Srinivasan Iyengar

Fatigue Fracture in Dual Modular Revision Total Hip Arthroplasty Stems: Failure Analysis and Computed Tomography Diagnostics in Two Cases.

We report on two patients with fracture of a modular, tapered and distally fixed, uncemented titanium revision hip stem, not previously described. A failure analysis revealed that the cause of the fractures was the development of fatigue cracks in the mid-stem cobalt-chromium modular junction ending in corrosion-fatigue failure. No material defects or stress risers were found in any of the implants. The diameter of the mid-stem modular junction might be undersized for use in heavy and active patients. We also report a new way of detecting an undisplaced fracture at the modular junction, using the scout image from a computed tomography (CT) scan; a technique that can be used when plain radiographs are inconclusive.

Characterization of Microstructure and Mechanical Properties of High Chromium Cast Irons Using SEM and Nanoindentation

The effects of composition changes and heat treatment on the microstructure and mechanical properties of high-chromium white cast irons were studied in order to characterize possible improvements in product performance and machinability. Materials characterization was performed using nanoindentation, SEM, and EDS techniques. Present results show that changes in carbon and silicon contents as well as heat treatment strongly affect the mechanical properties and their variation in the material. In the as-cast condition, the sample with relatively lower carbon and silicon contents has an austenite-martensite matrix and is much harder than the sample with relatively higher carbon and silicon contents having more eutectic carbides in a bainite matrix. Annealing leads to softening of the materials relative to the as-cast condition, with the relatively higher carbon-silicon material being marginally harder due to the presence of more eutectic carbides. A similar trend is seen after the hardening treatment, and the presence of primary carbide can restrict the extent of hardening due to the loss of alloying elements from the matrix.

Time-of-Flight Three Dimensional Neutron Diffraction in Transmission Mode for Mapping Crystal Grain Structures

The physical properties of polycrystalline materials depend on their microstructure, which is the nano- to centimeter scale arrangement of phases and defects in their interior. Such microstructure depends on the shape, crystallographic phase and orientation, and interfacing of the grains constituting the material. This article presents a new non-destructive 3D technique to study centimeter-sized bulk samples with a spatial resolution of hundred micrometers: time-of-flight three-dimensional neutron diffraction (ToF 3DND). Compared to existing analogous X-ray diffraction techniques, ToF 3DND enables studies of samples that can be both larger in size and made of heavier elements. Moreover, ToF 3DND facilitates the use of complicated sample environments. The basic ToF 3DND setup, utilizing an imaging detector with high spatial and temporal resolution, can easily be implemented at a time-of-flight neutron beamline. The technique was developed and tested with data collected at the Materials and Life Science Experimental Facility of the Japan Proton Accelerator Complex (J-PARC) for an iron sample. We successfully reconstructed the shape of 108 grains and developed an indexing procedure. The reconstruction algorithms have been validated by reconstructing two stacked Co-Ni-Ga single crystals, and by comparison with a grain map obtained by post-mortem electron backscatter diffraction (EBSD).

Heat Treatment of Low-Alloyed Steel up to Grade Q125

This work investigates the problem of how to reduce the prime cost of Grade Q125 Group 4 casing and tubing according to API 5CT and proposes process solutions to resolve it, consisting in the enhancement of series of mechanical properties without chemical alloying of steel with expensive elements such as chromium, molybdenum, vanadium, niobium and boron. During the investigation the process parameters of 9MnSi5 low-alloy steel heat treatment were developed, which confirmed the efficiency of this technology providing high values of strength and yield strengths which are minimum 931 MPa and 862 MPa respectively; at the same time maintaining the required cold resistance and resistance to hydrogen cracking of pipe. The suggested process parameters were based on heat cycle quenching with heating in induction furnace up to 1050–1100°С, 880–900°С and 740–780°С accordingly at each cycle and cooling rate in each case equal to 50°С/sec, during which the uniform martensite grain size ranging from 5 to 10 micron was obtained.

Fatigue Behavior of α/β Brass

Brass is a popular engineering material used in a variety of applications requiring good strength, ductility, corrosion resistance and machinability. This work focuses on the analysis of Stress – Strain Hysteresis Loops obtained in low cycle fatigue experiments in order to determine the internal and effective components of the stress amplitude which provide valuable information about damage mechanisms. Cyclic plastic straining has been studied using the SAP method (Statistical Approach of Plastic deformation). This approach allows the evaluation of the effective and internal stresses, effective elastic modulus and the probability density function, f(σic), which reflects the microstructure of the plastically deformed material. f(σic) obtained from each reversal exhibits two peaks in α/β brass, corresponding to the plastic deformation of each component phase of this material. At low strain amplitudes and during the initial loading cycles, a single peak has been generally observed. However, at high strain amplitudes and at the stabilization stage of the hysteresis loop, two distinct peaks could be distinguished. Further, relative to the effective stress, the internal stress shows a hardening effect with strain amplitude.