Can any one explain me the concept behind “LOCAL OVERSTRAIN”?
I haven’t got enough with the ASME B31.1 paragraph 119.3. I need a detail explanation
This will help the learners as me… Experienced Guys put light and share on this topic …please

Thanks in Advance

Hi Muth
Local overstrain leads to local yielding for steel pipe.
Once material yield it cannot come back to original position.
For steel we have yield stress value & tensile stress value (Check allowable stress ASME B 31.3).
Yielding does not mean failure but permanent deformation.
Sh ( sustain allowable stress) value derive from Yield stress value with safety margin.
Piping develop different kind of stress
1) Primary stress ( Sustain-weight & pressure) do not change with time---Allowable =Sh
2) Occasional stress ( Wind & Seismic)----Allowable 1.33Sh ( take advantage of safety margin between Sh & Sy)
3) Secondary stress ( thermal) that relax with time ( called self springing) . In this case local yielding is permitted for few starting cycle. After few cycle it take elastic path (Stress range). Stress range can exceed yielding limit but should be within metal tensile stress limit. Sa=f( 1.25Sc+0.25Sh) .
For primary & occasional load yielding is not permitted. Because that load do not relax . Once yielding start , it continue, continue & ultimately there will be failure.

Stress range concept came from stress relaxation due to yielding & creep .

Regards

Habib

It is a vast topic hence cannot be explained in few words. I will try to put it as precisely as possible.

As you know and mentioend in the previous post also , if a material is stressed within yield point, it can come back to its original shape.If it exceeds the yield point behavior can be elastic perfectly plastic ie w/o any strain hardening or hardening with either isotropic or kinematic type behavior.There can be different criteria for yield like Von Mises criteria etc.( and aasociated or non associated flow rules)

For primary ie load driven stresses B31 codes do not allow exceedance of yield stress ( nature of failure is by plastic collapse)but for displacement driven loading , exceedance of yield stress is allowable so long the magnitude does not exceed twice yield.The reason for this is can be traced to Melan's shakedown theorem ( shakedown is a phenomenon when a cyclic loading is superposed on a stady state loading. The failure can be excessive deformation leaidng to collapse or fatigue failure).

The other type of failure is fatigue which happens at discontinuities and concentrations.For thermal loading the fatigue is strain induced and for mechnaical vibrations the fatigue is stress induced ( the computed stress should be kept within yield point).

Having discussed the failure modes we can answer your specific question.

Local overstrain means exceedance of yield point strain at small regions.If the strain is not extended over a region ( or in other words if there is an elastic core ie a predominant part of the cross section is having elastic strain )the twice yield point limit can be exceeded ( see Elastic plastic analysis for Ratchetting, SEC VIII DIV 2 ANNEXE 5 or the paper by Arturs Kalnins). the concept is if major part of the component is elastically strained , the plastic region cannot spread.

There is also a simplified elastic plastic analysis to handle situations when 2yp limit is exceeded. This is use of strain concentration factors which can be defined as ratio of strain computed using elastic-plastuc analysis vs strain computed using elastic analysis only.The use of these factors is to increase the value of the peak stress.

Regarding fatigue failure, mechanical fatigue is stress controlled and high cycle and strains are within yield strain, as otherwise there will be collapse, where as thermal or displacement induced fatigue is strain controlled and the strain exceeds the yield strain.Hence analysis of stress controlled fatigue is an elastic analysis and analysis of strain controlled fatigue has inherent plastic component associated with it.

I would recommend few excellent text books to give you more insight into the above issues:

1) Theory of plasticity by Chakravarty

2) Mechanical behavior of materials by Norman Dowling.

3) Design of power plant structures by David Burgreen.

4) Design of Pressure vessels ; Chattopadhay

5) ASME SEC VIII DIV 2 ( 2007) ANNEXE 5

Regards

Anindya sir

Thank u very much