Measuring and testing – Specimen stress or strain – or testing by stress or strain... – By loading of specimen
Patent
1992-02-21
1994-09-27
Raevis, Robert
Measuring and testing
Specimen stress or strain, or testing by stress or strain...
By loading of specimen
374 47, G01N 300
Patent
active
053498704
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
The present invention relates to a method and apparatus for detecting stress in an object in response to an applied load. It is often desired to determine stress in an object to which a working load has been applied. For example, it is desirable to determine the effect of the application of working loads on automobile suspension parts to determine whether, in use, there will be concentrations of stress.
Many methods have been utilised in the past, including utilising transparent models of the component part, applying a working load, and from the polarisation effects, determining the stress across the object. With such an arrangement, apart from the fact that the object itself is not being tested, but a replica thereof, it can be difficult to determine whether tension or compression is present in a particular area of the object and difficult to quantitatively determine the stress.
Another conventional way of testing for stress is the use of stress gauges which are attached to particular parts of the object under test. Clearly the stress gauges can only test for stress between two particular points and there are a limited number of points which one can examine in this way. An unusual configuration of stress concentration might be missed by such a method.
SUMMARY OF THE INVENTION
According to one aspect, the present invention provides a method of measuring stress at a point on an object comprising:
determining a load applied to the object; collecting and measuring thermal radiation from said point over a plurality of sample time intervals t within a predetermined minimum period of interest T, wherein t<T;
and, from said collected and measured values of thermal radiation and said determined load providing a measure of stress at said point.
According to a second aspect, the present invention provides a method of measuring stress at a point on an object comprising:
applying a cyclically varying load of period T to the object;
collecting and measuring thermal radiation from said point over a sample time interval t, wherein t<T;
determining the phase of the first interval t with respect to the cyclic variation of thermal radiation produced by said point in response to said applied load;
and, from said collected and measured value of thermal radiation and said phase, providing a measure of stress at said point.
We may also compare the phase of the thermal radiation received from said point with the phase of the applied load to determine whether the application of load submits the object at said point to tension or compression.
Thus, the described embodiment of the invention is based on the Kelvin effect in which the application of a load to an object changes the temperature of that object.
Stress in the form of tension increases the temperature and stress in the form of compression decreases the temperature. This should be distinguished from the hysterisis effect.
In a typical example, the sample interval t can be 50 .mu.s and the frequency of the cyclically varying load up to 10 kHz (i.e. T=10.sup.-4 s). However we prefer the first predetermined interval t to be less than T/4 and even more preferably less than T/8.
We would normally determine the peak value of the cyclic thermal signal, i.e. determine the value of the cyclically varying thermal radiation from said point when the phase of t corresponds to the peak of the cyclic thermal signal. We can either measure the value of the cyclic thermal signal at its peak or we can extrapolate from other values if we know the phase of the measured value. Thus, for example, if we know that the cyclic thermal signal is sinusoidal and that t is displaced by a phase angle of A degrees from the peak value of the maximum thermal signal, then from the value measured during the interval t, we can calculate the maximum value which it would reach if A=0.
Alternatively, we can compare the value of the thermal signal at t with the instantaneous value of the applied load and assume that the thermal signal and the applied load are in phase.
However
REFERENCES:
patent: 4378701 (1983-04-01), Mountain et al.
patent: 4541059 (1985-09-01), Toshihiko
Optical Engineering, vol. 26, No. 1 (Jan. 1987).
Hopkinson Gordon R.
Webber Martin J.
Raevis Robert
Sira Limited
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