Mode II Fracture Analysis of GNP/Epoxy Nanocomposite Film on a Substrate

Epoxy resin with excellent mechanical properties, chemical stability, and corrosion resistance has been widely used in automotive and aerospace industries. A thin film of epoxy deposited on a substrate has great application in adhesive bonding and protective coating. However, the intrinsic brittleness of epoxy with a relatively low fracture toughness limits its applications. In this work, graphene nanoplatelets (GNP) were added to the epoxy resin to enhance its toughness, hardness, and elastic modulus. A series of nanocomposites with different loadings of GNP were fabricated. Ultrasonic sonication in combination with surfactant Triton X-100 were employed to disperse GNP in the epoxy matrix. A nanocomposite film with a thickness of 0.3 mm was deposited on an Al substrate using a spinning coating technology. The hardness and elastic modulus of the nanocomposite film on the Al substrate were experimentally measured by a nanoindentation test. Analytical expression of the mode II interfacial fracture toughness for the nanocomposite film on an Al substrate with an interfacial edge crack was derived utilizing the linear elastic fracture mechanics and Euler’s beam theory. End-notched flexure (ENF) tests were conducted to evaluate the mode II fracture toughness. It was found that the hardness, elastic modulus, and mode II fracture toughness of the nanocomposite film reinforced with 1 wt % of GNP were improved by 71.8%, 63.2%, and 44.4%, respectively, compared with the pure epoxy. The presence of much stiff GNP in the soft epoxy matrix prompts toughening mechanisms such as crack deflection and crack pinning, resulting in the improvements of the fracture toughness, hardness, and elastic modulus. Microscopic observation for the nanocomposite was examined by scanning electron microscopy (SEM) to investigate the dispersion of GNPs in the epoxy matrix. The performance of a nanocomposite film deposited on a substrate was rarely studied, in particular, for the interfacial fracture toughness of the film/substrate composite structure. Utilizing the theoretical model in conjunction with the ENF experimental test presented in this study, an accurate determination of the mode II interfacial fracture toughness of film/substrate composite structure is made possible.

Design Equations for Mixed-Mode Fracture of Dental Ceramic-cement Interfaces Using the Brazil-nut-sandwich Test

Dental interfaces are subject to mixed-mode loading. This study provides a practical guidance for determining interfacial fracture toughness of dental ceramic systems. We address interfacial fracture of a composite resin cement sandwiched between two dental ceramic materials. Emphasis is placed on sandwich disc specimens with cracks originating from elliptical-shaped flaws near the center, for which analytical fracture mechanics methods fail to predict. The interaction integral method is used to provide accurate finite element solutions for cracks with elliptical-shaped flaws in a Brazil-nut-sandwich specimen. The developed model was first validated with existing experimental data, and then used to evaluate three most widely used dental ceramic systems: polycrystalline ceramics (zirconia), glass-ceramics (lithium disilicate), and feldspathic ceramics (porcelain). Contrary to disc specimens with ideal cracks, those with cracks emanating from elliptical-shaped flaws do not exhibit a monotonic increase in interfacial toughness. Also, interfacial fracture toughness is seen to have a direct relationship with the aspect ratio of elliptical-shaped flaws and an inverse relationship with the modulus ratio of the constituents. The presence of an elliptical-shaped flaw significantly changes the interfacial fracture behavior of sandwich structures. Semi-empirical design equations are provided for fracture toughness and stress intensity factors for interfacial cracks. The developed design equations provide a practical guidance for determining interfacial fracture toughness of selected dental ceramic material systems. Those equations take into account four critical factors: size of the elliptical flaw, modulus ratio of constituent materials, loading angle and applied load.

Influence of the Aggregate Surface Conditions on the Strength of Quick-Converting Track Concrete

This experimental study investigates the effects of the aggregate surface conditions on the compressive strength of quick-converting track concrete (QTC). The compressive strength of QTC and interfacial fracture toughness (IFT) were investigated by changing the amount of fine abrasion dust particles (FADPs) on the aggregate surface from 0.00 to 0.15 wt% and the aggregate water saturation from 0 to 100%. The effects of aggregate water saturation on the compressive strength of the QTC and IFT were notably different, corresponding to the amount of FADPs. As the aggregate water saturation increased from 0 to 100%, in the case of 0.00 wt% FADPs, the IFT decreased from 0.91 to 0.58 MPa∙mm1/2, and thus, the compressive strength of the QTC decreased from 34.8 to 31.4 MPa because the aggregate water saturation increased the water/cement ratio at the interface and, consequently, the interfacial porosity. However, as the aggregate water saturation increased from 0 to 100%, in the case of 0.15 wt% FADPs, the compressive strength increased from 24.6 to 28.1 MPa, while the IFT increased from 0.41 to 0.88 MPa∙mm1/2 because the water/cement ratio at the interface was reduced as a result of the absorption by the FADPs on the surface of the aggregates and the cleaning effects of the aggregate surface.