Non-Destructive Evaluation (NDE) or Non Destructive Testing (NDT) is the technique of applying physical principles to determine the characteristics of materials, components, or systems. It is also used for detecting and assessing inhomogeneity and harmful defects without impairing the usefulness of such materials, components, or systems. NDT enables defects in the products to be examined without affecting the performance of the item being testing.
Non-Destructive Evaluation refers to a wide group of analysis techniques that are used to identify defects and evaluate the properties of a material, component or system without compromising its integrity or function. Because NDE does not harm the item being inspected, it can save time and money in product evaluation, troubleshooting, and research.
NDT plays an important role not only in the control of quality of products but also during various stages of manufacturing. NDT is also used in condition monitoring of various items during operation to predict and assess the remaining life of the component while at the same time retaining its structural integrity. It enables optimum utilization of components without sacrificing safety. Operator dependency for routine inspection is reduced thereby allowing personnel to concentrate more on technological aspects. The end result is savings in time, cost and improvement in precision and reliability of the output.
In the aerospace sector, NDE processes are used to find cracks, internal voids, leaks, contamination, and other defects in raw materials, components and assemblies. NDE processes can also be used for:
- Measurement of some physical properties, such as hardness
- Real-time monitoring of parts, such as internal stresses during production
- Measurement of dimensions (such as wall thickness or inside diameter)
- Periodic in-service monitoring for growth of previously detected defects.
The three stages where NDT is applied are (1) pre-manufacturing stage for inspection of raw materials (2) during manufacturing to check if the product meets its specifications and (3) during the service life of a product to check fitness for purpose and make life prediction. NDT offers the following advantages:
- Tests are made directly on the object. 100% testing on actual components is possible
- Many NDT methods can be applied on the same part, hence many properties of interest can be measured
- In-service testing of components is possible
- Repeated checks over a period of time are possible
- Very little preparation is sufficient to conduct NDT
- Most NDT methods are rapid
NDT methods range from simple to intricate. Visual inspection is the simplest of all NDT methods. The choice of a specific NDT method depends on factors such as availability, cost and suitability based on analysis and past experience. Another important factor to be considered is the type and origin of the defect in the product to be tested. The use of one NDT method is sometimes necessary to confirm the findings of another. Capabilities of one test method may overlap with those of another. No single method, for example, is capable of revealing all surface and subsurface discontinuities in all types of materials. For these reasons it is necessary to use a combination of tests to obtain complementary information helpful in evaluating the quality of the component.
NDE methods are verification tools that can be used as part of a project’s overall verification & validation plan at all system development stages, including:
- Design Development
- Design Qualification
- Acceptance of deliverable Flight Hardware
With NDE processes, hardware can be examined without compromising its integrity or function. Even though NDE detection capability may be imperfect, it may be advantageous to use NDE methods on all critical articles, rather than destructively examine just a small sample of them.
Of course, periodic destructive testing can be used in conjunction with more frequent NDE methods to qualify NDE process parameters and to monitor for ongoing control of NDE processes.
Today Non-Destructive Evaluation is becoming increasingly important in the design and production cycle for several reasons.
- As Flight Hardware grows increasingly complex, the consequences of undetected defects and failures increase rapidly.
- The cost of destructive Testing of critical and complex hardware increases as material and labor costs escalate.
- Destructive Testing can result in a sample size that is too small to provide a statistically sound basis for product acceptance.
Therefore, additional emphasis is being placed on the use of NDE processes early in the development of:
- Product designs
- Advanced materials
- Production processes.
The application of advanced Non-Destructive Evaluation technologies is most widespread in the aerospace, defense systems and nuclear power sectors. This is due to the critical and complex nature of the hardware and the risks associated with in-service failures. In order to keep pace with new materials and their design applications, NDE technologies are constantly being improved and new NDE methods developed. An example is the rapidly expanding use of composite materials in primary structures in critical Flight systems.
New piezo-electric materials and optical fibers allow for the development of so-called “intelligent” structures that can self-monitor and may also be able to respond to their environment.
Advances such as the use of lasers and various imaging technologies have made non-contact NDE methods more attractive in many applications. There is also a trend toward more automated NDE methods that can minimize operator-induced variability. More powerful computers permit signal enhancement and the use of software algorithms that improve the quality of the results obtained from both new and traditional NDE technologies.
There are literally hundreds of standards that govern the use of Non-Destructive Evaluation methods. The standards that NASA has deemed mandatory are:
- Federal Acquisition Regulations, Part 46, Quality Assurance
- National Aerospace Standard NAS 410, Certification and Qualification of Nondestructive Test Personnel.
- NASA-STD-5009 Nondestructive Evaluation Requirements for Fracture-Critical Metallic Components.
There are several valuable resources for more information on advanced NDE methods. NASA’s NDE Working Group seeks to innovate, standardize, and apply such advanced NDE methods across the Agency. For additional NDE information and to learn about the NDE Working Group’s specific activities that support each Center, please click on the link shown here http://www.nnwg.org.
Since NDE is a specialist field, two professional societies play major roles:
- The American Society for Nondestructive Testing (or ASNT) is a primary U.S. focal point for NDE research and standardization. http://www.asnt.org
- ASTM International, formerly the American Society for Testing and Materials (ASTM), is also engaged in the standardization of NDE and other Testing methods. http://www.astm.org
Before getting into the details of Non-Destructive Evaluation, let’s clarify the terminology we’re using. The terms Non-Destructive Evaluation, Non-Destructive Test, and Non-Destructive Inspection are all commonly used. NDE and NDT are recognized as interchangeable terms in NASA Standard 8709.22, Safety and Mission Assurance Acronyms, Abbreviations and Definitions. We will use the term Non-Destructive Evaluation in this course. Non-Destructive Inspection (or NDI) is a subset of Visual Inspection that uses borescopes, ultraviolet lights, and other methods.
Here are a few key points to remember: NDE is used to identify defects and evaluate the properties of a test item without compromising its integrity or function. It has become increasingly important during design and production due to:
- The complexity of flight hardware
- The cost of destructive testing
- Risks associated with in-service failures.
A good YouTube channel presenting various NDE methods can be found at: https://www.youtube.com/channel/UCSIb8aIGcp7Trr48SAUaCvQ