Nondestructive Evaluation Engineering

Nondestructive Evaluation (NDE) Engineering is the science and practice of safe design: designing inspection and verifcation processes to ensure safety and structural integrity. Where structural failure can cause serious injury or loss of life, we can't afford significant random failures, so we build significant strength margin into the structure. Unfortunately the only direct way to confirm the excess strength is by testing to failure (destructive testing).

NDE Engineering is a component of a larger safety engineering process. Safety engineering aims to reduce risk through qualitative and quantitative analysis of risks and processes for mitigating those risks. Safety culture is the organizational trait where safety and risk are taken seriously, and where speaking up is encouraged and valued.

Design of a structure centers around mitigating the various possible failure modes. Safety relies on significant margin before failure, but as long as the structure is intact you don't know how big that margin really is. Destructive testing can be used to determine the amount of margin, but is often impractical especially for larger or more expensive structures. By analyzing the structure as an assembly of components, the requirements on the structure can be translated to requirements of the components through a process known as systems engineering. However, one must be prepared for emergent behaviors where the system has new failure modes beyond those of the components.

Redundant structures can tolerate failures, but inspections may be needed to detect those failures and failure of one member can overload another. Some statistical analysis will be needed to keep the overall risk low enough.

The structural design must consider the materials used, types of failure, and degradation mechanisms. The design needs to consider overstress, fracture, and fatigue failure mechanisms in the context of material degradation such as corrosion and hydrogen embrittlement. Stress corrosion cracking is a particularly dangerous phenomenon of corrosion-accelerated fatigue.

Nondestructive testing (NDT) and Nondestructive Evaluation (NDE) are used to qualitatively and quantitatively confirm material integrity by detecting flaws such as cracks, presence of corrosion, or incorrect microstructure. NDT and NDE can be performed at manufacture and/or once the structure has been placed into service to detect developing damage or fatigue cracks.

Ensuring structural integrity requires identifying and mitigating all the possible ways the strength of the structure can degrade, for example from fatigue due to cyclic loads. The idea of "safe life" is to retire a part based on the number of cycles it has been exposed to, limiting overall degradation of the material. The safe life is based on some fraction of the expected number of cycles to failure at a given load, expressed in an S-N curve. Sometimes the safe life will be treated as infinite if the load is below the material's fatigue limit.

Material degradation is a very random process and it is possible for a crack to initiate even within a so-called "safe life". The growth rate of fatigue cracks in ductile materials can often be reasonably predicted through Paris Law. So long as the crack can be reliably detected for repair or replacement before the crack can grow to catastrophic failure, the structure is still safe.

Flaw sizes are important thresholds of crack length used to determine what is safe. To a structural engineer or materials scientist the critical flaw size is the length of the smallest crack in a structure that might cause immediate catastrophic failure. The capability or reliably detected flaw size of an NDE technique is the length of the smallest crack the NDE technique can consistently detect. The NDE engineer specifies the inspection interval based on the number of use cycles for a crack to grow from the reliably detected flaw size to the critical flaw size_ with a suitable factor of safety to accommodate uncertainty.

The reliability or probability of detection (POD) of an NDE technique is how consistently a flaw can be found. Such knowlege, with appropriate safety factors, can be used to select the inspection interval and NDE and NDT processes so as to ensure acceptible levels of risk. It is common to define the reliably detected flaw size as the crack length for 95% confidence of 90% POD, referred to as a90/95.

NDE engineers are responsible for NDT and NDE processes. The NDE engineer needs to understand the structural, strength, and material degradation aspects of a design, as well as the various sensing characteristics. The needed education must be both broad and deep and often acquired over the course of a career.

NDT and NDE processes are usually performed by technicians who are certified by their company and/or an outside certification agency such as the American Society for Nondestructive Testing (ASNT). Certification is usually in levels I-III based on experience and education.

The performance NDT and NDE inspections will never be perfect. Both equipment and human performance can vary. Equipment variability must be mitigated by procedures such as routine testing against calibration standards. Variations in human performance must be mitigated by considering human factors in the design of the inspection processes with the context of the organizational safety culture. Both equipment variability and human variability need to be included in the POD studies in the overall risk assessment.

The practice of NDE can also raise safety issues directly. One example is in radiography, due to the biological hazard of radiation. Engineered radiation safety uses quantitative engineering principles to mitigate the risks of radiographic NDE.