fully normalized grain structure and uniform hardness along the cross section. Unlike their welded counterparts, these do not suffer from any weak spots, discontinuities in material properties, or residual stresses along the perimeter of the section.

14. RESONANT VIBRATION METHOD FOR REDUCING RESIDUAL STRESSES IN WELDED

OR MACHINED FABRICATIONS For many people involved in the metalworking trades, the subject of stress relief is something they are not well versed in. As a result, stress relief is a subject they would just as soon like to avoid. With a little technical assistance, the average layman can get a basic understanding of residual stresses and how to deal with them. With this knowledge, he will be better prepared to evaluate shop problems and find a |solution that is effective. The following information is designed to answer some of the most frequently asked questions about stress relieving. Residual stresses, by definition, are those stresses in an elastic body that is free from external force or restraint and temperature gradients. An incompatibility of regions in the metal created by non-homogenous plastic deformation is the principal cause of these internal stress systems, whether they are in an individual part or in an assembly of parts. This mismatch or misfit between adjacent regions of the same part distorts the neighboring regions. This condition can be either extremely damaging or very beneficial to the part depending on the magnitude and direction. Compressive stresses

created by shot peening can be good while tensile stresses created during welding can be bad. Residual stresses are hard to visualise, difficult to measure and extremely difficult to calculate or predict, yet they are just as important in the function of a part as are externally applied forces that are more easily measured and calculated. Residual stresses are fundamentally introduced into the material in one or more of the following ways: thermal, metallurgical, mechanical and chemical. Since these are the processes that make up our metalworking trades, it is only right to assume that, at some point in time, a stress relief treatment may be required. "Formula 62" is a resonance based method of vibratory stress relief developed by Stress Relief Engineering Co. Workpieces are subjected to low frequency, high amplitude vibrations for a short period of time based on the weight of the workpiece. This allows the residual stresses to be reduced to a much lower level where static equilibrium is restored. The resonance method is used by researchers around the world in stress relief studies using vibration and is currently considered an industry standard. Resonant vibrations have been found to be the most effective means for reducing residual stresses by vibration. The resonant frequency vibration method has a much more pronounced stress redistribution compared to the subresonant (subharmonie) frequency methods. High amplitude resonant vibrations are very efficient in significantly reducing peak residual stresses in weldments. Parts may be stress relieved virtually at any point in the manufacturing process where the part is accessible. The most typical applications allow for stress relief at key junctures in the manufacturing process, i.e. after rough machining, boring, grinding, etc. For welded fabrications, stress relieving can be performed during welding which is very helpful in preventing residual stress build-up that can cause weld cracking or distortion of some sections. Because the fusion process produces large temperature gradients in a short period of time, residual stresses are more dynamically active which can require stress relief during welding, immediately after welding or in an ongoing program of routine stress relief on a daily basis. As the time to completion increases for a fabrication, so does the risk of distortion related problems. Since large magnitude tensile residual stresses can reduce the fatigue life of welded assemblies, ample thought should be given to stress relieving all welded assemblies.

In assembling and joining parts of a structure or of built-up members, the procedure and sequence of welding shall be such as will avoid needless distortion and minimize shrinkage stresses. Where it is impossible to avoid high residual stresses in the closing welds of a rigid assembly, such closing welds shall be made in compression elements. 15. FACTORS INFLUENCING FATIGUE BEHAVIOR The fatigue behavior of various types of structures, members and connections is affected by a large number of factors, many of which may produce interrelated effects. The parameters that influence fatigue behavior are: stress range, material, stress concentration, rate of cyclic loading, residual stressesresidual stressesresidual stressesresidual stresses, size, geometry, environment, temperature, and previous stress history. The effect of residual stress varies considerably, depending upon the material, state and magnitude of residual and applied stresses. The effect of compressive residual stress generally is to increase the fatigue resistance for lower levels of stress. But for higher levels of stress close to yielding, its effect is negligible. The residual tensile stresses do not affect fatigue resistance except in cases where residual tension reduces the stress range in cyclic loading. Welds invariably contain small crack-like defects; hence crack initiation stage does not exist. Only the number of cycles for the crack to grow to the point of unstable fracture constitutes the fatigue life. Residual stresses of yield stress level are always present in the vicinity of welds. Therefore stress cycling is always from the yield stress downwards and fatigue life is a function of stress range only. Fatigue life varies with the type of weld details due to the varying nature of the defects in the different details. Much of the current information on weld improvements has been derived from tests on small-scale specimens. In real structures, there will be large residual stresses that may affect fatigue life. Whereas peak stresses develop at the weld toe of a small joint, in a large multi-pass joint, peak stress may occur in any of the several beads and cracks initiate anywhere in this region. Peening is the application of repeated hammering, often with a round-headed punch or hammer, to produce local yielding of the material. The hammering is applied to the weld toe or other locations, where fatigue cracks are likely to initiate. It has the effect of reducing the local residual stresses.

� � � GENERAL SUGGESTIONS ARE LISTED BELOW FOR GUIDANCE WHILE DESIGNING

A WELDED STRUCTURE WITH RESPECT TO FATIGUE STRENGTH. · Adopt butt or single and double bevel butt welds in preference to fillet welds. · Use double-sided in preference to single sided fillet welds. · Aim to place weld, particularly toe, root and weld end in area of low stress. · Avoid details that produce severe stress concentration or poor stress distribution. · Provide gradual transitions in sections and avoid reentrant notch like corners. · Avoid abrupt changes of section or stiffness of members or components. · Avoid points so as to eliminate eccentricities or reduce them to a minimum. · Avoid making attachments on parts subjected to severe fatigue loading. If attachments in such locations are unavoidable, the weld profile should merge smoothly into the parent metal. · Use continuous rather than intermittent welds. · Avoid details that introduce localized constraints. · During fabrication, carry out necessary inspection to ensure proper deposition of welds. · Provide suitable inspection during the fabrication and erection of structures. · Intersection of welds should be avoided. · Edge preparation for butt welding should be designed with a view of using minimum weld metal so as to minimize warping and residual stress build up. · Ask for pre and post heating, if necessary, to relieve the build up residual stresses. · Fillet welds carrying longitudinal shear should not be larger in size than necessary from design consideration. · Deep penetration fillet welds should be used in preference to normal fillet welds. · Structures subjected to fatigue loading especially in critical locations should be regularly inspected for the presence of fatigue cracks and when such cracks are discovered, immediate steps should be taken to prevent their further propagation into the structure. · Any repair measures taken should be designed to avoid introduction of more severe fatigue condition.

· Provide multiple load path and / or structural redundancy in the structure to avoid overall collapse of the structure due to failure of one element in the structure in fatigue. · Provide crack arresting features in the design at critical locations to avoid propagation of cracks into the entire structure.

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