Measuring and testing – Specimen stress or strain – or testing by stress or strain... – By loading of specimen
Patent
1980-01-31
1983-04-05
Ciarlante, Anthony V.
Measuring and testing
Specimen stress or strain, or testing by stress or strain...
By loading of specimen
374 47, 374121, G01N 332
Patent
active
043787010
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to a method and apparatus for indicating stress in an object.
There are a large number of non-destructive testing techniques at present in use for the analysis of stresses in mechanical structures but many have problems associated with them. Some are difficult to apply and give chiefly qualitative results (eg. brittle lacquers and photo-elasticity), some require prior knowledge of the position and direction of the principal stresses and need careful application (eg bonded strain gauges) and others require complicated apparatus and expert interpretation (eg X-ray or Gamma-ray photography, acoustic emission techniques, laser holography, and moire fringe methods).
It has been found that heat is produced or absorbed at points of stress in a material undergoing any form of loading or vibration which is believed to be due to the reversible elastic deformation in the material and this produces small, localised temperature changes which may be positive or negative and which are generally proportional to the instantaneous stress magnitude. The present invention incorporates a method and apparatus for utilising this phenomenon.
The present invention provides a method for indicating and distinguishing tension and compression in an object comprising changing the stress in the object and measuring and distinguishing increases and decreases in temperature of a predetermined part of the object with change of applied stress.
In one method the change of stress may be uni-directional and in this case may be caused by a physical shock applied to the object such as a hammer blow.
In an alternative arrangement the stress change may be random and not in a particular direction and under these circumstances the stress change may arise naturally. For example the stress change may arise as random loading of a particular structure such as a bridge.
In a further preferred method, the stress change is cyclic. An impressed or natural cyclic stress change allows conventional signal analysis to be utilised. Thus in a preferred arrangement the cyclic stress change occurs at a predetermined frequency and then by filtering the signal resulting from the measurement of the temperature other effects such as changes in ambient temperature may be eliminated.
The stress change is preferably applied between two spaced points of the object and the predetermined part is arranged to be between these two spaced points. In a preferred arrangement the stress may be produced hydraulically or pneumatically.
In a special case, in which the object is hollow, the stress may be produced by internal hydraulic or pneumatic pressure.
A number of ways may be provided for measuring this temperature change such as contacting techniques using thermocouples or thermistors but we particularly prefer to measure the temperature change by a sensitive detection system which measures the change in thermal radiation emitted by surface areas within the field of view. Such an arrangement may utilise an infra-red detector which preferably may include optical components and which may be movable and of variable focus so that the predetermined part whose temperature is measured is variable.
In a particularly preferred arrangement, the predetermined part of the object whose temperature is measured is scanned across the object.
Before carrying out the method, we may apply a coating to the object having uniform thermal emissivity. The layer may be a thin chemically formed layer and if the object is of metal may be a salt or other compound of that metal.
The present invention also provides a method of determining discontinuities such as cracks, cavities and the like in an object comprising applying a symmetrically cyclic stress to the object and measuring the variation in temperature of a predetermined part of the object and determining any asymmetry in the wave form of the temperature variation.
The invention also provides a method for displaying stress in an object comprising applying a varying stress to the object and presenting a pictorial display of
REFERENCES:
patent: 1680589 (1928-08-01), Bock
patent: 3541851 (1970-11-01), Youmans
patent: 3934452 (1976-01-01), Prevorsek et al.
Allnutt Anthony J.
Baker Lionel R.
Cox Laurence J.
Mountain David S.
Picot Alan J.
Ciarlante Anthony V.
Sira Institute Limited
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