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残余应力有哪几类?它会产生什么后果?如何产生、消除?

答案

参考答案:

残余应力:引起附加应力的外因去处后,在物体内仍残存的应力叫残余应力,残余应力是弹性应力,不超过材料的屈服应力,也是相互平衡成对出现的。残余应力分为三类:第一类残余应力存在与变形体各区域之间;第二类残余应力存在于各晶粒之间;第三类残余应力存在于晶粒内部。残余应力引起的后果:⑴具有残余应力的物体再承受塑性变形时,其应力分布及内部应力分布更不均匀。⑵缩短制品的使用寿命,当外载作用下的工作应力与残余应力叠加超过材料的强度时,会使零件破坏,设备出现故障。⑶使在制品的尺寸和形状发生变化。当残余应力的平衡受到破坏时,相应部分的弹性变形也发生变化,从而引起尺寸和形状的变化。⑷增加塑性变形抗力,降低塑性、冲击韧性及抗疲劳强度。⑸降低制品表面耐蚀性,具有残余应力的金属在酸液中或其他溶液中的溶解速度加快。残余应力一般是有害的,特别是表面层中具有残余拉应力的情况。但当表面层具有残余压应力时,可以显著提高材料的强度和疲劳强度,反而可提高其使用性能。残余应力的消除方法:热处理法,机械处理方法

阅读理解

阅读理解。

     On December 18, 1620, passengers on the British ship Mayflower came on shore at modern-day

Plymouth, Massachusetts, to begin their new settlement, Plymouth Colony.

     The famous Mayflower story began in 1606, when a group of reform-minded people in England built

their own church, which was separate from the state-supported Church of England. Because of this, they

were forced to leave the country and settle in Netherlands. After 12 years of struggling to adapt and make

a hard living, the group sought financial backing from some London merchants to set up a colony in America.

     On September 6, 1620, one hundred and two passengers-called Pilgrims (朝圣者) by William Bradford,

a passenger who would become the first governor of Plymouth Colony-crowded on the Mayflower to begin

the long, hard journey to a new life in the New World. On November 11, 1620, the Mayflower landed at

Provincetown Harbor. Before going on shore, 41 male passengers-heads of families, single men and three

male servants-signed the famous Mayflower contract, agreeing to form a government chosen by common

election and to obey all laws made for the good of the colony.

     Over the next month, several small leading teams were sent on shore to collect firewood and hunt for a

good place to build a settlement. Around December 10, one of these groups found a harbor they liked on the

western side of Cape Cod Bay. They returned to the Mayflower to tell the other passengers, but bad weather

prevented them reaching the harbor until December 16. Two days later, the first group of Pilgrims went on

shore. After exploring the region, the settlers chose a cleared area once controlled by members of a local

Native American tribe (部落). The tribe had abandoned the village several years earlier, after an outbreak of

European disease.

     That winter of 1620-1621 was really hard, as the Pilgrims struggled to build their settlement, find food and

take care of the sick By spring, 50 of the original 102 Mayflower passengers were dead. The remaining settlers

made contact with returning members of the Native American tribe and in March they signed a peace treaty (条

约) with the tribe chief. In the spring time, helped by the locals, the Pilgrims were able to plant crops-especially

corn and beans-that were necessary to their survival.

     On April 5, 1621, the Mayflower and its crew left Plymouth to return to England. Over the next several

decades, more and more settlers made the voyage across the Atlantic to Plymouth, which gradually grew into

a successful shipbuilding and fishing center.

1. A group of English were forced to leave their country in 1606 because _____.[ ]

A.their belief was different from the official Church

B. they built their own churches in a foreign country

C. they lacked financial support from the government

D. they were ambitious to build the colony in America

2. How many days did the sea journey to Provincetown Harbor last? [ ]

A. 103

B. 96

C. 66

D. 35

3. Several leading teams were sent in order to _____. [ ]

A. hunt for food for the passengers

B. control local Native Americans

C. build a settlement for the passengers

D. explore the region for their settlement

4. From the passage we know that _____ between 1620-1621.[ ]

A. 52 Mayflower passengers died of some illnesses

B. Pilgrims had a good relationship with the locals

C. Plymouth turned a main center for shipbuilding

D. Pilgrims tried to plant corns and beans in winter

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