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Materials Testing

NON-DESTRUCTIVE TESTING (NDT)

To support Touchstone's failure analysis, industrial problem solving, and research and development activities, Touchstone utilizes a number of non-destructive testing techniques. Among them are:

A-Scan Ultrasound

A-Scan Ultrasound is useful to identify flaws in metal parts. It can also be used to evaluate the microstructure of some metal alloys.

Touchstone developed a technique which was adopted in a gray iron foundry in which the effectiveness of an inoculate could be evaluated by measuring the speed of sound utilizing an A-Scan ultrasonic instrument.

The theory of operation is as follows:

An ultrasonic transducer uses the piezoelectric effect to introduce a series of compressive strain pulses that propagates through the material. The speed at which the wave travels through a material depends upon both the modulus of elasticity and the density of the material. When a wave strikes a flaw, a portion of the wave is reflected back to the transducer. The information is transferred to an oscilloscope or through an analog to digital converter to a computer. The time required for the pulse to travel from the transducer to the flaw and back is measured. By knowing the speed of wave travel and the time of the travel distance from the transducer to the flaw evaluated.

Dye Penetrant and Fluorescent Penetrant Inspection

Dye Penetrant and Fluorescent Penetrant Inspection can be used to find discontinuities such as surface cracks that would otherwise be invisible. Performing the test is a four part process. The surface is cleaned. A liquid penetrant is placed on the surface and then cleaned off ­ leaving the penetrant trapped in the crack. A developer is then sprayed onto the surface. The penetrant migrates from the crack, reacts with the developer and causes a color change that identifies the crack location.

Dye Penetrant and Fluorescent Penetrant Inspection techniques differ in that Fluorescent Penetrant Inspection utilizes a fluorescent dye and a black light to perform the inspection.

Eddy Current

Touchstone personnel have substantial experience in the application of eddy current techniques. Eddy current can be used to detect difference in heat treated states of metals, as changes in material hardness also change eddy field permeability. Eddy current can also be used to detect delamination in clad structures, porosity or inclusions in castings or forgings, or incomplete infiltration in metal matrix composites. Different frequencies can be used to examine different depths of the workpiece.

Touchstone has an eddy current device which measures the thickness of organic coatings on both ferrous and non-ferrous metals.

Magnetic Particle Inspection

This technique is used to detect defects near the surface of ferro-magnetic materials. It operates as follows:

A magnetic field is induced in the material by placing it adjacent to a coil. The magnetic field can be thought of as producing flux lines. Any defect in the material will alter the flux density of the magnetic field leading to a reduction in the magnetic permeability of the material. Leakage of flux from the surface of the material creates North and South poles which attract magnetic powder particles. The magnetic powder particles indicate the presence of a surface or near surface flaw.

While this can be a powerful technique, it has some limitations. The sample being tested must be ferro-magnetic. The defects must ahave a lower magnetic permeability than the material being tested. And, the discontinuity must lie perpendicular to the lines of flux to be detected. It can be a challenge aligning the magnetic fields at the appropriate angles.

Replica Technique

Often in the process of evaluating surface defects or fractures, it is desirable to use the high resolution of a scanning electron microscope (SEM). While not normally included in the category of typical non-destructive tests, there is an SEM technique from which surface information can also be acquired from a part too large to fit in the SEM. With this technique a piece of acetate tape is first softened in acetone. The tape is then pressed against the surface to be evaluated to make a replica of the surface. This replica can then be coated with either gold, platinum, or carbon using a vacuum coater to produce a sample that can be examined using the SEM or TEM. Detail only visible at magnifications as high as 20,000X or greater can be accurately reproduced.


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