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Digital image correlation (DIC) has inherent limitations in large deformation measurement, such as speckle pattern decorrelation and high computation time cost. A new dots pattern with fixed size and randomly distributed dots is devised for DIC, rather than the traditional grayscale speckle pattern. Topology-based matching correlation (TMC) based on dots pattern is proposed, in which topology connectivity property is used in correlation calculation for images before and after deformation. Experiments on simulated tensile deformation with up to 300% elongation and nitrile-butadiene rubber tensile tests with 260% elongation were designed and results showed that, compared with the incremental-DIC method, the proposed TMC based DIC (TMC-DIC) method could correctly obtain large deformation with no need of updating reference image, less time consumption and smaller errors. The proposed method also has advantages in the applications like real time deformation observation or deformation measurement of large-scale structure. [ABSTRACT FROM AUTHOR]

• Laser speckle based Digital Image Correction method is proposed for deformation measurement in high temperatures. • A wavelet smoothing algorithm is proposed for filtering the noise while preserving displacement information. • A designed coaxially optical system with high-power laser, filters and polarizer is used for thermal radiation suppression. • Experiments on in-situ noncontact full-field deformation measurement of a nozzle throat sample under gas heating were carried out. The shape of nozzle throat in an engine could greatly change the amount of thrust or heat transfer performance. To in-situ evaluate nozzle throat deformation in high-temperature gas heating tests, a non-contact full-field deformation measurement system via laser speckle Digital Image Correlation with wavelet smoothing of displacement field is proposed. For high-temperature thermal radiation suppression, high-power laser speckle pattern is used for digital image correlation calculation and multi-step filters with polarizers are used. A displacement field wavelet smoothing algorithm is proposed to mitigate imaging distortion caused by airflow disturbance. Gas heating experiments on a nozzle sample made of carbon fiber high silica phenolic resin composites were carried out. The full-field displacement measurement results show that compared with the commonly used air blade method, the proposed method can provide smoother displacement field for deformation evaluation, the displacement measurement difference of the proposed method and a dial indicator was within 0.01 mm. [ABSTRACT FROM AUTHOR]

Highlights • The IR-DIC effectively removed noises from the noisy displacement field. • The laser speckle is used as speckle pattern in DIC at high temperature. • The proposed method has better robustness of strain measurement. • Experimental results at 2000 °C are consistent with those of contact extensometer. Abstract Background radiation and airflow disturbance at high temperature reduce the quality of captured images, resulting in a lot of noises in the displacement field obtained by digital image correlation (DIC) and decreased accuracy of strain measurement. A high temperature strain measurement method (IR-DIC) by combining digital image correlation of laser speckle and Improved Random Sample Consensus (IRANSAC) smoothing algorithm for uniform deformation is proposed. In the IR-DIC method, after obtaining the noisy displacement field by the digital image correlation of laser speckle, the IRANSAC algorithm is used to smooth the noisy displacement field by removing noises with an adaptive noise threshold selection for uniform deformation. Also, when fitting the displacement field, instead of using the minimal data model in the classical RANSAC algorithm, a subset with all the remaining effective data points after removing noises are used in the IRANSAC algorithm. An experiment on tensile test of a C/C composite sample at 2000 °C in the inert atmosphere was carried out. The results showed that, the strain curve calculated by the IR-DIC method is basically consistent with the result simultaneously measured by a high temperature contact extensometer. And compared with the least square algorithm, the mean deviation between the strain curve calculated by the IR-DIC method and the strain curve measured by the contact extensometer is reduced about 13.94%. [ABSTRACT FROM AUTHOR]

• Proposed index is a global and comprehensive quality assessment criterion for laser speckle. • The influence of external factors on the quality of laser speckle patterns is investigated. • The influence of material factors on the quality of laser speckle patterns is investigated. • Proposed index has better capability of assessing laser speckle than traditional indexes. • Proposed index is also suitable for the quality assessment of artificial speckle patterns. Digital image correlation based on laser speckle has its unique advantages in ultra-high temperature deformation measurement. The traditional quality assessment criteria of artificial speckle which consider only the contrast of the speckle pattern or the morphology of the speckle particles are not suitable for laser speckle. Here, a new index, called Multi-Factor Fusion Index (MFFI), which takes the inhomogeneity of gray distribution, the mean square deviation of gray and the standard deviation of speckle particles size into consideration, is proposed to evaluate the quality of laser speckle patterns. Multi-Factor Fusion Index was compared with three commonly used criteria, the mean intensity gradient, the mean intensity of the second derivative, and the pattern quality metric combining the sum of square of subset intensity gradients (SSSIG) and the secondary auto-correlation peak height. The results showed that the proposed index had better capability of assessing the quality of laser speckle patterns. And the influence by external factors such as apertures, laser powers and temperatures and material factors such as roughness and light reflectance were analyzed by MFFI. Experiments on artificial speckle showed that MFFI was also suitable for artificial speckle patterns in most cases. [ABSTRACT FROM AUTHOR]