Duality of fatigue failures of materials caused by Poisson defect statistics of competing failure modes

The statistical nature of failures of components subjected to fatigue loading is a major challenge in engineering applications. Here, we report an interesting fatigue phenomenon in which there is a 'duality' of the S-N fatigue data (S is stress and N is cycles to failure)--the grouping of data into two distinct S-N curves as a result of two competing failure modes. The proportions of failure by either mode are about the same and one cannot predict which failure mode will eventually dominate. This means that fatigue lives may be unpredictable in materials under certain application conditions and the consequences can be disastrous. Based on Poisson defect statistics and Monte Carlo simulations, we show that this phenomenon can arise from the Poisson distribution of low populations of microscale defects. An unexpected finding is that there should be a combination of average defect density and specimen area at which this phenomenon is observable in materials having sparsely populated defects.     

Moisture absorption and its effect on the electrical properties of high silica fabric

High silica fabric with a silica content >98% can be prepared by removing the non-siliceous ions from the E-glass fabric using HCl as the leachant. The presence of Si-OH groups makes the material hydrophilic. The extent of moisture absorption depends on the extent of leaching. The moisture present in the leached fabric decreases the volume and surface resistivity values.     

Microwave Characteristics of Polyaniline Based Short Fiber Reinforced Chloroprene Rubber Composites

This report presents the microwave characteristics of conducting polymer composites (CPCs) based on chloroprene rubber with special reference to dielectric properties. CPCs based on polyaniline (PANI), polyaniline-coated short nylon fiber (PANI-N) and chloroprene rubber (CR) were prepared by mechanical mixing. The important properties like dielectric permittivity, loss tangent, conductivity, and dielectric heating coefficient were evaluated and compared. It was found that PANI/CR composites had very good dielectric properties in the microwave range. The addition of PANI-N improved the mechanical properties of the composites with reasonably good dielectric properties. The CPCs were also found to have good microwave absorption.