Fracture Behavior of Layered Alumina Microstructural Composites with Highly Textured Layers

A new class of layered microstructural composites that combines equiaxed and textured alumina layers was fabricated. Template loading was used to change the texture fraction and porosity in the textured layers. Due to the thermal expansion anisotropy of the textured layers, residual compressive stresses as high as 100 MPa were achieved during cooling from the sintering step. Fracture experiments showed that the interface between the basal planes of highly oriented alumina grains in the textured layers changes from a “strong interface” to a “weaker interface” as the porosity changes from 1% to 5%. Composites with 5% porous textured layers show both crack bifurcation and crack deflection in the textured layers. Crack deflection is attributed to the anisotropic fracture energy of the oriented microstructures and crack bifurcation is ascribed to the compressive stresses that arise from the thermal expansion mismatch between adjacent layers.     

Some general considerations of a membrane/osmosis model for portland cement hydration

The proposed analogy between the process of membrane formation and osmosis in “silicate gardens” and Portland cement hydration is discussed. The major similarity is considered to be the formation of solids at a fluid interface at which the local supersaturation is high so that colloidal solids having no long-range order are produced. A membrane/osmosis model is then applied in explanation of various features of cement hydration, including the dormant period, the action of gypsum and morphological features such as “Hadley grains”.     

Morphology and composition of twin fracture surfaces of a crack in Portland cement paste

The microstructure in terms of morphological aspects and semi-quantitative elemental composition analysis of matching halves of a crack in a commercial Portland cement mature paste has been examined. Scanning electron microscope (SEM) and energy-dispersive X-ray analysis (EDXRA) results obtained for the “twin” regions of the two surfaces have given a more correct interpretation of the spatial texture both of the bulk massive paste and the typical localized regions. Hydrated phases “fingerprints” available from morphologies along with Ca/A1, Ca/Si, Ca/S and Ca/Fe count ratios are offered. A rather unusual feature of a relatively massive lamellar Ca(OH)₂ has been noted.     

A crack layer approach to fatigue crack propagation in high density polyethylene

Fatigue crack propagation (FCP) in high density polyethylene (HDPE) is observed to occur with an accompanying layer of damage ahead of the crack tip. The crack layer theory, which accounts for the presence of both the damage and the main crack, is applied to the problem. It is observed that the kinetic behavior of HDPE under fatigue consists of three regions: initial acceleration, constant crack speed (“deceleration”), and reacceleration to failure. Within the first two regions, crack propagation appears “brittle,” while in the third region “ductile” behavior is manifested. Ultimate failure occurs via massive yielding of the unbroken ligament. Two damage mechanisms are found to be responsible for HDPE failure: formation of fibrillated voids and yielding. Both mechanisms are present throughout the entire lifetime of the crack, but the former dominates the “brittle” crack propagation region, while the latter is more prominent in the “ductile.” Throughout the analysis the resistance moment R_t is approximated as the total volume of transformed material associated with crack advance. Crack layer analysis produces a satisfactory fit of the experimental data and yields a specific enthalpy of damage, γ^*, value in the 1–2 cal/g range.     

A novel method for fabricating brick-mortar structured alumina-zirconia ceramics with high toughness

The present work reports a novel and simple approach to prepare alumina-zirconia composites with superior toughness. Alumina microspheres were innovatively used as the raw materials, followed by coating zirconia and hot-pressing sintering to fabricate alumina-zirconia ceramics. The resultant ceramics are given a unique brick-mortar microstructure, in which the zirconia “mortar” layers continuously distribute around the alumina “brick” matrix, leading to outstanding fracture toughness of 7.34 MPa·m^1/2 and high strength of 635.84 MPa when prepared with zirconia contents of 10 wt%. The major explanation could be ascribed to that crack tips in sintered samples tend to propagate along the zirconia “mortar” layer, accompanied by deflection and branching, which effectively improve the fracture toughness of composites. The uniformity and integrity of the brick-mortar structure could be well tuned by varying the amount of zirconia. This method has reference significance for the preparation of high toughness alumina-based multiphase ceramics.     

Crack Healing by Thermal Treatment in Float Glass: The Effect of Tin

The evolution of radial cracks generated by Vickers indentation on “tin side” of float glass is analyzed when heat treated at 620°C under various atmospheres and results are compared with a previous similar study performed on the “air side” of the same glass. Results evidence that the presence and the oxidation state of tin ions has a strong influence on crack morphological changes. If the oxidation state of tin remains quasi unchanged during the heat treatment, the mechanism responsible of the crack evolution is the spheroidization and there is no strong difference between the crack morphological changes observed on the “air” and “tin side” in this case. For oxidative atmospheres, contrary to what is observed on the “air side”, no crack spheroidization occurs. The cracks stay quasi unchanged for dry air or close for humid air. Increase of the viscosity at the top surface due to Sn oxidation is proposed to explain this spheroidization impediment. It is also shown that in these two cases, there is an efficient blunting and healing of the cracks, as 4‐point bending tests indicate a significant sample strengthening with thermal treatment.     

Molecular weight distribution and environmental stress cracking of linear polyethylene

Molecular weight distributions of both Phillips and Zieglertype high density polyethylenes were determined following fractionation of the polymer samples using a column elution technique. The Wesslau log normal distribution function was used to describe the distributions of the resins investigated. Resistance to environmental stress rupture of speciments cut from compression molded plaques of these samples was measured by the constant tensile loading procedure. Data are presented showing annealed resins with “broad” molecular weight distributions, characterized particularly by a quantity of low molecular weigt material and a high molecular weight “tail,” to have poorer stress crack resistance than samples having a “narrow” molecular weight distribution. Stress crack resistance of specimens quenched from the melt, however, tends to improve for “broad” distribution resins, while decrasing for those polyethylenes having a “narrow” molecualar weigt distribution. Differences in crystal structure are used to explain the physical bassis for these effects.     

R-curve behaviour in weak interface-toughened SiC-C laminates by discrete element modelling

Tough ceramics can be made by introducing weak interfaces which deflect a growing crack. A simple discrete element model was employed to study crack growth in SiC-C laminate with graphite as weak interfaces. By modelling the three-point-bending test of the notched laminate, de-bonding, de-lamination and crack deflection were observed dynamically. The fracture features of the laminate as like “graceful failure” and R-curve behaviour were replicated both qualitatively and quantitatively. It has been revealed that the strength of the weak interface should be in a certain range to assure the appearance of the toughening effect. A minimum presence of the weak interface is required to increase the strength of the laminate. The employed model is explicit and simple, it can be used to reproduce the dynamic process of crack propagation in ceramic laminates and guide the design of the laminate's structure in the future.     

Microtexture and structure of high temperature massive pyrocarbons prepared on graphite substrates

CTEM techniques were used to study the texture and structure of high temperature massive pyrocarbons prepared on graphite substrates. Pyrocarbons were found to show a substructure at the CTEM scale (“micro-cauliflower” texture). In heat-treatment, they graphitize partially and suddenly in an increasing number of areas, as the micro-cauliflower texture disappears. The sudden graphitization by steps is explained and related to the specific texture.     

Fractography and failure mechanisms of particulate-filled thermoplastic polyester

Fractography has been used for the postfailure analysis of a filled thermoplastic polyester. The five fracture modes that were previously defined on the basis of macroscopic stress–strain behavior were distinguished by certain fractrographic features. These features were characteristic of the fracture mode and did not depend on filler type or filler content. The Mode A ductile fracture surface consisted of two regions: a pullout region of slower crack growth and a rosette region of faster crack growth. The Mode B ductile fracture surface contained only a ductile pull out texture. The Mode C quasi-brittle fracture surface exhibited secondary fracture features that sometimes included the herringbone pattern. The Mode D quasi-brittle fracture surface consisted of a stress-whitened dimple region and a brittle fracture region. The Mode E Fracture surface exhibited primarily the rough texture characteristic of brittle fracture. The failure mechanisms inferred from analysis of the fracture surfaces confirmed a microscopic failure model of the ductile-to-quasi-brittle transition in filled PETG that is based on the strain-hardening strength of the polymer ligaments between debonded filler particles. © 1994 John Wiley & Sons, Inc.     

Spiral Tunneling Cracks Induced by Environmental Stress Cracking in LaRC™-TPI Adhesives

Some currently-available formulations of LaRC?-TPI, a thermoplastic polyimide originally developed at NASA-Langley, were found to be highly susceptible to environmental stress cracking when exposed to solvents such as acetone, toluene, diglyme and methyl ethyl ketone. The combination of stress and solvent led to rapid cracking in films and adhesive layers of this material system. Residual cool-down stresses induced when the LaRC-TPI is used as an adhesive or coating led, in the presence of a solvent, to dense “mud crack” patterns which relieve a portion of the stored energy. Because these through-the-thickness cracks are not able to relieve the stored energy in the vicinity of the adherends, additional fractures in the form of curious spiral tunnel cracks initiated and grew inward within each adhesive fragment. Micrographs of the spiral fractures are given, along with a qualitative explanation for the failure process as observed in adhesives and coatings.     

Fatigue behavior of neat and long glass fiber (LGF) reinforced blends of nylon 66 and isotactic PP

This paper focuses on the study of the fatigue behavior of neat and long glass fiber (LGF) reinforced nylon 66/PP-blends. The fatigue was characterized using Parislaw plots in the stable crack growth acceleration range. The fatigue crack propagation (FCP) is presented as a function of the crack growth per cycle (da/dN), the amplitude of the stress intensity factor ΔK, and of the strain energy release rate ΔG. It was also of interest to compare the order of performance found in fatigue to that in the static fracture test. The fracture surfaces were characterized with SEM to determine the failure mechanisms. Further, thermographic camera recordings were used to study the size of a “heated” area (ΔT = 2°C) that developed around the crack tip during the cyclic loading of LGF-PP with different amounts of maleic anhydride grafted PP (PP-g-MAH). For the neat materials, a different order of performance was detected under static and cyclic loading. This was explained by the different failure mechanisms observed after static and cyclic fracture that were related to different stress states of the specimens during the fracture process. On the other hand, the LGF-blends showed a similar order of performance during the static and the fatigue test. This was explained by the observation that similar fiber related failure mechanisms occurred in the composite, both after failure caused by the static and cyclic loading, respectively. For the LGF-PPs with varying PP-g-MAH content, the order of performance in fatigue did not correspond to the size of the “heated area” around the crack tip. This was caused by a change in the composite failure mechanisms, which contributed differently to the size of the “heated area” and to the fatigue performance.     

Mecanique de la rupture appliquee au beton hydraulique

Test data from the fracturing of large specimens of traditional unreinforced aggregate concrete of the double cantilever beam (DCB) type have demonstrated the existence of an extensive damage length forward of the notch root (determined by the single-dimensional acoustic-emission location technique).It is observed that the energy necessary for fracture increases as the crack propagates. This situation is characterized by a curve “R” (fracture energy as a function of crack propagation). The stationary value of the fracture energy can be much greater than the crack initiation value at the notch root.     

Fracture toughness of impact-modified polymers based on the J-integral

The fracture toughness characterization of four impact-modified polymers based on the J-integral concept was studied. We have discovered that the use of the “crack blunting line” concept needs revision. Direct measurements of the crack growth can be made (as opposed to indirect readings from the fracture surface) that test the applicability of the crack blunting concept. Our results indicate that for rubber-toughened polymers the use of the blunting line fails to specify properly the critical J-integral value for crack initiation.     

APPLICATION OF THE TRANSFORM-ITERATIVE METHOD TO NONLINEAR CONCENTRATION BOUNDARY VALUE PROBLEMS

In this paper we present a “somewhat” new method that treats nonlinear concentration related boundary value problems, among other problems. The method uses a general convolution product to transform the nonlinear term, then it iterates with successive division inside the integral to arrive at the approximate solution. It will also include an explanation to the apparent success of what was recently termed “novel transform method” for such nonlinear problems, as being a very special case. As such, it represents the first mathematical basis of any kind, that would offer a reason for the limited success of the recent “novel” method. The present method is illustrated here, for a porous catalyst slab with an outline for other geometries and different non-linearities. The numerical results agree very well with the known exact results of these problems, and other problems in different fields.     

R-curve behavior and flexural strength of zirconia-toughened alumina and partially stabilized zirconia composite laminates

A composite-laminate formed by thick layers (~ 320?µm) of zirconia-toughened alumina (ZTA) with thin (~ 50?µm) interlayers of zirconia partially stabilized (Y-PSZ) has been fabricated by tape casting and pressureless sintering. Fracture behavior and strength has been investigated and compared to a “monolithic” reference, e.g. a stack of zirconia-toughened alumina (ZTA) without interlayers. The fracture behavior has been analysed using stable crack growth in V-notched specimens loaded in 3-point bending. The ZTA+Y-PSZ composite laminate presented a rising crack resistance with maximum values between 6 and 14?MPa?m^1/2. In contrast, the “monolithic” ZTA laminate shows a plateau R-curve behavior at 2.7?MPa?m^1/2. Several toughening mechanisms were identified in the ZTA+Y-PSZ composite laminate, such as, crack arrest/slow down, micro cracking and bifurcation. These toughening mechanisms are most likely caused by high tensile residual stresses that were estimated theoretically.     

Viscoelastic study of ionomers

Stress-relaxation curves were obtained for ionomers containing different cations, per cents of ionization, and thermal treatments. Differences in the rheological behavior were found to depend more on the ionization level than on the ion. A recently proposed model for ionomers is discussed and found to be consistent with these results. In terms of this model the degree of ionization in the polymer acts as a regulator for the growth of small oriented lamellar (crystalline) regions. In the most general terms, the mechanical behavior and strength of ionomers appears dominated by the existence of “hard” regions interspersed among “soft” regions. In the polymers studied here there was some slight crystallinity; however, similar effects and explanations are probably suitable for amorphous “ionomers.” Toughness was also found in some completely amorphous carboxylcontaining copolymers without added ionic salts. The same explanation of “hard” regions interspersed among soft regions is also valid here. The “blocky” nature of the copolymerization may play a role in setting up this type of structure.     

Comparison of the sandwiched beam (SB) and opposite roller loading (ORL) techniques for the pre-cracking of brittle materials

The “sandwiched beam” (SB) and “opposite roller loading” (ORL) methodologies suitable to introduce sharp through-thickness cracks in brittle materials are critically reviewed and compared in this work. In both cases a sharp crack is obtained in a notched specimen by means of a suitable loading. In the SB technique the specimen is placed between two support bars and bent in a 3- or 4-point configuration. The ORL procedure is based on the symmetrical loading by four rollers which induces a local tensile stress. Results show that both techniques are successfully usable on brittle materials: in both cases suitable specimens are obtained for fracture toughness measurements. The crack length can be reasonably controlled and varies in a wide range. The SB procedure typically provides cracks with α≌0.5, while shorter cracks are obtained by the ORL technique. Fracture toughness is measured on specimens prepared using the two techniques. The obtained values result in good agreement with literature data.     

A COMPARISON OF THE UNIFORMITY OF STRENGTH AND TEXTURE OF FIRE BRICK MADE BY DIFFERENT PROCESSES1

By assuming that the “deviations from the average” of the strengths and permeabilities of individual bricks of any one brand are governed by the laws of chance, a comparison of the uniformity of strength and texture of a number of brands of brick made by different processes is obtained. Of the brands examined, those made by the stiff-mud process are found to be more uniform in strength and less uniform in structure than those made by the dry-press process. The hand-made bricks show quite a variation in both strength and texture, whereas one brand of brick made by a semi-dry process (English type machine) shows good uniformity in both strength and texture.