Shunmugam M S

Fabrication of microparts with large aspect-ratio and complex shapes remains a huge challenge for industries and microforming is a potential solution to manufacture such microparts. However, similarity in specimen-deformation and microstructural length scales during microforming results in size effect, leading to unpredictable plastic behavior and increased process scatter. One approach to counter size effect is by engineering suitable microstructure in the material. In the present work, three grades of Al (AA1070, AA5083, AA2014) with unique alloy chemistry and microstructural profile are processed by cryorolling (CR) to 95% thickness reduction. By imparting controlled postprocess annealing on the CR materials, three distinctive microstructures – (i) ultrafine grained (UFG) with average grain size around 1 μm, (ii) fine grained (FG) with average grain size near to 5 μm, and (iii) coarse grained (CG) with approximate average grain size of 20 μm are engineered. The influence of alloy chemistry, grain boundary engineering and crystallographic texture on microformability are studied. For pure Al (AA1070), the UFG and FG microstructures show superior microformability than the CG counterparts. The equiaxed UFG grains present in these microstructures mitigate the size effect abnormalities by increasing the number of grains in the deformation volume and uniformly distributing the complex microforming strain via grain boundary mediated plasticity. Their corresponding texture containing strong Copper mixed with scattered Cube elements promotes in-plane strain condition and high resistance to localized thinning. Also, the material shows near-zero planar anisotropy that leads to a homogenous in-plane strain distribution. Unlike pure Al, the UFG and FG Al alloys suffer from increased strain localization due to presence of solute clouds and nanoprecipitates. They influence strain-aging (Portevin–Le Chatelier effect), strain gradient hardening phenomenon, and shear propensity during failure of the Al alloys. A composite texture consisting of a combination of Brass, Dillamore, S, and β-fiber elements in the Al alloys is found to be detrimental to their microformability. The Dillamore texture is contributed by formation of adiabatic shear bands during deformation of Al alloys.

Inverse dynamics analysis of prosthetic legs with polycentric knees is complex due to increased number of links. The present work proposes a simple and general method called equivalent system (ES) analysis. The ES analysis provides forces and moment at hip joint as well as at the functional knee centre (FKC), the instant centre of the polycentric knee. The input to the ES analysis is the motion data. For validation of the proposed method, synthetic motion data for the swing phase of walking with prosthetic legs having different knees are generated by simulations using ADAMS. The hip kinetics evaluated by the proposed method is compared with that from ADAMS. The root mean square errors of the ES analysis are lower than 17 (10−6) N for hip reaction forces and 2.6 (10−6) Nm for the hip moments, thereby validating the proposed method. In order to demonstrate the application of the proposed methodology, the motion data of two transfemoral amputees using single-axis and four-bar knee prostheses are obtained during gait trials. The hip kinetics as well as kinetics at FKC are computed using ES analysis. Hip power during the swing phase is also evaluated and compared. The results are presented in this paper and discussed. The ES analysis is shown to be a versatile tool to provide insights into the human-mechanism interaction.

Design and manufacturing of the high-precision meso deep drawing tools, especially in the low mesoscale range, demands close tolerance and high positional accuracy. Variation in environmental temperature affects the dimensions of individual components and final assembly due to thermoelastic deformation. This paper describes an approach to estimate the thermal deformation and positional errors of the tool components using finite element simulation. A shift of 52.74 and 61.79 μm with reference to central plane is observed for the guide push holes on left-hand and right-hand sides, respectively, at 42 °C. The thermal deformation along X-direction is validated through a dedicated measurement system with an expanded uncertainty of 2.8594 µm m−1. Based on thermal error estimation, CNC program is reconstructed to compensate dimensionally during the manufacturing of relevant tool components. A dimensional chain analysis reveals a maximum offset of 18 μm between punch and die axes in the tool assembly. Using the developed meso deep drawing tool, cups of 5.5 mm diameter are deep drawn from 0.8-thick sheet of IS513 low-carbon steel. This paper discusses the thermal deformation modeling, development of software system and experimental results in detail. The increased positional accuracy with desired clearances enhances the quality of the drawn parts, thereby ensuring improvement in the tool performance and its life. The proposed approach can be applied in the development of micro-deep drawing and other forming tools.

In this work on electrical discharge machining (EDM), pulse characteristics and pulse types are assessed from the pulse trains using a novel thresholding approach. For the first time, discharge energy is derived from the pulses, instead of using the setting voltage, setting current and duty cycle. Specific energy is computed as the discharge energy required for removing unit volume of work material. The experiments are carried out on D3 die steel and the specific energies are determined for conventional, powder-added and ultrasonic-assisted EDM. The maximum specific energy increases from 406.6 J/mm3 in conventional EDM to 463.1 J/mm3 with powder mix, while it reaches 486.8 J/mm3 with ultrasonic assistance. However, only the powder mix increases the maximum material removal rate (MRR) from 0.384 to 0.464 mm3/s. The ultrasonic-assisted EDM results in a marginally reduced maximum MRR of 0.369 mm3/s, even though the specific energy is the highest.

Introduction Typically, polycentric prosthetic knees have been designed with a focus on improved performance during stance. This study analyzes the influence of geometric aspects such as linkage dimensions of the polycentric knees and anterior-posterior (A/P) alignment on the swing phase of walking. Such a study would enable the development of polycentric knees with improved performance during both stance and swing phases. Methods A simulation-based approach was used to analyze the influence of geometric aspects on swing dynamics. A newly developed four-bar knee called IITM polycentric knee (IPK) was used as a baseline prosthesis. First, the swing was simulated with the baseline prosthesis. The IPK in the lower limb was then replaced with the linkage dimensions of four other commercial polycentric knees (Ottobock's 3R36, 3R55, and 3R70 and Ohio Willow Wood's Pendulum knee) for the simulation of swing phase. Effect of A/P alignment was analyzed by translating the socket adapter of the IPK along the A/P axis in the baseline prosthesis, which has the effect of moving the knee and foot in the opposite direction. A maximum of 20 mm translation of the socket adapter was applied on either side of its base location in the lower-limb model. The gait parameters from the simulations and the centrode of different polycentric knees were then analyzed to understand how linkage dimensions and A/P translations of the knee and foot affect the swing dynamics. Results All four-bar knees without extension assists extended to less than 6° before heel contact, which is significantly lower than the 24° obtained for a single-axis knee in a previous study. The swing performances of the different four-bar knees studied here were found to depend on their centrode. The instant centers (ICs) of the 3R55 and IPK, which were comparatively located more superior to the anatomical knee center during swing, enabled better knee extension and foot clearance than other knees. A/P translation of the knee and foot significantly affected the foot clearance. A 20 mm anterior translation increased toe and heel clearance by 1.6 cm and 1.4 cm, respectively, when compared with a 20 mm posterior translation. Conclusions This study shows that polycentric knees, which generally perform better than single-axis knees with improved stance stability and ease of push-off, can also perform well during swing in terms of knee extension before heel contact and foot clearance throughout swing. The extended moment arm for the weight of the shank and foot due to the superior location of the IC in polycentric knees is the reason for the improved swing performance of polycentric knees. Appropriate placement of the socket adapter, which affects A/P alignment, is crucial so that foot clearance during swing is not compromised. The insights obtained from the study would be useful in developing simpler polycentric knees, which can extend completely without any extension assist if proper alignment is used as recommended. Clinical Relevance The study provides better understanding of the effects of polycentric knee placement and the need for an extension assist during the swing phase of walking, which could help clinicians improve the selection of appropriate knees and their alignment for prosthesis users.

Background:Previously reported simulations comparing a new polycentric knee (called IPK) and a single-axis knee suggested that polycentricity could lead to improved performance during swing phase and negate the need for an extension assist. They also showed that an anteriorly translated socket adapter for enhanced stance stability compromises foot clearance during swing.Objectives:The objectives of this study are to validate those findings using gait trials to enable further improvement in the IPK design before mass production.Methods:Three subjects regularly using three different passive knees, single-axis knee without extension assist, single-axis knee with extension assist (SAK-EA), and polycentric knee with extension assist (Mobility India-sourced polycentric knee), participated in this study. Their gait with their regular prosthesis and with IPK (having no extension assist) were analyzed, compared, and broadly correlated with simulation results.Results:Extension assist in single-axis knee with extension assist improved swing performance, affected foot clearance in Mobility India-sourced polycentric knee, and was found to be unnecessary in the IPK. With an anteriorly placed socket adapter in the IPK, compensatory strategies were necessary for foot clearance. The IPK was found to provide better knee extension characteristics with lower hip effort (up to 42% reduction) than other knees.Conclusions:This case series confirmed previously reported simulation results on the swing phase behavior of passive prosthetic knees. The performance of the IPK during swing obviated the need for an extension assist, thereby simplifying the design. Appropriate design changes in the IPK's socket adapter location are required to achieve both stance stability and reduce gait compensations for foot clearance.

Introduction: A new passive polycentric knee called IITM polycentric knee (IPK), where IITM stands for the authors' institution, namely, Indian Institute of Technology Madras, was designed to provide better functionality at a reduced cost. The design of the knee was based on stance phase stability criteria and toe clearance during the swing phase for users in developing countries to handle uneven terrain. The present study features a simulation-based dynamic analysis of a passive single-axis knee (SAK) and IPK to understand the swing phase behavior of the IPK with respect to SAK. This study aims at understanding the advantages and disadvantages of the IPK against a passive SAK to improve its design criteria and enable further development to provide better functionality to the existing passive SAK users. Methods: Lower-limb models of a normal leg and prosthetic legs with SAK and IPK were created, their swing phases were simulated, and gait parameters were analyzed. The passive knees in the present study have only frictional damping and no active knee moments, implying the requirement of compensatory strategies for forward progression of the swinging leg. The required frictional damping at the knee joint to obtain near-normal knee motion and the compensations required while using the SAK and IPK were obtained by simulation. The lower-limb dynamics obtained from the simulations was analyzed to understand the performance of the SAK and IPK during swing. Results: This study shows that the IPK is superior to an SAK in terms of achieving knee extension in preparation for stance, even without an extension assist and with lower hip effort. The peak hip power during swing with the IPK was 58.7% less than with the SAK. A negative heel clearance was found in terminal swing with the IPK, which demands a compensatory strategy for foot clearance during the swing and shows the necessity for inclusion of a heel clearance parameter in the design of polycentric knees. Conclusions: The present work focuses on the swing behavior comparison of passive single-axis and polycentric knees. The study provides insight into the compensatory strategies required to achieve successful forward progression of the swinging leg with both types of knees. The analysis shows the potential for a well-designed passive polycentric knee to be a costeffective alternative to achieve near-normal swing during walking.