Papers by Keyword: Fatigue Limit

Abstract: The fatigue limit and fracture behavior of epoxy carbon fiber reinforced composites and hybrid composites were investigated. An aluminum foil and thin aluminum plate were incorporated with the carbon fiber to make the hybrid reinforced composites. Several specimens were prepared, and a series of tests were carried out to investigate the fatigue life and fracture behavior. Then the results were compared among epoxy carbon fiber reinforced composites and the hybrid composites. It is discussed whether the S–N curves shows almost the similar characteristics of these two types of hybrid composites or not. As far as the present result, the stress ratio and the type of hybrid composites effect on the fatigue life. The interfacial bonding plays an important role in the strength and fracture behavior of notched specimen of the fabricated composites.

Abstract: The fatigue limit and crack growth behavior of slit specimens of carbon fiber reinforced epoxy composites were investigated. The fatigue limit was defined by the maximum stress amplitude that the specimen endured 10⁶ times repeated stress when S-N curve was used. The highest fatigue limit was obtained when all the fiber directions were parallel to the load axis. The fatigue limits were evaluated in the cases of composites using alternately parallel and perpendicular to the load axis and compared with the result of the specimen having all the carbon fiber orientations were parallel to the load axis. When the measured value of the fatigue limit was lower, shear damage to the epoxy resin and peeling of fiber from epoxy resin occurred clearly. According to those results, it was expected that the fatigue limit of smooth specimens of carbon composites with long fibers can be evaluated from the results of the slit specimens.

Abstract: The carbon fiber reinforced polymer composite are widely used in industry as major structural materials. They represent the greatest interest for the production of gas turbine engines parts because of their high specific strength. But before adaptation these materials into the structure, it is necessary to conduct a number of tests, both on test coupon and on structurally equivalent samples for determining physical, in particular, fatigue properties of these materials. However, the high cost of manufacturing coupons for such tests has a negative impact on the adaptation of carbon fiber reinforced polymer composite into the composition of final products. In this paper it is presented a method for fatigue tests of test coupon and structurally equivalent samples of carbon fiber reinforced polymer, aimed at reducing the consumption of coupons which are necessary for obtaining fatigue properties. Based on the developed method, a series of carbon fiber coupons was tested and the fatigue limit was obtained. At the same time, the use of coupons was minimized.

Abstract: Fatigue crack initiation usually starts from defects or inclusion (s) in the material in the zone of stress concentration. The stress concentration zone can be at the surface of the specimen or inside the material. The fatigue crack starts to propagate, depending on stress amplitude, if the range of the stress intensity factor is higher than the fatigue threshold. The aim of this paper is to present the procedure for determining the fatigue crack initial stage by using the Wöhler S-N curve and the fatigue crack propagation parameters obtained by experiment for the loading range R=-1. Determination of the fatigue initial stage has been determined by the analysis of tensile smooth specimens and notched bending specimens. Results show that linear elastic fracture mechanics is applicable in order to establish a model for fatigue crack propagation.

Abstract: The paper presents a new thermographic method that enables simultaneous accelerated determination of the fatigue limit and the S-N curve. In the presented method, the fatigue limit was determined assuming a constant rate of temperature rise occurring in the second phase of a specimen fatigue life. The S-N curve was developed based on energy-related parameter with the assumption of its dependency on the stress amplitude. The tests made on C45 steel under reversed bending revealed that the fatigue limit value obtained from accelerated thermographic tests as compared to the value obtained using Staircase method differs by 10.0% maximum.The S-N curve obtained by accelerated thermographic method fits inside 95% confidence interval for the S-N curve obtained from the full test.

Abstract: In recent years, the core engineering components of high-speed train, automobiles and aircrafts are required to endure fatigue loads up from 10⁸ to 10¹⁰ cycles. The present study results show that in the very high cycle fatigue (VHCF) regimes of more than 10⁷ cycles, the fatigue failure of high strength steel materials can occur below the traditional fatigue limit, hence the VHCF investigations of high strength steels not only help to further understand the fatigue essence and mechanism, but also do research on the fatigue design and life assessment method. This paper summarizes works of VHCF researches for high strength steels in recent years, such as the characteristics of S-N curve, the observations on fish-eye, which is one of the typical characteristics of fracture surface, crack initiation, crack propagation, etc. The present work also analyzes the fatigue mechanisms and briefly discusses several factors that affect VHCF properties, such as hydrogen effect, inclusion effect, frequency effect. Some possible and prospective aspects of future researches are also proposed.

Abstract: Hardmetals, manufactured from powders by pressing and sintering, are the most important tool materials in service today. In many applications, such as milling or percussion drilling, they are subjected to fatigue with considerable loading cycle numbers. In the present study, the fatigue behaviour of hardmetals in push-pull loading was investigated up to N_max = 10¹⁰ using ultrasonic resonance fatigue testing. It showed that with all hardmetal grades investigated there is no fatigue “limit”, i.e. a horizontal branch of the S-N curve, but a consistent drop of the curve up to maximum N. Crack initiation was found to occur predominantly microstructure-controlled, as compared to defect controlled as typical for powder metallurgy tool steels.Keywords: gigacycle fatigue, WC-Co hardmetals, ultrasonic fatigue testing, fatigue limit

Abstract: The fracture behavior and fatigue limit in notched specimens of C/C composites were investigated. Also, the effect of specimen thickness on fatigue limit was discussed. Two plates of different thicknesses of plates of C/C composites using fine-woven carbon fiber laminates with α=0°/90° direction were used for testing. α is the angle between the carbon fiber direction and specimen axis. The crack growth behavior and failure mechanism of specimens are derived from the shear damage in the fiber bundle and matrix. Slits of several sizes were cut on both sides of a test section and different geometries of the specimens were prepared. Specimens with slits and blunt-notches were used to compare the fatigue strength. The fatigue limit is related to the method of making the plate of carbon composites. Large sizes of voids are observed in the case of specimens of thinner thickness. The fatigue limit was related to the void fraction, and thinner specimens showed a lower fatigue limit.

Abstract: The high cycle fatigue of super duplex stainless steel of SAF 2507 was investigated by rotating bending fatigue test in both air and 3.5% NaCl environment. The results showed that there is no much reduction of the fatigue life in corrosive environment, which is 90% of the air fatigue strength. In air fatigue, failure happens in ductile mode with austenite grains having finer and straighter fatigue striations than ferrite grains. Width and spacing of striations vary with the orientation and locations when the second cracking occurs. It is not reliable to identify the phase by morphology of striations. In 3.5% NaCl environment, the fracture exhibits a mixed mode of cleavage and quasi-cleavage in ferrites and ductile in austenite grains.

Abstract: According to the performed analysis of fatigue phenomena occurring in metals, the effects of fatigue appear in the form of lines and slip bands under loading conditions producing variable stresses with values below the fatigue limit of these metals. It is commonly accepted that variable stresses with constant amplitude of values below 0.4 of the fatigue limit do not cause plastic strain in grains (lines and slip bands), thus they do not affect the fatigue life. This study is an attempt of quantitative assessment of the influence of stresses with values below the fatigue limit on fatigue life by using tests with programed two-step loading (variable-amplitude). Tests were performed with the use of C45 steel specimens.