How to Perform a Tensile Test on Composite Materials According to ISO 527-4
Written by Ian McEnteggart
ISO 527-4 is an international testing standard for determining the tensile properties of fiber-reinforced plastic composites. ISO 527-4 covers test conditions for isotropic and orthotropic non-unidirectional composites and refers to ISO 527-1. This guide is designed to introduce you to the basic elements of an ISO 527-4 tensile test, including an overview of the equipment, software, and samples needed. However, anyone planning to conduct testing to ISO 527-4 should not consider this guide an adequate substitute for reading the full standard.
What Does ISO 527-4 Measure?
An ISO 527-4 test is performed by applying a tensile force to a specimen and measuring various properties of the specimen under stress. it is conducted on a universal testing machine (also called a tensile testing machine) at a fixed crosshead speed which depends on the geometry of the specimen and test type chosen. ISO 527-4 measures the following tensile properties:
- Poisson's ratio - the ratio of the change in transverse to axial strain between 0.05% and 0.25% strain
- Modulus of elasticity - the slope of the specimen's stress/strain response, usually determined between 0.05% and 0.25% strain
- Tensile strain at tensile strength - the tensile strain corresponding to the tensile strength
- Tensile strength - the maximum stress applied during the test (usually the stress at break)
Is ISO 527-4 the Right Standard for You?
A wide range of different tests are required in order to fully characterize anisotropic and heterogeneous composite materials. ISO 527-4 is one of the most basic tests performed on composite materials and is widely used to assess, qualify, and certify their tensile properties. ISO 527-4 can be used for composites with both continuous and discontinuous fiber reinforcements, while ISO 527-5 is intended for testing unidirectional continuous-fiber composites. Plastics reinforced by particles or short fibers can be tested to ISO 527-2. Pre-configured test methods for all of these standards, along with many more, can be found in Instron's Bluehill® Universal testing software.
ISO 527-4 defines three types of test specimen. Type 1B specimens are a molded “dog-bone” type suitable for testing short fiber reinforced materials with a thermoplastic matrix. Type 2 specimens are rectangular with a constant cross-section and are suitable for testing continuous fiber materials with thermoset and thermoplastic matrices. Type 3 specimens are similar to Type 2 specimens with the addition of end tabs, which are common in composite testing in order to prevent the specimen from being damaged by the grips.
All specimens must be measured before testing in accordance with ISO 16012 or ISO 23529. Most typical micrometers should be suitable for performing these measurements. In order for the test system to display stress measurements rather than just force measurements, operators will be asked to input the cross-sectional area (thickness and width) of the specimen, because stress is calculated by dividing the applied force by the specimen's cross-sectional area (this is shown in units of Psi, Pa, kPa, GPa, etc). While the thickness and width of rigid specimens require different measurement accuracies, it is common to use the same measuring device for both. Either cylindrical or rectangular micrometer tips can be used, assuming they meet the dimensions required by ISO 16012.
The Automatic Specimen Measuring Device feature in Bluehill Universal allows operators to connect up to two devices (micrometers or calipers) to the testing software. Measurement data will be uploaded directly into the testing software, eliminating the chance of input errors and increasing test efficiency.
Materials Testing System
ISO 527-4 may be performed on a table top or floor model universal testing machine. A 100 or 250 kN system is usually required when testing carbon fiber composites, though a 30 kN or 50 kN table model system may be sufficient for testing glass fiber composites.
A number of different devices are available for measuring strain during ISO 527-4 testing. When used with suitable instrumentation, bonded electrical resistance strain gauges provide a method of determining the strain below the active area (typically a few mm²) of the gauge. Strain gauges usually consist of a thin metal foil grid on an organic backing, and can be used for testing at a wide range of temperatures from cryogenic to over 200°C. Strain gauges require conditioning in order to generate a useful electrical signal, and an easy-to-use adapter is available with the standard electronics of an Instron test machine.
A range of extensometer options are also available depending on the needs of your laboratory. The simplest type is a clip-on device which must be clipped directly onto the specimen at the beginning of each test and removed before the specimen breaks. The fixed gauge length 2630 series clip-on extensometer is suitable for the measurement of axial strain. A biaxial clip-on extensometer is available for tests that require the measurement of both axial and transverse strain for the determination of Poisson’s ratio.
The AutoX750 is an extensometer that automatically attaches to the specimen without interference by the test operator. This is useful in labs with 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.
Composite materials are often destined for use in the aerospace industry and therefore must be resilient to extreme temperature conditions. To simulate these conditions, ISO 527-4 tests can be performed inside a temperature chamber where LN2 or CO2 are used to either heat or cool the chamber to application-specific temperatures. Strain gauges or clip-on extensometers can be used up to a maximum temperature of 200°C. A non-contacting Advanced Video Extensometer (AVE2) can also be used, which has the advantage of being placed outside of the temperature chamber. The AVE2 does not physically touch the specimen and therefore does not need to be removed from the specimen before failure. This eliminates the need for the test operator to open and close the chamber door during testing.
Temperature chambers use forced air convection and 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.
ISO 527-4 tests are conducted at a constant crosshead speed. The prescribed test speed depends on the specimen type and the type of testing being performed. For example, tests being done for routine quality control can be run at faster speeds (5 and 10 mm/min) than those being used for qualification (2 mm/min).
Calculations and Results
When presenting test results, it is important to ensure that the measurement terms are properly defined in order to ensure compliance with ISO 527-4 and facilitate data comparison between different laboratories.
Tensile Stress/Tensile Strength
Tensile stress is determined by dividing the force at a given point by the average specimen area. The tensile strength of a specimen is the maximum tensile stress achieved before failure.
Tensile Strain/Tensile Strain at Failure
Tensile strain is the engineering strain measured by the strain measuring device (strain gauge or extensometer). The tensile failure strain is the strain corresponding to the tensile strength.
Modulus is defined as the slope of the stress-strain response between 0.05% and 0.25% strain unless alternative values are given in the material specification. Because the modulus calculation starts at 0.05% strain, it is extremely important that appropriate pre-stresses are applied to the material to remove any slack or compressive forces induced from gripping the specimen. It shall not exceed 0.05% strain or 1% of the tensile strength of the material.
Poisson’s ratio is determined by taking the ratio of the change in transverse strain to the change in axial strain over the same axial strain range used to determine modulus (0.05% to 0.25%).