How NDT acceptance criterias are decided?

Do you mean "Non Destructive Testing"?

"How NDT acceptance criterias are decided?" They would be given to you by the customer, spelled out in the RFQ.

what is the science or physics behind this? IS fracture mechanics one of the factor to decide NDT acceptance criteria ? if yes, HOW?

You MUST provide more information, firstly do you mean non-destructive testing?

Secondly what is the testing in relation to specifically? Is it mechanical testing, electrical, civil, structural, consumer product, etc. Testing methods and procedures vary greatly depending on what you are actually wanting to test, test and verification standards, test and verification criteria, etc.

Exactly what are you wanting to test?

the thing i want is that how Non destructive criteria for various examination like visual, magnetic particle, ultrasonic testing are decided? which physics they are using...? according to ASME SECTION VIII-2 one can reject the material if it is having linear indication length more than 5 mm. so i want to know does they have tested componet with 5 mm linear indication...? do they consider it as crack? does it affect impact property of the material? so these are all i want....if you help me......

Ahh, your specific question is regarding "ASME Section VIII, Division 2" of the ASME Boiler and Pressure Vessel Code, with regard to the section on boiler and pressure vessel examination.

I do not have a copy of this particular standard but I would hazard a guess that the approved inspection methods for deciding if the vessel is cracked are clearly mentioned within the standard mentioned above (along with the inspection period between tests depending on the pressure vessel's application). Obviously a crack or defect in a pressure vessel could result in a rupture of the vessel under pressure (which is very dangerous), and a visual inspection would find a crack too late.

Do you have a copy of the standard?

yes i have....

There are two ways to decide the acceptance criterion

a) By calculation

b) By empirical or by previous experience.

My hunch, _sh... is that the method (b) is the predominant.

Most likely (in fact our designers I have seen doing it) : the zone is critical, heavily stressed, le us go to level 1 (or 01 or 001 as he feels the criticality, though the other two are very rare)

The sizes are initially decided on the method (a). But these are discrete steps, and the steps used for calculation is dependent on situation and not universally applicable as these are made out to be. More unfortunately the associated conditions, with the levels, I have tried to search out but could not get from the ASNT or any other areas, books etc.

The basis of the method (a) is the critical crack size (or critical defect size). On a material (variables its strength, toughness, grain size, condition of HT, retained stress etc), there is a defect , usually a crack (variable its position, orientation etc) , if the size of it is below this, on application condition (variables stress, temperature, stress cycle, its pattern, prestress/load, its direction etc ) will not propogate.

Hence for the given material and application condition, that is the maximum size of defect that can be tolerated, otherwise the equipment will eventually fail.

All the acceptance norms are usually supposed to base on it, however as you can see, it is highly specific and the CCS may be different in a given component at diferent places.

So I presume that based on some empirical assumptions, based on the worst case scenario these are taken and actual CCS will me much larger than this.

(Just for curiosity sake I did this quite a few years back that was the finding then, of course I could not find the weakest spot where the CCS was equal to the specified )

PS: Note the fatigue is the usual criteria for calculation (obviously that is the prime mover for crack growth)