Mechanical Characterization Techniques for Composite Materials
Partha Pratim Das and Vijay Chaudhary
Some of our new technologies need materials with specific combinations of properties that cannot be matched by traditional metal alloys, ceramics, and polymeric materials. This is especially true for applications in aerospace, under water, and in transportation. For example, aircraft engineers are increasingly looking for low-density, solid, robust, etc. materials. Combinations of material properties have been and still are being expanded through the production of composite materials. In general, any multiphase material that displays a large proportion of the properties of both constituent phases is considered to be a composite . The judicious combination of two or more distinct materials fashions properties according to this theory of joint action. The design of most composites requires trade-offs on properties.
Types of composite, including metal alloys, ceramics, and multiphase polymers, have already been discussed. For example, pearlitic steels have a microstructure composed of alternating layers of ferrite and cement. The ferrite phase is soft and ductile whereas the cement is hard and very fragile . The combined mechanical properties of the pearlite—reasonably high ductility and resistance—are superior to those of each of the constituent phases alone.
There are also composites that arise naturally. For example, wood is made up of strong and flexible cellulose fibers that are surrounded and kept together by a more rigid material called lignin. Similarly, bone is a mixture of strong but soft protein collagen and thin, hard mineral apatite. A composite is a multi-phase material that is artificially produced, as opposed to one that exists or is shaped naturally. In addition, the constituent phases must be chemically dissimilar and separated by another material .
In the manufacture of composite materials, scientists and engineers have ingeniously combined complex metals, ceramics, and polymers to create a new generation of extraordinary materials. Some composites were engineered to improve combinations of mechanical properties such as rigidity, resilience, and ambient and high temperature resistance . Many composite materials consist of only two phases: one is called the continuous matrix and covers the other, referred to as the scattered or reinforcing layer. The composite properties are a function of the properties of the constituent phases, their relative quantities, and the structure of the dispersed phase.
Mechanical Characterization Techniques
Characterization of composites using mechanical means is an important step before using the composites for various applications. Numerous experiments are conducted on a composite material to clarify its basic features (Figure 8.1) . The standard tests to study the mechanical behavior of the composites are friction tests, flexural tests, impact tests, and compression tests under different loading conditions.
The mechanical properties of materials are calculated by carefully planned laboratory experiments that reproduce the conditions of operation as near as possible. In real life, there are many factors affecting the way loads are applied to a material . Several specific examples of how to apply these loads are tensile, compressive, and shear, to name only a few'. Those properties are essential to mechanical design when choosing materials. The test is a destructive method using a specimen of traditional shape and dimensions (prepared in compliance w'ith D638/D3039 ASTM standards) .
FIGURE 8.1 Various mechanical characterization techniques 
ASTM D638 is usually used during the manufacture of composite materials, but when the strength of polymer is high then ASTM D3039 is required.
Stress is usually expressed in N/m2 or Pascal (1 N/m2 = 1 Pa). The stress value from the experiment is determined by dividing the amount of force (F) exerted by the device by its cross-sectional area (A) in the axial direction, which is measured before the experiment is carried out. Mathematically, it is expressed in Equation 8.1. Strain values that do not have units can be calculated using Equation 8.2. In the equation, L is the instantaneous length of the specimen and L0 its original length.
The stress-strain curve is characteristic of ductile metallic constituents. Another interesting aspect is that we usually speak about the “engineering stress-strain” curve . When a material approaches the stress-strain curve’s maximum, it will significantly reduce its cross-sectional area, a phenomenon known as necking. The computer program assumes when plotting the stress-strain curve that the cross-sectional area will remain constant during the experiment, even throughout the necking process, allowing the curve to slope downwards. The “real” stress-strain curve could be plotted by directly installing a gauge to calculate the change in the specimen’s cross- sectional area during the experiment.
When a force is released when the material is in its elastic zone, the material returns to its original form, the slope of the curve being a constant and an intrinsic property of a material, known as the elastic modulus, E (expressed in GPa). It gives a constant value and is given by:
FIGURE 8.2 Standard specimen drawn using AutoCAD software
A drawing of the standard specimen is shown in Figure 8.2. The higher the value of the youth module, the higher the stiffness value, as the structure would deform less at a higher stress value. This property contributes to a concept called material stiffness, which is an indicator of resistance to deformation under the elastic limit. The elasticity modulus is a very important property that is used in formulas that deal with beams and columns where stiffness is an important criterion.
In theory, even without calculating the specimen’s cross-sectional area during the tensile experiment, the real stress-strain curve could still be built by assuming the material volume remains the same. Using this definition. Equations 8.4 and 8.5 are used to measure both the true stress (oT) and the true pressure (eT). Within these equations, L0 refers to the specimen’s initial length, L the instant length, and у the instant tension .
A universal testing machine (UTM) is used for tensile testing.