History of Magnetic Particle Inspection

Magnetism is the ability of matter to attract other matter to itself. The ancient Greeks were the first to discover this phenomenon in a mineral they named magnetite. Later on Bergmann, Becquerel, and Faraday discovered that all matter including liquids and gasses were affected by magnetism, but only a few responded to a noticeable extent.

The earliest known use of magnetism to inspect an object took place as early as 1868. Cannon barrels were checked for defects by magnetizing the barrel then sliding a magnetic compass along the barrel's length. These early inspectors were able to locate flaws in the barrels by monitoring the needle of the compass. This was a form of nondestructive testing but the term was not commonly used until some time after World War I.

In the early 1920’s, William Hoke realized that magnetic particles (colored metal shavings) could be used with magnetism as a means of locating defects. Hoke discovered that a surface or subsurface flaw in a magnetized material caused the magnetic field to distort and extend beyond the part. This discovery was brought to his attention in the machine shop. He noticed that the metallic grindings from hard steel parts (held by a magnetic chuck while being ground) formed patterns on the face of the parts which corresponded to the cracks in the surface. Applying a fine ferromagnetic powder to the parts caused a build up of powder over flaws and formed a visible indication. The image shows a 1928 Electyro-Magnetic Steel Testing Device (MPI) made by the Equipment and Engineering Company Ltd. (ECO) of Strand, England.

In the early 1930’s, magnetic particle inspection was quickly replacing the oil-and-whiting method (an early form of the liquid penetrant inspection) as the method of choice by the railroad industry to inspect steam engine boilers, wheels, axles, and tracks. Today, the MPI inspection method is used extensively to check for flaws in a large variety of manufactured materials and components. MPI is used to check materials such as steel bar stock for seams and other flaws prior to investing machining time during the manufacturing of a component. Critical automotive components are inspected for flaws after fabrication to ensure that defective parts are not placed into service. MPI is used to inspect some highly loaded components that have been in-service for a period of time. For example, many components of high performance racecars are inspected whenever the engine, drive train or another system undergoes an overhaul. MPI is also used to evaluate the integrity of structural welds on bridges, storage tanks, and other safety critical structures (NDT Resource Center).

Magnetic Test Basic Principles

In theory, magnetic particle inspection (MPI) is a relatively simple concept. It can be considered as a combination of two nondestructive testing methods: magnetic flux leakage testing and visual testing. Consider the case of a bar magnet. It has a magnetic field in and around the magnet. Any place that a magnetic line of force exits or enters the magnet is called a pole. A pole where a magnetic line of force exits the magnet is called a north pole and a pole where a line of force enters the magnet is called a south pole.

When a bar magnet is broken in the center of its length, two complete bar magnets with magnetic poles on each end of each piece will result. If the magnet is just cracked but not broken completely in two, a north and south pole will form at each edge of the crack. The magnetic field exits the north pole and reenters at the south pole. The magnetic field spreads out when it encounters the small air gap created by the crack because the air cannot support as much magnetic field per unit volume as the magnet can. When the field spreads out, it appears to leak out of the material and, thus is called a flux leakage field.

If iron particles are sprinkled on a cracked magnet, the particles will be attracted to and cluster not only at the poles at the ends of the magnet, but also at the poles at the edges of the crack. This cluster of particles is much easier to see than the actual crack and this is the basis for magnetic particle inspection.

The first step in a magnetic particle inspection is to magnetize the component that is to be inspected. If any defects on or near the surface are present, the defects will create a leakage field. After the component has been magnetized, iron particles, either in a dry or wet suspended form, are applied to the surface of the magnetized part. The particles will be attracted and cluster at the flux leakage fields, thus forming a visible indication that the inspector can detect.

NDT Resource Center

Introduction to Magnetic Particle Inspection or Magnetic Test (MT)

Magnetic particle inspection (MPI) is a nondestructive testing method used for defect detection. MPI is fast and relatively easy to apply, and part surface preparation is not as critical as it is for some other NDT methods. These characteristics make MPI one of the most widely utilized nondestructive testing methods.

MPI uses magnetic fields and small magnetic particles (i.e.iron filings) to detect flaws in components. The only requirement from an inspectability standpoint is that the component being inspected must be made of a ferromagnetic material such as iron, nickel, cobalt, or some of their alloys. Ferromagnetic materials are materials that can be magnetized to a level that will allow the inspection to be effective.

The method is used to inspect a variety of product forms including castings, forgings, and weldments. Many different industries use magnetic particle inspection for determining a component's fitness-for-use. Some examples of industries that use magnetic particle inspection are the structural steel, automotive, petrochemical, power generation, and aerospace industries. Underwater inspection is another area where magnetic particle inspection may be used to test items such as offshore structures and underwater pipelines.

NDT Resource Center

Ultrasonic Test : Introduction

This technique is used for the detection of internal and surface (particularly distant surface) defects in sound conducting materials.

The principle is in some respects similar to echo sounding. A short pulse of ultrasound is generated by means of an electric charge applied to a piezo electric crystal which vibrates for a very short period at a frequency related to the thickness of the crystal. In flaw detection this frequency is usually in the range of one million to six million times per second (1 MHz to 6 MHz). Vibrations or sound waves at this frequency have the ability to travel a considerable distance in homogeneous elastic material, such as many metals, with little attenuation. The velocity at which these waves propagate is related to the 'Youngs Modulus' for that material and is characteristic of that material. For example the velocity in steel is 5900 metres per second, and in water 1400 metres per second.

Ultrasonic energy is considerably attenuated in air, and a beam propagated through a solid will, on reaching an interface (e.g. a defect, or intended hole, or the backwall) between that material and air, reflect a considerable amount of energy in the direction equal to the angle of incidence.

For contact testing the oscillating crystal is incorporated in a hand held probe which is applied to the surface of the material to be tested. To facilitate the transfer of energy across the small air gap between the crystal and the test piece, a layer of liquid, usually oil, water or grease, is applied to the surface.

As mentioned previously, the crystal does not oscillate continuously but in short pulses, between each of which it is quiescent. Piezo electric materials not only convert electri-cal pulses to mechanical oscillations, but will also transduce mechanical oscillations into electrical pulses; thus we have not only a generator of sound waves but also a detector of returned pulses. The crystal is in a state to detect returned pulses when it is quiescent. The pulse takes a finite time to travel through the material to the interface and to be reflected back to the probe.

The normal method of presenting information in ultrasonic testing is by means of a cathode ray tube, in which horizontal movement of the spot from left to right represents time elapsed. The rate at which the spot moves is such that it gives the appearance of a horizontal line on the screen. The system is synchronised electronically so that at the instant the probe receives its electrical pulse the spot begins to traverse the screen. An upward deflection (peak) of the line on the left hand side of the screen is an indication of this occurrence. This peak is usually termed the initial pulse.

Whilst the base line is perfectly level the crystal is quiescent. Any peaks to the right of the initial pulse indicate that the crystal has received an incoming pulse reflected from one or more interfaces in the material. Since the spot moves at a very even speed across the tube face, and the pulse of ultrasonic waves moves at a very even velocity through the material, it is possible to calibrate the horizontal line on the screen in terms of absolute measurement. The use of a calibration block, which produces a reflection from the back wall a known distance away from the crystal together with variable controls on the flaw detector allows the screen to be calibrated in units of distance, and therefore determination of origins of returned pulses obtained from a test piece.

It is therefore possible not only to discover a defect between the surface and the back wall, but also to measure its distance below the surface. It is important that the equipment is properly calibrated and, since it is in itself not able to discriminate between intended boundaries of the object under test and unintended discontinuities, the operator must be able to identify the origin of each peak. Further as the pulses form a beam it is also possible to determine the plan position of a flow.

The height of the peak (echo) is roughly proportional to the area of the reflector, though there is on all instruments a control which can reduce or increase the size of an indication - variable sensitivity in fact. Not only is part of the beam reflected at a material/air interface but also at any junction where there is a velocity change, for example steel/slag in a weld.

Probing all faces of a test piece not only discovers the three dimensional defect and measures its depth, but can also determine its size. Two dimensional (planar) defects can also be found but it is best that the incident beam impinges on the defect as near to right angles to the plane as possible. To achieve this some probes introduce the beam at an angle to the surface. In this manner longitudinal defects in tubes (inner or outer surface) are detected.

Interpretation of the indications on the cathode ray tube requires a certain amount of skill, particularly when testing with hand held probes. The technique is, however, admirably suited to automatic testing of regular shapes by means of a monitor - an electronic device which fits into the main equipment to provide an electrical signal when an echo occurs in a particular position on the trace. The trigger level of this signal is variable and it can be made to operate a variety of mechanical gates and flaw warnings.

Since the velocity of sound in any material is characteristic of that material, it follows that some materials can be identified by the determination of the velocity. This can be applied, for example in S.G. cast irons to determine the percentage of graphite nodularity. This process can also be automated and is now in use in many foundries. A typical equipment is the 'Qualiron'.

When the velocity is constant, as it is in a wide range of steels, the time taken for the pulse to travel through the material is proportional to its thickness. Therefore, with a properly calibrated instrument, it is possible to measure thickness from one side with an accuracy in thousandths of an inch. This technique is now in very common use. A development of the standard flaw detector is the digital wall thickness gauge. This operates on similar principles but gives an indication, in LED or LCD numerics, of thickness in absolute terms of millimetres. These equipments are easy to use but require prudence in their application.

Advantages of Ultrasonic Flaw Detection:

• Thickness and lengths up to 30 ft can be tested.
• Position, size and type of defect can be determined.
• Instant test results.
• Portable.
• Extremely sensitive if required.
• Capable of being fully automated.
• Access to only one side necessary.
• No consumables.

Disadvantages of Ultrasonic Flaw Detection:

• Indications require interpretation (except for digital wall thickness gauges).
• Considerable degree of skill necessary to obtain the fullest information from the test.
• Very thin sections can prove difficult.

source : www.insight-ndt.com

Introduction to Nondestructive Testing (NDT)

Nondestructive Testing (NDT) is the method used to examine or inspect a part or material or system without affected future usefulness. Nondestructive Testing (NDT) is utilized to investigate specifically, and it’s concerned in particular way with the performance of the test piece, how long the piece may be utilized and when it is necessary to be checked again. This is the main advantage of Nondestructive Testing (NDT), we can examine without destroying speciment and of course it is saving cost. There are several type of Nondestructive Testing (NDT) type, but commonly used are Magnetic Test(MT), Penetrant Test (PT) , Ultrasonic Test ( UT), and Radiography Test (RT).

Picture : Ultrasonic Testing

Modern Nondestructive Testing (NDT) used by manufacturer for several purposes.

• Ensuring the Integrity/Reliability of a Product

The users of a fabricated product have high expectation that it will give no trouble happen during service for a reason-able period of usefulness. Few of today’s products are expected to deliver decades of service but they are required to give reasonable unfailing value. Public has learned to expect better service and longer life, despite the increasing complexity of our everyday electrical and mechanical appliances.

• Preventing Accidents and Saving Lives

To make sure product reliability is very important because of the general increase in performance expectancy of the public. But reliability merely for convenience and profit is not enough. Reliability to protect human lives is a valuable end it itself. The railroad axle must not fail at high speed. The front spindle of the intercity but must not break on the curve.

• Ensuring Customer Satisfaction

While it is true that the most laudable reason for the use of nondestructive tests is that of safety, it is probably also true that the most common reason is that of making a profit for the user. The sources of this profit are both tangible and intangible.

• Aiding in Product Design

Nondestructive testing aids significantly in better product design. For example, the state of physical soundness as revealed by such nondestructive tests as radiography, magnetic particle or penetrant inspection of a pilot run of castings often shows the designer that design changes are needed to produce a sounder casting in an important section. The design may then be improved and the pattern modified to increase the quality of the product. This example is not academic; it occurs almost daily in many plants.

• Controlling Manufacturing Processes

Almost every nondestructive testing methods is applied in one way or another to assist in process control and so ensure a direct profit for the manufacturer.

• Lowering Manufacturing Costs

Most manufacturers could cut manufacturing costs by deciding where to apply the following cost reduction principle: A nondestructive test can reduce manufacturing cost when it locates undesirable characteristics of a material or component at an early stage, thus eliminating costs of further processing or assembly.

• Maintaining Uniform Quality Level

Once the quality level has been established, production and testing personnel should aim to maintain this level and not to depart from it excessively either toward lower or higher quality. In blunt language, a non destructive test does not improve quality. It can help to establish the quality level but only management sets the quality standard.