FastMouse
Member
- Feb 19, 2003
- 73
When performing a residual strength assessment of cracked structure of Part 23 & 25 aircraft, net section yielding is usually assessed. Should this be done using the material's yield stress, ultimate tensile stress, or something else?
DISCUSSION
I have encountered three approaches:
(1) use ultimate tensile stress,
(2) use yield stress, and
(3) use an allowable stress between yield and ultimate tensile stress.
An argument for using ultimate tensile stress is that while a static strength substantiation of undamaged structure should show that there is no detrimental deformation at limit load, when performing a residual strength assessment of cracked structure, we are beyond the conditions of no detrimental deformation since we might be dealing with a quite large critical crack (possibly up to circa two bays in the fuselage). The critical crack size calculated based on crack instability, perhaps using an R-curve approach, will return a crack size that is just at the point of instability and potentially catastrophic failure at limit load. To be consistent with this, the critical crack size calculated using net section strength criteria should use ultimate tensile stress, again to calculate a crack size that is at the point of potentially catastrophic failure at limit load.
A side issue is that this might indicate an inconsistency in the FARs/CSs. (That’s not a criticism of the regulations; it’s simply the result of a framework that has evolved over many decades, guided by many, many individuals with inputs from about a billion different sources.) A static strength analysis could be done and the net-section margin at limit load calculated using yield stress. Then, a residual strength analysis could be done and if the critical crack is small due to, say, thick material having low fracture toughness, G/N is not significantly affected, and higher net-section strength margins would be calculated at limit load using ultimate tensile stress. So you end up with two margins: one from the static strength assessment of undamaged structure at limit load, and a higher margin for damaged structure at limit load. It's a bit of a stretch, but it could happen.
Arguments for using yield stress include (1) strength at limit load is being considered, so ultimate tensile stress does not apply, (2) it is not unconservative, and (3) it avoids having to debate the conservatism with whomever has to concur with the findings.
The arguments for using an allowable stress between yield and ultimate tensile stress are not clear, at least not to me. This method accepts potentially catastrophic failure via crack instability, accepts potentially-large scale yielding via net section strength, but having done so, does not take full benefit of using ultimate tensile strength to demonstrate a critical crack size that is as large as possible.
That’s a lot of text. If you’ve stuck it out to the end, thank you! Any comments?
DISCUSSION
I have encountered three approaches:
(1) use ultimate tensile stress,
(2) use yield stress, and
(3) use an allowable stress between yield and ultimate tensile stress.
An argument for using ultimate tensile stress is that while a static strength substantiation of undamaged structure should show that there is no detrimental deformation at limit load, when performing a residual strength assessment of cracked structure, we are beyond the conditions of no detrimental deformation since we might be dealing with a quite large critical crack (possibly up to circa two bays in the fuselage). The critical crack size calculated based on crack instability, perhaps using an R-curve approach, will return a crack size that is just at the point of instability and potentially catastrophic failure at limit load. To be consistent with this, the critical crack size calculated using net section strength criteria should use ultimate tensile stress, again to calculate a crack size that is at the point of potentially catastrophic failure at limit load.
A side issue is that this might indicate an inconsistency in the FARs/CSs. (That’s not a criticism of the regulations; it’s simply the result of a framework that has evolved over many decades, guided by many, many individuals with inputs from about a billion different sources.) A static strength analysis could be done and the net-section margin at limit load calculated using yield stress. Then, a residual strength analysis could be done and if the critical crack is small due to, say, thick material having low fracture toughness, G/N is not significantly affected, and higher net-section strength margins would be calculated at limit load using ultimate tensile stress. So you end up with two margins: one from the static strength assessment of undamaged structure at limit load, and a higher margin for damaged structure at limit load. It's a bit of a stretch, but it could happen.
Arguments for using yield stress include (1) strength at limit load is being considered, so ultimate tensile stress does not apply, (2) it is not unconservative, and (3) it avoids having to debate the conservatism with whomever has to concur with the findings.
The arguments for using an allowable stress between yield and ultimate tensile stress are not clear, at least not to me. This method accepts potentially catastrophic failure via crack instability, accepts potentially-large scale yielding via net section strength, but having done so, does not take full benefit of using ultimate tensile strength to demonstrate a critical crack size that is as large as possible.
That’s a lot of text. If you’ve stuck it out to the end, thank you! Any comments?