8 - Fatigue Failure of Track - Crack Growth Flashcards
Phase 2 - large crack development process
Defects grow as wheels damage rail, and are accelerated when fluids such as water or flange lubricants enter the crack
Wear reduces defect size as rail surface is lost
If crack growth keeps ahead of surface wear, crack will grow and extend
If wear or grinding reduces crack size, it may stop growing and be worn away
Crack propagation
Contact stress is primary driver of growth as propagation begins
Linear elastic fracture mechanics can describe growth (cracks have grown away from plastic damage at surface)
Phase 2 RCF
Most structures crack due to tensile stress, rails crack due to compressive load
First addressed for contact of oil lubricated rollers, in which pits formed in surfaces as they ran together
Way of the American Society of Mechanical Engineers found in 1935 that the surfaces of contacting rollers were pitted when lubricated but not when dry
Discovery showed that fluid presence is crucial part of RCF
Stress intensity factors and linear elastic fracture mechanics
Repeated cycles of stress below yield can lead to growth of an existing crack (e.g. wheels crossing a damaged rail)
Stress intensity factor (K) can summarise stress state ahead of a crack so can predict growth
Three cracking modes
Opening mode
Shearing mode
Tearing mode
Modes of real cracks
Combination of all three simple modes - ‘mixed-mode’
Fracture mechanics for fatigue
Can only be applied after crack initiation phase
Simplest expression relating stress intensity factor range to crack growth rate is the Paris-Erdogan equation
Why model RCF?
To determine: if defect will grow; how fast it may grow; how long before the rail needs to be replaced
Potential crack growth mechanisms
Compressive wheel load won’t open the crack, but can apply shear, sliding faces as it passes
Application of a lubricant reduces friction between crack faces so they slide more easily, accelerating growth
This acceleration matches observation that fluid assists crack growth
Fluid can also be pressurised, causing opening or shearing mode crack
How can fluid be pressurised in cracks?
Hydraulic crack growth - fluid fills crack
Fluid entrapment crack growth - crack sealed by passing load, fluid is trapped under pressure
Squeeze film crack growth - fluid under pressure generated by motion of crack faces
How to measure fluid entering cracks
Apply water with UV fluorescent dye
Destructive examination
Cracked gauge corner area of rails cut out, and cracked section broken open with hydraulic press
Shows fluid is likely to be present from first wheel of passing vehicle
Converting stress intensity to crack growth rate
Even shear mechanism of growth will experience some opening stresses and compressive loading
Cracks in pure shear usually branch after only a fraction of a mm growth, whereas cracks in rails growth for tens of mm before branching as a results of the stress driving their growth
Usually, mode 1 opens crack (but not enough to grow it), allowing mode 2 to create growth
Exact shape and size of stress intensity output plots varies with crack size, applied load, and friction on rail surface and between crack faces
Interaction of modes was first predicted with FEA, then confirmed experimentally
Experimental validation of stress cycle
Applied to lab specimens to predict growth rate of rolling contact fatigue cracks
These biaxial fatigue tests produced crack growth data needed to convert stress intensity factors to growth rates
Modelling crack growth
Select crack size, wheel load and friction conditions
Choose crack growth model
Pass wheel over idealised crack
Generate stress intensity factor cycles
Combine values of stress intensity for modes 1 and 2
Calculate crack growth rate
Common features from modelling output in terms of crack growth rate
Rapid rise in growth rate at small crack sizes
Peak in growth at around 10mm crack size
Return to near zero crack growth rate by a 35mm crack size
Differences between runs due to speed and local track geometry giving different contact pressures
