Nondestructive Testing overview
Nondestructive Testing is an advanced method of inspection that has played a significant role for industry. Over the years, technological advancements and innovations have allowed for the introduction of many different types of revolutionary NDT methods. With access to multiple different types of inspections, it is common for users to outsource inspection to NDT labs. (Note: Also see NDI and NDE as the terms are interchangeable.)
What is NDT?
A set of analysis techniques utilized with intent to evaluate properties of a subject without applying pressure, causing damage or applying external forces. These nondestructive testing methods are utilized to ensure product reliability and quality by detective and evaluating flaws in materials, without destroying their usefulness. According to business dictionary, NDT is a quality control method that does not damage the product being tested.
What are the uses of NDT?
Non-destructive testing is a highly useful method of qualification for industrial applications. The following is a brief overview of the many uses of NDT:
- Inspect, test and evaluate materials
- Identify discontinuities or differences
- Ensure product integrity and reliability
- Control manufacturing process quality control
- Research & Development
- Maintain and improve quality of materials
- Detect service related conditions caused by wear, fatigue, corrosion or stress
- Product evaluation & troubleshooting
- Can be used for condition monitoring
- Residual life assessment
- Find, size, locate surface and internal flaws
- Quality assurance management tool
- Used as a part acceptance/rejection qualifier
- Accident prevention and to reduce costs
- Improve product reliability
- Determine acceptance according to specific requirements
- Provide insightful data and information on repair criteria
- Measurement of components and physical properties
- Asses equipment condition and reliability
- Ensure quality from raw material stage through manufacturing to pre-production/production inspection
When is NDT necessary?
Nondestructive testing can be applied in any stage of a given products life cycle. If applied, implemented and executed correctly, NDT can prove to be an efficient tool and useful during the following stages:
- Research & Development
- Product design and feasibility inspection
- Prototyping and pre-service inspection
- Pre-production inspection
- Manufacturing or fabrication inspection
- Processing product inspection
- In-service evaluation
- Production inspection
- Failure Investigation
Benefits of NDT
Nondestructive Testing’s main feature that provides additional value in the quality control process is the fact that it is able to test subjects without causing change, damage or destroying the subject. This feature is able to save users production and fabrication costs and help improve the quality of the part. Moreover, parts can be tested and qualified as accepted within specific tolerance or rejected if out of tolerance, without destruction. Measurements can be retrieved, providing extensive insight on the part being inspected. As an additional benefit, NDT methods can be applied at any time of the products life cycle.
Types of NDT methods
There are many different types of nondestructive testing methods. The six primary testing methods include magnetic particle testing (MT), liquid penetrant testing (PT), radiographic testing (RT), ultrasonic testing (UT), electromagnetic testing (ET) and visual testing (VT). Below, all nondestructive testing methods are briefly explained:
Surface Examination Methods
Visual Testing (VT) – The most common and basic method of inspections. Examiners check the surface integrity by looking at the part surfaces with the naked eye or with the assistance of camera systems, fiberscopes or power lenses.
Magnetic Particle Testing (MT) – Flaws which are on surface or near surface can be detected by this NDT method. The material of the part being tested must be ferromagnetic (steel/iron), where a magnetic field is induced, which would be distorted by any present discontinuity.
Liquid Penetrant Testing (PT) – A part is coated with fluorescent dye. Excessive dye is removed from the surface, while dye is consumed by and left in defective areas (cracks). A developer retrieves the remaining dye from defective areas.
Electromagnetic Testing (ET) – This method can only be used on electrically conductive materials because electrical currents are required, which are generated by an induced magnetic field. Shifting magnetic field generates an electric eddy current; any resistance caused by flaws or defects will be captured on the voltmeter.
Volumetric Examination Methods
Ultrasonic Testing (UT) – This NDT method determines flaws and defects within a part in accordance with varying acoustic nature. High frequency sound waves are induced within a part, to identify imperfections.
Radiographic Testing (RT) – Source of radiation captures internal defects and flaws on a radiographic film. The varying densities of the part material in comparison to the flaw, will be clearly identified on the radiography results. The location and orientation of the defect can be determined.
Condition Monitoring Methods
Infrared Testing (IR) – Variations of a part are determined by presenting the thermal profile of a part in graph form. This method allows user to retrieve working temperature assessment, variations are then identified.
Vibration Analysis (VA) – Determines condition of a machine by producing vibration noise and documenting the frequency.
Integrity Examination Methods
Leak Testing (LT) – Multiple different types of leak testing techniques exist including soap-bubble test, liquid and gas penetrant techniques or electronic listening devices to name a few. These methods can locate leaks in pressure absorbent parts, for example, pipes.
Acoustic Emission Testing (AE) – Emission is acoustic energy which is released when a part is under stress. Special receivers can detect the intensity, which can provide insight to the location of a defect.
Special NDT Methods
Guided Wave Testing (GW) – As detailed within ASNT, Guided wave testing on piping uses controlled excitation of one or more ultrasonic waveforms that travel along the length of the pipe, reflecting from changes in the pipe stiffness or cross sectional area.
Laser Testing Methods (LM) – Laser Testing includes three distinct methods: Holography, Shearography and Profilometry; using lasers, these techniques are utilized to perform the inspection and detect defects and flaws.
Magnetic Flux Leakage (MFL) – As detailed within ASNT, Magnetic Flux Leakage detects anomalies in normal flux patterns created by discontinuities in ferrous material saturated by a magnetic field.
Neutron Radiographic Testing (NR) – Neutron radiography is similar in process and application with the only main difference being the use of an intense beam of low energy neutrons as a penetrating medium rather than the gamma- or x-radiation used in conventional radiography.
Applications of NDT
Nondestructive testing is regularly applied in industries that are prone to accumulating failures in components which would ultimately cause substantial threat to safety or economic loss. Different areas where NDT methods can be applied include the following:
- Weld Verification
- Qualification of structural properties
- Research & Development
- Material thickness measurement analysis
- Manufacturing inspection to detect failures
- Application vary across multiple different industries, including but not limited to:
- Aerospace – ex. Castings
- Medical Devices – ex. Stints
- Automotive – ex. Piston head
- Military & Defense – ex. Ballistics
- Manufacturing – ex. Pre-production qualification of part
- Packaging – ex. Structural integrity/leak or failure analysis or package
History of NDT
Over time, there have been numerous improvements and notable advancements for nondestructive testing methods. Below is a brief observation of primary industry changing events:
- 1895 – Wilhelm Rontgen discovers X-rays
- 1924 – Lester uses radiography to examine castings
- 1926 – Release of the first Eddy Current for thickness measurement
- 1940 – Liquid penetrants tests developed
- 1944 – ultrasonic test method developed in the USA
- 1946 – First neutron radiographs produced
- 1950 – Introduction of Acoustic emission as a nondestructive testing method
The following is one form of NDT certification authorized by the American Society of Nondestructive Testing. There are two distinct approaches to NDT certifications that are recognized and accepted by the industry in accordance with ASNT.
Employer based certification: The employer develops a written practice which defines the responsibilities of the different levels of certification, highlights experience, training and examination requirements. The written examinations are usually based on industry practices (SNT-TC-1A) highlighted by the American Society for Nondestructive Testing.
Personal Central Certification: Certification is acquired from a certification authority which is recognized by employers, government and industry. Certification with this approach include standards (ISO 9712 & ANSI/ASNT CP-106) which involve training, experience under supervision, written exam.
Levels of NDT Certification
There are three distinct levels of certification designated by the following:
Level 1: Individuals certified with level 1 are restricted to perform only certain calibrations or tests under supervisions by a higher level individual. They are able to report results, and are expected to follow instructions regarding procedure.
Level 2: Individuals certified with level 2 are open to set up and calibrate inspection equipment, perform inspection according to standards and codes. These individuals are able to provide instruction to level 1 technicians, as well as report, interpret and evaluate results. Have knowledge of standards and codes.
Level 3: Individuals certified with level 3 are specialized, experienced and qualified engineers or technicians. These technicians are able to direct NDT labs, and have extensive knowledge about testing, service and manufacturing processes as well as codes and standards.
Reference: American Society of Nondestructive Testing (ASNT)