Quote:
Originally Posted by Evan Fullerton
Has anyone actually seen a Porsche Titanium rod have a fatigue failure? I know PMNA doesn't always replace them in 3.6L Cup motor rebuilds and with people running 996 Cup motors 200+ hours without rod failure and GT3s accumulating 100+k miles (slightly different rod to Cups but still Ti) with the rods intact, are we sure that they are actually operating beyond the fatigue limit?
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Quote:
Originally Posted by racing97
Porsche Titanium rods are usually replaced by PMNA at 60 hours yet several set are running at 100+ i and the street version of the GT3 is not pulled into the Dealer at any preconceived time and replaced Why?
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If the rods aren't breaking they simply haven't accumulated sufficient damage to cause failure, the issue is when will this happen?
This is a very difficult question to answer and I would reiterate that a decision to use a part beyond the life recommended by the manufacturer is a simple risk/reward judgement.
As we have no data in terms of levels of stress and no real fatigue test results you simply pay your money and make your choice.
The entire problem is based on the stochastic nature of the fatigue process, the variability of the material we are considering and the loading spectrum that creates the stresses that drive the fatigue process.
The concept of a Fatigue Endurance Limit is interesting and has been well accepted for many years and in practical terms seems to work - at least for the majority of conventional engineering steels.
The main reason for the development of a Fatigue Endurance Limit is concerned with a metallurgical process known as strain aging. In this process the dislocations present in the material's crystal structure are pinned into place by solute atoms which produces a very localised increase in yield stress and this effect is activated by the cyclic loading that causes fatigue. By using Electron Microscopy the development of dislocation 'tangles' around these solute atoms can be observed and this would suggest that this explanation is valid.
Engineering Steels exhibit high levels of strain aging but this behaviour is not so pronounced in 6AL4V.
It is, however, fair to say that this concept is routinely questioned and recent work published in Acta Metallurgia suggests that when we consider Very High Cycle Fatigue some of the assumptions don't add up. (Cycles of 10exp10 to 10exp12)
Some recent work carried out at UCLA on Ti Turbine Blades subjected to very high levels of vibration suggest that some of the previously applied properties may have been overestimated by at least a factor of 2 .
It is also fair to say that we are unlikely to see this number of cycles within a typical car engine.
Traditionally Ti Alloys were not considered to have a Fatigue Endurance Limit but
it has become customary for the manufacturers of Ti alloys to now quote Fatigue Endurance Limits of around 500MPa at 10exp8 cycles for 6AL4V but I believe if you design components with this level of gross stress you will suffer significant numbers of component failures.
If you read the 'small print' you will see that the 500MPa is an 'estimate' based on 50% of the yield strength of the material and that 'service and geometric' factors must be considered and that this figure should be down-rated between 1.5 and 4 times depending on the specific design.
Very helpful !
I started fatigue testing 6AL4V Ti on a commercial basis many years ago as it was essential to fatigue qualify material used in the manufacture of Sea Harrier components on a batch to batch basis due to material variability.
The thermo-mechanical processing of 6AL4V introduced significant variability which resulted in around 30% or the material supplied being inadequate for the duty cycle
I am sure that modern process control has improved this situation significantly and that by using FEA tools it is relatively straightforward to understand the stress/strain conditions applied to the rod and again using good quality published fatigue data it is possible to design a satisfactory con rod.
The reasons for the 'short' life is most likely due to the inevitable uncertainties and to completely eliminate the risk of a failure.
It is also worth pointing out that improvements in the Fatigue Limit of 6AL4V of up to 10% have been reported by using controlled shot peening and I would consider this to be a worthwhile cost on high value race engines.
Metal Improvements in the UK (part of Curtiss-Wright) have a CAA/FAA approved method for refurbishing Ti helicopter Rotor parts that has been backed up with fatigue test data.
It is also worth noting that it has become a common practice to coat Ti Rods using a Chromium Nitride (CrN) applied using a PVD chamber as an alternative to the plasma moly that used to be applied to the side faces of the big end.
I am not sure I like this process.
Some recent work has been carried out by Bodycote who have investigated the effect of PVD coatings on the fatigue life of 6AL4V Ti landing gear components.
They wanted to apply these coatings to reduce wear and eliminate galling. They evaluated Titanium Nitride, Chromium Nitride and a DLC based coating.
In all cases these coating
reduced the axial fatigue life of the components being evaluated and on this basis I would need to see some data before I used this process.
Ti rods will of also be more prone to failure if they are nicked or marked as they are likely to be less damage tolerant than a steel such as 4340.