Showing papers in "Strain in 1990"

Strain gauge reinforcement of plastics

Abstract: A major disadvantage in strain gauge work on plastics is the effect of strain gauge reinforcement, which induces signifcant errors in the quantitative evaluation of strain derived from the plastic. The parameters influencing reinforcement are well documented and procedures have been developed that permit the identification and correction of this effect in certain situations. The principal causes, effects and corrective procedures are reviewed and where appropriate compared with results obtained by the authors of this article.

Application of digital image analysis to strain measurement at elevated temperature

Abstract: Digital imaging techniques are applied to the measurement of surface displacement and strain at elevated temperatures. A simple surface preparation using high temperature paint together with a CCD video camera for digital imaging serve as the basis for the measurements. Mathemetical correlation of the surface intensity pattern between reference and deformed states is used to measure local surface deformation. The method is non-contacting and full field. Displacement and strain measurements have been made at temperatures to 600°. Coefficients of thermal expansion of three metals have been measured using this technique and compared to strain gauge and handbook values. Limitations and potential extensions of the method are discussed.

Inegrated autocorrelation method for plastic strain determination

Abstract: This paper presents an integrated autocorrelation method for the measurement of plastic deformation in metallic materials. The method employs a laser beam to illuminate the test objects. The diffracted images are digitised, recorded and processed on a computer based image processing system. The subsequent speckle pattern correlation analysis yields information on the surface deformation of the test objects. The technique is applied to brass tensile specimens and it is found that the integrated autocorrelation coefficient is linearly proportional to the plastic shear strain. For the range of plastic strain measured the error involved would be less than 10%.

Experimental method for determining through thickness residual hoop stresses in thin walled pipes and tubes without inside access

Abstract: The determination of through thickness residual stresses in pipes and pressure vessels is of growing interest because of emphasis placed on life prediction, design, and failure analysis of piping systems. Most of the through thickness residual stress measurement techniques require the placement of gauges on the outside and inside of the pipe. These methods are severely hampered when gauges cannot be placed on the inside of the pipe. This constraint could arise for small diameter pipes, long pipes or for pipes that have been used in a service condition causing corrosion or fouling of the inner surface. This paper focuses on the first step of a three step procedure for determining residual hoop stresses in thin walled pipes and tubes. The method described is designed for cases where it is impossible to place gauges on the inside of the pipe. The method yses biaxial strain gauges on the outside of the pipe and involves a through thickness axial cut of the pipe. Based on the change in strain on the outside of the pipe, changes in the hoop residual stress distribution due to the axial cut are obtained with the method presented here. The method provides a means to evaluate changes in stresses on both the outside surface and the inside surface of the pipe as well as an evaluation of the change in through thickness hoop stress distribution at any location in the pipe cross section. This paper further demonstrates that the problem of shortening long pipes to enable placement of gauges on the inside of the pipe can result in the loss of significant residual stress information.

The early days of the Saunders-Roe foil strain gauge

Abstract: Peter Stein wrote to me in 1987 enclosing advance information on his proposed celebrations to mark the 50th anniversary of the resistance strain gauge in Portland, Oregon, and inviting WSM to become a ‘sponsor’ which was a way of raising funds to pay for the key pioneers and personalities to attend. For a modest sum one could sponsor an individual, and it suddenly occurred to me that one man who ought to be at the ‘party’ was Peter Jackson, who invented the foil strain gauge at Saunders-Roe in the early 1950's. I joined Saunders-Roe shortly after he left, and have spent most of my working life involved in the design, development, manufacture, selling or application of foil strain gauges. I had never met Peter, and I wasn't even sure if he was still ‘with us’. I had heard that he had relatives still on the Isle of Wight and after some discreet enquiries I found that he was living in California. I promptly sent off the sponsors fee to Peter Stein and told him I was sponsoring the inventor of the foil strain gauge who was resident in California, and he tracked down Peter Jackson and invited him to the conference, although I didn't know he would be there. It was quite a moving moment when someone tapped me on the shoulder in Portland and said “are you Les Window? I'm Peter Jackson”….

Stress dependence of specific heat: observations on the TERSA technique

Abstract: Recently a new method of residual stress measurement, TERSA, was described1and it was suggested that there was a contribution to the effects reported from the stress dependence of specific heat. In this paper, experimental and theoretical evidence is presented demonstrating that the magnitude of the change of specific heat with stress is too small to make a significant contribution to the change in temperature resulting from a stress increment applied to a laser irradiated object.

Fatigue life prediction at a ship deck/superstructure intersection

Abstract: Difficulties arise in identifying an appropriate method to predict fatigue life for the weld joint at the intersection of a ship's deck and superstructure. The problem relates to the gross stress concentration and rapidly changing stress at this location. Variations on the ‘hot spot’ stress approach to fatigue life analysis are reviewed, and applied to results from an experimental programme of fatigue tests on welded joints. Finally tentative recommendations are made for a procedure to predict fatigue life of the structure.

The flexural fatigue performance of plain and fibrous concretes

Abstract: This article describes the results of flexural fatigue tests carried out on plain and fibrous concretes using a cyclic flexural loading machine. This machine is capable of fatigue testing large beams in 4 point bending with load cycling above zero for up to 10 million cycles at rates of up to 20 Hz. Some 50 fatigue tests were completed supported by a further 500 strength and control tests. The changes in strain profile, deflection, electro-dynamic modulus and ultrasonic pulse velocity were observed. Deflection measurements were made on the top surface of the test specimens at midspan using two linear variable transformers (LVDT) each situated about 10 mm from each side of the beams. Strain measurements were taken at various depths at midspan using LVDT's similar to those used for deflection measurement. A novel adaptation of the standard electrodynamic modulus procedure enabled concrete deterioration to be continuously monitored when subjected to either slow rate ramp loading or fatigue tests. These methods provided a complete series of strain, deflection and crack development histories. The results showed that the real stress ratio below which there will be no failure was about 30% for plain concrete (applicable to all rates of loading). Corresponding values for fibrous concrete were approximately 50% for steel fibre content of 1 or 2% by volume, rising to 60% for a concrete containing 3% fibre. There were few prior indications of failure of either plain or fibrous concrete in fatigue. When assessing a task in which concrete fatigue loading is a consideration, a knowledge of the range of loading rates may be as crucial as a knowledge of the stress being applied. (TRRL) (Author/TRRL)

Moiré interferometry for shear strain measurement

Abstract: Moire interferometry maps the in plane displacement components with very high sensitivity. However, in most applications only normal strain components are derived from these contour maps. With the advent of composite materials and adhesive bonded joints measurement of shear strain is becoming increasingly important. This paper looks at measurement errors likely to occur in shear strain derivation.

The Kelvin Hall fairground accident

Abstract: ‘The spread of information about failures within the technical community is essential and it is a sad refection on society that legal and political constraints are doing so much to prevent this spread’, Max S M Saltsman1. Details of such a failure are described here.

Stress corrosion and corrosion fatigue of metals in high temperature aqueous environments

Abstract: The dissolved oxygen content of water influences the susceptibility of austenitic stainless steels and reactor pressure vessel steels to environment assisted cracking by raising the electrochemical potential. The relevance of this fact to the integrity of welded stainless steel constructions, deaerator vessels and reactor pressure vessels steels is discussed. In particulal, the use of slow strain rate test procedures involving applied electrochemical potential control is discussed as a means of assessing the susceptibility to cracking of materials in a range of aqueous environments.

Prediction of stress and displacement fields in stress concentration regions by least squares asymptotic analysis of displacement data

Abstract: This paper demonstrates that highly accurate predictions of the stress fields, including the peak stress of a stress concentration region, can be obtained easily by a least squares asymptotic analysis (LSAA) of even a relatively sparse set of displacement data from points in this zone lying sufficiently far from the boundary to avoid ‘edge effects’.

The examination of heat effects on bond degradation in reinforced concrete using acoustic emission

Abstract: Experimental tests were carried out on pull-out concrete speciments with 8 mm dia. tor steel bars to examine the extent of deterioration of bond at the interface of steel and concrete afer exposure of the speciments up to 800°C furnace temperature. The bond stress-slip behaviour was examined and acoustic emission monitored simiultaneously. The bond stress-emission and slip-emission were compared and correlated with the bond stress-slip behaviour for speciments subjected to diferent levels of temperature both in the heated as well as the residual conditions. Random samples of waveforms of the emission for specific levels of temperature were compared with respect to the amplitude and time period of the pulses as well as the occurrence of reflections etc. Consequently attempts were made to utilise acoustic emission behaviour in establishing the trend of bond degradation in reinforced concrete by heat effects. The control of variable was the level of temperature exposure.