Debonding Properties of Residually Stressed Brittle-Matrix Composites

Trends in interface debonding have been calculated during fiber pullout for composites with interfaces subject to residual tension. The debond behavior is shown to depend sensitively on the thermal expansion mismatch. The results are used as the basis for designing a pullout test specimen suitable for measuring the mixed-mode fracture energy of bimaterial interfaces. The solutions also provide the background needed to assess the role of debonding in the toughening of ceramics by fibers.     

Quantitative Analysis of Brittle Fracture Surfaces Using Fractal Geometry

Fractal geometry is a non-Euclidean geometry which has been developed to analyze irregular or fractional shapes. In this paper, fracture in ceramic materials is analyzed as a fractal process. This means that fracture is viewed as a self-similar process. We have examined the fracture surfaces of six different alumina materials and five glass-ceramics, with different microstructures, to test for fractal behavior. Slit island analysis and Fourier transform methods were used to determine the fractal dimension, D , of successively sectioned fracture surfaces. We found a correlation between increasing the fractional part of the fractal dimension and increasing toughness. In other words, as the toughness increasing the fracture surface increases in roughness. However, more than just a measure of roughness, the applicability of fractal geometry to fracture implies a mechanism for generation of the fracture surface. The results presented here imply that brittle fracture is a fractal process; this means that we should be able to determine processes on the atomic scale by observing the macroscopic scale by finding the generator shape and the scheme for generation inherent in the fractal process.     

Effect of Electrode Configuration on Phase Retardation of PLZT Films Grown on Glass Substrate

Lead lanthanum zirconate titanate (PLZT) thick films were deposited on a glass substrate using a methoxyethanol-based sol–gel multicoating method. Two types of electrodes, a coplanar surface electrode and an embedded electrode, were deposited on the films to measure the phase retardation of the PLZT films by the Senarmont method. The quadratic electrooptic properties were measured as a function of the film thickness, for thicknesses ranging from 1 to 4 μm. The PLZT film with the embedded electrode structure showed a higher phase retardation value and enhanced electric breakdown resistance.     

A Scale Effect in the Durability of Oriented Thin-Wide Polypropylene Films during Oxidation under Load: Fracture Model of Stressed Polypropylene Films

Abstract

Studies have been made of the durability of oriented isotactic polypropylene (PP) films depending on their width during intensive oxidation (T = 130°C, P₀₂ = 20 kPa) under load. As the specimen width decreases under low stresses, mean durability is determined thermooxidative destruction rate at the nucleation sites the scale effect takes place. The durability values are found to be discrete multiples of integers for narrow specimens. The conclusion is drawn that the difference in polymeric specimen durability is determined by the concentration of defect zones, rather than by the versatility of their nature. The model of oriented PP films fracture under low-load oxidation conditions is proposed. This model is based on the dependence of durability on a number of oxidation destruction centers which are similar in nature (in the initiation rate) and cause specimen fracture while growing and joining together.     

Mixed-Mode Fracture Toughness of Ceramic Materials

An experimental technique whereby pure mode I, mode II, and combined mode I-mode II fracture toughness values of ceramic materials can be determined using four-point bend specimens containing sharp, through-thickness precracks is discussed. In this method, notched and fatigue-precracked specimens of brittle solids are subjected to combined mode I-mode II and pure mode II fracture under asymmetric four-point bend loading and to pure mode I under symmetric bend loading. A detailed finite element analysis of the test specimen is performed to obtain stress intensity factor calibrations for a wide range of loading states. The effectiveness of this method to provide reproducible combined mode I-mode II fracture toughness values is demonstrated with experimental results obtained for a polycrystalline Al₂O₃. Multiaxial fracture mechanics of the Al₂O₃ ceramic in combined modes I, II, and III are also described in conjunction with the recent experimental study of Suresh and Tschegg (1987). While the mode II fracture toughness of the alumina ceramic is comparable to the mode I fracture toughness K _Ic, the mode III fracture initiation toughness is 2.3 times higher than K _Ic. The predictions of fracture toughness and crack path based on various mixed-mode fracture theories are critically examined in the context of experimental observations, and possible effects of fracture abrasion on the apparent mixed-mode fracture resistance are highlighted. The significance and implications of the experimental methods used in this study are evaluated in the light of available techniques for multiaxial fracture testing of brittle solids.     

Properties of Modified Anodic-Spark-Deposited Alumina Porous Ceramic Films as Humidity Sensors

α-Alumina films formed by anodic spark deposition in the melt of NaHSO₄-KHSO₄ mixture (200°C) exhibited a porous structure. It has been found that the porosity and amount of α-phase varied with anodic current density. By electrical measurements, films were found to exhibit a continuous open-pore structure, causing failure of humidity sensors. This problem can be overcome by reanodization of the anodicspark-deposited alumina films in either borax aqueous solution or sulfuric acid solution for a short time to form an effective barrier layer at the pore base. The modified anodic-spark-deposited alumina films showed highly sensitive response to water vapor change in the range of −76° to +20°C dew point with much improved long-term stability. The humidity characteristics of the α-alumina films were qualitatively explained in terms of the BET theory and condensation theory.     

Experimental Observations of High Fracture Resistance in Silicon Carbide/Alumina Laminated Composites

Model laminated composites were fabricated with porous-Al₂O₃ interfaces between SiC bars. The porous Al₂O₃ was deposited using an aerosol spray deposition technique, and the sandwich specimen was fabricated by hot pressing. Residual thermal stresses were present in the interface because of the difference in the coefficients of thermal expansion of SiC and Al₂O₃. Crack deflection was observed with measured interfacial fracture resistances that were considerably higher than the deflection threshold predicted by the He–Hutchinson criterion. Examination of the fracture surface revealed a tortuous crack path and significant crack–flaw interaction.     

Films from oat spelt arabinoxylan plasticized with glycerol and sorbitol

The development of packaging films based on renewable materials is an important and active area of research today. This is the first extensive study focusing on film‐forming properties of an agrobiomass byproduct, namely, oat spelt arabinoxylan. A plasticizer was needed for cohesive film formation, and glycerol and sorbitol were compared. The tensile properties of the films varied with the type and amount of the polyol. With a 10% (w/w) plasticizer content, the films containing glycerol had higher tensile strength than the films containing sorbitol, but with a 40% plasticizer content, the result was the opposite. Sorbitol‐plasticized films retained their tensile properties better than films with glycerol during 5 months of storage. The films were semicrystalline with similar crystallinity indices of 0.20–0.26. The largest crystallites (9.5 nm) were observed in the film with 40% glycerol. The softening of films with 40% (w/w) glycerol started at a significantly lower relative humidity (RH) than that of the corresponding sorbitol‐containing films. The films with sorbitol also had lower water vapor permeability (WVP) than the films with glycerol. The films plasticized with 10% (w/w) sorbitol had a WVP value of 1.1 g mm/(m²·d·kPa) at the RH gradient of 0/54%. The oxygen permeability of films containing 10% (w/w) glycerol or sorbitol was similar: 3 cm³·μm/(m²·d·kPa) at 50–75% RH. A higher plasticizer content resulted in more permeable films. Permeation of sunflower oil through the films was not detected. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Dynamic Fracture Responses of Alumina and Two Ceramic Composites

A hybrid experimental-numerical procedure was used to characterize the dynamic fracture response of alumina (Al₂O₃) and TiB₂-particulate/SiC-matrix and SiC-whisker/Al₂O₃-matrix composites. Unlike metals and polymers, dynamic arrest stress intensity factors (SIFs) did not exist in the monolithic ceramics and the two ceramic composites considered. Thus a running crack in these materials cannot be arrested by lowering the driving force, i.e., the dynamic SIF. Fractography study of the alumina specimens showed that the area of transgranular failure varied from about 3% to about 16% for rapid crack extensions in statically and impact loaded specimens, respectively. The influence of kinematic constraints which enforces transgranular flat crack extension, despite the higher fracture energy of transgranular fracture, is discussed.     

Dimensionless Load—Displacement Relation and Its Application to Crack Propagation Problems

The dimensionless load (     ) and load-point displacement (û) are introduced to discuss crack propagation problems. The     −ũ relation for the case of equilibrium crack propagation in any linear elastic material is demonstrated to lie on a single universal fracture curve independent of the material. The concept of the universal     −ũ relation is extended to yield a direct and simple form of the dimensionless total energy (     _eq (α)) which is applicable to many types of instability problems for crack extension, when the shape factor ( Y ) of the specimen is known as a function of crack length ( a ). In addition, by applying the deviation of the experimentally observed     —ũ curve and the theoretical universal fracture curve, the evaluation of the nonlinear fracture resistance parameter of a polycrystalline graphite material, as an example, was demonstrated.     

Fracture Analysis of Chemically Strengthened Glass Disks

The increase in fracture strength in chemically strengthened, soda-lime glass disks compared with nonstrengthened ones is investigated analytically and experimentally. An empirical stress-relaxation function is proposed to predict the stress profile through the disk as a result of ion exchange, with parameters determined from experimental data. The function accounts for stress increase due to concentration rise and decrease caused by stress relaxation, which depends on concentration, treatment time, and exchange temperature. The surface-flaw closure distance caused by the compressive stress profile is computed using an existing model. The size of the most severe surface flaw for chemically strengthened disks is computed using fracture mechanics, assuming knowledge of the experimental failure strength. The predicted flaw size is slightly smaller than that calculated for a similar sample of nonstrengthened disks and the strengthening is underpredicted by a factor of 2. Reasons for the discrepancies are discussed.     

Fracture Resistance Behavior of Silicon Carbide Whisker-Reinforced Alumina Composites with Different Porosities

Fracture resistance behavior was characterized for SiC-whisker-reinforced alumina composites with porosities ranging from 0.6% to 11.5% The composites were hot-pressed from an Al₂O₃ powder with 25 wt% SiC whiskers. Strengths of individual specimens were measured in four-point flexure either for natural flaws or for Vickers-indentation flaws as a function of radial crack size. Indentation crack sizes were controlled with indentation loads which varied between 2 and 200 N. A novel method of analysis of these measurements indicates that the fracture resistance of these composites increases as a function of crack extension, a rising R curve. This behavior is interpreted in terms of tractions from both crack-bridging whiskers and interlocking grains, which develop in the wake of the crack tip as it extends. A decrease in porosity raises the level of fracture resistance, but has a negligible effect on the relative steepness of the R curve. The sizes of natural flaws which causes failure in flexure testing were also estimated from analysis of the data.     

Fracture mechanics of sharp scratch strength of polycrystalline alumina

The strength of a polycrystalline alumina containing controlled scratches introduced by translated sharp contacts is investigated and described by a multiscale fracture mechanics model. Inert strength measurements of samples containing quasi‐static and translated Vickers indentation contacts showed that scratches degraded the strength at normal contact loads an order of magnitude less than those for quasi‐static indentation. The fracture mechanics model developed to describe strength degradation by scratches over the full range of contact loads included toughening effects by crack‐wake bridging at the microscale and lateral crack‐based residual stress relaxation effects at the mesoscale. A critical element of the model is the nonlinear scaling of the residual stress field of a scratch with the normal contact load acting during scratch formation. The similarities and differences in the scratch model in comparison with prior indentation‐strength fracture mechanics models are highlighted by parallel development of both. Central to the scratch model is the use of easily controlled normal contact load as the scratch‐strength measurement variable. Scratch length and orientation are shown to have significant effects on strength. The distributions of scratch widths controlling the intrinsic strengths of as‐received samples are determined and agreement with the observed scratch dimensions is demonstrated.     

Mechanical Properties of Textured Alumina Made by High-Temperature Deformation

The mechanical properties of a textured alumina made by high-temperature deformation of normal-purity sintered alumina have been investigated. The textured alumina shows very high bending strength and extremely high fracture toughness. Fracture toughness of more than 10 MPa·m^1/2 was measured by the single-edge precracked beam method, and even using the single-edge V-notched beam method, toughness of over 8 MPa·m^1/2 was obtained. This high fracture toughness was attributed to a large number of aligned small platelike grains of the textured structure enhancing the grain bridging effect.     

Hydroxylation and Dehydroxylation Behavior of Silica Glass Fracture Surfaces

The hydroxylation and dehydroxylation behavior of amorphous silica fracture surfaces was studied using temperature-programmed static SIMS. The results show that vacuum heat treatments result in more extensive condensation of silanol groups on the silica glass fracture surface as compared to fumed silica (Cabosil). This is attributed to differences in the distribution of silanol groups on the two silica surfaces. The rehydration kinetics of the dehydroxylated silica fracture surfaces showed two distinct reaction rates—an initial rapid increase in the silanol concentration, followed by a slower rehydration for longer dosing times. The slower rehydration reaction was shown to follow first-order reaction kinetics with the reaction rate constant, suggesting hydrolysis of strained siloxane bonds on three-membered silicate ring structures. The much faster initial rehydration is attributed to the hydrolysis of extremely strained siloxane bonds in two-membered, edge-shared tetrahedral rings. The effect of the dehydration time and temperature (i.e., thermal history of the surface) on the rehydration kinetics is also discussed.     

Mechanical and Optical Properties of Polyamide 6/Clay Nanocomposite Cast Films: Influence of the Degree of Exfoliation

PA6 nanocomposite films with different nanoclay dispersion degrees are prepared by melt compounding and cast extrusion. The dispersion of the MMT platelets (homogeneity and degree of exfoliation) is evaluated qualitatively by TEM and quantitatively by rheology and NMR; it ranges from microcomposites to highly exfoliated nanocomposites. Compared to neat PA6, the optical properties (clarity, gloss, haze) are worse for the microcomposites and better for the nanocomposites. The mechanical properties depend strongly on the exfoliation level. Better exfoliation leads to higher stiffness. The strain at break decreases compared to neat PA6 films even in the case of highly exfoliated nanocomposites films. At low MMT content, the microcomposite has a higher ductility than well exfoliated nanocomposites films.

     

聚乙烯/矿物粉复合降解薄膜的机械性能及光降解性能

用2%钛酸酯偶联剂对滑石粉、膨润土粉粒进行表面改性,通过双螺杆挤出机将改性滑石粉和膨润土分别与聚乙烯树脂(PE)得填充母料,其中无机填料占80%;以不同比例的填充母料与PE混和,采用吹塑法制备了单一填充母料(含滑石粉或膨润土)和混和填充母料(既有滑石粉又有膨润土)的PE/矿物粉复合薄膜。研究了填充母料对PE复合薄膜的机械性能和光降解性的影响,实验结果表明:两种母料都具有较好的成膜性,对薄膜产生一定的增强减韧作用;另外,在填充母料用量一样的情况下,滑石粉母料填充薄膜的力学性能高于膨润土的,而目单一母料填充薄膜的力学性能优于混和母料填充薄膜的力学性能。在自然光照和紫外光照实验中,PE/矿物粉复合薄膜的机械性能下降和红外光谱的变化显示了滑石粉和膨润土能够极大的促进PE薄膜的降解。     

Silicon Carbide Whisker/Alumina Matrix Composites: Effect of Whisker Surface Treatment on Fracture Toughness

The fracture toughness of a 30 vol% SiC whisker/Al₂O₃ matrix composite was evaluated as a function of whisker surface chemistry. Two types of SiC whiskers (Silar-SC-9 and Tateho-SCW-1-S) were investigated. Modification of the whisker surface chemistry was achieved by subjecting the whiskers to thermal treatments under controlled atmospheres. Whisker surface chemistry, as determined by X-ray photoelectron spectroscopy, was correlated to the fracture toughness of the composites.     

Fracture Toughness of Ceramic Precracked Bend Bars

Fracture toughness was measured for four ceramic materials using precracked bend bar specimens. The effect of the precracking parameters, used for the bridge indentation method on fracture toughness values, was determined. Excellent agreement was obtained between fracture toughness values measured by this method and values obtained by other techniques.     

Fracture Analyses of Alumina Subjected to Mechanical and Thermal Shock Biaxial Stresses

Disks of commercial alumina were fabricated by slip casting and sintering. Two surface finishes were performed: coarse (denoted as "C") using a 70 grit diamond wheel and fine (denoted as "F") with 120 and 320 grit SiC papers. The machined surfaces were analyzed by SEM, profilometry, and residual stresses measurements. The fracture strength was evaluated in biaxial flexure, and the thermal shock resistance was tested by cooling with a high-velocity air jet. The fracture of the specimens under both conditions was studied analyzing crack patterns and fracture surfaces in relation to the surface machining and type of loading, i.e., mechanical and thermal stresses.