Table of Contents ECNDT '98 Session: Electromagnetic Methods

Measurement of Own Stress Using the Eddy Current Method C. Dybiec, S. Wlodarczyk, M. Dybiec Institute of Precision Mechanics UL. Duchnicka 3, 00-967 Warszawa

Introduction

The eddy current method allows to evalute the state of stress in ferromagnetic material. The given method is used for determining own stress as well as that formed in effect of outside load. With regard to physical principles of own stress analysis, the dependence between the magnetic permeability and the distance between atomic surfaces is utilized.
In effect of loads acting in the scope of elastic deformations (proportional) in polycristalline bodies, changes in distances between atomic surfaces of the net are observed, which change the initial magnetic permeability.

Devices

Equipment type "Wirotest 202 A and 301" are used for evaluating the degree of material relief in the joint zone(changes of stress).The saidequipment is characterized by high sensitivity of indication measurements regarding changes occuring in the material structure, as the current resonance phenomenon is used.
The measuring probe is characterized by indication directions, what allows for the precise defining of directions of maximum tensile as well as compressive stress activity.

Calibration of the measuring system

The measuring system composed of the "Wirotest" type equipment , the direction probe and reference sample are subject to calibration while using a strength machine.
The reference sample in form of a cylinder or bar with probe placed on top of it, is subject to tensile or compressive loading using strength machine ("Instron" type).

Tensometer indications illustrating the extension of the reference element constitute one coordinate of the dependence graphs of the indications and the size of stress, and the"Wirotest" indications constitute the second coordinate.
The obtained graph is the basis for evaluating the stress while applying the probe to controlled elements made of the same material and subjected to identical thermal processing as the reference sample.
When attempting the measurements the system should be zeroed with the aid of the reference element, which should remain close to the studied element for at least 8 hours.
The condition for zeroing the system before the measurement is caused by the need to diminish the influence of temperature change on the test result.

The test result

The shown in Fig. 1. results of stress measurement originating from external loading, have shown, that within the range of tensile deformations the dependence of indications on the loading is linear, whereas after exceeding the propotionality limit the angle of curve inclination changes.

Fig. 1: Dependence of indications "Wirotest () on the tensile stress in a bar Ø 33 mm made of steel 55.	Fig. 2: Dependence of "Wirotest () indications as a function of tensile stress, when applying different lengths between the probe and the tested surface.

In Fig. 2. it is shown, that the stress can be evaluated by placing the probe directly on the tested surface as well as at a certain distance from it. Test results indicate to the possibility of evaluating stress in parts covered with a protective layer (vanish or non-ferromagnetic electrolytic coating)

Fig. 3. shows results of "Wirotest 202 A" indications as a function of compressive stress for cylindrical samples Ø18 mm made of steel LH 15, what is turn indicates that the dependence of indications on compressive stress is linear in the range up to 180 MPa with regard of quenched (1) and up 120 MPa with regard to annealed (2) samples.

Fig. 3: Results of "Wirotest 202A" indications as a function of compressive stress of cylinder samples Ø18 mm LH 15 after quenching (1), annealing (2).

In the annealed cylinder the presence of own compressive stress has been observed, which was disclosed as lack of indications changes in the initial stretching phase, and their value may be evaluated on the basis of extrapolating the graphs to the coordinate axis.

Fig. 4: Measurement results of a pin of dimensions Ø 46/15,5 as a function of tensile and compressive stress using the "Wirotest".

In Fig. 4. measurement results of a pin of dimensions Ø 46/15,5 have been shown as a function of tensile and compressive stress using the "Wirotest" , where the course of the graph of these dependence on the load in the linear scope of indications is similar to the compressive as well as the tensile stress.
Whereas in the scope of plastic deformations differences are observed. Arc welding of pipes Ø 32 mm, wall thickness 6,5 mm has caused own tensile stress of 260 MPa in the jont, relief at 720 °C during 4 hours, has caused a lowering of stress to 60 MPa.

Results of own stress measurements in the pipe joints zone applied in high power, have been shown in Fig. 5 and 6.
The changing of welding method to the vibro-contact has caused a lowering of own tensile stress to 180 MPa, whereas in the joint the stress is close to zero. After annealing the maximum value of stress has reached 20 MPa .

Fig. 5: Course of stress in pipes with Ø 32 mm and wall thickness 6,5 mm after welding (1) and after annealing (2).

Fig. 6: Course of stress in pipes with Ø 32 mm and wall thickness 6,5 mm made of steel10H2M after vibro-contact welding.

The calibration graph for the probe using a strength machine, has been shown in Fig. 7. It can be observed that the dependence of indications of the device of "Wirotest" type on the loading is linear within the proportionality limit scope. After unloading the indications do not return to zero, but show own stress caused in effect of plastic deformation of the tested sample.

Fig. 7: The dependence between "Wirotest" and the tensile stress value.

Conclusions

The method of eddy currents allows to evaluate own stress very precisely, as well as those caused in effect of external loading. The use of the contact probe with a 10,00 mm diameter allows to for precise evaluation of stress in different control point of objects.
The "Wirotest" type equipment manufactured by the Institute of Precision Mechanics, allows for evaluating own stress of materials.

Literature

1. Szydlik W: Economic - technical test conditions presented by UD and RTG for welded joints.
   Materials of the 21 st. National Conference on Non-destroying methods, Szczyrk 1992, p. 127-132.
2. Johnston D.: Aboveground storage tank floor inspection using magnetic flux leakage Material Perform t.31 no 10 1990 p.36-39.
3. Wlodarczyk S., Dybiec Cz., Wlodarczyk W., The application of eddy currents and other techniques for measuring corrosion of metal tanks during exploitation. Materials CCI National Conference about "Science and practice in fighting against corrossion", Kule May 1994.
4. Dybiec Cz. Eddy current method for defecting intercrystalline corrossion. Materials XXIII KKBN, Szczyrk 1994.
5. Czeslaw Dybiec, Stefan Jankowski, Sylwia Wloarczyk, Control of stress and structural changes using the eddy current method. Materials XXX KKBN. Szczyrk 1996.