Measurement of residual stresses in austenitic stainless steel weld joints using ultrasonic technique

Abstract

A methodology has been developed using a non-destructive ultrasonic technique for measuring surface/subsurface residual stresses in 7 mm thick AISI type 316LN stainless steel weld joints made by activated tungsten inert gas and multipass tungsten inert gas welding processes. Measurement of residual stresses using an ultrasonic technique is based on the effect of stresses on the propagation velocity of elastic waves. Critically refracted longitudinal L _CR wave mode was employed and accurate transit time measurements were made across the weld joints. Quantitative values of the longitudinal residual stresses across the weld joints were estimated from the measured transit times and predetermined value of acoustoelastic constant for AISI type 316LN stainless steel. The nature of the residual stress profiles and their variations across the two types of weld joints were compared and interpreted.     

Application of the theory of linear elastic materials with voids to NDT of interfaces of bilaminates

Non-destructive testing (NDT) of interfaces in composite structural elements is mandatory in several industries. This calls for modeling of composite interfaces incorporating real world conditions of imperfect bonding. Theories of materials that modern continuum mechanics offer can be profitably employed to develop these models. For instance, distributed voids are often present in the adhesive zone of a structural element and these voids may lead to a failure of the element due to the imperfect bond. As the theory of linear elastic materials with voids (LEMV) provides a model for solids with properties akin to viscoelastic materials, it is employed in the present work to represent the thin adhesive viscoelastic layer with its minutes pores spread over the interface region of a symmetric bilaminate. The bilaminate is assumed to be insonified in water and the governing leaky Lamb wave (LLW) frequency equation is derived. The dispersion and attenuation of LLWs in an Al/adhesive/Al bilaminate are graphically-presented for various porosity-coupling parameter values and are compared with those in perfectly-bonded Al plates. The results presented in this paper showing the influence of pore-infested adhesive layer on the early leaky Lamb modes promises to be useful in identifying interfacial porosity.     

THE MARKET FOR ENCRYPTION

Abstract

In 1942, the United States Navy recognized that in the ranks of newly-enlisted WAVES lay the potential for much-needed assistance in processing German Navy Enigma messages that had been intercepted and deciphered. This is the improbable story of one of those WAVES.     

Measurement and modelling of hollow rubber spheres: surface-normal impacts

Abstract

The performance of natural rubber sports balls under impact conditions is dominated by the material's behaviour under high strain rate conditions dictated by the impact velocity and ball dimensions. To design improved products, sports ball manufacturers need to better understand the physical phenomena associated with ball impact against both rigid and deformable surfaces. This understanding will provide the foundation for performance prediction and optimisation design tools as well as more appropriate product and ultimately material testing techniques. Rebound characteristics of pressurised and pressureless tennis balls and their respective rubber cores subject to normal impacts are presented for a range of incident velocities. High-speed video analysis has been used to measure coefficient of restitution, impact duration and 'whole ball' deformation to validate a surface-normal impact finite element method based predictive model as the first step towards a more comprehensive oblique impact model. Accounting for strain rate dependent stiffness and damping material properties has achieved close correlations between model predictions and observed impact behaviour. The propagation of dominant bending and hoop-strain waves through the ball during the impact is revealed to illustrate the methodology's effectiveness in predicting ball performance associated with difficult to observe impact phenomena.     

Design of surface wave active beds based on human tissue physiology

A surface wave distributed actuation method and its proper design for safely transporting bedridden patients is explored in this paper. First, the basic principle of surface wave distributed actuation is presented, including a new kinematic feature that augments natural surface wave motion for enhanced transport efficiency of humans and elastic bodies. Kinematic modeling and analysis reveals that an object can be transferred by a simplified actuator architecture that makes the concept amenable to hardware realization. A proof of concept prototype demonstrates that heavily loaded rigid objects, elastic objects and humans can be transported. Human tissue physiology is studied to establish worst-case criteria for safe and healthy interactions between the human and the support surface that depends on the duration of interaction. Static models are developed and solved using finite element methods to calculate interaction stresses for realistic, worst-case human-surface wave interaction scenarios. Based on these results a new two-mode surface is designed to secure safe interactions for both long-term support and short term transport tasks.