The Effect of Artificial Fatigue‐crack Closure on Fatigue‐crack Growth

Abstract

An investigation of the effects of artificial fatigue-crack closure on the subsequent growth of fatigue cracks, at constant range of stress-intensity factor, was carried out for the following closure materials: (i) electroplated nickel; (ii) electroless nickel; (iii) ethylcyanoacrylate (ECA) adhesive; (iv) low-melting point (LMP) solder. It was observed that (a) the wedge thickness and area of crack surface covered by the artificial-closure material and (b) the distance of the tip of the closure wedge from the crack tip (i.e. the penetration of the closure material towards the crack tip) were the main parameters controlling the reduction in the subsequent growth rate of the fatigue crack. The results suggest that if the crack thickness (opening) is completely filled with a dense wedge of artificial-closure material, the wedge becomes effectively rigid and the precise mechanical properties of the closure material will have only a secondary influence on subsequent fatigue-crack growth in comparison to (a) and (b) above. This follows from the fact that the closure material is effectively a very thin strip compressed between the rough (high friction) sides of a crack. A rigid-wedge and elastic-crack model was found to successfully predict the changes in fatigue-crack growth rate following artificial crack-closure and gave a lower bound to the measured growth rates. NOMENCLATURE a = Crack length B = Specimen thickness c = Distance from the wedge tip of the artifical-closure material to the crack tip C = Material constant in the Paris equation K, = Stress-intensity factor due to artifical-closure wedge Kc, = Stress-intensity factor due to natural crack-closure K,, = Maximum stress-intensity factor K , , = Minimum stress-intensity factor K, =Theoretical stress-intensity factor due to a rigid wedge m = Material exponent in the Paris equation R = Load ratio (K,JKmX) UTS = Ultimate tensile stress W = Specimen width Y = Uniaxial yield stress 2h = Mean crack opening or closure-wedge thickness AK = Stress-intensity factor range AKa = Effective stress-intensity factor range

Self-healing of fatigue crack in epoxy materials with epoxy/mercaptan system

“…Compared with control specimen, the mechanisms of adhesive bonding and polymeric wedge involved in crack tip shielding of self-healing specimens are highlighted, regardless whether !K I is high or low. (2), it is known that if the fatigue crack opening load is purposely increased by a wedge with adhesive properties at the crack tip, the effective stress intensity factor range would be reduced [18,[20][21][22][23][24][25][26]. Keeping this idea in mind, researchers have made attempts to artificially introduce crack surface contact.…”

“…Effect of applied range of cyclic stress intensity Following the traditional crack closure concept [18,[20][21][22][23][24][25][26], it is known that if fatigue crack opening load is purposely increased at the crack tip of selfhealing specimens by the aforesaid four mechanisms (i.e. microcapsules induced-toughening, hydrodynamic pressure crack tip shielding, polymeric wedge and adhesive bonding), the effective stress intensity factor range, !K eff , would be reduced accordingly, see Equation (2): (2) where !K toughening denotes the stress intensity due to microcapsules induced increase of matrix ductility, !K liquid the crack-opening and crack-closure stress intensity from viscosity resistance of the liquid, !K bonding the stress intensity due to the combined (tensile) stresses in adhesives across the crack faces, and !K wedge the crack-closure stress intensity due to the wedge from adhesives gelling and hardening.…”

Healing of fatigue crack in epoxy materials with epoxy/mercaptan system via manual infiltration

“…With this idea in mind, a number of investigations were carried out by artificially introducing crack surface contact [4,[9][10][11][12][13][14][15][16]. It can be concluded that size and performance of the wedge are the most crucial factors affecting the crack closure effect.…”

“…It is hoped that the cracks can be autonomously eliminated soon after their emergence. Earlier studies on metals have shown that infiltrating proper substances into a fatigue crack could induce crack growth retardation and even crack arrest [9][10][11][12][13][14][15][16]. Recently, Brown et al [4] established a protocol to extend fatigue life of epoxy using dicyclopentadiene (DCPD).…”

“…As the crack grows past the wedge from the cured healing agent (refer to curve o in Figure 7), the crack growth rate increases with a rise in ΔK eff approaching the rate before healing until failure of the specimen. In previous studies, the adhesive infiltrants were generally allowed to be fully cured to form solid polymeric wedges under constant holding loads before resuming fatigue tests [4,[9][10][11][12][13][14][15][16]. Here the influence of the curing of the healing agent on fatigue crack growth is taken into consideration for evaluating the role of the wedge performance.…”