What is Inspection?

An inspection is, most generally, an organized examination or formal evaluation exercise. In engineering activities inspection involves the measurements, tests, and gauges applied to certain characteristics in regard to an object or activity. The results are usually compared to specified requirements and standards for determining whether the item or activity is in line with these targets, often with a Standard Inspection Procedure in place to ensure consistent checking. Inspections are usually non-destructive.

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Inspections may be a visual inspection or involve sensing technologies such as ultrasonic testing, accomplished with a direct physical presence or remotely such as a remote visual inspection, and manually or automatically such as an automated optical inspection. Non-contact optical measurement and Photogrammetry have become common NDT methods for inspection of manufactured components and design optimization.

 

What is NDT?

Nondestructive Testing (NDT) plays an important role in assuring that structural and mechanical components perform their function in a safe, reliable, and cost-effective manner. NDT technicians perform the necessary tests to locate the indicators and discontinuities that may cause failures or shutdowns in such systems. Nondestructive testing (NDT) is the process of inspecting, testing, or evaluating materials, components, or assemblies for discontinuities, or differences in characteristics without destroying the serviceability of the part or system. In other words, when the inspection or test is completed the part can still be used. The terms Non-Destructive Examination (NDE), Non-Destructive Inspection (NDI), and Non-Destructive Evaluation (NDE) are also commonly used to describe the non-destructive testing methodology.

In contrast to NDT, other tests are destructive in nature and are therefore done on a limited number of samples ("lot sampling"), rather than on the materials, components or assemblies actually being put into service. NDT is typically used at various points in a part’s life cycle. Today modern nondestructive tests are used in manufacturing, fabrication and in-service inspections to ensure product integrity and reliability, to control manufacturing processes, lower production costs and to maintain a uniform quality level. During construction, NDT is used to ensure the quality of materials and joining processes during the fabrication and erection phases, and in-service NDT inspections are used to ensure that the products in use continue to have the integrity necessary to ensure their usefulness and the safety of the public.

 

Selection of NDE Methods

The selection of a useful NDE method or a combination of NDE methods first necessitates a clear understanding of the problem to be solved. It is then necessary to single out from the various possibilities those NDE methods that are suitable for further consideration.

 

Nondestructive evaluation can be conveniently divided into nine distinct areas:

• Flaw detection and evaluation

• Leak detection and evaluation

• Metrology (measurement of dimension) and evaluation

• Location determination and evaluation

• Structure or microstructure characterization

• Estimation of mechanical and physical properties

• Stress (strain) and dynamic response determination

• Signature analysis

• Chemical composition determination

 

Flaw Detection and Evaluation

Flaw detection is usually considered the most important aspect of NDE. There are many conceivable approaches to selecting NDE methods.

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· The reason(s) for performing the NDE

· The type(s) of flaws of interest in the object

· The size and orientation of flaw that is rejectable

· The anticipated location of the flaws of interest in the object

· The size and shape of the object

· The characteristics of the material to be evaluated

 

Types of NDE Techniques

Test method names often refer to the type of penetrating medium or the equipment used to perform that test. The nondestructive testing methods most commonly use include:

• Visual Inspection (VT)

• Magnetic Particle Inspection (MT)

• Liquid Penetrant (LP)

• Radiographic Inspection (RT)

• Hardness Testing (HD)

• Ultrasonic inspection (UT)

• Immersion Ultrasonic Testing (IUT)

• Phased Array Testing (PAUT)

• Eddy Current Testing (ET)

• Digital Radiography (DR)

• Computed Radiography (CR)

• Acoustic Emission Testing (AE)

• Laser Testing Methods (LM)

• Neutron Radiographic Testing (NR)

• Thermal/Infrared Testing (IR)

• Vibration Analysis (VA)

 

The six most frequently used test methods are MT, PT, RT, UT, ET and VT. Each of these test methods will be described here, followed by the other, less often used test methods.

 

What is Visual Inspection (VT)?

Visual inspection involves using an inspector's eyes to look for defects. The inspector may also use special tools such as magnifying glasses, mirrors, or borescopes to gain access and more closely inspect the subject area. Visual examiners follow procedures that range from simple to very complex.

 

What is Magnetic Particle Inspection (MT)?

Magnetic Particle Examination is accomplished by inducing a magnetic field into a ferromagnetic material and applying iron particles to the surface of the item being examined. Surface and near-surface discontinuities affect the flow of the magnetic field within the part causing the applied particles to gather at locations of flux leakage, thus producing a visible indication of the irregularity on the surface of the material.

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MPI is fast and relatively easy to apply, and part surface preparation is not as critical as it is for some other NDT methods. These characteristics make MPI one of the most widely utilized nondestructive testing methods. The method is used to inspect a variety of product forms including castings, forgings, and weldments. Many different industries use magnetic particle inspection for determining a component's fitness-for-use. Some examples of industries that use magnetic particle inspection are structural steel, automotive, petrochemical, power generation, and aerospace industries. Underwater inspection is another area where magnetic particle inspection may be used to test items such as offshore structures and underwater pipelines.

 

What is Liquid Penetrant (LP)?

Penetrant Examination is performed with a dye solution. Once applied to the surface, the dye will effectively penetrate any surface-breaking cavity. Excess solution is removed from the object. A developer is then applied to draw out any penetrant that remains unseen. With fluorescent dyes, ultraviolet light is used to make the “bleed-out” fluoresce brightly, allowing imperfections to be readily seen. With visible dyes, a color contrast between the penetrant and developer makes the "bleed-out" easy to see. Liquid Penetrant Inspection (LPI) has been used in various fields for years to identify discontinuities/defects open to the surface, too small for the eye to detect. This method is one of the most portable of the surface inspection methods; LPI also has the advantage of being able to detect flaws in nonmetallic and non-ferromagnetic materials.

 

What is Radiographic Inspection (RT)?

Industrial radiography involves exposing a test object to penetrating radiation so that the radiation passes through the object being inspected and a recording medium placed against the opposite side of that object.  This method can be performed at our lab or in the field. This method is a volumetric inspection in which defects that are not open to the surface can be detected that may not otherwise be detectable. A vast array of material can be examined by this method which is an efficient and reliable way ranging from tiny electronic components to large vessels. Element has the capabilities to perform conventional film radiography to Computed and Digital radiography. Computed and Digital radiography is a highly sensitive method of radiography that produces an image in a digital format that can be viewed on any laptop or computer.

 

What is Hardness Testing (HD)?

Element has the capabilities to include a number of portable hardness testing for specific applications. Brinell and Rockwell have been common testing methods for years however recent advancements have given us new devices such as UCI (Ultrasonic Contact Impedance), TIV (Through Indenter Viewing), and improvements in the rebound method. This helps the technician’s perform hardness verification on parts with complex geometries that traditional hardness testing may not be able to perform.

 

What is Ultrasonic inspection (UT)?

Ultrasonic Testing (UT) uses high-frequency sound energy to conduct examinations and make measurements. Ultrasonic inspection can be used for flaw detection/evaluation, dimensional measurements, material characterization, and more. A typical UT inspection system consists of several functional units, such as the pulser/receiver, transducer, and display devices. A pulser/receiver is an electronic device that can produce high voltage electrical pulses. Driven by the pulser, the transducer generates high-frequency ultrasonic energy. The sound energy is introduced and propagates through the materials in the form of waves. When there is a discontinuity (such as a crack) in the wave path, part of the energy will be reflected back from the flaw surface. The reflected wave signal is transformed into an electrical signal by the transducer and is displayed on a screen. Signal travel time can be directly related to the distance that the signal traveled. From the signal, information about the reflector location, size, orientation and other features can sometimes be gained.

 

What is Immersion Ultrasonic Testing (IUT)?

Immersion ultrasonic testing, is an advanced form of ultrasonic testing and is a more effective method of inspecting than manual ultrasonic testing. It offers improved Probability of Detection (POD) of the smallest defects and can provide you with accurate reporting of the size and location of sub-surface irregularities and flaws in material or products.

Typically with conventional ultrasonic testing, a coupulent such as oil, gel or water is used on the tested item so that the transmission of the ultrasonic waves is more efficient and the transducer is manually moved over the tested item. With immersion ultrasonic testing, the tested part material and transducer are submersed typically in water and this allows for better sound travel from the transducer while maintaining consistent distance from the component.

 

What is Phased Array Testing (PAUT)?

Phased array ultrasonic (PA) is an advanced method of ultrasonic testing that has applications in medical imaging and industrial nondestructive testing. Phased array ultrasonic systems utilize multi-element probes, which are individually excited under computer control. By exciting each element in a controlled manner, a focused beam of ultrasound can be generated. Software enables the beam to be steered. Two and three-dimensional views can be generated showing the sizes and locations of any flaws detected. This method is an advanced NDT method that is used to detect discontinuities i.e. cracks or flaws and thereby determine component quality. Due to the possibility to control parameters such as beam angle and focal distance, this method is very efficient regarding the defect detection and speed of testing.

 

What is Eddy Current Testing (ET)?

Eddy current inspection is one of several NDT methods that use the principal of electromagnetism as the basis for conducting examinations. Several other methods such as Remote Field Testing (RFT), Flux Leakage and Barkhausen Noise also use this principle. Eddy currents are created through a process called electromagnetic induction. When alternating current is applied to the conductor, such as copper wire, a magnetic field develops in and around the conductor. This magnetic field expands as the alternating current rises to maximum and collapses as the current is reduced to zero. If another electrical conductor is brought into the close proximity to this changing magnetic field, current will be induced in this second conductor. Eddy currents are induced electrical currents that flow in a circular path. They get their name from eddies that are formed when a liquid or gas flows in a circular path around obstacles when conditions are right.

 

What is Digital Radiography (DR) & Computed Radiography (CR)?

Digital radiography is an advanced technology based on digital detector systems in which the x-ray image is displayed directly on a computer screen without the need for developing chemicals or intermediate scanning.  The incident x-ray radiation is converted into an equivalent electric charge and then to a digital image through a detector sensor. Compared to other imaging devices flat panel detector provides high-quality digital images with better signal-to-noise ratio and improved dynamic range, which in turn provides high sensitivity for radiographic application. Digital radiography is used in all industry sectors and, in particular, for assessing piping, pressure vessels and valves. The technique can detect discontinuities in a range of materials including aluminum, steel, plastics and composites.

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Computed Radiography (CR) uses reusable phosphor-coated imaging plates to capture images. Digital Radiography (DR) uses flat panel detectors to capture images.  Digital radiography has several important benefits as images can be enhanced and magnified for viewing and interpretation of findings.

 

What is Acoustic Emission Testing (AE)?

Acoustic Emission Testing is performed by applying a localized external force such as an abrupt mechanical load or rapid temperature or pressure change to the part being tested. The resulting stress waves in turn generate short-lived, high-frequency elastic waves in the form of small material displacements, or plastic deformation, on the part surface that are detected by sensors that have been attached to the part surface. When multiple sensors are used, the resulting data can be evaluated to locate discontinuities in the part.

 

What is Laser Testing Methods (LM)?

Laser Testing includes three techniques, Holography, Shearography and Profilometry.  As the method name implies, all three techniques user lasers to perform the inspections. Holographic Testing uses a laser to detect changes to the surface of a part as it deforms under induced stress which can be applied as mechanical stress, heat, pressure, or vibrational energy.  The laser beam scans across the surface of the part and reflects back to sensors that record the differences in the surface created by that stress.

Laser Profilometry uses a high-speed rotating laser light source, miniature optics and a computer with high-speed digital signal processing software.  The ID surface of a tube is scanned in two dimensions and the reflected light is passed through a lens that focuses that light onto a photo-detector, generating a signal that is proportional to the spot's position in its image plane.This technique can be used to detect corrosion, pitting, erosion and cracks in pipes and tubes.

Laser Shearography applies laser light to the surface of the part being tested with the part at rest (non-stressed) and the resulting image is picked up by a charge-coupled device (CCD) and stored on a computer.  The surface is then stressed and a new image is generated, recorded and stored. The computer then superimposes the two patterns and if defects such as voids or disbands are present, the defect can be revealed by the patterns developed.  Discontinuities as small as a few micrometers in size can be detected in this manner.

 

What is Neutron Radiographic Testing (NR)?

Neutron radiography uses an intense beam of low energy neutrons as a penetrating medium rather than the gamma- or x-radiation used in conventional radiography.  Generated by linear accelerators, betatrons and other sources, neutrons penetrate most metallic materials, rendering them transparent, but are attenuated by most organic materials (including water, due to its high hydrogen content) which allows those materials to be seen within the component being inspected.  When used with conventional radiography, both the structural and internal components of a test piece can be viewed.

 

What is Thermal/Infrared Testing (IR)?

An infrared thermographic scanning system can measure and view temperature patterns based upon temperature differences as small as a few hundredths of a degree Celsius. All objects emit electromagnetic radiation of a wavelength dependent on the object’s temperature. The frequency of the radiation is inversely proportional to the temperature. In infrared thermography, the radiation is detected and measured with infrared imagers (radiometers). The imagers contain an infrared detector that converts the emitting radiation into electrical signals that are displayed on a color or black & white computer display monitor.

 

What is Vibration Analysis (VA)?

Vibration analysis refers to the process of monitoring the vibration signatures specific to a piece of rotating machinery and analyzing that information to determine the condition of that equipment.  The vibration signature of a machine is the characteristic pattern of vibration it generates while it is in operation. It has been shown many times over that the vibration of an operating machine provides far more information about the inner workings of that machine than any other type of nondestructive test. A bearing that has a small developing fault will cause a tell-tale change in the machine's vibration, as will a weight imbalance condition, a shaft or coupling misalignment, or any of a myriad of other faults.