Training: Often the missing link in using NDT methods

Nondestructive test (NDT) methods are indirect methods that rely on the interactions of some type of mechanical or electromagnetic radiation with internal irregularities. Differences in response are used as indicators of the presence of these irregularities. Often the underlying physics are complex and not the type of information that most civil engineering students have studied, other than in basic physics courses. Yet civil engineers are called upon often to evaluate structures using various types of advanced NDT methods. Successful application of these advanced methods requires that persons taking the data and analyzing the data have a good understanding of the principles of the methods, how to set up data acquisition parameters for optimum results, and how to interpret recorded data. This requires investing resources in training, which is too often neglected because of budgeting or scheduling constraints. Lack of training can lead a user to arrive at incorrect conclusions from the NDT survey, which will cast a negative image on the NDT method. Thus when planning on acquiring NDT equipment, the budget should include sufficient funding for proper training. The benefits will outweigh the initial cost. This paper uses the impact-echo method as an example to illustrate the kind of basic information that should be understood by all members of the team conducting an evaluation using this method. The topics covered include: basic principles of stress-wave propagation and reflection; the vibrational response of plate-like structures and prismatic members; frequency domain analysis of waveforms; frequency content of impact force; selection of data acquisition parameters; and minimum detectable flaw size.