Extensive indentation tests were conducted for nineteen different engineering materials ranging from brittle to ductile materials, and including hard ceramics, ductile metals, and a soft organic polymer. Three tetrahedral pyramid indenters with specific face angles β [shallow pyramid (β = 10°), Vickers (β = 22°), and sharp pyramid (β = 40°) indenters] were used. All the materials tested were subjected to the quadratic load P and penetration depth h relationship P = k1h2 on loading, and most of the tested materials to the quadratic unloading relationship of P = k2(h − hr)2 with the residual depth hr after a complete unload. To determine the contact area at peak indentation load, a specially designed depth-sensing instrument was constructed, on which the contact behavior during loading/unloading was examined by through thickness observation of transparent specimens. All the characteristic indentation parameters were investigated on the basis of simple elastoplastic model, and correlated well with the nondimensional strain E′ tan β/H, in which the elastic modulus E′ was a measure for elasticity, true hardness H was a measure for plasticity, and the inclined face angle β characterized the indenter. The ratio of the conventional Meyer hardness HM to the true hardness H of the materials tested ranged from 0.2 to 0.9 as a function of E′ tan β/H. The cavity model suggested that true hardness H is expressed by the yield stress Y through a constraint factor C as H = C · Y with C ≈ 5.
Much interest has been paid to cyclic diaryliodonium salts as bis(electrophile)s in transition-metal-catalyzed annulation reactions to produce polycyclic (hetero)aromatic hydrocarbons. Herein, we report that dibenziodolium triflates smoothly react with potassium thioacetate in the presence of
A Vickers indenter, as an efficient mechanical microprobe, was applied to carbon materials heat-treated at temperatures in the range 880°-2600°C. The plasticity of the carbon materials, which was enhanced by increasing the heat-treatment temperature (HTT), was assessed from the relation between the indentation load, P, and the penetration depth, h. Using the concept of the true hardness, H, as a measure of plasticity and the experimental estimate of the H-value, the plasticity of the carbon materials was examined as a function of their crystallographic parameters. The residual impression of the carbons at HTT > 1800°C was hardly visible on the indented surface after unloading, because of the nearly complete elastic recovery of the indented surface, yielding a very unique indentation P-h hysteresis in the loading/unloading cycle. The microscopic processes associated with this unique elastic recovery during unloading are discussed here in relation to the reversible slip of the dislocation-network structures on the graphitic basal planes.
Ag-Pd-Cu-Au-Zn type alloys have been widely used as dental prosthetic materials. In general, the dental prosthetic products are fabricated by a dental casting method. The dental castings contain the casting defects such as micro shrinkages, pores, surface roughness, etc. Cyclic stress, that is, fatigue stress due to the mastification is applied to the dental prosthesis in the practical use. Therefore, the effects of the casting defects such as micro shrinkages, pores and surface roughness on the fatigue properties of the cast Ag-Pd-Cu-Au-Zn type alloy were investigated in the comparison with the fatigue properties of the drawn Ag-Pd-Cu-Au-Zn alloy in this study. Fatigue tests of Ag-Pd-Cu-AuZn alloy cast using a dental casting machine were carried out, and then the fatigue properties of the cast alloy were investigated in the relation with casting defects. The fatigue strength of the cast Ag-Pd-Cu-Au-Zn alloy is considerably smaller than that of the drawn Ag-Pd-Cu-Au-Zn alloy. The fatigue crack of the cast alloy initiates preferentially at the shrinkage near the specimen surface. The scatter of the fatigue strength of the cast alloy becomes to be small by relating the fatigue life with the maximum stress intensity factor calculated assuming the shrinkage that becomes to be a fatigue crack initiation site as a initial crack. This means that the size of the shrinkage affects the fatigue strength of this cast alloy strongly. The tolerant size of the shrinkage that satisfies the target value of the fatigue limit (230 MPa) of this cast alloy is calculated to be below 80 µm using the equation derived in this study, which describes the relationship between the maximum stress intensity factor and the number of the cycles to failure.
The elastoplastic surface deformation of a polycrystalline graphite was studied by examining the indenter's geometry dependence of load P versus penetration depth h relation (P-h relation) in instrumented pyramidal/spherical indentation tests. The tetrahedral pyramid indenters included inclined face angles  of 10.0°, 22.0°(Vickers pyramid), and 40.0°. The tip radius of spherical indenters used were 32 m, 200 m, 794 m, 1.59 mm, and 6.35 mm. The true hardness H as a measure for plasticity was singled out of the elastoplastic loading parameter k 1 in the quadratic expression of P ס k 1 h 2 and then quantitatively related to the yield stress Y that was determined from the mean contact pressure for spherical indentation at the onset of plastic yielding. The size effect of Y, decreasing with the increase in the tip radius of spherical indenter, is discussed using the model of geometrically necessary dislocations in terms of the material length scales for a plastic field with strain gradient.
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