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Old 15th Nov 2011, 08:35
  #1390 (permalink)  
blakmax
 
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NDI methods and their relevance to airworthiness

Sorry 9A+ I have been away on R and R, so I did not have a chance to reply.

In reality it does not matter what method is employed, the essence of the matter is that NDI depends directly on managing defect detection before the defect exceeds a critical value where failure may occur. Different methods may have different levels of accuracy and while that accuracy is sometimes important, it is even more important to understand the validity of the estimated "critical defect size". This is usually determined by the OEM by analysis or testing, typically based on artificial defects, usually teflon inserts for testing and disconnected elements for analysis. This procedure is known as Damage Tolerance Analysis (DTA).

So how valid is DTA? A fundamental feature of current DTA methods is that they all assume that the adhesive adjacent to the defect maintains the original properties (especially strength) of the original adhesive material. In reality, there is only ever one condition where DTA is valid, and that is in assessing the significance of defects identified in post-production inspections, and then only if the defects are large voids.

Let me be clear: If the defects are the micro-voids (porosity) exhibited in some AW139 disbonds, then DTA is inappropriate because the adjacent material will have a lower strength than the pristine bond material without porosity. Unless the testing or analysis is based on a reduction in localised strength, the analysis is invalid.

Next, if the defect is an in-service disbond, then again the localised material properties will be lower than for pristine material because in-service disbonds are due to interfacial degradation (or fatigue of micro-voided bonds such as those which have been reported in AW139 boom structure) so again DTA is invalid.

This is explained in my recent paper http://www.adhesionassociates.com/pa...d%20Joints.doc

So really the method of inspection is not really relevant; the validity of the defect size you are looking for is the really important issue. It is a bit like argueing about how many angels one can engrave on the head of a pin, when the real question is if the pin will hold up your trousers!

Regards

Blakmax
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