Crack Growth In Welds Research Articles

A Fluid-Solid-Magnetic Coupling Algorithm of Internal Crack Growth in the Weld of Oil and Gas Pipelines

In order to characterize the dynamic process of the crack growth in the weld of oil and gas pipelines, a mathematical model of fluid-solid-magnetic multifield coupling was constructed in this paper. Based on this model, the bidirectional fluid-solid coupling and unidirectional magnetic structure coupling caused by the weld deformation were achieved by dynamic application of the fluid permeation pressure, calculating the internal crack growth in the pipe weld, reconstructing the computational grid near the internal crack, and discussing the characteristics of the magnetic leakage field in the process of the internal crack growth in pipe weld. Thus, a fluid-solid-magnetic coupling algorithm for the internal crack growth in pipe welds considering fluid permeation pressure is established. According to the characteristics of the internal crack opening distance, internal crack growth length, crack tip energy release rate, peak values of magnetic induction intensity level, and vertical component, the process of the internal crack growth is measured. The results show that the fluid osmotic pressure accelerates the process of the internal crack growth and this algorithm can solve the problem of the characterization and evaluation of crack growth in pipe welds under fluid-solid-magnetic coupling action.

Fatigue crack growth in welds based on a V-notch model for the short crack propagation at the toe

This work presents a new fatigue crack growth prediction model for non-load-carrying fillet welded steel joints. For this joint configuration the fatigue cracks will emanate from the weld toe region. Due to the presence of a V-notch in this region the crack initiation point becomes a point of singularity for the stress field. This may in many cases make it difficult to determine the Stress Intensity Factor Range (SIFR) for small cracks by conventional methods based on Linear Elastic Fracture Mechanics (LEFM). The present approach solves this problem by using the Energy Release Rate (ERR) to determine the SIFR in the small crack growth regime. The model is fitted to crack growth curves from tests with cruciform steel welded joint subjected to constant stress ranges in both axial and bending loading mode. The Paris crack propagation law is adopted and the calculation of SIFR for larger cracks outside the material volume influenced by the V-notch singularity is carried out by the conventional approach. The model gives results in agreement with experimental facts and has also the potential of being extended to variable amplitude loading. The model is also well suited for taking into account the crack initiation phase that is significant for high quality welded joints

Microstructural Characterization and Fatigue Performance Evaluation of MIG-Welded Ship Hull Steel

The effectiveness of MIG welding with Argo-shield gas & ER70S-6 electrode in joining LRS (Grade-B) steel was investigated through structure–property correlation of the joint region. Microstructure, tensile and fatigue properties, and mode of fracture (SEM fractograph) were correlated. Fatigue behavior has been investigated in air and sea water with thin specimen at near-endurance stress amplitude up to 105 cycles. The crack growth rate (da/dN) maintained a non-linear relationship with logarithm of stress intensity factor range (logΔK) for the near-threshold values of ΔK. Considerable hardness and microstructural variation was observed across the weldment. Weld with more pearlite content was found to possess higher hardness and strength than the parent steel. Though, both in weld and in parent steel, either in air or in sea water, fatigue crack propagated at very slow rate with significant intermittent crack arrest, weld provided much higher resistance to crack growth in air. However, sea water accelerated the crack growth in weld and brought it closer to that in the parent steel. The morphologically complex microstructure of weld suffered much faster crack propagation in sea water than in air. While fatigue fracture in parent steel (both in air and sea water) and weld in air was found to occur through dimple rupture via microvoid coalescence, weld in sea water exhibited a mixed mode of failure.

Role of nonlinear fracture mechanics in assessing fracture and crack growth in welds

An experimental study was conducted to assess the structural performance of repair welds in an ex-service 1Cr–1Mo–0.25V steam turbine casing material. Material from two weld techniques, one involving a post-weld heat-treatment that produced undermatched welds and the other involving a temper bead welding technique that produced overmatched welds were tested. Both welding techniques were implemented in two base metal conditions giving rise to four different welds and two different base metal conditions. The tests conducted included tensile tests, creep tests, fracture toughness tests, fatigue crack growth tests, creep crack growth tests, and creep–fatigue crack growth tests on the base metal, weld metal and the weldment region. The yield strength of the weld metal in the undermatched condition was approximately 10% lower than the base metal, while the weld metal in the overmatched condition had a yield strength that was 30% higher than the base metal at 565 °C. The creep deformation rates in the undermatched welds were 60 times faster than the base metal at a stress of 207 MPa. In the overmatched welds, the creep rates at 207 MPa were about 2.8 times faster in one case and 2.8 times slower in the other. The crack path in fracture toughness specimens followed the interface between the transition layer and the weaker of the weld metal and the base metal. The J-resistance curves for the weldments at 565 °C showed significant variability among duplicate samples from the same welds. This scatter was caused by the variability in the location of the precrack with respect to the fusion line and the location of the low fracture toughness region in the weldment. This behavior was explained using a novel approach for characterizing the fracture of welds. The creep–fatigue crack growth rates at equivalent ( C t ) avg values in undermatched welds was higher than the crack growth rates in the overmatched weld samples. In all cases under creep–fatigue, the crack appeared to grow in the weaker of the base metal and the weld metal. Recommendations for future work are provided to enhance the theoretical underpinnings of the nonlinear fracture mechanics frame-work to rigorously address fracture and crack growth in welds.

Fatigue crack growth in a welded rail under the influence of residual stresses

The effect of welding residual stresses on fatigue crack growth in rail welds is studied. Finite element analysis is used to calculate residual stresses in a flash-butt welded rail. The calculated residual stresses are found to be in good agreement with experimentally determined residual stresses in a welded rail. The redistribution of residual stresses in the welded rail is simulated for a straight track, during heavy-haul operation conditions, using a train–track model. Fatigue crack growth of defects in the weld region is studied using fracture mechanics. In the investigation, a number of parameters such as the axle load, crack location, crack size and rail temperature are varied.

Residual stress effects on near-threshold fatigue crack growth in friction stir welds in aerospace alloys

Friction stir welding (FSW) is being explored as a potential tool for manufacturing aluminum aerospace structures. Several joint configurations, butt, lap and fillet joints have been made in the production of such exploratory structures. Research work on S–N fatigue and fatigue crack growth in the weld zone is required to provide an understanding and tools to assess the damage tolerance issues in friction stir welded joints and structures. In this work, results of a study conducted on near-threshold fatigue crack growth in friction stir welded aluminum alloy 7050-T7451 and a titanium alloy Ti–6Al–4V are presented. Tests were conducted on weld coupons as a function of specimen geometry (compact tension, eccentrically loaded single edge and center-crack tension) and stress ratio to understand the effects of residual stresses in the heat affected zone (HAZ) of the alloy. Residual stresses were measured on samples machined from the friction stir welded plates prior to testing. The crack growth results show that residual stresses play a key role in the crack growth parallel to the weld-path in the HAZ. Although friction stir welding process induced low residual stresses in the welds, they are found to produce large effects on the near-threshold fatigue crack growth. In general, the magnitude of the shift in the fatigue threshold in the friction stir welded coupons is a function of microstructure, residual stresses and specimen geometry. However, for a constant microstructure, fatigue thresholds at low stress ratios, were specimen geometry dependent. The thresholds were either higher or lower than those of the parent material. At high stress ratio, the differences due to the specimen geometry vanish. Stress ratio studies show that the center-crack tension geometry is less sensitive to the residual stress effects compared to the compact tension geometry. Crack growth analysis using equivalent residual stresses was used to determine the residual stress intensity factor and predict stress ratio effects. The role of microstructure on the fatigue threshold in the welds is also discussed.

Fatigue threshold and crack growth in an electron beam weld made of steel and bronze

In this research topic some experimental tests with single-edge notched beams were performed to determine the threshold load value for fatigue crack growth and to characterize fatigue crack behaviour of an electron beam weld made of steel and bronze. Subsequently, numerical analyses were done to estimate the threshold value Δ K th and to simulate the fatigue crack growth. The calculated crack path was compared to those determined experimentally. The objective was to find out the necessary fracture properties for an analysis of an electron beam welded worm wheel and to asses the capability of usual fracture analysis software to simulate fatigue crack growth in welds.

Creep crack growth in welds: a damage mechanics approach to predicting initiation and growth of circumferential cracks

The results of damage mechanics finite element analyses have been used to estimate the initiation and growth of type IV cracks in a series of internally pressurised circumferential pipe welds, in main steam pipelines made of 1/2CrMoV steel. The material properties used, for the various zones of new, service-aged and repaired welds, were produced from creep test data at 640°C. Damage distributions and accumulation with time within the HAZ are presented, from which the crack initiation times and positions for these welds, under a closed-end condition, and with additional axial (system) loading, were identified. By investigating the propagation of damage through the wall thickness, the remaining lives of the various weld types were estimated. The method provides a means for predicting the initiation and growth of type IV cracks in these CrMoV weldments, and for estimating the length of time a weld can safely be left in service, after damage, or type IV cracking, is identified during inspection.

Modeling of fracture and crack growth in welds operating at elevated temperatures

This paper critically examines the fundamental limitations of the available nonlinear fracture mechanics based models and procedures for predicting fracture and crack growth behavior in structural welds. It is shown that the weld fracture and crack growth data show significantly more scatter than the same data in homogeneous materials. The data scatter is linked to heterogeneity in the microstructure of the weld and the resulting differences in the deformation properties of the weld region causing cracks to meander from their expected path. Some preliminary ideas for dealing with this scatter are proposed and recommendations for future work in this area are also made.