Curing, composite resins stress

Resins are also used for permanent tooth-colored veneers on fixed prostheses, ie, crown and bridges. Compositions for this application include acryflcs, vinyl—acryflcs, and dimethacrylates, as well as silica- or quartz-microfilled composites. The resins are placed on the metallic substrates of the prostheses and cured by heat or light. These resins are inexpensive, easy to fabricate, and can be matched to the color of tooth stmcture. Acrylic facings do not chemically adhere to the metals and are retained only by curing the resin into mechanical undercuts designed into the metal substrate. They have relatively low mechanical strength and color stability, and poor abrasion and strain resistance they also deform more under the stress of mastication than porcelain veneers or facings. 

H.V. Chen, K. Manhart, K.-H. Kunzelmann, R. Hickel, Polymerization contraction stress in hght-cured packable composite resins. Dent. Mater. 19 (2003) 597-602. 

As the resin cures, specifically, in a geometrically constrained environment such as between plates, or within the spaces present between fibers in the case of composites, residual stresses are developed within the resin. These stresses may exceed the intrinsic strength of the resin which depends on its degree of conversion ... 

The primary resin of interest is epoxy. Carbon-fiber-epoxy composites represent about 90% of CFRP production. The attractions of epoxy resins are that they polymerize without the generation of condensation products that can cause porosity, they exhibit little volumetric shrinkage during cure which reduces internal stresses, and they are resistant to most chemical environments. Other matrix resins of interest for carbon fibers include the thermosetting phenolics, polyimides, and polybismaleimides, as well as high-temperature thermoplastics such as polyether ether ketone (PEEK), polyethersulfone (PES), and polyphenylene sulfide. 

A composite consists of 45% by volume of continuous, aligned carbon fibers and an epoxy resin. The tensile strength and modulus of the fibers is 3000 Mpa and 200 GPa, respectively, while the corresponding parameters of the cured epoxy are 70 MPa and 2.5 GPa, respectively. Determine (a) which component of the composite will fail first when the material is deformed in the fiber direction, and (b) the failure stress of the composite. 

Figure 4 shows typical failure surfaces obtained from tensile tests of the co-cured single and double lap Joint specimens. In the case of the co-cured single lap Joint, as the surface preparation on the steel adherend is better, a greater amount of carbon fibers and epoxy resin is attached to the steel adherend. Failure mechanism is a partial cohesive failure mode at the C ply of the composite adherend. In contrast with the co-cured single lap joint, failure mechanism of the co-cured double lap joint is the partial cohesive failure or interlaminar delamination failure at the 1 ply of the composite adherend because interfocial out-of-plane peel stress... 

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