Fractography

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

On the mechanism of fatigue failure in the superlong life regime (N>107 cycles). Part 1: influence of hydrogen trapped by inclusions

Abstract: The fracture surfaces of specimens of a heat-treated hard steel, namely Cr-Mo steel SCM435, which failed in the regime of N= 10 5 to 5 x 10 8 cycles, were investigated by optical microscopy and scanning electron microscopy (SEM). Specimens having a longer fatigue life had a particular morphology beside the inclusion at the fracture origin. The particular morphology looked optically dark when observed by an optical microscope and it was named the optically dark area (ODA). The ODA looks a rough area when observed by SEM and atomic force microscope (AFM). The relative size of the ODA to the size of the inclusion at the fracture origin increases with increase in fatigue life. Thus, the ODA is considered to have a crucial role in the mechanism of superlong fatigue failure. It has been assumed that the ODA is made by the cyclic fatigue stress and the synergetic effect of the hydrogen which is trapped by the inclusion at the fracture origin. To verify this hypothesis, in addition to conventionally heat-treated specimens (specimen QT, i.e. quenched and tempered), specimens annealed at 300 °C in a vacuum (specimen VA) and the specimens quenched in a vacuum (specimen VQ) were prepared to remove the hydrogen trapped by inclusions. The specimens VA and VQ, had a much smaller ODA than the specimen QT. Some other evidence of the influence of hydrogen on superlong fatigue failure are also presented. Thus, it is concluded that the hydrogen trapped by inclusions is a crucial factor which causes the superlong fatigue failure of high strength steels.

Effect of standard heat treatment on the microstructure and mechanical properties of hot isostatically pressed superalloy inconel 718

Abstract: Ni–Fe base superalloy, Inconel 718, was processed through powder metallurgy (P/M) hot isostatic pressing (HIP) route. In order to balance the strength and ductility, the HIPed material was given the standard heat treatment, viz. solution treatment at 980 °C for 1 h/water quenched (WQ) to room temperature and a two-step ageing treatment consisting of 720 °C for 8 h/furnace cooling (FC) at 55 °C h−1 to 620 °C and holding at 620 °C for 8 h before air cooling (AC) to room temperature. Optical microscopy and scanning electron microscopy (SEM) studies on the heat treated alloy have shown a homogeneous microstructure with fine grain size (25 μm) along with the presence of prior particle boundary (PPB) networks. Transmission electron microscopy (TEM) on the heat treated material has revealed the presence of oxides, MC carbides and δ-precipitates at the grain boundaries and a uniform precipitation of fine γ″ and γ′ strengthening phases in the matrix. Tensile and stress rupture tests were performed on the heat treated material. While the yield strength (YS) and ultimate tensile strength (UTS) of the HIPed and heat treated alloy at room temperature and 650 °C were comparable to those of conventionally processed wrought IN 718, its ductility was lower. The stress rupture life of the HIPed alloy improved marginally due to heat treatment and met the minimum specification requirement of life hours but the rupture ductility was found to be inferior to that of the wrought material. The fractography of the failed samples has revealed the transgranular ductile mode of fracture in the as-solution treated alloy, while intergranular mode of failure with the decohesion of PPBs occurred more predominantly in the aged condition. This change of fracture mode with ageing treatment shows the ductility dependence on the relative strength of the matrix and PPBs. The TEM studies on the deformed alloy have revealed that the brittle oxides and carbides at the prior particle boundaries coupled with the fine γ″ and γ′-precipitates in the matrix are responsible for low ductility at 650 °C. The investigations of the present study have led to better understanding of the structure–property relationships in HIP+heat treated alloy 718 and suggest that the standard heat treatment recommended for wrought IN 718 is not suitable for HIPed alloy and has to be modified to realise optimum properties.

A comparison of the tensile, fatigue, and fracture behavior of Ti–6Al–4V and 15-5 PH stainless steel parts made by selective laser melting

Abstract: In this work, Ti–6Al–4V and 15-5 PH steel samples were fabricated using selective laser melting (SLM) and their tensile, fatigue, and fracture properties were analyzed and compared. The tensile properties were compared with respect to the build orientation. The horizontally built samples showed relatively better tensile properties as compared with the vertically built samples. Fatigue performance was studied for the vertical build orientation and compared with standard wrought material data. The tensile and fatigue performance of SLM-built materials were comparable to their respective standard wrought materials. Fractography was carried out for all tensile and fatigue-tested samples. The fatigue fracture behavior of Ti–6Al–4V was different from 15-5 PH steel. For Ti–6Al–4V, the fatigue crack initiation occurred deep in the subsurface whereas for PH steel the fatigue crack was initiated from the surface.

Strength and ductile-phase toughening in the two-phase Nb/Nb 5 Si 3 alloys

Abstract: The effect of heat treatment on the mechanical properties of Nb-Nb5-Si3 two-phase alloys having compositions Nb-10 and 16 pct Si (compositions quoted in atomic percent) has been investigated. This includes an evaluation of the strength, ductility, and toughness of as-cast and hot-extruded product forms. The two phases are thermochemically stable up to ∼1670 °C, exhibit little coarsening up to 1500 °C, and are amenable to microstructural variations, which include changes in morphology and size. The measured mechanical properties and fractographic analysis indicate that in the extruded condition, the terminal Nb phase can provide significant toughening of the intermetallic Nb5Si3 matrix by plastic-stretching, interface-debonding, and crack-bridging mechanisms. It has been further shown that in these alloys, a high level of strength is retained up to 1400 °C.

Influence of Microstructure on Fracture Propagation in Rock

Abstract: Influence of Microstructure on Fracture Propagation in Rock This paper describes the results of research to correlate the fracture resistance with microstructural features of Salem limestone and Berea sandstone. Tests were conducted on wedge-loaded double-cantilever-beam specimens containing machined slots as crack starters. The fracture resistance of Salem limestone measured in terms ofR, the energy dissipated per unit area of projected surface, was found to increase in the initial stage of crack extension but finally reached a constant value which was strongly dependent on crack orientation with respect to the bedding plane. For this limestoneR ranges from about 50 joules/m2 to 230 joules/m2 (1 joule/m2 = 104 erg/cm2). The sandstone which is a more compliant rock exhibited similar fracture behavior while theR values ranged from 465 joules/m2 to 1580 joules/m2. In addition, tests in liquid nitrogen which were aimed at eliminating plastic deformation in the rocks during fracture showed little difference inR for the limestone but a substantial reduction inR for the sandstone which, in the latter case, may be caused by differential thermal expansion between the quartz grains and the calcite cement. Acoustic emissions were detected in both rocks at very early stages in the tests indicating the occurrence of microcracking near the initial slot tip at low loads. The mode of fracture and the fracture path in both materials were identified by fractography over a broad range of magnifications. The evidence gained from this work strongly points to the existence of an extensive array of microcracks produced in a region surrounding the main crack tip and which advances with it. The energy dissipated in fracturing of rock is associated with the creation of the large amount of surface area contained in this microcrack array. This picture provides a self-consistent explanation for the puzzling discrepancy between the measured tensile strength of rocks and the strengths predicted from measuredR values.