Key Engineering Materials Vols. 345-346

Abstract: Gray cast iron shows large asymmetrical features by the graphite flake when tensile and compressive stresses are applied. The plastic strain rage which is used in low-cycle fatigue life prediction by many researchers is hardly defined and gives very different values by the Standards in this case. From the results of this study, it is not reliable to use the plastic strain range as a low-cycle fatigue damage parameter. Therefore, the plastic strain energy density which is uniquely defined was suggested as a damage parameter and it showed good correlation in low-cycle fatigue in gray cast iron.

Abstract: Powder-metallurgical (P/M) produced components may be used as mass parts in a very large quantity. Due to the multistage manufacturing process which consists of the pressing of the prepared powder and the sintering of the green bodies also complex shaped components can be produced very economically and precisely. They can be utilised without any further post-processing if the whole production process is optimised. However, it is still difficult and only possible with considerable technical and financial effort to produce highly stressable components profitably using the sinter technique. Therefore, the examinations on hand had the intention to create the basis for the use of the sinter technology also for the production of highly stressable case-hardened components. To this, at first bending specimens were fabricated with modern pressing procedures and sinter facilities and first characterized in the as sintered state. Then the conditions for the case-hardening was analysed and the parameters for an optimised case-hardening procedure fixed. With these parameters specimens were case-hardened and their lifetime behaviour estimated under different bending loading conditions. Finally it should be checked whether the knowledge gained from the specimens could be applied to complex components. To this, gear wheels were produced using powder-metallurgy. The cyclical tooth foot strength of this gear wheels were analysed in the only sintered as well as in the case-hardened state. It could be demonstrated that the improvement of the fatigue strength of the bending specimens by case-hardening also appears at the tooth foot strength of the gear wheels.

Abstract: In this paper the fatigue crack growth properties of a nitrided shot-peened steel is dealt with: different peening intensities were considered and the resulting residual stresses measured by means of an X-ray diffractometer. Rotating bending fatigue tests were executed on specimens with a blind micro-hole, acting as a pre-existent crack. The results allowed determining the threshold of propagation of the nitrided and of nitrided and shot peened material. It was also possible to relate the crack initiation point with the applied and the residual stresses.

Abstract: Recent observations relevant to the early stages of the fatigue damage of crystalline materials are reviewed. Experimental evidence on the localization of the cyclic plastic strain and on the surface relief formation in cyclic loading of 316L austenitic stainless steel is presented. The focused ion beam is used for exposing three-dimensional evidence of persistent slip markings (PSMs). PSMs consist of extrusions and parallel or alternating intrusions which develop during cyclic loading. Monte Carlo simulations of vacancy generation within persistent slip band (PSB) and their migration to the matrix where they annihilate on the edge dislocations are used to simulate the growth of extrusions and intrusions. The results of the simulations are compared with experimental data and discussed in terms vacancy models of fatigue crack initiation.

Abstract: Cyclic strain control tests have been performed on cylindrical specimens of cast polycrystalline Inconel 792-5A superalloy at 23, 500, 700 and 800 °C in laboratory atmosphere to study the effect of temperature on the fatigue behavior. Cyclic hardening-softening curves and fatigue life curves were measured. Scanning electron microscopy was used to investigate the surface relief. Low amplitude straining was characterized by saturation of the stress amplitude. In room temperature high amplitude straining cyclic hardening was followed by marked saturation. Pronounced continuous hardening until failure was observed at 500 °C. Initial cyclic hardening was followed by softening at 800 °C. A systematic shift of the Manson-Coffin and of the Basquin curves to lower fatigue lives was found when temperature was increased. Fatigue cracks were observed to initiate from surface slip markings at all temperatures.

Abstract: The technical development of construction equipment such as wheel loader leads to an increase of not only amount of workload per hour, but also magnitude of allowable stress range. Since welded structures of the wheel loader occupy approximately 40-60% of its total weight and are subjected to incessant fatigue loads, fatal fatigue failures occur occasionally around the welded zone during its practical operation in the field. To reasonably evaluate the fatigue life, the effect of these geometries and welding residual stress on the fatigue life should be taken into account. In this paper, the modified notch strain approach developed previously by the authors is applied practically to assess the fatigue life of welded joints in boom and front frame structures of the wheel loader.

Abstract: The surface layer of binary Al-Zn alloy specimen, ever after a long time aging, remains softer than the interior when the specimens are age-hardened at around room temperature after quenching from high temperature. In this study, effects of the soft surface layer on the fatigue strength of Al-Zn alloys were studied under repeated tensile loading. Vickers microharhness test revealed that there existed less hardened region in the vicinity of grain boundary and specimens surface, and that the region extends 50 to 100μm from the surface inward. From the plot of the stress amplitude against the number of cycles to failure, it is concluded that the presence of less hardened surface layer increases fatigue resistance of age-hardened Al-Zn alloys containing 8 to 16 mass%Zn under the repeated tensile loading.

Abstract: There are different documents containing fatigue crack propagation limit or design curves and rules for the prediction of crack growth. The research work aimed to characterise the fatigue crack propagation resistance of different materials using limit curves and determination of limit curves under different loading conditions, based on statistical analysis of test results and the Paris-Erdogan law. With the help of the characteristic values of threshold stress intensity factor range (
Kth), two constants of Paris-Erdogan law (C and n), fatigue fracture toughness (
Kfc) or fracture toughness (KIc) two reliable method can be proposed. The limit curves calculated by both methods represent a compromise of rational risk (not the most disadvantageous case is considered) and striving for safety (uncertainty is known).

Abstract: Recently a European community funded thematic network project (participation of 17 countries) FITNET (www.eurofitnet.org) has completed a new and unified engineering assessment procedure (FITNET FFS Procedure) of flaws in metallic structures and welds. This newly developed procedure (under CEN Workshop Agreement WA22) provides assessment rules for flaws or damage due to fracture, fatigue, creep and corrosion to demonstrate the structural integrity of the component. This paper gives an overview of the FITNET Fitness-for-Service (FFS) Procedure and specifically presents the features and basic equations of the Fracture Module. It also presents two brief examples for the validation of the procedure using laser welded specimens.

Abstract: If multiple discrete flaws are detected, alignment and combination rules are used to determine whether the flaws should be treated as non-aligned or as coplanar, and independent or combined flaws. Alignment and combination rules for multiple flaws are provided in Fitness-for-Service (FFS) procedures of many codes and standards in the world. However, these alignment and combination rules defined in the FFS are different. This paper aims to give overview of alignment and combination rules for multiple flaws in FFS procedures. In addition, experimental data on flat plates with parallel planar flaws are compared with the alignment rules of these FFS procedures.