First a definition should be made since very often, this is not seen to be clear.
A wallthickness measurement method may also be seen as a flaw detection method. This is because wallthickness measurement is a defect monitoring device and has an "out of tolerance" function. However, it does not come under the remit of classical flaw detection. There are sometimes small wall thickness deviations, such tests are more or less relevant to Flaw Detection. Classic defect types such as conclusions and surface cracks obviously belong to the Flaw Detection term.
Flaws in Extrusions can arise because of:
Ultrasonics is well proven to detect such flaws and has in the last 40 years, greathy expanded in field of non plastics. Not only the manual testing, but also the in-line testing is widely used.
The investment, especially for in-line testing, is very high. The costs for such installations are in the range of 0.5 - 5 Mill. DM. The plastic industry is not ready to such high investment.
For this reason some pipe producers have already begun to use the present wall thickness system in conjunction with the flaw detection function. This can only be successful if the wall thickness system consists of some basic functions- listed later.
1. High Pulse Repetition Frequency (PRF)
The PRF is essential for the density test. A small flaw that runs with high pipe speed under the transducer, must be hit. For instant is by PRF of 1 KHz and a line speed of 60 meter/min., one pulse per millimeter is generated.
2. Test Mechanism
Reversible Scanner - scanning a spiral over the pipe surface. These are not useful for flaw testing.
Rotation Scanner - functional for a 100% test, but have high maintenance needs and low cost versions are not yet proven for the plastics industry. The mechanics of the steel market are expensive.
Fixed Transducers - scanning a line at pipe axis and gaining, depending on pipe diameter and flaw size, a certain test density. Example: by testing with 8 transducers, a pipe diameter of 18 mm and a flaw size of 1 - 2 mm, a 50% test can be gained. This method is less faulty and has the potential, by means of large transducers, to increase the test density by means of an inexpensive design.
3. Signal evaluation
The system must have the capability, by parallel processing, to operate the two basic evaluation methods.
1. Direct/Coincidence Flaw Detection - by reflection at the flaw. The flaw has good reflection characteristics, it over-excees the evaluation threshold and releases an alarm. Close surface flaws and bad reflecting flaw, cannot be detect.
2. Indirect/Anti-coincidence Flaw detection - by monitoring the back wall echo and it's shadow of the flaw. The flaw has a low reflection, and spreads the sound. The flaw is detected by means of under-exceeding the evaluation threshold. Surface flaws and close to the surface flaws can be detected too.
4. Adjustable sensitivity
This is a pre condition for successful use. If this item is not considered, the disadvantage of too many fault alarms will occur, or ignorance of the real flaw could transpire. Important parameters are: threshold, dynamic noise suppression, alarm counter.
5. Echo Signal-Display extension
The system should have at least an oscilloscope connection. Through this the echoes and the gates are displayed. This function is a pre-condition for the operator to carry out his work in relation to sensitivity adjustments, and evaluation of the natural or artificial flaws.
Wall Thickness defects and flaws must release an alarm, trend and record. Also the system must have an output for a marking device. A delay between test position and marking device should be considered. This is again a pre-condition to assist the operator when adjusting the optimum parameter settings.
If there is a need for a 100% test, this can only be decided by the users. Some have the opinion, that the great effort is not necessary. Also it's a statement in practice: the flaws often coming like an avalanche, and for that reason flaws can also be detected with less test density.
Skilled operators are necessary for the system displayed. The suppliere must deliver a good system's manual and also a operator training at customer's side. The training courses sometimes offered by NDT organizations, are often not well suited, because they are to general and focused on steel applications.
Rolf Diederichs 18. Dec 1995, email@example.com