GB2031589A

GB2031589A - Non-destructive testing techniques

Info

Publication number: GB2031589A
Application number: GB7930541A
Authority: GB
Original assignee: UK Atomic Energy Authority
Current assignee: UK Atomic Energy Authority
Priority date: 1978-09-08
Filing date: 1979-09-04
Publication date: 1980-04-23

Abstract

A material containing residual stresses is interrogated by a roving eddy-current probe B whilst an unstressed region of the material is interrogated by a static probe A. The differential of the signals is visually displayed to highlight the stressed regions. <IMAGE>

Description

SPECIFICATION Non-destructive testing techniques This invention relates to non-destructive testing techniques and has for an object an extension of the range of such techniques to include one for the evaluation of residual stresses in a material.

According to the invention a method for evaluating residual stresses in a material by a nondestructive testing technique comprises subjecting the material to alternating electromagnetic fields, comparing signals analogous to disturbances in the electromagnetic fields caused by residual stresses in the test material with reference signals analogous to zero stress in the material, and displaying the differential of the signals.

Apparatus for performing the method according to the invention may comprise an eddy-current search probe for interrogating a region of a material subjected to residual stress and an eddy-current reference probe for sensing in an unstressed sample of the material, the probes each comprising an exciting coil for generation of an electromagnetic field and at least one sensing coil for detecting disturbances in the electromagnetic field, the sensing coils being connected through a differential bridge circuit to apparatus for visually displaying the differential signal. In a preferred apparatus there is means for indicating the position of the search probe relative to the material in a manner such that the visual display indicates the position of the disturbances caused by the residual stresses in the material.

Three examples of experimental tests made in accordance with the invention are described, by way of example, with reference to the accompanying diagrammatic drawings wherein: Figure 1 is a view of apparatus for inducing a compressive stress in a test piece of material, Figure 2 is an isometric view of the test material, Figure 3 is an isometric view of a first test equipment, Figure 4 is a view of an oscilloscope display, Figure 5 is a plan view of the test material showing the path of a probe, Figure 6 is a view of a second oscilloscope display, Figure 7 is a plan view of a third test apparatus, and Figure 8 is a line diagram of electronic circuitry.

In the first experimental test a piece of mild steel 1/16" (1.6 mm) thick and 12 in. (300 mm) square was subjected to compression under the wheel of a fettling press as shown in Figure 1 to produce a strip of material containing residual compressive stress.

A second strip parallel to the first strip and containing residual tensile stress (Figure 2) was produced by bending the plate over a radius. As shown in Figure 3 a static eddy-current probe designated A comprising an exciting coil and a sensing coil was positioned at the centre of the plate and a matching eddy-current probe designated B was arranged to be traversed across the plate by a linear transducer Tin such a manner that the probe B passed successively over the two strips containing the residual stresses.

The sensed signals, being analogous to the disturbances in the electrom')agneticfields caused by the stresses, were fed to a differential bridge circuit and the resultant signal displayed on an oscilloscope. A baseline displayed on the oscilloscope was produced by a signal derived from the linear displace ment transducer T so that the position of the residual stresses relative to the test material could be indicated. The resultant visual display is illustrated in Figure 4 and it can be seen that the tensile and compressive residual stresses produced signals of opposite sense. The compressive stress produced a stronger signal than the tensile stress indicating a relatively greater stress as would be expected in view of the manner of inducing the stresses in the material.

In a second test a sine co-sine potentiometer was used and a circle was described by a roving probe B the static probe A being located in an unstressed corner region of the plate as shown in Figure 5. The circle described by the probe B passed over the stressed strips each in two places. The differential signal was displayed on a circular baseline produced by a signal from the potentiometer and each of the four areas of stress were displayed on the oscilloscope as illustrated in Figure 6. The broken lines t and c indicate the tensile and compressive stressed strips respectively and again it can be seen that the tensile and compressive stresses produced signals of opposite sense.

In a third test two linear transducers are used to describe any curve on the plane of a sheet and to relate the response of the moving probe B (mounted at the junction of the transducers) to the balanced condition obtained from the stationary probe A shown in Figure 7.

Circuitry for the third test is shown in Figure 8 and comprises an oscillator 1 of frequency variable between 500 Hz and 2.5 MHz, connected to a pair of Maxwell bridge systems 3 and 4 which are connected through amplifiers 5 and 6 to a pair of phase-sensitive amplifiers 7 and 8. Input signals from a reference unit which is a phase sensitive display with 3600 variable control are also fed to the phase-sensitive amplifiers 7, 8, the output signals therefrom being fed to a digital voltage display unit 9 by way of coherence function generator 10.

The arrangement of apparatus for the third test is applicable to the testing of a wider range of subjects, for example, those which are comprised in a complete geometrical form or shape which the sensors are able to follow so that analyses of stress in complex assemblies such as chain links, forgings and pressing can be made.

1. A method for evaluating residual stress in a material comprising subjecting the material to alternating electromagnetic fields, comparing signals analogous to disturbances in the electromagnetic fields caused by residual stresses in the test material with reference signals analogous to zero stress in the test material and displaying the differential of the signals.

2. Apparatus for evaluating residual stresses in a

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. SPECIFICATION Non-destructive testing techniques This invention relates to non-destructive testing techniques and has for an object an extension of the range of such techniques to include one for the evaluation of residual stresses in a material. According to the invention a method for evaluating residual stresses in a material by a nondestructive testing technique comprises subjecting the material to alternating electromagnetic fields, comparing signals analogous to disturbances in the electromagnetic fields caused by residual stresses in the test material with reference signals analogous to zero stress in the material, and displaying the differential of the signals. Apparatus for performing the method according to the invention may comprise an eddy-current search probe for interrogating a region of a material subjected to residual stress and an eddy-current reference probe for sensing in an unstressed sample of the material, the probes each comprising an exciting coil for generation of an electromagnetic field and at least one sensing coil for detecting disturbances in the electromagnetic field, the sensing coils being connected through a differential bridge circuit to apparatus for visually displaying the differential signal. In a preferred apparatus there is means for indicating the position of the search probe relative to the material in a manner such that the visual display indicates the position of the disturbances caused by the residual stresses in the material. Three examples of experimental tests made in accordance with the invention are described, by way of example, with reference to the accompanying diagrammatic drawings wherein: Figure 1 is a view of apparatus for inducing a compressive stress in a test piece of material, Figure 2 is an isometric view of the test material, Figure 3 is an isometric view of a first test equipment, Figure 4 is a view of an oscilloscope display, Figure 5 is a plan view of the test material showing the path of a probe, Figure 6 is a view of a second oscilloscope display, Figure 7 is a plan view of a third test apparatus, and Figure 8 is a line diagram of electronic circuitry. In the first experimental test a piece of mild steel 1/16" (1.6 mm) thick and 12 in. (300 mm) square was subjected to compression under the wheel of a fettling press as shown in Figure 1 to produce a strip of material containing residual compressive stress. A second strip parallel to the first strip and containing residual tensile stress (Figure 2) was produced by bending the plate over a radius. As shown in Figure 3 a static eddy-current probe designated A comprising an exciting coil and a sensing coil was positioned at the centre of the plate and a matching eddy-current probe designated B was arranged to be traversed across the plate by a linear transducer Tin such a manner that the probe B passed successively over the two strips containing the residual stresses. The sensed signals, being analogous to the disturbances in the electrom')agneticfields caused by the stresses, were fed to a differential bridge circuit and the resultant signal displayed on an oscilloscope. A baseline displayed on the oscilloscope was produced by a signal derived from the linear displace ment transducer T so that the position of the residual stresses relative to the test material could be indicated. The resultant visual display is illustrated in Figure 4 and it can be seen that the tensile and compressive residual stresses produced signals of opposite sense. The compressive stress produced a stronger signal than the tensile stress indicating a relatively greater stress as would be expected in view of the manner of inducing the stresses in the material. In a second test a sine co-sine potentiometer was used and a circle was described by a roving probe B the static probe A being located in an unstressed corner region of the plate as shown in Figure 5. The circle described by the probe B passed over the stressed strips each in two places. The differential signal was displayed on a circular baseline produced by a signal from the potentiometer and each of the four areas of stress were displayed on the oscilloscope as illustrated in Figure 6. The broken lines t and c indicate the tensile and compressive stressed strips respectively and again it can be seen that the tensile and compressive stresses produced signals of opposite sense. In a third test two linear transducers are used to describe any curve on the plane of a sheet and to relate the response of the moving probe B (mounted at the junction of the transducers) to the balanced condition obtained from the stationary probe A shown in Figure 7. Circuitry for the third test is shown in Figure 8 and comprises an oscillator 1 of frequency variable between 500 Hz and 2.5 MHz, connected to a pair of Maxwell bridge systems 3 and 4 which are connected through amplifiers 5 and 6 to a pair of phase-sensitive amplifiers 7 and 8. Input signals from a reference unit which is a phase sensitive display with 3600 variable control are also fed to the phase-sensitive amplifiers 7, 8, the output signals therefrom being fed to a digital voltage display unit 9 by way of coherence function generator 10. The arrangement of apparatus for the third test is applicable to the testing of a wider range of subjects, for example, those which are comprised in a complete geometrical form or shape which the sensors are able to follow so that analyses of stress in complex assemblies such as chain links, forgings and pressing can be made. CLAIMS

2. Apparatus for evaluating residual stresses in a material comprising an eddy-current search probe for interrogating a region of material subjected to residual stress and an eddy-current reference probe for sensing an unstressed sample of material, the probes each comprising an exciting coil for generation of an electromagnetic field and at least one sensing coil for detecting disturbances in the electromagnetic field, the sensing coils being connected through a differential bridge circuit to apparatus for visually displaying the differential signal.

3. Apparatus according to claim 2 wherein there is means for indicating the position of the search probe relative to the material in a manner such that the visual display indicates the position of the disturbances caused by residual stresses in the material.

4. A method of evaluating residual stresses in a material substantially as hereinbefore described with reference to the accompanying drawings.

5. Apparatus for evaluating residual stresses in a material substantially as hereinbefore described with reference to the accompanying drawings.