Non Destructive Testing Techniques

[Finley Chopin]

Inspectorssubject the material they are testing to different destructive test methods, which will deform or destroy the material completely, in order to gain insights about how the material performs under pressure.

DT methods are commonly used for failure analysis, process validation, materials characterization, and can form a key part of engineering critical assessments, which also involves non-destructive testing (NDT) techniques, such as digital radiography.

Destructive testing aims to deform or destroy a material to analyze its point of failure. On the other hand, non-destructive testing uses inspection methods that do not damage a material or asset in any way.

NDT is used to test an asset that is already in operation for early detection of damage and to prevent operational failures. This test method is performed to keep records of assets, to inform maintenance schedules, and to identify defects before they become worse.

There are several types of destructive testing methods, which are designed to simulate the environmental factors that materials may actually be exposed to once they are in use. These methods are designed to test the strength of a material under certain types of pressure or strain.

Aggressive environment testing is used to test fatigue and fracture points of a component when it is exposed to corrosive environments at different pressures and temperatures. Tests mimic the environment where components will be operating.

Weld fracture test is a test designed to reveal imperfections such as cracking due to inadequate width to height ratio, incomplete penetration, lack of fusion, porosity, and slag inclusion.

Pellini drop weight test is a test that determines the nil-ductility transition temperature (NDTT). NDTT is the test temperature in explosion bulge tests at which the plate remains flat after a fracture and crack propagation occurs in the presence of elastic strains only.

Hydrostatic pressure test is primarily an NDT method, but recently hydrostatic pressure tests have shown to exert strain within a material's elasticity, which only occurs micro structurally when the material being strained is slightly proportional to the pressure applied.

Hardness testing determines whether a component undergoes permanent deformation under stress using the Rockwell scale. How much a material resists indentation is what determines hardness. This test determines how well a component will perform over time and how long it can be in use.

Residual stress measurement measures the internal stress of a component and its effect on the surface stress. These measurements allow engineers to analyze residual stress distribution. Here are three methods that can be used in residual stress measurement:

Tensile (elongation) testing is a type of stress testing performed by elongating or compressing a component to determine the strength of the material. Breaking strength, maximum elongation and compression, and tensile strength are all measured to calculate physical properties and to determine which materials can withstand a great amount of force.

Torsion testing is a type of stress testing where twisting forces are applied to determine shearing of the material before it becomes deformed. Once the material succumbs to twisting, that is considered the failure point of the material.

Destructive testing is also used to test the strength of safety glass. Sandbags can be dropped at specified heights to simulate impactful forces for failure analysis, and fire can also be applied to determine flame resistance.

Destructive physical analysis is also used to determine which materials should be used in the construction of industrial boilers, which undergo extreme pressure and high temperatures, thus determining the pressure and temperature ratings of the boiler for safe operation.

Tensile testing is used to test weld-strength for construction materials. These tests ensure the structural integrity of a weld and of the building itself. For example, a skyscraper that is exposed to natural elements will use materials and components that are deemed safe to use by destructive testing methods to withstand conditions under expected limits.

NDT stands for Non-Destructive Testing. It refers to an array of inspection methods that allow inspectors to evaluate and collect data about a material, system, or component without permanently altering it.

For instance, when inspectors in industrial settings review the outside of a pressure vessel with their naked eye, that would fall under the NDT designation, since they are collecting data on the status of the boiler without damaging it. On the other hand, using a sophisticated tool like an ultrasonic sensor to look for defects in a certain material or asset would also be called NDT.

Savings. The most obvious answer to this question is that NDT is more appealing than destructive testing because it allows the material or object being examined to survive the examination unharmed, thus saving money and resources.

Accuracy. NDT methods have been proven accurate and predictable, both qualities you want when it comes to maintenance procedures meant to ensure the safety of personnel and the longevity of equipment.

"NDT is the life blood of a well-run facility, and the high importance of NDT is known by any trained inspector. NDT techniques and repeatable results depend on highly trained technicians with experience and integrity. Not only does the technician need to be certified in a specific NDT method, but they also need to know how to operate the equipment being used to gather data. Understanding equipment capabilities and limitations is the difference between making an accept or reject determination."

Rope access. The use of rope access also requires inspectors to work at height in order to collect inspection data. Learn more about rope access and how it's used by inspectors in this guide.

Drones. Inspectors can use drones to collect inspection data remotely, allowing them to remain safely outside of confined spaces or safely below locations at height. Read the next section to learn more about how drones can be used for NDT.

NDT techniques can be used for all kinds of inspections. But some of the most important types of NDT inspections are of assets like boilers and pressure vessels, which could be incredibly dangerous if not properly maintained.

Because proper maintenance of these assets is so important for the safety of those working nearby (or even at a distance, when it comes to nuclear power plants), most countries have laws requiring companies to adhere to specific inspection codes and standards when conducting inspections.

These standards and codes typically require inspections to be conducted periodically following specific guidelines. For the most assets that present the greatest risk, these inspections must be both conducted by a certified inspector and approved by a certified witness working for a formal inspection body.

3 point bend testing. 3 point bend testing examines the soundness and flexibility (or ductility) of a material by taking a sample of it, called a coupon, and bending it in three points to a specified angle.

Definition: Visual Non-Destructive Testing is the act of collecting visual data on the status of a material. Visual Testing is the most basic way to examine a material or object without altering it in any way.

Visual Testing can be done with the naked eye, by inspectors visually reviewing a material or asset. For indoor Visual Testing, inspectors use flashlights to add depth to the object being examined. Visual Testing can also be done with an RVI (Remote Visual Inspection) tool, like an inspection camera. To get the camera in place, NDT inspectors may use a robot or drone, or may simply hang it from a rope.

One of the most common Ultrasonic Testing methods is the pulse echo. With this technique, inspectors introduce sounds into a material and measure the echos (or sound reflections) produced by imperfections on the surface of the material as they are returned to a receiver.

Radiography Testing directs radiation from a radioactive isotope or an X-ray generator through the material being tested and onto a film or some other kind of detector. The readings from the detector create a shadowgraph, which reveals the underlying aspects of the inspected material.

Definition: Eddy Current Non-Destructive Testing is a type of electromagnetic testing that uses measurements of the strength of electrical currents (also called eddy currents) in a magnetic field surrounding a material in order to make determinations about the material, which may include the locations of defects.

To conduct Eddy Current Testing, inspectors examine the flow of eddy currents in the magnetic field surrounding a conductive material to identify interruptions caused by defects or imperfections in the material.

Definition: Magnetic Particle Non-Destructive Testing is the act of identifying imperfections in a material by examining disruptions in the flow of the magnetic field within the material.

To use Magnetic Particle Inspection, inspectors first induce a magnetic field in a material that is highly susceptible to magnetization. After inducing the magnetic field, the surface of the material is then covered with iron particles, which reveal disruptions in the flow of the magnetic field. These disruptions create visual indicators for the locations of imperfections within the material.

Inspectors conducting Acoustic Emission Tests are examining materials for bursts of acoustic energy, also called acoustic emissions, which are caused by defects in the material. Intensity, location, and arrival time can be examined to reveal information about possible defects within the material.

Definition: Dye Penetrant Penetrant Non-Destructive Testing (also called Liquid Penetrant Testing) refers to the process of using a liquid to coat a material and then looking for breaks in the liquid to identify imperfections in the material.

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