问题 问答题 简答题

什么是许用应力与安全系统?

答案

参考答案:

构件实际工作时,所允许产生的最大应力称许用应力。对锅炉承压元件不定来说,许用应力是指在工作条件下所允许的最小壁厚及最大压力时的应力。

构件工作时,其内部产生的应力既不能达到屈服极限,更不能达到强度极限,必须远小于它们才能保证安全。通常把和称危险应力。

危险应力与许用应力的比值n称为安全系数。N值的大小,不仅反映构件的安全程度。N值大,许用应力小,比较安全,但消耗材料多,构件也笨重;n值许用应力大,能节约材料,但安全程度就差。因此,在确定安全系数时,应在满足安全的前提下,充分考虑经济性的要求。

单项选择题
阅读理解

Taiwan police cannot decide whether to treat it as an extremely eleven act of stealing or an even cleverer cheat . Either way , it could be the perfect crime (犯罪), because the criminals are birds—horning pigeons !

The crime begins with a telephone message to the owner of a stolen car : if you want the car back, pay up then, the car owner is directed to a park, told where to find a bird cage and how to attach money to the neck of the pigeon inside . Carrying the money in a tiny bag , the pigeon flies off .

There have been at least four such pigeon pick-ups in Changwa. What at first seemed like the work of a clever stay-at-home car thief, however , may in face be the work of an even lazier and more inventive criminal mind—one that avoid (避免)not only colleting money but going out to steal the car in the first place . Police officer Chen says that the criminal probably has pulled a double trick: he gets money for things he cannot possibly return . Instead of stealing cars , he lets someone else do it and then waits for the car-owner to place an ad (启事) in the newspaper asking for help .

The theory is supported by the fact that , so far , none of the stolen cars have been returned . Also, the amount of money demanded-under 3,000 Taiwanese dollars –seems too little for a car worth many times more .

Demands for pigeon-delivered money stopped as soon as the press reported the story. And even if they start again, Chen holds little hope of catching the criminal . “We have more important things to do, ” he said .

1.After the car owner received a phone call. He          

A.went to a certain pigeon and put some money in the bag it carried

B.gave the money to the thief and had his car back in a park

C.sent some money to the thief by mail

D.told the press about it

2.The “lazier and more inventive” criminal refers to          .

A.the car thief who stays at home     

B.one of those who put the ads in the paper

C.one of the policemen in Changwa  

D.the owner of the pigeons

3.The writer mentions the fact that “none of the stolen cars have been returned” to show       .

A.how easily people get fooled by criminals

B.what Chen thinks might be correct

C.the thief is extremely clever

D.the money paid is too little

4.The underlined word “they” in the last paragraph refers to          .

A.criminals            B.pigeons

C.the stolen cars    D.demands for money

5.We may infer from the text that the criminal knows how to reach the car owners because     .

A.he reads the ads in the newspaper  

B.he lives in the same neighborhood

C.he has seen the car owners in the park 

D.he has trained the pigeons to follow them