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Crack Definition 3
- Thread starter Ruag
- Start date
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- #2
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.
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.
MiketheEngineer
Structural
In my book - if you can see it - it is cracked or punctured
SWComposites
Aerospace
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.
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
Aerospace
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
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
Aerospace
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
Mfgenggear
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.
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.
@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
Aerospace
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
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
- Thread starter
- #14
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.
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.
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 !!)
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 !!)
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- #16
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
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
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- #17
This is the standard that you need:
SAE AMS 2175 Castings, Classification and Inspection Of
SAE AMS 2175 Castings, Classification and Inspection Of
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