第12章动载荷与疲.ppt

工程力学
engineering mechanics
第12章动载荷与疲劳强度概述 summarize about mobile load and Fatigue strength
工程结构中还有一些构件或零部件中的应力这些应力的大小或方向却随着时间而变化,这种应力称为交变应力。在交变应力作用下发生的失效,称为疲劳失效,简称为疲劳。。
In engineering structure, value or direction of ponent stress is variational along with time, this kind of stress be called variety is called Fatigue invalidation on this condition of variety stress. Fatigue for short.
等加速度直线运动时构件上的惯性力与动应力inertial force and dynamic stress above constant ponent
以图 12-1 中的起重机吊重物为例,作用在钢丝绳
的总载荷是动载荷与静载荷之和:for example,crane suspend clog as diagram 12-1, total load effecting steel wire rope is sum of dynamic load and static load

(12-3)
式中,FT为总载荷total load ;Fst与FI分别为静载荷static load
与惯性力inertial force引起evocable的动载荷dynamic load 。
图12-1
按照单向拉伸时杆件的应力公式,钢丝绳横截面上的总正应力
According one-way pulled stretch pole of stress formula, Total positive stress above the steel wire rope cross section is .
旋转构件的受力分析与动应力计算 ponent force analysis and dynamic stress calculation
图12-2(a)中所示之以等角速度旋转的飞轮。飞轮材料密度为,轮缘平均半径为R,轮缘部分的横截面积为A。Revolving flywheel by constant angular velocity as diagram 12-2(a). Flywheel material density is , flange average radius is R, cross section acreage in sectional flange is A.
图12-2 飞轮中的动应力dynamic stress
a
b
弧长为 arc length IS
飞轮缘横截面上的轴力为 axial force above flywheel flange is
飞轮轮缘横截面上的总应力为 Total stress above flywheel flange is
疲劳强度概述 fatigue strength summarize
交变应力 alternating stress
1、交变应力的概念 the concept of alternating stress
一点的应力随着时间的改变而变化,这种应力称为交变应力。
A point stress is variational as time , this kind of strss be called alternating stress.
图12-4规则的交变应力 inerratic alternating stress
图12-5不规则的交变应力 anomalistic alternating stress