Stress Test Based on Planar Flexible Eddy Current Sensor

Transcription

1 5th International Symposium on NDT in Aerospace, 13-15th November 2013, Singapore Stress Test Based on Planar Flexible Eddy Current Sensor TANG Ying 1, LI Ji 1, ZHAO Jianqiang 1, HE Yunze 1, TAN Xianglin 2, CHEN Dixiang 1 ( 1 College of Mechatronics Engineering and Automation,National University of Defense Technology, Changsha ,China; 2 Wuhan Mechanical College,Wuhan,430075, China) Abstract: Stress is one of the main causes of metal parts fatigue and fracture, so it has important meaning to measure the stress of the metal key components. In this paper, the finite element modeling and simulation are used for the planar flexible eddy current sensor, and the sensor response is expressed for the planar measurement grid in the form of a database, and the designed sensor is used for tensile stress testing of aluminum alloy 3A21, then the relationship of aluminum alloy conductivity and the tensile stress is obtained. The experimental results show that the conductivity of the conductive material and the tensile stress has linear inverse relationship in the elastic range of aluminum alloy, and the method based on the planar flexible eddy current sensor s transfer impedance can be used for measurement of conductive material s conductivity and evaluation of its stress. Keywords: planar flexible eddy current sensor,finite element simulation, conductivity,stress measurement 1 Introduction The main cause for fatigue damage of metal parts is all kinds of micro and macro mechanical stress concentration. So stress is not only an important factor of parts fatigue and fracture, but also a key parameter representing its early quality. It is necessary for early failure prediction of key parts to effectively evaluate its stress and deformation state [1-2]. At present, the main nondestructive test methods being used for stress evaluation measurement are: X-ray diffraction, ultrasonic testing, magnetic memory testing, magnetic elasticity testing and eddy current testing [3]. The main shortage of X-ray diffraction is: The test equipment is expensive, the test operation is demanding and the tested residual stress in depth is insufficient [4]. The ultrasonic methods has higher testing depth than X-ray diffraction, but it must use couplant, which means high requirement on material surface. Also, it is restricted by thickness of the tested material [5]. The magnetic memory testing method doesn t need couplant and special magnetizing device, so it is suitable for outfield use. But as it is a weak magnetic signal testing method, it may be easily be interfered by the environment [6]. Stress testing using Barkhausen noise, magneto-elastic effect and magnetic acoustic emission in magnetic elasticity testing are all magnetic nondestructive testing methods which are convenient and quick but can only be used to magnetic material [7-8]. Piezoresistive effect of conductive material makes eddy current testing method more advantageous over other methods in stress evaluation. The planar flexible eddy current sensor manufactures the exciting coil and the testing unit on the same flexible substrate. It can test lift-off and conductivity independently at the same time. These characteristics make it easy to fit tested material with more complex surface. This paper studies the correlation of conductive material s conductivity and stress based on planar flexible eddy current sensor.

2 2 Piezoresistive effect The resistance R of rectangular conductor can be expressed as: R l S = (1) where is resistivity and its unit is Ω m; l is length and its unit is m;s is the conductor s cross-sectional area and its unit is m2. Conductivity is the reciprocal of with a unit of S/m. When the conductor deforms in the direction of length, l S change accordingly, which will lead to change of resistance finally. Suppose the strain in the direction of length is ε, the Poisson s ratio of the conductive material is µ, so the relative change of resistance can be expressed as equation (2): dr d = (1 + 2 µ ) ε + (2) R In equation (2), (1 + 2 µ ) ε is the material conductivity change caused by the change of the conductor s geometrical shape. As the thickness of tested conductor is larger than that of the eddy current penetration, this term can be neglected. d is decided by the relative change of resistivity caused by deformation of the conductor. So the conductivity of the tested material can be used to evaluate stress. When a one-way tensile stress is loaded on an isotropic conductor, the relation between its resistivity and the stress can be expressed by equation (3): ii o = ( π τ, π τ, π τ ) (3) 12 zz 12 zz 11 zz In (3), ii is resistivity in three directions of x\y\z, π ij is piezoresistive coefficient of the material which includes longitudinal piezoresistive coefficient π11 and transverse piezoresistive coefficient π Stress testing principle of planar flexible eddy current sensor 3.1 Planar flexible eddy current sensor The sketch of planar eddy current sensor is shown in figure 1. It has the same principle with traditional eddy current sensor, which is based on eddy current effect and made up of single-turn exciting coil and inductive coils. Under the effect of alternating current I D, the exciting coil produces spatially periodic magnetic field. The inductive coils distributed on both sides of the exciting coil will produce induced voltage V S. Response of the sensor has relations with geometrical and physical characteristics of the tested material and frequency of the exciting signal. Define the ratio of induced voltage and exciting current as transfer impedance, which is recorded as Z. Z is a complex number which can be expressed as:

3 Fig. 1 Sketch of planar flexible eddy current sensor. VS Z = = R + jx (4). I D The useful information, such as conductivity of the tested material, can be extracted from the transfer impedance measured by (4) and the planar measuring grid computed by numerical computation of the computer. 3.2 Finite-element simulation of the sensor Three-dimensional modeling and analysis on the planar flexible sensor which is shown in figure 2 is done with finite-element software ANSYS. Structure and size of the model are the same with real sensor. The width of the coil is 0.2mm, the thickness is 0.35mm. Space between the exciting coil and the inductive coil is 0.1mm. The frequency of exciting coil is the same with that in the experiment, which is set to f=500khz based on the result of the sweep experiment. After calculation response of the sensor can be got, namely the transfer impedance. Fig.2 Model of planar eddy current sensor 3.3 Planar measuring grid Before actual measurement, simulation calculation according to the predicted range of lift-off and change of conductivity is made, and the transfer impedances being got are formed into database. A definite mapping relation exists between the database and lift-off-conductivity. The database can be expressed as planar grid, as is shown in figure 3. The transfer impedance being got from simulation calculation is not the same with that in actual measurement. This is because the calculation result is based on ideal condition, for example, the effect of connecting wire is not considered. So the measurements should be calibrated.

4 Here the air calibration method is used. The sensor is put in air, and simulation calculation and actual experimental measurement are conducted separately. Calculate their differences of the real and imaginary parts separately, subtract these differences from all the measurements and get calibrated real and imaginary parts of the transfer impedances. Search and interpolate in the database according to these value and finally get conductivity and lift-off of the tested material [9] Lift-off 0.04 Conductivity Real part of transfer Impedance (Ω) Fig.3 Lift-off conductivity measuring grid 4 Experiment 4.1 Experimental equipments In order to analysis the correlation between response of the planar flexible eddy current sensor and stress of the tested material, we conduct a stress tensile experiment on aluminum alloy. The main experimental equipments include: 1) Impedance analyzer High precision impedance analyzer made by Wayne Kerr Electronics of UK is used for measuring transfer impedance. Its exciting frequency is 20HZ~20MHZ. 2) Planar flexible eddy current sensor, as is shown in figure 4. Frequency of the exciting signal is 500KHZ. 3) Loaded test piece. The sample in the experiment is rolled aluminum alloy 3A21 with 200mm in length, 40mm in width and 2mm in thickness. The yield strength is about 100MPa. 4) The model of the tensile testing machine is WDW-E100D, which is an electronic universal testing machine controlled by microprocessor. Its maximum loading capacity is 50 KN. As is shown in figure 4, in the tensile test, both ends of the sample are fixed on the testing machine. The sensor is fixed on one side of the sample, other faces of the sample are free. Adjust the machine to tensile state. The test is conducted in elastic range of the material. 4.2 Experimental result Increase the tension gradually from 0 to about 2.5KN. 5 repeated measurements of transfer impedance are conducted under each tension and mean of the measurements is recorded as the result. The tensions are, in order, kn kn kn kn kn, and the corresponding stresses are 2.415Mpa Mpa 7.54 Mpa Mpa Mpa respectively.

5 Fig.4 experimental equipments Use the difference of the measured air point impedance and that of the calculation as calibrating parameter, subtract it from the originally measured impedance and get calibrated transfer impedance, then label it in the corresponding planar measuring grid, as is shown in figure 5. Conductivity and lift-off can be found by using searching interpolation algorithm. Imaginary part of transfer impedance (Ω) Fig.5 Measured conductivity Draw out the conductivity and the stress, as is shown in figure6. Measurement of conductivity (MS/m) Fig. 6 Relation between stress and conductivity It can be concluded from figure 6 that conductivity decreases almost linearly with the increasing of tension. This is a typical characteristic of metal. Use linear fitting method with the following fitting equation: σ = σ The residual is C S

6 So the experiment proves that the planar flexible eddy current sensor can be used to evaluate distribution of conductivity and the stress state of the tested conductive material. 5 Conclusion This paper analyzes both piezoresistive effect and the relation between conductivity change and the stress of conductor. Conduct experiment on the tested aluminum alloy using planar flexible eddy current sensor. Combining with simulation computation, conductivity of the conductor is got according to transfer impedance. The experimental result shows that planar flexible eddy current sensor can be used to evaluate the stress state of conductive material. Reference 1. JAMES M N, HUGHES D J, CHEN Z, et a1. Residual stresses and fatigue performance [J]. Engineering Failure Analysis, 2007, 14(2):384~ ZHOU D Q, TIAN G Y, WANG H T, et al. Evaluation of applied stress using pulsed eddy current technology [J]. Chinese Journal of Scientific Instrument, 2010, (31):1588~ MENDIBIDE C, STEYER P, ESNOUF C. et a1. X-ay iffraction analysis of the residual stress state in PVD TiN/CrN multilayer coatings deposited on tool steel[j]. Surf. Coat. Techno, 2005(200): 165~ CHAIB M O S, DJELOUAH H, BOUTKEDJIRT T. Propagation of ultrasonic waves in materials under bending forces[j]. NDT&E Int. 2005(38): CHE J T, HOU Q, YU J. Contrast analysis of testing methods for tiny crack in parts of weapon [J].Ordnance Material Science and Engineering, 2005,9(28):44~ XU ZH X. Feasibility for predicting discontinuous fracture of aeronautical part by using the residual stress detecting method [J]. Aeronautical Manufacturing Technology, 2004(05): WILSON J W,TIAN G Y,BARRANS S. Residual magnetic field sensing for stress measurement [J].NDT&E Int. 2007(135):381~ ZHAO J Q, WENG F B, PAN M CH, et al. Finite-element model and experimental study on planar sensor array [J]. Measurement Technique, 2012,3 :3~7.