Joints In Steel Structures Research Articles

繰返し大ひずみ負荷後の高張力鋼溶接継手の疲労強度について

Earthquakes cause damages by fatigue at welded joints of steel structures. Structures which were not seriously damaged would be continuously used even after earthquakes. The fatigue strength of those structures, however, might have been decreased due to the cyclic straining during earthquake. In order to clarify the effect of large cyclic straining on the fatigue strength of the welded joint, low cycle fatigue crack propagation test under large straining as well as high cycle fatigue tests after large cyclic straining were performed on welded joint made of 780 MPa class high strength steel. Experimental results showed that the high strength steel welded joint exhibited an excellent fatigue limit characteristic in the case of reversal stressing. However, the fatigue limit is quite sensitive to mean strain. The application of positive mean strain substantially decreased the fatigue limit. Moreover, the cyclic application of large strain caused fatigue crack, which caused the substantial decrease of fatigue strength. The amount of reduction in fatigue limit was larger in the case of high strength steel compared to that in the case of low strength steel.

Effect of dynamic loading on fracture behavior of welded joints of structural steel

Effect of dynamic loading on fracture behavior of welded joints of structural steel

Hydrogen Diffusion Coefficient and Modelling of Hydrogen Behaviour in Welded Joints of Structural Steels

Modelling investigations, conducted to evaluate the influence of diffusion controlling factors on hydrogen behaviour in welded joints, are described. It was found that neglecting or exaggerating influence of these factors when plotting the temperature dependences of hydrogen diffusion coefficient could result in unreliable modelling solutions for hydrogen behaviour. Creation of databases with general diffusion data for small groups of steels and weld metals with similar alloying contents is suggested as a possible solution of the problem of correct accounting the influence of diffusion controlling factors.

Fatigue life prediction for AISI 304L butt welded joints having different bead geometry using local stress approach

It is indicated in the literature that the toe geometry affects the fatigue properties of butt welded joints in structural steels. In an attempt to study the above effect with respect to stainless steel sheet metal welded joints, butt welded joints having two bead geometries were fabricated using AISI 304L stainless steel using gas tungsten arc welding and gas metal arc welding processes. The objective of the present work was to predict the fatigue life of AISI 304L butt joints failing at the weld toe, using a local stress approach (a two stage model). The notch root stress method was applied to calculate the fatigue crack initiation life, whereas the fatigue crack propagation life was estimated using fracture mechanics concepts. Constant amplitude fatigue tests with a stress ratio R=0 were carried out using a 100 kN servohydraulic Dartec universal testing machine at a frequency of 30 Hz. The crack initiation points were determined using strain gauges, and crack propagation gauges were used to obtain the crack propagation data during the fatigue process. The predicted lives were compared with the experimental values.

Fatigue life prediction using two stage model for AISI 304L cruciform joints, with different fillet geometry, failing at toe

Reports in the literature indicate that the fillet geometry affects the fatigue properties of cruciform welded joints in structural steels. In an attempt to study the above effect with respect to stainless steel sheet metal welded joints, load carrying transverse fillet welded cruciform joints having a two fillet geometry were fabricated from AISI 304L stainless steel using gas tungsten arc welding and gas metal arc welding with 308L electrodes. The objective of the present work is to predict the fatigue life of AISI 304L cruciform joints failing at the weld toe using a two stage model. The local stress life method was applied to calculate the fatigue crack initiation life, whereas the fatigue crack propagation life was estimated using fracture mechanics concepts. Constant amplitude fatigue tests with stress ratio R ~ 0 were carried out using a 100 kN servohydraulic Dartec universal testing machine at a frequency of 30 Hz. An automatic crack monitoring system based on crack propagation gauges was used to obtain the propagation data during the fatigue process. The predicted lives were compared with the experimental values.

Local strain for fatigue strength of welded structures

Field criteria, which are usually applied in fracture mechanics to ensure the structural integrity of cracked components, are extended to the fatigue design of welded joints, whose weld toe can be assimilated to a notch with a small tip radius. In particular, fatigue strength in terms of strain rather than of stress has been determined by applying stress cycles with different stress ratios R = σmin/σmax. Finally, for the evaluation of the fatigue strength of welded joints in structural steel, a strain curve versus different R ratios is given to be compared with the service measured local strain at the weld toe.

Improving standard requirements for the quality of welded joints in steel structures ‐ part 3

Improving standard requirements for the quality of welded joints in steel structures ‐ part 3

Improvement of standard requirements on the quality of welded joints in steel structures. Part 2

Improvement of standard requirements on the quality of welded joints in steel structures. Part 2

Influence of stress concentrators on the fatigue and fracture characteristics of steels and welded joints

We study the effect of enhancement of the fatigue strength of welded joints in steel structures caused by the removal of the concentration of welding stresses. Main attention is given to the so-called method of contact treatment of the surface of welded joints close to the technology of arc welding. It is shown that the application of this method not only removes the defects of welded joints but also significantly increases their fatigue strength (by 20–100%).

Cold cracks in welded joints of structural steels

The present survey is devoted to the investigation of a dangerous phenomenon (quite often observed in the process of arc welding) of the formation of hydrogen-induced cold cracks in welded joints of structural steels. We briefly analyze the existing models of the mechanism of hydrogen embrittlement. It is shown that difficulties in understanding the processes of initiation and growth of cold cracks are explained not only by the necessity of determination of the principal factors which govern the embrittling action of hydrogen but also by the absence of reliable knowledge of the microscopic mechanisms of fracture processes in metals. We suggest a new model of hydrogen embrittlement of metals with bcc lattice developed at the Paton Institute of Electric Welding. According to this model, atomic hydrogen adsorbed on the metal surface in the form of anions affects the surface energy of submicrocracks appearing in dislocation clusters in the process of deformation whose initial period of propagation obeys the classical Griffith scheme. The proposed model enables one to explain all known distinctive features of the cold cracking of welded joints and forms a groundwork for the physical simulation based the use of mathematical methods and aimed at the quantitative evaluation of the contribution of each factor to the indicated processes.

An Evaluation Method on Fatigue Crack Initiation Life at Welded Joints in Steel Structures

This paper presents a simple model to evaluate fatigue crack initiation life. In the model, the stress concentration, welding residual stress and the effect of loading history are taken into account.

Investigation of fusion welded butt joints in structural steels

In the present paper research work is presented combining material related research, weld fabrication related research and research in the field of numerical analysis of welded joint performance. Elements belonging to this extensive combination are mechanical testing, fracture mechanics testing of small scale and full thickness specimens, component testing including engineering defect assessment and numerical modelling using finite element method. Examples are given for each of the above mentioned elements. Mechanische Untersuchungen an schmelzgeschweißten Verbindungen beinhalten einerseits werkstoffbezogene Untersuchungen zur Optimierung der mechanisch-technologischen und bruchmechanischen Eigenschaften schweißbarer Stähle. Außerdem gehört die Prüfung entsprechender Schweißgüter dazu. Beides erfolgt unter Berücksichtigung der schweißtechnischen Fertigungsbedingungen sowie der Randbedingungen der verschiedenen Prüfverfahren. Experimentelle wie auch numerische Untersuchungen führen – unter Berücksichtigung der mechanischen Inhomogenität – zur Optimierung des Beanspruchungsverhaltens von Schweißverbindungen.

Study on Residual Stress Relief of Welded Structural Steel Joints

This paper presents the results obtained from the residual stress relief tests on 21 welded structural steel joints: 17 butt-welded, two tee, and two corner joint specimens. The A572 grade 50 structural steel plates of two different thicknesses, 15 and 32 mm, were used. All specimens were welded by the submerged arc welding (SAW) process and were designed according to the prequalified welded joints specified by the American Welding Society. In this study, a controlled low-temperature stress-relief method was used for residual stress relief. The experimental results show that by using an appropriate combination of the heating temperature, heating distance, and cooling method, the original high-tensile residual stresses near the weld of the joint can be reduced effectively. The residual stresses before and after the heat treatment were measured by using the blind hole-drilling method. A reduction of more than 50% of the original tensile residual stresses adjacent to the weld was observed.

Characteristics of static and fatigue strength and of crack resistance of welded joints in structural steels at low temperatures. Communication 1

The article presents the results of experimental investigations of the effect of low temperatures (down to 77°K) on the resistance to static and low cycle deformation and rupture of steels 09G2S, 10KhSND, 06G2AF, 20KhGSA, 07Kh3GNMYuA, and ON6. It also presents the dependences of the theoretical characteristic of low cycle fatigue mTe of steels at low temperature under rigid loading on the index of ductility {ie022-01} under static loading and test temperatures down to 77°K.

Characteristics of static and fatigue strength and of crack resistance of welded joints in structural steels at low temperatures. Communication 2

Characteristics of static and fatigue strength and of crack resistance of welded joints in structural steels at low temperatures. Communication 2

Diagnostics of threshold characteristics of welded joints in structural steels

Diagnostics of threshold characteristics of welded joints in structural steels

Structural and mechanical heterogeneity of heat affected zone and impact toughness of welded joints in structural steels

Structural and mechanical heterogeneity of heat affected zone and impact toughness of welded joints in structural steels

Fatigue strength of a groove weld on steel backing

Groove welding made from one side is a common fabrication technique for joints in steel structures. If a steel backing bar is used, current (1988) North American specifications require that it be removed after welding and the weld ground flush if the detail is oriented transversely to the direction of stress and loaded in fatigue. This is an expensive, and in some cases impractical, procedure. British specifications permit the detail to be used but assign a very restrictive fatigue category. In the investigation reported herein, data concerning the fatigue strength of a groove weld with steel backing bar have been obtained experimentally. The backing bar was attached with intermittent fillet welds. A finite element analysis was used to assist in interpretation of the test results. The analysis showed that high local stresses are present at the toe of the fillet weld. However, high stresses also exist at the flush-ground face of the groove weld, and in the physical tests all fatigue cracks started at this side of the detail. When this detail is present in a structure that is subjected to a fatigue loading, Category C of the AASHTO or CSA specifications will provide a suitable basis for design. The selection of categories Class F or G from the British Standard (depending upon the proximity of the fillet welds to the edge of the plate) is too conservative.

Effect of high tempering on the structure and properties of the metal of the heat-affected zone of welded joints of structural steels

1. High tempering at 620–650°C with the object of reducing residual welding stresses leads to poorer strength characteristics of the base metal and of the metal of the HAZ of welded joints of steels VSt3sp, 09G2S, and 10KhSND and to increased plasticity; in steel with nitride strengthening 15G2AF the strength characteristics of the base metal and of the metal of the HAZ are somewhat improved, and plasticity is impaired. 2. The application of high tempering at 620–650°C does not increase the toughness of the metal of the HAZ of welded joints made with large heat input to steels VSt3sp, 09G2S, and 10KhSND. 3. In specifying high tempering for the purpose of reducing residual welding stresses, it is necessary not to be guided solely by the degree of reduction of the stress level but also to take into account the possible changes of the mechanical properties and of the toughness of the metal of the separate sections of the welded joints, and this also depends on the system of alloying the steel.

Regularities of deformation and failure of the metal of welded joints of structural steel at elevated temperatures under conditions of static and low-cycle loading. Communication 2

1. The low-cycle strength (N≤104 cycles) of welded joints in steel 09G2S under rigid loading in the range of elevated temperatures (293–873°K) is limited by the strength of the metal of the heat-affected zone in the temperature range of dynamic strain-aging (473–673°K). The difference in endurance at high temperature may attain a factor of 1.5–2 in dependence on the number of load cycles in comparison with the analogous characteristic at normal temperature. 2. Raising the test temperature to 823–873°K causes a doubling of the strain amplitude at failure of the base metal with ea>1% (N<103 cycles) in comparison with the strain amplitude under conditions of normal temperature. 3. The cyclic elastoplastic properties of the metal of welded joints in steel 09G2S depend on the test temperature in the following way: at T=293°K the base metal is cycli-cally stable, the metal of the heat-affected zone and the seam metal are cyclically slightly softening materials; at T=473–673°K (the temperature of dynamic strain-aging) the material of all zones of the welded joint become cyclically hardening; at T>823°K there is recovery of the initial cyclic strain properties of the material. 4. In the investigated range of elevated temperatures the static characteristics of strength and ductility of the metal of welded joints in steel 09G2S-EShP change to a considerable degree (by a factor of two to six) in a complex manner. 5. In the temperature range under consideration the fatigue crack propagates by the mechanism of formation of fatigue grooves. Signs of brittle failure were not detected in the fatigue zone or in the final fracture. 6. In specimens subjected to fatigue tests at 823°K the degree of development of processes of recovery and recrystallization is substantially higher than at 473°K; this is due to the increased rupture strain amplitude of low cycle fatigue.