Pavan Karra

Life expectancy of modular Ti6Al4V hip implants: influence of stress and environment.

Stress dependent electrochemical dissolution is identified as one of the key mechanisms governing surface degradation in fretting and crevice corrosion of biomedical implants. The present study focuses on delineating the roles of mechanical stress and chemical conditions on the life expectancy of modular hip implants. First, material removal on a stressed surface of Ti6Al4V subjected to single asperity contact is investigated experimentally to identify the influence of contact load, in-plane stress and chemical environment on mean wear rates. A range of known stress levels are applied to the specimen while its surface is mechanically stimulated in different non-reactive to oxidizing aqueous environments. Evolution of surface degradation is monitored, and its mechanism is elucidated. This phase allows estimation of Preston Constant which is later used in the analysis. Second phase of the work is semi-analytical and computational, where, based on the estimated Preston constant and other material and process parameters, the scratch propensity (consisting of magnitude of scratch depth and their frequency per unit area) due to micro-motion in modular hip implants is estimated. The third phase views these scratches as initial notches and utilizes a mixed-mode fatigue crack propagation model to estimate the critical crack length for onset of instability. The number of loading cycles needed to reach this critical crack length is then labeled as the expected life of the implant under given mechanical and chemical conditions. Implications of different material and process conditions to life expectancy of orthopedic implants are discussed. It is observed that transverse micro-motion, compared to longitudinal micro-motion, plays a far more critical role in determining the implant life. Patient body weight, as well as proximity of the joint fluid to its iso-electric point play key roles in determining wear rates and associated life expectancies of modular hip implants. Sustained aeration of joint fluid, as well as proper tolerancing of mating surfaces, along with a proper choice of material microstructure may be utilized to extend implant life.

Diffusion-Limited Agglomeration and Defect Generation during Chemical Mechanical Planarization

Chemical mechanical planarization (CMP) of copper involves removal of surface asperities with abrasive particles and polishing processes. This leads to copper-containing nanoparticles extruded into the solution. We model the diffusion-limited agglomeration (DLA) of such nanoparticles which can rapidly grow to large sizes. These large particles are detrimental because they can participate in polishing, causing scratches and surface defects during CMP. The agglomeration is much slower in the reaction-limited agglomeration process. Under realistic conditions the defect generation probability can increase significantly over time scales of {approx}10 to 20 min from DLA, unless prevented by slurry rejuvenation or process modification measures.

Assessment of Patellar Tendon Reflex Responses Using Second-Order System Characteristics

Deep tendon reflex tests, such as the patellar tendon reflex (PTR), are widely accepted as simple examinations for detecting neurological disorders. Despite common acceptance, the grading scales remain subjective, creating an opportunity for quantitative measures to improve the reliability and efficacy of these tests. Previous studies have demonstrated the usefulness of quantified measurement variables; however, little work has been done to correlate experimental data with theoretical models using entire PTR responses. In the present study, it is hypothesized that PTR responses may be described by the exponential decay rate and damped natural frequency of a theoretical second-order system. Kinematic data was recorded from both knees of 45 subjects using a motion capture system and correlation analysis found that the mean R2 value was 0.99. Exponential decay rate and damped natural frequency ranges determined from the sample population were −5.61 to −1.42 and 11.73 rad/s to 14.96 rad/s, respectively. This study confirmed that PTR responses strongly correlate to a second-order system and that exponential decay rate and undamped natural frequency are novel measurement variables to accurately measure PTR responses. Therefore, further investigation of these measurement variables and their usefulness in grading PTR responses is warranted.

Chip Segmentation in Machining: A Study of Deformation Localization Characteristics in Ti6Al4V

Chip segmentation by deformation localization is an important process in a certain range of velocities and might be desirable in reducing cutting forces and by improving chips’ evacuation, whereas few studies of practical criteria to calculate shear band spacing are available in literature. This paper extends nonlinear dynamics model for chip segmentation by allowing time varying orientation of the shear plane that are pronounced in strain hardening materials. The model extends the non-linear dynamics approach with additional state variables to the Burns and Davies approach. The model is simulated numerically to predict the shear bands of the chip. The numerical simulation of the model is compared with experimental observations and is in agreement with experimental observations in Ti6Al4V. This offers guidance to predict shear band spacing of other materials.Copyright