Single Edge Notched Beam Specimens Research Articles

Evaluation of fracture toughness of human dentin using elastic–plastic fracture mechanics

Dentin, the mineralized tissue forming the bulk of the tooth, lies between the enamel and the pulp chamber. It is a rich source of inspiration for designing novel synthetic materials due to its unique microstructure. Most of the previous studies investigating the fracture toughness of dentin have used linear-elastic fracture mechanics (LEFM) that ignores plastic deformation and could underestimate the toughness of dentin. With the presence of collagen (approximately 30% by volume) aiding the toughening mechanisms in dentin, we hypothesize that there is a significant difference between the fracture toughness estimated using LEFM ( K c) and elastic–plastic fracture mechanics (EPFM) ( K J c). Single-edge notched beam specimens with in-plane ( n=10) and anti-plane ( n=10) parallel fractures were prepared following ASTM standard E1820 and tested in three-point flexure. K J c of the in-plane parallel and anti-plane parallel specimens were found to be 3.1 and 3.4 MPa m 1/2 and K c were 2.4 and 2.5 MPa m 1/2, respectively. The fracture toughness estimated based on K J c is significantly greater than that estimated based on K c (32.5% on average; p<0.001). In addition, K Jc of anti-plane parallel specimens is significantly greater than that of in-plane parallel specimens. We suggest that, in order to critically evaluate the fracture toughness of human dentin, EPFM should be employed.

Three-point bending fracture characteristics of three-dimensional-C/SiC with single-edge notch beam specimens

Abstract Loading–unloading properties of three-dimensional-C/SiC composites at ambient temperature were studied by three-point bending experiments with single-edge notch beam specimens. Crack growth was examined by scanning electron microscopy. The whole propagation process of a notch consists of a transient state and a steady state. The crack growth resistance is derived based on energy evaluation by a graphical method taking into account the non-linear fracture of the composites. The maximum value of the crack growth resistance, corresponding to the transition of the notch into cracks, reaches about 303.7 kJ m−2 when the ratio of equivalent crack length to sample thickness amounts to 0.13. The energy for the steady crack growth is about 20 kJ m−2.

Evaluation of the Role of Pores during Strength Testing in Compacts Made from Different Particle Size Fractions of Sucrose

The purpose of this study was to investigate the role of pores in the fracture of circular compacts and to predict compact properties and critical crack lengths. Four different particle size fractions of sucrose, ranging from 20 to 500 microm, were compressed into circular discs (i.e. flat tablets) and rectangular beam specimens of porosity between 30 and 14%. Modelling of the relationship between the tensile strength of the circular discs and the compact porosity indicated extensive fragmentation during compaction for particles in the size range of 250-500 microm, accompanied by a change in densification mechanism for very coarse particles (355-500 microm). When determining the critical stress intensity factor from rectangular single edge notched beam specimens by 3-point bending, an apparent influence of particle size on the values could be seen, whereby here the results indicated that the critical particle size for fragmentation to occur is about 20-40 microm. It was possible to predict the critical stress intensity factor of the compacts from the median pore size and the tensile strength of the circular disc specimens by interpolation of the critical crack length for propagation to occur. The results indicated that for sucrose compacts regardless of their porosity, the pores themselves acted as stress concentrators, not as sharp cracks. For sucrose compacts, crack propagation hence proceeds most likely along grain boundaries.

An Evaluation of Fracture Toughness of Glass-Filled Ceramic Using Notched Specimen

The objective of this study is to evaluate the mechanical properties of static, quasidynamic and dynamic fracture toughness of glass-filled ceramic as promising structural material for a dome port cover of a ramjet engine system. Static and quasi-dynamic tests were carried out using SEPB (Single Edge Pre-cracked Beam) specimens. Static and dynamic fracture toughness tests were also performed using ASTM and strain gage methods with SENB (Single Edge Notched Beam) specimens machined with various notch radii. The critical notch radius was evaluated. Below the critical notch radius, the static fracture toughness of the SENB specimen well agreed with that of the SEPB specimen.

Fracture Toughness of Wood Fiber Gypsum Panels from Size Effect Law

This paper addresses the size effect of inplane bending strength as well as Mode I fracture toughness and process zone length of wood fiber-reinforced gypsum panels. Wood fiber gypsum panels represent an incombustible short fiber composite material composed of recycled paper fibers embedded in a gypsum matrix. The material, which is used for sheathing and bracing of timber frame constructions, exhibits marked fracture softening supposedly resulting in a considerable size effect. In the paper presented, in a first step Bažant’s size effect law for quasi-brittle materials is derived. The parameters of this size effect law are then determined by means of nonlinear regression analysis applied to a test series with scaled single edge notched beam specimens. Detailed consideration is given to the adequacy of linear confidence intervals of the model parameters in comparison to nonlinear inferential results. Finally, the probability densities of fracture toughness and fracture process zone length are determined f...

Effect of Annealing on the Microstructures and Mechanical Properties of Mo&lt;sub&gt;3&lt;/sub&gt;Si-Mo&lt;sub&gt;5&lt;/sub&gt;Si&lt;sub&gt;3&lt;/sub&gt; Eutectic

The microstructures and mechanical properties of arc-melting processed Mo3Si-Mo5Si3 eutectic have been investigated. The Vickers hardness of Mo3Si-Mo5Si3 eutectic alloy at room temperature is on the order of 1350Hv. The fracture toughness value of the alloy at room temperature is 1.39MPam1/2 measured by Single edge-notched beam specimen technique and 1.61MPam1/2 measured by Indentation technique. The compressive strengths at 1300 oC and 1400 oC under a strain rate of 10-4s-1 are about 550MPa and 300MPa respectively.

Surface Oxide Effects on Static Fatigue of Polysilicon MEMS

ABSTRACTStatic fatigue – crack growth causing delayed failure under constant stress and involving stress corrosion cracking – was investigated in polysilicon MEMS using two different surface-micromachined devices. One exploited residual tensile stresses to create stress concentrations at micromachined notches, and the other involved a single-edge notched beam specimen integrated with an electrostatic comb-drive microactuator. Tests with both devices revealed that polysilicon is not susceptible to static fatigue in humid environments. However, when a relatively thick (45 to 140 nm) surface oxide was thermally grown on the microactuator devices, the specimens demonstrated delayed fracture in a humid ambient, presumably due to static fatigue of the surface SiO2. The stress intensities at the resulting cracks in the SiO2were then sufficient to cause catastrophic crack propagation through the polysilicon specimens. The implications of our data on the issue of fatigue in polysilicon is discussed.

Adsorption and Kinetic Effects on Crack Growth in MnZn Ferrites

The variation of the fracture toughness of MnZn ferrite ceramics with varying loading rate and humidity was determined with the aid of the single edge notched beam (SENB) test. A strong decrease with increasing humidity and decreasing loading rate was observed. A model for subcritical crack growth incorporating kinetic and adsorption effects was formulated to analyze the data. The value of the adsorptioncontrolled fracture toughness was determined independently by double torsion experiments and agreed favorably with the values as determined from the SENB data using the model. The strength of the material was determined, and analysis showed a strength behavior similar to the fracture toughness behavior, as predicted by the model. The analysis presented can be used to assess the subcritical crack growth behavior using a limited number of SENB specimens.

Evaluation of crack-Growth resistance curve for a particulate ceramic–Metal composite

A technique is described to evaluate the crack growth resistance behaviour in brittle ceramic-base materials. In this method, the crack increment measurements during the stable crack propagation process are not required. The crack growth resistance curves are studied for a particulate ceramic–metal composite in the system lanthanium chromite–chromium. Experiments were performed with standard fracture mechanics single-edge notched beam specimens in a temperature range from room temperature up to 1100°C. Effect of temperature on crack growth resistance behaviour is discussed.

Fracture mechanism of cross-ply carbon/carbon composites

Mode I fracture toughness tests were carried out on (0/90) cross-ply carbon/carbon composite laminates in order to investigate the applicability of the concept of the fracture toughness based on linear elastic fracture mechanics. Tests were carried out by using both compact tension (CT) and single edge notched beam (SENB) specimens. Effect of the root radius of the starter notch was also investigated in order to find out its sensitivity on the fracture behavior. Although the load-displacement relation showed nonlinear behavior, the relation between the specimen compliance calculated by the straight line from the origin of the load-displacement axis and the measured crack length followed the polynomial equations based on the linear elastic fracture mechanics. Fracture toughness values were calculated by using the stress intensity factor. The relation between the fracture toughness and the increment of crack length (R-curve) was independent of the specimen shapes, the starter notch lengths, and the root radius of the starter notch. The fracture toughness values first increased rapidly until the increment of the crack length equaled 1 mm, and then leveled off. For the case of SENB specimens, tests without starter cracks were also carried out. These tests indicated that both the crack propagation behavior and the resultant R-curve were similar to those of normal SENB specimens. Comparison between the fracture toughness value and the bending strength yielded that the length of the latent crack is about 1 mm. X-ray observation showed that plenty of microcracks had already existed before the fracture load was applied. Microscopic observation showed that the crack growth process was characterized as pull-out of fibers. The length of fiber pull-out with the increment of crack growth was discussed in conjunction with the R-curve behavior.

Determination of Fracture Toughness in 2‐D Woven SiC Matrix Composites Made by Chemical Vapor Infiltration

An approach to the R‐curve behavior of 2‐D woven ceramic matrix composites is proposed. This approach takes into account the presence of a process zone at the crack tip. The strain energy release rate is determined using an equivalent system of two specimens free of process zones. The process zone size is determined independently by assimilating a notch tip region to a beam of bimaterial. Compact tension specimens having different dimensions and single‐edge notched beam specimens were used for the analysis. Important trends in toughening of ceramic matrix composites were anticipated.

R-curve measurement of silicon nitride ceramics using single-edge notched beam specimens

R-curve behaviour of three kinds of silicon nitride-based ceramics has been studied using the single-edge notched beam (SENB) technique. If the notch is deep enough, the specimen shows stable fracture during the bending test, even when the sample is a brittle material. The conditions required to obtain stable fracture in the bending test are clarified by the analysis. The crack length of the specimen was also calculated from the changing load during the fracture test. In this study, coarse-grained silicon nitride shows a large increase of theR-curve. On the other hand, silicon nitride with silicon carbide whiskers shows noR-curve increase. The rise of theR-curve should be related to the microstructure of the ceramics, and especially to the grain size of the specimen, because silicon carbide whiskers are not large compared to the silicon nitride grains, and silicon carbide can reduce the grain growth of silicon nitride during sintering.

Fracture toughness of single crystal alumina in air and a simulated body environment

The critical intensity factor KIC of a single crystal alumina (1100)-plane in air and in a simulated body environment was measured. Based on these measurements, the estimated durability of a single crystal alumina disk in a ceramic heart valve was determined. Standard single-edge notched beam specimens were prepared. The specimens were classified into three groups: a) stored in air (n = 10), b) immersed in Ringer's solution for 2 months (n = 11), and c) immersed in Ringer's solution for 4 months (n = 6). KIC was 2.345 +/- 0.072 MPam1/2 for specimens stored in air, 2.141 +/- 0.147 in Ringer's solution for 2 months, and 2.005 +/- 0.153 in Ringer's solution for 4 months. The KIC decreased for specimens stored in Ringer's solution (P < .01), but it did not differ significantly between 2 and 4 months. The decrease of KIC is the stress corrosion effect of Ringer's solution at the crack tip, and is not associated with an overall deterioration of strength. The stress intensity factor for possible cracks (length: 0.4 mm) in the single crystal alumina was calculated. Maximum stress was assumed 2.668 MPa (= 40 x 500 mm Hg). The stress intensity factor calculated was far less than the KIC. The single crystal alumina disk can endure a static pressure of 500 mm Hg.

The fracture behaviour of SiC reinforced SiAlYON ceramic matrix composites

This paper presents the results of a study carried out at the AEROSPATIALE Joint Research Center on SiAlYON Matrix Composites reinforced with 0, 10, 20 and 30% of SiC whiskers. Firstly the test methodology for measuring the K1c is described. The SENB (Single Edge Notched Beam) specimen pre-cracked in compression cycling is compared to the Chevron Notched bend bar test. In both cases the fracture toughness is measured in a bend test more adapted for tests at elevated temperatures. Then the crack propagation at elevated temperature of the composite for the four SiC w volume fractions is described and discussed in relahonship with the rheological behaviour of the matrix and the volume fraction of reinforcement

An Attempt for Measuring the Crack Extension Resistance of Ceramics by Fracture Test in Bending

R-curve properties for two kinds of SiC whisker reinforced Si3N4 ceramic composite were obtained by three-point bending test. Single edge notched beam specimens with sharp tip radius were fractured under a slow cross-head speed of 0.002mm/min in air. Crack extension Δa was calculated from the change of the specimen compliance. Crack extension resistance KR was also calculated from the crack extension Δa and the fracture load during stable fracture. The extent of KR increase in the R-curve was more remarkable in the composite containing whiskers with larger diameter. Some theoretical and technical problems for measuring the crack extension resistance of toughened ceramics were discussed in this letter.

Fracture toughness measurement with fatigue-precracked single edge-notched beam specimens of WC-Co hard metal

The plain-strain fracture toughness of WC-8%Co hard metal, KIC, was measured using single edge-notched beam (SENB) specimens with fatigue precrack. The fatigue precrack was introduced with compressive fatigue cycling in four-point bending at room temperature. Since stable fatigue-crack propagation was obtained from the notch tip, it was easy to control the fatigue-precrack length. A reasonable KIC value of 13.3 MPa m1/2 was obtained with the fatigue-precracked SENB specimens in four-point bending. The compressive fatigue-precracking technique in four-point bending was simple and convenient, and is therefore applicable to precracking in a variety of brittle materials prior to fracture-toughness measurements.

A fracture mechanics approach to ceramo-metal bond evaluation

The incidence of mechanical failure of cast metal/porcelain dental restorations has promoted a study to develop reliable methods of characterizing and improving bond strengths when produced under standard dental laboratory conditions. Single-edge notch beam specimens were prepared by firing porcelain from a single blended frit to either side of a central metal coupon and subjecting these to four-point bending at 20 ± 2° C and 0.5 mm min−1 crosshead speed. Specimens of three representative dental alloys and an experimental alloy were prepared in two specimen sizes and three notch widths. The fracture toughness (KIc) values were independent of notch width and specimen size over the range of these variables examined and good discrimination was obtained. The fracture toughness of all bonds was improved by a commercial hot isostatic pressure cycle. SEM examination of the fractured surfaces suggested that this improvement was due to the reduction in microporosity at the interface.

Fracture toughness of silicon nitride determined by cyclic-fatigue-cracked specimens

An experimental technique for determining fracture toughness has been developed. In this method, a fatigue precrack is introduced in single-edge notched beam specimens by cyclic fatiguing in four-point bend at an elevated temperature. The resulting fatigue precracks satisfy all conditions required by the ASTM Standard Test for plane-strain fracture toughness of metallic materials. The applicability of this technique to provide reproducible fracture toughness values is demonstrated by experimental results obtained for silicon nitride sintered in two different ways in comparison with those obtained by means of the indentation technique for the same materials.

Fracture toughness of pressureless sintered silicon carbide: A comparison of Klc measurement methods

Four different K lc measurement methods, all of them using small bars, were compared. The materials studied in this paper were pressureless sintered α-and ß-SiC. K lc values obtained by the direct crack measurement method depended on the indentation load. The single edge notched beam method yielded a higher value than others. Indentation strength bending and chevron notched beam methods showed almost the same K lc values: both methods measured K lc with a sharp crack as controlled defect. Effect of notch width was observed on single edge notched beam specimens as the strain rate and span length dependence on chevron notched beam specimens; chevron notched beam K lc values were also strain rates and span length dependent. Influence of the temperature was also studied. The fracture toughness of sintered SiC increases at high temperature (1350°C, in air).

構造用セラミックスの強度および破壊じん性評価モデルについて

In order to elucidate the notch width or notch root radius dependence of the critical stress intensity factor evaluated by SENB (Single Edge Notched Beam) specimens, 4-point bending tests were carried out on hot-pressed SiC (with 2wt% AlN) and sintered Si3N4. The Experimental results showed that the critical stress intensity factor Kc determined with SENB specimens coincides with a valid fracture toughness KIc value only when notch root radius ρ is smaller than about 10μm. For larger root radii, the variation of Kc with ρ was observed, and this can not be attributed only to the stress concentration effect of notch.In the present study, by introducing a micro-damage concept a method was proposed for evaluating KIc from Kc values given by SENB specimens having rather blunt notch.On the basis of the published strength data for SiC, the relation between strength and microstructure was examined. It was assumed that a micro-damage existed at the machine finished surface, and its size should be the sum of the machined damage depth hmax and the maximum grain diameter dmax. Here, hmax can be estimated from the average grit size of the diamond disk used. An analysis was made of the dependence of Kc on ρ by considering the micro-damage at the notch root, and a theoretical expression was formulated. The experimentally observed Kc-ρ relation agreed quite well with the theoretical prediction.