Internal Void Formation Research Articles

Strengths of Ceramic Fibers at Elevated Temperatures

Tensile strengths were measured and Young's moduli were estimated for two SiC‐based and three oxide ceramic fibers for temperatures from 25° to 1400°C. The SiC‐based fibers were stronger but less stiff than the oxide fibers at room temperature and retained more of both strength and stiffness to high temperatures. High‐temperature strengths of the SiC‐based fibers were limited by internal void formation and oxidation; those of the oxide fibers were limited by softening of an intergranular glassy phase.

The Prevention of Central Bursts During Rolling

An upper bound analysis has been developed to model the flat rolling of a perfectly plastic material. A rigid body rotational velocity field was assumed for the deformation region. This field permits the possible formation of an internal void. The process conditions under which it is energetically favorable for a void to develop have been determined. It has been found that internal void formation (central burst) is more likely to occur in thick strips and sheets when small reductions are taken. Both front and back tension tend to promote the development of a central burst with the back tension reducing the safe, noncentral burst domain to a greater degree. Criteria curves for the central burst domain and the safe domain are presented for a wide range of process variables.

Bulk modulus determination of solid propellant void content

Nitrate ester decomposition in propellant systems leads to a slow buildup of internal gas pressure which potentially can create internal porosity, void formation, and fracture. While related experimental techniques currently available can provide information on ultimate propellant fracture by monitoring gross property changes, few procedures were available to monitor prefracture changes. This study used bulk compressibility measurements to define changes in internal void content (porosity) in solid propellants due to gas evolution. The rate of internal void formation was determined in three propellant families: composite modified double-base (CMDB), crosslinked double-base (XLDB), and nitrate ester polyether (NEPE) systems. Good correlation was found between void content and location in a 16-in. cube and bulk compressibility measurements as well as Xray examination. Elevated temperature storage at 158 and 176 °F was used to accelerate the void formation in the three propellant systems. The CMDB propellant exhibited the highest void formation rate of the three propellants. The utility of recent developments in fracture mechanics applied to gas generation induced voids was also explored in this study.

Hydrogen Induced Delayed Failure of High Strength Alloy Steel Wires

Abstract Delayed failure tests, In which the samples were cathodically charged with hydrogen while under sustained tensile loads, were conducted upon two medium carbon alloy steels which had been quenched, tempered, and cold drawn to very high strength levels. The results were correlated with the microstructure and fractographic features of the steels. The resistance of the steels to hydrogen degradation is attributed to trapping of hydrogen on defects, alignment of elongated dislocation cell walls with the wire axis and decreased transverse wall spacing, all effects which are accentuated by increasing drawing strain. The beneficial effects of increased strain are thought to be limited eventually by the formation of internal fissures and voids.

The origin of microscopic planetary ring particles

The microscopic planetary ring particles around Jupiter are possibly the result of internal void formation and fracture of initially larger particles. The void formation is thought to be the result of high electron and proton fluences attained by the particles during their lifetimes in orbit. The constant irradiation produces defect species which coalesce to form voids, which provide fracture paths for the particles.

Intergranular oxidation and internal void formation in Ni-40% Cr alloys

Internal void formation and intergranular oxidation behaviour have been studied during the oxidation of two Ni-40Cr alloys in 1 atm oxygen at 1000° to 1200°C. The development of an external Cr 2O 2 scale causes vacancies to be generated in the alloy at the alloy-scale interface as chromium diffuses into the scale, and others to be generated in the alloy due to the different diffusion rates of chromium towards the interface and of nickel back into the bulk alloy. At 1200°C, internal void formation results from condensation of such vacancies at inclusions in the grains and at the grain boundaries. The intergranular oxidation observed at 1000°C, 1100°C and to a lesser extent. 1200°C results from preferential condensation of vacancies to form voids in the alloy grain boundaries. Significant depletion of chromium in the alloy adjacent to the scale facilitates the supply of oxygen from the scale and its penetration into the alloy grain boundaries to form intergranular oxide. Such intergranular oxide develops deep into the alloy following diffusion of this oxygen through a porous network in the oxide, which arises because of the vacancy condensation, and oxidation of chromium at the tip of the intergranular penetration.

Ultrahigh modulus polyethylene. II. Effect of drawing temperature on void formation and modulus

AbstractThe maximum degree of molecular orientation and deformation obtained by ultradrawing of high‐density polyethylene in air is limited by formation of internal voids (both longitudinal separation of fibrils and perpendicular cracking), and thus values of Young's moduli which are achievable by ultradrawing techniques are also limited to values much below the theoretical limit for fully extended chains. Temperature has a significant effect on the critical draw ratios at which intensive void formation begins, and also on the draw ratio at which failure occurs during the ultradrawing. The temperature effect is observed only for high‐density polyethylene having a wide molecular‐weight distribution, and which can be drawn at higher temperatures (30–40°C below its melting point), e.g., Dow Chemical polyethylene LP51.1. As a result of ultradrawing at higher temperatures, transparent, ultrahigh modulus samples having draw ratios of order of 40 have been obtained. The higher drawing temperatures significantly reduce fibril separation, and perpendicular cracking is shifted toward higher draw ratios. Hence, with LP51.1 the highest Young's moduli (65–70 GPa) have been exhibited by the samples which were ultradrawn at 100–105°C.

A Microscopic Examination Of Void Formation In Superplastic Materials

SUMMARYThere are two distinct groups of superplastic materials, depending on whether the final fracture surface has a large cross‐sectional area at optimum ductility or the specimen gradually pulls down to a very fine point. Experiments on a commercial superplastic copper alloy and the Zn‐22% Al eutectoid indicate that the formation of internal voids is important in materials of both groups. In addition, the results show that the size and shape of the internal voids is dependent both on the material and the testing conditions.

The role of bulking in brittle failure of rocks under rapid compression

Many rocks fail in a brittle way when submitted to a compressive state of stress. During the process of brittle failure in compression, cracks break the material up into subregions. These subregions, which consist of still intact material, must separate in order that full unloading can take place. The unloading process through separations of these subregions is accompanied by frictional sliding and the formation of internal voids, which result in a non-elastic bulking of the failing material. Under dynamic loading such as is encountered in the split Hopkinson bar or an impact experiment, it has been observed that rocks can carry larger compressive loads than their static strength for a considerable length of time before unloading through failure. This apparent strengthening of the material under dynamic loading is shown to be a consequence of the limited velocity of the unloading process, which in turn is controlled by bulking and frictional losses. Cylinders of Bohus granite and Solenhofen limestone were made to impact on a steel bar of equal diameter, which was instrumented with strain gauges to measure the average pressure over the contact surface during the failure process. The rock cylinders were accelerated in a compressed air gun to velocities of up to 115 m/s. For Bohus granite a maximum axial stress of 6·0 kbar was measured. The experimental results are compared to calculations with a simple unloading model, which is based on radial inertia and takes into account both frictional losses and bulking. In the presence of bulking radial inertia becomes very important because the reaction forces to the radial acceleration act as the confining pressure for the not yet unloaded material.

Corrosion of Superalloys at High Temperatures In the Presence of Contaminating Salts

Abstract The corrosion of Inconel X, Inconel 702, Rene 41, M-252, and WF-11 (Haynes 25) by potassium chloride and lithium fluoride at 1600 to 1900 F was studied. Thin coatings of the salts (1.5 mg/cm2) caused severe corrosion of the alloys in air, which resulted in accelerated failures of thin sheet specimens in creep-rupture testing. Rankings for the alloys based on creep-rupture tests were similar for uncoated and salt-coated materials: WF-11, Rene 41, and M-252 best, Inconel 702 poorer, and Inconel X poorest. The corrosion products consist mainly of oxides and spinels, and also contain small amounts of chromates. Only very little corrosion, if any, occures without oxygen. The presence of the salt prevents the normal formation of a protective oxide film. X-ray diffraction studies showed differences between the normal oxidation products and the oxide corrosion products produced with salt present. The types of corrosion include severe surface attack, intergranular penetration, and internal void formation. All of the alloys were susceptible to each of these types of corrosion. Grain boundary separation effects due to stress (2,500 to 10,000 psi) were also found. 4.3.6, 6.3.10