RESIDUAL STRESS ANALYSIS IN THE TITANIUM BY EDDY CURRENT METHOD

Transcription

1 RESIDUAL STRESS ANALYSIS IN THE TITANIUM BY EDDY CURRENT METHOD S.Mekhlouf, M. Zergoug,Y.Benkedda, Division des Procédés Electriques et Magnétiques Centre de soudage et de contrôle, Route de Dely Ibrahim, B.P:64, Chéraga, Alger Tel: (213) (21) à 55, Fax: (213) (21) , Abstract Control by eddy current is one of techniques that are very used in nondestructive evaluation of materials. These processes find a significant application in the determination of the mechanical and metallurgical parameters of materials. The object of this work is to carry out an analysis of the residual stresses in the material created by external solicitations and analyses by eddy currents as a non destructive testing method. The analysis of the eddy current results will be compared to the results obtained by the traditional methods (X-rays diffraction ) in order to determine a relation between the non-destructive tests and destructive. It was shown in this work that all microstructure modifications of the samples were detected and they can be quantified by eddy current impedance measurement. The impedance analysis by eddy current will be correlated with the microstructure changes observed in the material because of plastic damage and fatigue. Keyword: NDT, residual stress, eddy current, impedance, diffraction X Introduction The ability to control the working stresses level in mechanical components and structures is an important factor in engineering industries. The characterization of microstructures, mechanical properties, deformation, damage initiation and growth by Non-Destructive Evaluation (NDE) techniques plays a vital role in various industries because of the growing awareness of the benefits that can be derived by using NDE techniques for assessing the performance of various components. Fracture mechanics based analysis of component integrity requires quantitatively characterization of microstructure defects as well as stresses. Any alteration in the microstructure, which reduces the life or performance, should be predicted sufficiently in advance in order to ensure safe, reliable and economic operation of the components. This prediction is possible with NDE techniques; so far the interaction of the non-destructive probing energy with the material depends on the sub structural / micro structural features such as point defects, dislocations, voids, micro and macro cracks, secondary phases, texture and residual stress. The stress sensitivity plays a very important role with respect to the different material properties.

2 Physical approach Various non-destructive techniques are available for the measurement of either applied and/ or residual stresses [3-8]. Eddy current testing is also sensible to changes in micro structural characteristics and the stress state of the material as well and can be used to evaluate these materials characteristics. Eddy current testing [1-2] allows to evaluate the state of stress in ferromagnetic material. The method can be used for determining residual stress, also named inner stress, as well as stress induced by external loads. In the study by Dybiec et al. [1] eddy current inspection is used to evaluate the state of stress in ferromagnetic material. Because a notable change in the magnetic characteristics can be observed, even at small values of strain degree, the technique has high sensitivity. Experimental approach The parts carried out were subjected to a definite experimental procedure in order to obtain different mechanical modification in material. We have chosen Titanium as non ferromagnetic material, for our study. The dimension samples are 250x40x2, 7 mm3. They were taken on the same sheet for the same structure. The test-specimens were cut out and were machined in conformity with the French standard. These test-specimens were subjected to tensile stresses. In order to be in strong solicitation case in the critical zones, we have taken two charges: - A load higher to the elasticity load (status 1) corresponding to F 1. - A load higher to the plasticity load (status 1) corresponding to F 2 - A load near to the breaking limit (status 2) corresponding to F 3. These forces were applied during two hour. Measurements by probe with eddy current were taken every 15 minutes. Acquisition systems were located on the critical zone level. The data acquisitions were done by a PC connected by an interface IEEE 488. Optimum condition works were taken in consideration in order to avoid systematic or experimental errors. In eddy current testing, it is necessary to carry out the characterization of the probe according to studied case. Study feasibility depends strongly on the probe used and the material treated. In this work, we will quote the impedance diagram only, because it summarizes in itself the stability of the probe and material used. Moreover it enables us to determine the optimal frequency allowing a maximum energy exchange between the probe and material and a satisfactory penetration depth. The comparison was done between the NDE results and X-rays diffraction. The data acquisition was done on the PC, using software carried out in graphic programming language. Results and Interpretation An NDT eddy current is very significant in the microstructures evaluation. We will study the behaviour of the impedance on the various solicitations applied to materials. The figure 1, 2, 3 shows a great stability in the diagram impedance. Figures (1-2-3) prove that the deformations are null. The trajectory of each load according to the frequency does not present a fluctuation.

3 For the plastic load, the material is in the microstructure agitation and in the impedance diagram, the loci is different for each time charge. For the plastic load and after a few time the material keep the new stability that explain the small variation in the impedance or the phase.the number of oscillation decrease for the F3 load and the material is near the rupture, the movement of the microstructure is very important and the material is the instability that is explained by the opening of the trajectories of impedance according the charge time. In this case the conductivity of this specimen increases and then the impedance decreases comparatively to the Plastic load. (Figures 1-2-3). A stability of the impedance with low oscillation is perceived for the load F2, contrary to the load F1 we observe the impedance decrease but the oscillations are more significant and the high amplitude. (Figure ). Figure -1-: Graph of impedance for a load F1=36347,6 N (Elastic zone) Figure-2- : Graph of impedance for a load F2=60000 N (Plastic zone) Figure -3-: Graph of impedance for a load F3 =80000 N (Zone near to the breaking limit) The method continues confirms the conclusion quoted for titan to see that the electromagnetic movements are random. The internal stress undergoes by the mechanical solicitation differ from a sample to another without differentiating in the content. By the constraint, micro structural movements are created in the metal which is accompanied by electromagnetic movements and consequently by the impedance oscillations. The movement of the magnetic fields is in a slight link with the internal constraints in the microstructure. We got to show that the fluctuations of impedance in function of the solicitation (load) either the plastic or the one near the breaking can provide information s on the materials status. These results are important in a way that we will be able to define that a material put under critical solicitations is near to the breaking.

4 z(ohm) z=f(t) t(min) z (o hm ) z=f(t) t(min) Figure -4-: Impedance according to solicitation time (charge F1) Figure -5-: impedance according to Solicitation time (charge F2) z=f(t) z(ohm) t(min) Figure -6-: impedance according to Solicitation time (charge F3) The X diffraction analysis shows that lines variation position in the 2θ indicates that residual stresses exist in material. This result is significant because it shows that the existence of the residual stress can be determined and thus quantified after, indeed the same shifts of the peak principal towards the right according to the applied load is noticed. The second remark is that the amplitude increases if the load F1 or F2 or the continuous test is applied (Figure-7-, Figure-8-, and Figure-9- ). Figure -7-: Diffractometry X, measure intensity according to the angle 2θ (Elastic test) Figure -8-: Diffractometry X, measure intensity according to the angle 2θ (Plastic test)

5 Figure -9-: Diffractometry X, measure intensity according to the angle 2θ (Continues test) Conclusion The investigation in this field being very competitive, the eddy currents NDT techniques can give by their sensitivities a significant place to solve complex mechanical and metallurgical problems in industry and the aerospace in particular. The simplicity of this technique and the various advantages which they offer in the determination of the intrinsic properties of materials allow the evaluation of material. Analysis NDT results confirm that the oscillations obtained by eddy current due to constraints in the material in the real-time caused by the tensile stress. Results obtained by NDT was confirmed by X diffraction. The same manner as x-diffraction, the NDT techniques shows fluctuations on the peaks amplitudes caused by the uniaxial stress. The most significant result treated by eddy current explain clearly that the samples are in unstable states by the constraint presence.this constraint would be quantified in the future and allows the installation prediction life analysis. References [1] Dybiec, C., Wlodarczyk, S. and Dybiec, M., "Measurement of Own Stress Using the Eddy Current Method", proceedings of the ECNDT, Vol. 3, December 1998 [2] C.Dybiec and all Determining Residual Austenite With Eddy Current Method, proceedings of the ECNDT, Vol3, December 1998 [3] K. Jassby, D. Saltoun, Use of ultrasonic Rayleigh waves for the measurement of applied biaxial surface stresses in aluminium 2024-T351 Alloy, Materials Evaluation 40 (1982) [4] N.Ye.Nikitina, L.A.Ostrovsky. An ultrasonic method for measuring stresses in engineering materials Ultrasonic.35. (1998)

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