Thermal measuring and testing – Thermal testing of a nonthermal quantity
Patent
1986-06-24
1989-01-17
Yasich, Daniel M.
Thermal measuring and testing
Thermal testing of a nonthermal quantity
374 5, 374121, G01N 2117, G01N 2500
Patent
active
047984775
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to thermo-elastic stress analysis and in particular to the remote measurement of stress by non-contacting means.
Thermoelastic Stress Analysis, as described in U.S. Pat. No. 4,378,701 issued Apr. 5, 1983, is a system whereby stress in the material of a structure or component can be measured remotely and by non-contacting means, whilst subjected to dynamic loading. It makes use of the property of thermoelasticity by which very small amounts of heat are generated or absorbed when the material undergoes a compressive or a tensile stress, thereby producing small changes in temperature. This phenomenon is perhaps better appreciated in the case of a gas, it being well known that under adiabatic compression there is a rise in temperature and conversely, under expansion, the gas cools. The method of measuring the minute temperature changes involved employs an extremely sensitive infra-red detector, specially developed signal processing, and a scanning mechanism to provide, for example, a real time map of the stress pattern of the object under test on a colour TV monitor. The performance achieved can discriminate temperature down to 0.001.degree. C. (representing a stress discrimination of 145 lb/in.sup.2 in steel) with a spatial resolution of better than one mm.
Thermoelastic Stress Analysis as described in the above patent is an extremely valuable mechanical engineering tool by measuring stress under dynamic loads such as are often encountered in service conditions and supplements and in some cases supersedes previously employed methods such as brittle lacquers, strain gauges and photo-elastic techniques. Static stress, however, can only be measured using these existing techniques if the loads are capable of being applied and removed at will, albeit in slow time. However examination in both a stressed and unstressed state is often impossible, for example in the case of stresses produced during a manufacturing process, eg residual stresses on welding, or as the result of assembly and although a knowledge of the usable strength remaining may be vital to the safety of a structure or component, there are no methods at present available to the engineer or designer which can satisfactorily provide this information.
The object of the present invention is to provide a method and apparatus for measuring static stress in a body.
The present invention provides in one form a method for indicating static stress in a predetermined part of an object comprising the steps of: predetermined surface part of the object; part of the object; and stress in the predetermined part.
The invention relies upon the dependence of the specific heat of an object under inspection on local stress. Thus by determining the specific heat from the temperature increase or observing variations in the specific heat across the surface of an object it is possible to obtain an indication of the static stress at a point on the surface or a map of the variation of static stress across the surface. The stress-dependence of the specific heat is such that a very sensitive measuring technique is required. The quantity of heat applied to a surface part must be accurately known and the temperature sensor desirably should be capable of measuring temperature changes down to 0.001.degree. C.
Preferably the temperature is measured by detection of changes in thermal radiation emitted by the surface area of the predetermined part of the object. Advantageously the surface area of the object is scanned such that the predetermined part whose increase in temperature is measured is variable.
Variations of temperature caused by factors other than by stress related variations in specific heat should thus be minimised.
As a first step with some materials it may be desirable to coat the object with a material to make the energy absorption and emissivity as uniform as possible.
In a preferred arrangement the energy is applied cyclically to the predetermined part of the object. Advantageously the source of energy is a pulsed laser operating in the opt
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EPO document No. 0105078, (Vanzetti), 4/11/84.
WO 79/01156 to SIRA Institute, 12/27/79; GOIN 25/72, (Stevenson et al. applicants).
The Secretary of State for Defence in Her Britannic Majesty's Go
Yasich Daniel M.
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