Bend testing determines a material’s flexural properties, which are critical for quality control and specifications purposes. Properties measured include flexural stress, flexural strength, flexural offset yield strength, flexural strain, strain at a given strain, Tangent Modulus of Elasticity, Secant Modulus, and Chord Modulus.
During a bend test, a test specimen rests on two supports and is loaded by means of one or multiple loading noses. The stress-strain or load-deflection curves obtained from a bending test provide useful information on a material in addition to equations that can be used to calculate its flexural property values. Some materials give a load deflection curve with a break point and no yield. Other materials exhibit both a yield and a break point which can be visualized on a yield deflection curve.
Definitions
Flexural strength is the maximum bending stress that the test specimen can sustain before it yields. Flexural offset yield strength, which may be a different value than the flexural strength, is the stress at which the stress-strain curve deviates by a given strain (offset) from the tangent to the initial straight line portion of the curve.
Flexural strain is the nominal fractional change in the length of an element of the outer surface of the test specimen at midspan, where the maximum strain occurs.
Definitions and table can be found on ASTM D790 Standard Test Methods for Flexural Properties of Unreinforced and Reinforced Plastics and Electrical Insulating Materials.
Tangent Modulus of Elasticity is the ratio of stress to the corresponding strain value within the elastic limit of the material.
Secant Modulus is the ratio of stress to the corresponding strain at any point on the stress-strain curve. The point selected is chosen at a pre-defined stress or strain value in accordance with the appropriate material specification.
Chord Modulus is calculated from two discrete points on the load deflection curve that are chosen at two pre-defined stress or strain values.
Definitions and table can be found on ASTM D790 Standard Test Methods for Flexural Properties of Unreinforced and Reinforced Plastics and Electrical Insulating Materials.
Bend Test Setup
A typical bend test setup includes a testing machine capable of running tests at constant rates of crosshead motion and a bend fixture. Picture to the left is from Wikipedia, three-point flexural test article. The deflection may be measured and calculated utilizing the crosshead position or a deflectometer. Values obtained from tests utilizing the crosshead position and the deflectometer may vary.
Depending on the test procedure followed, the bend fixture may be a three-point or a four-point configuration. The three-point configuration has one loading nose whereas the four-point configuration has two top loading noses that apply the force to the specimen. The bottom part of the bend fixture with the two supports where the test specimen rests on is mounted on the fixed member of the testing machine. The loading nose is mounted to the movable member of the testing machine that applies the force.
The span length of the bend fixture, or the support span, must be selected based on the specimen dimensions. Selection of the support span compared to the depth of the specimen, or the support span-to-depth ratio should be done carefully as with large support-span-to-depth ratios, significant end forces will develop at the support noses. This affects the moment in a simple supported beam specimen, thus the calculated flexural values. Support and load spans are shown on the right, based on ASTM specifications.
In addition, selection of the right loading nose diameter and width is important as excessive indentation, or failure due to stress concentration directly under the loading noses may occur while force is applied to the specimen. Â Loading noses and supports that come in contact with the specimen may be cylindrical, pointed, or, flat, depending on the specimen and the test method being followed. Click here for more information.
Bend Test Standards
Common bend test method for plastics include ASTM D790 Standard Test Methods for Flexural Properties of Unreinforced and Reinforced Plastics and Electrical Insulating Materials.
Bend Testing Machines
Common bend (flexural) test machines include Instron, ADMET, MTS, and Tinius Olsen testing systems.
