Conventional composite materials, in which a matrix material is reinforced by particulates, whiskers, and/or continuous fibers, have long been of interest as potential materials solutions to engineering needs. Typically, the benefits of these reinforcements are observed for cases of tensile loading, with only minimal improvement for cases of compressive loading. As a result, recent attention has grown in the use of hollow spheres as a potential reinforcement in metallic systems.

Schematic drawing of the stress-strain behavior of porous materials under impact loading

Commonly known as syntactic foams, the hollow spheres display a prolonged region under compressive loading in which the spheres deform by crushing, thereby absorbing a large amount of deformation energy. Furthermore, as an added benefit, the inclusion of hollow spheres also serves to lower the weight of the final component, thereby offering the possibility of improved performance with a simultaneous reduction in system weight. Thus, these materials are primarily being considered for applications that require a high capacity for absorbing energy (bumpers, struts, etc.).

When subjected to a compressive load, the hollow spheres (as well as composites based on these materials) typically display a characteristic stress-strain relationship with 4 main areas, as seen in the accompanying Figure. After an initial region characterized by a linear-elastic response (i), the cellular materials experience buckling, plastic deformation and collapse of intercellular walls as they enter the transition zone (ii). Under further loading, the mechanism of buckling and collapse becomes even more pronounced, which is manifested in large strains at almost constant stress (iii). The stress level σpl indicates the beginning of this plateau region in which the gradient of stress plateau is denoted as plateau modulus P. Cellular structure densification is observed after reaching some critical strain σd, and the stress level increases exponentially thereafter (iv). The cellular material is able to absorb a significant amount of impact energy through its elastic and plastic deformation during the loading process, as represented by the area under the strain-stress curve.

Most of the early research in metal syntactic foams has focused on the use of fly ash spheres that were produced as a by-product of fuel combustion. Although composites reinforced with such materials showed improved energy absorption capabilities, the manner in which these spheres are produced resulted in a large (and mostly random) distribution of size, geometry, and composition. This inherent variability in ash-based spheres made it practically impossible to produce repeatable foams over a long processing time frame. As such, definitive conclusions regarding the influence of sphere parameters on performance could not be reached.

More recently, a process has been developed that can produce hollow metal and/or ceramic spheres with consistent and repeatable properties. In this process, a suspension of metal and/or ceramic powder is sprayed onto a polymer support (e.g., spheres) to form a green body that is then sintered to produce the final sphere material. As can be envisioned, the ability to tailor the process (suspension composition, coating time, etc.) directly translates into the ability to produce spheres with customized—and repeat-able—compositions and/or geometries. This, in turn, allows one to create composite materials specifically designed for a given application.

In a project designed to evaluate the potential of such materials for use in Army-relevant applications, the US Army Research Laboratory entered into a collaborative effort with Deep Springs Technology (DST) (Toledo, Ohio). The focus of this joint effort was to identify and develop processing method(s) for incorporating the hollow spheres into a light metal (aluminum [Al], magnesium [Mg]) alloy matrix. Several mechanical and physical tests were then used to determine the performance of the resulting hollow metal sphere composites.

This work was done by Oliver Strbik III and Vincent H Hammond for the Army Research Laboratory. ARL-0206


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Processing and Characterization of Lightweight Syntactic Materials

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This article first appeared in the December, 2017 issue of Aerospace & Defense Technology Magazine.

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