ASME VIII2-2025 5.5.4 疲劳分析

5.5.4 FATIGUE AN ALYSIS 

5.5.4.1 Overview. Three fatigue assess ment methods are provided (see paragraph 5.5.1.3) to determine the permis sible number of cycles for the time varying loadings specified in the User’s Design Specification. 5.5.4.2 Method A — Fatigue Assess ment Using Elastic Stress An alysis and Equivalent Stresses. 5.5.4.2.1 Overview.The effective total equivalent stress amplitude obtained from a linear elastic stress a nalysis is used to evaluate the fatigue damage. The controlling stress for the fatigue evaluation is the primary plus secondary plus peak equivalent stress amplitude. The controlling stress is defined as one-half of the primary plus secondary plus peak equivalent stress range, Δ(PL + Pb + Q + F), calculated for each cycle in the loading time history (see Figure 5.1 and 5.2.2.4). Examples of this stress category for typical pressure vessel components are shown in Table 5.6. The primary plus secondary plus peak equivalent stress range is the equivalent stress, derived from the highest value across the 603 AS ME BPVC.VIII.2-2025 5.5.4.2.1– 5.5.4.2.2 thickness of a section, of the combination of all primary, secondary, and peak stresses produced by specified cyclic op erating pressures and other mechanical loads and by general and local thermal effects and including the effects of gross and local structural discontinuities. Twooptions are provided to determine the primary plus secondary plus peak equivalent stress range. Refer to Table 5.11 for implementation details. (a) Option 1. The local thermal stress range is not separated from the total stress range and any stress concentration factors, SCFij, fatigue strength reduction factors, Kf, and fatigue penalty factors, Ke, are applied to the total stress range directly. (b) Option 2. The local thermal stress range is separated from the total stress range. Stress concentration factor, SCFij, fatigue strength reduction factor, Kf, and fatigue penalty factor, Ke, are applied to the range of equivalent primary plus secondary plus peak stress minus the local thermal stress range. The plastic Poisson’s ratio adjustment factor, Kv, is ap plied only to the local thermal stress range. If the range of primary plus secondary equivalent stress range exceeds SPS, the fatigue penalty factor, Ke, is required. Note that the primary plus secondary equivalent stress range, ΔSn,k deliberately excludes the peak stress, F, and is rep resented by the stress categories Δ(PL + Pb + Q). The primary plus secondary stress range, ΔSn,k is determined by lin earizing the stress across the thickness of the section of interest, excluding the peak stresses, F, produced by all relevant cyclic operating pressures, and other mechanical and thermal loads, then performing a tensorial subtraction at the re levant times for each cycle being an alyzed. Once each stress range at all relevant times in the loading history for each section of interest of the component is determined, and the aforementioned stress linearization computation is per formed, the equivalent stress range is calculated per eq. (5.36). A flow diagram example for the Method A Fatigue calculation procedure is shown in Figure 5.13.

5.5.4 疲劳分析

5.5.4.1 概述

本节给出三种疲劳评定方法（见 5.5.1.3），用于确定在用户设计规格书规定的交变载荷下，构件的许用循环次数。

5.5.4.2 方法 A — 采用弹性应力分析与当量应力的疲劳评定

5.5.4.2.1 概述

本方法采用线弹性应力分析得到的有效总当量应力幅评定疲劳损伤。

疲劳评定的控制应力为：一次 + 二次 + 峰值当量应力幅

该控制应力定义为载荷历程中每个循环的一次 + 二次 + 峰值当量应力范围 Δ(Pₗ + Pᵦ + Q + F) 的 1/2（见图 5.1 及 5.2.2.4）。典型压力容器构件的该应力类别示例如表 5.6 所示。

一次 + 二次 + 峰值当量应力范围，是由规定循环运行压力、其他机械载荷、整体及局部热效应产生的全部一次、二次、峰值应力组合后的当量应力，取截面厚度方向上的最大值，并包含整体与局部结构不连续效应。

确定一次 + 二次 + 峰值当量应力范围提供两种选项，实施细节见表 5.11：

(a) 选项 1局部热应力范围不从总应力范围中分离，所有应力集中系数 SCFᵢⱼ、疲劳强度降低系数 Kբ、疲劳惩罚系数 Kₑ直接作用于总应力范围。

(b) 选项 2将局部热应力范围从总应力范围中分离。应力集中系数 SCFᵢⱼ、疲劳强度降低系数 Kբ、疲劳惩罚系数 Kₑ作用于 当量一次 + 二次 + 峰值应力范围 − 局部热应力范围。塑性泊松比修正系数 Kᵥ仅作用于局部热应力范围。

若一次 + 二次当量应力范围超过 SPS，则必须采用疲劳惩罚系数 Kₑ。

注：一次 + 二次当量应力范围 ΔSₙ,ₖ 刻意不包含峰值应力 F，表示为应力类别 Δ(Pₗ + Pᵦ + Q)。一次 + 二次应力范围 ΔSₙ,ₖ 通过以下步骤确定：

·对关注截面沿厚度方向进行应力线性化，剔除由相关循环压力、机械及热载荷产生的峰值应力 F；

·对每个分析循环在相应时刻进行张量减法运算。

构件各关注截面在载荷历程中所有相应时刻的应力范围确定，并完成上述应力线性化计算后，按公式 (5.36) 计算当量应力范围。

方法A 疲劳计算流程示例如图 5.13 所示。

还有另外两种疲劳分析方法，请大家查阅标准。