Fatigue design method of encased composite beams with welded shear studs

Fatigue evaluation and design methods were developed in a recent project for steel highway bridges. The evaluation method enables an engineer to estimate the safe remaining life of an existing bridge. The estimate of safe remaining life is needed to make cost-effective decisions regarding inspection, repair, rehabilitation, and replacement. The design method is consistent with the evaluation method. The reliability-based concepts and data base behind the development of these methods are discussed and illustrated through some examples. The evaluation and design methods were calibrated for both redundant and single-load-path bridge components using safety (reliability) indices computed with existing fatigue specifications. The proposed procedures lead, however, to more uniform and consistent safety indices. The evaluation and design methods also follow to a large degree present procedures familiar to highway agencies. These methods, however, contain options to use site-specific traffic and inspection data and improved structural analysis. Both the evaluation and fatigue design methods have been incorporated in two recent guide specifications published by AASHTO.

This paper addresses the issue of fatigue in grid structures, a topic of interest in engineering and academia. The goal is to establish a practical fatigue design calculation method for weld toes in welded hollow spherical joints (WHSJs). The study focuses on commonly used steel tube-WHSJs in grid structures, conducting 25 constant amplitude and four variable amplitude fatigue tests on tube–sphere joints (TSJs) to derive corresponding S-N curves. Using ANSYS, the hot spot stress concentration coefficient Kh at the weld toes in 22 TSJs was calculated, resulting in a numerical solution for Kh ranging from 2.0550 to 4.8600. Based on this, fatigue design methods were established using nominal stress amplitude and hot spot stress amplitude as fundamental parameters. Within a fatigue design reference period of two million cycles, the allowable nominal stress amplitude for TSJs is 22 N/mm2, and the allowable hot spot stress amplitude is 66 N/mm2. The study also conducted macroscopic and microscopic analyses on fatigue fractures of TSJs, revealing that the weld toe in the sphere of TSJs is the primary site for fatigue crack initiation. This research provides practical calculation methods for fatigue design in WHSJ grid structures, contributing to their broader application.

Agricultural equipments, such as linkage graders, haymakers, swing-ploughs, bale handlers and road transport equipments, such as road trailers, traffic sign supports and lighting poles are manufactured using welded thin-walled (t < 4 mm) hollow section connections. There are currently no fatigue design rules for nodal joints made up of thicknesses less than 4 mm in existing fatigue design standards such as CIDECT Design Guide No.8. Previous research on welded thin-walled joints covered plate-to-SHS (square hollow section) and SHS-to-SHS T-joints. This paper describes fatigue tests on T-joints made up of thin-walled (t < 4 mm) SHS chords and circular hollow section (CHS) braces, namely CHS–SHS T-joints. The joints were tested under constant stress amplitude cyclic in-plane bending in the brace. In all the 18 CHS–SHS T-joints tested, chord-tension-side failure mode was observed. Using the experimental stress concentration factors (SCFs) obtained through stress distribution measurement at weld toes in the chord and SCFs from existing parametric equations, fatigue design recommendations are proposed.

Nowadays, the fatigue design of ship structural members is performed based on each classification society rule. However, the considerable size of crack damage is reported sometimes soon after the service. And in the same type of ships, fatigue damage occurs in one and not occurs in the other. These facts cannot be analyzed and explained by the current fatigue design method.Then, authors have proposed new fatigue design method. The differences from the current fatigue design method are the following three points.(1) Use of fatigue crack growth analysis as the basis for fatigue strength evaluation instead of the Miner rule.(2) Implementation of author' s new storm model for the time history of stress.(3) Proposal of the new sailing model of the ship instead of the all headings model.The developed method and proposed fatigue strength charts are applicable to the initial fatigue design instead of the current fatigue design method, and also the residual fatigue life prediction in cracked structural members when fatigue cracks are found after a certain service period.

This study is part of a project regarding the accuracy of life predictions based on finite element analyses and four different fatigue design methods. Different fatigue design codes, such as nominal stress, geometrical stress, notch stress and linear elastic fracture mechanics are compared regarding work effort and analysis accuracy. The paper also describes the process of developing and verifying a finite element (FE) model as well as other practical work such as load data acquisition, static strain measurements and full-scale fatigue tests under spectrum loading. The component that was used was a stay from an articulated hauler. The results from the fatigue tests showed that all cracks started from relatively large root defects and that a large scatter of the weld dimensions can be expected when the welds are manually performed. The comparison between the different fatigue design codes showed a large scatter in the estimated fatigue life. The verification of the FE-model showed some interesting and in some cases unexpected results. The non-linear force-strain behaviour close to the weld toe was not expected and caused a large amount of work with the FE-model. Since also the manufacturing tolerances were shown to influence the measurements, the verification of the FE-model became quite hard to accomplish. The influence of different parameters such as weld geometry, defects, misalignments, pre-tension forces and installation accuracy of the gauges should be studied further.

The subject paper presents the methods and results of fatigue tests for both torch-cut and machined racks using 40.4 mm module and finite-element elastic stress analyses for these racks. Further, the results of the analysis of fluctuating jack load range distribution over a one-year period for a jack-up rig is presented. The author proposes a fatigue design method of racks for jack-up units based on the foregoing results. The proposed fatigue design method was compared with those of ASME Boiler and Pressure Vessel Code, DNV’s Classification Notes, and the draft proposal of ISO for the strength design of gears, resulting in the conclusion that a single conventional fatigue design method as presented by ASME, ISO and DNV is insufficient for the fatigue design of these racks.

The results of a calibration study of the new simplified fatigue design procedure adopted in API RP2A are presented. Detailed fatigue analyses were performed on seven structures with different geometries, natural periods, and water depths ranging from 58 feet to 314 feet. The calibration results along with realistic descriptions of the long-term wave climate for different water depths in the Gulf of Mexico were used to develop curves of allowable hot spot stress. These curves will be included in the 17th Edition of API RP2A as new recommended design values. INTRODUCTION The 16th edition of API RP2A (Reference 1) states that template type structures in the Gulf of Mexico with natural periods less than three seconds may be designed for fatigue without a rigorous fatigue analysis. The fatigue design method specified limits the peak nominal brace stress due to the "design environmental loading (wind, wave, etc.)" to 20 ksi and the corresponding cyclic punching shear to 10 ksi. Alternatively, if stress concentration factors are known, the peak hot spot stress should be limited to 60 ksi. These criteria can be traced back to research and development carried out in the late 1960's, with the limit on brace nominal stress first appearing as a design guideline in the 3rd Edition of API RP2A (1971). This fatigue design method is an attempt to predict fatigue behavior using the design wave event. The accuracy depends upon how well the particular platform characteristics and force levels match those of the structures used to calibrate the method. Given the operator's choice in the selection of wave height, wind, and current, there could easily be a factor of two difference in total force level from that used in the calibration. In fact, the original guidelines were developed when the deepwater design wave in the Gulf of Mexico was believed to be 60 feet. Marshall and Luyties (Reference 2) showed that a comparable hot spot stress for a 70-foot wave, which is consistent with the current API reference level wave height, would be 75 ksi. This method has several other shortcomings. It does not distinguish between platforms in different water depths where the relationship between design wave and fatigue wave climate varies, nor between different members within a structure that might be more fatigue sensitive. In addition, the method implicitly assumes an S-N curve and fatigue life, and doesn't allow the operator to select appropriate values. There is also no guidance given for structures located outside the Gulf of Mexico. In 1983, the API Subcommittee on Fixed Structures organized a task group to study the problem of fatigue and propose updated guidelines. This task group, which was comprised of representatives from major oil companies and design consulting firms, accomplished several goals.

Experimental results for the cyclic pressure fatigue of several pressure vessels with flat heads are reported and compared against various fatigue design methods. The geometry is based on a welded joint presented in a recent paper by Kalnins, Bergsten, and Rana (2005). Weld designs between the head and shell include full penetration welds completed from a single side and both sides of the shell. Unique characteristics of this test include thin plate design, a large D/t ratio, and a low membrane-to-bending ratio. These are aspects of flat head geometries which have not been widely reported in the literature. Allowable fatigue cycles for ASME Section VIII, Division 2, IIW, PD 5500, EN 13445, API 579, and the Master S-N method (proposed for the ASME Section VIII, Division 2 rewrite project) are presented. Results show that several of these design method produce non-conservative fatigue life predictions. In addition, the fatigue results demonstrate that size effects and plate thickness effects have diminishing influence for thin plates failing at less than 100,000 cycles. Finally, the fatigue strength of stainless steel is compared to carbon steel and the lack of a unified approach to stainless steel fatigue design is discussed.

Intelligent tower crane has large structure deadweight. The service life of the whole machine is dependent on the life of the structure. As a result, the metal structure should be designed by using correct and reasonable methods to obtain excellent fatigue properties. And it can guide the design, manufacturing, use and maintenance process for crane. And it has great significance to prevent the fatigue fracture accident. The theoretical mechanics and material mechanics theory is a semi-theoretical semi-empirical traditional design method, but it is not the suitable design method. Now, the finite element method, fracture mechanics, the boundary element method, the optimization design method, reliability design and fatigue design methods are widely used in the structure design of crane. In this paper, the intelligent tower crane structure is analyzed by finite element modeling. According to the actual situation, crane structure is studied by finite element statics analysis. Then mechanical property of this kind of structure is analyzed in detail. And the weak links of the structure are found out. The residual life can be estimated finally. As a result, designers can comprehensively understand the fatigue life distribution of different parts in the crane. Therefore, they can provide intuitive and comprehensive basis for comparing the advantages and disadvantages of different design schemes and fatigue performances for design improvement.

Research on multidisciplinary fatigue optimization design method in structural design of high speed train

Recently, a new issue in the design of spot-welded structures has been economical prediction of a fatigue design criterion without additional fatigue tests. In general, the most typical and traditional method is the use of a Δ P- Nf curve. However, since the fatigue data on the curve vary according to welding conditions, materials, geometry and fatigue loading conditions, it is necessary to perform an additional fatigue test in order to determine a new fatigue design criterion for a spot-welded lap joint of different dimensions and geometry. This is, of course, a very time consuming and costly task. Thus, in this paper, an economical and advanced design method is proposed, the reliability and performance of which have been verified with the theory of the Weibull probability distribution. As a mechanical parameter to predict the fatigue design criterion, a maximum stress equation has been defined using artificial neural networks. By using the maximum stress equation and the fatigue data previously obtained from fatigue tests, a reasonable fatigue design criterion for a spot-welded lap joint could be predicted without any additional fatigue tests. The results predicted by this method showed very good agreement with actual fatigue data

In order to improve reliability of an automotive engine, a fatigue design method was studied based on the fatigue mechanisms in flake graphite cast iron. In the present study, fatigue test specimens were cut from an actual engine component. The material used showed nonlinear behavior in stress–strain relationship from lower stress level during tensile testing. Rotating bending fatigue testing and axial load fatigue testing were performed. Fatigue crack initiation and propagation behavior were observed during the fatigue tests by a replication technique. Fatigue cracking initiated from the tip of flake graphite in early stage of the total fatigue life when the applied stress was higher than the fatigue limit. On the other hand, fatigue cracking was not observed in the specimen tested with applied stress below the fatigue limit. The threshold stress intensity factor obtained with a small surface crack was lower than that obtained with a through-thickness crack. Fatigue limits under various loading conditions such as bending loading and axial loading were predicted well based on fracture mechanics using the $$ \sqrt {\text{area}} $$ parameter. Threshold stress intensity factor for a small surface crack and the maximum graphite size assumed as an initial crack were used for the prediction. Axial load fatigue tests with different stress ratios were performed. The fatigue limit diagram obtained by the fatigue tests did not follow the modified Goodman relation. On the other hand, fatigue limits prediction based on fracture mechanics approach showed good agreement with fatigue limit obtained with fatigue tests in a wider range of mean stresses σmean varying from − 150 to 50 MPa, in negative stress ratio. In case of σmean higher than 50 MPa, in a positive stress ratio, a smaller effect of stress ratio in the fatigue limit diagram was observed. It was considered that this phenomenon could be explained by the lower effect on the crack closure in a small surface crack initiated from the tip of graphite flake and also influence of local plastic behavior of the matrix around the tip of graphite flake. It can be summarized that the fracture mechanics approach is an effective way to predict fatigue limits of an engine component produced by flake cast iron in different loading conditions and in wider range of mean stress conditions.

Experimental results for the fatigue testing of several welded flat head geometries are reported. These tests are similar to those previously reported by Hinnant (2006) [1] and focus on the fatigue behavior of full penetration welds with cover fillet welds. Fatigue calculations according to several fatigue design methods are compared against the experimental results, as are the mean fatigue curves of several of the design methods. Of particular interest for these new tests is the effect of plate thickness, testing environment, and geometric effects. Nominal plate thickness values ranging from 0.0625" (1.59 mm) to 0.1875" (4.76 mm) have been tested and correlated. Four additional fatigue tests were conducted using air to determine if previous testing in room temperature tap water resulted in decreased fatigue life.

The suspension system of vehicle is directly influenced to ride and handling. Therefore, suspension part should have enough endurance during its lifetime to protect passenger. Spring is one of major suspension part of vehicle. Thus, in this paper, a fatigue design method for leaf spring based on proving ground response was proposed. At first, stress and displacement of leaf spring are measured through the proving ground test. And next, the maximum load acting on leaf spring assembly under driving condition was defined from the road load response. On the base of these results, fatigue tests for leaf spring assembly and 3-point bending fatigue tests for material of leaf spring were carried out. From the above, the maximum load-fatigue life relation of leaf spring material and assembly was defined, and 3-point bending test result has good agreement with leaf spring assembly fatigue test result. Thus, it is expect that economical fatigue design criterion for leaf spring assembly can be determined from fatigue data of simple smooth specimen by 3 point bending fatigue tests.

Fatigue behavior and design of welded tubular T-joints with CHS brace and concrete-filled chord