Enhanced Digital Image Correlation at Cryogenic Temperatures: Novel PDMS-TiO2 Speckle Fabrication for Accurate Strain Measurement Under Large Deformations

Significance 

The accurate measurement of deformation and strain is critical for the development and evaluation of materials intended for cryogenic applications. Traditional measurement techniques, such as extensometers, though widely used, suffer from limitations including contact-based measurements, uniaxial response capabilities, and an inability to provide full-field observations. These limitations have spurred the adoption of digital image correlation (DIC) technology, which offers non-contact, full-field measurements with high spatial and temporal resolution. DIC technology, developed in the 1980s, has revolutionized the way shape, motion, and deformation are measured across various scales and environments. Its advantages include simplicity in setup, robustness to environmental vibrations and light changes, and a wide application range. Central to the success of DIC measurements is the quality of speckle patterns, which act as carriers of deformation information. The accuracy and reliability of DIC are significantly influenced by the quality and stability of these speckle patterns, especially under extreme conditions. One of the primary challenges in utilizing DIC technology at cryogenic temperatures is the brittleness and hardness of speckle patterns. As temperatures approach those of liquid helium, materials tend to become more brittle and harder, leading to issues such as speckle cracking and shedding during large deformation processes. These issues severely compromise the accuracy of strain measurements, making it difficult to capture the true mechanical behavior of materials under such extreme conditions. Another challenge is the thermal radiation effect, which can introduce significant errors in strain measurements. At extremely low temperatures, even small amounts of external heat can cause sharp temperature increases in the sample, altering its mechanical properties and behavior. Thus, controlling and minimizing thermal radiation from the experimental setup is crucial for obtaining accurate DIC measurements.

New study published in Experimental Mechanics and conducted by J. Yang, Y. Li, J. Deng, Z. Zhang, J. Zhou & Professor Xingyi Zhang from Lanzhou University developed novel speckle patterns and fabrication methods that can withstand the harsh conditions of extremely low temperatures and large deformations.  They improved the stability and performance of speckle patterns, and by this enhanced the accuracy and precision of DIC measurements, which can provide deeper knowledge into the mechanical behavior of materials in cryogenic environments.

In their experiments, the authors proposed a novel approach using PDMS (Polydimethylsiloxane) silicone and TiO2 spherical particles. The speckle mixture was prepared with 45% PDMS silicone rubber liquid, 5% curing agent, and 50% TiO2 particles. The mixture was thoroughly blended, degassed in a vacuum chamber to eliminate air bubbles, and then spin-coated onto 316LN stainless steel samples. The samples were baked to solidify the silicone and fix the particles on the surface. Then they evaluated the fabricated speckle patterns using a cryogenic loading system and an optical path designed to minimize thermal radiation. The researchers demonstrated that the PDMS-TiO2 speckle patterns exhibited excellent stability during large deformations at both room temperature (300 K) and cryogenic temperature (20 K). Unlike other commonly used speckle patterns, such as black spray paint, UV printing, and hydrographic printing, the PDMS-TiO2 speckles did not crack or shed even at strains exceeding 20%. This stability was essential for accurate strain measurements in the DIC experiments. Afterward, the researchers conducted tensile tests on 316LN stainless steel samples using the PDMS-TiO2 speckle patterns to observe strain distribution and slip band evolution. The tests were performed at 20 K and 300 K. The strain distribution was monitored at various strain levels: 0%, 1%, 5%, 10%, and 20% and found that all samples maintained a non-uniform strain distribution during the entire deformation process at both temperatures. Strain localization occurred in the form of cross-slip bands, which propagated along the loading direction as the strain increased. At strains above 2%, these slip bands became more pronounced and spread throughout the observation area and the local strain differential could exceed 10% during significant deformation which shows the importance of full-field and in-situ measurements.

The team also investigated the impact of thermal radiation on the samples. They used heat insulation glass and polarized light paths to reduce the thermal radiation effects. They found that the LED polarized light path had the least impact on the sample temperature, with only a minimal increase of about 2 K. In contrast, fluorescent light caused more noticeable thermal radiation heating. These findings highlighted the effectiveness of the heat-insulating polarized light path combined with the PDMS-TiO2 speckle patterns in minimizing thermal radiation effects during DIC measurements. To gain deeper insights into the slip band propagation and phase transformation, the researchers examined the formation and evolution of slip bands at both temperatures. They selected specific regions for metallographic observation after the tensile tests and their results showed that at cryogenic temperatures, the originally yellow-white austenitic phase in 316LN stainless steel gradually transformed into a black-green martensite phase, particularly at the intersections of the slip bands. This phase transformation contributed to the higher stress-bearing capacity of the material at lower temperatures. The slip band propagation displacement was measured at both temperatures, revealing that the expansion velocity of the slip bands was almost unaffected by the low-temperature environment at the same loading rate. The local strain of a new slip band gradually increased to about 8.8% and then stabilized. Other new slip bands also formed in nearby regions, with more pronounced local strain concentration at room temperature. The authors also compared the performance of various speckle patterns under the same experimental conditions. The black spray paint, fluorescent speckles, and the PDMS-TiO2 speckles met the strain observation requirements of more than 20% at room temperature. However, at cryogenic temperatures, the black spray paint speckles cracked and shed at strains of 5%, leading to significant errors in the strain results. The hydrographic printing and UV printing speckles cracked at strains exceeding 1%, failing pattern matching in most areas. In contrast, the PDMS-TiO2 and fluorescent speckles exhibited the most stable performance, with no cracking or shedding throughout the deformation process. Additionally, the authors used the mean intensity gradient (MIG) method to evaluate the quality of the speckle patterns. The PDMS-TiO2 speckles had a MIG value comparable to that of black spray paint and fluorescent speckles, indicating good speckle quality. The UV printing and hydrographic printing speckles had higher MIG values but were less stable under large strains and cryogenic conditions. According to the authors, the overall strain localization was observed to be similar for each sample, emphasizing the reliability of the PDMS-TiO2 speckle patterns in capturing accurate strain distribution during large deformations at low temperatures.

In conclusion, the new study by Professor Xingyi Zhang and colleagues advanced the capabilities of DIC technology under extreme conditions and successfully developed a novel speckle fabrication method using PDMS silicone and TiO2 spherical particles, and by this provided a robust solution for accurately measuring strain and deformation at cryogenic temperatures and under large deformation conditions. This is significant because the novel speckle patterns exhibit exceptional stability and do not crack or shed during large deformations, ensuring high accuracy in DIC measurements. This is also vital for understanding the true mechanical behavior of materials in cryogenic environments. Moreover, the improved speckle patterns extend the applicability of DIC technology to more extreme environments, such as near liquid helium temperatures, where traditional methods and existing speckle patterns fail. This broadens the scope of materials and conditions that can be studied using DIC. Furthermore, the study provides valuable data on the strain distribution and slip band evolution in 316LN stainless steel, a material widely used in cryogenic applications and with understanding these behaviors at both room and cryogenic temperatures is important in designing and developing more reliable and efficient cryogenic systems. Additionally, the authors’ findings have several practical engineering implications, for instance, the accurate measurement of strain and deformation at extremely low temperatures is essential for designing and evaluating cryogenic systems used in aerospace, medical, and energy sectors. The novel speckle fabrication method reported in the study will enhance the reliability of these measurements and lead to improved design and safety of cryogenic equipment. Researchers and engineers now can develop new materials for cryogenic applications that can use the enhanced DIC measurement capabilities to better understand and optimize material properties. This can accelerate the development of materials with superior performance in extreme environments. The improved DIC techniques can also be applied to monitor the structural integrity of components operating under extreme conditions, such as space exploration equipment, superconducting magnets, and cryogenic pipelines. Accurate strain measurements help in early detection of potential failures, thereby preventing catastrophic events.

About the author

Dr. Jinbo Yang is a recent Ph.D. graduate in Solid Mechanics from Lanzhou University. Dr. Yang’s research interests include experimental solid mechanics and extreme mechanics. Dr. Yang has published three papers in the journals Experimental Mechanics, Measurement Science and Technology, and Review of Scientific Instruments. One of these papers, concerning strain observation in large deformations at low temperatures, was selected as the cover article for the first issue of Experimental Mechanics in 2024. He also holds one invention patent. Dr. Yang have led and completed an Outstanding Graduate Student Innovation Project in Gansu Province.

Reference

Yang, J., Li, Y., Deng, J. et al. Novel Speckle Preparation and Heat Insulation Method for DIC Strain Measurement at Cryogenic Temperature and Large Deformation Environment. Exp Mech 64, 73–84 (2024). https://doi.org/10.1007/s11340-023-01006-0

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