ASTM D790 3-point flexure tests on plastics
The 3-point flexure test to ASTM D790 is one of the most important test methods for determining the flexural modulus, flexural strength and flexural stresses of plastics, fiber-reinforced materials and insulating materials.
In practice, however, measurement deviations often occur. With coordinated testing technology from ZwickRoell, test procedures that comply with standards, and automated data collection, these influencing factors can be significantly reduced.
Common sources of error Running a test and test equipment FAQ Downloads Request a consultation
ASTM D790: Key facts at a glance
- ASTM D790 describes a 3-point flexure test to determine the flexural properties of unreinforced and reinforced plastics, including high fiber-reinforced materials and electrical insulating materials. Typical test results according to ASTM D790 include the flexural modulus, stresses and strains at the yield point, the flexural strength and the flexural strain at break. The tests are used for rigid and semi-rigid plastics, however only up to a 5% strain limit.
- The main causes of deviating test results are often not the materials testing machine itself, but the specimen dimensions, the ratio of the support span to the specimen thickness, the alignment of the test arrangement, and the selection of test speed.
- Even a measurement error of 0.1 mm for a 3.2 mm specimen can lead to a deviation of more than 5% in the calculated flexural stress.
- ASTM D790 allows for various methods of deflection measurement. Their accuracy has a direct impact on modulus determination.
- Adjustable flexure test kit, precise displacement transducer and automated test sequences help reduce operator influence and scatter.
- The testXpert testing software supports with standard compliant parameterization, automated speed calculation and centralized evaluation of test data.
ASTM D790 in practice: Common sources of error and their effect on test results
The 3-point flexure test to ASTM D790 seems easy to run, but several influencing factors lead to increased scatter, incorrect modulus values or incomparable results in practice:
- Specimen measurement: Since the specimen thickness is included quadratically in the calculation of the flexural stress in ASTM D790, even small measurement deviations have a disproportionate effect on the result. Even a measurement error of 0.1 mm with a specimen thickness of 3.2 mm can skew the calculated flexural stress by more than 5% and thus affect the flexural strength and modulus values.
- Thickness-to-support ratio: If the standard-specified ratio of support span to specimen thickness is not observed, shear stresses increase, and there is a risk of shear fractures and inaccurate modulus values. This is especially true for fiber-reinforced materials with low shear strength.
- Alignment of the flexure test kit: Angular errors between the supports or between the supports and die result in point contact instead of line contact at the start of the test. As a result, the specimen is initially torsionally deformed, which becomes visible as the "curve foot" in the stress strain curve and can particularly affect the secant modulus and chord modulus.
- Deflection measurement: If the elasticity of the test arrangement is not corrected, machine deformations are included in the result. The actual material deformation is lower, flexural modulus are distorted, and different testing systems cannot be compared.
- Test speed: The strain rate specified in ASTM D790 must not be equated directly with the crosshead speed, but must be calculated from the specimen thickness, the span and method A or B. Errors in this conversion can skew modulus values, yield points, flexural stresses and the reproducibility of the results.
- Limitations of normative calculation models: The formulas in ASTM D790 only apply to small deflections. For larger deformations, geometric non-linearities can skew the stress and elongation calculation – therefore, ASTM D790 limits method B to a maximum of 5% flexural strain.
Performing the 3-point flexure test to ASTM D790 with ZwickRoell
Critical test steps and test solution from ZwickRoell:
ASTM D790 is used wherever the flexural properties of plastics or fiber-reinforced materials need to be reliably assessed and compared – from material development to quality control in series production to acceptance testing of injection molding materials or the evaluation of aging and process changes. Precisely because the results are compared in such different contexts, the appropriate equipment is needed for the crucial test steps. This is how ZwickRoell helps to avoid typical sources of error right from the start:
Selection of the load frame Specimen dimensions Alignment of the test arrangement Deflection measurement Determining the test speed Automation options testXpert testing software
How ASTM D790 testing is performed
In the 3-point flexure test to ASTM D790, a cuboid specimen is positioned on two supports and loaded by means of a centrally mounted die using a materials testing machine at a specified test speed. During the test, the force exerted by the die and the deflection of the specimen under the die are continuously measured. The flexural modulus, flexural strength, flexural stresses and flexural strains can be determined from the measured values as well as the specimen dimensions and the span.
Depending on the material, the test is performed to method A or B with different strain rates. The test ends when the specimen breaks or, at the latest, when the flexural strain reaches 5%. For reliable results, the specimen geometry, support ratio, test speed and deflection measurement must be coordinated according to the standard.
Selecting the load frame
The zwickiLine, ProLine and AllroundLine can be used to perform the 3-point flexure test to ASTM D790. Generally, a zwickiLine is already sufficient for pure flexure tests on plastics. However, the maximum test force is not the only factor that plays a role when selecting the materials testing machine. In practice, various plastics tests, such as tensile and flexure tests, are often to be performed on a common platform. In addition, the required degree of automation and the extensometer solution used or available influence the selection of system. ASTM D790 requires a force measurement accuracy of ±1% of the measured value. Due to the high measurement accuracy of the modular ZwickRoell universal testing machines, which is achieved starting from as little as 1/1000 of the measuring range, a 10 kN ProLine or AllroundLine, for example, can be used with a suitably dimensioned load cell for tensile and flexure tests.
Precise measurement of specimen dimensions
Since the specimen thickness has a direct influence on the calculated characteristic values, its exact measurement is one of the most important steps in the test. The direct transfer of measured values from micrometers or calipers to testXpert reduces transfer errors and improves the traceability of the results.
Precise alignment of the test arrangement
The ZwickRoell flexure test kit has adjustable supports with carefully designed distance gauges and additional tools for exact centering the specimens. This reduces angular errors and prevents typical distortions in modulus determination. This contributes to reproducible results, especially for modulus evaluations.
Appropriate measurement of deflection
According to ASTM D790, the deflection can be measured in the center between the supports either via the position of the crosshead (type I) or by measuring it directly with a separate displacement transducer system (type II), which is the most precise form of measurement and is essential for reproducibility between laboratories. The crosshead travel measurement is mainly used for simple quality assurance tasks.
- To measure deflection via the crosshead position according to type I, the testXpert testing software provides automatic correction of the inherent deformation of the test arrangement, in which the actual deformation curve of the test arrangement is determined and stored in the testing software.
- Thanks to the modularity of our testing systems, ZwickRoell has various type II displacement transducer options that meet these requirements and depend on the possibility of combining them with other testing methods: Mechanical displacement transducers for compression and flexure tests attached directly between the supports on the specimen or clip-on extensometers, macroXtens sensor arm extensometer, multiXtens or optical videoXtens systems, which can also be used for tensile tests.
Automatic calculation of test speed
Depending on the selected test method A or B and the specified strain rate, the testXpert test software automatically calculates the test speed to the standard and guides you through the test step by step, including consistent parameterization when switching between methods A and B.
Automation of ASTM D790 tests
Especially when there are large numbers of specimen, scatter often occurs due to manual process steps. Automated testing systems reduce operator influences, standardize testing procedures and increase reproducibility. At the same time, they relieve laboratory personnel of routine tasks. This allows employees to take on more complex tasks, such as interpreting test results, evaluating new materials or participating in other research activities.
For flexure tests according to ASTM D790, an automated solution may prove to be worthwhile starting at 10 specimens per day. Our ALEX testing system handles simple pick-and-place-tasks and offers cost-effective support in the testing laboratory. For higher testing volumes and more extensive requirements, the roboTest N, roboTest L and roboTest R systems from ZwickRoell offer an alternative.
The combination of different tests, for example tensile tests to ASTM D638 and flexure tests to ASTM D790, is possible by simple conversion or by including several test fixtures in one system.
Comparison of our automated testing systems for plastics Request a consultation
Looking to automate ASTM D790 testing?
Your local ZwickRoell expert can help you select the right level of automation for your application.
Request your no-obligation consultation today
ASTM D790: Efficient and reproducible with the testXpert testing software
The testXpert testing software supports the performance of ASTM D790 tests compliant to standards through standardized test templates and guided workflows, reduces operator errors and makes test sequences up to 30% faster:
Key functions:
- Standard-compliant test sequence with clear user guidance: The standard test program in testXpert contains all relevant ASTM D790 characteristic values and guides users through the test step by step. User management displays only the test steps required in each case – for fewer operator errors and standardized processes.
- Direct transfer of specimen dimensions from measuring equipment saves time and reduces errors.
- Centralized data storage and evaluation: With testXpert analytics, all test data – even beyond ASTM D790 – can be evaluated centrally, and characteristic values from different test series can be compared clearly.
- ERP and QA integration: Standard interfaces enable the fast, error-free transfer of test specifications and results between testXpert and your systems.
- With our Trend Analysis feature, you have the ability to recognize longterm deviations with a simple longterm evaluation option
Frequently asked questions about 3-point flexure test to ASTM D790
The 3-point flexure test to ASTM D790 is a traditional standardized characterization method for rigid and semi-rigid plastics. The flexural properties determined with these tests are of great importance to designers, engineers and material manufacturers to ensure that the plastic material used meets the requirements for its intended application.
Typical test results according to ASTM D790 are:
- Flexural modulus: The flexural modulus is a measure of the stiffness of the material and indicates how well it can resist flexural loads. The ASTM D790 standard provides three calculation methods for the flexural modulus, each of which leads to different results:
- Tangent Modulus (Modulus of Elasticity)
The ratio, within the elastic limit, of stress corresponding to strain - Secant Modulus
Ratio of stress to corresponding strain at any selected point on the stress-strain curve - Chord Modulus
Slope between two specified points on the stress-strain curve
- Tangent Modulus (Modulus of Elasticity)
- Flexural strength: Flexural strength provides information about the maximum load that the material can withstand in flexure before it breaks.
- Flexural stress at a specified flexural strain
- Flexural stresses and flexural strains at the yield point and at specimen break
The ZwickRoell flexure test kit enables tests up to maximum forces of 20 kN with a maximum deflection of 36 mm. The distance gauge can be used to set the span and centering stops according to the standards for both ISO 178 and ASTM D790. The supports can be adjusted in distance and angle and can be precisely centered and aligned with each other and with the die. The specimen stops are also positioned using the distance gauge. This facilitates precise and reproducible specimen positioning.
Both the ASTM standard and the ISO standard describe flexure tests on plastics. However, they differ in terms of range of application, specimen geometries, test speed and modulus evaluation. Therefore, the results are not directly reproducible.
The key difference between method A and method B according to ASTM D790 lies in the strain rate or test speed – and thus in how quickly the specimen is bent.
- Method A uses a lower strain rate and is the standard method for most plastics.
- Method B operates at a higher strain rate and is used when method A would take too long for certain materials or when no meaningful fracture/limit value is reached within the intended test conditions.
Test methods according to ASTM D790 and their influence on the test speed:
- For materials that break at small deflections, the preference is to use a strain rate of 0.01 min-1 according to method A.
- Materials that neither break nor show a yield point up to an elongation of 5 % are tested according to method B with a ten times higher strain rate of 0.1 min-1.
For the operator, this information means that the test speed to be set on the testing machine must be calculated on the basis of the specimen thickness and the support span, or the ratio of support span to specimen thickness. This function can be conveniently stored in the testXpert testing software and the calculation process can be automated.
For the standard specimen with 3.2 x 12.7 x 127 mm and a support ratio of 16, the test speed according to ASTM D790 is:
- To method A: 1.4 mm/min
- To method B: 13.7 mm/min
The results of flexure tests according to ISO 178 and ASTM D790 on plastics are typically similar in magnitude, but not directly comparable. The reasons for this are different strain rates in the flexure test, different calculation methods for the modulus value and differences that can arise during specimen production.
- The flexural modulus describes the stiffness of a material under flexural loads.
- The flexural strength indicates the maximum load that a material can withstand in flexure before it breaks.
Flexural stress is calculated according to ASTM D790 and in ISO 178 as the extreme fiber stress of the specimen in the center between the supports. The formula provided in the standard is based on an ideal load case. Angular errors or frictional forces at the supports, which result from the increasing deflection of the specimen during a flexure test, are not taken into account. The calculation is only sufficiently accurate for small bending angles.
To test plastics according to ASTM D790, the specimens are produced by injection molding or machined from sheets, plates or molded parts by mechanical processing.
- According to ASTM D790, test specimens with a cross-section of 3.2 mm x 12.7 mm and a length of 127 mm are usually used for plastic molding compounds. The specified thickness to support ratio of 16 leads to a support span of 51 mm. In addition, the standard defines a series of additional requirements for specimens with a thickness greater than 3.2 mm and less than 1.6 mm.
- To test laminated thermosets and sheet materials for electrical insulation, the specimens may have to be machined to a thickness of 25.4 mm or 12.7 mm. If the shear strength is very low compared to the tensile strength, the thickness-to-support ratio must be increased to 32 or even 40 to avoid shear fractures.
- Testing of high-strength fiber composites is carried out according to the tensile shear-strength ratio with a thickness-support distance ratio of 16, 32 or 40. For highly anisotropic materials, a ratio of 60 may even be required for correct modulus determination.
Extreme care is needed when measuring specimen dimensions for a flexure test. Since specimen thickness is calculated quadratically in the flexural stress, the measurement error result is also a quadratic function. A measurement error of only 0.1 mm with a specimen height of 3.2 mm (nominal) produces an error in flexural stress of more than 5%. For this reason, the thickness of the test specimens is measured using a micrometer that meets the requirements of ASTM D5947.
Angular errors between the supports, or between the supports and the die, lead to point contact between the specimen and the test tool at the start of the test. As the load increases, the specimen is deformed in torsion and linear support is achieved. These initial deformations are visible in the stress-strain diagram as a so-called curve foot, that is an increasing gradient in the initial measuring range. The values of the secant modulus and chord modulus can be significantly affected by this, while the tangent modulus according to ASTM D790 is less sensitive to this effect.
Indentation takes place at the support points and in the area of the die(s), which is dependent on the hardness of the material, the magnitude of the acting force and the radius of the loading nose and the support. If the deflection is measured by the movement of the dies compared to the supports, the indentation seems to increase the measured deflection. This can normally not be compensated by any compliance compensation. By using a centrally attached displacement transducer, the indentation of the die is compensated.
Downloads related to ASTM D790
- Product Information: 3-point Flexure Test Kits PDF 246 KB
- Industry Brochure: Plastics & Rubber PDF 9 MB
- Product information: zwickiLine Up to Fmax 5 kN PDF 820 KB
- Product Information: ProLine, Fmax 5-100 kN PDF 1 MB
- Product Information: AllroundLine, Fmax 5-100 kN PDF 1 MB
- Product information: videoXtens 2-150 HP PDF 1 MB
- Product Information: multiXtens II HP extensometer PDF 1 MB
- Product Information on makroXtens extensometer PDF 637 KB
- Product Information: Displacement transducer for compression and flexure tests PDF 1 MB
- Product Information: Clip-on extensometers PDF 525 KB
