Physics of Nondestructive Evaluation

Nondestructive evaluation is the science of determining the condition of an object without having to damage it. The various methods of nondestructive evaluation rely on physics to be able to sense problems in an object. The types of physics range from capillary action (dye/fluorescent penetrant methods) to wave propagation (ultrasound, microwave, and terahertz methods) to high energy interactions of elementary particles (radiography).

A proton and neutron surrounded by a diffuse electron

We start by discussing the fundamental structure building blocks of matter, the atom and elements. Atoms are composed of protons, neutrons, and electrons. Sharing electrons can bind atoms together into molecules.

In some cases the electrons that otherwise bind molecules together can flow in the form of electricity. Flowing electrical currents also induce magnetism. It is electricity and magnetism that power all our modern devices.

Electric current flowing back and forth can create waves. There are a large number of different kinds of waves, including the familiar interface waves we see on the surface of water. Sound is a form of wave that can move through fluids and solids. Sound, also known as ultrasound when it is beyond the range we can hear, can be used to find flaws in materials by transmitting the sound into the material and then listening for reflected echos. Light, terahertz, microwaves, and radio waves are different frequencies of low-energy electromagnetic waves. We can obviously inspect objects visually, but other frequencies of electromagnetic waves can penetrate better inside materials (generally non-conductors) to let us see inside.

X-Rays are electromagnetic waves of extremely high energy and frequency (much higher than light). X-Rays are good for nondestructive evaluation because they penetrate differently through different materials, casting a shadow like the X-Ray image of your bones that you might get at your doctor's office. Unlike lower energy electromagnetic waves, X-Rays do not significantly refract.

To understand flaws, we have to understand the materials we build things with. This includes the different types of materials, such as metals and plastics, and the structures of those materials as well as their properties. We also need to know and how materials degrade and fail if we want to detect signs of impending failure.

Fluid flows are also important to nondestructive evaluation because fluid getting sucked into cracks through capillary action is how we find cracks using penetrant testing.