Search

Modern foundries increasingly use chemically bonded no-bake sand cores and molds because they provide ease of molding, good surface finish, and collapsibility. One of the most popular binder systems is alkyd oil urethane no-bake system comprising three parts: alkyd resin, catalyst, and crosslinking agent. Their amounts and ratios can influence mold shrinkage, and thereby dimensional quality of the resulting casting. With rising emphasis on net-shape casting, there is a need to optimize the binder composition to minimize dimensional errors, while achieving the desired bench life, stripping time and hardness. This work investigates the effect of binder composition on the dimensions of chemically bonded sand cores with respect to time. The maximum shrinkage of 0.15% over length was observed when resin content was 2.4% by weight of sand. The rate of shrinkage increased with the amount of catalyst. Measurement of core hardness and reduction in weight, followed by SEM studies, provided a better understanding of the underlying phenomena, especially formation of resin bridges between compacted sand particles and evaporation of solvent. This work is expected to aid in selection of the most appropriate binder composition for a given set of molding process constraints and cast product requirements. [ABSTRACT FROM AUTHOR]

Abstract: The paper addresses the damage tolerance of sandwich structures, where the prevention and limitation of delamination failure are highly important design issues. Due to the layered composition of sandwich structures, face–core interface delamination is a commonly observed failure mode, often referred to as peeling failure. Peeling between the sandwich face sheets and the core material drastically diminishes the structural integrity of the structure. This paper presents a new peel stopper concept for sandwich structures. Its purpose is to effectively stop the development of debonding/delamination by rerouting the delamination, and to confine it to a predefined zone in the sandwich structure. The suggested design was experimentally tested for different material compositions of sandwich beams subjected to three-point bending loading. For all the tested sandwich configurations the suggested peel stopper was able to stop face–core delamination and to limit the delamination damage to restricted zones. [Copyright &y& Elsevier]

Abstract: Fatigue failure of a gas-nitrided 4140 steel under axial cyclic loading results from a competition between surface crack initiation in the nitrided case and internal “fish-eye” cracking inside the core material. When nitriding is deep enough, the internal mechanism prevails in smooth specimens and fatigue strength improvement as compared to base metal is about 20% at 106 cycles. In the present study, three V-notched specimens (blunt, medium, and severe) are designed to be representative of the stress gradient: (i) in a small rotary bending specimen, (ii) at the root of a gear tooth, and (iii) at the root of a very sharp notch. The cracking mechanism depends on the notch severity. The nitrided blunt notch fails from a fish-eye nucleated at the case/core boundary, whereas the medium and sharp notches fail from surface cracks. The high-cycle fatigue strength improvement varies from 80% for the blunt notch and to more than 100% for the sharp notch. The notch fatigue behaviour of nitrided steel is discussed by comparing the evolutions of internal and surface fatigue strengths with relative stress gradient. [Copyright &y& Elsevier]

Polymeric aerogels are used in this study to bind foundry sand forming thus a new kind of mould and core material called AeroSand. A core and mould material in general shall be able to withstand all handling stresses in a foundry shop, withstand the thermal and mechanical stresses exerted onto them during casting, but should also easy removable from a casting. Therefore mechanical properties are important besides others and this paper therefore concentrates on bending and compression strength of AeroSands. It is shown that the strength levels obtainable with aerogel binders are comparable to conventional binder systems. One essential advantage of AeroSands, namely the easy core removal, is not payed off by a loss in strength. The mechanical properties can be explained using a simple extension of the Griffith criterion taking into account the microstructure of the sand aerogel composite and the fracture path. [ABSTRACT FROM AUTHOR]

The objectives of this study are to investigate the basic mechanical properties of TEEK-L polyimide-foam of a balloon type and TEEK-L filled aramid-honeycomb core-materials for sandwich structures and to compare their mechanical properties with those of AIREX R82 and ROHACELL WF foam core-materials. Compression, tension, and plate-shear tests provided their stress–strain relationships, elastic modulus, and strength. The test results are compared and discussed. The major results obtained are: (1) Filling the aramid honeycomb cells with TEEK-L can improve the basic mechanical properties of an aramid-honeycomb core-material, in particular making them higher than the values expected from the principle of superposition for compression tests and equal to the values expected from superposition for plate-shear tests. (2) The basic mechanical properties of the TEEK-L filled aramid honeycomb are superior to those of AIREX R82 110 and equivalent to those of ROHACELL WF 110, where the three materials have equal densities. (3) The basic mechanical properties of TEEK-L are inferior to those of AIREX R82 60 and ROHACELL WF 51, when materials with equivalent densities are compared. (4) TEEK-L filled aramid honeycomb has high potential as a new core material for aerospace sandwich structures. [ABSTRACT FROM AUTHOR]