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Non-destructive examination NDE is used to detect cracks and other flaws in a part without rendering the part unusable by performing destructive testing. These inspections can increase the reliability of the part by providing early detection of flaws that could lead to a failure.
The purpose of this article is to describe these basic NDE techniques and discuss some of the advantages and disadvantages of the different methods. Liquid penetrant testing Liquid penetrant inspection is the most popular method to check for surface cracks because it is simple and no special equipment is required. With this technique, liquid penetrant is applied to the surface of a material and is drawn into surface discontinuities such as cracks, pits and seams by capillary action.
This technique can be used on a variety of ferrous and non-ferrous materials, including steel, stainless steel, aluminum, bronze, tungsten carbide, ceramics and plastics.
ASTM E165 02 – Español
In most cases, the discontinuities are visible to the naked eye without liquid penetrant testing. The liquid penetrant examination just makes the discontinuities easier to see.
An example of a gear tooth crack that was made more visible using a dye penetrant examination is shown in Figure 1.
The visible dye penetrant technique involves the following steps:. Another technique involves the use of an ultraviolet light to view the surface discontinuities.
This method substitutes fluorescent penetrating oil for the visible dye penetrant. After application of the developer, the visual inspection is done with a black light. The penetrant that is trapped in any discontinuities will glow when viewed under a black light. One of the most critical aspects of liquid penetrant testing is interpreting and evaluating the discontinuities. It is a common misconception to label a discontinuity as a crack.
The correct term for a discontinuity is an indication, which is not necessarily a crack or a flaw.
A scratch or a pore can easily be misinterpreted as a crack or flaw, and this can lead to unnecessary and costly repairs. To avoid this, any indications should be investigated by polishing and re-evaluating the indication. Also, there are no universal acceptance criteria on the maximum allowable size of an indication. Some equipment manufacturers allow some surface indications, depending on their size and location on a particular part.
Advertisement d165-02 Go to Page 1 2 3 4 Edit Story. Box Dubuque, IA Phone: Energy-Tech Magazine Contact us. Some of the most popular NDE methods include: The visible dye penetrant technique involves the following steps: Pre-clean the surfaces to be inspected. This might involve sandblasting, wire brushing, etc.
The surface then needs to be solvent cleaned to remove any greases or oils. Penetrating oils typically contain a red dye and are sprayed or brushed onto the area of the material being tested.
ASTM E(02) – [PDF Document]
A suitable waiting time allows the penetrant to enter the surface discontinuities, and then excess e165-0 can be removed. This is typically done with a special cleaning solvent.
Care must be taken not to overly clean and remove the penetrant from the discontinuities. Then the surface must be allowed to dry. As soon as the material is dry, the d165-02 is applied. The developer brings the penetrating oil out of the discontinuities by a blotting action. This can take minutes after the developer is applied.
Cracks typically show up as red lines and pits usually asttm like red dots. The part then needs to be cleaned to remove the penetrant and the developer. A common recommendation is to use solvent and not potable water when cleaning stainless steel parts, such as turbine rotors, that will be highly stressed when in service.
The reason is that some potable water sources contain chlorine, which can leave a residue on the surface and can lead to cracking in some stainless steels.
Non-destructive examination basics Patrick J.