Structural Stress Method Research Articles

Fatigue behavior of typical details of orthotropic steel bridges in multiaxial stress states using traction structural stress

In this study, the fatigue property of arc-shaped cutout region in orthotropic steel bridge crossbeam in multiaxial stress states is investigated. The analytical equation of traction structural stress method considering normal structural stress and shear structural stress is applied for analyzing multiaxial fatigue of crack initiation. It is found that the arc-shaped cutout in OSB crossbeam is in multiaxial stress states and 9th spot in arc-shaped cutout region under specified loading conditions is evaluated as crack initiation position.

Fatigue strength assessment of TIG-dressed ultra-high-strength steel fillet weld joints using the 4R method

Post-weld treatments can enhance the fatigue strength of weldments by means of improving their geometry or residual stresses, or both. In this study, the fatigue strength of TIG-dressed non-load-carrying cruciform joints made of ultra-high-strength steel were analyzed using stress-based approaches, i.e. nominal, structural, and effective notch stress (ENS) methods, and a multi-parametric approach, entitled the 4R method. Comparisons were made between the methods, along with the applied parameters, and the results showed the 4R and ENS approaches to have the best correspondence with the experimental test data, whereas the nominal and structural stress methods resulted in conservative fatigue strength estimations.

Evaluation of magnesium weldment fatigue data using traction and notch stress methods

Recent magnesium weldment fatigue test data (Karakas et al., 2008), which involve partial- and full-penetration butt welds, as well as non-load carrying fillet welds, have been analyzed using both traction structural stress and equivalent notch stress methods. Both methods were compared for their effectiveness in correlating the test data by incorporating fracture mechanics-based load ratio parameters and was successfully used in conjunction with the load ratio parameter. Furthermore, the equivalent traction stress parameter, once converted into an equivalent structural strain parameter, enables comparison between magnesium weldment test data with the master S-N curve scatter band adopted in ASME Div. 2.

High strength welding of Ti to stainless steel by spot impact: microstructure and weld performance

Vaporizing foil actuator (VFA) spot welding, a type of spot impact welding, was used to weld a titanium alloy (Ti-1.2ASN) to a stainless steel (436 SS). The interfacial microstructures and fracture surfaces were characterized using scanning electron microscopy (SEM). Lap shear tests that strained the samples to failure with digital imaging correlation (DIC) were conducted to study the mechanical performance of these welds. A mesh-insensitive structural stress method was used to understand the stress distribution and model the failure modes of VFA welds in ABAQUS. Despite experimental scatter in this developing joining method, most samples failed through the base metal, but multiple failure modes coexisted, including interface failure. These failure modes were used to classify the results. The failure process can be best understood through the lens of the spatial variation that is natural in this type of weld. The center is naturally unwelded, and there is an annulus of high strength material surrounding this unwelded zone that has a wavy morphology. The mesh-insensitive structural stress method could naturally provide a link between the joint structure and the mechanical properties of the spot impact welds. This could show that despite varied failure modes and nugget strength, strength itself is not usually affected adversely by the size of the central unbonded zone.

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.

A comparative study on the fatigue life of the vehicle body spot welds using different numerical techniques: Inertia relief and Modal dynamic analyses

Among different parts of a vehicle, the body is the main load-bearing component and as a result, its durability is critical. Fatigue analyses are typically divided into different categories, the quasi-static methods and the dynamic methods. The aim of this paper was to compare the inertia relief and modal dynamic approaches for their formulation, accuracy and computation time. The chosen case study is the fatigue life of the vehicle body. By utilizing multi-body dynamics model and driving the vehicle on different standardized roads and by different velocities, the force and moment time histories which act on the body were calculated and later used by the finite element model for the stress analysis. Then, by using the structural stress method, the fatigue life of the vehicle spot welds were calculated and the results were compared for both quasi-static and dynamic approaches. The findings reveal that the modal dynamic method is almost 37 times more time-consuming than the inertia relief approach, but if accuracy is desired, it can be up to 96% more accurate. Also as predicted, at low frequency loading (less than 10% of the first nonzero frequency of the structure), there is no difference between the results of both methods.

Fatigue life prediction of spot-welded joints with a notch stress approach

Spot-welded joints are widely used in the construction of vehicle structures and frequently become critical locations for fatigue failure. Hence, it is essential to have reliable fatigue life prediction method for the spot-welded joints during vehicle structure design. In this paper, a new notch stress approach is developed for fatigue life prediction of the spot-welded joints. Currently, structural stress methods are widely used in automotive industry for fatigue life prediction of spot-welded joints. However, these methods are not well considering local geometry information. This paper introduces a notch stress based method to overcome the limitation of the structural stress methods. In the notch stress method, stress concentration factors for spot-welded joints are calculated from stress intensity factor equations. Then, the notch stress method is validated with fatigue test results of lap-shear and coach peel specimens.

Analysis of Fatigue Performance of U-Rib to Deck Connections in Orthotropic Steel Bridge Structures

The stress condition of the transverse rib, bridge deck, and crossbeam in orthotropic steel bridge decks (OSBD) is complicated. Existing fatigue test specimens and the fatigue performance of components in OSBD differ significantly. In this paper, the numerical analysis method using traction structural stress is validated by the comparison with fatigue test results. The evaluation results of the fatigue behavior using the traction structural stress method are proven to be accurate. The comparative results give the fatigue performance of various types of typical test specimens in terms of the equivalent structural stress and initiation locations.

Fatigue Performance of T-joint with Complete Weld Details

The fatigue properties of T-joint of Q345 steel commonly used in construction machinery are studied. The fatigue life of T-joint with complete weld details is analyzed by nominal stress method and the equivalent structural stress method respectively, and compared with the test results. The results show that under the condition of stress ratio R=0.5, the fatigue crack of T-joint with complete weld details often occurs at the weld root of the arc extinguishing and propagate along the weld throat. In production, arc extinguishing should be avoided in key parts. The nominal stress method used to predict the fatigue life of T-joints with complete weld details is dangerous, while the fatigue life of T-joint analyzed by the equivalent structural stress method is basically consistent with the test results.

Traction structural stress analysis of fatigue behaviors of girth butt weld within welded cast steel joints

Due to its good mechanical properties, aesthetic and structural advantages, welded cast steel joints are widely utilized in engineering applications. The fatigue failure mode of girth butt weld between the cast steel node and the connecting steel tubular member was not well characterized. In this paper, fatigue tests of typical types of welded cast steel joints with different welding details were conducted. Fatigue failure mode and nominal S–N curves obtained from these tests were compared with that of an additional type of welded joint obtained from the literature. Then the equivalent structural stress parameter was selected to analyze the fatigue performance of the welded cast steel joint. The following conclusions can be drawn: (1) The traction structural stress concentration factor is higher at the weld root than at the weld toe for all five types of welding details, so the crack initiates at the weld root, which is consistent with test results; (2) Different lab-testing setups have an effect on the fatigue life of welded cast steel joints, and the traction structural stress can capture this test condition effect efficiently; (3) All fatigue data of 5 different test conditions fall within the 99% confidence interval of the master S–N curve. Furthermore, the scatter band of fatigue data processed by the traction structural stress method, is smaller than that processed through the nominal stress method. This suggests that the traction structural stress method is more suitable for the fatigue design for welded cast steel joints.

Analysis of fatigue behavior of welded joints in orthotropic bridge deck using traction structural stress

In this study, the fatigue behavior of welded joints in an orthotropic steel bridge is simulated and analyzed. The traction structural stress method is proven to be more accurate and effective, and the predicted results agree well with the test results compared to traditional assessment methods, including the nominal stress, hot-spot stress, and effective notch stress methods. The traction structural stress concentration factor curves of welded joints under cyclic tensile and bending moment loading are obtained. The accuracy and validity of finite-element simulation methods for welded joints in orthotropic steel bridge are verified by comparing simulation results with full-scale tests results. Equivalent nominal stresses predicted by the master S–N curve method based on traction structural stress results agree well with the fatigue test results with small standard deviation. The fatigue behavior of orthotropic steel bridge analysis specifications Eurocode3 and GB50017 is combined with the traction structural stress method using unified traction structural stress concentration factor curves. This approach provides practical guidance for the fatigue behavior design of orthotropic steel bridges.

Analysis of the Fatigue Crack Evolution of Corrugated Web Girders

Based on linear elastic fracture mechanics (LEFM), the fatigue crack evolution process and behavior of corrugated web girders were studied. The global finite element analysis (FEA) model of corrugated web girders was first developed and the equivalent structural stress method was used to reveal the dangerous locations along the weld under the bending load. The weld toe between the tension flange and the web weld, which is near the intersection of the inclined fold and the parallel fold, was determined as the fatigue crack easy-initiating location. Then a small region containing the crack-prone site was extracted as the sub-model for a crack propagating simulation. A semi-circle initial crack with 0.1 mm radius was inserted at the crack easy-initiating location. The stress intensity factors (SIFs; KI, KII, and KIII) of some discrete points along the crack front were calculated by the M-integral method. Based on Nasgro law, the geometry of the new crack front with a given extension length was obtained. Finally, the complete evolution process of the crack propagation was simulated. Results showed that the dominant crack propagating mode is open type (Mode I) and KI is the most important propagating driving force. According to the crack front shape evolution, the whole propagating process was divided into 6 stages. An obvious kink of the crack was found in stage 1, which covered only a very short time. The stages 3, 4 and 5 accounted for the majority of life, among which the stage 3 accounted for as high as 81% of the total life. Therefore, the cycles of the weld toe crack propagating from 0.1 mm to the thickness of the flange can be defined as the prediction life of this kind of structures.

Optimization research on S‐N curve of ring welding structure based on structural stress method

AbstractIn engineering, △F‐N curves are usually used to predict the fatigue life of ring welding, which is time‐consuming, laborious, and not universal. To improve the above inadequacies, an S‐N curve for evaluating the fatigue life of the ring welded specimen is proposed. The fatigue life of ring welded specimens with different materials, plate thicknesses, and hole diameters is obtained by tensile and shear fatigue tests. Shell elements, CBar beam elements (a kind of beam element in Nastran that can simulate bending), and rigid elements are used to establish the finite element model of ring welding. The stress of the ring welding structure under tensile shear load is calculated according to the structural stress method. The stress range △σs of the welding core is taken as the longitudinal coordinate and the experimental life N as the horizontal coordinate, using two‐parameter log‐log model and the least square method of the fatigue data for linear fitting to obtain the S‐N curve equation of fatigue life evaluation. Most of the data are located within five times of the life span, which proves that the predicted life is close to the actual life of the test, and it can provide a certain reference for design and life prediction of the ring welding structures.

The application of load identification model on the weld line fatigue life assessment for a wheel loader boom

The external loads on the wheel loader are essential for structural fatigue life assessment. However, the external loads are hard to be acquired, because the contact force between the soil and the working device is very complex. Most researchers obtain the external loads using dynamic simulation and theoretical analysis, whereas there is a big gap between the simulation and the reality. In order to obtain the precise external loads of the wheel loader, a load identification model verified by test is proposed in this paper. The mathematical relationship between the external loads and loads at the hinged points of the wheel loader is deduced from mechanical balance equation, and the load identification theoretical model is established. Based on the load identification model, a load test system is designed and the test is carried out. Based on the time history of strain measurement near the fatigue weak points and the Miner's damage theory, the relative damage is calculated to ensure that the relative damage ratio of the weak life point between the measured and simulated value is less than 0.8. Finally, the external loads obtained by the load identification model are taken as the input condition, and the life of the boom weld structure is evaluated by the mesh-insensitive structural stress method (MSS method). The results show that the calculated weak point life can truly reflect the actual weld failure position and the error ratio between predicted and actual lives is within one magnitude, which can meet the needs of engineering applications.

An equivalent structural stress-based fatigue evaluation framework for rib-to-deck welded joints in orthotropic steel deck

Rib-to-deck welded joints in orthotropic steel decks have demonstrated different fatigue failure modes that have different fatigue resistance. Among the different failure modes, the mode corresponding to the lowest fatigue resistance governs the fatigue resistance of the welded joint. The ability of determining the governing failure mode is essential for predicting the fatigue resistance of the welded joints. This research establishes a fatigue evaluation framework for determining the dominant failure mode of rib-to-deck welded joints. The framework is established based on the equivalent structural stress method and validated using model test data. The results indicate that the proposed framework provides accurate prediction of the dominant fatigue failure mode of rib-to-deck welded joints. Based on the presented framework, the fatigue resistance of the welded joints is reasonably predicted, and the effect of weld penetration ratio on the fatigue failure mode is investigated.

Structural Stress Method to Evaluate Fatigue Properties of Similar and Dissimilar Self-Piercing Riveted Joints

In this paper, we discuss the application of a simple Battelle structural stress model to evaluate the fatigue life of a self-piercing riveted (SPR) carbon-fiber-reinforced polymer (CFRP) composite to aluminum AA6111. The analytical model accounts for the forces and moments acting on the rivets to determine the structural stresses which were then plotted against the laboratory-generated fatigue life data. The master S-N curve determined in this study thus accounts for various factors such as the stacking configuration, rivet head height, and fatigue load ratios. The analytical model used in this study was able to collapse a large number of fatigue life data into one master S-N curve irrespective of stack-ups, rivet head height, and load ratios. Thus, the master S-N curve derived from the model can be used to predict the fatigue life of the SPR joints.

A notch stress method for fatigue life prediction of spot-welded joints

Spot-welded joints are widely used in the construction of vehicle structures and frequently become critical locations for fatigue failure. Hence, it is essential to have reliable fatigue life prediction method for the spot-welded joints during vehicle structure design. In this paper, a new notch stress approach is developed for fatigue life prediction of the spot-welded joints. Currently, structural stress methods are widely used in automotive industry for fatigue life prediction of spot-welded joints. However, these methods are not well considering local geometry information. This paper introduces a notch stress based method to overcome the limitation of the structural stress methods. In the notch stress method, stress concentration factors for spot-welded joints are calculated from stress intensity factor equations. Then, the notch stress method is validated with fatigue test results of lap-shear and coach peel specimens.

Design recommendations for fatigue-loaded hollow section K-joints with gap

The existing and approved rules for the fatigue design of hollow section joints have origin in the CIDECT Design Recommendations, which are based on experimental investigations made in the 1990s and earlier. Conclusions of recent research, based on fatigue tests and numerical investigations as well, are used to extend the scope to high strength steels up to S700 and various load cases. The nominal stress method as well as the structural stress method is considered. Drawbacks and opportunities of existing design recommendations are exemplified. Further instructions for fatigue resistant constructions are given to enable a safe and economic design of K-joints with gap and its girders.

Random vibration fatigue of welded structures - Applications in the automotive industry

The automotive industry is facing its most significant transformation since the first Ford factory. The rise of electrical vehicles alongside autonomous driving and additive manufacturing increase demand for lightweight vehicles, which requires proper numerical analysis tools in place in order to keep the costs low and to achieve durability targets. Vibration in heavy and light-duty vehicles poses many challenges in the design phase mainly due to the uncertainties associated with the loads and material properties. Therefore stochastic methods have attracted the attention of engineers responsible for the fatigue assessment of truck chassis and their welded attachments. In order to address the mesh scale sensitivity in the finite element method (FEM), Dong et al created the Battelle Structural Stress Method, an elegant alternative to the Volvo nodal force/moment method which was named Verity®. For fatigue life evaluation of welds Verity® is used in conjunction with a master SN curve also developed by the same authors. The present paper proposes a frequency domain variant of the Verity® Method to be used in random vibration fatigue applications along with Dirlik and T&B methods. Frequency domain approaches enjoy the statistical advantage of using probability density functions instead of rainflow cycle counters, which offers computational benefits (in terms of time) in multi-channel long history scenarios. This paper thoroughly describes the proposal and shows an example of its application to a brake chamber support attached to the rear axle of a light-duty truck. Verity is used in the both the time and frequency domains and the fatigue results are compared.

Multiaxial fatigue analysis of complex welded joints in notch stress approach

The notch stress method is often recommended for fatigue assessment of complex welded joints as it can consider the influence of weld toe geometric parameters on local stress. However, the application of the notch stress method is still in the exploratory stage in multiaxial fatigue compared with the traditional nominal stress method and structural stress method. Considering the geometric effect at the weld toe, the notch stress method describes the local stress with an accurate geometry model and the complex stress states at the damage point under multiaxial loading are determined in the local coordinate system. Using the notch stress method, six classical multiaxial fatigue criteria are re-evaluated for a series of typical welded joints in the literatures under both proportional and non-proportional loads. In order to evaluate the different criteria quantitatively, a statistical analysis is performed and the probability of non-conservative prediction is determined for each multiaxial fatigue criterion. The results show that the S-N curve for notch stress under multiaxial loading has a lower inclination and higher fatigue class compared with the uniaxial curve recommended by IIW.