Crack Definition 3

[Ruag]

Good day guys,
Do you know if there is a std definition of crack?
I have heard that in NDT there is a crack when the length of the defect is three times more than its width. Unfortunately I have not found any document stating that.

Thanks,
D.

 

[TVP]

Terms like crack, defect, etc. tend to be described in very broad terms in consensus standards from SAE, ISO, ASTM, etc. Usually it is a product standard for a fastener, forging, casting, etc. that sets the acceptable limit for a crack, etc. Here are a few definitions from various standards:

SAE AMS2647
Discontinuity - Any interruption in the normal physical structure or configuration of a part, such as cracks, laps, seams, inclusions, or porosity. A discontinuity may or may not be a relevant defect.

ISO 6157, ASTM A788, etc.
Crack — A crack is a clean (crystalline) fracture passing through or across the grain boundaries and may possibly follow inclusions of foreign elements. Cracks are normally caused by overstressing the metal during forging or other forming operations, or during heat treatment. Where parts are subjected to significant reheating, cracks usually are discolored by scale.

ASTM E1316
defect, n — one or more flaws whose aggregate size, shape, orientation, location, or properties do not meet specified acceptance criteria and are rejectable.

discontinuity, n — a lack of continuity or cohesion; an intentional or unintentional interruption in the physical structure or configuration of a material or component.

flaw, n — an imperfection or discontinuity that may be detectable by nondestructive testing and is not necessarily rejectable.

 

[rb1957]

why do you need one ?

 

[MiketheEngineer]

In my book - if you can see it - it is cracked or punctured

 

[monkeydog]

There are flaws that are inclusions, which are not cracks or punctures.

 

[swall]

I am thinking that for a discontinuity to be designated as a crack, it had to have been formed due to tensile stress.

 

[SWComposites]

swall - probably true for metallic materials, not necessarily for other materials; besides, often there is no way to determine how a damage was initiated (at least not without destructive cut-up and inspection).

Ruag - why do you need a definition? what materials are you dealing with? in what context do you need to do an inspection and need a definition? many aircraft SRM's have definitions for different defect / damage types.

 

[mfgenggear]

Mil-Std-1907 free to view on Assisted Docs search

It's interesting someone would need a definition of a crack.
It would in my mind would be for metals.
& it is not allowed for aerospace/aviation critical applications regardless of it's size.
It's probably taken for granted by most what a crack is.
There's welding cracks, heat treat cracks, grinding cracks, straightening cracks, bending cracks & on.

but it's possible for other non metallic materials.
or like plastics, stones or what ever.

see attached Mil-std-1907 definitions

Mfgenggear

 

[mfgenggear]

In addition if there is in process NDT procedures, it can be discovered in the process. & can be isolated to which manufacturing process is causing the defects. early in the manufacturing cycle.

Mfgenggear

 

[der8110]

If you have to ask, it probably is.

Whatever you call it, crack, flaw, or discontinuity, the result is the same: things are no longer held together. Which depending on the loading and remaining residual strength, could result in a problem (and does not matter if metal, CFRP, or stone). Safety is at risk if the flaw/crack cannot be detected before failure.

The back story must be very interesting. Please divulge if you can.

 

[rb1957]

@mfg, the statement that cracks are not acceptable at any size is not, IMHO, 100% accurate. cracks are acceptable in highlu unusual circumstances. typically you need to demonstrate that the residual strength exceeds ultimate load, including crack growth untill the next inspection. Canadian C130s have been flying with known cracks, very carefully monitored.

 

[mfgenggear]

Rb1957

Yes thats true I should add more to my comment.
My statement is in regard to the mfg details, sub assembly.
& final assembly, with very very critical applications.
My experience doe not extend to all ready flying aircraft.

to my knowledge no cracks where ever permitted.

there is always exception to the rules. especially for non stress parts or Assemblies. I normally worked with flight articles.
I am surprised that a flying aircraft allows for cracks regardless if it still passes stress calc's.

Thanks

Mfgenggear

 

[Taz99]

Some Australian Sea King Helicopters flew with cracks in main gearbox mounting feet. Flight envelope was limited to "pilot shall feel no appreciable g's" lol

 

[Ruag]

Dear all,
Thanks a lot for your support.

Actually I am dealing with a component that is made of magnesium-casting and afterwards milled to the final geometry.
The production procedure requires dye penetrant inspection after the milling, but this procedure is not very clear in defining the acceptable size of defects.
As you know casting parts are prone to pores and applying the mentioned NDT I get several indications. Then my question: how to distinguish between pores and cracks? Is there a std. definition of crack that can help me to solve my doubt?

Thanks.

 

[rb1957]

1) i'd've thought that raw (in process) castings were inspected with ultra sonics, xray (not my personal favourite),

2) maybe talk to a casting manufacturer ?

3) maybe take pix to show inclusion/pore (acceptable) or crack (unacceptable) ? part of this may be "it's a surface detect, that'll be removed in final dressing" ?

4) don't stand under/near old military a/c (particularly Canadian C130, and Ozzie SH3s, and Canadian SH3s ... and a long list of others !!)

 

[CastMetal]

Who ever made the specification for die penetrant should also specify an allowable size and or a specific sensitivity level of die penetrent to be used. Penetrents vary in viscosity to set a sensitivity level i.e. size of cracks/pores that will allow the die to enter. ALL castings have porosity it is merely a question of size. In terms of cracks vs "pores", defects in castings are typically caused by air entrainment, inclusions of foreign material such as crucible ceramic, or shrinkage. Under magnification air pockets/pores will appear very smooth while those caused by shrinkage are jagged and crack growth will likely propagate from those areas.

Comprehension is not understanding. Understanding is not wisdom. And it is wisdom that gives us the ability to apply what we know, to our real world situations

 

[TVP]

This is the standard that you need:

SAE AMS 2175 Castings, Classification and Inspection Of

http://standards.sae.org/ams2175/