Increase In KIC Research Articles

Mechanical Properties of SiC Whisker-Reinforced TiB<sub>2</sub> Composites Fabricated by Hot-Pressing

SiC whisker reinforced TiB2 matrix composites containing 10 to 30vol% whisker were prepared by hotpressing at 1600 to 1900°C. The flexural strength of 867±77MPa and KIC of 5.8±0.12MPa√m were obtained for the 1800°C hot-pressed samples containing 10vol% whisker. No significant increase in KIC was observed by the whisker reinforcement because of the chemical reaction between matrix and whisker, which enhanced strong interfacial bonding and whisker agglomeration.

単結晶Ｓｉの高温破壊じん性

The fracture toughness (KIC) on the major planes of single crystal silicon was measured by the controlled surface flaw (CSF) and indentation fracture (IF) methods at high temperatures. The KIC values obtained by the CSF method for the surface orientations {100}, {110}, {111} and {112} of silicon up to about 500°C decreased with increasing temperature. In the low temperature region, the linear decrease of KIC for {110}, {111}, and {112}, excepting for {100}, was explained on the basis of an elastic model. The KIC values for {100}, {110}, {111} and {112} planes of silicon increased rapidly over the temperatures of 500, 600, 700 and 800°C, respectively.It was found that the rapid KIC increase for those planes of silicon at the individual critical temperatures was attributable to extensive plastic yielding in the vicinity of the crack tips relating the Schmid factor and numbers of slip systems for each planes. The KIC values obtained by the IF method in the range from 400 to 600°C were larger than those obtained by the CSF method for all planes. The trend of increase in KIC values by the former method in the high temperature region agreed with that by the later method.

Metabolic interaction between skeletal muscle and liver during bacteremia.

To study the effects of bacteremia on skeletal muscle leucine (LEU) metabolism, mongrel dogs were infused with normal saline or Escherichia coli (10(9)/kg). After a bolus dose (3.6 microCi), L(1-carbon 14) LEU (0.3 microCi/min) was infused directly into the isolated, constant-flow, in vivo gracilis muscle. Arteriovenous differences for amino acids, labeled and unlabeled LEU and alpha-ketoisocaproic acid (KIC), and labeled carbon dioxide were measured at ten-minute intervals for one hour. Bacteremia increased the net release of amino acids and total N2 from muscle. Moreover, plasma LEU that was deaminated and released as KIC was increased, and there was also an increase in decarboxylated plasma LEU during bacteremia. Despite the marked increase in KIC release from skeletal muscle during bacteremia, arterial concentrations were not significantly different from those of controls. An unchanged arterial plasma KIC concentration associated with a marked increase in KIC released from skeletal muscle indicates an increase in LEU metabolism, most likely in the liver. Thus, the increased skeletal muscle catabolism is not a futile cycle but rather an essential event to meet the increased metabolic needs of the body during bacteremia.

Transformation Toughening in Composites Containing Dicalcium Silicate

By taking into account solid‐state Compatibility relations in the system CaO‐SiO2‐ZrO2, a series of dense, tough CaZrO3‐β‐Ca2SiO4 composites were obtained. The increase in KIC and σf values observed is interpreted in terms of the β‐Ca2SiO4→γ‐Ca2SiO4 polymorphic transformation.

Chemical modification of matrix Resin networks with engineering thermoplastics

Functionally terminated bisphenol-A polysulfone oligomers were used in the modification of Epon Resin 828/4,4′-diamino-diphenylsulfone (DDS) network system. Phenolic hydroxyl terminated PSF oligomers were first capped with a large excess of bisphenol-A diglycidyl ether or Epon Resin 828 at both ends and then the resulting system was cured with DDS, in a two-step process. During these studies molecular weight and the amount of PSF oligomers incorporated into the network were varied and their effect on the overall properties of the resulting systems were investigated. The capping and curing reactions were followed by using FT-IR and NMR spectroscopy, GPC, HPLC and DSC techniques. As a function of the oligomer molecular weight, SEM studies showed the formation of two-phase structures with ductile PSF particles dispersed in the continuous epoxy matrix. Mechanical characterization and fracture toughness measurements showed a remarkable increase in KIC or gIC values of the modified networks over that of control, without significant loss in the modulus. This work would appear to be one of the first studies where well bonded ductile glassy modifiers have significantly improved the fracture toughness of highly crosslinked networks.

Mechanical Properties of Mullite

The mechanical properties of alkoxy‐derived, high‐purity, translucent, theoretically dense mullite (3AI2O3.2SiO2) were investigated over the temperature range room temperature to 1500°C. Large agglomerates were found to contribute to the formation of porosity nests which act as strength‐controlling flaws at room temperature as well as at high temperatures. Despite the slow crack growth above 1300°C, a slight increase in fracture stress and a large increase in KIc were observed up to 1500°C. These increases are explained by the dominance of energy dissipation through plastic relaxation in the plastic zone over grain‐boundary sliding due to the presence of the glassy phase.

Fracture toughness and fatigue of polystyrene, polyphenylene oxide and their blends

AbstractBlends of atactic polystyrene (PS) and poly(2,6‐dimethyl‐1,4‐phenylene oxide) (PPO) represent a compatible system that exhibits a transition from a crazing to a shear bending mode of deformation as a function of composition. PS (Mw = 30,000; Mw/Mn = 1.06) exhibited a value of \documentclass{article}\pagestyle{empty}\begin{document}$ K_{IC} = 1.00 \pm 0.20{\rm }MPa\sqrt m $\end{document}. Blends of up to 60 wt.% PPO exhibited \documentclass{article}\pagestyle{empty}\begin{document}$ K_{IC} \sim 1.00{\rm }MPa\sqrt m $\end{document}. However, there is a very rapid increase in KIC with increasing PPO content in the vicinity of 75 percent PPO. Fatigue behavior of the blends parallels that under static loading.

Study of ferroe lectric ceramic fracture by indentation

1. The indentation method is applicable to fracture toughness KIc measurement in standard industrially produced piezoceramic elements for material having an average grain size in the range 3–7 μm. Values of KIc have been obtained for a series of compounds taking account of transverse-isotropic symmetry of material properties, 2. Polarization leads to an increase in KIc for the plane of crack development along the residual polarization axis and to a reduction in KIc in the perpendicular direction. Data have been obtained pointing to the presence of irreversible changes in IPEs after polarization. 3. The contribution of internal stresses due to polarization in fracture depends on crack length due to the suggested macroinhomogeneity for distribution of these stresses.

The effect of minor additions of titanium on the fracture toughness of Fe-12 at% Ni alloys at?196� C

The reasons for the improvement in the fracture toughness of an Fe-12 at% Ni base alloy at −196° C by the addition of small amounts of Ti were investigated employing transmission electron microscopy (TEM), scanning electron microscopy (SEM) and Auger electron spectroscopy. Ti additions ranging from 0.18 to 0.99 at% and heat treatments of 2 h at 550, 685 and 820° C respectively followed by a water quench were considered, since previous work by Witzke and Stephens had shown that maximumKIC occurred for an Fe-12 at% Ni-0.18 at% Ti alloy heat treated at 685° C. It was found here thatKIC at −196° C for all the alloys and heat treatments correlated with the fraction of ductile fracture compared to intergranular and cleavage fracture, the latter modes being predominant in the Fe-12 at% Ni base alloy without Ti additions. Cubic and rectangular shaped inclusions were noted in the SEM fractographs of the alloys with the Ti additions. A fine precipitate was observed by TEM for the Fe-12 at% Ni-0.18 at% Ti alloy heat treated at 550° C; this precipitate was not observed for the 685 and 820° C heat treatments of the same alloy. Auger mappings of the fracture surfaces indicated a weak to moderate association of the interstitials C, N and O with Ti, the degree of which depended on the particular interstitial and the heat treatment temperature. It was concluded that the increase inKIC due to the initial 0.18 at% addition of Ti was due to a scavenging of interstitials which normally segregate at the grain boundaries and to the refinement of the microstructure. The subsequent decrease inKIC with further Ti additions was attributed to the increase in flow stress and slight lowering of the fracture stress resulting from these additions. It was further inferred that the Ti additions and heat treatment on the flow and fracture stresses may be due in a large part to their influence on the amount, size and distribution of the precipitate which was observed.

Strength and Toughness of a Cold Rolled 13Ni-15Co-10Mo Maraging Steel

A study was conducted to determine the effect of cold rolling prior to aging on the strength and toughness of an ultrahigh strength 13Ni-15Co-10Mo maraging steel. The smooth tensile strength of cold rolled and aged maraging steel increased continuously with increasing amount of cold reduction. The cold rolled maraging steel exhibited anisotropy in properties; the tensile strength in the transverse direction was higher than in the longitudinal direction, whereas the reduction of area and elongation showed lower values in the transverse direction. The notch tensile strength and plane strain fracture toughness KIC also increased with increasing amount of cold reduction, taking maximum value at the 60-70% reduction. For example, longitudinal KIC value increased from 34Mpa•m1/2 for 0% reduction to 63Mpa•m1/2 for 70% reduction. The large amount of KIC increase was attributable to the formation of delaminations running parallel to the rolling plane. In the presence of delaminations, it is considered that the KIC test specimen behaved as the sum of a number of thin plates and each thin plate fractured under the plane stress condition. Based on these results, it was concluded that the good combination of the strength and toughness could be obtained at the 60% cold reduction.

The effects of stress cracking on the fracture toughness of polycarbonate

AbstractThe growth of crazes from a sharp crack in extruded polycarbonate sheets immersed in ethanol was measured. Below a critical level of the stress intensity factor craze growth was controlled by solvent diffusion through the end of the notch and fracture was prevented by craze arrest. Above a critical level, growth was controlled by either end diffusion or a combination of end diffusion and diffusion through the faces of the extruded sheet, and in both cases the final result was brittle fracture. The effects of annealing and quenching was studied at various sheet thicknesses. In thin specimens annealing and/or quenching had a significant effect on crack growth rate, which was predictable in terms of the state of stress. As the specimen thickness increased, causing a transition from plane stress to plane strain conditions, the previous thermal history had a diminishing effect on craze growth rate. The effects of thermal history and thickness on the fracture toughness of polycarbonate was also investigated. It was found that thickness was the more important variable and that at a ½ in. thickness the effects of thermal history were statistically insignificant. The effect of ethanol exposure on fracture toughness was studied. It was found that exposure to solvent initially caused an increase in kIC with time to a maximum value, followed by a substantial decrease with time which eventually led to brittle fracture. This behavior was explained as a competition between plasticization of the crack tip and coalescence of crazes to form microcracks.

冷間圧延した13Ni-15Co-10Mo系マルエージ鋼の強度と靱性

A study was conducted to determine the effect of cold rolling prior to aging on the strength and toughness of an ultrahigh strength 13Ni-15Co-10Mo maraging steel. The smooth tensile strength of cold rolled and aged maraging steel increased continuously with increasing amount of cold reduction. The cold rolled maraging steel exhibited anisotropy in properties: the tensile strength in the transverse direction was higher than in the longitudinal direction, whereas the reduction of area and elongation showed lower values in the transverse direction. The notch tensile strength and plane strain fracture toughness KIC also increased with increasing amount of cold reduction, taking maximum values at the 60-70% reduction. For example, longitudinal KIC value increased from 34 MPa·m1/2 for 0% reduction to 63 MPa·m1/2 for 70% reduction. The large KIC increase was attributable to the formation of delamination cracks running parallel to the rolling plane. The KIC test specimen might behave as the sum of a number of thin plates in the presence of the delamination cracks and each thin plate might fracture under the plane stress condition. From these results, it was concluded that the good combination of the strength and toughness could be obtained at the 60% cold reduction.

The grain size dependence of fracture toughness in an Al-6.0% Zn-2.5% Mg alloy

The grain size dependence of the fracture toughness (KIC) of an aged Al-6.0% Zn-2.5% Mg alloy was studied experimentally. KIC depended strongly upon grain size (LG) in two ways. In the small grain size region KIC decreased with increasing average grain size. In contrast, KIC increased with increasing average grain size for large grain sizes. The increase in KIC with increasing grain size arose as a result of the presence of abnormally large grains compared to the average grain size in the large-grained specimens.

Relationship Between Plane-Strain Ductility and KIc for Various Steels

In most structural steels, the critical plane-strain stress-intensity factor, KIc, which represents the inherent ability of the material to resist crack propagation, increases with increasing test temperature. In many steels, the rate of increase of KIc with test temperature increases markedly above a temperature region which is designated the fracture-toughness transition region. Because of this transition behavior with temperature and the inherently high fracture toughness of many steels, very thick specimens must be tested to determine valid KIc values. However, the large size of these specimens and the cost of conducting the tests minimize the usefulness of this procedure as a research tool for analyzing the fracture behavior of steels under plane-strain conditions. Therefore, as part of a long-range program to obtain KIc values from small specimens, the relationship between KIc and the ductility measured in a small plane-strain tension test was investigated. The results showed that the temperature transition of the plane-strain tensile ductility at fracture, εf.ps, paralleled that of the KIc, whereas the temperature transition of the axisymmetric ductility (measured in a conventional round tension specimen) was quite different from that of the KIc. At any temperature, the relationship between these properties for steels ranging in yield strength, σy, from 80 to 250 ksi is approximated by the equation KIc=Aσyε2f.ps where A is a constant for a given steel. This relationship indicates that for certain steels the KIc value approaches an upper limit or shelf value with increasing temperature, because as the test temperature increases, the plane-strain tensile ductility approaches an upper limit and the decrease in yield strength becomes negligible. The existence of a KIc shelf would imply that in thick sections the material could fail suddenly, even at elevated temperatures. However, because of the rapid rate of increase of the plastic-zone size with temperature (and thus the through-thickness plastic flow at the crack lip), steels in conventional thicknesses would lose plane-strain constraint at temperatures approaching that for the KIc shelf, and therefore only ductile behavior would be expected at shelf temperatures.