Crack length

[corus]

What methods are available for measuring the change in length of a crack, or for that matter if the crack is growing? The crack itself is normally inaccessible without major dismantling although the crack opening at the surface can be seen.
Is it also possible to determine the age of a crack?

 

[Carburize]

Several methods exist to monitor crack growth such as ultrasonic, acoustic emission and potential drop techniques.
The age of a crack can sometimes be determined depending on the environment or the growth mechanism. If it is a fatigue crack some estimate can be made via the striation pattern, if it is in a corrosive environment the thickness of the corrosion product can be used and I have seen forensic data presented using the types of pollen trapped in the surface to determine the age of a crack.

 

[FRChem]

This is very important Q, we need more expertise to add to this subject.

 

[corus]

Thanks for the suggestions Carburize. The enivronment that the structure operates in excludes continuous measuring methods such as acoustic emission, and I've been advised that the other 2 methods wouldn't work either. I have heard of measuring the crack opening, rather than the crack length, by measuring the distance between two marks made either side of the crack opening, but access is a problem. I'm also not sure that the measuring equipment is sensitive enough to measure a change in the crack opening and I'm also not sure if it changes from one unloaded state to the next.

 

[Metalguy]

What kind of loading and environment are involved? What is the material?

 

[corus]

The environment is noisy, hot, and steamy. The material is a cast steel and the crack is subject to both tensile and bending.

 

[Maui]

Liquid dye penetrant will probably be the most effective method that you will be able to use, based on your description of the problem. It's a simple technique, and works well. But it will only give you information regarding the crack length on the surface, and will not provide you with any information about the internal geometry of the flaw. If you need to know how the flaw is propagating internally (which I suspect that you do) radiography may be your only other practical option.


Maui

 

[Carburize]

It sounds like your best option would be a routine UT inspection during shut-downs. This is used in nuclear reactor inspection to check for any growth of flaws.

 

[5240]

Try Time of Flight Diffraction (TOFD).This ultrasonic base inspection was developed for in service inspections to monitor crack growth to maximise the usage of that component.

 

[SMF1964]

I agree with 5240 - we've used TOFD on high temperature piping during outages with reasonable success. I've also had ultrasound used to measure cracks using what the inspector called crack tip diffraction (?) method.

Have you considered radiography?

 

[corus]

Liquid dye penetrant was used to discover the crack. It is now proposed to use an endoscope to look internally without dismantling everything, and thus check the crack growth on the internal surfaces visually. I'll check on radiography and TOFD. Many thanks for the suggestions.

 

[MOB1]

Radiography is not generally too clever at measuring or even detecting cracks.
I would agree that ultrasonic inspection in the hands of a good operator is generally the best technique so long as the casting has been heat treated correctly and therefore does not attenuate the sound excessively.