Magnetic Particle Testing (MPT) Paper Example

Introduction

Magnetic particle inspection can be defined as a non-destructive method used in detecting discontinuities present at the surface of a ferromagnetic material. Some of the materials include iron, steel, cobalt, nickel and their alloys. This technique is fast and relatively easy to utilize, and the preparation of the test surface is not as critical as in other methods. The testing process starts with magnetizing a component and then the surface is coated with ferromagnetic particles. These are usually iron fillings which can either be dry or wet suspensions (Du et al., 2018). Surface discontinuities create flux leakage filed in the test material which is magnetized. These particles move to the flux leakage and cluster at the discontinuities. The particle cluster is more noticeable compared to the actual crack enabling examination. This paper seeks to examine the principle behind magnetic particle testing and inspection procedure used during the examination of components.

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The principle of Magnetic Particle Examination

In theory, magnetic particle inspection is described as a straightforward concept. It is often referred to as a collaboration of tow nondestructive testing techniques namely the magnetic flux leakage testing, and visual inspection. To accomplish magnetic particle testing, the test component is first magnetized. Direct magnetization is done by passage of an electric current through the material being tested and thereby creating magnetic fields. Indirect magnetization, on the other hand, entails placement of the test material near a strong magnetic field resulting in the induction of a magnetic field in the test material. If the surface to be inspected does not have discontinuities, the magnetic flux does not change (Hijazi, n.d.). This because of the uniformity of the material being tested. If the material being inspected has flaws, the magnetic flux flow with being interrupted. This is because of the variation of permeability with the material such as air in a crack or presence of a non-magnetic inclusion in the component being examined. When the magnetic flux is interrupted, the outcome is the creation of a flux leakage around the discontinuity (Hijazi, n.d.). This leads to the creation of local magnetic poles at the edge of the discontinuities and iron filings are attracted on the surface of the component.

The intensity of magnetization determines the size of the leakage flux in addition to the flaw orientation. This means that if the flaw is parallel to flux direction, there will be no noticeable variation in the magnetic leakage flux. However, if the testing material has discontinuities at right angles to the direction of flux, then there will be significant magnetic flux leakage. It is so since the flux lines must cross the air gaps because the flux lines disperse as they repel each other within the air gap (Hijazi, n.d.). This happens because the flux lines tend to follow the path which has minimal resistance in contrast to the air characterized by high reluctance. It is important to note that to enable noticeability of discontinuities, it should be within 45 to 90 degrees of the magnetic flux direction. The accumulation of the iron filling particles which is also called indication needs interpretation to establish the cause and its importance in the inspection process. Also, the evaluation of the flaws should conform to the predetermined acceptable standards. Figure 2 below shows magnetic field lines that have been disturbed and the concentration of particles because of a flaw.

The Procedure of Magnetic Particle Inspection

Magnetic particle testing requires the utilization of fine ferromagnetic particles. The colour of dry particles used should have a great contrast with the material being inspected. Magnetic particle testing should not be conducted when the surface temperature is more than 51 degrees Celsius. In case, wet particles are used, they should have a significant contrast with the surface being inspected (Gallego, 2016). A suitable liquid should be used as suspense for the ferromagnetic particles. To obtain reliable and consistent results, it is essential that the bath strength which is defined as the concentration of the ferromagnetic particles should be upheld at appropriate levels after ensuring the necessary inspection material. There is a general operation procedure for magnetic particle examination.

The primary steps of performing magnetic particle examination include surface preparation; introduction of the magnetic field; introducing magnetic media; and examination of indications. During the inspection process, the component that requires examination should be cleaned and dried to ensure the magnetic particles have an uninterrupted path of movement to both weak and strong magnetic leakage fields ("Magnetic Particle Inspection Procedure" 2009). It is essential that the contaminants such as grease, oil or scales are eliminated to attract the magnetic particles to be attracted to the magnetic leakage fields. Also, such contaminants may negatively affect the interpretation of indications. Figure 3 shows an inspector cleaning the surface to enable effective magnetic particle testing.

The second step of magnetic particle inspection is the introduction of the magnetic field. The introduction of the magnetic field can be accomplished in several ways. For instance, a performant magnet or an electromagnet can be used (Gallego, 2016). Another method can include flowing an electric current through the component to be inspected. Another way would involve flowing an electric current through a wire coil wound around the material being tested. A longitudinal magnetic field is formed by placing the material being tested near the inside of a coil. This leads to the production of magnetic lines of force. These lines of forces and the long axis of the component being tested are parallel to each other. In most cases, permanent magnets and yokes are utilized to form a longitudinal magnetic field and they travel from one magnetic pole to the next ("Magnetic Particle Inspection Procedure" 2009). These are in such a way that the discontinuities run normal to these magnetic lines of force. Figure 4 shows magnetic flux from a Yoke magnet and crack indications.

The third step in the inspection process is using the magnetic media. Magnetic particle inspection is conducted using dry particles or wet particles. When using dry particles, they are dusted on the component surface (Gallego, 2016). When using wet particles, a component surface is flooded with a solution containing the ferromagnetic particles. During the inspection, the dry method is considered more portable. Wet method, on the other hand, is more sensitive because there is additional mobility brought by the liquid carrier.

The fourth step is interpreting the magnetic particle indications. In this step, the inspector visually examines the part for defect indications. Discontinuity indications are usually found in an orientation which is perpendicular to the axis of the magnetic flux. For instance, Figure 5 shows indications of flaws in a weldment.

References

Du, W., Zhao, Y., Roy, R., Addepalli, S., & Tinsley, L. (2018). A Review of Miniaturized Non-Destructive Testing Technologies for In-situ inspections. Procedia Manufacturing, 16, 16-23. doi: 10.1016/j.promfg.2018.10.152

Gallego, J. (2016). Magnetic Particle Testing (MPT). Retrieved from https://www.feis.unesp.br/Home/departamentos/engenhariamecanica/maprotec/lmcm2_aula8.pdf

Hijazi, A. (n.d.) Magnetic ParticleTesting. Retrieved from https://eis.hu.edu.jo/ACUploads/10526/Magnetic%20Particle%20Testing.pdf

Magnetic Particle Inspection Procedure. (2009). Retrieved from http://www.bmt-tank.com/cmpydocs/NDEP/MT-1.pdf

Uludag, A. (2016). The Magnetic Particle Inspection Examinationof Aircraft Propeller Mounting Bolts. Journal of Multidisciplinary Engineering Science and Technology (JMEST), 3(12), 6337-6341.