The Study Of Array Eddy Current Probes Used To Detect The Defects At The Root Of Bolts

Bolts are standard parts in industrial product, they are usedcommonly. Therefore, the quality of bolts is very important. Due to theunique structure and the complex stress, it is highly possible for defectsto occur at the root of bolts. However, magnetic particle test does notcover those defects, such as the cracks of the bolt root. Therefore, eddycurrent test has been introduced. Though, single eddy current probe canalready test the cracks of the blot root, it has complex mechanical designand the probes are prone to damage and other shortcomings. Therefore,in this paper, research on array eddy current probes so as to producea static test probe which can accurately and effectively detect crackoccurences at bolt roots is conducted. This paper has solved one of theissues in traditional testing. Therefore, it provides a solution tominimize the defect rate of bolts which are popularly used in aircrafts,automotives and cruises. The probes solve the problems that puzzledpeople.In this paper, a method combining numerical and experimental methodsis used to design eddy current array probes. First, based on the sizeof the tested bolt and the expected cracks, the number of probes in thearray is determined. Then, based on the conductivity of the testedmaterial and the size of the bolt cracks, the test frequency is determined.Then, electromagnetic theories and Maxwell's equations are used to builda model of one single probe, to estimate the distribution of the eddy field, and to identify the diameter of the single probe. Finally, theprobe framework to fit the bolt root is designed. The experiment isdivided into two parts. During the first part, our samples are testedby the array eddy current probes. The samples are twenty-one galvanizedbolts, and are numbered from 0 to 20. We start the test from eight ofthe samples, record the maximum and the minimum voltage values of eachbolt And then cut a crack at the bolt root by WEDM, with the width of5mm and with the depth of 0.2mm, 0.4mm, 0.6mm, 0.8mm and 1.2mmcorrespondingly (No. 0 is a standard sample, No. 1-4 are 0.2mm; No. 5-8are 0.4mm; No. 9-12 are 0.6mm; No. 13-16 are 0.8mm; No. 17-20 are 1.2mm.)Then the bolts are tested. And again we record the maximum and minimumvoltage values of each bolt. With the second part, sixty-six SFC8.8blackening bolts offered by the manufacturer are tested. The numbers are1-60, NO. 1 is a standard sample. Then, the maximum and minimum voltagevalues of each bolt are recorded. The expected defective bolts are usedfor metallographic analysis.The eddy current array probes discussed in this paper are designedas follows: (1) eight single probes; (2) the single probe diameter is 1.7mm; (3) the inductance of single probe is 30mh, impedance is 188.5Ω; (4)the number of turns is 110; (5) detection frequency is 100Hz; (6) theframework is made of nylon. First part of the test results are as follows:depth is 0.8mm, average voltage is 11.9V, SNR is 3.09, gain is 9.79db. Inthe second part, 9V is set as a threshold. Fourteen failed bolts areselected. Five of them are blackening incomp]ete. Cracks are found inall the others through the metallographic microscope. And all of thequalified ones have passed the tensile testing.The eddy current array probe described in this paper is able to testthe root of bolts effectively and is able to avoid blind spots. 9db ofgain is achieved in static test when the depth of cracks equals 0.8mm. It satisfies the national requirements for automatic test. Compared withsingle probe test, the array eddy current probes have longer life andmore simple mechanical design. By adjusting parameters it can greatlyaccelerate detection efficiency and stability. If they are used afterheat treatment and before galvanized or blackening, the impact of unevengalvanization or imcomplete blackening on the test results can be avoided.Therefore, it enhances the accuracy of tests. Eddy current array probeshave many characteristics that conventional probes do not have. Thedevelopment and application of array probe are at the initial stage inChina. It should be noted that the application of array probes in crackdetection is novel in China. This project has contributed to the theoryand technical application of the defect detection methodology which usessingle eddy current array probes. It provides high confidence for thereliability of the bolts used in real world.