In NDT we often talk about the concept of defect. In fact what is a defect ? Generally a defect can be defined as a region of a material or a structure that has different physical properties and existed from the start (construction defect) or a region who its physical properties has been altered compared to its neighborhood over time. When it comes to trying to detect a defect we use different NDT methods. The appropriate method is the one that adapts to the physical nature of the test structure and to its size. If we take for example a metal piece in which there is an internal microcracks,we can detect this microcrack (depending on its size) using ultrasound methods, on the other hand we cannot detect this defect using thermographic images because it is difficult to achieve a thermal gradient on conductor metals and so difficult to achieve a temperature contrast to be detected by infrared camera which can lead to say that of ultrasound point we have a defect and on the other hand thermally speaking there is no defect. All that to say that the existence or not in a given structure is particularly related to the control method used. What I said may seem obvious but it deserves a discussion refer to reality I opened the discussion by asking colleagues are you agree?
Dear Sir Dr Sougrati,
your view of the defects are inline with the normal definition of defects.
The NDT techniques use one of the effects of presence of the defects to detect them.
The ultrasonic testing, magnetic particle inspection, eddy current testing, X ray radiodraphy, ringing test are a few test for detection of voids.
A defect can also be defined as a discontinuity in the properties of a material within the material volume.
I 'm Ok Dear colleague, but to detect a defect we need a sensor and the detection depend on the its nature (method, eddy current testing, X ray radiodraphy, ultrasonic...) and on its sensitivity ... and what we can detect with one sensor, we cannot detect it with an other sensor of different type..
NDT is called as non destructive testing.
ultrasonic method, x ray radiography, eddy current and so on will help to identify and locate the defects.
We should take care of the terminologies while using them. IN NDT,
According to ASTM E1316-17a, a flaw is defined as ‘‘an imperfection or discontinuity that may be detectable by nondestructive testing and is not necessarily rejectable”.
On the other hand a defects are defined as ‘‘one or more flaws whose aggregate size, shape, orientation, location, or properties do not meet specified acceptance criteria and are rejectable”.
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