# 第三章 零件的疲劳强度

> 本章对应 邱宣怀 《机械设计：第四版》 第三章
>
> 及 濮良贵 《机械设计：第十版》 第三章

## 3-1 疲劳断裂特征

疲劳源

疲劳发展区

脆性断裂区

前沿线

垄沟纹

## 3-2 疲劳曲线和疲劳极限应力图

### 疲劳曲线

表示循环次数 $$N$$ 与疲劳极限间的关系曲线，称为疲劳曲线，$$\sigma - N$$ 或 $$\tau -N$$ 曲线。

有限寿命区

无限寿命区

### 疲劳极限应力图

反映材料在相同循环次数和不同循环特性下疲劳极限变化情况

近似呈抛物线分布

简化为几条折线段

## 3-3 影响机械零件疲劳强度的主要因素

## 3-4 零件的许用疲劳极限

$$\overline{AB}$$ :

$$
\sigma\_{-1} = \sigma\_a + \varphi\_{\sigma} \cdot \sigma\_m
$$

$$\overline{A^{\prime} B^{\prime}}$$

$$
\sigma\_{-1} = K\_{\sigma} \sigma\_{ae}^{\prime} + \varphi\_{me}^{\prime}
$$

## 3-5 零件的疲劳强度计算

单向应力

复合应力

$$r = C$$

求 $$N^{\prime}$$ 坐标

$$
\sigma\_{me}^{\prime} = \frac{\sigma\_{-1} \cdot \sigma\_m}{K\_\sigma \cdot \sigma\_a + \varphi\_{\sigma} \cdot \sigma\_m}
$$

$$
\sigma\_{ae}^{\prime} = \frac{\sigma\_{-1} \cdot \sigma\_m}{K\_{\sigma} \cdot \sigma\_a + \varphi\_{\sigma} \cdot \sigma\_m}
$$

安全系数计算

1）按照 $$\sigma\_\max$$ 求

$$
S = \frac{\sigma\_{\lim}}{\sigma\_{\max}} = \frac{\sigma\_{\max}^{\prime}}{\sigma\_{\max}} = \frac{\sigma\_{-1}}{K\_{\sigma} \cdot \sigma\_a + \varphi\_{\sigma} \cdot \sigma\_m} \geq \[S]
$$

2）按 $$\sigma\_a$$ 求

$$
S\_a = \frac{\sigma\_{-1}}{K\_{\sigma} \cdot \sigma\_a + \varphi\_{\sigma} \cdot \sigma\_m} \geq \[S]
$$

3）按屈服求

$$
S\_{\sigma} = \frac{\sigma\_s}{\sigma\_m + \sigma\_a} \geq \[S]
$$

## 3-6 规律性非稳定变应力时机械零件的疲劳强度

## 3-7 低周循环疲劳概述

## 3-8 疲劳裂纹寿命概述


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