Papers by Keyword: Hot Spot Stress

Abstract: The stress concentration of trimaran’s cross-deck structure is serious and the problem of its fatigue strength appears particularly important. In this paper, the responses of ship motion and hydrodynamic pressure on ship’s surface in regular waves are calculated using 3D linear potential flow theory, then the stress responses of the hot-spots in regular waves with different wave angles are evaluated by finite element analysis of the global trimaran structure. Based on linear cumulative damage theories, the fatigue damage is calculated according to the direct calculate method used spectral analysis. The result can offer the reference for the trimaran’s design and development.

Abstract: As the loading conditions for the railway vehicles are becoming more and more complex, nominal stress has been unable to fully satisfy the requirements for assessing fatigue strength for weld structures. Hot spots stress is much more closer to the actual stress of a weld structure, it can suit the demand of high speed trains’ fatigue evaluation better. In this paper, on the basis of UIC615-4 standard, the nominal stress of the welded bogie frame was calculated and its fatigue strength was assessed using FEA method. Also, set the joint section between drawbar seat and the transom bottom flange as the object of the study, succeeded in evaluating the hot spot stress of the welded bogie frame. Compared the hot spot stress calculation result with the nominal stress from shell elements, the result shows that hot spot stress is higher than the nominal stress. Using the hot spot stress to assess fatigue has the higher reliability

Abstract: Hot spot stress (HSS) approach is more applicable for fatigue evaluation, because it is more closer to the real stress at the welded notch than nominal stress.This paper studies a typical welded joint, fillet welded joint, which is used widely in high-speed car. Hot spot stress concentration factor is calculated under different nominal stress and loading methods. The S-N curves belong to nominal stress and hot spot stress were analysized. The result show that the difference between the S-N curves under different load style can be reduced greatly by hot spot stress. The hot spot stress characteristic variable was put forward, which is used to describe the influence from the welding size of the joint.

Abstract: The structural stress (SS) method developed by BATTELLE has been studied based on small or mid-size scale specimens. In order to apply the new method, such as SS, on an actual project, it should have application results on actual project. However, SS method didn’t have a lot of application data compared to class procedure using hot spot stress (HSS). In order to find out whether the SS method, for the evaluation of fatigue life, can give reasonable results when it is applied under the same loading suggested by classification societies, it was compared with fatigue lives derived by class. ABS & DNV’s simplified fatigue analysis method were adopted to check the validity of SS method. Before applying complicated loading of class, static loading case was applied, since the class method has their own correlation factor for wave loading. And then, simplified fatigue analysis was performed with more complicated loading cases. From the results of fatigue life calculation, it can be said that SS shows reasonable fatigue lives with respect to HSS or notch stress based fatigue lives.

Abstract: The suspension bridge has more flexibility and repetitive vehicles produce stress cycles in members. Then fatigue of the member is accumulated with the daily traffic loadings. In order to evaluate the working condition of the Tsing Ma Bridge, the online monitoring health system has been installed in long suspension bridge. The location of the strain sensor is not exactly at the critical member locations. The hot spot stress analysis for critical members is necessary for accurate fatigue evaluation of the bridge. The global finite element analysis of the Tsing Ma Bridge under traffic loading is performed to determine the critical fatigue member locations. A detailed local finite element analysis for the welded connections is performed to determine the hot spot stress of critical fatigue location. As a case for study, the calculated stress concentration factor is combined with the nominal representative stress block cycle to obtain the representative hot spot stress range cycle block under traffic loading from online health monitoring system. The comparison result shows that the nominal stress approach cannot consider the most critical stress of the fatigue damage location and the hot spot stress approach is more appropriate for fatigue evaluation.

Abstract: Fatigue strength assessments with two types of load carrying fillet weldment under out-of-plane bending load have been carried out by using both hot spot stress and structural stress methods. Basis for the derivation of structural stress method is discussed in detail. Finite element analyses using shell elements models have been performed for the fatigue strength assessment of weldments. As a result of the fatigue strength evaluation for load carrying transverse fillet weldment, hot spot stress method is found to be consistent with structural stress method as well as measurement. Hot spot stress, however, estimated for the load carrying longitudinal fillet weldment exhibit large variation with respect to mesh size and element type while the calculated structural stress for the longitudinal fillet weldment is relatively independent of mesh size. The fatigue life estimation according to structural stress has been introduced with the master S-N curve.

Abstract: Fatigue strength of duplex stainless steel welded joints with longitudinal fillet welded guess and with longitudinal flat side guess welded on plate was evaluated by hot spot stress, hot spot stress S-N curves were produced and compared with normal stress S-N curves. The results show that the hot spot stress fatigue strength of the two kinds welded joints should be expressed by only single S-N curve while nominal stress S-N curves can only be regressed respectively. The test data are greatly above the S-N curves with slope m=3, recommended by IIW, which indicates that the slope m=3(IIW) has great safety factor in practical engineering application. But there is no appreciable difference in fatigue strength between duplex stainless steel and common structural steel with similar welded joints when fixed the test data to slope m=3.

Abstract: In order to strengthen or repair the welded structural members or fatigue damaged areas, various surface treatment methods such as grinding, shot peening and/or hammer peening are commonly employed among other methods available. While the weld toe grinding method is known to give 3~4 times of fatigue strength improvement, this improvement may significantly vary according to weld bead shapes and loading modes. In this context, a series of fatigue tests is carried out for three types of test specimens that are typically found in ship structures. Weld burr grinding is carried out using an electric grinder in order to remove surface defects and improve weld bead profiles. The test results are compared with the same type of test specimen without applying the fatigue improvement technique in order to obtain a quantitative measure of the fatigue strength improvement. Moreover, structural stress method is employed to evaluate the effectiveness of the method in evaluating the fatigue strength improvement of welded structures.

Abstract: A fatigue detecting sensor that enables easy and precise estimation of fatigue damage of machines and structures was developed, based on the crack growth characteristics of metal. The sensor is composed of a metal foil with a single edge notch and a base metal foil. The two metal foils are bonded at the both far ends. The sensor is attached with an adhesive to the surface of structural member which is subjected to cyclic loads. The fatigue damage of the member is detected as the fatigue crack initiation from the notch tip of the sensor. The fatigue crack growth rate is independent of the crack length. The metal foil of the sensor is given pre-tension, so the sensor has stable crack growth characteristics independent of the average strain. The crack growth length is measured in a certain period after the installation. The length is converted to the fatigue damage of the member during the sampling period of sensor installation. Since no measuring instrument and signal wiring is necessary, the diagnosis procedure becomes easy and the cost for structural health monitoring could be reduced. With such small size of the sensor as conventional strain gages, the pin-pointed application of the sensor to the hot spot of structural stress concentration, where is important for the fatigue strength estimation of welded structures, is possible. Therefore, precise fatigue remaining life assessment could be carried out. Sensors with several strain ranges were developed, so application to vast strain range is possible. Even such a small strain range as 1/10000 is able to be detected. The sensors have been applied to various products like steel bridges, rolling stocks, ships, etc.