The Definitive Guide to ISO 178 Flexure Testing of Plastics

Due to their low cost, light weight, resistance to corrosion, and general versatility, plastics have established a significant presence in many different industries. It is important to determine the material properties of plastics in order to characterize their expected behavior in real-world applications. One important property is the relationship between stress and strain while a plastic sample is being bent or flexed: in other words, the flexural properties of the material. To regulate an internationally standardized method for determining this property, the International Organization for Standardization (ISO) developed ISO 178. 

This guide is designed to introduce you to the basic elements of an ISO 178 flexure test and will provide an overview of the testing equipment, specimens, and results involved in the standard. However, anyone planning to conduct testing compliant with ISO 178 should not consider this guide an adequate substitute for reading the full standard.

What Does it Measure? 

ISO 178 is a test method for determining the flexural properties of rigid and semi-rigid plastics by performing a three-point bend test on a universal testing system. A three-point bend test applies force at the midpoint of a rectangular specimen, which is freely supported at either end. The applied force is measured by a load cell, and the resulting deflection is measured by either the system’s crosshead displacement (with results corrected for system compliance) or by a direct strain measurement device. There are four test types outlined in ISO 178, each specifying a deflection measurement method (i.e. via crosshead or direct strain transducer) and an associated calibration accuracy requirement.

The type of ISO 178 test a lab performs is prescribed by the requirements of its customers, whether they are internal, such as an R&D lab, or external, such as a third party testing lab. It is important to ensure that the specific requirements for your lab are identified and agreed upon by your customers prior to purchasing test equipment and performing testing.

Is ISO 178 the Right Standard for You?

ISO 178 is very similar to ASTM D790, though it differs in several key points:

• ISO 178 requires the use of either a deflectometer or compliance correction in order to determine modulus. ASTM D790 allows modulus to be calculated by crosshead displacement alone.
• Preferred specimen sizes are different, and because test speed is dependent on specimen depth, test speeds between the standards may vary. The preferred depth for ISO 178 specimens is 4 mm while the preferred depth of ASTM D790 specimens is 3.2 mm. 
• ASTM D790 allows only one test speed, whereas ISO 178 allows a second (faster) test speed to be used after modulus is measured.

Specimens

ISO 178 specimens are either machined from sheets in accordance to ISO 2818 or prepared via compression or injection molding in accordance to ISO 293/295 or ISO 294-1/10724-1, respectively. The preferred specimen dimensions are specified as:

• Length: 80 mm (± 2)
• Width: 10 mm (± 0.2)
• Thickness: 4 mm (± 0.2)

If the preferred dimensions are not achievable, ISO 178 offers alternative widths dependent on the specimen’s thickness. If the material is anisotropic, both directions are to be tested.

Procedure

ISO 178 testing is usually performed on table model universal testing systems due to the relatively low force capacity requirement. The test machine must be able to maintain a constant test speed between 1 and 500 mm/min within tolerances specified in the standard. All models of Instron’s 3300 Series and 5900 Series testing machines meet the necessary speed accuracy requirements. The force measurement device, or load cell, must meet Class 1 of ISO 7500-1 over the range of forces to be reported. It is important to ensure that your lab is aware of the minimum and maximum test forces so that they select a load cell with an appropriate force capacity and verification range.

Extensometer

ISO 178 allows the use of different deflection measurement systems depending on the desired results and their associated accuracies. From least to greatest accuracy, the types of deflection measurement allowed are crosshead displacement, crosshead displacement with compliance correction, and use of a direct strain measurement device. Prior to the 2010 update of ISO 178, errors in deflection measurement accuracy could not exceed 1% of the value. However, since the 2010 update, operators must use a Class 1 deflectometer per ISO 9513, or use software capable of removing machine compliance from the results.

Whichever deflection accuracy your lab requires, Instron has a solution to meet it. All 3300 Series and 5900 Series test frames, equipped with Bluehill^® Universal software, are capable of measuring deflection from the crosshead with or without compliance correction. Instron also provides a range of solutions for direct strain measurement. One common solution is a clip-on extensometer mounted to a plunger, or deflectometer. The most advanced and highest throughput solutions include the AutoX750 and the Advanced Video Extensometer (AVE2). 

Similar to the system’s load cell, the deflection measuring device must also be verified over the operated range of the reported results. Flexural modulus, in particular, is calculated at exceptionally small deflections (under 0.1 mm). It is helpful to identify the minimum and maximum displacements of the test to be able to properly instruct the verification provider.

Fixtures

A three-point bend fixture is necessary for testing to ISO 178. This fixture consists of a loading, upper anvil mounted to the moving crosshead and a fixed, lower support beam with two adjustable anvils. Instron's 2810-400 flex fixture is fully compliant with the fixture requirements, and is adaptable for use with the 2.0 mm radius anvils for testing specimens up to 3.0 mm thick, or 5.0 mm radius anvils for thicker specimens. The support span requirement is based on a ratio of the specimen thickness, which varies depending on the rigidity of the material being tested. The adjustable anvils allow the operator to set the distance of the support span, and feature graduated length units on the support beam to allow for accurate positioning of the anvils. It also allows for easy centering of the deflectometer when using strain devices. Because specimen alignment can cause significant variation in results, proper care should be taken to ensure that the specimens are aligned consistently for each test. Instron bend fixtures also come with alignment arms that are adjustable for the width of the specimen.

Specimen Measurement

ISO 178 calls for width and thickness measurements to be taken at the center of the specimen’s length in accordance with ISO 16012. At least three measurements are taken, and their mean is recorded for both width and thickness. Instron’s Automatic Specimen Measuring Device (ASMD) feature in Bluehill Universal allows operators to connect up to two micrometers or measurement devices to the computer and have them input the mean of the measurements directly into the software. This eliminates the chances of operator input errors and increases efficiency.

Test Method

The span of the three-point bend fixture (distance between lower anvils) is configured as a function of the specimen’s thickness. The environmental conditions of the test - low temperature or high temperature, for example - are dependent on the material being tested and should be agreed upon by the lab and their customer. ISO 291 provides guidance in the absence of this information. Instron offers fully-integrated environmental chambers to accommodate non-ambient testing.

ISO 178 specifies Method A and Method B to set the crosshead’s speed. Method A uses a single speed throughout the entire test, whereas Method B uses one speed during the initial, modulus region and a second, higher speed for the remainder of the test. The constant crosshead speeds used are intended to estimate specified strain rates. Following a pre-load routine, the test specimen is deflected until break or 5% flexural strain, whichever occurs first. Force and deflection data are recorded throughout the test at a sufficient data acquisition rate.

When setting up the test, having an adequate pre-load is highly recommended to ensure accurate and consistent strain measurements. The amount of force that is applied to the specimen prior to starting the test directly impacts the repeatability of calculations.

Results

Below are a list of calculations and results that can be reported for ISO 178:

• Flexural stress – A function of the applied load, span, specimen width, and specimen thickness. This differs from tensile or compressive stress, which is force per unit area.
• Flexural strain – A function of the deflection, span, and specimen thickness. This differs from tensile or compressive strain, which is the change in gauge length over the original gauge length.
• Flexural modulus – A function of flexural stress and flexural strain between 0.05% and 0.25% flexural strain. A chord modulus can be fitted at those points on a flexural stress vs. flexural strain curve.
• Flexural strength – The maximum flexural stress obtained during a bend test.
• Flexural stress at break – The flexural stress when a specimen breaks. For some materials, the specimen breaks before a yield point in which case the flexural strength would equal the flexural stress at break.
• Flexural stress at 5% flexural strain – The flexural stress at the end of a test, for tests where the specimen does not break within 5% flexural strain.

Throughput

For labs looking to increase their throughput, several modifications can be made to the system setup. Automatic specimen measuring devices and specimen alignment devices both increase test efficiency by reducing the amount of manual input needed from the test operator. Fully automated test systems such as Instron's AT3 and AT6 are also available and are designed to incorporate specimen measurement, specimen loading, testing, and removal. These systems can run for hours without requiring any operator interaction. In addition, these systems help reduce variability due to human error.