What is allowable stress?

Allowable stress (σ_allow or f or S) is the maximum stress permitted in a design. It equals material strength divided by a safety factor defined by the design code (e.g. ASME VIII, EN 13445).

How is allowable stress calculated per ASME VIII?

ASME VIII Div.1: S = min(Rp0.2/1.5, Rm/3.5) at room temperature. At elevated temperatures, time-dependent (creep) properties may govern. The 2007 edition reduced the UTS factor from 4 to 3.5.

How is allowable stress calculated per EN 13445?

EN 13445-3: f = min(Rp0.2/1.5, Rm/2.4) for ferritic steels. For austenitic: f = max(Rp0.2/1.5, Rp1.0/1.2) but ≤ Rm/3. Temperature-dependent values from EN 10028.

When does creep become important?

Creep is significant above ~350°C for carbon steel, ~400°C for low-alloy, ~450°C for austenitic stainless, ~500°C for high-alloy. Above these temperatures, time-dependent rupture stress governs the allowable.

What is weld joint efficiency?

Weld joint efficiency (E or z) reduces allowable stress based on NDE level: E=1.0 for full radiographic examination, E=0.85 for spot RT, E=0.7 for no volumetric NDE. It accounts for potential weld defects.

Free Engineering Tool

Allowable Stress by Material

Calculate design allowable stress per ASME VIII Div.1 or EN 13445-3. Temperature-dependent properties, creep rupture check, weld joint efficiency factor. 12+ material grades.

ASME VIIIEN 13445σ_allowCreep CheckWeld Factor

Material Grade

Design Code

Design Temperature (°C)

Weld Joint Efficiency

Load Type

Quick presets

Design Allowable Stress

Allowable Stress σ_allow × E

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Rp0.2(T) / Safety Factor

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Rm(20°C) / Safety Factor

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σ_allow (before weld factor)

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Governing Criterion

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Creep Range Check

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Load-Adjusted Allowable

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Material Properties at T

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EN 13445-3 (Ferritic Steels)

ASME VIII Div.1

Temperature Derating

Both yield and UTS decrease with temperature. This calculator uses linearized derating curves based on EN 10028 / ASME II-D data:

Below 100°C: properties are at room-temperature values
100–400°C: gradual reduction (yield drops faster than UTS)
Above 400°C: significant reduction; creep may govern

Creep Regime

When the design temperature exceeds the creep onset temperature, time-dependent rupture stress governs. Typical creep onset:

Material Type	Creep Onset (°C)	Typical Max Design T
Carbon steel (S235, S355)	~350	400°C
C-Mn pressure (P265GH, P355GH)	~380	450°C
Mo steel (16Mo3)	~400	500°C
CrMo steel (13CrMo4-5)	~450	550°C
2¼Cr-1Mo (10CrMo9-10)	~470	600°C
Austenitic SS (304, 316L)	~500	700°C

Practical Example

Example — P265GH at 250°C, EN 13445

Given: P265GH, Rp(20)=265 MPa, Rm=410 MPa, T=250°C, weld E=1.0

Rp(250°C) ≈ 265 × 0.85 = 225.3 MPa (derating for temperature)

f_yield = 225.3 / 1.5 = 150.2 MPa

f_UTS = 410 / 2.4 = 170.8 MPa

f = min(150.2, 170.8) = 150.2 MPa (yield governs)

With E=1.0: σ_allow = 150.2 MPa

Material Comparison at Room Temperature

Material	Rp0.2	Rm	f (EN)	S (ASME)
S235JR	235	360	150	103
S355JR	355	510	213	146
P265GH	265	410	171	117
P355GH	355	490	204	140
16Mo3	280	450	187	129
13CrMo4-5	295	460	192	131
10CrMo9-10	275	450	183	129
304 SS	210	520	140	138
316L SS	200	500	133	133
2205 Duplex	450	620	258	177

⚠️ Note: This calculator provides approximate values for preliminary design. Always verify with the actual code tables (ASME II-D, EN 10028, EN 10222) for your specific material grade, product form, thickness, and temperature.

Allowable Stress Calculator | ASME VIII / EN 13445 | Temperature Derating | Free Tool

Published by admin on February 15, 2026 February 15, 2026