问题 问答题 简答题

简述影响疲劳寿命的因数

答案

参考答案:

1、应力集中:如果存在应力集中处,如缺口、孔、键槽或截面突变处,疲劳强度会大大降低。图824所示的疲劳失效就起源于钢轴的键楷。如果精心设计,尽可能避免出现应力集中处,就可以使疲劳失效减到最少。

2、表面粗糙度:一般地说,金属试样表面加工愈光滑,疲劳强度就愈高。租糙的表面会造成应力集中处,有利于疲劳裂纹的形成。

3、表面状态(化学成分):由于大多数疲劳失效起源于金属表面,任何表面状态的主要变化都会影响到疲劳强度e例如,钢的表面硬化处理(渗碳、氮化)使表面硬度提高,从而提高了疲劳寿命。反之,脱碳造成热处理钢件表面软化而使疲劳强度降低。在金属表面形成残余压应力层也会提高疲劳寿命。

4、环境:如果金属在承受周期应力时还有一个腐蚀性环境,所造成的腐蚀会大大加速裂纹扩展速率。腐蚀和周期应力对金属的综合作用又称腐蚀疲劳。

填空题
阅读理解

根据短文内容,从短文后的选项中选出能填入空白处的最佳选项,并在答题卡上将该项涂黑。选项中有两项为多余选项。

There was once a lonely girl who longed so much for love. One day while she was walking in the woods she found two starving birds.  小题1:  She cared them with love and the birds grew strong.Every morning they greeted her with a wonderful song. The girl felt the great love from the birds. 小题2: . The larger and the stronger of the two birds flew from the cage. The girl was so frightened that he would fly away. As he flew close, she grasped him wildly. Her heart felt glad at her success in capturing him.  小题3: . She opened her hand and stared in horror at the dead bird. Her desperate love had killed him.

She noticed the other bird moving back and forth on the edge of the cage. She could feel his great need for freedom. He needed to fly into the clear, blue sky.   小题4: . The bird circled once, twice, three times.

The girl watched delightedly at the bird’s enjoyment.   小题5: .She wanted the bird to be happy. Suddenly the bird flew closer and landed softly on her shoulder. It sang the sweetest songs that she had ever heard.

The fastest way to lose love is to hold on it too tight, while the best way to keep love is to give it wings!

A.She gave them some delicious food and left.

B.Suddenly she felt the bird go softly.

C.Her heart was no longer concerned with her loss.

D.One day she left with the two birds at home alone.E. She took them home and put them in a small cage.

F. One day the girl left the door of the cage open.

G. She lifted him from the cage and threw him softly into the air.