Tensile tests are used to determine how materials will behave under tension load. In a simple tensile test, a sample is typically pulled to its breaking point to determine the ultimate tensile strength of the material. This blog post covers the frequently asked questions around tensile testing that help predict a material’s behavior.
What is the strength of a material?
In material science, strength refers to the ability of a structure to resist loads without failure because of excessive stress or deformation. When a specimen material is tested properties measured often include: ultimate tensile strength (UTS), offset yield strength, the point of fracture, as well as the percent elongation and reduction of area.
What are the principles of operation for an electromechanical and a hydraulic testing machine?
Electromechanical testing systems use an electric motor, gear reduction system, and one, two, or four screws for crosshead movement. Changes in the speed of the motor allows changing the speed of the crosshead and consequently the loading rate.
Electromechanical testing machine motor control system
Hydraulic testing machines use either a single or a dual-acting piston for crosshead movement. Manually operated hydraulic testing equipment relies on the operator adjusting the needle valve to control the rate of loading whereas hydraulic systems with closed loop controls have an electrically operated servo-valve for precise control.
Electromechanical testing machines are capable of a wide range of testing speed and long crosshead displacements. Hydraulic machines are often preferred for high capacity testing applications.
How does the loading rate affect tensile properties?
The rate at which a test is performed can have a significant effect on tensile properties. Materials such as plastics, polymers, steels exhibit tensile properties that are very sensitive to testing rates. Consequently, these types of materials have higher tensile strengths and lower elongation values at faster speeds.
Standardized test methods developed for these materials often specify a loading rate or a range within certain limits. In order to obtain accurate and repeatable results, it is critical to maintain the same loading rate when testing these materials.
We recommend keeping up with the recommended calibration schedules to ensure load and strain measuring devices provide accurate and repeatable results.
How does specimen alignment affect tensile properties?
When a specimen is properly aligned, the top and bottom test fixtures are in one straight line with the direction of force, the loading train, and with one another. In addition, the specimen is clamped between the top and bottom grip jaws at the same centerline.
On the contrary, specimen misalignment creates bending stresses and lower tensile stress readings. It may also cause the specimen to break outside the gauge section.
If the specimen is incorrectly machined, fracture may occur outside the gauge length and result in erroneous strain readings as well as incorrect stress measurements. Similarly, if the test equipment is out of alignment, data will be skewed and the recorded results will not be correct.
How does grip-specimen engagement affect tensile properties?
When a test specimen is fully engaged in grip jaw faces, it is supported inside the grip jaws, which should be correctly supported inside the grip pockets.
When a test specimen is fully engaged in the grip jaw faces but the jaws are not fully supported inside the grip pockets, the grip pockets may be permanently damaged resulting in specimen slippage.
When the test specimen is partially engaged in grip faces, even if grip faces are correctly supported inside the grip pockets, grip jaws may become permanently damaged leading to specimen slippage and incorrect results.
What is the correct way to measure small strains?
Measuring small strains -0.0001 inch or less- that are typical of materials such as high-strength metals, an extensometer should be used.
Tensile testing with extensometer per ASTM D638
If an extensometer is used but the yield values are still incorrect, the stress-strain diagram should be reviewed. The stress-strain curve below shows the extensometer slipping from the specimen. To help prevent extensometer slippage, make sure the clamping force and the zero point are checked regularly and the extensometer parts, such as fingers, arms, knife edges, are checked and replaced, as needed.
Behavior that can occur if the specimen slips in the grips and disturbs the extensometer as the force is applied (ASTM E8)