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Technical Discussions
Graham Stephen
Graham Stephen
09:02 Sep-21-1998
Fault detection

I am curious to know if anyone has been involved
in detecting flaws before phiysical damage is shown
(ie. "pre-crack" fault detection in steel).



    
 
 Reply 
 
Godfrey Hands
Consultant,
PRI Nadcap, United Kingdom, Joined Nov 1998, 298

Godfrey Hands

Consultant,
PRI Nadcap,
United Kingdom,
Joined Nov 1998
298
08:40 Sep-22-1998
Re: Fault detection

: I am curious to know if anyone has been involved
: in detecting flaws before phiysical damage is shown
: (ie. "pre-crack" fault detection in steel).


I assume here that Graham is talking about condition monitoring structures or components.
I have been involved now for some years with Resonant Inspection. This technique analyses the component's or structure's Eigen modes, and these are affected by dimensions and material properties.
Material properties is a very general term, and includes things like microstructure, alloying elements and RESIDUAL STRESS, which I assume is responsible for the onset of cracking.
This means that by monitoring the resonances of a component or structure, we should be able to detect changes in residual stress, perhaps before cracking is evident.
Resonanat Inspection, as we are applying it, is very new, and I myself have no experiences with monitoring Residual Stress. Perhaps you can get more information from Quasar International in New Mexico, USA.
E-Mail info@quasarintl.co

Another technique that is used to monitor Residual Stress is Barkhausen noise, but I have no experiences there.


    
 
 Reply 
 
Ed Ginzel
R & D, -
Materials Research Institute, Canada, Joined Nov 1998, 1261

Ed Ginzel

R & D, -
Materials Research Institute,
Canada,
Joined Nov 1998
1261
03:44 Sep-22-1998
Re: Fault detection
: I am curious to know if anyone has been involved
: in detecting flaws before phiysical damage is shown
: (ie. "pre-crack" fault detection in steel).

Graham:
Could you please be a bit more detailed about the application
the question is based on. Nearly all NDT is based on the
principle that the test is to locate areas of "potential"
failure. This is normally dealt with as acceptance criteria
and examples of how they are derived are seen in many Codes
such as ASME Boiler and Pressure Vessels, CSA Z662 and
API 1104 (plus many more). Fracture mechanics will
indicate that not all flaws are reason for concern
and you will need to know several things about the
product, such as, material properties, dimensions,
service conditions (stress direction and magnitude),
is loading static or cyclic, etc, etc, etc.
Fracture mechanics equations then usually want some
sort of input on the flaw. Mostly flaw size perpendicular
to the direction of stress and where the flaw is with
respect to the surface.

One then has to assess how reliable the NDT test is and
the probability that the most critical flaw conditions
have been detected. E.g. did you select the best test
for detecting the most likely flaws and does the test
provide you with a useful means of sizing it for fracture
mechanics.

Therefore, simply "detecting flaws before physical
damage is shown (ie. "pre-crack" fault detection in steel)"
is not sufficient information.




    
 
 Reply 
 

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