Fundamental Types Of Distortion Engineering Essay
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The high localised heating required in welding joint edges cause non-uniform stresses in the component and lead to expansion and contraction of the heated material. Initially, compressive stresses are created in the surrounding cold parent metal when the weld pool is formed due to the thermal expansion of the hot metal (heat affected zone) adjacent to the weld pool. However, tensile stresses occur on cooling when the contraction of the weld metal and the immediate heat affected zone is resisted by the bulk of the cold parent metal.
The magnitude of thermal stresses induced into the material can be seen by the volume change in the weld area on solidification and subsequent cooling to room temperature. For example, when welding CMn steel, the molten weld metal volume will be reduced by approximately 3% on solidification and the volume of the solidified weld metal/heat affected zone (HAZ) will be reduced by a further 7% as its temperature falls from the melting point of steel to room temperature.
If the stresses generated from thermal expansion/contraction exceed the yield strength of the parent metal, localised plastic deformation of the metal occurs. Plastic deformation causes a permanent reduction in the component dimensions and distorts the structure.
Fundamental Types of Distortion
Three fundamental dimensional changes that occur during the welding process cause distortion in fabricated structures:
1. Transverse shrinkage perpendicular to the weld line
2. Longitudinal shrinkage parallel to the weld line
3. Angular distortion (rotation around the weld line)
These dimensional changes are shown in 1 and are classified by their appearance as follows:
(a) Transverse shrinkage. Shrinkage perpendicular to the weld line
(b) Angular change (transverse distortion). A non-uniform thermal distribution in the thickness direction causes distortion (angular change) close to the weld line.
(c) Rotational distortion. Angular distortion in the plane of the plate due to thermal expansion.
(d) Longitudinal shrinkage. Shrinkage in the direction of the weld line.
(e) Longitudinal bending distortion. Distortion in a plane through the weld line and perpendicular to the plate.
(f) Buckling distortion. Thermal compressive stresses cause instability when plates are thin.
Figure - Various types of weld distortion
Contraction of the weld area on cooling results in both transverse and longitudinal shrinkage.
Non-uniform contraction (through thickness) produces angular distortion in addition to longitudinal and transverse shrinkage.
For example, in a single V butt weld, the first weld run produces longitudinal and transverse shrinkage and rotation. The second run causes the plates to rotate using the first weld deposit as a fulcrum. Hence, balanced welding in a double side V butt joint can be used to produce uniform contraction and prevent angular distortion.
Similarly, in a single side fillet weld, non-uniform contraction produces angular distortion of the upstanding leg. Double side fillet welds can therefore be used to control distortion in the upstanding fillet but because the weld is only deposited on one side of the base plate, angular distortion will now be produced in the plate.
Longitudinal bowing in welded plates happens when the weld centre is not coincident with the neutral axis of the section so that longitudinal shrinkage in the welds bends the section into a curved shape. Clad plate tends to bow in two directions due to longitudinal and transverse shrinkage of the cladding; this produces a dished shape. Dishing is also produced in stiffened plating. Plates usually dish inwards between the stiffeners, because of angular distortion at the stiffener attachment welds.
In plating, long range compressive stresses can cause elastic buckling in thin plates, resulting in dishing, bowing or rippling.
Distortion due to elastic buckling is unstable: if you attempt to flatten a buckled plate, it will probably 'snap' through and dish out in the opposite direction.
Twisting in a box section is caused by shear deformation at the corner joints. This is caused by unequal longitudinal thermal expansion of the abutting edges. Increasing the number of tack welds to prevent shear deformation often reduces the amount of twisting.
(Hirai and Nakamura 1955) conducted an investigation to determine the values of the angular change in a free joint and the coefficient of rigidity for angular changes under various conditions. 2shows the values of angular change as a function of plate thickness, t (mm), and weight of electrode consumed per weld length, w (g/cm). In order to convert from w to the size of the fillet weld, Df (mm), the following formula may be used:
Where ? = density of weld metal,
?d = deposition efficiency.
The fillet size, Df, is commonly used in design work, while w is easy to determine in a welding experiment.
Figure - Angular change of a free fillet weld in steel
The results shown in 2 were obtained using covered electrodes 5mm in diameter. The maximum angular changes were obtained when the plate thickness was around 9mm. Then the plate was thinner, the amount of angular change was reduced with the plate thickness. This is because the plate was heated more evenly in the thickness direction, thus reducing the bending moment. When the plate was thicker than 9mm, the amount of angular change was reduced as the plate thickness increased because of increased rigidity.
Previous two dimensional investigations (Duffy 1970; Shin 1972) of out-of-plane distortion of welded panel structures have shown that distortion increases with span length, and size of fillet weld. The investigations also indicated that there is a peak in distortion around 10mm plate thickness with lower distortion for thicknesses of 6mm and 14mm.
When thin plates are welded, residual compressive stresses occur in areas away from the weld and cause buckling. Buckling distortion occurs when the specimen length exceeds the critical length for a given thickness in a given specimen size. It is important to determine whether distortion is caused by buckling of bending. Buckling distortion differs from bending distortion in that:
1. There is more than one stable deformed shape
2. The amount of deformation in buckling distortion is much greater
Since the amount of buckling distortion is large, the best way to avoid it is to properly select such structural parameters as plate thickness, stiffener spacing and welding parameters.
Extensive experimental and analytical investigations described in (Masubuchi 1970) conducted at Kawasaki heavy Industry clearly indicate the existence of a critical buckling heat input for given conditions.
The critical buckling heat input decreases as plate thickness decreases and free span increases.
For a given panel size the critical values for the heat input, are not affected by plate thickness.
The critical heat input for buckling is little affected by the difference in welding process.
Longitudinal and Transverse Shrinkage
Twisting Contraction of the weld area on cooling results in both transverse and longitudinal shrinkage, whereas non-uniform contraction (through thickness) produces angular distortion. For example, in a single V butt weld, the first weld run produces longitudinal and transverse shrinkage and rotation. The second run causes the plates to rotate using the first weld deposit as a fulcrum. Hence, balanced welding in a double side V butt joint can be used to produce uniform contraction and prevent angular distortion. Similarly, in a single side fillet weld, non-uniform contraction produces angular distortion of the upstanding leg. Double side fillet welds can therefore be used to control distortion in the upstanding fillet but because the weld is only deposited on one side of the base plate, angular distortion will now be produced in the plate.
The temperature distribution in the weldment is not uniform as a result of local heating (by most welding processes), and changes that take place as welding progresses. Heat-affected zones of the weldment and the base metal immediately adjacent to the welded area are at a temperature substantially above that of the unaffected base metal. Compressive stresses are created in the surrounding cold parent metal, when the weld pool is formed due to the thermal expansion of the hot metal (heat affected zone) adjacent to the weld pool. As the molten pool solidifies and shrinks, it begins to exert shrinkage stresses on the surrounding weld metal and heat-affected zone. However, tensile stresses occur on cooling when the contraction of the weld metal and the immediate heat affected zone is resisted by the bulk of the cold parent metal.
Residual stresses in weldments have following two major effects: First, they produce distortion. Distortion is caused when the heated weld region contracts non-uniformly, causing shrinkage in one part of the weld to exert eccentric forces on the weld cross-section. The weldment strains elastically in response to these stresses. The distortion may appear in butt joints both as longitudinal and transverse shrinkage and as angular change (rotation) when the face of the weld shrinks more than the root. The latter change produces transverse bending in the plates along the weld length. Distortion in fillet welds is similar to that in butt welds. Transverse and longitudinal shrinkage as well as angular distortion results from the unbalanced nature of the stresses in these welds. Since fillet welds are often used in combination with other welds in a weldment, the specific resulting distortion may be complex.
Secondly, residual stresses may be the cause of premature failure in weldments. If the stresses generated from thermal expansion/contraction exceed the yield strength of the parent metal, localised plastic deformation of the metal occurs. Plastic deformation causes a permanent reduction in the component dimensions and distorts the structure.
The residual stresses in a component or structure are stresses caused by incompatible internal permanent strains. They may be generated or modified at every stage in the component life cycle, from original material production to final disposal. Welding is one of the most significant causes of residual stresses and typically produces large tensile stresses whose maximum value is approximately equal to the yield strength of the materials being joined, balanced by lower compressive residual stresses elsewhere in the component.
Tensile residual stresses may reduce the performance or cause failure of manufactured products. They may increase the rate of damage by fatigue, creep or environmental degradation. They may reduce the load capacity by contributing to failure by brittle fracture, or cause other forms of damage such as shape change or crazing. Compressive residual stresses are generally beneficial, but cause a decrease in the buckling load.
Residual stresses may be measured by non-destructive techniques, including X-ray diffraction, neutron diffraction and optic magnetic and ultrasonic methods; by locally destructive techniques, including hole drilling and the ring core and deep hole methods; and by sectioning methods including block removal, splitting, slicing, layering and the contour method. The selection of the optimum measurement technique should take account of volumetric resolution, material, geometry and access.
Prediction of residual stresses by numerical modelling of welding and other manufacturing processes has increased rapidly in recent years. Modelling of welding is technically and computationally demanding, and simplification and idealisation of the material behaviour, process parameters and geometry is inevitable. Numerical modelling is a powerful tool for residual stress prediction, but validation with reference to experimental results is essential.
Allowing for residual stresses in the assessment of service performance varies according to the failure mechanism. It is not usually necessary to take account of residual stresses in calculations of the static strength of ductile materials. Design procedures for fatigue or buckling of welded structures usually make appropriate allowances for weld-induced residual stresses, and hence it is not necessary to include them explicitly. Residual stresses have a major effect on fracture in the brittle and transitional regimes, and hence the stress intensity, K, or energy release rate, J, due to residual stresses must be calculated and included in the fracture assessment. K or J may be obtained as a function of stress distribution, crack size and geometry by various methods, including handbook solutions, weight functions, and finite element analysis.
Residual stresses in as-welded structures may be minimised by appropriate selection of materials, welding process and parameters, structural geometry and fabrication sequence. Residual stresses may be reduced by various special welding techniques including low stress non-distortion welding (LSND), last pass heat sink welding (LPHSW) or inter-run peening. They may be relaxed by thermal processes including postweld heat treatment and creep in service, or by mechanical processes including proof testing and vibratory stress relief. Different stress relief treatments are appropriate in different applications. The effectiveness of the treatment may be reduced or the residual stresses may be increased if the treatment is not applied properly. Specialised processes are available for inducing beneficial compressive residual stresses, including peening, shot blasting, induction heating stress improvement (IHSI), low plasticity burnishing (LPB) and mechanical stress improvement procedures (MSIP).
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