Fractography

About: Fractography is a research topic. Over the lifetime, 5043 publications have been published within this topic receiving 86068 citations.

Investigation of cold rolling influence on the mechanical properties of explosive-welded Al/Cu bimetal

Abstract: In this study, effect of cold rolling process on the mechanical properties and bond strength of Al/Cu/Al bimetal has been investigated. The bimetal raw material has been fabricated by the explosive welding process. Then, cold rolling process was used to reduce the thickness of explosive-welded plates. The mechanical properties of the raw materials and cold-rolled samples were experimentally measured using the tensile, tensile-shearing and hardness tests along the thicknesses of the samples. Moreover, the fractography of the surfaces after the tensile tests were examined by the electron microscope (SEM). The obtained results show that, with the increase of thickness reduction, the ultimate strength and hardness have been increased significantly, while the elongation percentage has been diminished. Also, the bond strength confirms the relevant known standard criterion which is higher than the strength of the aluminum layers. Examination of the fracture surfaces reveal that, due to the brittle nature of the intermetallic compounds at the joining interface, the nucleation and propagation of micro cracks have been accelerated under tension and plastic deformation.

Fractography of fatigue crack propagation in 2024-T3 and 7075-16 aluminum alloys in air and vacuum

Abstract: Electron fractography and transmission electron microscopy were used to study fatigue crack propagation processes in 2024-T3 and 7075-T6 aluminum alloys in vacuum and air. There was evidence that crack growth occurs cycle-by-cycle in vacuum, but only for fa tigue in air was it possible to relate dislocation substructure band spacings, immediately below the fracture surface, to cyclic crack growth. A discussion of crack propagation mechanisms suggested that the Tomkins and Biggs model of striation formation comes closest to explaining the fractographic features.

Friction Welding of Titanium to 304 Stainless Steel with Electroplated Nickel Interlayer

Abstract: Friction welding is a solid state joining process and it is best suited for joining dissimilar metals. It overcomes the problems associated with the conventional fusion welding processes. The joining of dissimilar metals using fusion welding processes produce brittle intermetallic precipitates at the interface which reduce the mechanical strength. Various aerospace, nuclear, chemical and cryogenic applications demand joints between titanium and stainless steel. Direct joining of these metals results in brittle intermetallics like FeTi and FexTiy, at the weld interface, which is to be avoided in order to achieve improved properties of the joints. Present study involves joining of two industrially important dissimilar metals such as commercially pure titanium and 304 stainless steel by friction welding with electroplated nickel coating as interlayer that can prevent the brittle intermetallic formation. Microstructural details of the interfaces of the friction welded joints were studied by optical microscopy, scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS) technique and X-ray diffraction (XRD). Microhardness survey was carried out across the joints and tensile test was conducted to assess the mechanical properties of the joints. Fractography studies were carried out on the fracture surfaces of the joints to know the region of failure as well as the mode of failure. XRD patterns indicate the presence of intermetallics in the friction welded joints. These two metals were successfully joined by having electroplated nickel as interlayer. The weld interface on titanium side contained Ti-Ni intermetallics layers, in which the hardness of the weld metal showing the higher value than the base metals. Fractography study conducted on the fracture surfaces created due to pull test reveals that the failure is by brittle fracture and occurred at the intermetallics layer. The maximum strength of the joints achieved for 30 μm and 50 μm thick electroplated nickel interlayers are 242 MPa and 308 MPa, respectively.

A fractographic study of hydrogen-assisted cracking and liquid-metal embrittlement in nickel

Abstract: Metallographic and fractographic studies of crack growth in nickel polycrystals and single crystals in a number of environments are described. “Brittle” intercrystalline and transcrystalline cleavage-like fractures were observed for specimens tested in liquid mercury, liquid lithium, liquid sodium, gaseous hydrogen, and for hydrogen-charged specimens tested in air. “Brittle” fractures were associated with considerable slip, and dimples/tear ridges were observed on fracture surfaces, suggesting that crack growth occurred by localized plastic flow. There were remarkable similarities between adsorption-induced liquid-metal embrittlement and hydrogen-assisted cracking which, along with other observations, suggested that adsorbed hydrogen at crack tips was responsible for hydrogen-assisted cracking. It is concluded that adsorbed atoms weaken interatomic bonds at crack tips thereby facilitating the nucleation of dislocations and promoting crack growth by localized plastic flow.