Fractography

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

Crack path selection in adhesively bonded joints: the roles of external loads and speciment geometry

Abstract: This paper investigates the roles of external loads and specimen geometry on crack path selection in adhesively bonded joints. First, the effect of mixed mode fracture on crack path selection is studied. Using epoxy as an adhesive and aluminum as the adherends, double cantilever beam (DCB) specimens with various T-stress levels are prepared and tested under mixed mode fracture loading. Post-failure analyses on the failure surfaces using X-ray photoelectron spectroscopy (XPS) suggest that the failure tends to be more interfacial as the mode II fracture component in the loading increases. This fracture mode dependence of the locus of failure demonstrates that the locus of failure is closely related to the direction of crack propagation in adhesive bonds. Through analyzing the crack trajectories in failed specimens, the effect of mixed mode fracture on the directional stability of cracks is also investigated. The results indicate that the direction of the crack propagation is mostly stabilized when more than 3% of mode II fracture component is present at the crack tip regardless of the T-stress levels in the specimens for the material system studied. Second, using a high-speed camera to monitor the fracture sequence in both quasi-static and low-speed impact tests, the effect of debond rate on the locus of failure and directional stability of cracks is investigated. Post-failure analyses including XPS, Auger electron spectroscopic depth profile, and scanning electron microscopy indicate that as the crack propagation rate increases, the failure tends to be more cohesive and the cracks tend to be directionally unstable. Last, as indicated by the finite element analyses results, the T-stresses, and therefore the directional stability of cracks in adhesive bonds, are closely related to the thickness of the adhesive layer and also the thickness of adherend. This specimen geometry dependence of crack path selection is studied analytically and is verified experimentally.

Deformation and fracture of a particle-reinforced aluminum alloy composite: Part I. Experiments

Abstract: Mechanical tests were performed on a powder-metallurgically processed 7093/SiC/15p discontinuously reinforced aluminum (DRA) composite in different heat-treatment conditions, to determine the influence of matrix characteristics on the composite response. The work-hardening exponent and the strain to failure varied inversely to the strength, similar to monolithic Al alloys, and this dependence was independent of the dominant damage mode. The damage consisted of SiC particle cracks, interface and near-interface debonds, and matrix rupture inside intense slip bands. Fracture surfaces revealed particle fracture-dominated damage for most of the heat-treatment conditions, including an overaged (OA) condition that exhibited a combination of precipitates at the interface and a precipitate-free zone (PFZ) in the immediate vicinity. In the highly OA conditions and in a 450 °C as-rolled condition, when the composite strength became less than 400 MPa, near-interface matrix rupture became dominant. A combination of a relatively weak matrix and a weak zone around the particle likely contributed to this damage mode over that of particle fracture. Fracture-toughness tests show that it is important to maintain a proper geometry and testing procedure to obtain valid fracture-toughness data. Overaged microstructures did reveal a recovery of fracture toughness as compared to the peak-aged (PA) condition, unlike the lack of toughness recovery reported earlier for a similar 7XXX (Al-Zn-Cu-Mg)-based DRA. The PA material exhibited extensive localization of damage and plasticity. The low toughness of the DRA in this PA condition is explored in detail, using fractography and metallography. The damage and fracture micromechanisms formed the basis for modeling the strength, elongation, toughness, and damage, which are described in Part II of this work.

On the mechanism of fatigue crack growth in silicon nitride

Abstract: The mechanism of fatigue crack growth in silicon nitride under the experimental conditions utilized is found to be of a cyclic nature, as contrasted to a form of static fatigue observed in some other ceramic systems. Conventional methods of analysis of the rate of fatigue crack growth in terms of ΔKeff are not applicable, because the results of the experimental portion of this investigation show that ΔKeff can decrease as the rate of fatigue crack growth increases. A mechanism which involves the formation of microcracks caused by a wedge effect which develops during the unloading portion of a cycle is discussed and evaluated. The wedge effect results from crack closure, which arises due to the roughness of the intergranular fracture surface as well as to debris trapped between the opposing fracture surfaces. In the proposed mechanism, the extent of crack advance per cycle is limited because of the decrease in stress intensity factor with crack advance in a given cycle associated with the wedge effect. The quantitative results of a semiempirical analysis of tests carried out in either air or vacuum are in agreement with some unusual experimental trends.

Evaluation of a high fracture toughness composite ceramic for dental applications.

Abstract: Purpose: The introduction of yttrium partially stabilized zirconia polycrystals (Y-TZP) has pushed the application limits of all-ceramic restorations. The mechanical properties of these materials can be further improved by the addition of a secondary dopant phase. The aim of this work was to evaluate the properties of a new nano-composite ceramic used as a dental framework material. Materials and Methods: The properties of a new ceria-stabilized tetragonal zirconia polycrystal co-doped with alumina (Ce-TZP-Al) were investigated. Y-TZP was used as control. Sixty bars (20 × 2.5 × 1.5 mm3) from each material were prepared by cutting CAD/CAM milling blocks. Twenty specimens were used to measure the 4-point flexural strength and the modulus of elasticity of the tested materials. The remaining specimens were used to measure the fracture toughness using indentation strength (IS), single edge notched beam (SENB), and fractography (FR). The thermal expansion coefficient (TEC) was measured using temperature expansion diagrams. The bond strength of the two framework materials to two esthetic veneer ceramics was tested using the microtensile bond strength test (MTBS). Finally, scanning electron microscopy (SEM) and energy dispersive X-ray microanalysis (EDX) were used to analyze the internal structure of the materials. One- and two-way analysis of variance (ANOVA) and Bonferroni post hoc tests were used to analyze the data (α= 0.5). Results: The flexural strength and modulus of elasticity of Ce-TZP-Al (856 MPa, 170 GPa) were significantly weaker (p < 0.001) than those of Y-TZP (1003 MPa, 215 GPa). The (IS) fracture toughness of the former (19.02 MPa m1/2) was significantly higher (p < 0.001) than SENB (12.6 MPa m1/2) or FR (12.8 MPa m1/2) values. These values were significantly higher (p < 0.001) than the fracture toughness of Y-TZP (7.4 MPa m1/2), which showed statistically similar values using the same three techniques. The measured TEC for the two materials was relatively similar, 10.1 μm/°C and 10.4 μm/°C, respectively. Regarding MTBS values, Ce-TZP-Al had significantly lower bond strength values (p < 0.001) and a higher percentage of interfacial failure than Y-TZP, which failed completely cohesively with the two used veneer ceramics. SEM analysis revealed zirconia grains pull out and structural defects at the core-veneer interface for Ce-TZP-Al material, which explained its weak bond to the two used veneers. Conclusion: Despite the promising mechanical properties of Ce-TZP-Al nano-composite ceramic, its very low bond strength to esthetic veneers leaves such layered restorations highly susceptible to delamination and chipping under function. Further studies are needed to enhance the surface stability of this high fracture toughness ceramic.