There are many different non-destructive testing techniques recognized by the American Society for Non-destructive Testing (ASNT) including Acoustic Emission Testing (AE), Electromagnetic Testing (ET), Ground Penetrating Radar (GPR), Laser Testing Methods, Leak Testing (LT), Liquid Penetrant Testing (PT), Magnetic Flux Leakage (MFL), Magnetic Particle Testing (MT), Microwave Technology, Neutron Radiography Testing, Radiographic Testing, Thermal/Infrared Testing, Ultrasonic Testing, Vibration Testing, and Visual Testing. It is interesting to note, however, that most of these techniques process non-destructive testing information in the same sequence to determine the final conclusion on an indication or to answer the existential question; is it a defect or is it not a defect?
ASTM E1316, Standard Terminology for Nondestructive Examinations is good starting point from which key non-destructive testing terms are defined. Additionally, this standard provides a simple but useful chart that presents the basic decision making steps that follow an non-destructive testing (NDT) indication through interpretation to acceptance or rejection. First let’s review the some basic non-destructive testing definitions and see where they fit into the decision making process.
Non-destructive testing indication: The response or evidence from a nondestructive examination. This is located at the top of the process.
Non-destructive interpretation: The determination of whether indications are relevant, non-relevant, or false.
False indication: A non-destructive testing (NDT) indication that is interpreted to be caused by a condition other than a discontinuity or imperfection.
Relevant indication: A non-destructive testing (NDT) indication that is caused by a condition or type of discontinuity that requires evaluation.
Non-relevant indication: A non-destructive testing (NDT) indication that is caused by a condition or type of discontinuity that is not rejectable.
Evaluation: Determination of whether a relevant non-destructive testing indication is cause to accept or to reject a material or component.
Applying the Process to Magnetic Particle Testing
The process may be applied to visual magnetic particle testing and basic physics and/or material properties may be further used to confidently discriminate between a relevant or non-relevant indication. Considering the non-destructive testing data presented below. A36 steel, 0.375” thick, is welded together using a single-vee configuration.
The weld is inspected using visible magnetic particle testing, red particles, and an AC yoke. Two indications are clearly observed. The first is along the weld toe, located towards the top of the plate, and is approximately 1” long. The second magnetic particle indication is located on the bottom of the plate and is less than 0.5” long. Starting at the top of the non-destructive testing process diagram, magnetic particle testing was selected to inspect the weld for surface discontinuities. Great choice and a requirement in most common codes include AWS D1.1 Structural Welding Code – Steel, AWS D1.5 Bridge Welding Code, API 653 - Tank Inspection, Repair, Alteration, and Reconstruction, and other codes. Clearly, two indications are observed and it is necessary to move into the interpretation stage. We must now ask the existential question and determine whether these indications are relevant or non-relevant. First, let’s consider if the non-destructive testing indication is even in the location of interest. In weld inspections, the area of interested includes the weld metal, heat affected zone, and adjacent base metal. So we can apply a location based filter. If the indication is outside this area it will be non-relevant. Clearly the upper indication is in the heat affected zone and the lower indication is in the weld metal.
We now move forward into the intepretation stage. Is this top magnetic particle indication a false indication, a relevant indication, or a non-relevant indication? Could the top indication possibly be a false indication? Recall the definition of a false indication, “A non-destructive testing (NDT) indication that is interpreted to be caused by a condition other than a discontinuity or imperfection”, and consider some potential sources for a false indication. These may include grease on the surface or other contaminants that were not eliminated during the important pre-inspection cleaning process. Note that the most diligent non-destructive testing professional using the best equipment will not detect material defects if the part tested in not prepared properly. In fact, for visual techniques the argument can be made that this is the most important step in the non-destructive testing process. So let’s add pre-inspection cleaning to our non-destructive testing process flowchart. Based on the appearance of the part, the pre-inspection cleaning was thorough so the false indication option can be ruled out with confidence.
Moving forward, it must be determined if the non-destructive testing indication is relevant or non-relevant. Starting with the top indication, there are two likely scenarios. First and more commonly, the indication is related to the change in geometry at the base plate – weld reinforcement. Less likely, the indication is from a weld toe crack. The major difference between former and latter is that the toe crack is surface breaking. From basic physics and material science, we know that a residual magnetic field will only be present if the discontinuity is surface breaking. In other words, after we have stopped applying a magnetic field to the plate and weld, does a small residual magnetic field remain? Inspector skill may be used to answer this question. By gently blowing the magnetic particle indication with a magnetic particle blower or bulb, it is possible to determine a non-relevant geometric indication from a relevant surface breaking indication. Even with a very subtle application of air pressure, the red magnetic particles will be displaced from the non-relevant geometric indication. However, the residual magnetic field from a surface breaking discontinuity prevents the red particles from being displaced under the same air pressure.
Let’s assume both indications were not displaced by a subtle application of air pressure and are correctly classified as relevant indications. Can we move forward into evaluation or is further investigation required? Have we, with 100% certainty, determined the surface breaking discontinuity is surface crack or is it simply a deep scratch? A scratch may be buffed out using an appropriate grit sand paper or wire brush wheel up to an acceptable depth. No indication will be present upon retesting. Once the surface scratch versus crack question is resolved, it is possible to move into the evaluation stage. The evaluation stage consults the appropriate code or standard that outlines the accept/reject criteria based on defect type and length. For example, in most codes any type of crack regardless of length is cause for rejection. Some codes allow for a certain amount of porosity, however. Presented below is the non-destructive testing process customized for magnetic particle testing scenario presented in this article.
Non-destructive testing is a process that should be undertaken in a series of logical steps that provides the inspector with the greatest chance for success. Regardless of the non-destructive testing process selected and in the information processed, the general process is similar. The process should be customized for the inspection goals established by the client, non-destructive testing service supplier, and, and relevant code or standard with non-destructive testing accept/reject criteria.