土木工程 外文翻译 外文文献 英文翻译

Stress Limits in Design

How large can we permit the stresses to be? Or conversely:

How large must a part be to withstand a given set of loads what

are the overall conditions or limits that will determine the size

and material for a part?

Design limits are based on avoiding failure of the part to

perform its desired function. Because different parts must satisfy

different functional requirements, the conditions which limit

load-carrying ability may be quite different for different

elements. As an example, compare the design limits for the floor

of a house with those for the wing of an airplane.

If we were to determine the size of the wooden beams in a

home such that they simply did not break, we would not be very

happy with them; they would be too ‘springy’. Walking across

the room would be like walking out on a diving board.

Obviously, we should be concerned with the maximum

‘deflection’ that we, as individuals, find acceptable. This level

will be rather subjective, and different people will give different

answers. In fact, the same people may give different answers

depending on whether they are paying for the floor or not!

An airplane wing structure is clearly different. If you look

out an airplane window and watch the wing during turbulent weather, you will see large deflections; in fact you may wish

that they were smaller. However, you know that the important

issue is that of ‘structural integrity’, not deflection.

We want to be assured that the wing will remain intact. We

want to be assured that no matter what the pilot and the weather

do, that wing will continue to act like a good and proper wing.

In fact, we really want to be assured that the wing will never fail

under any conditions. Now that is a pretty tall order; who knows

what the ‘worst’ conditions might be?

Engineers who are responsible for the design of airplane

wing structures must know, with some degree of certainty, what

the ‘worst’ conditions are likely to be. It takes great patience and

dedication for many years to assemble enough test data and

failure analyses to be able to predict the ‘worst’ case. The

general procedure is to develop statistical data which allow us to

say how frequently a given condition is likely to be

encountered—once every 1000 hours, or once every 10000

hours, etc.

As we said earlier, our object is to avoid failure. Suppose,

however, that a part has failed in service, and we are asked; Why?

‘Error’ as such can come from three distinctly different sources,

any or all of which can cause failure: 1. Error in design: We the designers or the design analysts may

have been a bit too optimistic: Maybe we ignored some loads;

maybe our equations did not apply or were not properly applied;

maybe we overestimated the intelligence of the user; may we

slipped a decimal point.

2. Error in manufacture: When a device involves heavily

stressed members, the effective strength of the members can be

greatly reduced through improper manufacture and assembly:

May the wrong material was used; maybe the heat treatment was

not as specified; maybe the surface finish was not as good as

called for; may a part was ‘out of tolerance’; may be surface was

damaged during machining; maybe the threads were not

lubricated at assembly; or perhaps the bolts were not properly

tightened.

3. Error in use: As we all know, we can damage almost anything

if we try hard enough, and sometimes we do so accidentally: We

went too fast; we lost control; we fell asleep; we were not

watching the gages; the power went off; the computer crashed;

he was taking a coffee break; she forgot to turn the machine off;

you failed to lubricate it, etc.

Any of the above can happen: Nothing is designed perfectly;

nothing is made perfectly; and nothing is used perfectly. When failure does occur, and we try to determine the cause, we can

usually examine the design; we can usually examine the failed

parts for manufacturing deficiencies; but we cannot usually

determine how the device was used (or misused). In serious

cases, this can give rise to considerable differences of opinion,

differences which frequently end in court.

In an effort to account for all the above possibilities, we

design every part with a safety factor. Simply put, the safety

factor (SF) is the ratio of the load that we think the part can

withstand to the load we expect it to experience. The safety

factor can be applied by increasing the design loads beyond

those actually expected, or by designing to stress levels below

those that the material actually can withstand (frequently called

‘design stresses’).

Safety factor=SF=failure load/design load

=failure stress/design stress

It is difficult to determine an appropriate value for the safety

factor. In general, we should use larger values when:

1. The possible consequences of failure are high in terms of life