A high-speed polarization imaging instrument is demonstrated to be capable of measuring the collagen fiber alignment orientation and alignment strength during high-displacement rate dynamic loading at acquisition rates up to 10 kHz. The implementation of a high-speed rotating quarter wave plate and high-speed camera in the imaging system allows a minimum measurement acquisition time of 6 ms. Sliced tendon-to-bone insertion samples are loaded using a modified drop tower with an average maximum displacement rate of 1.25 m∕s, and imaged using a high-speed polarization imaging instrument. The generated collagen fiber alignment angle and strength maps indicate the localized deformation and fiber realignment in tendon-to-bone samples during dynamic loading. The results demonstrate a viable experimental method to monitor collagen fiber realignment in biological tissue under high-displacement rate dynamic loading.
A high-speed, quantitative polarized light microscopy (QPLM) instrument has been developed to monitor the optical slow axis spatial realignment during controlled medium to high strain rate experiments at acquisition rates up to 10 kHz. This high-speed QPLM instrument is implemented within a modified drop tower and demonstrated using polycarbonate specimens. By utilizing a rotating quarter wave plate and a high-speed camera, the minimum acquisition time to generate an alignment map of a birefringent specimen is 6.1 ms. A sequential analysis method allows the QPLM instrument to generate QPLM data at the high-speed camera imaging frequency 10 kHz. The obtained QPLM data is processed using a vector correlation technique to detect anomalous optical axis realignment and retardation changes throughout the loading event. The detected anomalous optical axis realignment is shown to be associated with crack initiation, propagation, and specimen failure in a dynamically loaded polycarbonate specimen. The work provides a foundation for detecting damage in biological tissues through local collagen fiber realignment and fracture during dynamic loading.
Femtosecond laser fabrication outperforms the traditional fabrication techniques with high precision, high efficiency, low collateral damage and wide applicability, which has shown to be a powerful tool in precision machining. Imaging the ultrafast dynamics of femtosecond laser fabrication is necessary for understanding the processing mechanism and for establishing the corresponding physical models. Up to now, ultrafast measurement techniques based on the pump−probe strategy are the most used methods. However, they are limited by laser energy stability and materials surface uniformity, which have a heavy impact on the dynamic measurement precision of femtosecond laser fabrication. To overcome this limitation of the traditional pump−probe techniques, we developed chirped spectral mapping ultrafast photography (CSMUP), which can achieve single-shot real-time ultrafast imaging with a frame rate of about 250 billion frames per second (temporal frame interval of 4 ps) and a spatial resolution of less than 833 nm. We experimentally imaged the dynamics of femtosecond laser ablation in silicon under a 400 nm femtosecond laser exposure with CSMUP, and the experimental result agreed well with previous theoretical models. CSMUP provides a new strategy to improve the efficiency and accuracy of femtosecond laser fabrication by a single-shot dynamic measurement of the interaction between the femtosecond laser and materials, and it is expected to work as a real-time detection method for various ultrafast phenomena.
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