Home > Research > Functionally Graded Materials and Structures
Functionally Graded Materials and Structures
Back in the Middle Ages (and even earlier in China), weapons and tools were made of iron; either wrought iron, which was fairly soft and wouldn’t hold an edge for long, or cast iron, which was extremely hard, unable to deform, and would break quite easily. Introducing a precise amount of carbon in the smelting process produced steel, which combined the useful qualities of wrought and cast iron, making it infinitely more useful and much more valuable.
Materials science has come a long way since the Middle Ages, but one thing remains the same—as our technology improves we increase our demands on structural materials, subjecting them to greater loads and more severe environments. In the same way that steel was a big improvement over iron, today’s metal alloys are giving way to advanced materials that can perform better under a variety of demanding conditions, from outer space to thousand-degree jet engines. Professor K.T. Ramesh is the director of JHU’s new Center for Advanced Metallic and Ceramic Systems (CAMCS), where faculty from Hopkins’ Mechanical Engineering and Materials Science departments design, fabricate, and study state-of-the-art materials for a variety of defense- and industry-related applications.
An ideal material combines the best properties of metals and ceramics—the toughness, electrical conductivity, and machinability of metals, and the low density, high strength, high stiffness, and temperature resistance of ceramics. Take away some of the brittleness of ceramics and make strong metals lighter and stiffer, and the material becomes really useful. You’ve got a material that is hard but won’t break; one that will conduct electricity but can withstand high temperatures. These materials, known as Metal Matrix Composites (MMCs) or Ceramic Matrix Composites (CMCs) have incredible promise in many engineering applications. Demand for such materials comes from the automotive industry (lightweight and strong materials would increase fuel efficiency and last longer), electronics, telecommunications, and the aerospace and defense industries.
Such advanced materials can be “functionally graded” to provide the exact combination of characteristics desired. Functionally Graded Materials (FGMs) are materials or structures in which the material properties vary with location in such a way as to optimize some function of the overall FGM. The matrix alloy (the metal), the reinforcement material (the ceramic), the volume, shape, and location of the reinforcement, and the fabrication method can all be tailored to achieve particular desired properties. In MMCs, for example, ceramic reinforcements in the form of either fibers, whiskers, or particulates are introduced into the metal; the structure is controlled at scales varying from 100 nm to several millimeters. The design of FGMs requires an explicit understanding of the material behavior at each location and over all these length scales.
However, the responses of such advanced materials to dynamic and impact loadings (severe mechanical environments) are generally unknown. Prof. Ramesh runs the Laboratory for Impact Dynamics and Rheology (LIDAR), an offshoot lab of CAMCS, in which he subjects these materials to impact loadings in an effort to understand their mechanical properties under high strain rates. With ultra-high-speed instrumentation, lasers, a dynamic temperature sensing system, and a camera capable of taking 100 million pictures per second, Prof. Ramesh can characterize dynamic fracture and failure processes in these materials. Using the results of these experiments in combination with finite-element models developed by Prof. Molinari and scanning and transmission electron microscopy techniques developed by Prof. Hemker, Prof. Ramesh hopes to be able to pinpoint how failure evolves in advanced materials. Knowing exactly how a material will behave under a certain kind of loading will allow engineers to tailor the FGM precisely to the kind of load or environment it will need to withstand in service.