As the core equipment for material mechanical property testing, tensile testing machine is widely used in quality inspection and R&D in many industries such as metals, plastics, rubber, textiles, and building materials. Proper operation is fundamental to ensuring the accuracy of your test data and the longevity of your instrument. However, in daily use, some operational misunderstandings stemming from habit or lack of cognition still occur from time to time. This article aims to analyze three common misunderstandings and provide professional advice based on relevant technical standards and practical experience.
Myth 1: Specimen clamping and centering operations are arbitrary
Many operators believe that the test can be started by simply placing the specimen in the fixture and clamping it. In fact, the clamping state and neutrality of the specimen are key preliminary steps that affect the test results, especially the fracture position and strength values.
Incorrect clamping may cause: 1) the specimen breaks in or near the jaws in advance, invalidating the test results; 2) Generate additional flexural stress, causing the measured tensile strength value to deviate from the true value. According to the principle of material mechanics, the ideal tensile should be in a unidirectional stress state, and its stress calculation formula is σ = F/A, where σ is the engineering stress, F is the real-time load, and A is the original cross-sectional area of the specimen. Poor alignment will introduce non-axial force components, making the prerequisites of the above formula untenable.
Professional advice:First, the fixture type and jaw face that match the specimen should be selected according to the standards (such as GB/T 228.1, ISO 6892-1 for metal materials). Secondly, the position of the specimen must be carefully adjusted to ensure that its longitudinal axis coincides with the force axis of the testing machine. For rectangular or flat specimens, use auxiliary tools such as alignment blocks. Before formal loading, a small preload (usually no more than 1% of the expected maximum force) can be applied, and the force value can be observed to show whether it is stable or not to preliminarily judge the alignment situation.
Myth 2: The test speed setting ignores the standard and material properties
"Set a faster speed to save time" or "All materials at the same speed" are common misconceptions. The test speed, i.e., the displacement rate or stress rate of the beam, directly affects the shape of the stress-strain curve and the measured performance indicators (such as yield strength, elastic modulus) of the material.
For plastic materials, too fast a rate may make the measured yield strength too high. For viscoelastic materials such as polymers, the rate effect is more significant. The relevant standards specify clear rate control modes (stress control, strain control) and scopes for different materials. For example, room temperature testing of metal materials is typically performed within a specified stress rate range or at a strain rate based on gauge length.
Professional advice:Before operation, the technical standards on which the test is based must be clarified, and the settings must be strictly in accordance with the rate requirements specified in the standards. If no specific criteria are available, the rate values used and their control methods should be clearly indicated in the report. Understanding the impact of rate on test results is fundamental to conducting comparability testing and analysis of results.
Myth 3: Neglecting daily calibration and system inspection
Treating tensile testing machines as "ready-to-use" equipment, relying solely on annual external measurements, and ignoring daily or pre-use rapid verification is a potential risk point. Sensors, measurement systems, and even mechanical components of instruments can undergo small changes or drift over time, environment, and use.
Common oversight points include: zero drift of the force value sensor, calibration validity period of the extensometer, desynchronization of beam movement, insufficient lubrication of moving parts, etc. These factors systematically affect the accuracy of all test data.
Professional advice:Establish and implement a strict daily inspection and period verification system. It is recommended that the operator perform the following simple inspections daily or after each power-on:
| Inspect the item | Brief operation and purpose |
| The force value is zero | When the specimen is not installed and the fixture is empty, the confirmation force value is displayed within a very small error range. |
| Emergency stop function | Test whether the emergency stop button is effective to ensure safety. |
| The beam runs without load | Run at different speeds to check whether the movement is smooth and noise-free. |
| Fixture status | Check whether the jaws are worn or loose, and whether the clamping mechanism is flexible. |
In addition, regular internal calibration should be carried out using traceable reference standards (e.g., standard force gauges, standard length blocks) in between official measurement cycles to monitor the stability of the instrument state.
Summary
Standardizing the operation of tensile testing machines and avoiding the above misunderstandings is the prerequisite for obtaining reliable testing data and ensuring the safety and life of equipment. This requires operators not only to be familiar with the equipment itself, but also to have a deep understanding of testing standards, material mechanics principles, and quality control requirements. Only by combining rigorous standard procedures with meticulous daily maintenance can we truly exert the effectiveness of the instrument and provide solid support for material evaluation and product development.