The Definitive Guide to ISO 178

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 3400 Series and 6800 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.

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.

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.

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.

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.