Tensile Testing - M117

Tensile Testing
Foundational knowledge method
Document Type	Method
Document Identifier	117
Relevant Class	Material
Tags	Method Mechanical properties Material characterization Outcome parameters

Introduction

This test is designed to find the in-plane properties of fibrous composites that have a balanced and symmetric layup with regards to the test direction. This page is based on the testing standard ASTM D3039: “Standard Test Method for Tensile Properties of Polymer Matrix Composite Materials” The test procedure uses a thin rectangular strip of composite material with a constant cross section (specimen), which is loaded into a test frame by grips clamping each end of the specimen. The load frame applies a constant strain in tension and records the force. This can then be used to determine the ultimate strength of the material. A strain gage may also be attached to the sample to determine the stress-strain response, to then find the elastic modulus, Poisson’s ratio, ultimate tensile strain and transition strain.

Below is an example of a tensile test being performed on unidirectional carbon specimen as seen in the webinar on Parameters for Structural Analysis of Composites.

For more information on test methods, see the following webinar: Composite materials engineering webinar session 12 - Testing

Example of a tensile test being performed on a unidirectional composite specimen at 0 degrees to the fiber orientation with multiple strain gages attached.

Test Specimen

Example of a Tensile Test with a Carbon Specimen Loaded into an 100kN Load Frame with Mechanical Grips

For tensile testing of homogenous materials, the test specimens are machined into a "dog bone" shape, where the top and bottom are wider. This is to concentrate the forces away from the grips into the region where gages are attached. This is, however, not feasible for unidirectional composites as axial splitting will occur along the fibers causing the sample to prematurely fail. To avoid this, composite tensile testing specimens need to be rectangular with an even cross section and/or have tabs bonded to the ends, see section on tabs below. For more on challenges with the composite tensile test, see CompositesWorld: Tensile Testing of Composites

The geometry of the specimens can be varied depending on the material being tested. There are very few mandatory requirements for the geometry, but there are some non-mandatory suggestions for dimensions. Below are two tables of the mandatory and suggested dimensional requirements.

Parameter	Requirement
Shape	Constant rectangular cross section
Minimum Length	Grip area + 2 x width + gage length
Width	As needed ±1 % of width
Thickness	As needed ±4 % of thickness
Flatness	Flat with light finger pressure

When determining the dimensions of a test specimen the purpose is to have the failure occur in the gage section of the specimen. There should also be a representative number of fibers in the cross section of the bulk material. The length of the specimen must be longer than the minimum set in table, but it is recommended to aim for significantly longer as this will minimize bending stresses if the grips are slightly misaligned or there is torsion in the specimen. The gages should be placed centrally away from the grips. The table below is a conglomeration of a number of test labs and their best practice that have yielded reliable results.

Parameter	0° Unidirectional	90° Unidirectional	Discontinuous or Balanced Layup
Width (mm)	15	25	25
Length (mm)	250	175	250
Thickness (mm)	1	2	2.5
Tab Length (mm)	56	25	Emery cloth
Tab Thickness (mm)	1.5	1.5	-
Tab bevel Angle (°)	7 - 90	90	-

Tabs

There are many different methods of clamping the test specimens to the load fixture and one of the options is to bond tabs onto the ends of the specimen. Tabs are usually made from E-glass and are used to transfer loads into the specimen and prevent premature failure. The tabs also prevent the specimen from being damaged by the grips as they are tightened.

The tab geometry and its success is dependent on the specimen material and fiber orientation, as well as the types of grips used in the test. For example, the bevel angle has an effect on the success of the test with different grips. Pressure-operated non-wedge grips show higher success with 90° edges, whilst wedge operated grips show success with low angle edges, 7 to 10°. It is important that the tab thickness is kept the same on both sides so that the specimen is positioned in the center of the grips.

To bond tabs onto the specimen it is recommended to use a high-elongation adhesive. The adhesive should have a minimal thickness and a uniform bond line.

Procedure

Firstly, measure the specimen cross sectional area, measure both the width and thickness in three areas and take the average. Secondly, ensure the load frame is calibrated.

The test is to be performed as a constant strain procedure, where the test is to be performed in a time window of 1 to 10 minutes. For a test setup with the ability to apply a constant strain, the rate should be 0.01 min-1. If this is not possible, then a constant crosshead speed of the load frame is recommended to be set to 2 mm/min. When loading the sample into the grips it is important that the specimen is aligned precisely with the grips and not loaded at an angle. Misalignment will drastically alter the outcome of the results as the material strength rapidly changes due to its anisotropic nature.

To validate the experiment, ensure that the failure has not occurred in the region of the grips or at any obvious flaw in the sample. It may be necessary to alter the way the specimen is gripped due to that all loads being transferred into the sample is done through the friction between the grips and the specimen. The tensile loading of the sample is presented though the shear force from the grip and specimen interface, which is equal to the clamping force times the coefficient of friction. It is not recommended to alter the surface of the specimen as it may damage it, but instead alter the surface of the grips to provide more friction or place emery cloth between the clamps and the specimen. It has also been suggested that the surface of the clamps may be coated in rubber or tungsten carbide spray in order to increase friction as well. Five repeat tests are generally recommended to get somewhat statistically significant results.

Calculations

Examples of Tensile Test Specimens with Woven Carbon Fabric with and without Tabs Attached

Below are the calculations that can be performed with the test results to find the material properties of the laminate.

To read more about how to predict the material properties of laminae and laminates see Micro-Mechanics and Macro-Mechanics.

Ultimate Tensile Strength

\[F^{\mathrm{tu}}=\frac {P^{\max } }{ A}\] \(F^{\mathrm{tu}}\) = Ultimate tensile strength (MPa)

\(P^{\max}\) = Maximum force before failure (N)

\(A\) = Average cross sectional area (\(mm^2\))

Tensile Strain

\[\varepsilon_i=\frac {\delta_i }{ L_g}\] \(\varepsilon_i\) = Tensile strain at \(i_{th}\) data point

\(\delta_i\) = Extensometer displacement at \(i_{th}\) data point

\(L_g\) = Extensometer gage length (mm)

Elastic Modulus

\[E=\frac{\Delta \sigma}{\Delta \varepsilon}\] \(E\) = Tensile modulus of elasticity (GPa)

\(\Delta \sigma\) = Differnece in applied tensile stress between two strain points (MPa)

\(\Delta \varepsilon\) = Difference between two strain points

Poisson's Ratio

\[v=-\frac{\Delta \varepsilon_t }{ \Delta \varepsilon_l}\] \(v\) = Poisson's ratio

\(\Delta \varepsilon_t\) = Difference in lateral strain between the two longitudinal strain points

\(\Delta \varepsilon_l\) = Difference between the two longitudinal strain points

Related pages

Page type	Links
Introduction to Composites Articles
Foundational Knowledge Articles	Basic Definitions of Material Properties - A211 Composite properties - A214 Macro-Mechanics - A271 Material properties - A150 Micro-Mechanics - A245 Polymer properties - A212 Reinforcement properties - A213
Foundational Knowledge Method Documents	How to measure curing time and degree of cure - M100 How to measure reinforcement content (and corresponding matrix content) - M109 Tensile Testing - M117
Foundational Knowledge Worked Examples
Systems Knowledge Articles
Systems Knowledge Method Documents
Systems Knowledge Worked Examples
Systems Catalogue Articles
Systems Catalogue Objects – Material
Systems Catalogue Objects – Shape
Systems Catalogue Objects – Tooling and consumables
Systems Catalogue Objects – Equipment
Practice Documents
Case Studies
Perspectives Articles	Effect of cure on mechanical properties of a composite (Part 1 of 2) - A319 Effect of cure on mechanical properties of a composite (Part 2 of 2) - A320 Parameters for Structural Analysis of Composites - A247 Resin Behaviour During Processing: What are the key resin properties to consider when developing a manufacturing process? - A257

References

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