Design and teaching application of mechanical property testing device based on indentation method

Conventional mechanical property testing methods, including uniaxial tensile and impact tests, are predominantly destructive. These methods are inherently disruptive, cumbersome to perform, and often impractical for the in-service inspection of pressure vessels in engineering environments. While the indentation technique has demonstrated potential as a micro-destructive testing method that derives mechanical properties from load-displacement curves, its application remains largely limited to industrial evaluations. Consequently, current engineering education frameworks are not fully aligned with modern practices, resulting in students receiving limited exposure to modern mechanical property testing technologies. To address this critical discrepancy, the present study developed an integrated mechanical testing apparatus based on the continuous spherical indentation method. The objective was to establish a unified platform that integrates laboratory validation with real-world field operations, thereby enhancing the practical and innovative skills of engineering students.

To achieve these objectives, the experimental device was designed with a highly modular configuration, encompassing mechanical, control, and data processing modules. The mechanical module features a lightweight alloy frame that is integrated with versatile fixtures, including U-shaped and magnetic clamps. These fixtures are suitable for both flat and curved specimens commonly encountered in engineering environments. The control module employs a hybrid software-hardware configuration, integrating a microcontroller with high-precision sensors for displacement and tension-compression measurements. This configuration ensures accurate signal capture while mitigating electromagnetic interference. The data processing unit incorporates a continuous spherical indentation algorithm that facilitates the automated conversion of raw load-depth data into true stress-strain curves. This capability enables the direct extraction of critical mechanical parameters. Validation was conducted through a series of comparative experiments on three representative pressure vessel steels. The following materials were utilized: Q235B, 45 steel, and Q345R. A spherical indenter with a diameter of 1.5 mm was used under a preload of 5 N, and the loading rate was maintained at 0.4 mm/min. Eight consecutive loading-unloading cycles were executed, and the outcomes were systematically compared with those of standard uniaxial tensile tests. Furthermore, a simulated field experiment was designed to assess a large-scale Q245 carbon steel pressure vessel to evaluate local mechanical properties across the base metal, heat-affected zone, and weld seam.

The validation experiments demonstrated the high accuracy and reliability of the developed indentation testing device. A comparison was made between the indentation-derived data and the corresponding standard uniaxial tensile test outcomes for the three selected vessel steels. The relative errors for yield strength ranged from −4.29% to 4.80%, while those for tensile strength ranged from −2.68% to 3.32%. The maximum recorded deviation remained below 5% threshold, thereby satisfying the stringent precision requirements for both academic experiments and engineering applications. Furthermore, the on-site simulation experiment successfully differentiated the mechanical properties variations within the welded joints of the Q245 steel vessel. Micro-destructive testing revealed the performance gradient, indicating a stable and uniform base metal, as well as pronounced performance fluctuations within the weld seam due to thermal processing. These findings were accompanied by intermediate properties observed in the heat-affected zone. From a pedagogical perspective, these practical implementations enabled students to develop a nuanced understanding of the discrepancies between idealized laboratory conditions and the intricate dynamics present in complex field environments. This enhanced their awareness of engineering norms and rapid deployment strategies, thereby fostering a more comprehensive and nuanced understanding of engineering practices.

The development and deployment of this indentation-based testing apparatus effectively addresses the prevalent limitations associated with conventional destructive testing in educational settings. The integration of interdisciplinary methodologies and advanced sensing technologies enables the device to serve a dual purpose, supporting fundamental experimental teaching and complex engineering field simulations. The high-precision acquisition of load-depth information, in conjunction with flexible structural adaptations, demonstrates significant practical engineering value. The integration of this apparatus into the curriculum has been demonstrated to have a substantial impact on students’ technical proficiency and problem-solving skills. It offers a robust and innovative instructional paradigm that has been tailored to the contemporary engineering education landscape.