Fractography

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

On the relation of microstructure and impact toughness characteristics of DSAW steel of grade API X70

Abstract: In this research, mechanical characteristics of double submerged arc-welded line pipe steel of grade API X70 (70ksi yield strength) were investigated. Different experimental examinations including chemical analysis, microstructural investigation, microhardness, tensile and Charpy tests and standard fractography were carried out on different zones of test material. Impact properties of the fusion zone, the heat-affected zone (HAZ) and the base metal were measured on an instrumented Charpy rig and correlated then to their microstructural features. The experimental results showed that the base metal was the toughest, and fusion zone was the least tough region with average Charpy energy of 222 and 128 J, respectively. The latter fulfilled service requirements set by API 5L, despite its relatively low impact energy. Interestingly, the cast microstructure and presence of grain boundary phases such as proeutectoid ferrite in fusion zone confirmed its low energy characteristics. Reduction in HAZ impact energy (compared to base metal) was observed too, together with grain coarsening and the associated HAZ softening adjacent to the weld.

Observations on Mode I ductile tearing in sheet metals

Abstract: Cracked ductile sheet metals, subject to Mode I tearing, have been observed to display a variety of fracture surface morphologies depending on the material properties, and a range of studies on the fracture surface appearance have been published in the literature. Whereas classical fractures such as cup-cone, cup–cup and slanting are widely observed, the phenomenon of a slanted crack which systematically “flips” back and forth in a roughly 45° orientation has only recently been reported. The present study aims to add details and understanding to this crack growth phenomenon – through experimental testing and comparison with published results. A series of crack propagation tests have been carried out, where cracks are driven many plate thicknesses under global Mode I loading. The current study employs both the edge crack specimen (ECS) loaded in combined in-plane bending and extension, and the double edge notched tension (DENT) specimen loaded in extension. Clear evidence of the alternating slant failure is observed for a normal strength steel and details on the phenomenon are brought out by grinding and polishing cross-sections in successive steps along the crack growth direction. Moreover, these results are compared to a soft aluminum (1050A) which predominantly shows cup–cup or slant failure. In general, both materials fail by the mechanism of void growth to coalescence (fractographs are included), and evidence points toward cup–cup being favored by materials that experience extensive amounts of void growth (thus severe thinning in the process zone), whereas slanting is typical in high strength materials that rapidly nucleate large populations of smaller voids.

Microstructure controlled bending response in AA6016 Al alloys

Abstract: A contemporary approach in the car weight reduction is the use of low weight and high strength Al alloys sheets for hang-on body panels production. The final step in the forming route of such panels is the attachment of the outer skin to the inner part of the panel by applying a hemming operation. This joining method is cheap, easy to perform and environment-friendly, but requires severe 180° bending of the edges of the outer skin which quite often results in cracking or complete tearing of the bend surface. Such kind of failure restricts the further application of the hemmed products. The microstructures after solution heat treatment and pre-aging (T4P temper state) of two grades age-hardening AA6016-type aluminium alloy sheets were studied in this work by means of optical microscopy, scanning electron microscopy and electron backscatter diffraction. The obtained results were related to the hemming response of the grades. It was found that the alloy composition is one of the main parameters controlling the bendability of these grades through the amount of the formed strengthening phases. However, the applied thermal treatment remains the key factor responsible for the favorable distribution of these phases into the microstructure. The grain size and the volume fraction of the constituent particles were found to play secondary role in forming the material bending properties and can be only used for their fine tuning. The presence of Mg 2 Si (β-phase) and/or Al 1.9 CuMg 4.1 Si 3.3 (Q-phase) particles in the grain boundaries structure was recognized as a critical microstructural feature causing severe reduction in the bending ability of the sheets by promoting an intergranular fracture. The possibility of grain boundaries failure exponentially raises with the time passed due to the natural aging.

Mechanical properties of aluminium-copper solid-phase welds

Abstract: The mechanical properties of aluminium-copper solid-phase welds have been examined in the as-bonded condition, and after heat treatment to produce intermetallic layers of up to 50 μm in thickness at the weld interface. The layers were grown both by continuous annealing and by thermal cycling. The presence of the intermetallic layers led to a reduction in the tensile, shear, and impact strengths of the joint. The impact strength was observed to be negligible for layer thicknesses greater than 2 μm. Thermal cycling had a more detrimental effect than an equivalent heat treatment by continuous annealing, and this was attributed to the build-up of thermal strains. Fractography showed that the fracture propagated between the brittle CuAl2 and CuAl phases, and it was concluded that the existence of these brittle phases was principally responsible for the reduction in properties. Contrary to an earlier hypothesis, it was shown that porosity was not responsible for the poor joint properties after heat trea...