Papers by Keyword: Fatigue Limit

Abstract: Selected results of experimental research showing possibilities to combined accelerated methods, i.e. thermographic determination of the fatigue curve and estimating the fatigue limit, were presented in this paper. The tests were performed using the reversed bending fatigue-testing machine with a rotational frequency of 77 Hz. The main item of the test station was the thermographic camera. Test specimens were made of C45 (1.0503) steel drawn bars. The full tests results were presented for the fatigue S-N curve (23 specimens at 10 load levels) and fatigue limit by the Staircase method (25 specimens) based on 10⁷ cycles to failure was determination. The accelerated test for determining the slope of the S-N curve and the fatigue limit were made by a single specimen based on gradually increasing loading. Methods previously presented in the literature are based on the phenomenon of temperature stabilization during most of the fatigue process. The results presented in this work did not show the occurrence of such a phenomenon. For that reason, the new approach to the determination of the slope of S-N curve was proposed. Combined application of both experimental methods, Locati and thermographic, with a new approach to the determination of this slope might allow one to develop automatic equipment for the accelerated determination of the fatigue limit which would contribute to significant reduction of the test costs and time, thus increasing availability of such tests.

Abstract: The truck that is used as running equipment for freight car support is a core structural part that supports the load of the car body and that greatly influences the safety of freights and vehicles, as well as their running performance. The running equipment is composed of truck frames, wheels and wheel axles, independent suspensions, and brakes. Among these components, the truck frame supporting the load of the vehicles and freights may be the most important one. This study was carried out to analyze the structure of truck frames and to determine whether they are safe when the maximum vertical load, breaking load, and front and rear loads are applied to them. This was done by subjecting the truck frames to stress tests and then measuring the stress on each of their parts. Before the load test, a structural-analysis program was used for the stress distribution analysis of the truck frame. To measure the stresses based on the results of the structural analysis, strain gages were attached to the surfaces of truck frames [1].

Abstract: The fully reversed long life fatigue cycle behaviour of shot peened steels has been investigated. In the case of air cooled forged 0.4%C and 0.7%C steels, shot peening resulted in a relatively small effect on fatigue life (+2.2% and-2.0% respectively) owing to cyclic softening. Fatigue cracks in the shot peened specimens have been observed to initiate in sub-surface layers, reducing the detrimental effect of surface roughness. Neither cyclic softening nor hardening occurred in the smooth non shot peened samples cycled under the same conditions. Shot peening quench and tempered 0.5%C steel samples resulted in a reduced fatigue limit of 12.0% due to cyclic softening. Relaxation of the residual stresses occurred quickly in these steels due to adjustment and rebalancing of the residual stresses caused by the plastic strain. The effect of cyclic softening and shift in crack initiation site rather than the residual peening stresses was significant in determining the fatigue life of these shot peened steels.

Abstract: According to the present situation of short service life of high grade highway the domestic gradually developed the design and study of long-life pavement in order to improve the operational performance of the road and extend the use fixed number of year. This paper discusses the design concept of long-life pavement and also analyzes the present situation of permanent road in the Europe, the United States and other countries.The paper proposes the domestic design research direction of long-life pavement to control the early destruction of the pavement structure and improve the road performance.

Abstract: Aspect ratio is a key factor to calculate stress intensity factor (SIF) K using fracture mechanics. Cracks are approximated to be semi-circle or semi-ellipse in simple calculation, however, their empirical shapes are changed by stress concentration. In this study, in order to calculate K simply and precisely, a new method to predict fatigue crack initiation stress, σ_w1 of austenitic stainless steel with modified aspect ratio is discussed. The new method succeeded to predict σ_w1 within 20% error from empirical values using average austenite grain sizes.

Abstract: Increasing attention has been paid on fatigue performance evaluation in the recent years. More efficient fatigue testing methods are imperatively needed to accomplish the test requirement. The modified staircase theory is introduced in this paper. Traditional staircase method and modified staircase method are respectively used to evaluate the fatigue performance of pressure vessel steel Q345R. Results show the fatigue limits are 194MPa and 193MPa respectively, which indicates the modified staircase method can enhance the testing efficiency about 50% and keep the test veracity.

Abstract: In general, aluminum alloy does not exhibit distinct fatigue limit (knee point) in the S-N diagram. The growth of a small fatigue crack of precipitation-hardened Al-Mg-Si system alloy (6061-T6) was investigated to clarify the mechanism of non-appearance of distinct fatigue limit (knee point) in the S-N diagram. The small crack was analysed in detail by replica method, scanning electron microscope (SEM), and Electron Back Scatter Diffraction Patterns (EBSD). On the other hand, the existence of distinct fatigue limit (knee point) of new developed aluminum alloy by adding excess Mg to the 6061 alloy was found. In this study, the resistance of small crack growth of the developed alloy was compared with standard 6061 alloy. It was revealed that the resistance of crack growth of new developed alloy was higher than that of standard 6061 alloy in short crack region (l<1.0 mm).

Abstract: Crack size measurement is necessary in order to calculate stress intensity factor (SIF), K. Definition of the initial crack size in plain specimens is more complex than notched specimens because artificial defects are not introduced. In this study, the effect of difference between initial crack size calculated with both average grain size and maximum shear mode crack size on K, and fatigue limit value, σ_w were examined. A new alternative method to predict endurance limit of plain specimen, σ_w with a combination of threshold stress in shear mode and in Mode I was proposed.

Abstract: Aspect ratio is a key factor to calculate stress intensity factor (SIF) K using fracture mechanics. While cracks are approximated to be semi-circle or semi-ellipse for simply evaluation, their shapes are changed by stress concentration source. In this study, a new method to modify aspect ratio of a crack at a notch root is proposed. Modified aspect ratio in this method succeeded to decrease prediction error of fatigue crack initiation stress, σ_w1 which was calculated using experimental value.

Abstract: This paper aims at a deeper understanding of mechanisms leading to crack initiation in ductile metals in Very High Cycle Fatigue (VHCF). The VHCF regime is associated with stress amplitudes lower than the conventional fatigue limit and numbers of cycles higher than 10⁹. Tests were conducted using an ultrasonic technique at loading frequency of 20 kHz. The mechanisms leading to crack initiation express via slip bands at the specimen surface and self-heating due to intrinsic dissipation. Thermal maps were used to estimate the mean dissipation and its change with number of cycles and stress amplitudes in case of pure copper polycrystals. At the same time, the surface relief changes due to plasticity were characterized using optical and scanning electronic microscopes. A good correlation was found between slip band initiation and dissipation. Dissipation and slip band amount always increased over the number of cycles. At very small stress amplitudes, no slip band appeared up to 10⁸ cycles but the material was found to dissipate energy. Results derived from tests performed at high loading frequency on pure cupper specimens showed a drift of dissipative regimes incompatible with concepts of fatigue limit and/or asymptotic cyclic stability. These results reveal that the material never reached a steady state. Therefore it could break at higher number of cycles.