How to Perform a Tensile Test on Composite Materials to ASTM D3039
Written by Ian McEnteggart
ASTM D3039 is a widely-used testing standard for determining the tensile properties of composite materials. Due to their lightweight properties and high tensile strength, composite materials are increasingly favored by the aerospace and automotive industries, and are being used to replace metals in many applications. Though there are many different types of composites, ASTM D3039 applies only to those that consist of a polymer matrix reinforced by high-modulus fibers. This guide is designed to introduce you to the basic elements of an ASTM D3039 tensile test, including an overview of the equipment, software, and samples needed. However, anyone planning to conduct ASTM D3039 testing should not consider this guide an adequate substitute for reading the full standard.
What Does it Measure?
ASTM D3039 testing is performed by applying a tensile force to a specimen (coupon) and measuring various properties of the specimen under stress. It is conducted on a universal testing machine (also called a tensile testing machine) and combines both inch-pound and SI units in a single standard. ASTM D3039 measures the following tensile properties:
- Tensile strength - the maximum stress applied during the test (usually the stress at break)
- Ultimate tensile strain - the strain at break
- Tensile modulus - how much the material can deform (stretch) in response to stress.
- Poisson's ratio: the ratio of the change in transverse to longitudinal strain between two longitudinal strain points (usually the same points as those used for the modulus determination i.e. 0.1 to 0.3%)
- Transition strain: in cases where the material shows a yield behavior (displayed as a slope change in stress-strain response), the transition strain is the strain value where the slope change occurs.
- Failure mode: Broken specimens must be examined and their failure type, area, and location must be recorded by means of a three character code.
Is ASTM D3039 the Right Standard for You?
ASTM D3039 is specific to plastics reinforced with particles or short fibers, and is one of the most basic tests used to characterize, qualify, and certify the tensile properties of these materials. However, a wide range of other tests are necessary in order to fully characterize the many different mechanical properties of anisotropic and heterogeneous composite materials. Anyone needing to test the compression properties of a composite should refer to ASTM D695. Those who require in-line shear testing of a composite should follow ASTM D3846. A pre-configured test method for D3039, along with many other composites test methods, is included in the Bluehill Universal Composites Application Module.
ASTM D3039 specimens are rectangular in shape with a constant cross-section. The minimum length of the specimen is equal to the total gripping length + 2 x width + gauge length, but greater lengths are recommended in order to minimize bending stresses. The specimen width and thickness should be representative of the bulk material in respect to quantity of fibers. There are four recommended specimen geometries for 0⁰ unidirectional, 90⁰ unidirectional, balanced, and symmetric and random-discontinuous material types. ASTM D3039 can be used for both continuous and discontinuous fiber reinforcements, but the lay-up of the laminate specimen must be balanced and symmetric with respect to the test direction.
It is common practice in composites testing to use tabs to protect the specimen material from being damaged by the grips. ASTM D3039 does not require tabs, but it does recommend their use when testing unidirectional materials. Tabs can either be bonded to the specimen or held in place by friction (e.g. emery cloth).
The specimen's cross-sectional area must be measured before testing – this is done by measuring the width and thickness at three places within the gauge length and averaging them. When one or both surfaces are irregular, a micrometer with a ball interface is required to measure thickness. If both surfaces are flat, then a micrometer with a ball or flat interface can be used. A micrometer or caliper with a flat anvil interface is required to measure the specimen width. The Automatic Specimen Measuring Device feature in Bluehill Universal allows operators to connect up to two devices (micrometers or calipers) to the computer and input the data directly into the software. This eliminates the chances of input errors and increases efficiency.
Materials Testing System
ASTM D3039 testing is performed on a table top or floor model universal testing machine. A 30 kN or 50 kN system may be sufficient for testing glass fiber composites, but a 100 or 250 kN system is needed for testing carbon fiber composites.
Grips and Specimen Alignment
The grips used to hold composite specimens must provide a sufficiently strong and even pressure to prevent the specimen from slipping during testing. The jaw face patterns should be suitable for the material and be in good condition. The alignment of the grips should limit the bending strain to within 3-5% at moderate strain levels (>1000µɛ).
Suitable grip designs for ASTM D3039 include the Instron® precision manual wedge grips (2716-028 / 2716-030) and hydraulic wedge grips (2742-401 / 2742-501). Both of these grips use a moving body design and incorporate specimen location stops to provide reliable gripping of composites and other materials while achieving the required alignment.
A number of different devices are available for measuring strain during the test. The most common are extensometers, which are available in a variety of different options depending on the needs of your laboratory. The simplest is a fixed gauge length 2630 clip-on extensometer to measure axial strain. An operator must clip this directly onto the specimen at the beginning of each test and remove it before the specimen breaks.
If testing for Poisson’s ratio, a transverse extensometer must also be added to measure the change in width throughout the elastic region of the specimen. A standalone transverse extensometer can be used to supplement an existing clip-on or automatic extensometer, or a biaxial device can be used to measure both axial and transverse strain simultaneously.
The AutoX750 automatically attaches to the specimen without interference by the test operator. This extensometer is useful in labs that have high throughput needs, as it eliminates the time-consuming need for manual manipulation by the operator and also provides more consistent placement on a large number of specimens. Consistent placement results in more repeatable modulus values.
Oftentimes, the composites being tested are ultimately destined to be used under non-ambient conditions. To simulate these end-use applications, ASTM D3039 is performed inside a temperature chamber where heating or cooling (LN2 or CO2) can be used. Strain gauges or clip-on extensometers can be used up to a maximum temperature of 200 °C. Alternatively, a non-contacting advanced video extensometer (AVE2) can be used. The AVE2 is mounted outside of the temperature chamber and uses a camera to track deformations in the specimen throughout the test, with the advantage that test operators do not need to open and close the chamber door during testing.
Electrical resistance strain gauges are also useful in determining strain during ASTM D3039 testing. Unlike extensometers, strain gauges are consumable items that can be used to measure strain at failure. These gauges typically consist of a thin metal foil grid which is bonded to the specimen with an adhesive. Strain gauges can be used in environmental conditions from cryogenic temperatures to over 200°C, but require conditioning in order to generate a useful electrical signal. An easy-to-use adapter is available for use with the standard electronics in an Instron test machine. Because composite materials do not substantially deform before failure, the extreme precision of a bonded strain gauge can sometimes be preferable to the use of an extensometer for ASTM D3039 testing.
Testing composite materials under non-ambient conditions is normally performed inside a temperature chamber. These chambers use forced air convection along with resistive heating elements to achieve high temperatures and liquid nitrogen or carbon dioxide cooling to achieve low temperatures. The Instron range of 3119-600 series environmental chambers provides extensive temperature testing capabilities for evaluating material properties under non-ambient testing conditions. A full range of complementary grips, pullrods, and extensometers is available.
ASTM D3039 requires that the test should produce a failure within 1 to 10 minutes. The suggested test speeds are either a strain rate of 0.01 min-1 or a constant crosshead speed of 2 mm/min (0.05 in/min). The general practice is to conduct the test at a constant crosshead speed.
Calculations and Results
When presenting test results, it is important to ensure that the terms are properly defined in order to ensure compliance with the standard and facilitate data comparison between different laboratories.
ASTM D3039 requires testers to record and report the mode and location of specimen failure. Bluehill software can be configured to present the operator with the recommended list of three-part failure codes once the test has been completed.
Tensile Stress/Tensile Strength/Ultimate Tensile Strain
The tensile stress is force / average specimen area and the tensile strength is the maximum force before failure / average specimen area. The ultimate tensile strain is the strain value at failure.
For materials that have a linear stress-strain response and demonstrate no change in the slope of the stress-strain curve, the modulus is defined as a chord modulus over a strain range of 1000 to 3000 µɛ (0.1 to 0.3%) strain. If the material's stress-strain response shows a slope change within the 1000 to 3000 µɛ (0.1 to 0.3%) range then a more suitable strain range should be used. Other definitions of elastic modulus may also be used.
Poisson’s ratio is normally determined by taking the ratio of the transverse to axial strain over the same axial strain range as that used to determine modulus, i.e. 1000 to 3000 µɛ (0.1 to 0.3%). Other strain ranges can be used, as can other definitions of Poisson's ratio.