Hot Spot Stress Method Research Articles

A generalised approach to rapid finite element design of notched materials against static loading using the Theory of Critical Distances

The aim of this paper is promoting a simple approach whose use - together with conventional linear-elastic Finite Element (FE) analysis - results in estimates that are more accurate than those obtained by applying the classic Hot-Spot Stress Method. The generalised formulation of the Theory of Critical Distances (TCD) being proposed calculates the required critical distance from two readily available material properties, namely, the ultimate tensile strength, and the plane strain fracture toughness. This alleviates the need for further testing normally required in the conventional TCD methods which can be costly. An extensive search through the technical literature has resulted in a data base storing approximately 800 experimental results, which have been used to validate this simplified TCD methodology. The investigated test samples contained a range of notch root radii from 0.01mm up to 7mm. The specimens were made of a variety of engineering materials, exhibiting brittle, quasi-brittle and ductile mechanical behaviour, and were tested under uniaxial as well as multiaxial static loading. This extensive validation exercise demonstrates that the proposed simplified methodology is a powerful engineering tool which allows static strength to be estimated more accurately than with the classic Hot-Spot Stress Method.

Fatigue behavior and strength evaluation of vertical stiffener welded joint in orthotropic steel decks

Abstract This study focuses on the fatigue strength and crack behaviors at weld toe of vertical stiffener and deck in orthotropic steel decks. A total of 20 specimens have been carried out in fatigue tests in relation to the two types of welded joints and diverse stress conditions. The crack initiation and propagation behaviors were comparative analyzed based on four stages of cracking process. In addition, the geometry properties of welded joints were measured in cross sections, and their effect on stress concentration factors was investigated by test and FE analysis. Based on these, the stress concentration factors were calculated and compared by hot spot stress method. Finally, the fatigue strength of this structural detail had been investigated by comparing the fatigue test results and the corresponding regression S N curves. Fatigue strength of partial penetration welded joint is slightly larger than full penetration welded joint before toe crack initiation. However, based on the crack behaviors in the propagation process on the whole, it is acceptable to evaluate these two kinds of welded joint by the same S N curve in practical engineering application.Using the same S N curve JSSC-G (50 MPa) for the allowable fatigue strength evaluation of this structural detail in China was recommended.

Distortion induced fatigue of deck plate at rib intersection with diaphragm in orthotropic steel deck

Toe-deck (TD) fatigue due to wheel induced distortion of orthotropic steel decks (OSDs) was reported by laboratory tests. The existing OSDs could probably suffer the TD fatigue and is evaluated using the hot spot stress (HSS) method. The wheel induced stresses of the trough-deck welded joint are analyzed using the shell model. The influences of HSS with mesh size, stress extrapolation, element shape function and the hot spot location variation are checked. TD fatigue durability using the monitored traffic data and the design specifications are discussed. Results demonstrate that the weld toe of the joint suffers wheel induced stress concentration and frequent stress cycles. The refined shell model using the 4-node-shell elements results in nearly constant HSS. TD fatigue durability of the 12mm deck with 50mm asphalt pavement falls below the designed service life that might explain the observed fatigue cracks in the vicinity of the deck toe by laboratory tests and the in-site observation. The TD fatigue evaluation using the HSS method may be utilized to get a conservative OSD design.

Study on Fatigue Life Assessment of Offshore Platform Lattice Crane in Service

In this paper, the hot spot stress method was applied to the life assessment of offshore platform lattice crane in service for the first time. The research of residual life of crane was carried out for the problems such as long service time and poor working condition. Taking a platform crane in Bohai Sea for example, the finite element analysis was carried out and drew a conclusion that the danger zone was at the bottom of A-frame and boom. The stress test was taken using strain gauge measuring method under different working conditions to acquire the hot spot stress of the danger zone. According to usage record on the spot, more accurate statistics has been gathered to obtain practical cycles of the crane. Using hot spot stress method and Miners linear accumulative theory to evaluate the residual life of the crane successfully. The residual life of the crane is 17 years.

Fatigue life assessment for a composite box girder bridge

Two conventional methods, the nominal stress method and the hot spot stress method, are examined for the fatigue assessment of a steel box girder bridge while considering the effect of the modeling options on the estimated fatigue life. Two analytical models, a simplified global analytical model and a refined local finite element analytical model, are developed for the evaluation of the stress ranges induced by a fatigue truck. The field tests are conducted for validating the analytical models as well as for direct fatigue assessment via an ambient vibration test. The local modeling options that are examined are the inclusion of the fillet weld itself in the refined analytical model and the mitigation of the concentrated stress at the web plate, considering stress dispersion over the width of the attachment. The fatigue life is estimated for three different nonexceedance probabilities, considering different partial factors according to the accuracy of the analysis method as well as the reliability of the estimated fatigue loading for a specific bridge site. Through a comparative analysis, the potential differences in estimated fatigue lives according to the assessment methods and the modeling options are quantitatively discussed.

Numerical Evaluation of Toe-Deck Fatigue in Orthotropic Steel Bridge Deck

AbstractSome toe-deck crack cases in China have been reported after about 10 years of service in suspension and beam bridge steel orthotropic decks with trough stiffeners. In this paper, a reference stress method used in design practice and a hot-spot stress method provided by the International Institute of Welding are used to investigate toe-deck fatigue of the newly built Taizhou Yangtze River Bridge. Shell and solid finite element models with different mesh sizes are compared. The hot spot at the weld toe and the point at the trough-deck intersection are discussed. One is a shell-model hot spot with no weld. The other is a solid-model hot spot with the weld. Hot-spot stress calculated with the shell model is larger than that with the solid model. Different extrapolation methods of hot-spot stress are discussed; stress calculated with the linear extrapolation method is almost the same as with the quadratic method. The stress of the joint under a moving unit wheel load is studied. Fatigue strengths again...

Study on the Correction of S-N Distribution in the Welding Fatigue Analysis Method Based on the Battelle Equivalent Structural Stress by Rough Set Theory

In welding techniques, failure is a key problem that is related to the stability and safety of the welded structure. In the commonly used welding fatigue analysis methods, the Master S-N curve method based on Battelle structural stress solves the problem of inconsistencies in stress calculation and S-N curve selection better than the nominal stress method and hot spot stress method. In this paper, rough set theory was employed to study the S-N distribution based on the Battelle equivalent structural stress. Firstly, rough set analyses of the S-N distribution based on the nominal stress, structure stress and Battelle equivalent structure stress were carried out. Then the S-N distributions based on the three stresses were studied. The results indicated that structural stress rearranges the S-N point making it much more concentrated in the region near the mean S-N curve and that equivalent structural stress places the S-N point more uniformly on both sides of the curve. Subsequently, the corrections from the plate thickness and stress ratio in the Master S-N curve method were studied. It was concluded that with two steps of corrections the decision-making degrees of welding factors are weakened and harmonized in the Battelle equivalent structural stress. This made the S-N distribution much more concentrated and uniform, which allows more accurate welding fatigue prediction by the Master S-N curve.

1mm 응력 기법을 적용한 EH 강재 필릿 용접 이음부 피로 강도 평가

In this study, Non-load-carrying EH Grade steels in fillet welded joints were evaluated with both the hot spot stress method and the 1mm stress method. The thickness effect criterion for fatigue strength evaluation of welded of welded steel structures recommendations of the IIW was used to evaluate the fatigue strength of EH40 and EH36 and Both EH40 and EH36 have been compared with FAT 125 curve recommended in the IIW. Furthermore, fatigue strength of the welded tow and the ground conditions for Non-load-carrying EH36 based on the 1mm stress method has been discussed.

Development of Fatigue Life Improvement Technology of Butt Joints Using Friction Stir Processing

Burr grinding, tungsten inert gas (TIG) dressing, ultrasonic impact treatment, and peening are used to improve fatigue life in steel structures. These methods improve the fatigue life of weld joints by hardening the weld toe, improving the bead shape, or causing compressive residual stress. This study proposes a new postweld treatment method improving the weld bead shape and metal structure at the welding zone using friction stir processing (FSP) to enhance fatigue life. For that, a pin-shaped tool and processing condition employing FSP has been established through experiment. Experimental results revealed that fatigue life improves by around 42% compared to as-welded fatigue specimens by reducing the stress concentration at the weld toe and generating a metal structure finer than that of flux-cored arc welding (FCAW). Hot-spot stress, structural stress, and simplified calculation methods cannot predict the accurate stress at the weld toe in case the weld toe has a smooth curvature as in the case of the FSP specimen. On the contrary, a finite element calculation could reasonably predict the stress concentration factor for the FSP specimen because it considers not only the bead profile but also the weld toe profile.

A Simplified Model of a Reinforced Square Hollow Section (SHS) T-Joint for Stress Evaluation in Bus Superstructures

This study aims to create a simplified model of a reinforced square hollow section (SHS) T-joint found in bus superstructures. The approach is to use a combination of one- and two-dimensional finite element models to represent a reference three-dimensional finite element (solid) model of the joint and determine stress concentration factors (SCFs) as functions of the geometrical variables of the joint. This approach requires the stiffness of the simplified model to be equivalent to the stiffness of the reference solid model. Trial models, therefore, must be proposed and their stiffnesses must be evaluated against the stiffness of the reference solid model. The best trial model is then selected based on the stiffness error function defined to represent the deviation of the simplified model's stiffness from the reference model's stiffness. After a trial model with minimum stiffness error is selected, its SCFs, relating the maximum stress in the simplified model to the maximum stress in the reference solid model, are determined. Since the maximum stress is assumed to be at the weld toe where structural discontinuity exists, the maximum stresses on both simplified model and reference solid model are evaluated based on a hot spot stress (HSS) method. In this study, three trial models, namely Model A, Model B, and Model C, were investigated. Model B, consisting of beam and shell elements with particular constraints on the joint-reinforcement geometry, was found to provide the minimum stiffness errors of 8.09%, 6.87%, and 6.44% for three different joint dimensions. The SCFs were then determined as a function of the thickness-to-width ratio of the joint under static in-plane bending load. The resulting simplified model allows the stress evaluation on the bus superstructures to be done more quickly compared to a solid model while maintaining the accuracy of the solutions. Consequently, the designs of bus superstructures can be explored more thoroughly, leading to a better design.

Fatigue assessment of high frequency mechanical impact (HFMI)-improved fillet welds by local approaches

Local fatigue assessment methods like the structural hot spot stress and effective notch stress methods as defined by the International Institute of Welding are widely used by design engineers and researchers to assess the fatigue strength of welded components. This paper provides a comprehensive evaluation of published data for welded joints which had been improved using high frequency mechanical impact (HFMI) treatment. All of the published data for HFMI-treated welds are presented in terms of nominal stress. The goal of the current paper is to establish local fatigue assessment procedures for improved fillet welds. In total, 160 published experimental results for longitudinal and cruciform welds subjected to R=0.1 axial loading are evaluated. Local stress quantities for each joint were assessed based on the finite element analyses and reported nominal stress values. A correction procedure for yield strength that was previously verified for nominal stress-based fatigue assessment is also applied to the local stress methods studied in this paper. For both the structural hot spot stress and effective notch stress methods, sets of characteristic fatigue strength curves as functions of yield strength are proposed and verified. The structural hot spot stress method includes one set of fatigue strength curves for load-carrying welds and a second set for non-load carrying welds. The effective notch stress method includes a single set of curves for all welds. All of the design curves proposed in this study are conservative with respect to available fatigue test data.

Robust Mesh Insensitive Structural Stress Method for Fatigue Analysis of Welded Structures

The design of welded structures and careful fatigue design evaluation is especially important in structural design since the stress concentrations at the welds have significant impact on the overall fatigue lives of the structures. Currently, the nominal stress or hot spot stress methods are widely used in the field for fatigue design evaluation. However, design based on these methods requires constructing separate S-N curves from the fatigue testing based on the joint geometry, loading, thickness, etc. It is also difficult to evaluate the fatigue performance using finite element analysis, where the calculated stress can vary according to element size, type, etc. To overcome these challenges, Battelle has developed a novel, mesh insensitive structural stress method (Verity ® ). The stresses are calculated using the balanced nodal forces and moments obtained at the weld toe location from the finite element solutions. Working with industry partners, Battelle has also developed a unified master S-N curve that combines the effects of joint geometry, loading modes and thicknesses. Battelle's structural stress procedure has been adopted as design weld fatigue codes by ASME Sec. VIII Div. 2 (ASME Boiler and Pressure Vessel Code) and API 579/ASME FFS-1 (Fitness-for-Service Evaluation). As a case study, the fatigue life prediction of a rectangular hollow section joint using a new structural stress method is presented in this paper.

Fatigue tests and design of welded thin-walled RHS–Channel and Channel–Channel cross-beam connections under cyclic bending

Research on fatigue strength of welded galvanized cross-beam connections has been carried out and reported. Previous research focused on cross-beams made up of rectangular hollow sections (RHS) and Angle sections; RHS–RHS as well as RHS–Angle cross-beam connections and was published in Thin-Walled Structures, 48 (2010) 159–168. This paper reports on the fatigue strength of cross-beam connections made up of rectangular hollow sections (RHS) and Channel sections; RHS–Channel and Channel–Channel cross-beam connections. Fatigue tests on RHS–Channel and Channel–Channel cross-beam connections were carried out under cyclic bending in the bottom member. The resultant failure modes during the fatigue tests are reported. The crack growth patterns that are observed leading to failure are studied in relation to the stress concentration factors (SCFs) from sample specimens. The stress concentration factors (SCFs) are determined from stress measurements around the hot spots of the sample specimens. The resultant RHS–Channel and Channel–Channel S–N data is compared to S–N data of cross-beam connections from previous research. A comparison of the S–N data with existing fatigue design curves in the Australian Standard, AS4600, gives an indication of the class that can be adopted for the fatigue design of RHS–Channel and Channel–Channel cross-beam connections under cyclic bending using deterministic methods. Statistical analyses of the S–N data are also carried out to determine recommended design curves for the RHS–Channel and Channel–Channel cross-beam connections for both the classification and hot spot stress method.

Fatigue of Tubular Joints: Hot Spot Stress Method Revisited

A series of well-known tubular joints tested in UKSORP II have been re-evaluated using the mesh-insensitive structural stress method as a part of the on-going Battelle Structural Stress JIP efforts. In this report, the structural stress based analysis procedure is first presented for applications in tubular joints varying from simple T joints, double T Joints, YT joints with overlap, and K joints with various internal stiffening configurations. The structural stress based SCFs are then compared with those obtained using traditional surface extrapolation based hot spot stress methods. Their abilities in effectively correlating the fatigue data collected from these tubular joints are demonstrated. These tests are also compared with the T curve typically used for fatigue design of tubular joints as well as the structural stress based master S-N curve adopted by ASME Section VIII Div 2. Finally, some of the implications on fracture mechanics based remaining life assessment for tubular joints are discussed in light of the results obtained in this investigation.

Fatigue strength evaluation of transverse fillet welded joints subjected to bending loads

Three methods of fatigue strength evaluation, i.e., the hot spot stress (HSS) method, 1 mm stress method and linear elastic fracture mechanics (LEFM) method, are used to evaluate the fatigue strength of transverse fillet welded joints in bending and the results are compared with those of fatigue experiments. It is found that the HSS is overly conservative in predicting the fatigue strength of fillet welded joints in bending. The 1 mm stress catches the geometric influence of attachments very well. A reference strength range is suggested for applying the 1 mm stress method to the fatigue strength evaluation of fillet welded joints in bending on the base of analysing available test data. LEFM is an established method for fatigue strength evaluation of welded joints and its applicability is much dependent on the ease and accuracy of quantifying the stress intensity factor (SIF) along the crack front. In this study, the SIF of fillet welded joints in bending is estimated in different ways and the resulting fatigue strengths are compared with fatigue test data.

Correcting accounting results of tensions using fem by Hss method

The usage of the Hot Spot Stress (HSS) method by means of linear surface extrapolation (LSE) approach was analyzed for the correction of results of the Finite-Element Method (FEM) in case of singularity of stresses. The given examples of structures and testing examples were computed on the base of design-and-computation software SCAD for Windows (version 11.3).

Fatigue Strength Study on Different Structure Type of Hatch Corner

The fatigue strength study of the hatch corner is carried out ,based on the S-N curve method and the assumption of the linear cumulative damage of Palmgren-Miner's rule, using the hot spot stress method. Study on the effect of the structure different type to the fatigue strength of the hatch corner is carried out through changing its construction details and thickness. By comparing, the reasonable structure is obtained, and the scale effect should be put into consideration in fatigue design.

Square hollow section (SHS) T-joints with concrete-filled chords subjected to in-plane fatigue loading in the brace

In recent years, there has been an increased use of concrete-filled composite tubular joints in bridge construction. The reliability of these joints under fatigue therefore need to be investigated to avoid any catastrophic failures. In this paper, hollow section T-joints made up of square hollow section (SHS) chords and braces are investigated. The chords of the SHS–SHS T-joints were concrete-filled thereby forming welded composite tubular T-joints. The SHS–SHS T-joints with concrete-filled chords were strain gauged and tested under static loading to determine stress concentration factors (SCFs) at hot spot locations, where cracks are likely to propagate. Fatigue tests of the welded composite joints were also carried out under cyclic in-plane bending in the brace to obtain stress range vs. number of cycles ( S– N) data. The maximum stress concentration factor (SCF) at hot spot locations in a welded composite tubular T-joint was found to be generally lower than the maximum SCF in an empty (or called unfilled) hollow section SHS–SHS T-joint. Fatigue failure in welded composite tubular T-joints occurred through the initiation and propagation of cracks at weld toes in either the chord or brace member, with the majority of tests exhibiting the first visual crack at weld toes in the chord. The welded composite tubular T-joints were found to have better fatigue strength compared to the empty hollow section SHS–SHS T-joints. Design rules are recommended for the SHS–SHS T-joints with concrete-filled chords through analysis of fatigue test data using the hot spot stress method.

Fatigue design of welded joints using the finite element method and the 2007 ASME Div. 2 Master curve

Fatigue design of welded structures is primarily based on a nominal stress; hot spot stress methods or local approaches each having several limitations when coupled with finite element modeling. An alternative recent structural stress definition is discussed and implemented in a post-processor. It provides an effective means for the direct coupling of finite element results to the fatigue assessment of welded joints in complex structures. The applications presented in this work confirm the main features of the method: mesh-insensitivity, accurate crack location and life to failure predictions.

Fatigue of welded aluminium hollow section profiles

This paper presents a study of the fatigue properties of welded connections in extruded aluminium hollow section profiles based on the use of the hot spot stress method. The study includes experimental fatigue test data and analytical results, with a focus on the hot spot stress concentration factors (SCF hs). A welded aluminium rectangular hollow section T-joint forms the basis of the study covering numerical stress analysis (finite element analysis); 3-D laser profile scanning of the welds; experimental stress analysis (strain gauges); and fatigue testing. Finite element analysis was performed on both idealised and actual (scanned) weld geometries. Values of SCF hs were computed for three different load configurations and a comparison made of the effects of the idealised and scanned weld geometries. Differences ranging up to 17.5% were found from the variations in weld geometry alone. The dominant effect lay in the choice of standards-based guidance for calculating the hot spot stress – causing variations as high as 38%. An experimental programme of some 40 new fatigue tests was compared with tentative design guidance previously drawn from earlier test results. The published database for RHS joints in aluminium remains small and further expansion of the database would clearly be beneficial.