| Early physical damage caused by elastic stress concentration, mirco plastic deformation, etc. will certainly lead to lower loading capacity of engineering materials and structures, and sometimes even to catastrophic accidents. Therefore, effective monitoring is the key to ensure the industrial safety. Although the traditional magnetic flux leakage detection has already been widely used in practical engineering, such strong magnetic detection technology is not suitable for the detection of early damages. In the most recent decade, the metal magnetic memory (MMM) method is developed as a passive weak magnetic detection technology, which identifies the damage locations and conditions by detecting the surface spontaneous magnetic leakage field due to the localized stress concentration of damage in the measured object. Theoretically, the MMM method is a prospective method that can realized early nondestructive detection for ferromagnetic materials and structures. However, due to short developing period and various influencing factors, this method, in practical engineering, is only applied as a preliminary testing technique for determining the possible location of defects. This thesis uses the MMM method to detect the magnetic flux leakage signals in uni-axial tensile of a ferromagnetic metal specimen with a hole and contact of nonferromagnetic-ferromagnetic materials and ferromagnetic-ferromagnetic materials, respectively. Furthermore, on the basis of magneto-mechanical analysis of magnetic flux leakage signals, we propose the corresponding damage criteria and get the evaluation parameters based on the gradient of the magnetic flux leakage signals. Finally, we present numerical simulation and discuss the image of the damage shapes by using the damage criteria and evaluation parameters. Detailed contents and main conclusions are as follows:(1) Magnetic flux leakage experiments for a45#steel specimen with a small hole under the uni-axial tensile are performed. A criterion of plastic damage is developed which states:at the stress-concentration zone, the normal magnetic flux leakage signal has obvious fluctuation, with its gradient exhibiting a peak. Meanwhile, a clear peak appears in the tangential magnetic flux leakage signal; and its gradient follows a peak-peak change with a value of zero at the center of the stress-concentration zone.(2) Contact test between a cylindrical nonferromagnetic (copper) indenter and a45#steel specimen are performed. The criterion of damage states:along the direction perpendicular to the loading, the normal magnetic flux leakage gradient exhibits a peak, and the tangential magnetic flux leakage gradient follows a peak-peak change with a value of zero at the center of the stress-concentration zone. Meanwhile the contact tests between a cylindrical ferromagnetic indenter and a45#steel specimen are performed. It is found that the features of the magnetic flux leakage signals are totally different from those of a cooper cylindrical indenter. The criterion of damage in this case becomes:the normal magnetic flux leakage signal exhibits a peak, which its gradient following a peak-peak change with a value of zero at the center of the stress-concentration zone. Moreover, tangential magnetic flux leakage signal follows a peak-peak change with a value of zero, and its gradient exhibits a peak.(3) Contact test between a flat nonferromagnetic (cooper) or ferromagnetic indenter and45#steel specimen are performed. For the contact tests of a flat cooper indenter or demagnetized ferromagnetic indenter, the magnetic flux leakage signal curves can be created as superpositions of the results of cylindrical indenter under the same condition. The criterion of damage may be developed based on those for a cylindrical indenter. The magnetic flux leakage signals in the contact test of a non-demagnetized ferromagnetic indenter are quite different. The criterion of damage is: the normal magnetic flux leakage gradient follows two reverse peak-peak changes with two values of zero. Also, the tangential magnetic flux leakage signal exhibits two peaks, and its gradient shows a peak and a valley.(4) From the curves of the magnetic flux leakage signals and their gradient, some characteristic parameters can be obtained to evaluate damage degree and region. However, the gradient curves can avoid influence of external magnetic fields, and can highlight the localization of damage. The inhomogeneity of damage can be evaluated through the magnetic flux leakage gradient curves by the peak-peak values, peak-valley values and peaks; and the damage area can be evaluated by the peak-peak widths, peak-valley widths and horizontal distances of zero crossings.(5) Numerical results of the magnetic flux leakage signals of a specimen which damage are presented. The results show that the MMM method is more sensitive to the damage at the surface of a specimen. In the test, the sensor should approach the specimen surface as closely as possible. The testing direction is of no influence on the magnetic flux leakage gradient curves. The size and shape of the damage area can be imaged by the evaluation parameters. |
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