Effect of Fragile Speckle Patterns on Accuracy of Digital Volume Correlation

Due to challenges in generating high-quality 3D speckle patterns for Digital Volume Correlation (DVC) strain measurements, DVC experiments often utilize the intrinsic texture and contrast of composite microstructures. One common deficiency of these natural speckle patterns is their poor durability under large deformations, which can lead to decorrelation and inaccurate strain measurements. Using syntactic foams as a model material, the effects of speckle pattern degradation on the accuracy of DVC displacement and strain measurements are assessed with both experimentally-acquired and numerically-generated images. It is shown that measurement error can be classified into two regimes as a function of the percentage of markers that have disappeared from the speckle pattern. For minor levels of damage beneath a critical level of damage, displacement and strain error remained near the noise floor of less than 0.05 voxels and 100 mu epsilon, respectively; above this level, error rapidly increased to unacceptable levels above 0.2 voxels and 10,000 mu epsilon. This transition occurred after 30%-40% of the speckles disappeared, depending on characteristics of the speckle pattern and its degradation mechanisms. These results suggest that accurate DVC measurements can be obtained in many types of fragile materials despite severe damage to the speckle pattern.