Digital shearing speckle pattern interferometry-driven quantification of bolt preload: A bolt-vision-free metrology paradigm via surface strain-to-preload mapping

Accurate measurement of bolt preload remains a persistent engineering challenge. Conventional vision-based methods, which merely assess loosening status based on bolt angular/height changes without establishing mathematical relationships between these parameters and preload, exhibit inherent limitations. Leveraging the fundamental phenomenon of coupling surface deflection induced by bolt preload, this paper proposes a novel approach employing digital shearing speckle pattern interferometry to quantify bolt preload by measuring the corresponding strain directly. Experimental verification across three distinct coupling structures confirmed a robust linear correlation (coefficient of determination, R2>0.99) between strain and preload. The resolution of the method achieves 50 N, and the relative error is less than ±2%, which are analytically derived and experimentally validated. Crucially, our technique achieves a wide detection angle range (30°-90°) without requiring direct bolt visibility and successfully detects non-rotation loosening scenarios, significantly enhancing applicability in constrained structural environments. Furthermore, the method is independent of bolt length and length-to-diameter ratios and demonstrates broad applicability across diverse structural configurations. Finally, the mathematical relationship between strain and preload was derived based on thin plate theory and validated with experimental results. The proposed method established a novel measurement paradigm for high-precision bolt preload quantification.