Fractography

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

Fracture resistance of polyblends and polyblend matrix composites: Part II Role of the rubber phase in Nylon 6,6/ABS alloys

Abstract: A comprehensive study was undertaken on the specific role of rubber on toughening when other rigid polymer or non-polymer phases were present. Nylon 6,6/SAN blends of various SAN concentrations ranging from pure SAN to pure nylon 6,6 were investigated with and without fibre reinforcements. These results could be compared with the toughness values of unreinforced and fibre-reinforced nylon 6,6/ABS alloys from a previous study in order to elucidate the role of rubber. Fracture behaviour was investigated rigorously by characterizing the fracture initiation toughness, JIC, and the steady-state fracture toughness, Jss. These were then related to the microstructure and failure modes determined by microscopy and fractography methods. It was found that rubber increased both fracture initiation and propagation toughness in the presence of the rigid phase, while the rigid phase toughened the alloy only when the rigid phase/matrix interface was strong enough. The role played by glass fibres was found to be critically related to the fibre/matrix interfacial strength. Toughening was generally observed, both in the presence and absence of rubber, when the interface was strong. In all cases toughening could be related to the enhancement of plasticity in the crack tip by the presence of the rubber phase or the reinforcing glass phase.

Surface Integrity and Fatigue Behavior for High-Speed Milling Ti–10V–2Fe–3Al Titanium Alloy

Abstract: Influence of cutting parameters on surface integrity when milling Ti–10V–2Fe–3Al is investigated based on high-speed cutting experiments. Surface integrity measurements, fatigue fractography analysis, and fatigue life tests are conducted to reveal the effect of surface integrity on crack initiation and fatigue life. The results show that under given experiment conditions, surface roughness decreases linearly when increasing cutting speed or decreasing feed per tooth. The latter has a greater impact on surface roughness than the former. Compressive stress can be detected on all machined surfaces. With the increase of feed per tooth or cutting speed, respectively, residual stress presents a linear increase. Cutting parameters have no significant impact on micro-hardness. When the surface roughness ranges from 0.5 to 1.0 μm, the effect of surface residual stress on fatigue life is more than that of surface roughness. When the surface residual compressive stress increases, the fatigue life improves significantly. Compared with 60 m/min, when cutting speed is 100 or 140 m/min, the surface roughness decreases, the surface residual compressive stress increases, and the fatigue life improves by 124 and 59%, respectively. Under a tensile load, fatigue crack on machined surface of Ti–10V–2Fe–3Al titanium alloy originates at the cross-edge of the specimen surface. With the increase of surface roughness, the area ratio of fatigue crack propagation zone, and fatigue fracture zone decreases.

The effects of microstructure evolution on the fracture toughness of BT-25 titanium alloy during isothermal forging and subsequent heat treatment

Abstract: Isothermal compression with four different height reductions (0, 30%, 50% and 80%) and subsequent heat treatment was applied to BT25 alloy. Microstructure evolution following deformation and heat treatment was studied, meanwhile, the effect of various microstructure on fracture toughness of BT-25 alloy was analyzed in this paper. The microstructure observation and chemical analysis shows that volume fraction of globularized α phase increases with the increasing deformation, and the amount of Mo in α phase decreases with globularization. The fracture toughness exhibits a decreasing trend as the increasing volume fraction of globularized α phases. It is noteworthy that the fracture toughness of basket-weave structure has a significant decrease even though a small amount of α phases are globularized. The mechanism of this phenomenon could be explained by the different crack propagation mode between the microstructure with weaving tightly lamellar α phases and that with partially or fully globularized α phases. Fractography shows that fracture surface of undeformed material is characterized by deep secondary cracks and big fracture steps, and the surface becomes flatter with increasing volume fraction of globularized α phases. In addition, a model that can predict KIC using tensile properties is utilized. It can provide a relatively reliable prediction for fracture toughness of BT-25 alloy with globularized α phase. The model has a little error for prediction of fracture toughness with basket-weave structure due to underestimation of the secondary cracks.

Mechanical Property and Fracture Behavior of Squeeze-Cast Mg Matrix Composites

Abstract: The present study aims to investigate the microstructure and fracture properties of AZ91 Mg matrix composites fabricated by the squeeze-casting technique, with variations in the reinforcement material and applied pressure. Microstructural and fractographic observations, along with in situ fracture tests, were conducted on three different Mg matrix composites to identify the microfracture process. Two of them are reinforced with two different short fibers and the other is a whisker-reinforced composite. From the in situ fracture observation of Kaowool-reinforced composites, the effect of the applied pressure on mechanical properties is explained using a competing mechanism: the detrimental effects of fiber breakage act to impair the beneficial effects of the grain refinement and improved densification as the applied pressure increases. On the other hand, for the composites reinforced with Saffil short fibers, microcracks were initiated mainly at the fiber/matrix interfaces at considerably higher stress intensity factor levels, while the degradation of fibers was not observed even in the case of the highest applied pressure. This finding indicates that the higher applied pressure yields better mechanical properties, attributable to the Saffil short fibers having relatively high resistance to cracking. Although an improved microstructure was obtained by accommodating the appropriate applied pressure in the short fiber-reinforced composites, their mechanical properties were far below those of conventional A1 matrix composites. In this regard, the Alborex aluminum borate whisker is suggested as a replacement for the short fibers used in the present investigation, to achieve better mechanical properties and fracture toughness.