Contact fatigue cracking

Fig. 21 In situ visualization pictures showing damage a in a fretting contact between an unmodified DGEBA/DDM epoxy network and a glass sphere, and b in a similar contact with a DGEBA/DDM network modified with the acetamide derivative shown in Table 1. The magnitude of the maximum tensile stress at the edge of the contact was found to be similar in both cases. Contact fatigue cracks were only observed in the neat DGEBA/DDM system...

A. F. Bower, The influence of crack face friction and trapped fluid on surface initiated rolling-contact fatigue cracks. Journal of Tribology-Transactions of the ASME, 110 (1988)704-711. 

Garnham, J.E., Beynon, J.H., 1991, "The Early Detection of Rolling-Sliding Contact Fatigue Cracks", Wear, 1991, Vol. 144, PP103-116. 

YOSHIOKA,T. Detection of Rolling Contact Sub-Surftice Fatigue Cracks Using Acoustic Emission Technique. Lubrication Engineering, Vol.49, No.4, 1993, p.303-308. 

Crack extension is often observed to vary significantly at the same nominal value of AK (= Y AOpos Tta) for different values of R-ratio. Elber [26] was the first to explain this observation for metals in terms of the crack closure phenomenon. He determined, by measuring specimen compliance, that fatigue cracks open and close at the crack tip at positive values of stress due to contact between crack surfaces behind the crack tip. For elastic fatigue conditions it is generally found that P p = P, and Kop = K, where P is the applied load. 

Fig. 7 Development of fatigue cracks in an epoxy/glass contact under gross slip condition (1Hz, displacement amplitude 60 xm) (from [97]). White arrows indicate the occurrence of crack initiation and propagation at the edge of the contact under the action of tensile stresses. The lateral contact stiffness, K, is essentially a measurement of the elastic response of the epoxy substrate within the contact zone. Brittle crack propagation is associated to a drop in stiffness due to the additional accommodation of the imposed displacement provided by crack opening mechanisms...

Fig. 8 Post mortem optical observation of a contact cross-section showing crack depth and orientation within an epoxy polymer after contact fatigue...

Fig. 10 Measured crack lengths, L, in an epoxy polymer as a function of the number of cycles and the imposed lateral displacement. The crack length was measured in the contact plane from in situ observations. Amplitude of the relative displacement Black circles 15 xm, white circles 20 pm, white squares 25 pm, black squares 30 pm, white diamonds 60 pm. Dotted lines indicate the occurrence of britde failure events during the course of the contact fatigue experiments...

This combined analysis of AoA and Ar, therefore establishes that the main cracks that nucleate close to the contact edge correspond to predominantly tensile fatigue cracks. This conclusion remains valid whatever the contact condition (partial slip or gross slip). In addition, the distribution of within the contact plane is of interest (Fig. 15). The maximum amplitude... 

As for the epoxy polymers, a quantitative comparison of the contact fatigue behaviour was attempted on the basis of an estimate of the maximum tensile stress at the edge of the contact. The coefficient of friction of the copolymers increased as the tests proceeded, with a variation which was dependent upon the level of the normal loading. As a first approach, the value of //. at crack initiation was taken into account in the calculation of a . The results are reported in a S-N fatigue diagram giving the maximum applied tensile stress as a function of the number of cycles to crack initiation (Fig. 23). These data show a marked increase in the contact fatigue resistance of the GIM copolymers compared with the MIM material. 

Fig. 23 Contact fatigue behaviour of methylmethacrylate random copolymers under fretting conditions. White circle Glutarimide co-monomer (76%), black circle cyclohexyl malemimide co-monomer (25%). The maximum value of the tensile stress has been calculated at the edge of the contact, where cracks initiate, and for values of the coefficient of friction measured at crack initiation...

Experimental and Theoretical Investigation of the Contact Fatigue Behaviour of an Epoxy Polymer 57 Theoretical analysis of the location and orientation of cracks... 

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