The production process of welding defects is very complex, due to both metallurgical reasons and the effects of stress and deformation. Defects have a significant impact on the bearing capacity of welded structures, and more importantly, stress and deformation coexist with defects. Welding defects are prone to occur in the weld seam and its vicinity, and those areas are where the tensile residual stress in the structure is the largest. The main reason why welding defects can reduce the strength of welded structures is that they reduce the effective area of the load-bearing cross-section of the structure and create stress concentrations around the defects. In general welded structures, due to improper design or construction, stress concentration and changes in load-bearing cross-section can also occur. Welding defects generally include incomplete penetration, incomplete fusion, cracks, slag inclusions, air holes, undercuts, weld penetration, and poor weld formation. Welding defects are planar or three-dimensional, and planar type defects have a much greater impact on stress increases than three-dimensional type defects, and are therefore much more dangerous. The former includes cracks, incomplete penetration, incomplete fusion, etc; The latter includes pores and slag inclusions.
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Mechanism of stress concentration from welding defects
Material due to the sudden change in the transfer of load cross-section and the local stress increases, this phenomenon is called stress concentration, the shape of the defect is different, causing a different degree of cross-sectional changes, different angles to the direction of the load, will make the stress concentration around the defect be very different. An ellipsoidal cavity defect, for example, the cavity is surrounded by isotropic infinite elastomer, and the role of stress, when the ellipsoidal cavity gradually becomes a piece of crack, the result is that the stress concentration becomes very serious. In addition to the cavity type of porosity, cracking and failure to weld through, there is also slag is also a common welding defect, when the distance between multiple defects is small (such as dense porosity and slag, etc.), there will be a high stress concentration in the defect area, so that these places appear inter-defective cracks will be connected between the holes.
In this case, the largest stress concentration occurs at the edges of the two outer holes. In welded joints, geometric discontinuities such as weld height increments, misalignment and corner deformations, some of which are allowed by the current code, can produce stress concentrations. In addition, due to the difference in the form of the joint will also appear different stress concentrations, in the form of joints commonly used in welded structures, butt joints have the smallest degree of stress concentration, corner joints, T-joints and frontal lap joints have similar degrees of stress concentration. T-shaped joints in important structures, such as dynamic load work under the H-shaped plate beam, can be used to open the bevel of the plate edge to make the joint in the stress concentration degree be greatly reduced, it is impossible to do this for lap joints, side lap welds along the entire length of the weld stress distribution is very uneven, and the longer the weld, the more serious the unevenness, so the general steel design specifications are specified side lap welds Calculated length may not be greater than 60 times the size of the weld foot. Because beyond this limit even after increasing the length of the side lap weld, it is not possible to reduce the peak stress at both ends of the weld.
02
The effect of welding defects on the static load non-brittle damage of the structure
Welding defects have different degrees of influence on the static load damage of the structure, in general, the destruction of the material is mostly in the form of plastic fracture, when the reduction in strength caused by the defect is roughly proportional to the reduction in the bearing cross-sectional area it causes. In general standards, the weld is allowed to have individual, non-string or non-intensive pores, if the total pore cross-section of only 5% of the working section, pores on the yield limit and tensile strength limit of the impact is not significant, when the total cross-section of bunches of pores more than 2% of the weld cross-section, the intensity of the joint limit is sharply reduced. The main cause of this situation is due to the interruption of the protective atmosphere during welding, so that the emergence of bunches of pores while the weld metal itself decreases in mechanical properties. Therefore, limiting the amount of porosity also serves to prevent deterioration of the weld metal properties. Weld or adjacent surface porosity is more dangerous than deeply buried porosity, and bunches or dense porosity is much more dangerous than individual porosity.
Slag or inclusions, depending on the size of their cross-sectional area proportionally reduce the tensile strength of the material, but the impact on yield strength is smaller. The size and shape of such defects have a greater impact on strength, a single intermittently small spherical slag or inclusions is not more dangerous than the same size and shape of the pores. The linear arrangement, small and perpendicular to the direction of the force direction of the continuous slag is more risky.
Geometrical discontinuity defects, such as galling, poorly formed welds or weld penetrations, not only reduce the effective cross-sectional area of the member, but also produce stress concentrations. When these defects overlap with high residual tensile stress zones in the structure or coarse brittle grain zones in the heat affected zone, they often trigger brittle unstable extensional cracking.
Unboned and undeveloped welds are more harmful than porosity and slag. The effects of unfused and undeveloped welds may not be very obvious when the weld has an increased amount or is made with a welded joint that is superior to the base material. In fact, many welded structures in use have been working for many years without serious accidents caused by unfused and unbroken welds buried inside the seam. But such imperfections may become the initiation point for brittle fracture under certain conditions.
Cracking is considered to the most dangerous welding defect and its existence is not allowed in general standards. As sharp cracks are prone to produce tip notch effect, emergence of three-way stress state and temperature reduction, cracks may be destabilised and extended, causing the fracture of the structure. Cracks are generally generated in the tensile stressed field and poor heat affected zone microstructure segments. Under static load non-brittle damage conditions, if plastic flow occurs before the crack destabilisation and expansion, the residual tensile stress in the structure will have little deleterious effect and will not produce brittle fracture. Unless there is a combination of unfavorable conditions such as sharp deterioration of material properties at the crack tip, poor microstructure in the nearby region, high residual tensile stresses, and a working temperature below the critical temperature, in general even if cracks appear in the material, they are often stopped after they leave the tensile stress field or the deteriorated microstructure zone.
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Conclusion
Mechanism of stress concentration from welding defects
Material due to the sudden change in the transfer of load cross-section and the local stress increases, this phenomenon is called stress concentration, the shape of the defect is different, causing a different degree of cross-sectional changes, different angles to the direction of the load, will make the stress concentration around the defect be very different. An ellipsoidal cavity defect, for example, the cavity is surrounded by isotropic infinite elastomer, and the role of stress, when the ellipsoidal cavity gradually becomes a piece of crack, the result is that the stress concentration becomes very serious. In addition to the cavity type of porosity, cracking and failure to weld through, there is also slag is also a common welding defect, when the distance between multiple defects is small (such as dense porosity and slag, etc.), there will be a high stress concentration in the defect area, so that these places appear inter-defective cracks will be connected between the holes.
In this case, the largest stress concentration occurs at the edges of the two outer holes. In welded joints, geometric discontinuities such as weld height increments, misalignment and corner deformations, some of which are allowed by the current code, can produce stress concentrations. In addition, due to the difference in the form of the joint will also appear different stress concentrations, in the form of joints commonly used in welded structures, butt joints have the smallest degree of stress concentration, corner joints, T-joints and frontal lap joints have similar degrees of stress concentration. T-shaped joints in important structures, such as dynamic load work under the H-shaped plate beam, can be used to open the bevel of the plate edge to make the joint in the stress concentration degree be greatly reduced, it is impossible to do this for lap joints, side lap welds along the entire length of the weld stress distribution is very uneven, and the longer the weld, the more serious the unevenness, so the general steel design specifications are specified side lap welds Calculated length may not be greater than 60 times the size of the weld foot. Because beyond this limit even after increasing the length of the side lap weld, it is not possible to reduce the peak stress at both ends of the weld.
02
The effect of welding defects on the static load non-brittle damage of the structure
Welding defects have different degrees of influence on the static load damage of the structure, in general, the destruction of the material is mostly in the form of plastic fracture, when the reduction in strength caused by the defect is roughly proportional to the reduction in the bearing cross-sectional area it causes. In general standards, the weld is allowed to have individual, non-string or non-intensive pores, if the total pore cross-section of only 5% of the working section, pores on the yield limit and tensile strength limit of the impact is not significant, when the total cross-section of bunches of pores more than 2% of the weld cross-section, the intensity of the joint limit is sharply reduced. The main cause of this situation is due to the interruption of the protective atmosphere during welding, so that the emergence of bunches of pores while the weld metal itself decreases in mechanical properties. Therefore, limiting the amount of porosity also serves to prevent deterioration of the weld metal properties. Weld or adjacent surface porosity is more dangerous than deeply buried porosity, and bunches or dense porosity is much more dangerous than individual porosity.
Slag or inclusions, depending on the size of their cross-sectional area proportionally reduce the tensile strength of the material, but the impact on yield strength is smaller. The size and shape of such defects have a greater impact on strength, a single intermittently small spherical slag or inclusions is not more dangerous than the same size and shape of the pores. The linear arrangement, small and perpendicular to the direction of the force direction of the continuous slag is more risky.
Geometrical discontinuity defects, such as galling, poorly formed welds or weld penetrations, not only reduce the effective cross-sectional area of the member, but also produce stress concentrations. When these defects overlap with high residual tensile stress zones in the structure or coarse brittle grain zones in the heat affected zone, they often trigger brittle unstable extensional cracking.
Unboned and undeveloped welds are more harmful than porosity and slag. The effects of unfused and undeveloped welds may not be very obvious when the weld has an increased amount or is made with a welded joint that is superior to the base material. In fact, many welded structures in use have been working for many years without serious accidents caused by unfused and unbroken welds buried inside the seam. But such imperfections may become the initiation point for brittle fracture under certain conditions.
Cracking is considered to the most dangerous welding defect and its existence is not allowed in general standards. As sharp cracks are prone to produce tip notch effect, emergence of three-way stress state and temperature reduction, cracks may be destabilised and extended, causing the fracture of the structure. Cracks are generally generated in the tensile stressed field and poor heat affected zone microstructure segments. Under static load non-brittle damage conditions, if plastic flow occurs before the crack destabilisation and expansion, the residual tensile stress in the structure will have little deleterious effect and will not produce brittle fracture. Unless there is a combination of unfavorable conditions such as sharp deterioration of material properties at the crack tip, poor microstructure in the nearby region, high residual tensile stresses, and a working temperature below the critical temperature, in general even if cracks appear in the material, they are often stopped after they leave the tensile stress field or the deteriorated microstructure zone.
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Effect of welding defects on brittle damage of structures
Welded structures often cause a catastrophic damage by initiating brittle fracture at flawed locations or structural discontinuities. It is generally accepted that the worse the stress concentration caused by defects in a structure, the more the risk of brittle fracture. Cracking is the most hazardous because the sharpness of the crack tip is much sharper than defects such as underweld, underfusion, nibbling and porosity. When the presence of volume type defects such as porous and slag is less than 5%, they are not harmful to the safety of the structure if the working temperature of the structure is within the plastic-brittle transition temperature of the material. The threshold temperature of a member with cracking is much higher than that of a member containing slag. In addition to the transition temperature to measure the effect of various defects on brittle fracture, many important welded structures use fracture mechanics as the basis for evaluation, because fracture mechanics allows the relationship between fracture stress and crack size and fracture toughness to be determined. Many welded structures have brittle fractures initiated by small cracks, and in general, the structure does not rupture immediately after operation because the small cracks do not attain a critical size. However, small welding defects and discontinuities are likely to grow at a steady rate during use and eventually reaches a critical value and brittle fracture occurs. Therefore, periodic inspection during the service of the structure, timely detection and monitoring of defects close to the critical conditions are the most efficient measures to prevent brittle fracture of welded structures. When welded structures are subjected to impact or local occurrence of high strain and harsh environmental factors, are susceptible to brittle break triggered by welding defects, such as fatigue loads and corrosive environments can make defects like cracks become more acute, so that the size of the crack gradually increases, accelerating it to reach critical values.
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Conclusion
Understanding and mastering the impact of various welding defects on the strength of the structure is necessary for us to correctly grasp the safety of the welded structure, but also to clarify which welding defects may bring catastrophic consequences to the welded structure, which welding defects exist is not a large impact on the strength of the welded structure, which provides a good reference for the determination of our welding quality inspection standards.
