Important Aspects of the Flexure Test
Flexure tests, also called bending tests, are used to test or compare plastics, including their compounds. Flexure tests provide a reliable test method with a relatively simple test arrangement. They are used to determine the stress-strain behavior of a material in the range of low specimen strain.
The most common result is the flexural modulus, but yield points, maximum flexural stress or flexural strain at break can also be measured on low-ductility materials. The direct measurement of deflection using a displacement transducer presents the most precise form of measurement, reliably leading to true test results.
Why is accurate cross-section measurement so important when performing flexure tests?
Determination of the specimen dimensions, especially the specimen thickness, is of particular significance since the specimen thickness value has a quadratic effect in the calculation of the flexural stresses. A measurement error as small as 0.1 mm causes an error of approximately 5% in the calculation of flexural stress. An accurate cross-section measurement is therefore critical for reliable test results.
Why is it so important to pay close attention to proper alignment of supports, loading nose and specimen in flexure testing?
Poor alignment of the flexure test fixture is often displayed in the form of a non-linear beginning of the stress strain curve. This should be avoided by all means, since it leads to incorrect measurement of the flexural modulus.
For optimal alignment we have available tools. For example, with the appropriate adjustment gauge, both the support span and the alignment can be set quickly and reliably.
Does the material indentation from the loading nose and the supports play a role in the test results?
Indentation takes place at the support points and in the area of the loading nose(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 loading nose 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 loading nose is compensated.
What benefit does a displacement transducer provide when measuring deflection?
Direct measurement of the deflection using a displacement transducer attached centrally between the supports presents the most precise form of measurement, reliably leading to true test results.
This test arrangement is required for the Type 2 method according to ASTM D790, as well as for correct and precise measurements according to ISO 178. This applies if, for example, data sheets are to be created or if comparisons are to be made between different laboratories.
Alternatively, the standards provide scenarios for simple quality assurance tasks, for which crosshead travel measurement can be used.
What do I have to consider when working with a displacement transducer?
A very important factor in obtaining accurate and reliable test results is to ensure minimum influence of the displacement transducer on the test. ZwickRoell displacement transducers T15, T25 and T50 ensure reliable test results through secure attachment, precise axial alignment and tracking, and a small increase in contact force that does not affect the test process or the test results.
Deformation effects from the load frame and load cell must also be excluded. ZwickRoell displacement transducers prevent these influences by being mounted directly onto the flexure table.
These displacement transducers measure with high accuracy, independent of the test temperature. All temperature-related accuracy deviations are automatically compensated in every ZwickRoell testing machine.
What results are obtained from a flexure test on plastics?
The flexure test produces a stress-strain curve and different characteristic values including flexural modulus, yield point and if applicable, the point of break. The standards normally differentiate among curves a, b, and c and provide a suitable evaluation in each case.
A flexural modulus can be determined for all three curve types. According to ISO 178, the measurement is taken between the specified limits of 0.05 % and 0.25 % flexural strain. ASTM D790 defines the modulus measurement as secant (chord modulus) or as a tangent to the slope of the curve.
Additional results include the maximum flexural stress, the flexural stress at break, the flexural strain at break, the strain at maximum flexural stress and, if applicable, the flexural stress at the defined deflection limit.
What is the difference in stress and strain measurement when comparing between tensile and flexure tests?
Unlike in the tensile test, flexural stresses cannot simply be determined from the ratio between force and cross-sectional area. The deflection applied to the specimen generates bending moments and shear forces. The bending moment increases steadily between the support and the loading nose, while the shear forces in this range remain constant. In a three-point flexure test, the highest bending moment occurs directly under the loading nose. In a four-point flexure test, the bending moment is constant between the loading noses. This range remains free of shear stresses, which is a benefit that this method provides for materials with low shear strength.
