Electricity: measuring and testing – Impedance – admittance or other quantities representative of... – Lumped type parameters
Patent
1991-01-25
1993-04-13
Strecker, Gerard R.
Electricity: measuring and testing
Impedance, admittance or other quantities representative of...
Lumped type parameters
324718, 324713, 324603, G01R 2714, G01N 2720
Patent
active
052026418
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The present invention concerns improvements in and relating to methods, test probes and apparatus for the measurement of alternating current potential drop such as adopted for example in the monitoring and measurement of the depth of cracks generated in materials under strain.
Among the various techniques available for monitoring crack growth, the potential drop (PD) method has been widely used because it is relatively simple yet accurate. The PD technique depends on the relationship of the electrical resistance of a cracked body to and depth of the crack. The voltage drop across the crack generated by passing a constant current through the specimen is used to monitor the crack depth. Either direct or alternating current may be used to generate the voltage across the specimen (DCPD or ACPD respectively). The DCPD technique is more commonly used because of its apparent simplicity. The DCPD measurement however has several disadvantages which are effectively overcome by using ACPD.
The accuracy of measurement using the DCPD technique is limited by several errors as described below. few .OMEGA.. Large currents of the order of 30 to 100 amperes are therefore necessary to get a measurable signal. If the PD is to give a direct measure of resistance, then for accurate reproducible measurement of crack depth the drift in the value of this large current has to be kept to a minimum. To achieve a resolution of 0.01 mm in crack depth measurement, with a specimen geometry generally used, the current has to be constant to within .+-.0.01%. between the specimen and the measuring leads due to temperature differentials can be of the order of .mu.V. In the case of iron/copper for example for a difference of only 1.degree. C. between the measuring leads the thermal EMF is about 11 .mu.V. This can cause an error of the order of 1 mm in crack depth measurement. PG,5 amplifier can be of the order of 1 .mu.V per degree centigrade. A 5.degree. C. variation of temperature during a test can therefore cause an error of about 0.5 mm in crack depth measurement. temperature coefficient of resistance of steel which is about 5.times.10.sup.-3 per degree centigrade. The error caused by a 5.degree. C. change in temperature would be about 2.5%. point, the crack tip, causing undesirable conditions leading to further errors in measurement.
Some of these errors can be minimized by interrupting or reversing the large d.c. current at regular intervals and subtracting out the error voltages. This however adds considerably to the complexity of the already bulky power supplies.
When a conductor carries an alternating current at high frequencies, the current flows near the surface of the conductor due to the "skin effect". The strength of this effect is given by the depth of penetration 6 which is termed the "skin depth". The stronger the skin effect the smaller the skin depth The skin depth is given by the relation ##EQU1## where .rho. is the resistivity, .mu. is the magnetic permeability and f is the frequency.
Advantages of ACPD over DCPD are: has a much greater signal to current ratio. Also phase sensitive detection may be employed readily to enhance signal over noise ratio. sensitivity of ACPD (i.e. high signal to crack growth ratio) and its sensitivity to the onset of elastic and plastic strain.
It is however essential to observe certain precautions whilst using ACPD. It is a problem that the output from the PD sensing leads in addition to the specimen potential drop also contain voltages picked up from the electromagnetic field radiated by the supply current to the specimen. If the electromagnetic pick-up is an appreciable fraction of potential drop voltage, it can affect the initial calibration and can cause discrepancies. This error becomes serious for non-magnetic materials (e.g. stainless steel or aluminium) where higher frequencies have to be used with a consequent increase in electromagnetic radiation and pick-up.
The accuracy of measurement can also be limited by crack geometry. For a short deep crack, the current can f
REFERENCES:
patent: 2094234 (1937-09-01), Drain
patent: 3699436 (1972-10-01), Shigematsu et al.
patent: 4266185 (1981-05-01), Charlesworth et al.
patent: 4683419 (1987-07-01), Neuelmann et al.
Do Diep
Goldberg Richard M.
Matalect Limited
Strecker Gerard R.
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