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

Abstract

Abstract. The present work verified the capability of epoxy/mercaptan/tertiary amine system for retarding and/or arresting fatigue cracks in epoxy materials subjected to cyclic loading at room temperature. By using static and dynamic manual infiltration methods, the effects of hydrodynamic pressure crack tip shielding, polymeric wedge and adhesive bonding of the healing agent were revealed. Depending on the applied stress intensity range and the competition between polymerization kinetics of the healing agent and crack growth rate, the above mechanisms exerted different influences on crack retardation under different circumstances. On the whole, the epoxy/mercaptan/tertiary amine system proved to be very effective in obstructing fatigue crack propagation. It formed a promising base for developing self-healing epoxy materials that enable in-situ autonomic rehabilitation of fatigue crack. Vol.4, No.10 (2010) 644-658 Available online at www.expresspolymlett.com DOI: 10.3144/expresspolymlett.2010.79 dling strength in minutes at room temperature and develop useful bond strengths at ambient temperatures as low as -20°C. Therefore, it should be favorable for repairing fatigue damages under cyclic loading. In our previous work, self-healing epoxy composites containing dual encapsulated healant, i.e. two types of microcapsules that respectively include epoxy prepolymer as the polymerizable component and mercaptan/tertiary amine catalyst as the hardener, were made [18][19][20][21]. Upon fracture the unreacted epoxy can be bled into damage sites together with the hardener fluid and then polymerized to repair cracks. The system proved to work in the case of monotonic fracture as characterized by the attractive healing effect even below room temperature. As a continuation of our project, the present work is focused on examination of the performance of the epoxy/mercaptan/tertiary amine system in suppression and rehabilitation of fatigue crack in epoxy materials via manual infiltration. Effect of adhesive curing process on fatigue crack propagation was systematically studied. The results are expected to provide a knowledge frame for the subsequent in-situ self-healing that has the practical value for engineering application. Keywords: smart polymers, fracture and fatigue, self-healing, epoxy eXPRESS Polymer Letters Experimental 2.1. Materials and specimen preparationTapered double cantilever beam (TDCB) specimens were cast from the mixture of epoxy resin (EPON 828, diglycidyl ether of bisphenol A, Hexion Specialty Chemicals, USA) and 12.5 pph curing agent (diethylenetriamine, DETA, Shanghai Medical Group Reagent Co., China). The mixture was degassed, poured into a closed silicone rubber mold and cured for 24 h at room temperature, followed by 48 h at 40°C. Table 1 shows the material properties.The healing agent consists of epoxy (1:1 mixture by weight of EPON 828 and diglycidyl ether of resorcin (J-80, Wuxi Resin Factory of Bluestar New Chemical Materials Co., China)) and the hardener (pentaerythritol tetrakis (3-merca...

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

“…The results agree with those observed in the case of infiltration of epoxy prepolymer (or the hardener, i.e. mixture of PMP and DMP-30) alone [12]. Secondly, the dependence of crack length on fatigue cycle of control specimen with 10 wt% epoxy-loaded capsules and 10 wt% polythiol-loaded capsules (no tertiary amine catalyst is included) was measured (curve c in Figure 6).…”

“…It means that both microcapsules induced-toughening and hydrodynamic pressure crack tip shielding mechanisms have taken effect, while the former makes more contribution to retardation of fatigue crack than the latter. When the healing agent flowing out of the broken microcapsules contain catalyst and could rapidly react, polymeric wedge and adhesive bonding mechanisms [12][13][14] are involved, which can greatly extend fatigue life of the material (see curve d in Figure 6). More details of this aspect will be discussed in the next section.…”

“…All the fracture toughness values of self-healing specimens were obtained from monotonic fracture tests. The data of degree of cure were estimated from isothermal polymerization of the healing agent conducted in DSC at 25°C [12]. The KIC of virgin self-healing specimen is higher than that of neat epoxy specimen as a result of toughening effect induced by the embedded microcapsules, which will be discussed in sub-section 3.2. fatigue experiment ( Figure 6).…”

“…In the latter case, cyclic loading was stopped after a small amount of crack growth and healing was allowed under different steadystate stress intensities for different times. In the case of manual infiltration, about 0.5 µl of pre-mixed epoxy and polythiol (keeping the stoichiometric ratio while excluding catalyst) was injected into the crack plane of unfilled epoxy specimen with a microsyringe after a crack growth increment of 8 mm, and fatigue loading was not interrupted throughout the process [12]. The healing efficiency, !, was defined by fatigue life extension [4], see Equation (1): (1) where N Healed and N Control denote the total number of cycles to failure of the self-healing specimen and that of a similar control specimen without healing, respectively.…”

“…The degree of fatigue life extension was found to be dependent on the relative magnitude of mechanical kinetics of crack propagation and chemical kinetics of healing. In our previous work [12], the epoxy/mercaptan/ tertiary amine healing system proved to be able to suppress and rehabilitate fatigue crack in epoxy materials via manual infiltration. Effect of adhesive curing process on fatigue crack propagation was studied in detail.…”