Variation of fractographic appearance for different microstructures in welded joints having the same fatigue crack propagation properties

Fatigue fracture surfaces were examined with a scanning electron microscope to investigate the influence of the different microstructure between weld metal and heat affected zone. The specimens were centre-cracked type transverse butt welded joints. The relationship between macroscopic fatigue crack propagation rate and the stress intensity factor range is the same in spite of the difference in microstructure for both materials. It is shown that the fractographic appearance changes with microstructure even in the very low growth rate region near fatigue threshold. This suggests that fractographic appearance is not necessarily a guide to the rate of fatigue crack growth.     

Consideration of the residual stress distributions in fatigue crack growth calculations for assessing welded steel joints

To better understand the crack closure and propagation, an analytical model is established. The residual stress effect on fatigue crack growth equations has been considered using the residual stress intensity factor (SIF) (K_res). The joint geometries, residual stress distributions (σ_res) and residual stress ratio (R_res) were considered also. K_res are calculated using the analytical weight function (WF) method and different residual stress distributions. It is to be emphasized that the current approach is little investigated. This is because the WF has already been developed to calculate SIF for an existing crack. The current approach calculates K_res for the crack that initiates and propagates until failure. Different stress distributions have been used, and R_res is defined. The validity of using the WF has been shown. SIF due to applied load (K_app) and applied stress ratio (R_app) have been considered. Fatigue crack growth rate was investigated in accordance with the current approach. The results have been verified and benchmarked.     

Experimental and analytical investigation of fatigue crack propagation of T‐welded joints considering the effect of boundary condition

T‐welded joints are commonly employed in ship and ocean structures. The fatigue failure of structure components subjected to cyclic loading always occurs in T‐welded joints because of the metallurgical differences, tensile residual stress fields and stress concentrations. The former researches about T‐welded joints fatigue have in common that the boundary condition needs to be taken into account as an influencing parameter to predict the crack propagation during cyclic loading. In this paper, the crack growth behaviour in T‐welded joint processed with Q345D steel (Pingxiang Iron & Steel Co., Ltd, Hukou, Jiangxi Province in China) under the fatigue loading was analysed via analytical model and verified via experiment. The results show that the influence of boundary condition should be considered in T‐welded joint structure during crack propagation in weld toe area. The correction factor concerning the effect of boundary condition and modified Paris' equation was proposed according to the experimental results and verified by the following repeated experiments.     

Fatigue crack propagation in tensile residual stress fields of welded joints under fully compressive cycling

Fatigue crack propagation rates in centre-crack-typed transverse butt-welded joints were measured at a constant stress intensity factor range obtained by decreasing the applied and mean loads on test specimens. The propagation rate was maintained constant except at extremely compressed stress ratios. Fatigue crack propagation properties under compressive loading were found to be similar to those under tensile loading. Only under highly compressive cycling did crack propagation rates decrease.     

Fatigue strength of steel and aluminium welded joints based on generalised stress intensity factors and local strain energy values

Weld bead geometry cannot, by its nature, be precisely defined. Parameters such as bead shape and toe radius vary from joint to joint even in well-controlled manufacturing operations. In the present paper the weld toe region is modelled as a sharp, zero radius, V-shaped notch and the intensity of asymptotic stress distributions obeying Williams’ solution are quantified by means of the Notch Stress Intensity Factors (NSIFs). When the constancy of the angle included between weld flanks and main plates is assured and the angle is large enough to make mode II contribution non-singular, mode I NSIF can be directly used to summarise the fatigue strength of welded joints having very different geometry. By using a large amount of experimental data taken from the literature and related to a V-notch angle of 135°, two NSIF-based bands are reported for steel and aluminium welded joints under a nominal load ratio about equal to zero. A third band is reported for steel welded joints with failures originated from the weld roots, where the lack of penetration zone is treated as a crack-like notch and units for NSIFs are the same as conventional SIF used in LEFM. Afterwards, in order to overcome the problem related to the variability of the V-notch opening angle, the synthesis is made by simply using a scalar quantity, i.e. the mean value of the strain energy averaged in the structural volume surrounding the notch tips. This energy is given in closed form on the basis of the relevant NSIFs for modes I and II and the radius R_C of the averaging zone is carefully identified with reference to conventional arc welding processes. R_C for welded joints made of steel and aluminium considered here is 0.28 mm and 0.12 mm, respectively. Different values of R_C might characterise welded joints obtained from high-power processes, in particular from automated laser beam welding. The local-energy based criterion is applied to steel welded joints under prevailing mode I (with failures both at the weld root and toe) and to aluminium welded joints under mode I and mixed load modes (with mode II contribution prevailing on that ascribable to mode I). Surprising, the mean value of ΔW related to the two groups of welded materials was found practically coincident at 2 million cycles. More than 750 fatigue data have been considered in the analyses reported herein.     

From a local stress approach to fracture mechanics: a comprehensive evaluation of the fatigue strength of welded joints

This paper investigates the possibility of unifying different criteria concerned with the fatigue strength of welded joints. In particular, it compares estimates based on local stress fields due to geometry (evaluated without any crack-like defect) and residual life predictions in the presence of a crack, according to LEFM. Fatigue strength results already reported in the literature for transverse non-load-carrying fillet welds are used as an experimental database. Nominal stress ranges were largely scattered, due to large variations of joint geometrical parameters. The scatter band greatly reduces as soon as a 0.3-mm virtual crack is introduced at the weld toe, and the behaviour of the joints is given in terms of Δ K _I versus total life fatigue. Such calculations, not different from residual life predictions, are easily performed by using the local stress distributions determined near the weld toes in the absence of crack-like defects. More precisely, the analytical expressions for K _I are based on a simple combination of the notch stress intensity factors K ₁^N and K ₂^N for opening and sliding modes. Then, fatigue strength predictions, as accurate as those based on fracture mechanics, are performed by the local stress analysis in a simpler way.     

A study of fatigue crack growth from artificial corrosion pits at welded joints under complex stress fields

The paper studies the effects of artificial corrosion pits and complex stress fields on the fatigue crack growth of full penetration load‐carrying fillet cruciform welded joints with 45° inclined angle. Parameters of fatigue crack growth rate of welded joints are obtained from S‐N curves under different levels of corrosion. A numerical method is used to simulate fatigue crack growth using different mixed mode fatigue crack growth criteria. Using polynomial regression, the crack shape correction factor of welded joints is fitted as a function of crack depth ratios. Because the maximum circumferential stress criterion is simple and easy to use in practice, fatigue crack growth rate is modified using this criterion. The relationship of effective stress intensity factor, crack growth angle and crack depth is studied under different corrosion levels. The simulated crack growth path obtained from the numerical method is compared with the actual crack growth path observed by fatigue tests. The results show that fatigue cracks do not initiate at the edge or bottom of pits but at the weld toes where the maximum stress occurs. The artificial corrosion pits have little effect on the effective stress intensity factor ranges and crack growth angle. The fatigue crack growth rates of welded joints with pits 1 and 2 are 1.15 times and 1.40 times larger than that of the welded joint with no pit, respectively. The simulated crack growth path agrees well with the actual one. The fatigue life prediction accuracy using the modified formulation is improved by about 18%. The crack shape correction factor obtained using the maximum circumferential stress criterion is recommended being used to calculate fatigue life.     

A notch stress intensity approach to assess the multiaxial fatigue strength of welded tube-to-flange joints subjected to combined loadings

Full penetration T butt weld joints between a tube and its flange are considered, subjected to pure bending, pure torsion and a combination of these loading modes. The model treats the weld toe like a sharp V‐notch, in which mode I and mode III stress distributions are combined to give an equivalent notch stress intensity factor (N‐SIF) and assess the high cycle fatigue strength of the welded joints. The N‐SIF‐based approach is then extended to low/medium cycle fatigue, considering fatigue curves for pure bending and pure torsion having the same slope or, alternatively, different slopes. The expression for the equivalent N‐SIF is justified on the basis of the variation of the deviatoric strain energy in a small volume of material surrounding the weld toe. The energy is averaged in a critical volume of radius R_C and given in closed form as a function of the mode I and mode III N‐SIFs. The value of R_C is explicitly referred to high cycle fatigue conditions, the material being modelled as isotropic and linear elastic. R_C is thought of as a material property, independent in principle of the nominal load ratio. To validate the proposal, several experimental data taken from the literature are re‐analysed. Such data were obtained by testing under pure bending, pure torsion and combined bending and torsion, welded joints made of fine‐grained Fe E 460 steel and of age‐hardened AlSi1MgMn aluminium alloy. Under high cycle fatigue conditions the critical radius R_C was found to be close to 0.40 mm for welded joints made of Fe E 460 steel and close to 0.10 mm for those made of AlSi1MgMn alloy. Under low/medium cycle fatigue, the expression for energy has been modified by using directly the experimental slopes of the pure bending and pure torsion fatigue curves.     

Prediction of fatigue crack growth path in mechanical joints using a weight function approach

Cracks often initiate from the mechanical joints which are widely used in structural components. It has been reported that cracks in mechanical joints are under mixed‐mode condition and there is a critical angle at which mode I stress intensity factor becomes maximum. The crack propagates in an arbitrary direction and the prediction of fatigue crack growth path is needed to provide against crack propagation and examine safety. In this study, mixed‐mode fatigue crack growth tests are performed for horizontal and critical inclined cracks in mechanical joints. Fatigue crack growth paths are predicted using a weight function approach and maximum tangential stress criterion.     

Overload effect on the fatigue crack propagation in large‐scale tubular joints

This paper examines the overloading effect on the fatigue crack propagations monitored in a large‐scale tubular X‐joint specimen under two separate cyclic tests. The first cyclic test applies a constant‐amplitude brace in‐plane bending to the joint, with a single cycle of 150% overload before the crack depth reaches the mid‐thickness of the chord. The second fatigue test applies two batches of cyclic loads, with the amplitude of the second batch at 66% of the former. The X‐joint specimen experiences a 150% overload cycle during the first batch of loading, followed by the second batch after it has recovered from the overload effect. The experimental results reveal that deep surface cracks experience more significant overload retardation than does a shallow fatigue crack. The Paris law estimation indicates that the single overload cycle applied in the first specimen leads to a 7% increase in the fatigue life of the X‐joint.     

Research on fatigue crack propagation behaviour of 4003 ferritic stainless steel based on infrared thermography

Fatigue crack propagation (FCP) behaviour of 4003 ferritic stainless steel was investigated using infrared thermography. Four stages of superficial temperature evolution were observed during the FCP tests: an initial temperature decrease stage, a temperature equilibrium stage, a slow temperature increase stage and an abrupt temperature increase stage; a thermal model is developed to explain the observed temperature evolution. The experimental results indicate that: when the range of stress intensity factor (ΔK) is at a low level where the crack is located in slow propagation region, thermoelastic effect will be in dominant status; when the ΔK is at a high level where the crack is located in stable propagation region, the temperature rise can be used to describe FCP rate. The fatigue fracture surfaces were examined using scanning electron microscope (SEM) in order to understand the effect of the fatigue mechanisms on temperature variation.     

Fatigue crack growth analysis of welded aluminium RHS T-joints with manipulated residual stress level

The fatigue life of a welded aluminium T‐joint made from beams with rectangular hollow section (RHS) has been predicted using a crack propagation analysis and compared with experimental results from joints with different residual stress levels. To include the effect of the residual stresses, the stress ratio was calculated at the weld toe and, via Walker's equation, introduced into the analysis. How to obtain the Walker exponent has been discussed in detail. The introduction of a stress ratio at the weld toe provides good agreement between the experimentally and analytically found S–N curves. The effect of the residual stress was successfully included in the analysis.     

Fatigue crack propagation rates and threshold stress intensity factors for welded joints of HT80 steel at several stress ratios

Fatigue crack propagation rates and threshold stress intensity factors were measured for welded joints and base metal by using 200 mm wide centre-cracked specimens. The fatigue crack propagation properties of welded joints were similar in spite of the different zones in which the cracks propagated (ie, in the heat-affected zone and in the weld metal) and the different welding process used (submerged arc welding and gas metal arc welding). They were, however, inferior to those of the base metal. It was revealed by observation of the crack closure that the fatigue cracks were fully open during the whole range of loading, due to the tensile residual stress distribution in the middle part of the welded joints. This observation also explains the lack of a stress ratio effect on the fatigue crack propagation properties of welded joints, and their inferiority to those of the base metal.     

Fatigue crack propagation in API 5L X‐70 pipeline steel longitudinal welded joints under constant and variable amplitudes

Fatigue crack propagation (FCP) under constant and variable amplitude loading in base metal (BM), weld metal (WM) and heat affected zone (HAZ) of longitudinal welded joints of an API X‐70 pipeline steel was investigated. Constant amplitude loading tests were performed at R = 0.1 and 0.5, whereas for variable amplitude testing single peak tensile overloads (OLs) alternating between 75 and 100% of maximum load were applied at 2.5 mm intervals in crack growth. Results of SE(B) specimens tested under constant and variable amplitude loading revealed that BM, WM and HAZ regions subjected to R = 0.5 and low ΔK‐values presented the highest crack growth rates. At higher ΔK values FCP rates in all the studied regions were similar and the R effect on FCP rate was no more observed. Crack growth retardation due to OLs was observed at the three studied regions, showing a decrease on the FCP delay with a decreasing on ΔK.     

Measuring fatigue cracks in fillet welded joints

Accurate measurement of short (<1 mm depth) elliptical fatigue cracks that grow from the toes of fillet welds has proved to be an obstacle to the application of fracture mechanics principles to welding fatigue. This paper reports a DC potential drop technique which allows continuous measurement of the depth of such elliptical cracks. A delicate compromise between sensitivity and accuracy, combined with superior electrical stability displayed by the measurement apparatus, has allowed detection of: 1 — crack growth less than 0.01 mm and; 2 — crack growth rates less than 10^?7 mm/cycle for cracks less than 1 mm deep.Preliminary results have indicated the relative importance of stress ratio, defect size and material variation on the growth of these short elliptical cracks. When the weld toe is subject to high stress ratios the phenomenon may be considered propagation dominated whereas low stress levels increase the influence of threshold and initiation mechanisms.     

Influence of effective stress intensity factor range on mixed mode fatigue crack propagation

ABSTRACT The behaviour of fatigue crack propagation of rectangular spheroidal graphite cast iron plates, each consisting of an inclined semi‐elliptical crack, subjected to axial loading was investigated both experimentally and theoretically. The inclined angle of the crack with respect to the axis of loading varied between 0° and 90°. In the present investigation, the growth of the fatigue crack was monitored using the AC potential drop technique, and a series of modification factors, which allow accurate sizing of such defects, is recommended. The rate of fatigue crack propagation db/dN is postulated to be a function of the effective strain energy density factor range, ΔS_eff. Subsequently, this concept is applied to predict crack growth due to fatigue loads. The mixed mode crack growth criterion is discussed by comparing the experimental results with those obtained using the maximum stress and minimum strain energy density criteria. The threshold condition for nongrowth of the initial crack is established based on the experimental data.     

Fatigue crack initiation assessment of welded joints accounting for residual stress

The impact of residual stresses on the fatigue crack initiation life of welded joints is evaluated by the finite element method. The residual stresses of nonload‐carrying cruciform joints, induced by welding and ultrasonic impact treatment, are modelled by initial stresses, using the linear superposition principle. An alternative approach of using modified stress‐strain curves in the highly stressed zone is also proposed to account for the residual stress effect on the local stress‐strain history. An evaluation of the fatigue crack initiation life of welded joints based on the local strain approach is carried out. The predicted results show the effect of residual stresses and agree well with published experimental results of as‐welded and ultrasonic impact treated specimens, demonstrating the applicability of both approaches. The proposed approaches may provide effective tools to evaluate the residual stress effect on the fatigue crack initiation life of welded joints.     

Fatigue crack propagation in welded joints under variable-amplitude loading in synthetic sea water

Fatigue crack propagation rates and the fatigue threshold in transverse butt-welded joints of centre crack type were measured in synthetic sea water under variable-amplitude loading. The fatigue threshold was about a half of that in air, and the propagation rates near the fatigue threshold were high compared with those in air. The propagation rate for each cycle under variable-amplitude loading could be estimated from the constant-amplitude data, which were obtained in the crack-closure-free condition.     

Evaluation of fatigue crack propagation by mode I and mixed mode in 1050 aluminium

The mechanism of mixed‐mode fatigue crack propagation was investigated in pure aluminum. Push‐pull fatigue tests were performed using two types of specimens. One was a round bar specimen having a blind hole, one was a plate specimen having a slit. The slit direction cut in the specimen was perpendicular or inclined 45 degrees relative to the centre of the specimen axis. In both cases, cracks propagated by mode I or by the mixed mode combining mode I and shear mode, depending on the testing conditions. In these cases the crack propagation rate was evaluated with a modified effective stress intensity factor range. Crack propagation retardation was observed in some specimens. However, it was found that the crack propagation rate could also be evaluated by the effective stress intensity factor range independent of the crack propagation mode.     

Unique fatigue threshold and growth properties of welded joints in a tensile residual stress field

The fatigue threshold and high growth rate region properties were investigated on several kinds of welded joints. These properties became unique in spite of the variation of steels (ferrite-pearite, martensite, austenite), welding method, heat input and stress ratio. It was revealed that the unique properties occurred from the fully opened fatigue crack due to the tensile residual stresses. Based on these results, the equation of the fatigue crack growth curve for the design and inspection of welded structures was proposed. It is also suggested that the inducement of compressive residual stress at the fatigue critical zone is effective in improving the fatigue properties of welded structures.