Notched Beam Technique Research Articles

Techniques for characterizing the mechanical properties of aerogels

In this paper, we present the different characterization techniques used to measure the mechanical properties of silica aerogels. The mechanical behaviour of aerogels is generally described in terms of elastic and fragile materials (such as glasses or ceramics) but also in terms of plastic media in compression testing. Because of these very different mechanical behaviors, several types of characterization techniques are proposed in the literature. We first describe the dynamic characterization techniques such as ultrasounds, Brillouin scattering, dynamic mechanical analysis (DMA) to measure the elastic properties: Young's modulus (E) , shear modulus (G), poisson ratio (υ) but also attenuation and internal friction. Thanks to static techniques such as three-point bending, uniaxial compression, compression we also access to the elastic modulus (E) and to the rupture strength (σ). The experimental results show that the value of the elastic and fracture moduli measured is several orders of magnitude lower than that of a material without porosity. With regard to the brittleness characteristics, Weibull's analysis is used to show the statistical nature of the fracture resistance. We also present the SENB (single edge notched beam technique) technique to characterize toughness (K1C) and the stress corrosion mechanisms, which are studied in ambient conditions and temperature by the double-cleavage drilled compression experiment (DCDC). In the last part of the paper, we show how, during the isostatic compression test, aerogels behave like plastic materials.The data allow calculating the bulk modulus (K), the amplitude of the plastic deformation and the yield strength (σel), which is the boundary between the elastic and plastic domains. These different techniques allow understanding which parameters influence the overall mechanical behavior of aerogels, such as pore volume, but also pore size, internal connectivity and silanol bounds content. It is shown that pore size plays a very important role; pores can be considered as flaws in the terms of fracture mechanics.

Influence of surface treatments and repair materials on the shear bond strength of CAD/CAM provisional restorations.

PURPOSETo evaluate the effect of surface treatments and repair materials on the shear bond strength and to measure the fracture toughness of CAD/CAM provisional restoration materials.MATERIALS AND METHODSFour CAD/CAM (3D printing: Nextdent C&B and ZMD-1000B Temporary, CAD/CAM resin block: Yamahachi PMMA disk and Huge PMMA block) and four conventional (monometacrylate: Jet and Alike, dimetacrylate: Luxatemp and Protemp 4) materials were selected to fabricate disk-shaped specimens and divided into six groups according to surface treatment (n=10). CAD/CAM materials were repaired with Jet or Luxatemp, while conventional materials were repaired with their own materials. The shear bond strength was measured by using universal testing machine. Ten rectangular column-shaped specimens for each material were fabricated to measure the fracture toughness by single edge v notched beam technique. Statistical analysis was performed by one-way ANOVA.RESULTSThe highest shear bond strength of CAD/CAM materials was achieved by SiC paper + sandblasting. It was also accomplished when repairing 3D printing materials with Luxatemp, and repairing CAD/CAM resin blocks with Jet. Yamahachi PMMA disk showed the highest fracture toughness. Nextdent C&B showed the lowest fracture toughness value but no statistically significant difference from Alike and Luxatemp (P>.05).CONCLUSIONIn order to successfully repair the CAD/CAM provisional restoration, mechanical surface treatment and appropriate repair material according to the CAD/CAM material type should be selected. The CAD/CAM provisional materials have proper mechanical properties for clinical use as compared to conventional materials.

Microstructural and mechanical characterisation of biomorphic SiC ceramics synthesised from coir fibreboard preform

Fibreboards made of coir fibres were converted to carbon templates by controlled thermal processing and were further infiltrated and reacted with liquid Si at 1550°C under vacuum to produce biomorphic SiC ceramics. Microstructural analysis including quantitative characterisation of microstructure was undertaken, employing optical microscopy, scanning electron microscopy and X-ray diffraction techniques. Mechanical properties were investigated at room temperature. Testing included measurement of flexural strength and elastic modulus by the three-point bending method and fracture toughness by the single edge notched beam technique. Properties measured on this material are typical of the biomorphic ceramics made from the natural and artificial (processed) bio-precursors.

Fracture behaviour of alumina and zirconia thin layered laminate

Abstract Laminates with strong bonds between thin layers were examined in this work to explore the influence of developed internal stresses on the fracture behaviour. A set of laminates having different level of internal stresses were prepared. Alumina and zirconia were the model materials for evenly alternating layers. The electrophoretic deposition technique was used for manufacturing of the laminates. The basic mechanical properties as elastic modulus and flexural strength were determined for all prepared materials. The crack propagation changes due to effect of internal stresses, elastic mismatch and surface effects were investigated using modified single edge notched beam technique. An extensive fractographical analysis of fracture surfaces was undertaken using laser confocal microscopy. The changes of the crack direction when crack propagates through alternating layers under different angels were described. Further, the effect of the internal stresses level within individual layers was reported.

Carbon nanotubes: do they toughen brittle matrices?

The development of a model CNT-brittle matrix composite system, based on SiO2 glass containing well-dispersed CNTs at up to 15 wt%, allows a direct assessment of the effect of the nanoscale filler on fracture toughness (KIC). Samples were prepared by colloidal heterocoagulation followed by spark plasma sintering. Detailed KIC measurements, using both indentation and notched beam techniques, show a linear improvement with CNT content, with up to a twofold increase of fracture toughness at maximum loading. The results from the two methods used in this study show equivalent trends but differing absolute values; the relative merits of these two approaches to measuring nanocomposite toughness are compared. Possible toughening mechanisms associated with CNT pull-out, crack bridging, and crack deflection are identified, and discussed quantitatively, drawing on conventional short-fibre composite theory and the potential effects of scaling fibre diameter.

Slow crack growth in aerogels

The chemical susceptibility was measured by a dynamic method on aerogels immediately after supercritical drying and after oxidation treatment. Five loading speeds between 10 μm/min and 100 mm/min were used. After oxidation treatment, the Weibull modulus increased, which indicates a narrowing of the strength distribution. Toughness of the aerogels was measured by the single edge notch beam technique. ‘As-prepared’ aerogels did not show slow crack propagation. However, as the oxidized materials exhibit a stress corrosion phenomenon, the crack propagation rate was evaluated using the median value of strengths obtained with the highest crosshead speed. The lifetime prediction is estimated as a function of failure probability and from a proof testing experiment.

Fracture toughness of CaO-P2O5-B2O3 glasses and glass-ceramics determined by indentation

The fracture toughness, (K IC) of CaO-P2O5-B2O3 glasses and glass-ceramics was investigated using both Vickers indentation and the notched beam technique (NBT). Five representative equations were applied and it was found that for the variation of K IC with B2O3 content, the Lawn and Fuller equation showed the best correspondence with the NBT. The values of fracture toughness obtained from the Lawn and Fuller equation showed the same trend with B2O3 content as that determined by NBT, although the values from indentation were on average 33% lower. The determination of absolute fracture toughness by indentation requires a correction factor which can be obtained by calibration using NBT. A significant increase in K IC occurred after a 37CaO-37P2O5-20B2O3-6Al2O3 (mol%) glass was converted to a glass-ceramic. The much higher K IC for the glass-ceramic measured by NBT (1.32 MN m−3/2) compared with that from indentation (0.89 MN m−3/2) is attributed to internal stresses due to thermal expansion differences between the crystalline and residual glass phases leading to additional microcrack toughening.

Simple Notch Preparation Technique for Fracture Toughness Measurements in Partially Stabilized Zirconia

Fracture toughness is an important property of transformation‐toughening materials. Since indentation techniques are not reliable, notched‐beam techniques using slicing wheels on grinding machines are often used. Notching can induce residual stress as well as cracks, the effect of which cannot be completely nullified by annealing. Hence, the fracture toughness estimation methods may not yield realistic values for transformation‐toughening materials. The present paper reports on a reliable technique for notch preparation which can completely exclude the detrimental effects of stress/cracking that may arise due to notching, using slicing wheels on grinding machines.

A simple preparation method to determine fracture toughness using the single edge notched beam technique for porcelain ceramics

A simple preparation method to determine fracture toughness using the single edge notched beam technique for porcelain ceramics

Grain Size Dependence of Fracture Toughness in Polycrystalline Alumina

To investigate effects of grain size and temperature on the fracture toughness (KIC) of polycrystalline alumina ceramics, the KIC of high purity polycrystalline alumina having various grain sizes was measured using a single edge notched beam technique from room temperature to 1500°C. The material with grain size of 2μm revealed the largest KIC values over the temperature range studied. The KIC values of polycrystalline alumina decreased with increasing grain size. The decreasing rate of KIC values with increasing grain size changed around 22μm at room temperature and 500°C and around 16μm from 1000° to 1500°C. The KIC values of polycrystalline alumina having grain sizes over the range studied decreased with increasing temperature. For the grain size up to 42μm, the decreasing rate of KIC values at temperature up to 1000°C was smaller than that above 1000°C. On the other hand, the KIC values of the polycrystalline alumina having grain size of 70μm decreased slightly with increasing temperature and tended to increase at 1500°C. This trend might be attributed to be related to the interaction between fracture surfaces.

Evaluation of fracture toughness by a single-edge V-notched beam method.

We have developed a single-edge notched beam technique using a very sharp V-notch for evaluating fracture toughness of structural ceramics. The technique has several advantages such as simple testing procedures, applicability to a wide variety of materials and suitability to high-temperature testing. Results obtained by the technique agree quite well with the values of SEPB (Single Edge Precracked Beam) technique for several ceramics by using the modification proposed by Takahashi et al. except for alumina.

Effect of inclusions on size of surface flaws in glass-crystal composites

Indentation fracture studies were conducted on three sodium borosilicate glasses containing a dispersed phase of alumina inclusions with different degrees of thermal expansion mismatch between the glass matrices and the alumina. The alumina inclusions were found to cause a significant decrease in the size of the indentation cracks compared to those in the glass. This effect was greatest at the higher values of indentation load, which resulted in cracks of dimensions of sufficient size that their propagation was impeded by the tougher alumina dispersions. The fracture toughness for the composite samples calculated from the indentation data showed a significant increase with increasing crack size. For the smallest cracks in these composites, the value for fracture toughness was well below the value obtained in an earlier study by the single-edge notch-beam technique. The fracture toughness for the larger crack sizes which interacted with the alumina dispersions showed excellent agreement with the notch-beam data. The residual stresses due to the thermal expansion mismatch appeared to lead to a slight increase in the mean crack size regardless of the direction of thermal expansion mismatch.

Strength, fracture toughness and Vickers hardness of CeO2-stabilized tetragonal ZrO2 polycrystals (Ce-TZP)

Dense Ce-TZP ceramics containing about 7 to 16 mol% CeO2 were fabricated using fine powders prepared by the hydrolysis technique. The mechanical properties of these ceramics were evaluated. The bending strength of sintered bodies with 10 to 12 mol% CeO2 content and small grain-size was about 800 MPa. Fracture toughness was measured by two different methods; a micro-indentation technique and the chevron notched beam technique. A high fracture toughness was obtained for sintered bodies with 7 to 10% CeO2 content and large grain-size. Fracture toughness and hardness were dependent on CeO2 content and grain-size. These mechanical properties are discussed on the basis of the stability of the metastable tetragonal phase depending on CeO2 content and grain-size.

Fracture toughness measurements of polymer concretes

SYNOPSIS This paper describes the use of linear elastic fracture mechanics to examine the behaviour of premix and impregnated polymer concretes. Results are given for three monomers, viz. styrene, methyl methacrylate and butyl methacrylate, and two strengths of concrete, as well as for appropriate control series. The notched-beam technique is used to determine fracture parameters, together with compliance measurements to determine slow crack growth. Experimental results indicate that thermal treatment to polymerize the monomer in the premix concrete has a marked influence upon fracture toughness. Irradiation on the other hand has little influence. Polymer impregnation increases the stress-intensity factor and the strain at failure, the latter increase being influenced by the extensibility of the polymer. The results also show that the plain and the premix polymer concretes are partially notch-sensitive, whereas the impregnated polymer concrete is a notch-sensitive material.

Measuring the fracture toughness of cement paste and mortar

Synopsis This paper describes two methods that have been used to measure the effective fracture toughness of cement pastes and mortars. The first is a notched-beam technique, combined with compliance measurements to measure the slow crack growth prior to instability. The change of toughness is measured for separate increments of crack growth as the crack propagates. The second method, using a double-cantilever beam, avoids the slow crack growth problem by making a specimen of variable web width such that the length of crack front increases with and exactly compensates for the effect of crack growth. Tests of both pastes and mortars show that the fracture toughness of cement paste is independent of crack growth but that the toughness of mortar increases as the crack propagates. For both materials, the stress intensity required to initiate crack growth was less than that to maintain crack growth at the loading rates used.