BEND AND FLEXURAL TESTING
Bend testing, sometimes called flexure testing or transverse beam testing, measures the behavior of materials subjected to simple beam loading. It is commonly performed on relatively flexible materials such as polymers, wood, and composites. At its most basic level, a bend test is performed on a universal testing machine by placing a specimen on two support anvils and bending it through applied force on one or two loading anvils in order to measure its properties.
Why Perform a Bend/Flex Test?
Engineers often want to understand various aspects of a material’s behavior, but a simple uniaxial tensile or compression test may not provide all necessary information. As the specimen bends or flexes, it is subjected to a complex combination of forces including tension, compression, and shear. For this reason, bend testing is commonly used to evaluate the reaction of materials to realistic loading situations.
Flexural test data can be particularly useful when a material is to be used as a support structure. For example, a plastic chair needs to give support in many directions. While the legs are in compression when in use, the seat will need to withstand flexural forces applied from the person seated. Not only do manufacturers want to provide a product that can hold expected loads, but the material also needs to return to its original shape if any bending occurs.Types of Bend/Flex Tests
FLEX / BEND TESTING MACHINE
Components and Parts
Bend tests are commonly performed on universal testing machines. These systems consist of a test frame that is equipped with a load cell, testing software, and application-specific grips and accessories, such as extensometers. The type of material being tested will determine the type of accessories needed, and a single machine can be adapted to test any material within its force range simply by changing the fixturing.
|Flex Test Setup
Universal testing machine load frames come in single or dual column configurations and are available in force capacities up to 2,000 kN.
Testing software allows users to configure test methods and output results.
The load cell is a transducer that measures the force applied to the test specimen. Instron load cells are accurate down to 1/1000 of load cell capacity.
Flex testing requires upper and lower anvils to apply force to key points of the specimen. The number of anvils is determined by the test type being performed.
Universal testing machines are available in a variety of different sizes and force capacities ranging from 0.02 N to 2,000 kN. Most low force testing is performed on an electromechanical single-column or dual-column tabletop machine, while higher force applications require floor model frames. Instron's 6800 Series systems are available in capacity ranges up to 300 kN and can perform a wide range of different test types, including tensile, compression, bend, peel, tear, shear, friction, torsion, puncture, and more. Instron's Industrial Series servohydraulic systems are designed for even higher capacity testing of high strength metals, alloys, and advanced composites, and the ElectroPuls Series is designed for dynamic fatigue testing.
For Testing Plastics, Metals, Alloys, Composites, Microelectronics, and Components
FLEXURAL TESTING DATA ANALYSIS
Understanding the Mechanical Properties of Materials
Flexural tests are typically performed to ISO, ASTM, or other recognized standards, which will prescribe variables such as the required test speed and specimen dimensions. Specimens are generally rigid and can be made of various materials such as plastic, metal, wood, and ceramics. The most common shapes are rectangular bars and cylindrical-shaped specimens.
A flex test produces tensile stress in the convex side of the specimen and compression stress in the concave side. This creates an area of shear stress along the midline. To ensure that primary failure comes from tensile or compression stress, the shear stress must be minimized by controlling the span to depth ratio: the length of the outer span divided by the height (depth) of the specimen. For most materials, S/d=16 is acceptable. Some materials require S/d=32 to 64 to keep the shear stress low enough.
Maximum fiber stress and maximum strain are calculated for increments of load. Results are plotted on a stress-strain diagram. Flexural strength is defined as the maximum stress in the outermost fiber. This is calculated at the surface of the specimen on the convex or tension side. Flexural modulus is calculated from the slope of the stress vs. deflection curve. If the curve has no linear region, a secant line is fitted to the curve to determine slope.
Calculated values such as maximum force and maximum extension can be recorded just like a normal tension or compression test based on load cell and extension readings. Stress and strain values are calculated differently, as they incorporate the flex fixture support span and loading span (for 4-point bend testing). It is just as important to record these measurements as it is to properly record the specimen’s dimensions. Once these values are entered into Bluehill Universal, calculations such as flexural modulus are automatically calculated when requested.
Wood and Composites
Wood and composites are most commonly tested with the 4-point bend test. The 4-point test requires a deflectometer to accurately measure specimen deflection at the center of the support span. Test results include flexural strength and flexural modulus.
When a 3-point bend test is done on a brittle material like ceramic or concrete, flexural strength is often called modulus of rupture (MOR). This test provides flex strength data only, not stiffness (modulus). The 4-point test can also be used on brittle materials, though alignment of the support and loading anvils is critical in these cases, and the test fixture for these materials usually has self-aligning anvils.
BEND / FLEX TESTING STANDARDS
Standards for Testing Plastics, Elastomers, and Metals
Most testing is performed to established standards published by organizations such as ASTM and ISO. These standards prescribe acceptable test parameters and results for different types of raw materials such as metals, plastics, elastomers, textiles, and composites, as well as for finished products such as medical devices, automotive parts, and consumer electronics. These standards ensure that materials and products entering the supply chain display predictable mechanical properties and are not likely to fail in their expected end use. Since the cost and safety implications of product failure cannot be overstated, companies are encouraged to invest in high-quality, accurate testing equipment that is designed to help them easily determine whether or not their products meet applicable standards.
- ASTM C1550 | Flexural Toughness of Fiber Reinforced Concrete
- ASTM C1609 | Flex Testing of Fiber Reinforced Concrete
- ASTM C880 | Flexural Strength of Dimension Stone
- ASTM C99 | Modulus of Rupture of Dimension Stone
- ASTM D143 | Flexural Properties of Wood
- ASTM D6272 | Flexural Properties of Plastics and Electrical Insulating Materials
- ASTM D790 | Flexural Testing of Plastics
- ASTM E190 | Guided Bend Testing of Welds
- ASTM E290 | Bend Testing of Material for Ductility
- ASTM F2606 | Three-Point Bending Balloon Expandable Vascular Stents and Stent Systems
- EN 12089 | Determining the Bend Behavior of Thermal Insulation Products