Design of Fretting Rig and Thin Contact Displacement Sensor For Fretting Experiments

Abstract

The present work mainly focuses on understanding the fretting phenomenon and its effects on life of the components/assemblies. To accomplish this, the suitable fretting setup which comprises of proper fretting rig/fixture and an instrument/technique to measure relative contact slip between the tightly contacted pair is needed. The design and fabrication of fretting rig begins with One-Dimensional (1-D) and Three-Dimensional (3-D) finite element analysis on proposed model of fretting rig. From the analysis results, the fretting rig model which produces 50% Load transfer Ratio (LTR) for the given load is selected for the fabrication alongwith with the consideration of machine bed dimensions and difficulties arised when mounting the rig on the machine bed. To ensure this condition, initially eleven cases of simple one- dimensional (1-D) fretting rig models have been designed and Finite Element (FE) analysis of these models was carried out. Afterwards four more different fretting rig models have been designed and analyzed with the inclusion of machine constraints and its results are also presented here. Among these models, the finite element model with optimum LTR was selected to conduct fretting experiments and the detailed three- dimensional (3-D) finite element study of the selected model has been performed and its results were validated. Following this, the selected fretting rig was fabricated and tested. These tests revealed that the fabricated fretting rig failed to maintain constant normal load during fretting experiments. So, the necessary modifications have been made in the fabricated fretting rig to conduct fretting experiments by applying constant normal load on the specimen. The fretting rig thus designed would be used for conducting fretting experiments. Also, this work proposes a new thin curved sensor/strip to measure the relative slip between pad and specimen under fretting condition. Since the relative contact displacement is an important parameter to categorize the fretting process, the measurement of contact displacement between pad and specimen is necessary. Because of high yield strength and the ability to return to its initial position even with notable deflection, the spring steel have chosen to fabricate the thin curved strip. Before the fabrication, the detailed Finite Element Analysis (FEA) of the thin curved sensor was ii carried out. The strip consists of different shapes (rectangular, circular and elliptical) of slots and the number of slots in each strip is varied from 2 to 6. The Strain Energy Approach (SEA) has been used to calculate the displacement for the curved strip and it was compared, verified and validated with its FEA and Experimental results. From FEA study of thin curved strip with slots, four configurations were chosen to measure micro level displacement between pad and specimen under fretting experiments. The study reveals that the increasing number and size of holes presented in the curved strip indicate that the increased in displacement and von-Mises stress values which offers the higher flexibility to the strip. The reduction in area and minimum thickness of the curved strip could be the reason for the decrease in the stiffness of the curved strip. This study explores the use of new simple and novel instrument/sensor to capture the micro level relative displacement between the pad and specimen under fretting condition.