Fatigue strength dependence on the ultimate tensile strength and hardness in magnesium alloys

The relationships between fatigue strength and ultimate tensile strength as well as hardness have been studied in high-strength NZK alloys (Mg–yNd–zZn–xZr) and other magnesium alloys. In the absence of casting defects, clear linear relationships have been found between the fatigue strength and the ultimate tensile strength and the hardness values in these magnesium alloys in both T4 (solutionized) and T6 (solutionized and aged) conditions. The fatigue strength models developed in this work alloys can be directly applied to other defect-free magnesium alloys.     

Temperature dependence of the tensile strength of polymers and metals at elevated temperatures

We have studied the temperature dependence of the tensile strength for polymers at temperatures above the glass transition temperature and for metals, above about half of the melting temperature. An empirical relation is found between the strength of these materials, the temperature, and the time to fracture.     

Fatigue crack propagation of bainitic nodular cast iron

The effects of austempering temperature and isothermal transformation time on fatigue crack growth rate in a ductile iron with a bainitic structure have been studied. Crack growth rates in austempered samples were compared with those in materials with a ‘bullseye’ casting structure. Using scanning electron microscopy, the mechanism of the fatigue crack growth can be understood by observing the fracture surface of a fatigue specimen. X-ray diffractometry was used to determine the volume fraction of retained austenite. It can be concluded that the volume fraction of retained austenite, the fracture mode and the matrix microstructure are closely related to the fatigue crack propagation rate and the fracture mode.     

Abrasive wear study of bainitic nodular cast iron

The abrasive wear properties of the bainitic nodular cast iron under different austenitizing and isothermal transformation temperature and time were studied in the present paper. Using Optimol Schwingung reibung verschleiß wear tester, the point contact is adopted to compare the effect of those various heat-treatment conditions on wear properties. The weight loss, wear-scar diameter and friction coefficient were measured to evaluate the wear properties. Using scanning electron microscopy, the wear mechanisms can be understood by observing the worn surface. From the results of weight loss, wear-scar diameter and friction coefficient, it can be concluded that the wear resistance increased with increasing austenitizing temperature and the most suitable austempering temperature of the material used in the present study is from 270–300 °C for 4 h.     

Influence of defects on fatigue strength and failure mechanisms in the VHCF-region for quenched and tempered steel and nodular cast iron

Investigations are presented in this paper on quenched and tempered steel 42CrMoS4 from two batches, with two different tensile strengths (R_m = 1100 MPa, 1350 MPa) but with similar microstructure, and a nodular cast iron EN-GJS-900-2 (R_m = 930 MPa). Fatigue tests with smooth (K_t = 1) and notched (K_t = 1.75) specimens were performed at R = −1 and R = 0 up to the number of cycles N = 2·10⁹ in order to determine the fatigue strength behaviour and failure mechanisms, especially in the VHCF-region. Failure in smooth specimens often initiated at material defects such as oxides in the quenched and tempered steel and shrinkage holes in the nodular cast iron. Firstly, a fatigue strength analysis was performed that did not consider these defects. A possibility of analysis of experimental data including VHCF-results has been discussed. Next, a linear elastic fracture mechanics analysis was performed in order to describe the defect behaviour, assuming that the defects act like cracks. The results showed that there are lower limit or threshold values of the stress intensity factor range ΔK for crack propagation in both materials. Analysis of defects and defect distribution in run-out specimens confirmed this conclusion. From the comparison of the results with an S–N curve from the design code FKM-Guideline Analytical strength assessment of components, recommendations for design and assessment of components have been derived.     

Effect of the cooling rate on the dendrite arm spacing and the ultimate tensile strength of cast iron

This paper describes a series of microstructural and strength studies performed on hypereutectic cast iron which was sand cast using a variety of end chills (metallic, non-metallic, water cooled and liquid nitrogen cooled, respectively). The effects of cooling rate on the dendrite arm spacing (DAS) and the ultimate tensile strength (UTS) were evaluated. Attempts were also made in the paper to explain these effects and to correlate the UTS with the DAS.     

Fracture analysis of a camshaft made from nodular cast iron

The camshaft is a shaft having some semi-oval protrusions which are designed to control the open and close intervals of the inlet and exhaust poppet valves in the gasoline and diesel engines. Rotation of the cam which takes its movement from the crankshaft via a chain or a trigger belt causes its profile to slide against the smooth flat closed end of a cylindrical member known as a follower. In this study, fracture analysis of a camshaft of an automobile engine is carried out. The analyzed camshaft is fractured after a very short period of usage of the car. For the determination of the failure reason, the microstructure and chemical compositions of the camshaft material are determined. Some fractographic studies are carried out to asses the fracture conditions. A stress analysis is also carried out by the finite element technique for the determination of highly stressed regions on the camshaft.     

Fatigue life assessment of nodular cast iron containing casting defects

The fatigue behaviour of a nodular cast iron containing casting defects has been investigated in the high-cycle fatigue regime. In this paper, we propose a fatigue life assessment model for flawed materials based on a fracture mechanics approach which takes into account the position and size of the defect, short crack behaviour and the notch effect introduced by the defect. The fatigue behaviour of smooth samples, and long and short crack behaviour have been experimentally determined in order to identify the relevant mechanical parameters; these being introduced into the model. An experimental study has been made both in air and in vacuum in order to account for the position of the defect, noting that internal defects are supposed to be under vacuum conditions. Experimental results, which are based on a two-crack front-marking technique specially developed for this study, show that the propagation of natural cracks is controlled by the effective stress intensity factor in air as well as in vacuum. The K calculation for a short crack in the stress field of a notch is analysed using numerical elastic–plastic results. Comparison between experimental results and the computation of fatigue life for fatigue lives less than 10⁶ cycles shows that the fatigue behaviour of nodular cast iron is controlled by a propagation process. The model proposed is thus relevant for fatigue lives less than 10⁶ cycles so that the defect can be considered as a crack and the initiation stage neglected. Closer to the fatigue limit, this study shows that the initiation stage should be considered in the assessment of fatigue life of nodular cast iron, because a single macroscopic propagation assessment is not enough to describe the whole fatigue life. The defect cannot be considered as a pre-existent crack in the high-cycle fatigue range (>10⁶ cycles), and the initiation stage that contains microcrack propagation around the defect should be evaluated when assessing the high-cycle fatigue life of nodular cast iron.     

Comparison of fatigue properties of nodular cast iron production and Y-block castings

Monotonic, cyclic, low and high cycle axial fatigue and region I and II fatigue crack growth behaviour were obtained and compared for a nodular iron Y-block casting and a heavy, off-highway, ground vehicle production casting. Despite the greater size and incidence of micro-discontinuities in the production iron, the two castings had essentially the same monotonic, cyclic and fatigue properties. SEM fractography indicated the same fatigue mechanisms for both casting types. Crack closure did not completely account for mean stress effects found in both cast irons.     

Role of composition heterogeneity on fracture micromechanisms of nodular cast iron

Abstract

The aim of the present study is to explain the role of chemical composition and heterogeneity within the microstructure of nodular cast iron on the micromechanisms of fracture. The heterogeneity in chemical composition is revealed by a thorough study of the microstructure using scanning electron microscopy, Auger electron spectroscopy and colour etching. The ferritic phase surrounding the graphite globules is enriched by silicon and depleted of manganese in the range of micrometres. Maximum manganese concentration is found in regions distant from these particles and in pearlitic regions. Simultaneously, ferritic grain boundaries in the vicinity of the graphite particles are enriched in phosphorus in the nanometer range. This interfacial segregation, reaching nearly 15 at.-%P, is most probably responsible for an embrittlement of these regions and the appearance of intercrystalline decohesion. An elastic-plastic finite element analysis of the ferritic/pearlitic microstructure demonstrates that a heterogeneous microstructure is locally subjected to heterogeneous stresses and strains, which are also expected to affect fracture mechanisms.     

Abrasive wear property of bainitic nodular cast iron in laser processing

The effects of the different austenitizing and isothermal transformation temperature and time on the wear resistance in bainitic ductile iron has been investigated and compared with surface hardening by laser processing, to find the best wear resistance for the bainitic ductile iron. It was found that the wear resistance of the lower bainitic ductile iron was better than upper bainitic ductile iron, and that the factor affecting hardness and wear properties the most was the isothermal transformation temperature. After surface hardening by laser processing, the hardening reached RC 55 or more, no evidence of any difference between the lower and the upper bainitic ductile iron being found.     

Strain-rate dependence of the tensile strength of glass fibers

It is well known that the strength of glass fibers increases with increasing strain rate. Consequently, impact strength of glass fiber is competitive with that of carbon fiber. This strengthening phenomenon is well recognized for bulk glass. Strain-rate dependence of the strength for bulk glass was described by considering slow crack growth in glass. The analytical model that considered the slow crack growth of glass is proposed to predict the strength of glass fibers. The proposed model considered the stress corrosion limit and a constant crack velocity region. Calculations showed almost same results with the previous model, however, some differences were confirmed. To discuss the validity of the analysis, tensile tests of E-glass fiber bundles were conducted at various strain rates. It was observed that the fracture behaviors differ with the strain rates. Experimental results showed that the strength of E-glass fibers increased with increasing strain rate. Furthermore, we confirmed that the analytical results were in good agreement with the experimental results. The strain-rate dependence of the strength of glass fibers was successfully predicted by considering the slow crack growth in glass.     

Factors influencing the tensile strength, hardness, and ductility of hydrogen-cycled palladium

A systematic study of the tensile strength, hardness, and ductility of palladium has been performed on foil specimens that have undergone hydrogen absorption and desorption under a variety of conditions. The experimental parameters under control in the various hydrogen treatments were: (i) the amount of hydrogen absorbed, (ii) the isotope of hydrogen (protium versus deuterium) absorbed, (iii) the number of hydrogen absorption/desorption cycles, and (iv) the hydrogen absorption/desorption temperature. In all instances hydrogen absorption/desorption cycling significantly alters the tensile strength, hardness, and ductility of well-annealed palladium. The results show that, in general, the strength and hardness of palladium increases as a result of hydrogen cycling while the ductility decreases. The extent of the respective increases and decreases has been found to differ depending on the parameter being varied. The most sensitive parameter was found to be the amount of hydrogen absorbed during cycling.     

玻璃模具用球墨/蠕墨复合铸铁的研究

介绍了用稀土和Ｋ－Ｎａ复合变质处理而形成的球墨／蠕墨复合铸铁玻璃模具材质的组织,力学性能及热特性,探讨了影响这些性能的因素,试用表明,这种材和于玻璃具有着良好的使用性能和较高的经济价值。