NON-DESTRUCTIVE TESTING,  FRIEND OR FOE?

Abstract:

Non-Destructive Testing systems are frequently regarded as necessary evils in the tube and pipe industry. The processes involved are mysterious, and of course the test systems have nothing whatever to do with the manufacture of tubular goods.  Some insights are offered to better understand NDT and view these test systems as a product enhancement tool rather than an annoyance.

NON-DESTRUCTIVE TESTING-FRIEND OR FOE?           A.C. Richardson

Introduction:

Tube manufacturers who follow a quality testing program during production should do so with two objectives in mind. The first and foremost objective should be to ensure that only acceptable material is shipped out of their doors. If this first objective is met, then the second should be not to scrap any good material.

The implementation of such a program should be simplicity itself. Select and install suitable Non-Destructive Testing (NDT) equipment, and relax, because both of the objectives will be met.

Sad to relate, the NDT industry cannot offer this utopia to the Tube manufacturer, in fact, as we shall discuss, we are still a very long way away from it.

Existing Methods:

The main feature of any NDT system involves using a phenomenon or physical process to which common engineering materials are "transparent". For example, if we can pass energy through steel we can examine the effects on the energy field which might be caused by a singularity within the steel.

The choice of words here has been very deliberate. A singularity refers to an anomaly which may be present in an otherwise homogenous medium.  Therefore, a singularity may represent a defect in the product, on the other hand, it may not!

An obvious example of the transparency criterion is is in the use of high energy x-rays, which will pass through most materials and produce an image of varying density, allowing direct internal views of an otherwise totally opaque object.

X-ray technology is of course used in the tube industry. Other methods used by the industry include the following:

Ultrasonic Testing, in which pulses of very high frequency sound pass though the material.  Singularities in the material produce echoes, which are interpreted much like radar signals.

Eddy current testing, in which high frequency electric fields are generated in the test object using a coil. Singularities in the material disturb the otherwise uniform electric field, which are then detected by the same or another adjacent coil.

Flux Leakage Testing, in which a magnetic field is induced in the part, with singularities causing distortions of the otherwise uniform field. A detector measures the resulting leakage of magnetic flux from the part.

If an NDT system is used, and any and all product containing detectable singularities is rejected, one would probably meet the first objective (not to ship defects). One would however, fail miserably in the second objective, (not to scrap acceptable material).

If no NDT is applied, or if it is mis-applied, one can expect failure to meet even the first objective, an equally disturbing situation.

It is clear that a tube producer who is following a successful quality program will be optimizing at some point between these extremes.

Unfortunately, it is equally clear that some overlap between the two objectives is inevitable, even in the best regulated situations.    

Capabilities and Limitations:

The preceding section tends to put the entire discipline of NDT into a questionable light; and indeed this is true unless some care is taken to evaluate the methodology and its application.

The NDT system response to various anomalies is highly variable. For example, in ultrasonic testing a very large reflector which is adversely orientated will produce a small echo, while a small but well positioned anomaly will produce a very large echo. Similarly, in eddy current testing, a small surface effect can give a larger signal than a major deep rooted anomaly. Any attempt to increase electronic gain to make the smaller signals read as significant will of course have the effect of further enhancing the insignificant indications.

Operator training and aptitude are equally significant in determining the overall response of an NDT system, and operator motivation is sometimes taken for granted even when it is absent.

The fundamental points being made here are as follows:-

1) NDT systems are non qualitative.

      (They just find anomalies)

2) NDT systems are mostly non quantitative

      (Size of signal is not      related to size of anomaly)

3) NDT systems are only as good as the operator and the set-up.

      (This may be obvious but it is far from trivial)

Advanced techniques do exist, using computer analysis of data to attempt to qualify and quantify raw data, but these are very exotic and at this time, not suitable for data analysis "on the fly" as would be needed for a tube mill operation.

By this point, it should be clear that any tube producer who is implementing an NDT programme should realize that this is not a case of spending dollars and seeing quality problems evaporate as a result of that investment. Instead the chances are  that putting in an on-line test system is often the precursor to an entirely new set of quality problems.

Application of NDT:

The word "defect" may now be introduced into the discussion. The question then becomes how to use an NDT system to find rejectable defects but to ignore those indications which have their origins in trivial anomalies.

To put this in better engineering terms, if the probability of finding a defect is P_D, and the probability of finding something spurious is P_S, then the objective is to fine tune the test system  to the point where

                     P_D=1,  and

                     P_S=0. 

Using currently available technology this is a grand fantasy, and it is likely to remain so for many years to come.  The real issue is how close one can come to this ideal.

This is where the choice of technique and its proper utilization come into play.

Choice of Technique and Application:

Figure 1 is an application chart which we use as a guide to technique selection. It is far from totally definitive, and of course there are exceptional situations which are outside the scope of the chart. 

Based on our experience, the chart illustrates the best method of testing according to product size, material and method of manufacture.

The established practice to calibrate the test equipment is to use artificial defects of known dimensions. This method is often mandated by codes of practice: the API code for oilfield tubulars is a good example of a rigid code.  The various ASTM codes are much less rigorous, often allowing the standards to be established by agreement between customer and supplier.

In cases where no code governs the product being tested, the various ASTM codes can be used to set internal quality standards.

The codes however, specify artificial defects which should be used to set alarm levels on the test system, and which therefore represent the minimal size of defect which should be detected and alarmed on-line. Recognizing that the test system is likely to perform less well on line than on the calibration sample, many operators will increase the signal gain level to compensate. This is not discussed in most codes of practice and it can lead to a hyper sensitive system which rejects too much.  Conversely, other operators often reduce the original calibration gain settings to eliminate "nuisance" alarms.

So, while the codes might be quite explicit in what should be done, the shop floor practices may be wildly at variance with code.

To combat these problems, data loggers are frequently built into NDT systems. These loggers if used correctly, will show all settings used on the equipment, when calibrations were done and record all signals which exceeded the alarm threshold. More recently, we have arranged some systems such that the data logger must be enabled in order to run the unit.

Once the test system has been calibrated, and the data logger is in use, the final step in the process can be taken.  This is to segregate the material marked as defective, to section those indications where there is no obvious defect; and to decide whether the system is operating at the correct sensitivity to suit the need or to meet specification as the case may be.

Provided the test system meets any code-related sensitivity requirements, it is possible to fine tune the process to meet properly the user's needs.

Unless this last vital step is taken, the purchase and use of NDT equipment can result in disappointment and frustration.