Effect of mis-match ratio (MMR) on fatigue crack growth behaviour of HSLA steel welds

Welding of High Strength Low Alloy Steels (HSLA) involves usage of low, even and high strength filler materials (electrodes) than the parent material depending on the application of the welded structures and the availability of the filler material. In the present investigation, the fatigue crack growth behaviour of under matched (UM), equal matched (EM) and over matched (OM) weld metals has been studied. The base material used in this investigation is HSLA-80 steel of weldable grade. Shielded Metal Arc Welding (SMAW) process has been used to fabricate the single “V” butt joints. Centre cracked tension (CCT) specimen has been used to evaluate the fatigue crack growth behaviour of the welded joints. Fatigue crack growth experiments have been conducted using servo hydraulic controlled fatigue testing machine at constant amplitude loading (R=0). Effect of mis-match ratio (MMR) on fatigue crack growth behaviour of HSLA steel welds has been analysed in detail.     

Influences of MMR,PWHT and notch location on fatigue life of HSLA steel welds

Welding of High Strength Low Alloy Steels (HSLA) involves usage of lower, more even and higher strength filler materials (electrodes) than the parent material depending on the application of the welded structures and the availability of the filler material. In this investigation an attempt has been made to study the influences of mis-match ratio (MMR), Post Weld Heat Treatment (PWHT) and notch location on fatigue life of HSLA steel welds. A Shielded Metal Arc Welding (SMAW) process has been used to fabricate the butt joints. A Centre Cracked Tension (CCT) specimen has been used to evaluate the fatigue life of welded joints. Fatigue experiments have been conducted using a servo hydraulic controlled fatigue testing machine at constant amplitude loading (R=0). The Design of Experiments (full factorial design) concept has been used to optimise the number of experimental conditions. Analysis of Variance (ANOVA) method has been used to identify the significance of main and interaction effects. A mathematical model has been developed to predict the fatigue life of HSLA steel welds using Regression analysis.     

Effect of welding processes on fatigue crack growth behaviour of AISI 409M ferritic stainless steel joints fabricated using duplex stainless steel fillers

The present investigation aims to study the effect of welding processes such as shielded metal arc welding (SMAW), gas metal arc welding (GMAW) and gas tungsten arc welding (GTAW) on fatigue crack growth behaviour of the ferritic stainless steel (FSS) conforming to AISI 409M grade. Rolled plates of 4 mm thickness were used as the base material and AISI 2209 grade duplex stainless steel (DSS) was used as filler metal, for preparing single pass butt welded joints. Centre cracked tensile (CCT) specimens were used to evaluate the fatigue crack growth behaviour. From this investigation, it is found that the GTAW joints showed superior fatigue crack growth resistance compared with SMAW and GMAW joints. The reasons for the superior performance were discussed in detail.     

Establishing criteria for root and toe cracking of load carrying cruciform joints of pressure vessel grade steel

In this investigation an attempt has been made to establish a criterion to forecast the possible crack initiation region (toe or root) in double fillet welded load carrying cruciform joints and also to know the probable failure mode. Cruciform joints were fabricated from pressure vessel grade (ASTM 517 ‘F’ grade) steel using shielded metal arc welding (SMAW) and flux cored arc welding (FCAW) processes. Fatigue crack growth experiments were carried out in a mechanical resonance vertical pulsator (SCHENCK 200 kN capacity) with a frequency of 30 Hz under constant amplitude loading (R=0). It was found that the fatigue crack initiation lives (root crack or toe crack) were relatively lower in the joints fabricated by FCAW process than the joints fabricated by SMAW process.     

Fatigue of high strength steel joints welded with low temperature transformation consumables

In the present paper constant (CA) and variable amplitude (VA) fatigue testing have been carried out on out-of plane gusset fillet welded high strength steel joints. The joints were welded with conventional weld filler material and martensitic low transformation temperature weld filler, LTT, in order to study the influence of the residual stress on the fatigue strength. Residual stress measurements were carried out close to the weld toe using X-ray diffraction technique in order to study the relaxation due to VA fatigue. The residual stress showed different level of relaxation depending on the VA spectrum loading used. The LTT joints show 40% increase in mean fatigue strength compared to the conventional joints in CA. The LTT joints show 12% increase in mean fatigue strength compared to the conventional joints. The LTT joints show 33% increase in mean fatigue strength in CA compared to VA testing. However, the improvement of the fatigue strength is less significant in variable amplitude testing mainly due to the relaxation of the compressive residual stresses.     

Modelling of ductile fracture initiation in strength mismatched welded joint

In this paper, the effect of strength mismatch and width of the welded joints on the stress–strain distribution in the crack tip region has been discussed. The single-edge notched bend (SENB) specimens (precrack length a₀/W = 0.32) were experimentally and numerically analysed. The model of local approach to fracture, proposed by Gurson, Tvergaard and Needleman, was used. High-strength low-alloyed (HSLA) steel was used as a base metal in quenched and tempered condition. The flux-cored arc-welding process in shielding gas was used. Two different fillers were selected to make over- and undermatched weld metal. The experimental analysis of fracture behaviour of the over- and undermatched welded joints was followed by numerical computations of void volume fraction in front of the crack tip. The critical void volume fraction, f_c, used in prediction of the crack growth initiation on the SENB specimen had been previously determined on a round smooth specimen. Three widths of weld metal were considered: 6, 12 and 18 mm. A comparison of the crack tip opening displacement (CTOD) values corresponding to crack initiation in the SENB specimens is given, as determined both experimentally and using the GTN model.     

Rißfortschritt in ultrafesten Stählen und deren Schweißverbindungen bei dynamischer Beanspruchung

Crack propagation in ultra-high-strength steels and their welded joints under dynamic loading . Reported are results of investigation into the propagation of cracks in the base metal and weld metal of an ultra-high-strength steel. The material used in the investigations was a Ni? Co? Mo? alloy maraging steel with a yield point of 170 kp/mm². The steel was arc welded and TIG welded. The joints exhibited a drop of static strength in the range of 5 to 8 percent related to the base metal. Under zero-to-tension stress cycles the fatigue strength corresponded that of other high-strength steels, under tension-compression stress cycles the steel exhibited a higher fatigue strength. It was possible to show striations with the aid of scanning microscopy. Comparing the track propagation calculated in the microscopic range with the results obtained from the crack growth curves produced approximate agreement.     

Strain-based approach to fatigue crack initiation and propagation in welded steel joints with arbitrary notch shape

In this paper, a novel strain-based approach for the fatigue strength modelling of welded steel joints is introduced. The actual weld notch geometry and the variation in the microstructure characteristics of the material are considered, and thus, the approach enables the fatigue crack growth simulation from the crack initiation to the critical crack length before the final fracture. The predicted fatigue strength is in line with the experimental results. By considering the crack tip plasticity and stress triaxiality, the approach is able to describe the different crack growth periods of the fatigue life: the short crack, long crack, and tearing-related long crack growth periods. For a welded joint with a smooth notch shape, the short crack growth period is observed to be dominant and to have a significant influence on fatigue life.     

A parametric fracture mechanics study of welded joints with toe cracks and lack of penetration

The existing design rules give quite general guidelines to the fatigue assessment of different types of welded joints. The goal of this investigation was to give designers some tools, which would allow more precise assessment of the effect of dimensional variations on the fatigue strength. Therefore the fatigue behaviour of 12 common types of welded joints has been studied parametrically. Two-dimensional (2D) finite element models of the joint were made and evaluated using plane strain linear elastic fracture mechanics (LEFM) calculations. The as-welded condition was assumed with the result that no crack initiation period was considered and stress ranges were fully effective. A maximum tangential stress criterion with the Paris’ crack growth law was used to predict the growth rate and direction of root and toe cracks under mixed mode K_I-K_II conditions. The effects of weld size and joint dimension ratios on the fatigue strength were systematically studied. In addition to tensile loading, bending and combined tension/bending moment loading in both directions are examined for positive and negative mean stress.     

Fatigue crack growth resistance of gas tungsten arc,electron beam and friction stir welded joints of AA2219 aluminium alloy

AA2219 aluminium alloy square butt joints without filler metal addition were fabricated using gas tungsten arc welding (GTAW), electron beam welding (EBW) and friction stir welding (FSW) processes. The effect of three welding processes on fatigue crack growth behaviour is reported in this paper. Transverse tensile properties of the welded joints were evaluated. Microstructure analysis was also carried out using optical and electron microscopes. It was found that the FSW joints are exhibiting superior fatigue crack growth resistance compared to EBW and GTAW joints. This was mainly due to the formation of very fine, dynamically recrystallised grains and uniform distribution of fine precipitates in the weld region.     

Assessment of In-Service Degradation of Mechanical Properties of NPP Reactor and Piping Materials

An experimental investigation has been carried out to clarify the influence of operating factors on static and cyclic strength of NPP reactor vessels and piping. The authors present the results of analysis of crack growth resistance characteristics of the base material and welded joints in WWER-1000 reactors (steel 15Kh2NMFA) in fatigue failure. The mechanisms of thermal aging of steels of NPP piping in long-term operation (up to 10,000 hours) have been studied.     

Influence of the filler metal on the mechanical properties of Ti-6Al-4V electron beam weldments

The influence of various filler metals on the mechanical properties of 17 mm thick Ti-6Al-4V electron beam welded joints has been analysed. Autogeneous welded joints exhibit higher toughness when compared to the parent plate but this improvement was less marked than that observed in plasma arc welded joints. To achieve better toughness, without suffering unacceptable losses of strength, different morphologies of commercially pure titanium filler metals have been employed. Using 0.50 mm thick sheet as filler metal leaded to maximum toughness but as counterpart a significant decrease in strength was observed. To obtain high toughness while maintaining a high strength level 0.25 mm sheet and 1 mm diameter filler metals are recommended. Fractographic examination of the failed specimens helped to explain the fracture behaviour of the different welded joints.     

Effects of gap filler and brazing temperature on fracture and fatigue of wide-gap brazed joints

The influence of gap filler content on the fracture, fatigue crack initiation and propagation of AISI 316 stainless steel wide-gap brazed with nickel-based filler metal has been investigated. The brazed joints were found to consist of eutectic, intermetallic compound and solid solution. The volume of solid solution was observed to depend on the gap filler content and brazing temperature. Tensile tests with extra small strain gauge bonded at the centre of the joints showed that the strength and elongation of the brazed joints increased with brazing temperature, and the addition of gap filler was able to improve the load-carrying capacity of the brazed joints only when the brazing temperature was high enough. Fatigue crack initiation and growth under displacement amplitude control were also carried out. Crack closure in the brazed joints was determined by means of back face strain on the compact tension specimen used. The introduction of gap filler was able to increase the fatigue and fracture resistance of the brazed joints when a suitable brazing temperature was used. Crack deflection, branching and uncracked ligament bridging behind the crack tip were observable along the crack paths. Experimental results showed that gap filler was able to enhance the crack closure caused by roughness and ligament bridging.     

An implicit gradient application to fatigue of complex structures

This paper presents a procedure to evaluate the stress gradient effect on the fatigue strength of steel welded joints and notched components. An effective stress is calculated by solving a second-order differential equation over all the component (the implicit gradient approach) independently of its geometric shape. The solution is obtained by assuming the isotropic linear elastic constitutive law for the material and the maximum principal stress as equivalent stress. The fatigue behaviour of geometrically complex steel welded joints is analysed and compared with previous fatigue scatter bands obtained for two-dimensional joints. In complex details, the actual critical point is derived from the analysis and is not assumed a priori. Implicit gradient analysis is also used to investigate high-cycle fatigue behaviour in the case of notches.In addition, it is shown that critical distance approaches can be obtained from the non-local theory by proper choice of the weight function.     

Effect of mechanical mismatching on the ductile-to-brittle transition curve of welded joints

The effect of mechanical mismatching (ratio between the yield strength of base and weld metal) on the toughness of welded joints at different temperatures was analysed and the ductile-to-brittle transition curves of these welded joints were experimentally obtained. The filler metal of the joints was always the same, varying the base metal and the width of the welded zone. Two base metals were selected, one with a higher strength than the filler metal (undermatched joint) and the other with a lower strength than the filler metal (overmatched joint). In addition, the joints were made using two different weld widths, 20 and 10 mm.The fracture behaviour of the joints were determined at different temperatures using SE(B) specimens provided with short cracks (a/W = 0.22). Besides, long crack specimens (a/W = 0.5) were also used for comparison. In the case of overmatched joints, the J-values for ductile crack growth are larger than for the undermatched joints. In addition, the ductile-to-brittle transition curve is displaced towards lower-temperatures and higher-toughness values and the toughness for cleavage fracture is also larger for overmatching than for undermatching. All these effects are more significant as the weld width decreases and have been explained in terms of constraint modifications.     

Fatigue behaviour of friction welded medium carbon steel and austenitic stainless steel dissimilar joints

This paper reports the fatigue behaviour of friction welded medium carbon steel–austenitic stainless steel (MCS–ASS) dissimilar joints. Commercial grade medium carbon steel rods of 12 mm diameter and AISI 304 grade austenitic stainless steel rods of 12 mm diameter were used to fabricate the joints. A constant speed, continuous drive friction welding machine was used to fabricate the joints. Fatigue life of the joints was evaluated conducting the experiments using rotary bending fatigue testing machine (R = −1). Applied stress vs. number of cycles to failure (S–N) curve was plotted for unnotched and notched specimens. Basquin constants, fatigue strength, fatigue notch factor and notch sensitivity factor were evaluated for the dissimilar joints. Fatigue strength of the joints is correlated with microstructure, microhardness and tensile properties of the joints.     

Tensile and Impact Properties of Shielded Metal Arc Welded AISI 409M Ferritic Stainless Steel Joints

The present study is concerned with the effect of filler metals such as austenitic stainless steel, ferritic stainless steel and duplex stainless steel on tensile and impact properties of the ferritic stainless steel conforming to AISI 409M grade. Rolled plates of 4 mm thickness were used as the base material for preparing single pass butt welded joints. Tensile and impact properties, microhardness, microstructure and fracture surface morphology of the joints fabricated by austenitic stainless steel, ferritic stainless steel and duplex stainless steel filler metals were evaluated and the results were reported. From this investigation, it is found that the joints fabricated by duplex stainless steel filler metal showed higher tensile strength and hardness compared to the joints fabricated by austenitic and ferritic stainless steel filler metals. Joints fabricated by austenitic stainless steel filler metal exhibited higher ductility and impact toughness compared with the joints fabricated by ferritic stainless steel and duplex stainless steel filler metals.     

Application of different concepts for fatigue design of welded joints in rotating components in mechanical engineering

Several concepts are used for the fatigue design of welded joints. In this paper investigations are presented, which were carried out in a joint project between five research institutes [1]. The aim is to investigate currently applied fatigue concepts with respect to their limitations, compatibility and reliability, in order to improve the accuracy of lifetime estimation and to simplify the choice of the optimum fatigue concept. Here, the results of the investigation of welded joints in rotating universal joint shafts are shown [2]. In the critical weld, a structural steel and a quenched and tempered steel are joined. In practice, stresses result from rotating bending, torsion and also residual stresses are sometimes present. Several welding techniques, MAG, TIG and laser welding, and two seam geometries were investigated with regard to their influence on fatigue strength. Experiments were conducted with welded tube specimens representative of the actual component application and with derived flat specimens as detail specimens. The welded sheet thickness was 5.5 mm. Fatigue strength was investigated from 10⁴ to 10⁷ numbers of cycles. In numerical analyses, nominal stress, structural hot spot stress and elastic notch stress with reference radii of 0.3 mm and 0.05 mm were calculated. In the comparison of the concepts, their respective advantages and disadvantages have been demonstrated. A comparison of the results with the IIW recommendation for fatigue design of welded joints and components [3] has been carried out and improvements have been suggested.     

Effect of microstructural features on the failure behavior of hybrid laser welded AA7020

Detailed investigations of microstructural feature, mechanical property, fatigue strength, and damage mechanism were conducted on hybrid laser welded 7020‐T651 aluminum alloys used into high‐speed railway vehicles. The results show that the hybrid laser welding process can induce significant changes of microstructures and alloying elements, together with numerous gas pores. Such local modifications degrade the fatigue performance. The tensile strength of welded joints was approximately 74% with respect to the base metal, thus satisfying the design standard. The fatigue property was determined in the low and high cycle regimes. It was found that the fatigue strength of welded joints was fairly inferior to that of the base metal, but far higher than the IIW recommended value. Furthermore, welding defects were well believed to contribute to the shorter fatigue life. The small fatigue crack growth presented highly discontinuous and inhomogeneous due to microstructure and porosity. By contrast, the crack stable growth stage was less sensitive to microstructural features of hybrid welded joints.     

Fatigue and fretting of mixed metal self-piercing riveted joint

The fatigue behavior of self-piercing rivet (SPR) joints joining differing thicknesses of AA6111-T4 aluminum and HSLA340 steel sheets in lap shear geometry was investigated in this paper. Crack initiation in the aluminum sheet was the dominant failure mode, while unexpected rivet shank failure tended to occur at high loading levels. Fretting wear was also observed at interface between aluminum and steel sheets as well as between the rivet and sheets under sinusoidal cyclic tension–tension loading. An Energy Dispersive X-ray (EDX) analysis of fretting debris revealed the presence of oxides of aluminum and zinc. Fretting was shown to be critical to crack initiation. For initiations in the aluminum sheet, micro cracks were found to nucleate early in the fatigue life, and crack initiation life was found to be much shorter than crack growth life.