Effect of Reinforcement on Flexural Properties of Brittle Composites

The flexural properties in terms of ductility of laminated brittle composites have been studied experimentally in the laboratory. Test specimens consisting of variable layers of mesh were prepared and tested under third point loadings. It was observed that the brittle matrix altered it properties and behaved as the ductile matrix even with small amount of ductile materials. The causes of the change in the properties have been clarified by analyzing the experiment data. The experimental results were verified by the theoretical formulation. The formulation showed that the flexural modulus of the entire section of brittle composites obtained by section analysis is equal to the modulus of ductile layers plus a factor of the difference of modulus of ductile and brittle layers. This factor is third order of ductile layer causing flexural properties in brittle composite even with 10% of ductile materials.

Evaluation of Brittle Layers Obtained by Boriding on AISI H13 Steels

The fracture toughness of AISI H13 borided steel and the strength adhesion of the coated system were estimated in the present work. The formation of the layers was carried out by the powder pack boriding process at 1273 K with 8 h of treatment. The fracture toughness (KC) of the layer is estimated at 25 and 45 m from the surface using four different Vickers indentation loads. The KC values were estimated by the extension of Palmqvist cracks parallel and perpendicular to the surface obtained at the indentation corners. The adherence of the layer/substrate was evaluated in qualitative form through the Rockwell-C indentation technique. The results obtained by both techniques, show, in first instance, that the fracture toughness of the boride layer can be expressed in the form (KC) (π/2) > (KC) > (KC) (0). Also, high delamination is obtained around the Rockwell-C indentation prints that denote poor adhesion in the coating-substrate interface.

Fracture modes in curved brittle layers subject to concentrated cyclic loading in liquid environments

Damage response of brittle curved structures subject to cyclic Hertzian indentation was investigated. Specimens were fabricated by bisecting cylindrical quartz glass hollow tubes. The resulting hemicylindrical glass shells were bonded internally and at the edges to polymeric supporting structures and loaded axially in water on the outer circumference with a spherical tungsten carbide indenter. Critical loads and number of cycles to initiate and propagate near-contact cone cracks and far-field flexure radial cracks to failure were recorded. Flat quartz glass plates on polymer substrates were tested as a control group. Our findings showed that cone cracks form at lower loads, and can propagate through the quartz layer to the quartz/polymer interface at lower number of cycles, in the curved specimens relative to their flat counterparts. Flexural radial cracks require a higher load to initiate in the curved specimens relative to flat structures. These radial cracks can propagate rapidly to the margins, the flat edges of the bisecting plane, under cyclic loading at relatively low loads, owing to mechanical fatigue and a greater spatial range of tensile stresses in curved structures.

Modeling Ductile/Brittle Behavior in Polymeric Microlaminates: Effect of Volume Fraction

In this work we present a micromechanical model for two-phase ductile/brittle laminates that captures the macroscopic behavior, as well as the underlying micro-mechanisms of deformation and failure, in particular the synergy between the inelastic deformation mechanisms of crazing and shear yielding. The finite element implementation of our model considers a three-dimensional representative volume element (RVE), and incorporates continuum-based physics-inspired descriptions of shear yielding and crazing, along with failure criteria for the ductile and brittle layers. The interface between the ductile and brittle layers is assumed to be perfectly bonded. The model successfully explains the volume fraction effect on the micro and macromechanics of ductile/brittle microlaminates subjected to uniaxial tension.

Competing Damage Modes in All-Ceramic Crowns: Fatigue and Lifetime

A study is made of fracture from cyclic loading of WC spheres on the surfaces of brittle layers on compliant substrates, as representative of repetitive occlusal contact on dental crowns. Several damage modes—radial cracks at both top surface and cementation interface, and classical cone cracks as well as deep penetrating cone cracks from the top surface—have been identified and analyzed. The most dangerous fractures are radial cracks that initiate from either the top or bottom surfaces of the brittle layers and spread laterally to failure. In fatigue, these cracks are driven by chemical forces associated with the intrusion of water into the crack. Also dangerous are deep penetrating cone cracks which, unlike their classical cone crack counterparts, are mechanically driven by hydraulic pumping and can evolve rapidly with cyclic loading, threatening the lifetime of a dental crown veneer.