Fatigue Failure Location Research Articles

Implementation of the Peak Stress Method for the automated FEM-assisted design of welded joints subjected to constant amplitude multiaxial fatigue loads

The Peak Stress Method (PSM) is a local approach to the fatigue strength assessment of welded structures. Starting from FE-calculated opening, in-plane shear and out-of-plane shear peak stresses at the weld toe and weld root, the PSM defines an equivalent peak stress for estimating the fatigue failure location and fatigue life of welded structures, in compliance with properly defined design curves. An interactive tool has been developed in Ansys® Mechanical to automate all calculation tasks required to apply the PSM to generic welded structures. The developed application allows to identify and analyse all weld toe and weld root lines of the structure in a full-automated way, performing fatigue life estimation on each analysed node. Finally, fatigue life results can be visualized directly on the model’s geometry through dedicated contour plots. In this work, some joint geometries taken from the literature and related to steel welded joints subjected to multiaxial fatigue loads are re-analysed taking advantage of two design procedures: (i) manual application of the PSM, (ii) automated implementation of the PSM. Taking advantage of the tool developed in Ansys®, a remarkable reduction in analysis time and effort can be achieved, with respect to manual PSM analyses.

Static and fatigue behavior of hydraulic components produced by different additive manufacturing processes

This study focuses on hydraulic components’ behavior to significant pressure loads both in static and fatigue conditions. This hydraulic block commonly used on large machines (earthmoving, mining, agriculture…) was initially designed to be machined but has been topologically optimized for additive manufacturing methods: laser powder bed fusion (L-PBF), metal binder jetting (MBJ) and lost-wax casting. The material chosen was a martensitic precipitation-hardening stainless steel that provides good corrosion resistance and high mechanical properties. Most of the samples were obtained with laser powder bed fusion and underwent static and fatigue pressure tests, exceeding the technical specifications of the end user. The tested samples have then been screened and went through various fatigue finite element analyses. This approach demonstrated that all the critical fatigue failure locations obtained experimentally were predictable through software simulation, local non uniform roughness and residual stress in failure areas being key factors for good correlation.

Coexistence of two distinct fatigue failure mechanisms in Super304H welded joint at elevated temperatures

Super304H steel is a promising candidate for boiler tube materials in thermal power plants operating under ultra-supercritical conditions, and the fatigue properties of its welded joint play a key role in the structural reliability of the thermal power plants. Herein, we report that there is a transition in the fatigue failure location (i.e., fatigue failure mechanism) from the base metal at a high strain amplitude (≥~0.4%) to the weld metal at a low strain amplitude (≤~0.3%) at a constant temperature in the operating range of 500–700 °C, and this leads to a significant reduction in the fatigue resistance of the welded joint as compared to that of the base metal. We found that a longer fatigue test time at a low strain amplitude induces a thermal aging effect that promotes different microstructural evolutions in the base metal and weld metal, deepening material inhomogeneity in the welded joint, and thereby triggering strain localization in the weld metal. With increasing fatigue test time (i.e., thermal aging time), Cr23C6 carbides precipitated in the interdendritic region of the weld metal and at the austenitic grain boundary of the base metal, and Nb (C, N) phases precipitated in the interior of austenitic grain in the base metal, which increased local hardness, whereas there was no significant microstructural change in the dendrite core of the weld metal, retaining its initial hardness. The intensified local hardness inhomogeneity caused the softest zone in the welded joint, wherein strain localization occurred, serving as a crack nucleation site and propagation path, to shift from the base metal to the weld metal (dendrite core). This induced a shift in the fatigue failure location from the base metal to the weld metal with decreasing strain amplitude.

Fatigue life analysis of steel bicycle frame according to ISO 4210

The study estimates the fatigue life of a bicycle frame after fatigue failure due to the operational loads. Mechanical properties were determined for a tubular frame made of 25CrMo4 steel. High-cycle fatigue tests were carried out for mini-specimens taken from the location of fatigue failure (joint material) and the base material. The experimental data were compared using a normal distribution for the S-N area and an analysis of the slopes for linear regressions for two data populations. The bicycle frame was evaluated using a fatigue strength approach and nominal stress values. The conditions of the analysis conformed to the safety requirements laid down in the international ISO 4210 standard. The stress was calculated using a finite element model consisting of the surface elements. The highest stress points were selected to compare different fatigue evaluations. The FEA and material data results were compared using a probabilistic Weibull model (P-S-N curve). A mean stress effect was allowed for using the Walker model. The highly stressed volume model and weakest link theory model were used to analyse the size effect. The fatigue life was estimated for the maximum stress determined using FEA. The maximum stress point corresponds to the fatigue failure location on the bicycle frame. The fatigue life at 10% failure probability is below the value recommended in accordance with ISO 4210. A key factor affecting the results is the operating conditions, including the angle of force applied to the bicycle pedals.

Fatigue life prediction of 316L stainless steel weld joint including the role of residual stress and its evolution: Experimental and modelling

Weld residual stresses have a great effect on the fatigue strength of welding structures. As an internal stress, the residual stress is unfounded to be treated as the conventional mechanical load during the fatigue safety assessment due to its relaxation and redistribution under cyclic loading. Thus, establishing an accurate model considering the effects of residual stress is critical for fatigue strength design. Therefore, this paper develops a sequential coupling residual stress analysis and a cyclic viscoplastic constitutive model to predict the residual stress evolution and its role on the fatigue life, which has been validated by an experimental measurement of residual stress and a SEM-based observation fatigue fracture morphology. The results show that the role of residual stress on fatigue life is mainly reflected by the increase of mean stress, and its increased magnitudes depends on the stress redistribution. The redistribution of residual stresses is closely correlated to the fatigue fracture behavior of weld joints. And a fatigue life prediction model of weld joints is proposed based on the experimental data of base metal by further considering the residual stress and its redistribution. Finally, the experimental results verify the proposed model on predicting both the fatigue failure location and lives of weld joints, and the predicted lives are within the 1.5 error band.

Tensile and fatigue properties of Super304H welded joint at elevated temperatures

Super304H is a promising candidate for heat exchangers in boiler systems used in thermal power plants operating at ultra-supercritical conditions, and the mechanical properties of its welded joint play a decisive role in the reliability of the thermal power plant. Herein, we report the tensile and fatigue properties of Super304H welded joint at elevated temperatures up to 750 °C, focusing on the operating temperature range of 500–700 °C. Our results showed that the tensile strength of the welded joint was almost the same as that of the base metal at all test temperatures, meeting the requirement for weld integrity, but a significant reduction in the elongation was induced in the welded joint due to non-uniform deformation that promotes deformation localization, leading to an early occurrence of necking. As for fatigue resistance, both the welded joint and base metal had comparable fatigue lives at high strain amplitudes (≥~0.4%) in the temperature range of 500–700 °C, whereas a drastic reduction in the fatigue resistance was observed in the welded joint at low strain amplitudes (<~0.4%), which was associated with the transition of the fatigue failure location (mechanism) from the base metal to the weld metal. As both the welded joint and base metal cyclically hardened during fatigue deformation, the plastic strain energy density was found to be a suitable fatigue parameter. To predict the fatigue life behavior and its temperature dependence, an energy-based unified life prediction model was developed, and its validity was demonstrated with both the welded joint and base metal.

The effect of thickness variation and pre-strain on the cornering fatigue life prediction of a DP600 steel wheel disc

Cornering fatigue test of an automotive wheel is one of the essential tests to ensure the structural integrity of a wheel disc. Forming operation of wheel disc introduces forming history such as thickness reduction, pre-strain, and mean residual stresses. In this work, a methodology is proposed to consider the factors affecting the cornering fatigue performance of DP600 automotive wheel disc. The cyclic elastic-plastic Finite Element (FE) simulations are performed using the Chaboche kinematic hardening model. Local stress-strain states of critical element evaluated from FE simulations and fatigue parameters obtained from symmetric strain and stress controlled tests are coupled to predict the cornering fatigue life. Maximum equivalent stress and stress triaxiality at the cup radius region indicates a possible site for fatigue crack initiation. The incorporation of thickness variation and pre-strain effect is found to be the two most significant parameters for the accurate estimation of cornering fatigue life and failure location of wheel disc.

Fatigue Performance of Different Rib-To-Deck Connections Using Traction Structural Stress Method

The fatigue performance of an orthotropic steel bridge deck is significantly influenced by the type of the rib-to-deck connection considering the crossbeam. Fatigue fracture of the weld seam at the rib-to-deck connection has been a serious problem in such decks. In this paper, numerical models are developed for the orthotropic steel bridge decks and are analyzed for the fatigue behavior. The traction structural stress method is proven to be more accurate and effective in predicting the fatigue life. Fatigue behavior of three typical rib-to-deck connections are obtained by using traction structural stress method and by considering the effect of crossbeams. Compared to the bridge deck with weld seam of a large root, the fatigue performance of the deck with single-sided weld seam is much better with lower equivalent structural stress. The results indicate that the weld seam size should be strictly controlled for better fatigue resistance. The fatigue performance of the bridge deck with double-sided seam is significantly better than that of the bridge deck with single-sided weld seam. An increase in the thickness of the inner weld seam in the rib-to-deck connection optimizes the distribution of the equivalent structural stress and shifts the fatigue failure location from the weld root of the outer weld seam to the weld toe of the inner weld seam thus demonstrating the effect of the crossbeam. The trends of equivalent structural stress with geometric parameters of the weld seam in the rib-to-deck connection are obtained in this study. The fatigue behavior of the components and the equivalent structural stress are significantly influenced by the bridge deck thickness.

Вдосконалення литих деталей візків вантажних вагонів на основі результатів визначення ймовірних місць виникнення втомних руйнувань

Вдосконалення литих деталей візків вантажних вагонів на основі результатів визначення ймовірних місць виникнення втомних руйнувань

Fatigue Estimation based on Stress RMS under Stochastic Excitation

The thesis presents a case study of fatigue estimation based on stress response root mean square under stochastic excitation. The cantilever beam with a hole is subjected to a stochastic vibration test firstly, during which measured the time history of stress at critical locations using strain gauges. Then the characteristics of measured locations’ stress response RMS is discussed and the stress RMS curve of fatigue failure location is fitted. Finally, fatigue estimation is taken out based upon the stress RMS curve compared with experimental test life. The result indicates that it is practical to use stress RMS to estimate fatigue life under stochastic excitation, providing a method for fatigue analysis in engineering applications.

Experimental Research on Fatigue Failure for 2219-T6 Aluminum Alloy Friction Stir-Welded Joints

The fatigue experiment was executed for the 2219-T6 aluminum alloy friction stir-welded joints at the rotation speed of 800 r/min and the welding velocity of 150 mm/min. Most fatigue failures occurred in the weld nugget zone (WNZ), the thermo-mechanical affected zone and the nearby areas. The experimental results demonstrated that the sudden hardness gradient increases sites corresponding to the fatigue failure locations. The high-angle grain boundaries with the highest concentration were scattered within the WNZ. The microcracks initiated at the intersection of the soft grains. More than one crack initiation site was observed within the WNZ and the thermo-mechanical affected zone, when the fracture occurred in these areas. The rough surface of the welding area should be one of the main reasons for the fatigue failure occurrence. The fatigue crack growth rate in the WNZ at the first stage was fastest in comparison with the fatigue crack growth rate in the other areas of the joint.

Strain energy density based fatigue cracking assessment of load-carrying cruciform welded joints

This paper presents fatigue failure crack initial points and the fatigue failure mode transition relationship between the weld toe and root of load-carrying cruciform welded joints on the basis of LEFM and Williams’ notch stress intensity theory. The notch stress intensity factors (NSIFs) of different failure locations were calculated quantitatively. The averaged strain energy density (SED) in a control volume near the notches is used to unify the scalar quantity and rectify inconsistencies of NSIFs units for the weld toe and root. The failure mode transition relations of different geometrical parametric models (such as main plate thickness, attachment plate thickness, weld length, and penetration size) were compared and analyzed based on the SED values from weld toe and root locations, and the effect of unequal main plate thickness ratio in a cruciform joint on failure mode was also investigated. The results show that SED approach can be used to accurately predict the transition region of equal base plate thickness. The weld length and penetration size are related to significant changes in the fatigue failure location of cruciform joints. The SED approach effectively reflects failure characteristics and can be used to assess fatigue failure behaviors for load-carrying cruciform welded joints.

Fatigue Performance Evaluations of Vehicle Toroidal Liquefied Petroleum Gas Fuel Tanks

In this study, fatigue performances of the vehicle toroidal liquefied petroleum gas (LPG) fuel tanks were examined to estimate the fatigue life and its failure locations using both experimental and finite element analysis (FEA) methods. The experimental investigations performed as accelerated fatigue tests were carried out using a hydraulics test unit in which the tanks were internally pressurized by hydraulic oil. The LPG tanks were subjected to repeated cyclic pressure load varying from zero to service pressure (SP) of the tank. The computerized FEA modeling of these tanks were developed in three-dimensional (3D) form using nonuniform geometrical parameters and nonlinear material properties. These models were also subjected to zero-based high cycle fatigue pressure load considering the stress life approach. The FEA modeling process was also simulated in nonhomogeneous material conditions. Therefore, the fatigue life performance and failure location of the toroidal LPG fuel tanks were predicted using the computer-aided simulations and compared with the experimental results.

Fatigue failure transition analysis in load‐carrying cruciform welded joints based on strain energy density approach

AbstractThis paper details a study of the application of notch stress intensity theory to the fatigue failure mode analysis of the transition in load‐carrying cruciform welded joints. The weldment fatigue crack initiation point is difficult to predict precisely because it usually occurs in the vicinity of the weld toe or weld root. To investigate the relationship between fatigue failure location and the geometry of the weldments, we analysed the weld toe and root asymptotic notch stress fields were analysed using the notch stress intensity factors on the basis of the Williams' solution in Linear Elastic Fracture Mechanics (LEFM). Numerous configurations of cruciform joints of various plate thicknesses, transverse plate thickness, weld sizes and incomplete penetration size were used to investigate the location of the fatigue failure. The strain energy density (SED) surrounding the notch tip was introduced to unify the scalar quantity and preclude the inconsistency of the dimensionality of the notch stress intensity factors for various notch opening angles. The results of the investigation showed that the SED approach can be used to determine the transition zone for a variety of joint geometries. The validity of the SED criteria was verified by comparing the experimental results of this study with the complied results for load‐carrying cruciform welded joints reported in literature.

Fatigue behaviors of 2.5D woven composites at ambient and un-ambient temperatures

In-depth understanding of the fatigue behavior of woven composites at un-ambient temperatures is critical for their engineering applications, especially in aero-engine. Here, fatigue tests were conducted on 2.5D woven composites at an ambient temperature (AT) and an un-ambient temperature (U-AT) to investigate the fatigue characteristics and related damage mechanism. Results show that the fatigue life of this material at U-AT has a sudden increase phenomenon, alongside an abnormal behavior existing in residual strength. Additionally, the gradual right shift of the hysteresis loop was exhibited, which unambiguously reveals that during the cyclic loading, the samples were stretched alongside the increase in inclination of warp, causing the strengthening of mechanical properties in warp direction. The fatigue failure locations were mostly located on the intersection of warps and wefts, which is similar with the static fracture locations. Finally, based on the test data, fatigue life and residual stiffness models taking temperature into account were established.

Effects of plate stiffness on the fatigue resistance and failure location of pipe-to-plate welded joints under bending

A series of tests have been carried out using specimens made of a tube, having a thickness of t=10mm, joined to a plate by fillet welding. Two different kinds of specimen were employed, differing in the plate geometry (stiffness). Both kinds of specimen were tested under bending (prevalent load) and shear loading in as welded conditions.Different initiation regions for the fatigue cracks were found and significantly different fatigue resistances were obtained for the two geometries in terms of the nominal stress approach (or in terms of applied load vs cycles to failure). Two local methods for the fatigue life assessment were then applied to independently analyse the experimental results: the fictitious notch rounding approach proposed by Radaj, which is also recommended by some international standards and the more recently proposed peak stress method, which is based on the NSIF concept.It is shown that the nominal stress method, which is by far the simplest method among those recommended in standards for analysing the joint under study, fails to explain the observed different endurances. On the other side, the methods based on local stresses account for the different joint stiffness and provide a reduced scatter in the results. However, even if local approaches, accounts for differences in the structural behaviour of the joint, the knowledge of the actual geometry of the weld need to be accounted for, in order to be able to identify the fatigue crack initiation region.For a design purpose, a safe prediction of the fatigue endurance of the joint can be obtained by all the analysed methods, if the corresponding recommended design curve is used.

Effects of mechanical heterogeneity on the tensile and fatigue behaviours in a laser-arc hybrid welded aluminium alloy joint

The effects of mechanical heterogeneity on the tensile and high cycle fatigue (104–107 cycles) properties were investigated for laser-arc hybrid welded aluminium alloy joints. Tensile–tensile cyclic loading with a stress ratio of 0.1 was applied in a direction perpendicular to the weld direction for up to 107 cycles. The local mechanical properties in the tensile test and the accumulated plastic strain in the fatigue test throughout the weld’s different regions were characterized using a digital image correlation technique. The tensile results indicated heterogeneous tensile properties throughout the different regions of the aluminium welded joint, and the heat affected zone was the weakest region in which the strain localized. In the fatigue test, the accumulated plastic strain evolutions in different subzones of the weld were analyzed, and slip bands could be clearly observed in the heat affected zone. A transition of fatigue failure locations from the heat affected zone caused by accumulated plastic strain to the fusion zone induced by fatigue crack at pores could be observed under different cyclic stress levels. The welding porosity in the fusion zone significantly influences the high cycle fatigue behaviour.

An investigation of the location of fatigue initiation – Deterministic and probabilistic aspects

Fatigue limit tests where there are two competing fatigue failure locations, two notches of different size, are performed in this paper. The normal distribution is used for describing the experimental outcome, where the largest occurring stress value or the gradient adjusted stress is used as a stress measure. Another model for describing the experimental outcome is the Weakest Link (WL)-integral where the integration is performed over the specimen surface area or the specimen volume. In addition, the positions of the fatigue failure initiation sites are measured and investigated. The predictive capabilities of the different models are evaluated with respect to determination of the longitudinal failure location.A notable result is that the longitudinal location of fatigue failure differs considerably from one experiment to the other, even when they are numerically equal. Nominally the stress at the fatigue failure site is much lower than the maximal stress. The load cases where there should be an equal amount of failures in both notches according to the models used do not agree with the experimental findings. Neither the point stress model nor the gradient adjusted point stress, nor the WL-integral can explain the experimental outcome from the competing notch tests.

Experimental and Modeling Study of the Effect of Corrosion Pitting on Fatigue Failure Locations in Aircraft Components

It is well established that corrosion pits reduce the fatigue life and structural integrity of aluminum alloy aircraft components. A great deal of research has been conducted in this area in the last 20 years. This problem is not unique to aluminum alloys or aircraft however. Similar problems have been observed in the steel components of other engineered structures such as steel pipelines and steam turbine blades. However the effect of pitting corrosion on the probable location of fatigue failures has been overlooked. This is problematic as corrosion pits have caused fatigue failures in locations and components where they were unexpected, such as the trailing edge flap lug of the F/A 18 fighter aircraft. DSTO have called this problem ‘Corrosion Criticality’. This paper reports the development of Monte-Carlo models of how pitting corrosion affects the location of fatigue failures in two fatigue specimen geometries that have different stress concentration factors (kt). These specimens are a low-kt fatigue life specimen and a high-kt fatigue life specimen with three holes arranged along its centerline. The modeling results for the low-kt specimen are then compared with experimental results for that specimen. The low-kt model produces good estimates of fatigue life and of the probability of fatigue failure at any given location in the specimen’s gauge section. The process that will be followed to develop the high-kt model is outlined. The paper includes a discussion of using the Corrosion Criticality models to reduce the cost of corrosion maintenance by (i) identifying areas in which corrosion inspections are critical and (ii) identify aircraft components for which pitting corrosion will not be a threat to airworthiness during the life of an aircraft.

Fiber optic sensors for assessing strains in cold in-place recycled pavements

This study aims to investigate aspects related to the assessment of strains in the body of asphalt pavements that have been cold in-depth recycled utilising foamed asphalt as a stabilisation treatment for base works. For this purpose, the critical tensile strains within the pavement structure were measured using a fiber optic sensor (FOS) system at four different test sites recycled utilising the mentioned technique. Initially, an investigation was undertaken to define the critical fatigue failure locations within the structure of the test pavements where the FOS systems were eventually installed. The measured strains were verified and documented through falling weight deflectometer loading. The collected data were analysed thoroughly. The results of the analysis prove that the recycled material, except for its sustainability characteristics, could be also characterised as a material durable against fatigue failure.