Design Stress Calculator
Allowable Stress from Material Properties per Engineering Code
This page is an explanation on how the design stress is calculated in mechitcalc.
The design stress is the maximum stress a material is allowed to be under in design conditions for our calculations. Almost every calculation is based on calculating the stress from a certain pressure/design condition and comparing this to the design stress. Code compliance depends on if the calculated stress is lower than the design stress.
Most codes define the design stress on a certain safety factor from a yield and tensile strength. Primary loads (like internal pressure) can cause thinning when the material starts to yield, so in those kinds of calculations you always have a safety factor from the yield strength.
Different materials also have their inherent failure modes. For example Stainless steel increases in strength as it yields and does not have a clear yield point. but also tends to have a lower yield strength. Because of this, often codes will allow you to use a 1% deformation yield stress instead of a 0.2% deformation yield. But this depends on the application as well, for applications where a small deformation can have a negative effect, you would not want to use the 1% yield strength.
Sometimes this 1% yield strength is also not available. Mechitcalc automatically substitutes it for 0.2% in that case.
Material Properties Used in Design Stress
Design stress calculations draw on up to five material properties, all functions of temperature and nominal size:
| Symbol | Name | Description |
|---|---|---|
| Proof strength (yield) at room temperature | Minimum specified 0.2% proof stress at ambient temperature | |
| Proof strength at temperature | 0.2% proof stress at the design temperature | |
| 1% proof strength at temperature | 1.0% proof stress at design temperature — used for austenitic steels | |
| Tensile strength (ambient) | Minimum specified ultimate tensile strength | |
| Tensile strength at temperature | Ultimate tensile strength at the design temperature | |
| Elongation at fracture | Minimum elongation %; determines material ductility category |
EN 13480
EN 13480-3 (industrial piping) uses a stress methodology that depends on the material category:
Non-Austenitic Steels (elongation below 30%)
For carbon steel, low-alloy steel, and other ferritic materials:
At temperatures up to 50 °C, the room-temperature proof strength (defined at 20C) may also be considered if it is higher than the elevated-temperature value but only for non austenitic/austenitic-ferritic steel.
Design stress is the lower of the tensile-based and yield-based limits.
Austenitic Steels — Elongation ≥ 35%
When elevated-temperature tensile strength is available:
When is not available, the calculation falls back to:
Austenitic Steels — Elongation 30%–35%
Austenitic Steels — Elongation below 30%
Austenitic-Ferritic (Duplex) Steels
Duplex stainless steels follow the same formula as non-austenitic steels.
External Pressure, Elastic Limit Stress
For external pressure calculations, EN 13480 requires a separate elastic limit stress used in the buckling check:
| Material | Elastic limit formula |
|---|---|
| Austenitic | |
| All others (ferritic, duplex) |
EN 13458, Cryogenic Vessels
EN 13458 covers static vacuum-insulated cryogenic vessels. For the inner vessel design pressure, austenitic steels are predominantly used and the 1% proof strength governs:
This factor of 1.5 applies at the vessel design pressure. Different safety factors apply for test pressure, external pressure buckling (Sp and Sk), and the knuckle region — these are not covered by this calculator’s general design stress output.
EN 14460, Pressure Shock Resistance (Explosion Protection)
EN 14460 covers equipment intended to resist internal explosions. Design stress is based on the plastic deformation capacity of the material rather than elastic limits:
Austenitic Steels
For austenitic materials with an value available:
If is not available, a correction on is used:
Brittle Materials
For materials classified as brittle (low elongation), the design stress accounts for the limited ductility by including elongation in the denominator:
Where is the elongation at fracture in percent. A brittle material with 5% elongation will have a meaningfully lower design stress than one with 15% elongation at the same tensile strength.
Ductile Non-Austenitic Materials
DNV / Lloyd’s / Bureau Veritas, Ship Classification
Ship classification societies regulate design stress for gas tankers, gas-fuelled ships, and similar marine applications. Two factors are in common use, depending on the society and whether material test results were attended by the society: In some cases it might be allowed to use Rp1% in the calculation, but not currently part of the mechitcalc program.
Factor 1.8 (standard)
Applies to:
- DNV Liquified gas tankers it comes from IGC Code CH 5.11.3.1
- NR529 BV gas fuelled ships part A-1 7.3.4
- LLoyds register LR-RU-008 Classification of Ships for the Carriage of Liquefied Gases in Bulk
- LLoyds register gas fuelled ships
- RINA if material strength test results are not atttended by the society
- BV general if material strength results are not attended by the society
Factor 1.6
Applies to:
- LLoyds register class part 5 chapter 12 section 2 (general use)
- RINA general, if values result from tests attended by society
- BV general, if values result from tests attended by the society
Both variants use the same tensile strength factor of 2.7. The difference lies only in the yield-based denominator.
ASME B31.3 Process Piping
ASME B31.3 uses a material lookup table as the primary source of design stress. When the material appears in Table A-1, the tabulated value is used directly — no formula is applied, and the table already encodes the code committee’s safety factor decisions.
When a design stress is not available in the table, the following formula applies:
The higher safety factor on tensile strength (3.5 vs. 2.4 in EN codes) reflects the ASME B31.3 philosophy of maintaining a larger margin against failure by fracture.
External Pressure ASME B31.3
For external pressure, B31.3 uses the same external pressure tables as Asme VIII Div 8 and that has the design stress built in. For thick walled pipe, an additional calculation is done using the following design stress.
This effectively limits the external pressure design stress to 90% of the yield strength or twice the internal design stress, whichever is lower for thick walled pipes.
Comparison of Safety Factors by Code and Material
| Code | Material | Yield factor | Tensile factor |
|---|---|---|---|
| EN 13480 / EN 13445 | Ferritic | 1.5 on | 2.4 on |
| EN 13480 / EN 13445 | Austenitic (A ≥ 35%, Rm/T available) | 1.2 on | 3.0 on |
| EN 13480 / EN 13445 | Austenitic (A ≥ 35%, no Rm/T) | 1.5 on | — |
| EN 13480 / EN 13445 | Austenitic (30% ≤ A < 35%) | 1.5 on | 2.4 on |
| EN 13458 | Austenitic (inner vessel) | 1.5 on | — |
| EN 14460 | Austenitic | — | 1.0 × 1.1 on |
| EN 14460 | Ductile non-austenitic | 1.0 on | — |
| Ship 1.8 | Non-austenitic | 1.8 on | 2.7 on |
| Ship 1.6 | Austenitic | 1.6 on | 2.7 on |
| ASME B31.3 | All (no table) | 1.5 on | 3.5 on |
Why Design Stress Depends on Temperature
Material strength decreases with increasing temperature. The and values in the material database are temperature-dependent curves derived from standard material specifications (EN 10216, EN 10217, ASTM, etc.). As temperature rises:
- Proof strength drops — reducing the yield-governed design stress
- Tensile strength drops more slowly at first, then accelerates above the creep range
- Above the creep threshold (typically ~370 °C for carbon steel, ~550 °C for austenitic), long-term creep rupture strength governs instead of proof strength
Always use the highest expected design temperature instead of the operating temperature. A good evaluation has to be done to designate what the design temperature is. This design temperature should be reflected in all the documentation like piping drawings and Line lists.
Frequently Asked Questions
Why does EN 13480 treat austenitic steels differently?
Austenitic stainless steels have no well-defined yield point and strain-harden continuously. Their value is relatively low compared to actual load-carrying capacity, so the code permits use of the 1% proof strength ( ) which better represents the material’s usable strength. For highly ductile grades (A ≥ 35%), the code further allows a lower safety factor (1.2 instead of 1.5) on when elevated-temperature tensile data is available.
What is the difference between Rp0.2 and Rp1.0?
Both are proof strengths — the stress at which a material exhibits a specified permanent plastic strain after unloading. corresponds to 0.2% permanent strain and corresponds to 1.0% permanent strain. Austenitic steels work-harden between 0.2% and 1.0% strain, so is meaningfully higher and is used as a more representative measure of their elastic-plastic boundary.
Why does ASME B31.3 use a tensile factor of 3.5 vs. 2.4 in EN codes?
The two code families use different design philosophies. EN 13480 uses a factor of 2.4 on , targeting roughly the same safety margin as the yield factor when material properties are balanced. ASME B31.3 uses 3.5, reflecting the historical ASME approach of maintaining a larger margin against rupture. In practice, for most common pipe materials the yield strength governs anyway, so the tensile factor often does not control the design stress.
Is design stress the same as the yield strength?
No. Design stress is always lower than the yield strength, divided by a code-mandated safety factor. The safety factor accounts for uncertainties in material properties, manufacturing quality, load estimation, and analysis accuracy. The actual allowable stress in a component may be further reduced by weld joint efficiency, stress concentration, or other factors.