5. Allowable Stress
1. Introduction
This write-up gives particulars of the allowable stress as per the ASME.

2. Allowable Stress as per ASME-1 (Power Boilers)
The basic allowable stress (Sm) is the minimum of the following seven values:

1.ultimate tensile strength at room temperature/3.5
2.1.1 x ultimate tensile strength at metal design temperature/3.5
3.yield stress at room temperature/1.5
4.yield stress at metal design temperature/1.5
5.minimum stress to produce a creep strain of 0.01% in 1,000 hours
6. average stress to produce rupture in 100,000 hours/1.5
7.minimum stress to produce rupture in 100,000 hours/1.25

3. Allowable Stress for Welds as per The ASME-I (Power Boilers)
groove-weld in tension = 74%
groove-weld in shear = 60%
fillet-weld in shear = 49%
where, Sm = basic allowable stress

4. Allowable Stress as per The ASME-B31.1 (based on faigue and creep) – Power Piping
SA = f(1.25 Sc + 0.25Sh)
Where, SA = allowable stress range for expansion stresses
Sc = allowable stress at room temperature
Sh = allowable stress at metal design temperature
f = metal fatigue stress range reduction factor
= 1.0 for 1 to 7,000 metal fatigue cycles
= 0.9 for 7,001 to 14,000 metal fatigue cycles
= 0.8 for 14,001 to 22,000 metal fatigue cycles
= 0.7 for 22,001 to 45,000 metal fatigue cycles
= 0.6 for 45,001 to 100,000 metal fatigue cycles
= 0.5 for 100,000 and over
SA+SL = f(1.25 Sc + 1.25Sh)
Where, SL = longitudinal stress due to pipe/tube internal pressure 