Remember corrosion exacerbates the potential for fatigue failure

AUGUST 2017

Fatigue of engineering materials and their degradation through micro-cracking, as a result of cyclic loading, is extremely common in materials, especially metals and alloys, accounting for up to eighty percent of all structural failures. Real world engineering structures are subjected to a range of environmental conditions, which can and usually do exacerbate the fatigue circumstances and accelerate the fatigue crack advancement process.

This corrosion-induced increase in crack growth rate is best described by means of Fracture Mechanics, as covered in this Tech Tip.  Even at the level of traditional old fashioned characterisation of fatigue by so called SN curve representation – of stress level S versus number of cycles to failure N – the effect of environment is clear.  In conventional non corrosive fatigue, SN plots are characterised on a log–log plot of Stress, S, versus Number of cycles to failure, N, by a decreasing straight line on such a log plot, concluded, at high lifetimes, by a ‘constant’ fatigue stress limit below which fatigue life is no longer dependant on the magnitude of the cyclic stress and fatigue life is ‘infinite’.  When environmental effects and mechanisms are present, together with fatigue, the whole SN curve is effectively lowered and shifted to the left.  Furthermore, the ‘fatigue limit’ aspect is no longer apparent and ‘infinite’ fatigue life is not realised.

 

The fracture mechanics formulation of fatigue is nowadays much more useful and applicable, where the lifetime number of cycles N, and in particular the crack growth rate is related to the cyclic stress intensity amplitude ΔK through the well-known Paris equation:

        da/dN  = CΔKm

                         Where:
                         da/dN is the crack growth rate per cycle
                         ΔK is the cyclic stress intensity
                         C and m are constants generally related to the material,
                         and m is (between 2 and 4) typically about 3.

The Paris representation thus results in a linear relationship between da/dN and ΔK, with typically a threshold region at the bottom left,ΔKth below which the fatigue crack growth rate is effectively insignificant.  However, when fatigue occurs in an environmental regime, additional corrosive mechanisms occur to increase the rate of crack advancement, and in effect shift the entire Paris curve to the left, implying a faster crack growth rate under corrosive conditions.  There are however different types of such corrosive effects, which are conventionally separated into four categories, as illustrated in the figure (sourced from Fracture Mechanics by Ewalds and Wanhill)  below. 



In case ‘a’, the entire Paris curve is generally shifted to the left, indicating the corrosion is affecting all aspects of crack advance, right from threshold up to propagation, even at the top range of propagation.  In case ‘b’, in addition to a leftward shift of the Paris curve there is an additional rapid acceleration in crack growth rate above a certain ΔK value, which is consistent with (liquid) stress corrosion cracking (SCC).  In this case there can be a brief regime of very high growth rates, where the index ‘m’ (of the Paris equation) is significantly enhanced (to values typically in excess of 10, but can be as high as 60).  In case ‘c’, the behaviour is similar to type ‘b’, but without the acceleration right down to threshold regions, and is generally related to gaseous corrosion.  Finally there is type ‘d’, where again there is a general acceleration of crack growth rate except at threshold, where the behaviour at threshold shifts slightly to the right due to ‘closure effects’.  In this case oxidation/corrosion product forms in the crack wake and can inhibit the metal component experiencing the full cyclic applied amplitude, (rather like a nutcracker effect) and thus reducing the effective cyclic stress intensity ΔK, with a natural reduction in crack growth rate in accordance with conventional Paris effects.

It is also important to understand that when considering corrosion fatigue, the effect of cyclic frequency can play a significant role. Recognising that corrosive mechanisms enhance the crack growth rate, it is hardly surprising that a reduction in the cyclic frequency also leads to enhanced crack growth rates because the crack is ‘open for longer periods’ and thus more susceptible to the corrosion component, rather than the plasticity (fatigue) component.  Thus at low frequencies the Paris curve shifts further to the left over the range from say, 20 Hz (20 cycles per second) through 1Hz to 0.1 and 0.01Hz (1 cycle every 0.6seconds).  At these lower frequencies the crack remains open for a sufficient time to exacerbate the growth rate (yet beyond a period of about ten minutes the effect does not get any worse).  Such behaviour becomes very important for slow frequency operations in sea water for example, and should be taken into account, but the effect can be ameliorated by cathodic protection using implied current or sacrificial anodic systems.

In the fatigue of engineering components, especially in adverse liquid or gaseous environmental conditions, it is important to take into account corrosion fatigue for lifetime evaluations, and at the very least understand the sources of crack initiation and propagation and advance in such circumstances, and how to overcome these challenges. 


Published in Technical Tips by Origen Engineering Solutions on 1 August 2017