The tensile testing machine's test speed (i.e., the rate at which the specimen stretches when subjected to stress) is one of the key control parameters in material mechanical property testing. Because different materials exhibit varying mechanical response timelines (e.g., the elastic and plastic deformation rates of metals and the viscoelastic properties of polymers), changes in test speed can directly affect core test results such as tensile strength, elongation at break, and elastic modulus. The specific impact can be analyzed by material type and test parameter:
I. Impact on Material Strength Indices
Strength indicators (such as tensile strength and yield strength) reflect a material's ability to resist failure. Faster test speeds generally result in higher strength measurements. Test speeds affect strength measurements by altering the time it takes for stress transfer and deformation to coordinate within the material.
Metallic Materials
Plastic deformation in metals (such as steel and aluminum) relies on dislocation motion. Faster test speeds prevent dislocations from fully moving, forcing the material to withstand higher stresses before yielding or breaking, resulting in higher measured yield strength and tensile strength. For example, when testing Q235 steel at a speed of 5mm/min, the yield strength is approximately 235MPa. If the speed is increased to 50mm/min, the yield strength may increase to 245-250MPa, with a deviation of 4%-6%.
For brittle metals (such as cast iron), the impact of test speed on strength is relatively small due to minimal plastic deformation, and the deviation is generally controlled within 2%.
Polymer Materials (Plastics, Rubber)
Plastics (such as PP and ABS) exhibit "viscoelasticity." At low stretching speeds, the molecular chains have ample time to orient and slip, resulting in good plasticity and lower strength measurements. At high stretching speeds, the molecular chains have less time to adjust, leading to brittle fracture and a significant increase in the tensile strength measurement. For example, the tensile strength of PP plastic at a speed of 10mm/min is approximately 30MPa. At a speed of 100mm/min, the strength may increase to 35-38MPa, with a deviation exceeding 15%.
When rubber materials (such as natural rubber) are stretched at low speeds, the elastic network fully expands, resulting in stable strength values. However, when stretched at high speeds, localized stress concentrations are likely to occur within the rubber, resulting in inflated tensile and tear strength values, which may also mask the material's true fatigue properties.
II.Impact on Material Plasticity
Plasticity (such as elongation at break and reduction of area) reflects the material's ability to deform before fracture. Faster test speeds generally result in lower plasticity values. Test speed, by affecting the material's deformation sufficiency, negatively impacts plasticity measurement results:
Metallic Materials: When stretched at low speeds, metals undergo sufficient plastic deformation (e.g., significant necking), resulting in higher elongation values. When stretched at high speeds, plastic deformation is incomplete, necking is less pronounced, and elongation is lower. For example, the elongation at break of 6061 aluminum alloy is approximately 12% at a speed of 2 mm/min. When the speed is increased to 20 mm/min, the elongation may drop to 9%-10%, with a deviation of 17%-25%.
Polymer Materials: When plastics are stretched at low speeds, the molecular chains can fully orient along the stretching direction, resulting in a high elongation at break (for example, PP plastic has an elongation of approximately 300% at 5 mm/min). At high speeds, the molecular chains have no time to orient, causing the material to break rapidly. The elongation may drop sharply to 100%-150%, with a deviation exceeding 50%, or even "brittle fracture" (elongation close to zero).
Rubber Materials are extremely sensitive to speed: at low stretching speeds, rubber can stretch to 5-10 times its original length (elongation at break 500%-1000%). At high stretching speeds, elastic deformation accounts for a greater proportion, while plastic deformation decreases, potentially reducing elongation at break to 300%-400%, and resulting in a smoother fracture surface (no obvious signs of plastic deformation).
III.Impact on Material "Elastic Modulus"
Elastic modulus (reflecting a material's ability to resist elastic deformation) is an inherent property of the material. Test speed has a relatively small impact on it. However, excessively low or high speeds can still lead to deviations and should be kept within the standard range:
Metallic Materials: During the elastic deformation stage (stress not exceeding the yield point), material deformation is primarily due to adjustments in interatomic distances, resulting in a fast response. The impact of test speed on the elastic modulus is typically less than 1%. For example, the elastic modulus of 45 steel remains stable between 200 and 205 GPa within a speed range of 1-50 mm/min.
Polymer Materials: The elastic modulus is slightly more affected by speed. At low speeds, the material is prone to "creep" (elastic deformation followed by slow plastic deformation), resulting in lower elastic modulus values. At high speeds, creep is less pronounced, and the elastic modulus is closer to the true value. However, excessively high speeds may introduce "dynamic effects" (such as testing machine vibration), which can cause data fluctuations. For example, the elastic modulus of ABS plastic at 5 mm/min is approximately 2.5 GPa, and at 100 mm/min it is approximately 2.7 GPa, with a deviation of 8%.
IV. Impact on "Test Repeatability and Standard Conformity"
If multiple tests on the same material or tests conducted by different laboratories do not use a uniform speed, even if the material properties themselves are identical, the test results will vary due to the speed differences, making them incomparable.
For example, a company tests a batch of PET plastic. Laboratory A, using a speed of 5 mm/min, measures a tensile strength of 28 MPa and an elongation at break of 200%. Laboratory B, using a speed of 50 mm/min, measures a strength of 35 MPa and an elongation of 120%. The strength deviations are 25% and the elongation deviations are 40%. If the speed is not specified, the material properties can be mistakenly judged as unqualified.
Material testing standards in various countries (such as GB/T 228.1 Metal Tensile Standard and GB/T 1040 Plastic Tensile Standard) clearly define a test speed range: 0.005-50 mm/min for metals (depending on material thickness/diameter), and 5-500 mm/min for plastics (depending on material type). This standardization ensures repeatability and fairness of results.
V. Test Speed Selection Principles for Different Scenarios
Selection by Material Type
For metal materials, prioritize the standard's stress rate (rather than displacement rate). For example, for mild steel, a stress rate of 2-20 MPa/s should be used to ensure a smooth stress transition during the yield phase.
Polymer Materials: Selection should be based on the intended use. For example, for plastics used in high-speed stress scenarios (such as car bumpers), a higher speed (such as 100mm/min) can be selected to simulate actual stress. For materials used in low-speed stress scenarios (such as everyday plastic parts), a lower speed (such as 10mm/min) is more realistic.
Rubber Materials: Due to their sensitive viscoelastic properties, testing should strictly adhere to standard speeds (such as the 500mm/min specified in GB/T 528) to avoid speed fluctuations that could lead to biased results.
Select by Testing Purpose
Quality inspection: Select the "medium speed" specified in the standard (such as 5mm/min for metals and 20mm/min for plastics) to balance efficiency and accuracy.
Material R&D: Performance should be tested at multiple speeds (such as 1, 10, and 100mm/min), and a "speed-strength-plasticity" curve should be plotted to comprehensively evaluate the material's dynamic mechanical properties. Failure Analysis: Only by simulating the stress speeds experienced by a material in actual use (e.g., high-speed stretching during a car crash, or low-speed stretching of everyday items) can the cause of failure be accurately identified.
Summary: Test Speed Is a Key Variable for Accurate Results.
If you're looking for tensile testing machines or other material testing equipment, the SISCO online store offers a wide range of models, from benchtop to floor-standing models, to meet your testing needs for various materials, including metals, plastics, rubber, and composites.
The core functionality of a tensile testing machine and its coordinated control of test speed ensure that it's more than just a "loading tool." It's also a key device that simultaneously monitors "speed, deformation, and force" through its precise mechanical structure and intelligent control system. The correlation between its core functions and test speed is reflected in the following aspects:
Speed Control Accuracy: High-end testing machines utilize a servo motor and ball screw drive (such as the SISCO-LD series), achieving speed control accuracy of ±0.1mm/min. This ensures that the set value (e.g., 50mm/min) is consistent with the actual tensile rate, preventing strength/plasticity data drift caused by speed fluctuations.
Real-time Feedback Mechanism: Using a force sensor (0.5-level accuracy) and a displacement encoder (0.001mm resolution) to collect real-time data, software algorithms dynamically adjust the loading rate (especially during the yield phase) to ensure that the material deforms at the "target speed" rather than relying solely on mechanical inertia.
Multi-mode Adaptability: In addition to constant-speed tensile testing, it also supports stress rate control (e.g., 2-20MPa/s during the yield phase of metals). Adjustment), strain rate control (suitable for viscoelastic testing of polymer materials), users can flexibly switch modes based on material properties. The servo system of the tensile testing machine must have millisecond-level response capabilities to achieve precise control.
Expandable Design: Modern tensile testers often integrate modules such as environmental chambers (for high/low temperature/corrosion) and extensometers (for measuring local deformation). The test speed must be calibrated in conjunction with these expanded functions (for example, in high-temperature environments, material response accelerates, so the speed must be appropriately reduced to ensure data validity).
The test speed of a tensile strength testing machine is not a matter of "faster, better" or "slower, better." Instead, it must be determined comprehensively based on material properties, testing standards, and actual application scenarios. The core principle is that the speed must match the material's "mechanical response speed" to ensure that the test process truly reflects the material's stress-deformation behavior in actual use. Ignoring the influence of speed can produce results that deviate from actual performance, even with the highest equipment accuracy, and can even mislead production and R&D decisions.
