ASME B31.1 & B31.3 codes, failure criteria 5

[techmx]

Hi
I would like to know what is the failure criteria that ASME B31.1 & B31.3 codes recomend when designing piping systems. I red on a textbook that the Maximum normal stress criterion is the basis for those codes. As far as I know, that criteria applies to brittle materials only.

thanks

 

[hacksaw]

techmx,

the mode of failure is taken into account(along with other factors) is establishing the maximum allowable stress and various "design factors".

the subject is quite complex. one of the simplest expositions is in the appendix of the BW book called "Steam" where it goes through the modes of failure, failure criteria, etc.

the maximum stress criteria is conservative when properly applied.

good luck,

 

[subsp]

hi

wandering what would be the diffrence in using B31.3 and B31.8 in stress analysis with regards to the %SMYS.

thank you

 

[hacksaw]

dnv,

speculation is not an issue. the codes are set up for specific classes of service.

the stress calculations (within a given gode) used are designed to result in a minimum thickness requirement, one that is conservative and you can always make it thicker.

the allowable stresses are developed from materials tests, experience and a "design factor", etc.

using the requirements of a code outside of its designated range of applicability is a no-no and would surely invite professional problems down the road,

good luck

 

[JohnBreen]

Hello Techmx,

The B31 Pressure Piping Codes use the Tresca failure theory in the equations provided for calculating the combined stresses (ranges). These (Tresca) stresses are to be compared to the allowable stress (ranges) as prescribed in the specific B31 Code you are using.

Regards, John.

 

[techmx]

thanks to all the people that have taken the time to answer this thread. JohnBreen, Could you indicate in what part of the B31.1/B31.3 code I can find the reference to Tresca failure theory you mention?

again, thanks

 

[hacksaw]

John,

I believe that Tresca's theory is based on shear failure and is the basis for the pressure vessel codes.

Shear stress failure is never a controlling factor in piping designs and the piping codes (B31) is based on a maximum prinicpal stress mode of failure.

 

[GSDGoBucks]

Just to clarify, the code equations for B31 are based on two failure theories. As mentioned above, the TRESCA theory (i.e., maximum shear stress failure theory) is one theory used when the maximum shear stress exceeds one-half of the yield strength of the material at temperature, failure will occur.

One other theory used in the piping code is the maximum principal stress failure theory where any one of the three principal stresses (i.e., longitudinal, circumferential and radial) exceed the yield strength of the material at temperature, failure will occur.

As mentioned above the maximum principal stress mode of failure is almost always the conrolling factor, but the code does take into consideration the above two theories.

 

[hacksaw]

Well said

 

[srk]

Hai Techmx ,

The following explanation is helpful to you


HOW PIPING AND COMPONENTS FAIL (MODES OF FAILURES)

There are various failure modes, which could affect a piping system. The piping engineers can provide protection against some of these failure modes by performing stress analysis according to piping codes.

„« FAILURE BY GERNRAL YIELDING: Failure is due to excessive plastic deformation.

ć Yielding at Sub Elevated temperature: Body undergoes plastic deformation under slip action of grains.

ć Yielding at Elevated temperature: After slippage, material re-crystallizes and hence yielding continues without increasing load. This phenomenon is known as creep.

„« FAILURE BY FRACTURE: Body fails without undergoing yielding.

ć Brittle fracture: Occurs in brittle materials.

ć Fatigue: Due to cyclic loading initially a small crack is developed which grows after each cycle and results in sudden failure.

WHEN PIPING AND COMPONENTS FAIL (THEORIES OF FAILURE)

Various theories of failure have been proposed, their purpose being to establish the point at which failure will occur under any type of combined loading.

The failure theories most commonly used in describing the strength of piping systems are:
„« Maximum principal stress theory
This theory states that yielding in a piping component occurs when the magnitude of any of the three mutually perpendicular principle stresses exceeds the yield point strength of the material.

„« Maximum shear stress theory
This theory states that failure of a piping component occurs when the maximum shear stress exceeds the shear stress at the yield point in a tensile test.
In the tensile test, at yield, S1=Sy (yield stress), S2=S3=0.So yielding in the components occurs when

Maximum Shear stress =Ċmax=S1-S2 / 2=Sy / 2


The maximum principal stress theory forms the basis for piping systems governed by ASME B31.3.

Note: maximum or minimum normal stress is called principal stress.


STRESS CATEGORIES
The major stress categories are primary, Secondary and peak.

PRIMARY STRESSES:
These are developed by the imposed loading and are necessary to satisfy the equilibrium between external and internal forces and moments of the piping system. Primary stresses are not self-limiting.

SECONDARY STRESSES:
These are developed by the constraint of displacements of a structure. These displacements can be caused either by thermal expansion or by outwardly imposed restraint and anchor point movements. Secondary stresses are self-limiting.

PEAK STRESSES:
Unlike loading condition of secondary stress which cause distortion, peak stresses cause no significant distortion. Peak stresses are the highest stresses in the region under consideration and are responsible for causing fatigue failure.

Hope you are clear.right!!!

SRK