High-resolution 3D weld toe stress analysis and ACPD method for weld toe fatigue crack initiation

Weld toe fatigue crack initiation is highly dependent on the local weld toe stress-concentrating geometry including any inherent flaws. These flaws are responsible for premature fatigue crack initiation (FCI) and must be minimised to maximise the fatigue life of a welded joint. In this work, a data-rich methodology has been developed to capture the true weld toe geometry and resulting local weld toe stress-field and relate this to the FCI life of a steel arc-welded joint. To obtain FCI lives, interrupted fatigue test was performed on the welded joint monitored by a novel multi-probe array of alternating current potential drop (ACPD) probes across the weld toe. This setup enabled the FCI sites to be located and the FCI life to be determined and gave an indication of early fatigue crack propagation rates. To understand fully the local weld toe stress-field, high-resolution (5 mu m) 3D linear-elastic finite element (FE) models were generated from X-ray micro-computed tomography (mu-CT) of each weld toe after fatigue testing. From these models, approximately 202 stress concentration factors (SCFs) were computed for every 1 mm of weld toe. These two novel methodologies successfully link to provide an assessment of the weld quality and this is correlated with the fatigue performance

Magnetic properties of silicon steel after plastic deformation

The energy efficiency of electric machines can be improved by optimizing their manufacturing process. During the manufacturing of ferromagnetic cores, silicon steel sheets are cut and stacked. This process introduces large stresses near cutting edges. The steel near cutting edges is in a plastically deformed stress state without external mechanical load. The magnetic properties of the steel in this stress state are investigated using a custom magnetomechanical measurement setup, stress strain measurements, electrical resistance measurements, and transmission electron microscopic (TEM) measurements. Analysis of the core energy losses is done by means of the loss separation technique. The silicon steel used in this paper is non-grain oriented (NGO) steel grade M270-35A. Three differently cut sets of M270-35A are investigated, which differ in the direction they are cut with respect to the rolling direction. The effect of sample deformation was measured—both before and after mechanical load release—on the magnetization curve and total core energy losses. It is known that the magnetic properties dramatically degrade with increasing sample deformation under mechanical load. In this paper, it was found that when the mechanical load is released, the magnetic properties degrade even further. Loss separation analysis has shown that the hysteresis loss is the main contributor to the additional core losses due to sample deformation. Releasing the mechanical load increased the hysteresis loss up to 270% at 10.4% pre-release strain. At this level of strain, the relative magnetic permeability decreased up to 45% after mechanical load release. Manufacturing processes that introduce plastic deformation are detrimental to the local magnetic material properties

The development of high-resolution crack monitoring methods to investigate the effect of the local weld toe geometry on fatigue crack initiation life

The effect of weld toe geometry on the fatigue performance of welded joints is investigated, with a primary focus on the fatigue crack initiation (FCI) life of the joints. Available standards prescribe global approaches for estimating total life and crack propagation life, as FCI life has historically been considered to be negligible. However, experimental results from the literature show that FCI can take up a significant portion of the total life of welded joints, especially in the case of good quality welds under high-cycle fatigue loading conditions. Weld toe fatigue crack initiation is highly dependent on the local weld toe stress-concentrating geometry, including the inherent flaws such as undercuts, spatter, cold-laps and inclusions. Such flaws, as well as the inhomogeneous geometry, promote premature fatigue crack initiation. Thus, it is essential to investigate methods of resolving such flaws and characterising them in terms of their geometry and stress concentration. The weld toe stress concentration factor (SCF) is an important parameter for characterising the stress-concentrating features that act as fatigue crack initiation sites.In this work, interrupted fatigue tests were performed on arc-welded joints manufactured by different welders with different electrodes to obtain a variety of weld toe geometries. To obtain fatigue crack initiation lives (corresponding to the growth of a technical crack of depth 250 µm), a novel multi-probe array of alternating current potential drop (ACPD) probes and strain gauges were positioned across the weld toe. These bespoke systems also located fatigue cracks. The effect of stress on the ACPD measurements was observed to influence results, and once cracks approached through-thickness they were difficult to characterise using this method. Further research to investigate this phenomenon has been identified. Both crack growth monitoring techniques (ACPD and strain gauges) were seen to correlate well with crack depth for isolated cracks. However, due to the large number of crack coalescence events in high stress-range fatigue loading conditions, both techniques proved inconsistent at the onset of crack coalescence. FCI lives and early crack propagation rates were correlated with the local weld toe stresses, which were obtained from a high-resolution (5-10 µm) stress analysis. 3D linear-elastic and elastic-plastic finite element models were developed from X-ray micro-Computed Tomography (µ-CT) scans of each fatigue tested weld toe. From these models, over 5000 SCFs were extracted for approximately every 50 mm of weld toe, thus providing comprehensive SCF distribution maps for each of the interrupted-fatigue tested and scanned weld toes. In obtaining FCI lives in welded joints, the local notch stress-strain approach has been used from the available literature. In this study, multiple variations of the approach have been investigated to potentially identify improvements to the approach when using state-of-the-art technology (industrial µ-CT) for resolving the weld toe geometry and bespoke data-rich crack monitoring techniques. The SCF distribution maps, along with the experimental FCI data, were used to identify the robustness of each of the variations in the literature. The use of fatigue notch factor (Kf) is seen to give significantly non-conservative results when combined with high-resolution SCF data. Elasticplastic SCF data was observed to give satisfactory results in terms of predicting the FCI lives. However, more computation time is required to get elastic-plastic SCF distributions compared to linear-elastic stress analysis. The number of tests performed was low and so were not statistically significant to draw firm conclusions but provide an indication of the methods that have the most potential

Test Methods for Corrosion-Fatigue of Offshore Structures

The increasing demand for renewable energy, necessitates offshore wind turbine structures to be installed in deeper waters and more remote areas. They are subjected to challenging conditions, i.e., the combination of cyclic loads from wind, waves, and currents, and the corrosive nature of the seawater environment. The literature lacks experimental data on the corrosion-fatigue of structural steels, especially concerning the short crack propagation regime. This study involves developing test methods and equipment for corrosion-fatigue testing of welded structural steel and quantifying the propagation rate of short cracks (starting from corrosion pits). The developed models (calibrated by the experiments) allow accurate prediction of the remaining lifetime of steel structures exposed to corrosion damage

Calibration and validation of extended back-face strain compliance for a wide range of crack lengths in SENB-4P specimens

Compliance equations based on back-face strain or crack mouth opening displacement, and potential drop techniques are widely used to calculate fatigue crack growth rate. Standard ASTM E647 for fatigue crack growth rate testing does not include compliance based equations for single edge notched four-point bending (SENB-4P) specimens. Equations developed based on finite element (FE) analysis have been reported in literature; however, they are limited to the long crack propagation regime (i.e., relative crack length ratios a/W > 0.15). No compliance relations for crack growth in the physically short crack regime were found in literature. This work reports on a complementary numerical-experimental study towards the development of compliance equations with an extended application range in terms of relative crack length. FE simulations have been used to calibrate a back-face strain compliance equation for calculation of relative crack length in the range 0.05 ≤ a/W ≤ 0.5 for SENB-4P specimens. Four point bending fatigue tests were performed on SENB specimens extracted from 50 mm thick welded steel plates. Direct current potential drop (DCPD) for crack monitoring was used as benchmark and validation of the crack lengths determined from a back-face strain gage

Front face strain compliance for quantification of short crack growth in fatigue testing

Short fatigue crack growth investigation is of considerable scientific interest as it comprises a significant portion of the total fatigue life of a structure. It is very challenging to accurately quantify this stage of fatigue crack growth experimentally. In this article, a novel front face strain compliance technique for single-edge notched specimens subjected to four-point bending is proposed. Finite element analysis is performed to determine the correlation between crack length and strain change near the crack. This relationship is then validated by experiments in which strains are measured by strain gauges attached near the short crack, and crack length is quantified by examining beachmark lines at the fracture surfaces. Based on the numerical and experimental results, it is concluded that the strain measured near the notch allows quantifying short crack growth for normalised crack lengths in the range 0.01 <= a/W <= 0.06 (a/W being the ratio of crack length over specimen width). A compliance equation based on the front face strain is finally presented

Fracture mechanics and hot spot stress‐based fatigue life calculation : case study for a crane runway girder

This work presents the case study of a welded overhead crane runway girder. A full-shell global finite element model was developed and validated using strain measurements. The global model drives solid submodels of different scales. Using load spectra based on real operational data, hot spot stress and XFEM-based fracture mechanics simulations are performed to assess the fatigue properties of two critical joints. For the fracture mechanics-based approach, it is shown that small cracks quickly converge to the same aspect ratio. Hence, the choice of the initial crack aspect ratio was found to be inconsequential. But the aspect ratio must not be assumed to be constant during the fatigue crack growth simulations. Finally, it is shown that the size at which a crack is transferred from one submodel to a subsequent is critical for the accuracy of the fatigue crack growth simulations

Test methods for corrosion-fatigue of offshore structures

The increasing demand for renewable energy, necessitates offshore wind turbine structures to be installed in deeper waters and more remote areas. They are subjected to challenging conditions, i.e., the combination of cyclic loads from wind, waves, and currents, and the corrosive nature of the seawater environment. The literature lacks experimental data on the corrosion-fatigue of structural steels, especially concerning the short crack propagation regime. This study involves developing test methods and equipment for corrosion-fatigue testing of welded structural steel and quantifying the propagation rate of short cracks (starting from corrosion pits). The developed models (calibrated by the experiments) allow accurate prediction of the remaining lifetime of steel structures exposed to corrosion damage