Papers by Keyword: S-N Curve

Abstract: Today in many cases ultrasonic testing machines with a frequency of f ≈ 20 kHz are used for investigations of the fatigue behaviour up to the very high cycle regime (VHCF-regime). A question that arises is if the results of these high frequency fatigue tests are comparable to conventional fatigue tests. This paper compares the fatigue behaviour of a quenched and tempered steel 50CrMo4 in two different tempered conditions investigated at low frequencies (f ≤ 400 Hz) on a servohydraulic testing machine and at a high frequency (f ≈ 20 kHz) on an ultrasonic fatigue testing machine. Effects which can occur because of the different testing techniques and testing frequencies are investigated. A concept is derived to describe the frequency effect caused by the strain rate. The estimations are compared with results of the fatigue tests.

Abstract: The aim of this study is to develop a statistical estimation method of S-N curve for iron and structural steels by using their static mechanical properties. In this study, firstly, the S-N data for pure iron and structural steels were extracted from "Database on fatigue strength of Metallic Materials" published by the Society of Materials Science, Japan (JSMS) and S-N curve regression model was applied based on the JSMS standard, "Standard Evaluation Method of Fatigue Reliability for Metallic Materials -Standard Regression Method of S-N Curve-". Secondly, correlations between regression parameters and static mechanical properties were investigated. As a result, the relationship between the regression parameters and static mechanical properties (e.g. fatigue limit E and static tensile strength σ_B) showed strong correlations, respectively. Using these correlations, it is revealed that S-N curve for iron and structural steels can be predicted easily from the static mechanical properties.

Abstract: The paper presents the analytical method of determining the S-N curve based on the static material properties. To verify the procedure algorithm, an experiment has been carried out to define the reference fatigue characteristics of material 42CrMo4. The research was performed with the use of the rotary-bending test stand made according to ones own design. The proper operation of the material testing machine was verified compliant with the ISO 1143 standard and presented in the previous paper [7]. To compare the proposed method, different characteristics according to analytical methods from literature have been determined. To verify this approach, statistical calculation has been performed. Statistical method was derived from paper [3]. The best results were achieved by the proposed method. As analytical method can lead to large error, which was presented in the papers [4,7,8], it was decided to use hybrid method. That method is based on analytical method supported by a simple experiment. Verification of this approach has produced satisfactory results.

Abstract: Machine and equipment safety is the most essential factor that determines the choice of a particular material used in the construction phase. Failure analyses in engineering praxis demonstrate that nearly 90 % of all cases of failures are caused by fatigue. For popular technical applications such as cars and trains, the durability expected for some components ranges from 10⁸ to 10¹⁰ loading cycles. However, only few studies have been carried out for more than 10⁷ cycles. The SN curve in the ultra-wide life region must be determined in order to ensure actual fatigue strength and safety of these components. This paper presents the results obtained from fatigue tests carried out by means of a high-frequency fatigue testing machine for the three grades of ductile iron: with ferritic-pearlitic matrix (GGG50), with pearlitic-ferritic matrix (GGG60) and ADI cast iron in the range from 10⁶ up to 10¹⁰ cycles.

Abstract: Low cost, low material waste and good accuracy in components with complex geometry are the main reasons for powder metallurgy to be considered as a promising manufacturing process for the future. Like wrought steel, sintered steel can also be heat treated to increase surface hardness and to improve strength. This paper compares mechanical properties of the hardened sintered steel with the sintered steel of the same powder metal SINT D30. Firstly, the static strength of both samples is determined by quasi-static tensile tests. Results are compared in stress strain diagram and they show that the tensile strength of the hardened sintered steel SINT D30 can surpass 700 MPa. The main focus of this study is however fatigue behaviour of the sintered steel. Both sets of samples are tested on a pulsating test machine with the load ratio of R = 0. The first sample is subjected to a load that corresponds to 90 % of the yield strength and is then gradually lowered to achieve one million stress cycles without breakage. Obtained results are then presented as Wöhler curves and compared in S-N diagram.

Abstract: Fatigue properties of canopy PMMA with typical damage, which are two different depth scratches of 0.3mm and 0.4mm, are studied by experimental method. In order to gain the fatigue strength about the depth scratches of 0.3mm in the designated life, experiments based on up and down method had been conducted. The S-N curves of the PMMA about the depth scratches of 0.3mm were acquired based on three-parameter exponential function. Fatigue life of the PMMA with the depth scratches of 0.4mm under different stress levels is obtained by grouping method. Fatigue properties of the canopy PMMA of the two different depth scratches were discussed. The results of analysis showed that compared with the depth scratches of 0.3mm, fatigue properties of canopy PMMA with the depth scratches of 0.4mm decrease significantly.

Abstract: Fatigue properties of the newly-formed and served canopy PMMA, which was applied in a certain airplane, were studied by experimental method. In order to gain the fatigue strength in the designated life, experiments based on up and down method had been conducted. The S-N curves of the PMMA were acquired based on three-parameter exponential function. Fatigue properties of the canopy PMMA in the two different condition of service were discussed. The results of analysis showed that fatigue properties of the served PMMA did not decrease significantly compared to the newly-formed PMMA.

Abstract: By using of a micro mechanical fatigue testing system, low tension-tension cycle fatigue properties of 301 stainless steel thin sheets with a thickness of 0.1 mm were studied. The effects of loading frequency and stress ratio were considered in the tests. The results show the S-N curves descend continuously in the low cycle regime. Cyclic σ-N curve was obtained according to the traditional fatigue theory. It agrees well with the experimental data, showing that the traditional fatigue research methods are also suitable to describe thin sheets in a certain extent. With the increase of loading stress ratio, the fatigue strength of thin sheets is increased. There is an evident effect of frequency on the fatigue behavior of the thin sheets.

Abstract: The key ingredient for any turbine runners design procedure and failure analysis is the material and its mechanical and fatigue properties. Starting from this observation, a series of static and dynamic tests were performed on the T10CuNiCr180 stainless steel after more than three decades of service. The fatigue tests were carried out under constant amplitude at a stress ratio of 0.1 and a frequency of 20 Hz. In the high cycle fatigue range, the S-N median curve was plotted and the fatigue limit for 310⁶ cycles to failure was determined at 93.35 [MPa] expressed in stress amplitude. The casting and welding defects that were observed on fracture surfaces explained this lower fatigue limit. The fatigue crack growth tests were performed on compact tension specimens and a three-parameter model was used to plot the crack length versus number of cycles curves. In addition, the material constants from Paris law were determined. Finally, the experimental results are compared against published data for another stainless steel.

Abstract: In order to ensure the reliability of the fatigue design, the dispersion of S-N data obtained by ultrasonic fatigue test is usually researched by statistical assessment. Life distribution of fatigue strength of steel materials follows the lognormal distribution or Weibull distribution in general. In this paper, 50 axle steel is researched under symmetric tension and compression loading, and the ultra-high cycle fatigue test data of 50 axle steel are researched by the Weibull distribution, 50 axle steel is described by equation of Basquin, the mode of probability S-N curve are established. The results shows that very high cycle fatigue S-N data of 50 axle steel under symmetric tension and compression loading obey the Weibull distribution. Similar to the S-N curve of 50 axle steel, the probability S-N curves display the characteristic of “the multi-stage type” under the Weibull distribution.