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Sep 2, 20225 min read

Mechanical Testing of Composites

Updated: Sep 16, 2022

Contents

Overview

Compared to traditional structural materials, polymer matrix composites offer greatly enhanced performance and flexibility. However, these benefits come at the cost of increased material complexity, and it is easy to overlook the challenges of producing high-quality test data to support the needs of materials development, design, and quality control.

This article below covers the unique nature of composite material briefly before diving into the individual composite testing methods, highlighting the process and outcome. Summarizing the details with key aspects to keep in mind.


Introduction

Composite materials are being used in an ever-increasing variety of products and applications, as more and more industries realize the benefits that these materials offer. As the demands for lightweight composite structures for aerospace, ground transportation, and environmentally sustainable energy systems develop, so do the mechanical testing requirements for composite materials, components, and structures. Full characterization of the properties of anisotropic and inhomogeneous composite materials, for use in demanding structural applications, requires a wide range of mechanical tests. Determination of bulk properties requires tension, compression, and shear tests.


Why does each configuration require a new test?

Basic characterizing and evaluation of any of the materials is defined by mechanical testing. But characterizing composites involves various key defining parameters like Fibres, matrix phases, volume fractions, orientations, and direction of the load applied. Mechanical properties of composite materials like strength, stiffness and other properties are influenced by the orientation of the fiber, the concentration of the fiber, and the matrix. These materials exhibit different mechanical behavior in different directions which is dissimilar to metals.


Stress-Strain curve in composites

A general study of the stress-strain behavior of the composite considers the fiber is aligned in the longitudinal direction of the stress-strain curve of the composite in the Figure 1 below

Figure 1: Stress-strain for Fiber, Matrix, Composite [1]
Figure-1: Stress-strain for Fiber, Matrix, Composite [1]

From the Stress-stress Figure-1 failure of the fibers and matrix defines that fibers are brittle, and a matrix is ductile. Where the composite stress curve is discussed in two stages. In stage I, Fibre and matrix are deformed in elasticity In stage II fiber elongates, and the matrix yields and deforms plastically, and failure happens exactly concerning the strain(ef)of the fiber. Since fibers are the only load-carrying member in the composite materials the failure will depend on the type of fiber.


Composites Test Standards

Based on the applications and structural requirements leads to various mechanical test requirements. Mostly for evaluation of quality and mechanical failure behavior materials are defined by Tensile strength, compressive strength, flexural strength, and shear strength. These experimental results play key input parameters for Finite element analysis of the composite materials.

Composite materials mechanical tests are carried out ASTM, ISO, and DIN standards

  • Tensile Test - ASTM D3039

  • Compression test - ASTM D3410

  • Flexural test - ASTM D7264

  • In-plane shear test - ASTM D3418

  • Interlaminar shear stress test -ASTM D2344

  • Fatigue testing - ASTM D3479 (Tension-Tension)


Tensile test -ASTM D3039

This test determines the properties like tensile strength, young’s modulus, and poisons ratio of the composite materials. This test can be carried out in two directions longitudinal direction and transverse direction. A Unidirectional fiber specimen geometry for testing in the longitudinal and transverse direction as per ASTM D3039 is shown in Figure 2.

Figure 2: Tensile test specimen of unidirectional fiber in a longitudinal direction (a) and transverse direction (b) [2]
Figure 2: Tensile test specimen of unidirectional fiber in a longitudinal direction (a) and transverse direction (b) [2]

Specimen geometry like overall length, gauge length, width, and thickness play an important factor in tensile properties. Specimens’ geometry mainly depends on the fiber direction. In specimen end tabs are bonded the main function of the end tabs is to protect the specimen from load transfer from the grips tensile testing machine. Another specimen geometry is mentioned in Table 1. To determine the Poisson ratio of the material, strain gauges or extensometers are used while testing. From the testing data for stress-strain cure young’s modulus is calculated.

Table 1:Speciemen geometry based upon fiber orientation [3]
Table 1:Speciemen geometry based upon fiber orientation [3]

Out comes the test

  1. Tensile strength

  2. Tensile chord modulus of elasticity

  3. Tensile strain (%)

  4. Poisson’s ratio (Consider strain gauges or extensometer while testing)

Compression test-ASTM D3410

This test determines the properties like the compressive strength of the materials. This test is most compilated in the testing greatest efforts have been expended to design the test fixture to attain a pure compression without introducing the buckling of the specimen while testing. This is a test carried out as per ASTM D3410. This test can be carried out in two directions longitudinal direction and transverse direction

In this specimen geometry, there is a correlation between thickness and the gauge length of the specimen based on the gauge length consideration the thickness of the specimen is considered. The required specimen geometry of the specimen is shown in Figure 3.

Figure 3: Compression test specimen geometry [3]
Figure 3: Compression test specimen geometry [3]

Out comes the test:

  1. Compressive strength

  2. Compressive Poisson’s Ratio(Consider strain while testing)

  3. Compressive chord modulus of elasticity

Flexural test ASTM D7264

This test was carried out to determine the flexural properties of the composites. Generally, this test is also called a three-point and four-point bending test of the composite beam. This specimen is geometry in main depending on the span length and the thickness(l/t) of the specimen, span length to thickness is 32:1 as per the standard. Since the specimen is considered as simply a supported beam, for supporting span is considered 20% extra from the span length. Loading nose radius and fixes support have to be constant to give cylindrical contact to the specimen. Schematic representation of the Three-point and four-point bending test as shown following figure


(a) Three point bending test


(b) Four-point bending test
(b) Four-point bending test

Out comes the test:

  1. Flexural Strength

  2. Flexural Chord Modulus of Elasticity

In-plane shear test ASTM D3518

In-plane shear test is carried out to determine the shearing behavior of the laminate in the same plane. This test and specimen geometry were tested with reference to the Tensile test ASTM D3039. But the specimen has to manufacture in +45°/-45 fiber orientation as shown in the figure with a symmetric layup.

Figure 5: Specimen representation of in-plane shear test
Figure 5: Specimen representation of in-plane shear test

Out comes the test:

  1. Shear Strength

  2. Shear Strain (strain gauges are considered while testing )

  3. Shear Modulus of Elasticity

Interlaminar shear test-ASTM D2344

This test is carried out to determine the shear strength between the layer under the shear force acting parallel to the layer. This test is also called as short beam shear test.

Specimen geometry

Width: Thickness x 2

Length: Thickness x 6



Figure 6: Schematic representation of Interlaminar shear test[4]


Out comes the test:

  1. Shear Beam Strength

Fatigue testing

Composite fatigue testing varies from fatigue testing of metals because composite fibers are oriented, which means the fatigue properties depend on direction, layup, and failure mode.


Generally, fatigue properties are not a critical consideration for fiber-dominated materials, as fatigue cracks arrest when reaching the next fiber. However, certain highly fatigued parts or structures, like rotor blades or propellers, are an exception. For these components, composite fatigue testing may be performed to better understand the fatigue life.


Figure: Tension tension Fatigue testing specimen
Figure: Tension tension Fatigue testing specimen

Methods of composite fatigue testing

Element offers different test methods for evaluating the fatigue properties of composite materials, including:

  • Tension-tension

  • Tension-compression

  • Compression-compression

  • Bending fatigue

  • Fatigue crack growth

Summary

The mechanical testing of composite materials is complex, involving a range of test types, a plethora of standards, and the need to condition and test in a variety of different environments. Life is made easier by well-aligned test machines and grips, interchangeable test fixtures, and test software with pre-configured test methods.

References:

  1. Callister, William D. Materials Science and Engineering: An Introduction. New York: John Wiley & Sons, 2016.

  2. Fikry, M. J., Shinji Ogihara, and Vladimir Vinogradov. "The effect of matrix cracking on mechanical properties in FRP laminates." Mechanics of Advanced Materials and Modern Processes 4.1 (2018): 1-16.

  3. Gibson, R.F. (2016). Principles of Composite Material Mechanics (4th ed.). CRC Press. https://doi.org/10.1201/b19626

  4. ILSS https://dokumen.tips/documents/norme-astm-d-2344-d-2344m-01pdf.html

  5. On the tension–tension fatigue behaviour of a carbon reinforced thermoplastic part I: Limitations of the ASTM D3039/D3479 standard


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