###### Abstract  
Gas and oil pipeline networks require periodic inspections to detect cracks or notches that can cause industrial accidents and environmental contamination. For these inspections, the metal magnetic memory (MMM) method could be used as a non-destructive testing (NDT) technique, which does not need expensive equipment and high-skilled operators. However, more investigations are required to quantify the size and shape of defects in ferromagnetic pipes using the MMM signals. We present experimental measurements of MMM signals around five small V-shaped notches of an ASTM-A36 steel pipe using a three-axis magnetoresistive sensor. The V-shaped notches have different values of depth (500 \(\mu\)m, 1000 \(\mu\)m, 1500 \(\mu\)m, 2000 \(\mu\)m and 2500 \(\mu\)m) and width (1000 \(\mu\)m, 1500 \(\mu\)m, 2000 \(\mu\)m, 3000 \(\mu\)m and 3500 \(\mu\)m). We measured the variations of tangential and normal MMM signals around these defects and their relationships with the size of each defect. The first V-notch defect (500 \(\mu\)m depth and 1000 \(\mu\)m width) registers variations of the tangential and normal MMM signals of 14.32 \(\mu\)T \(\pm\) 1.62 \(\mu\)T and 27.95 \(\mu\)T \(\pm\) 1.14 \(\mu\)T, respectively. On the other hand, the fifth V-notch defect (2500 \(\mu\)m depth and 3500 \(\mu\)m width) has variations of the tangential and normal MMM signals of 68.75 \(\mu\)T \(\pm\) 1.10 \(\mu\)T and 71.37 \(\mu\)T \(\pm\) 0.72 \(\mu\)T, respectively. The MMM method could be used for real-time monitoring of V-shaped notches in steel pipes. This method does not require special treatment of steel pipes. gas and oil pipeline network; magnetic field variations; metal magnetic memory method; magnetometer; non-destructive testing; V-shaped notches +
Footnote †: journal: Applied Sciences  
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Footnote †: journal: Applied Sciences  
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to acquire the measurements. In addition, X-ray inspections can detect small flaws of pipes using expensive equipment and operators with extensive technique experience [15; 16]. On the other hand, liquid penetrant testing can be employed for inspections of pipes using liquids around defects. However, this NDT technique requires pipes with surfaces free of contaminants [17]. Additionally, magnetic flux leakage testing uses an external source of magnetic flux close to surface of ferromagnetic pipes, in which the magnetic field changes around discontinuities [18; 19]. Several of these NDT techniques need special treatment on the surfaces of pipes and expensive equipment. In addition, several of these techniques have problems for real-time monitoring of defects in ferromagnetic pipes. An alternative NDT technique is the metal magnetic memory (MMM) method, which could be used for real-time monitoring of defects in ferromagnetic samples [20; 21]. This method is based on the self-magnetization field of ferromagnetic samples [22; 23]. The MMM method can be used for detecting early damage to ferromagnetic structures [24; 25]. It is mainly used in soft ferromagnetic materials, such as medium carbon steel [26]. This method does not require an external magnetic field source, expensive equipment and special treatment on the surfaces of samples. The MMM method can be used to detect cracks in ferromagnetic structures, based on variations in residual magnetic field signals. Generally, the MMM method is employed to detect the locale of surface defects in ferromagnetic materials. However, the MMM method requires more investigations regarding the quantitative relations between the shape and size of the defects, and the variations of their tangential and normal MMM signals. A few investigations [27] have been reported about MMM signals around V-shaped notches in steel pipes. In order to study the behavior of the MMM signals of V-shaped notches, we measured the tangential and normal MMM signals around five different V-shaped notches in an ASTM-A36 steel pipe.