The dual-field-of-view optical extensometer based on telecentric lens imaging enables a long gauge length that far exceeds the camera’s resolution and offers low image distortion, allowing for high-accuracy strain measurement. However, its strain measurement range is limited by the displacement of the gauge points, making it difficult to apply in large strain measurements. To address this, we propose a large-strain optical extensometer based on segmented strain superposition, referencing the existing concept of segmented measurement. The image sequence is divided into segments based on the displacement of the gauge points, with strain calculations performed for each segment. A strain superposition algorithm is then employed to avoid displacement matching errors caused by excessive deformation between images, enabling high-accuracy strain measurement in large strain scenarios.

Epoxy resin and glass fiber materials are used in this study. Two axial strain gauges are attached to one face of each specimen, while the other face is coated with a matte white primer and black speckle pattern. Uniaxial tensile tests are conducted on each specimen using a universal testing machine, and the strain data are collected through strain gauges connected to a strain meter. The strain values measured by the strain gauges are corrected using a correction formula. A camera equipped with a field-of-view (FOV) splitting device and a telecentric lens is used to capture test images, and the strain superposition algorithm is applied to calculate large strains. The strain results obtained through the optical extensometer and electrical measurement methods are analyzed.

The uniaxial tensile test results for three groups of epoxy resin specimens, obtained using both the large-strain optical extensometer and the electrical measurement method (Fig. 10), show high consistency. For loads below 5000 N, the error between the two methods is less than 20 με, indicating excellent measurement stability. As the load increases, the strain of the specimens increases, causing greater fluctuations in the measurement error. However, the absolute error remains below 61 με until specimen failure. A comparison of the elastic modulus and relative error from the three tests (Table 3) reveal that the error between the two methods is within 0.4%. Similarly, the uniaxial tensile test of the glass fiber specimen (Fig. 11) shows an elastic modulus measurement error of 0.09%, further confirming the high accuracy of the proposed method.

Optical extensometers have been widely used in many fields due to their non-contact testing capabilities and ability to measure large strains. However, achieving high accuracy in large strain measurements is challenging due to the camera’s resolution limitations. This study presents a large-strain, high-accuracy optical extensometer based on the FOV-splitting technique for large strain measurement. By adopting the concept of segmented displacement superposition from digital image correlation (DIC), we propose a method for grouping image sequences and superimposing large strains to improve measurement accuracy. Additionally, to address strain errors in electrical measurement during large strain testing, we introduce a strain correction method. Based on these methods, uniaxial tensile tests are performed on three groups of epoxy resin specimens and one group of glass fiber specimens. Load-strain curves and strain errors are obtained using both the large-strain optical extensometer and the corrected electrical measurement method, and the specimens' elastic modulus is calculated. The results demonstrate that the strain results obtained using the proposed large-strain method are highly consistent with those from the electrical measurement method, with a root-mean-square error (RMSE) of approximately 20 με. This validates the high measurement accuracy of the large-strain optical extensometer based on segmented strain superposition and the FOV-splitting technique, highlighting its potential for large deformation measurement in materials.