# Inspection of Semi-Elliptical Defects in a Steel Pipe Using the Metal Magnetic Memory Method
###### Abstract
Another NDT technique is based on the eddy current, which can detect cracks in ferromagnetic pipelines [20; 21]. This NDT technique requires an electromagnetic energy source for the eddy current induction on the pipe surface and high-skilled personnel to analyze the results. Moreover, magnetic flux leakage is another NDT technique for monitoring flaws in ferromagnetic pipes, in which the pipe is magnetized, employing an external magnetic field [22; 23; 24]. Finally, the X-ray inspection NDT can be used for monitoring surface or internal discontinuities in ferromagnetic pipes. However, this NDT technique requires high-cost equipment and skilled personnel [25]. Flaws on ferromagnetic pipes can be detected using conventional NDT techniques, although several NDT techniques require expensive specialized equipment and sample treatments. Furthermore, these NDT techniques require highly skilled personnel to study their results. Also, conventional NDT techniques have problems in monitoring early stage damage on ferromagnetic pipe surfaces, such as small defects at the micrometer scale and in stress concentration zones (SCZs) [26]. On the other hand, magnetic properties analysis has been used in NDT techniques to characterize ferromagnetic materials. For instance, the micromagnetic characterization of ferromagnetic steels in various heat treatment conditions can be measured using a hysteresis frame device [27]. Magnetic force microscopy can be employed to assess the magnetic properties of nanomaterials [28]. The metal magnetic memory (MMM) method is an NDT technique used to detect flaws in ferromagnetic structures and it was introduced by [PERSON] [29]. The MMM method is based on the magneto-elasticity property associated with the magnetic fields of ferromagnetic materials under stress as well as with the geomagnetic fields. The MMM method can measure the magnetic field shifts on ferromagnetic surfaces [30; 31; 32; 33; 34; 35; 36; 37]. This method does not need artificial magnetic field sources, which is in contrast with magnetic flux leakage (MFL) testing [38; 39; 40]. In ferromagnetic materials, the magnetic field measured with the MMM method is related to the residual magnetic leakage field (RMFL) around surface defects [41; 42; 43]. The surface defects on ferromagnetic samples can be measured using the variations of the MMM signals around the defects without skilled personnel and expensive equipment [44; 45]. The ferromagnetic materials may have residual magnetic fields due to their fabrication processes, flaws, SCZs, and applied magnetic fields. Small variations in the magnetic field around the surface defects of ferromagnetic structures can be measured by employing highly sensitive and high-resolution magnetometers. This MMM method can detect flaws and SCZs on ferromagnetic structure surfaces. According to mathematical models reported by [46; 47], the MMM signals around flaws on ferromagnetic surfaces register a tangential \(H_{t}(x)\) component peak value and a normal \(H_{n}(x)\) component zero value at the center of the ferromagnetic material defect or SCZ [48], as is depicted in Figure 1.  
The MMM method is used in diverse fields due to its easy and low-cost implementation. This method can assess the size and shape of the defects on a ferromagnetic pipe surface [34]. Various researchers have reported models and experimental tests to quantify the relationship between the defect size and the variation in their MMM signals [30; 31; 32; 40; 47; 49]. Nevertheless, this method requires more experimental tests to quantify the shape and size of different defects on pipe surfaces. Herein, we report the analysis of the behavior of the MMM signals related to the size and shape of five different semi-elliptical defects on the surface of a ferromagnetic pipe. The MMM signals are measured using a rotatory structure, a magnetoresistive sensor, a microcontroller board, and a laptop.