The difference between Destructive and Non Destructive testing:

Non Destructive Testing (NDT):

NDT is a vast array of technical/industrial methods for analyzing the inherent characteristics of material or finding the discontinuities in the applicable/associated fabrication methods, without damaging the material.

NDT is otherwise called Non Destructive Evaluation (NDE) or oft Non Destructive Inspection (NDI) per the industry or sector describing the scope of work involved.  Almost always defined as a quantitative analysis method.

Destructive Testing (No known acronym - feel free to fill in the blanks) : )

Destructive Testing is almost always a qualitative analysis method, used to ascertain certain qualities of the materials and unions such as tensile strength, elongation properties, hardness values, impact toughness (ductility), etc....

The above mentioned qualitative properties cannot always be found using NDT methods.  Often, in order to analyze such mechanical properties, it is required to apply load on the material itself.  The load deforms the material making them unusable.

But NDT methods are used in such a manner that the material can be reused after the testing.

A select few of the NDT Methods which are commonly used in industries for discontinuity detection in materials are given here:

Ultrasonic Testing (UT) uses high frequency sound not audible to human ears, to find internal defects of virtually any material.

Phased Array Ultrasonic Testing (PAUT) is an advanced Ultrasonic Testing method in which multiple sound emitting elements arranged in a particular manner send sounds at different angles and different timings.  The received sound gives more detailed views of the discontinuities.

Liquid Penetrant Testing (PT) uses liquid/dye penetrant and developer to find surface defects in virtually any material.

Magnetic Particle Testing (MT) uses a magnetic field to find surface and near surface discontinuities in predominantly permeable ferromagnetic materials.

Radiographic Testing (RT) uses penetrating electromagnetic radiation (gamma or X-ray) of high frequency to capture images in virtually any material in order to view discontinuities at the surface, near surface, and internally.

Eddy Current Testing (ET) is a method used to detect surface and shallow sub-surface defects in conductive materials by use of electromagnetic induction using alternating current (AC).

Accoustic Emmision Testing (AE) uses sensors which monitor minute elastic deformation in material during testing and in-service.

Visual Testing (VT) is the process of visually inspecting materials/fabrication for discontinuities with the use of countless visual aids such as mirrors, rules, boroscopes, calipers, fiberscopes, pit gauges, micrometers, etc......

All perfectly viable methods depending on the task/project/material at hand.

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Did you know?

Non-Destructive Testing (NDT) is nearly 200 years old.

The earliest recorded Non-Destructive Testing was way back in 1820………One of the earliest surface inspection techniques involved rubbing carbon black on glazed pottery. The carbon black would settle in surface cracks rendering them visible.

Soon after, it became the practice in railway workshops to examine iron and steel components by the "Oil and Whiting" method. In this method, a heavy oil common to railway workshops was diluted with kerosene in large tanks so that locomotive parts such as wheels could be submerged. After removal and careful cleaning, the surface was then coated with a fine suspension of chalk in alcohol so that a white surface layer was formed after the alcohol evaporated. The test object was then struck with a hammer, causing the residual oil solution in any surface discontinuities to seep out and stain the white coating (a crude PT examination had been performed).

The “Oil and Whiting” method was in use from the latter part of the 19th century to approximately 1940, when the magnetic particle method (MT) was introduced and found to be more sensitive for ferromagnetic iron and steels.