Aircraft Fatigue and Creep 1

If you do not 1. understand the definitions of creep and fatigue and 2. do not understand the loading environment on an aircraft rudder assembly, then why would you be asked to comment about the situation? For your own sake, you should not become involved in this.

I've never heard of fatigue in a cable! Most cables are made up of many individual strands. If one strand fatigues apart, there are still some 48 odd strands remaining.

My understanding of creep is it involves temperature as well. I doubt your control cable sees the kind of temperature required for creep.

The biggest problem I've seen in cables is wear. This wear can weaken the cable which gives way to a static failure at a lower load.

Some basic descriptions.

Creep - Plastic strain induced by temperature and stress for a period of time - normally for steels below 400 deg C not a problem. Failure = accumataled creep strain = strain at rupture. Creep more responsive to temperature than stress. Not easily analysed as stress redistribution and sometimes stress relaxation occurs.

Fatigue - Material degradation as a result of cyclic loading. Sensitive to mean tensile stresses, notches, temperature, number of cycles, stress range, etc. etc. etc
More easily analysed by linear damage theories etc, etc.

Creep - Fatigue - complex interaction of creep and fatigue very conservative approaches exists in the pressure vessel industry. Not very nice.

Regards.

Gerrie

Generally, I don't think control cables fatigue or creep. If any cable will creep, it will most like be engine control cables running close to the exhaust system. The maintenance manual calls out lubricating the cable when the input required exceeds a certain amount. My experience tells me that the pressure vessel, in the case of pressurized fuselages, or other major structural elements would fatigue much much sooner than any control cables. As I recall, an FAR specifies a certain amount of load to be assumed when analyzing the structural integrity of a control system (200 Lbs I think for elevators). There might be one for rudders controls. The cable manufacturer should provide fatigue data if applicable. Other wise, you can use the FAR number. My guess would be the load on the cable would fall under "No fatigue load". i.e: the amount of load is not high enough to cause fatigue if applied infinitely.

Guvnor1313uk,

I don't mean to be offensive but if you need to ask these questions in relation to this problem, you should not be the person conducting this engineering investigation. This field carries far too much legal liability for anyone but experienced, qualified aircraft engineers to answer. Pass it on.