Magnetic particle inspection is performed on ferrous alloys to locate both surface and sub-surface flaws. This method utilizes electromagnetism produced from electrical current to generate north and south poles within the test article, similar to that of a standard magnet. Extremely fine fluorescent particles of iron suspended in solution cover the test part. When viewed with the black (ultraviolet) light, flaws will present themselves as bright, fine, sharp indications.

Practice:
Radiographic testing is used as a nondestructive method for detecting internal defects in thick and complex shapes in metallic and nonmetallic materials, structures, and assemblies.
Benefit:
Unlike most other nondestructive testing methods, radiographic testing provides a permanent visual record of the defects for possible future use. It can also be used to determine crack growth for use in fracture mechanics to determine critical flaw size in a particular component.

In ultrasonic testing (UT), very short ultrasonic pulse-waves with center frequencies ranging from 0.1-15 MHz and occasionally up to 50 MHz are launched into materials to detect internal flaws or to characterize materials. The technique is also commonly used to determine the thickness of the test object, for example, to monitor pipework corrosion.
Ultrasonic testing is often performed on steel and other metals and alloys, though it can also be used on concrete, wood and composites, albeit with less resolution. It is a form of non-destructive testing used in many industries including aerospace, automotive and other transportation sectors.

Dye penetrant inspection (DPI), also called liquid penetrant inspection (LPI) or penetrant testing (PT), is a widely applied and low-cost inspection method used to locate surface-breaking defects in all non-porous materials (metals, plastics, or ceramics). The penetrant may be applied to all non-ferrous materials, but for inspection of ferrous components magnetic-particle inspection is preferred for its subsurface detection capability. LPI is used to detect casting and forging defects, cracks, and leaks in new products, and fatigue cracks on in-service components.

Eddy-current testing uses electromagnetic induction to detect flaws in conductive materials. There are several limitations, among them: only conductive materials can be tested, the surface of the material must be accessible, the finish of the material may cause bad readings, the depth of penetration into the material is limited, and flaws that lie parallel to the probe may be undetectable.
In a standard eddy current testing a circular coil carrying current is placed in proximity to the test specimen (electrically conductive).The alternating current in the coil generates changing magnetic field which interacts with test specimen and generates eddy current.Variations in the phase and magnitude of these eddy currents can be monitored using a second 'search' coil, or by measuring changes to the current flowing in the primary 'excitation' coil. Variations in the electrical conductivity or magnetic permeability of the test object, or the presence of any flaws, will cause a change in eddy current flow and a corresponding change in the phase and amplitude of the measured current. This is the basis of standard (flat coil) eddy current inspection, the most widely used eddy current technique.
However, eddy-current testing can detect very small cracks in or near the surface of the material, the surfaces need minimal preparation, and physically complex geometries can be investigated. It is also useful for making electrical conductivity and coating thickness measurements.

Self explanatory section.