Functionally graded material

In materials science functionally graded material (FGM) may be characterized by the variation in composition and structure gradually over volume, resulting in corresponding changes in the properties of the material. The materials can be designed for specific function and applications. Various approaches based on the bulk (particulate processing), preform processing, layer processing and melt processing are used to fabricate the functionally graded materials.

History

The concept of FGM was first considered in Japan in 1984 during a space plane project. Where a combination of materials used would serve the purpose of a thermal barrier capable of withstanding a surface temperature of 2000 K and a temperature gradient of 1000 k across a 10 mm section [1]. In recent years this concept has become more popular in Europe, particularly in Germany. A transregional collaborative research center (SFB Transregio) is funded since 2006 in order to exploit the potential of grading monomaterials, such as steel, aluminium and polypropylen, by using thermomechanically coupled manufacturing processes [2].

General information

The basic unit for FGM representation is maxel. The term maxel was introduced in 2005 by Rajeev Dwivedi and Radovan Kovacevic at Research Center for Advanced Manufacturing (RCAM). The attributes of maxel include the location and volume fraction of individual material components.

A maxel is also used in the context of the additive manufacturing processes (such as stereolithography, selective laser sintering, fused deposition modeling, etc.) to describe a physical voxel (a portmanteau of the words 'material' and 'voxel'), which defines the build resolution of either a rapid prototyping or rapid manufacturing process, or the resolution of a design produced by such fabrication means.

Applications

There are many areas of application for FGM. The concept is to make a composite material by varying the microstructure from one material to another material with a specific gradient. This enables the material to have the best of both materials. If it is for thermal, or corrosive resistance or malleability and toughness both strengths of the material may be used to avoid corrosion, fatigue, fracture and stress corrosion cracking.

The transition between the two materials can usually be approximated by means of a power series. The aircraft and aerospace industry and the computer circuit industry are very interested in the possibility of materials that can withstand very high thermal gradients [3]. This is normally achieved by using a ceramic layer connected with a metallic layer.

The Air Vehicles Directorate has conducted a Quasi-static bending test results of functionally graded titanium/titanium boride test specimens which can be seen below [4]. The test correlated to the finite element analysis (FEA) using a quadrilateral mesh with each element having its own structural and thermal properties.

Advanced Materials and Processes Strategic Research Programme (AMPSRA) have done analysis on producing a thermal barrier coating using Zr02 and NiCoCrAlY. Their results have proved successful but no results of the analytical model are published.

The rendition of the term that relates to the additive fabrication processes has its origins at the RMRG (Rapid Manufacturing Research Group) at Loughborough University in the United Kingdom. The term forms a part of a descriptive taxonomy of terms relating directly to various particulars relating to the additive CAD-CAM manufacturing processes, originally established as a part of the research conducted by architect Thomas Modeen into the application of the aforementioned techniques in the context of architecture.

Modeling and simulation

Numerical methods have been developed for modeling and simulating mechanical behavior of FGM. Methods that allow material property varying continuously in elements was proposed by researchers in Japan in 1990 [5] [6]. Numerical methods were also used to analyze surface waves in piezo FGM. [7]

References

[1] Andrew Ruys and D. Sun, Functionally Graded Materials (FGM) and Their Production Methods, last visited 27 April 2011

[2] Hans-Peter Heim (Spokesman), Sonderforschungsbereich Transregio 30 (SFB/TR 30)

[3] J. Aboudi, Prof. M.J. Pindera, and Dr. Steven M. Arnold, Higher-Order Theory for Functionally Graded Materials, last visited 27 April 2011

[4] Wright-Patterson Air Force Base, Ohio, AFRL Researchers Perform Functionally Graded Material Bending Tests, last visited 27 April 2011

[5] Liu, G R and J Tani, Lamb Wave Propagation In Anisotropic Functionally Gradient Material Plates in Proceedings of the First International Symposium on Functionally Gradient Materials, edited by MasaoYamanouchi etal, pp. 59-64. Sendai: Functionally Gradient Materials Forum, 1990.

[6] Liu, G.R., Tani, J. and Ohyoshi, T., Lamb Waves in a Functionally Gradient Material Plates and its Transient Response, Part 1: Theory, Part 2: Calculation Results Transactions of the Japan Society of Mechanical Engineers, Vol. 57(A), No. 535, 1991, pp. 603-611.

[7] Liu, G.R., Tani, J., Characteristics of Wave Propagation in Functionally Gradient Piezoelectric Material Plates and its Response Analysis, Part 1: Theory, Part 2: Calculation Results. Transactions of the Japan Society of Mechanical Engineers, Japan, Vol. 57(A), No. 541, 1991, pp. 2122-2133.