Fatigue fracture surface analysis in C45 steel specimens using X-Ray fractography

X-Ray fractography is a useful technique to analyse the mechanisms operating in fracture and involves an examination of the fracture surface. In the present investigation, this technique has been employed to study the fatigue fracture behaviour of a medium carbon steel of C45 grade in different heat treated conditions. The different trends observed in the residual stress (σr) and diffraction profile full width at half maximum intensity (B) relationships with the maximum stress intensity factor (Kmax) on the fracture surface have been correlated to the differences in flow characteristics of these materials. The root mean square value of microstrain , and the coherent domain (particle) size, D, were determined through single line diffraction profile (Voigt's) analysis. It has been observed that contribution of microstrain to profile broadening is more significant than that due to domain size. However, at higher Kmax values an influence of D was found on line broadening, to a small extent. Results of sub-surface stress measurements were compared in two conditions. A good correlation was noticed between the depths below the fracture surface, designated as ymax, at which the measured σr, reaches the base material value and the corresponding monotonic plastic zone size (rp) obtained by calculation. The observed depths below the fracture surface (ypeak) at which an increase in σr or a decrease in B was noticed, appear to be related to the conditions of the near-tip regions where the material has undergone severe fatigue damage and cyclic softening

Failure analysis based on microvoid growth for sheet metal during uniaxial and biaxial tensile tests

The aim of the presented investigations is to perform an analysis of fracture and instability during simple and complex load testing by addressing the influence of ductile damage evolution in necking processes. In this context, an improved experimental methodology was developed and successfully used to evaluate localization of deformation during uniaxial and biaxial tensile tests. The biaxial tensile tests are carried out using cruciform specimen loaded using a biaxial testing machine. In this experimental investigation, Stereo-Image Correlation technique has is used to produce the heterogeneous deformations map within the specimen surface. Scanning electron microscope is used to evaluate the fracture mechanism and the micro-voids growth. A finite element model of uniaxial and biaxial tensile tests are developed, where a ductile damage model Gurson-Tvergaard-Needleman (GTN) is used to describe material deformation involving d`amage evolution. Comparison between the experimental and the simulation results show the accuracy of the finite element model to predict the instability phenomenon. The advanced measurement techniques contribute to understand better the ductile fracture mechanism

Creep degradation in oxide-dispersion-strengthened alloys

Oxide dispersion strengthened Ni-base alloys in wrought bar form are studied for creep degradation effects similar to those found in thin gage sheet. The bar products evaluated included ODS-Ni, ODS-NiCr, and three types of advanced ODS-NiCrAl alloys. Tensile test specimens were exposed to creep at various stress levels at 1365 K and then tensile tested at room temperature. Low residual tensile properties, change in fracture mode, the appearance of dispersoid-free bands, grain boundary cavitation, and internal oxidation in the microstructure were interpreted as creep degradation effects. This work showed that many ODS alloys are subject to creep damage. Degradation of tensile properties occurred after very small amounts of creep strain, ductility being the most sensitive property. All the ODS alloys which were creep damaged possessed a large grain size. Creep damage appears to have been due to diffusional creep which produced dispersoid-free bands around boundaries acting as vacancy sources. Low angle and possibly twin boundaries acted as vacancy sources

Fractography of composite delamination

Studies were made of the fractography of mode 2 delamination and the impact damage of carbon fiber reinforced polymer composites. Laminates fractured under mode 2 loading were potted in a clear epoxy polymer, sectioned and polished and examined using transmission and reflection light microscopy. There were only occasional fibers bridging the mode 2 cracks. These cracks were not always visible probably because the crack opening displacement was too small as to be resolved using light microscopy. A study was made of the effect of repetitive impacts on laminates of AS4/3501-6 and IM6/3501-6. Plots of cumulative impact energy vs cumulative absorbed energy exhibited a sharp change in slope which corresponded to the damage area reaching the edges of the specimen. The initial slope was highly reproducible for both composite materials. On the other hand, the intersection point between the two slopes, where the damage area reaches the specimen edges, was highly variable between specimens. This variability is tentatively ascribed to differences in laminate quality

Fractography of the high temperature hydrogen attack of a medium carbon steel

Microscopic fracture processes were studied which are associated with hydrogen attack of a medium carbon steel in a well-controlled, high-temperature, high-purity hydrogen environment. Exposure to a hydrogen pressure and temperature of 3.5 MN/m2 and 575 C was found to degrade room temperature tensile properties with increasing exposure time. After 408 hr, yield and ultimate strengths were reduced by more than 40 percent and elongation was reduced to less than 2 percent. Initial fissure formation was found to be associated with manganese rich particles, most probably manganese oxide, aligned in the microstructure during the rolling operation. Fissure growth was found to be associated with a reduction in carbide content of the microstructure and was inhibited by the depletion of carbon. The interior surfaces of sectioned fissures or bubbles exhibit both primary and secondary cracking by intergranular separation. The grain surfaces were rough and rounded, suggesting a diffusion-associated separation process. Specimens that failed at room temperature after exposure to hydrogen were found to exhibit mixed mode fracture having varying amounts of intergranular separation, dimple formation, and cleavage, depending on exposure time

Slurry-pressing consolidation of silicon nitride

A baseline slurry-pressing method for a silicon nitride material is developed. The Si3N4 composition contained 5.8 wt percent SiO2 and 6.4 wt percent Y2O3. Slurry-pressing variables included volume percent solids, application of ultrasonic energy, and pH. Twenty vol percent slurry-pressed material was approximately 11 percent stronger than both 30 vol percent slurry-pressed and dry-pressed materials. The Student's t-test showed the difference to be significant at the 99 percent confidence level. Twenty volume percent (300 h) slurry-pressed test bars exhibited strengths as high as 980 MPa. Large, columnar beta-Si3N4 grains caused failure in the highest strength specimens. The improved strength correlated with better structural uniformity as determined by radiography, optical microscopy, and image analysis

Fractography can be used to analyze failure modes in polytetrafluoroethylene

Fractographic principles used for analyzing failure in metals are applied to the analysis of the microstructure and fracture of polytetrafluoroethylene. This material is used as seals in cryogenic systems

Investigations on the Material Efficacy of Failed Helical Gears in a Gear Train

An investigation on the material efficacy of failed helical gears in the gearbox of an automobile has been carried out. Two helical gear samples taken as representatives of the whole of six in the gearbox were denoted as samples A and B. Methods employed in the failure investigation include visual examination with the unaided eye and fractography; compositional analysis; hardness measurements and microstructural analysis. The results obtained showed that Gear sample A failed by oxidative wear essentially caused by insufficient lubrication. On the other hand, Sample B exhibited no outward sign of failure. Processes culminating in the formation of a wear particle were however noticed few micrometers below the surface of Gear Sample B. Furthermore, in the absence of sufficient lubrication, numerous carbide precipitates formed in both samples contributed to wear of the gear material. It was concluded that the premature service failures of the gears was caused by inadequate lubrication and not inadequate material selection