How is allowable stress calculated per ASME?

ASME BPVC Section VIII Div.1 uses σ_allow = min(Rp0.2/1.5, Rm/2.4) for austenitic stainless, or min(Rp0.2/1.5, Rm/3.5) for carbon and low-alloy steels at room temperature. This provides a safety margin against both yielding and fracture.

What is the fatigue endurance limit?

For steels with Rm < 1400 MPa, the endurance limit Se ≈ 0.5 × Rm (unnotched, polished specimen). For Rm ≥ 1400 MPa, Se ≈ 700 MPa. Actual fatigue strength depends on surface finish, size, stress concentration, and environment.

What safety factors are used for different load types?

Typical: Static loads n=1.5-2.0 against yield. Alternating/fatigue loads n=2.0-4.0 against endurance limit. Pulsating loads n=1.5-2.5. Impact loads n=3.0-5.0. Factors depend on consequence of failure and load uncertainty.

What is yield strength Rp0.2?

Rp0.2 (or proof stress) is the stress at which 0.2% permanent (plastic) deformation occurs. Below this stress, deformation is elastic and recoverable. It is the primary design parameter for pressure vessels and structural components.

Do properties change with heat treatment?

Yes, significantly. Values shown are typical for standard delivery condition. Q&T (quench and temper) generally increases strength but decreases ductility. Actual properties depend on section size, austenitizing temperature, and tempering conditions.

免費工程工具

鋼材特性與允許應力計算器

選擇鋼材等級以查看機械特性。計算不同負載類型的 ASME 允許應力、疲勞耐力極限及安全係數。

ASME BPVC Rp0.2 / Rm 疲勞 Se

特性與允許應力

ASME 允許應力 σ_allow

—

Rp0.2 屈服強度

—

抗拉強度 Rm

—

疲勞耐力極限 Se

—

伸長率 A%

—

硬度 HB

—

密度

—

安全係數 (n)

—

ASME 允許應力

根據 ASME BPVC 第八篇第一分篇，室溫下的允許應力為：

對於奧氏體不鏽鋼（具有顯著的加工硬化），ASME 使用 Rm/2.4 而非 Rm/3.5。

疲勞耐力極限

對於無缺口、拋光的鋼材試樣在旋轉彎曲下：

實際疲勞強度需要針對表面粗糙度 (ka)、尺寸 (kb)、可靠度 (kc) 等進行修正係數：

依負載類型的安全係數

負荷類型	係數 n	設計應力
靜態（屈服）	1.5	Rp0.2 / 1.5
靜態（斷裂）	2.4–3.5	Rm / 2.4 或 Rm / 3.5
脈動疲勞	2.0	Se / 2.0
完全反向疲勞	2.5	Se / 2.5
衝擊 / 震動	3.0–5.0	Rp0.2 / n

實務範例

範例 — 42CrMo4 Q&T

特性： Rp0.2 = 900 MPa, Rm = 1100 MPa

σ_allow (ASME) = min(900/1.5, 1100/3.5) = min(600, 314) = 314 MPa

Se = 0.5 × 1100 = 550 MPa （未修正）

脈動疲勞：550/2.0 = 275 MPa

⚠️ 注意： 所示特性為典型值。實際特性取決於熱處理、截面尺寸及材料測試證書。設計計算時請務必使用經認證的數值。