When comparing liquid penetrant testing with other methods of nondestructive testing, the primary difference is the type of the discontinuity that can be detected. Liquid penetran testing can only detect discontinuities that are open to the surface.
Magnetic particle testing and Eddy current testing are limited in the depth of subsurface discontinuities they can detect. Ultrasonic testing and radiographic testing are routinely used to detect subsurface discontinuities.However, when economy is a factor, penetrant has a primary advantage over the other methods.
Penetrant does not require special equipment and does not always require electrical power, which makes it a great field testing method.
Another valuable feature of penetrant testing is the lower inspector training and experience hours required compared to other methods of nondestructive testing. For many managers, this is a factor in determining what nondestructive testing method will be used. It is difficult for a production manager to invest a great deal of time and money to qualify a technician to a level II in electromagnetic testing when the same technician can be qualified to level II in liquid penetrant testing at a fraction of the time and expense. While a level III ay se the value of investing in training and certifying a level II electromagnetic testing technician, production managers are often faced with the tight production schedules and limited budgets.
Penetrant as a complementary method.
Just like liquid penetrant testing, the other nondestructive testing methods have improved over years. They are able to detect extremely small discontinuities with a high degree of reliability. However, liquid penetrant testing is still viewed by many as the primary method for verifying surface indication detected by these penetrant testing is used properly, it has performed exceptionally well.
Selection of a penetrant method.
The selection of the proper liquid penetrant testing technique for a particular part is a function of a level III inspector. This is the area where the level III inspector must understand all the factors involved in selecting the proper penetrant process.
The following are some of the factors that should be considered when selecting the liquid penetrant testing process.
The location of testing should be another consideration in the selection process. Testing in the field somewhat limits the process because it is performed under less than ideal conditions. Shop testing always allows the most detail when processing parts.
The number , weight and size of the parts are factors that should be considered when estimating the hours and materials that will be used. If there are only a few of small parts to be tested, a Method C penetrant test may be the ideal process. If there are hundreds of small parts, Method D testing may be the ideal process. The weight of parts is a factor because heavy parts require special handling.
The type of discontinuity expected to be detected must be consider. A small crack will require a very high sensitivity penetrant, while a deep, large indication will require a less sensitive discontinuity, its criticality must be considered to determine the reliability and safety requirements of determine the reliability and safety requirements of the part. In addition, the size of the discontinuity that can be allowed the reliability and safety requirements are exceeded must be know and considered. The type and size of the discontinuity will also influence the penetrant dwell time. For instance, penetrant testing for stress corrosion cracking can require extended penetrant dwell time.
The surface condition of the part also influences the selection process. On a very rough surface, a water washable penetrant with less sensitivity may be required. A rough part will also require special cleaning measures, thus increasing processing time and hours. An aircraft engine part with a smooth surface will require a high sensitivity, postemulsifiable penetrant.
The selection of the developer may have the highest sensitivity, it also has the highest cost and can be difficult to remove. As with penetrant selection, the number, size and condition of the parts are factors in determining the type of developer.
The entire selection process is invalid without acceptance and rejection criteria. It is important to match the penetrant process and materials to the acceptance and rejection criteria.
Acceptance and rejection criteria are based on stress and fracture mechanics and engineering evaluations. These evaluations are designed to analyze the critical points of a material or part and provide accurate discontinuity tolerance data. Equipped with this data, the proper selection of penetrant, developer and process procedures can be made.