Fractography

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

Fatigue fracture mechanisms and fractography of short-glassfibre-reinforced polyamide 6

Abstract: An adaptation to existing failure models for fatigue fracture of short-fibre-reinforced thermoplastics is presented. This was based on results using some new experimental methods. These results led to the conclusion that cracks in glassfibre-reinforced polyamide 6 (conditioned to equilibrium water content) remain bridged by plastically drawn matrix material and/or fibres until just prior to final fracture. In this article, emphasis will be on the fractographic evidence for the existence of this failure mechanism. Also some other phenomena in glassfibre-reinforced polyamide will be mentioned. Apart from the normal fractographic investigations, specimens were cryogenically fractured after fatigue, revealing the structure of damage, before failure. Both fracture surfaces were compared, showing that only a small fraction of the fibres is broken in fatigue; mostly the fibres are pulled out. The mechanism consists of the following steps: damage begins with void formation, mainly at fibre ends; these voids coalesce into small cracks. These cracks, however, do not grow into one full crack, but the crack walls remain connected at several points. This is contrary to the fracture mechanism for the dry as-moulded material. When the material is dry as moulded, the matrix material cracks, without showing much ductility, and no bridges are formed.

Effect of Y, Sr, and Nd additions on the microstructure and microfracture mechanism of squeeze-cast AZ91-X magnesium alloys

Abstract: This study aims to investigate the effects of Y, Sr, and Nd additions on the microstructure and microfracture mechanism of the four squeeze-cast magnesium alloys based on the commercial AZ91 alloy. Microstructural observation, in situ fracture tests, and fractographic observation were conducted on the alloys to clarify the microfracture process. Microstructural analyses indicated that grain refinement could be achieved by small additions of alloying elements, although the discontinuously precipitated Mg17Al12 phases still existed on grain boundaries. From in situ fracture observation of an AZ91-Sr alloy, it was seen that coarse needle-shaped compound particles and Mg17Al12 phases located on the grain boundary provided easy intergranular fracture sites under low stress intensity factor levels, resulting in the drop in toughness. On the other hand, the AZ91-Y and AZ91-Nd alloys showed improved fracture toughness, since deformation and fracture paths proceeded into grains rather than to grain boundaries, as the planar slip bands and twinnings actively developed inside the grains. These findings suggested, on the basis of the well-developed planar slip bands and twinnings, that the small addition of Y or Nd was very effective in improving fracture toughness.

Mechanical properties and microstructure of biomorphic silicon carbide ceramics fabricated from wood precursors

Abstract: Silicon carbide based, environment friendly, biomorphic ceramics have been fabricated by the pyrolysis and infiltration of natural wood (maple and mahogany) precursors. This technology provides an eco-friendly route to advanced ceramic materials. These biomorphic silicon carbide ceramics have tailorable properties and behave like silicon carbide based materials manufactured by conventional approaches. The elastic moduli and fracture toughness of biomorphic ceramics strongly depend on the properties of starting wood preforms and the degree of molten silicon infiltration. Mechanical properties of silicon carbide ceramics fabricated from maple wood precursors indicate the flexural strengths of 3441+/-58 MPa at room temperature and 230136 MPa at 1350C. Room temperature fracture toughness of the maple based material is 2.6 +/- 0.2 MPa(square root of)m while the mahogany precursor derived ceramics show a fracture toughness of 2.0 +/- 0.2 Mpa(square root of)m. The fracture toughness and the strength increase as the density of final material increases. Fractographic characterization indicates the failure origins to be pores and chipped pockets of silicon.

Failure analysis of ceramic clinical cases using qualitative fractography.

Abstract: PURPOSE To educate dental academic staff and clinicians on the application of descriptive (qualitative) fractography for analyses of clinical and laboratory failures of brittle materials such as glass and ceramic. MATERIALS AND METHODS The fracture surface topography of failed glass, glass fiber-reinforced composite, and ceramic restorations (Procera, Cerestore, In-Ceram, porcelain-fused-to-metal) was examined utilizing a scanning electron microscope. Replicas and original failed parts were scrutinized for classic fractographic features such as hackle, wake hackle, twist hackle, arrest lines, and mirrors. RESULTS Failed surfaces of the veneering porcelain of ceramic and porcelain-fused-to-metal crowns exhibited hackle, wake hackle, twist hackle, arrest lines, and compression curl, which were produced by the interaction of the advancing crack with the microstructure of the material. Fracture surfaces of glass and glass fiber-reinforced composite showed additional features, such as velocity hackle and mirrors. The observed features were good indicators of the local direction of crack propagation and were used to trace the crack back to an initial starting area (the origin). CONCLUSION Examples of failure analysis in this study are intended to guide the researcher in using qualitative (descriptive) fractography as a tool for understanding the failure process in brittle restorative materials and also for assessing possible design inadequacies.