M.R. Bayoumi

Effect of surface finish on fatigue strength

Abstract The components surfaces used in lubrication, friction and wear vary in their initial surface topography according to the particular use involved. In practice a real component from a machine or structure will generally have various surface finishes and different fatigue strengths. The usual stand by which various surface conditions are judged is the polished laboratory specimen. Irregular and rough surfaces generally exhibit interior fatigue properties. The present study represents a neat investigation which is carried out to correlate the various surface finish parameters with the endurance fatigue strength of a commercial aluminium alloy. The flow strain in both horizontal and vertical directions is monitored using two attached transducers during the testing of specimens grouped according to their surface finish features. Accordingly, the fatigue initiation life as interpreted from the generated flow curves is correlated with the surface finish parameters. Results indicate a great effect for all surface finish parameters. It leads to a significant correlation between the fatigue initiation life, final separation life and the fatigue endurance limit. Implementing the regression analysis leads to exponential correlations between the number of cycles for initiation of microcracks and applied cyclic stress, also between total life and alternating stress. Moreover, polynomial equations have been developed to relate the endurance stress to the various surface finish parameters.

The mechanics and mechanisms of fracture in stress corrosion cracking of aluminium alloys

Abstract A fracture mechanics approach to stress corrosion cracking is highlighted. The mechanisms of stress corrosion cracking is presented. Experiments on 2024 and 7075 aluminium alloys are carried out to determine their mechanical properties, microstructure and plane strain fracture toughness ( K IC ). Stress corrosion cracking tests, namely, cantilever beam tests as well as wedge opening loading tests using sea water as a corrosive medium, are conducted to establish the critical stress intensity factor for stress corrosion cracking ( K ISCC ) for each aluminium alloy. It is found that the K ISCC is in the range of ( 1 5 ) to ( 1 6 ) of the plane strain fracture toughness, K IC , depending on the alloy. The scanning electron microscopy of fracture surfaces reveals a great dependence of the cracking and/or pit severity on the applied stress intensity factor. A brief discussion on the dislocations role in stress corrosion cracking is given.

The observation of dislocation structures during the fracture of prestrained AISI 4340 steel

Abstract The role that dislocation structures in a low alloy steel which had been subjected to various amounts of prestraining play in the fracture toughness of this steel has been investigated previously. In the study reported in this paper the presence and development of dislocation structures in tensile specimens of the same steel which were loaded to fracture are examined. The material used is an AISI 4340 steel which was predeformed, before tensile pulling, to values ranging from 0% to 15% prestrain by cold rolling. It was found that small amounts of prestraining introduce mobile dislocations into the material. These mobile dislocations contribute to the initial process of cell formation. They also have an effect on the extent of cell size reduction which occurs until final fracture takes place.

Characterization of the effect of alloying elements on the fracture toughness of high strength, low alloy steels

Abstract The effect of small alloy additions on the ductile fracture toughness of ferritic high strength, low alloy steels was characterized by determining the fracture toughness parameter J Ic and by microstructural analysis on six compositions of these steels. Small additions of niobium (0.10 wt.%) or titanium (0.08 wt.%) resulted in material with significantly improved J Ic values. Additions of vanadium (0.085 wt.%), however, resulted in little improvement in J Ic values compared with the base steel and the other two alloying elements. The improvements in fracture toughness due to the additions of niobium and titanium are explained in terms of the dependence of ductile fracture toughness on the effects of grain size, inclusion diameter and inclusion volume fraction.

Fracture topography of HSLA steels

Abstract The topography of the fracture surfaces of several high strength, low alloy (HSLA) steels was determined using scanning electron microscopy. The steels contained various additions of titanium, vanadium and niobium. Two types of specimens were investigated. They were either fractured tensile specimens which had undergone uniform strain followed by necking to fracture or fractured three-point bending specimens broken during determination of the J-integral fracture parameter. The distribution of the dimple size from both types of specimens was determined. It was found that the average dimple size for the tensile specimens is about 1.5 μm for all steels. For the three-point bending specimens, however, the average dimple size is about 15 μm. Also, the relative depth of the dimples is higher for the tensile specimens than it is for the three-point bending specimens. These results are interpreted in terms of the plastic strain in the specimens where, in the tensile specimens, the large uniform strain contributes to the formation of elongated dimples and, in the three-point bending specimens, the state of triaxity at the crack tip contributes to large shallow dimple formation.

Fatigue behaviour of a commercial aluminium alloy in sea water at different temperatures

Abstract Fatigue tests of a commercial aluminium alloy are conducted in sea water at different temperatures using a specially designed experimental environmental chamber installed on a standard rotating bending fatigue testing machine. The tests are carried out in air at 20°C to establish the σ - N curve for this alloy as a reference curve, while tests in sea water are at 20, 30, 50 and 80°C to investigate the role of temperature on the fatigue behaviour. Changing the medium from air to sea water during fatigue results in a significant decrease in the fatigue endurance limit. Increasing the temperature from 20 to 80°C reduces the endurance limit from 0.58 to 0.3 σ γ . In a generalized empirical estimation formula, the present study suggests an incorporation of an environmental factor ( C E ) which depends on both the working medium and the temperature. Fracture surface examination on the scanning electron microscope indicates a great dependence of the pitting and/or intergranular corrosion fatigue cracks on the testing temperature and the applied cyclic stress levels.

Production of Aluminum-Silicon Carbide Composites Using Powder Metallurgy at Sintering Temperatures Above the Aluminum Melting Point

The extensive utilization of aluminum reinforced with silicon carbide in different structural applications has motivated the need to find a cost effective technological production method for these composites. Homogeneity, machinability, and interfacial reaction of the constituents represent the significant problems pertaining to these composites. Production of a homogenous, high strength, and net-shape structural components made from aluminum-silicon carbide composites can be achieved using powder metallurgy (PM) technology. In the present work the problem of low strength of the aluminum-silicon carbide produced by powder metallurgy is solved by raising the sintering temperature of the composite above the melting temperature of the aluminum. This method produces a local fusing and welding of the aluminum particles. Using aluminum powder with a thick oxide layer surrounding the particles prevents the total melting of the composite. Green compacted specimens containing 0, 5, 10, 15, 20, 25, and 30 wt % silicon carbide were prepared. Samples from each composition were sintered at 650, 700, 750, 800, 850, and 900°C separately, while other specimens were left without sintering for comparison. Microstructure examination, a microhardness test, and a compression test were carried out for each of the 49 combinations of SiC contents and sintering temperatures to study the effect of sintering temperature and SiC contents on the composite properties and to detect the optimum sintering temperature for each SiC weight percent. Generally the results show the tendency for both strength and ductility to increase upon increase in the sintering temperature. These specific sintering temperature levels are found to be 650°C for the aluminum with no silicon carbide content, 700°C for composites containing both 5 and 10 wt % SiC, 750°C for composites containing 15 wt % SiC, 800°C for composites containing 20 wt % SiC, 850°C for composites containing 25 wt % SiC, and 900°C for composites containing 30 wt % SiC.

Low-cycle fatigue behaviour of notched AISI 304 stainless steel specimens

Low-cyclic fatigue tests were conducted on semi-circle notched and V-notched specimens made of AISI 304 stainless steel. Extensive scanning electron microscopic examination of the fracture surface was also carried out to correlate the microscopic fracture surface features with the macroscopic fatigue loading parameter for this steel. The elastic-plastic fatigue test results indicated a noticeable cyclic hardening phenomenon and also a great influence of the maximum cyclic stress, the mean stress and the notch geometry on both the fatigue life and the fatigue behaviour process. Using careful sensitivity and regression analysis correlations between the macroscopic fatigue parameters on the one hand and the macroscopic and the microscopic fracture surface features on the other, these correlations are presented and clearly documented and discussed for the two notch geometries investigated.

Mechanisms of plastic deformation prior to ductile fracture in a low alloy steel

Abstract The fracture toughness of metallic materials is related to their ductility. Thus the fracture process is characterized by the mechanisms of plastic deformation which take place prior to fracture. In this study, such a characterization was obtained on tensile specimens of a low alloy steel containing niobium. The dislocation cell structures formed in the necked sections of tensile specimens were examined as a function of local strain, expressed as reduction in area. It was found that these cell structures decrease in size as the strain is increased up to a certain diameter where the diameter stays constant with further increase in strain. This behavior is related to observations on the variation in the fracture criterion J Ic with grain size to provide qualitative understanding of the dependence of fracture toughness on microstructural and dislocation arrangements in metallic materials.

Effect of loading rate on fracture toughness of bonded joints

Abstract One of the main factors affecting the strength of bonded joints is the loading rate. The objective of this study is to investigate the role of loading rate on fracture toughness of bonded joints. Cleavage strength tests were carried out at seven different loading rates using an epoxy resin adhesive and two adherend materials, aluminium and brass. The results indicate that the strain rate has a significant effect on the fracture strength.