# Study of Crack Growth of Transparent Materials Subjected to Laser Irradiation by Digital Holography
###### Abstract  
The crack growth of transparent materials after laser wavelength irradiation was studied. It is known that laser irradiation is used in many applications for the ablation of undesired material and/or coatings. The impact of laser irradiation on cracks was studied using the digital holography (DH) technique. Transparent samples were irradiated using near-ultraviolet, visible, near-infrared, and infrared light. The DH system is able to detect cracks and crack growth of the transparent samples irradiated by a range of laser wavelengths. Results also show that light with infrared to near-infrared wavelengths has a great effect on crack growth. High-resolution photomechanical effects of laser irradiation on material expansion or/and generation of defects due to specific wavelengths are also illustrated. The DH system with a multispectral laser has practical applications for laser cleaning of painted artworks. digital holography; laser irradiation; photomechanical effects; tip crack; transparent materials +
Footnote †: journal: Applied  
**Citation:** [PERSON], [PERSON]; [PERSON]; [PERSON]; [PERSON]; [PERSON]; [PERSON] Study of Crack Growth of Transparent Materials Subjected to Laser Irradiation by Digital Holography. painting. Cracks of a painting are generated on the surface of the varnish layer due to multiple factors, including aging over time, external mechanical vibrations [14], gradients or drastic changes in humidity [15; 16] and temperature [7; 17], and chemical contamination. It is also known that laser cleaning, widely adopted for oil painting, is actually capable of generating cracks [18; 19; 20; 21; 22; 23]. Furthermore, laser irradiation itself is also able to generate cracks [24; 25; 26; 27]. Laser cleaning is a process for restoring the aesthetic of the painting through the photothermal, photochemical, and photomechanical effects [28; 29]. The unwanted layer in paintings could be removed by laser of different wavelengths, including ultraviolet, visible, and infrared light [30]. A real-time monitoring system of short-term and long-term effects of laser cleaning could better explore which frequency bands of light may cause harm to achieve safe laser cleaning [31; 32]. Deterioration of precious works of art cracking can result in loss of aesthetic appearance, depreciation of value, and destruction of irreplaceable cultural heritage and identity. Previous methods for crack detection have been proposed, including the nanoindentation technique [33] and the three-dimensional (3D) scanning technique [14]; however, measurement errors are common for nanoscale techniques [34; 35], which may be caused by trace wear of the measurement including microscopic defects on the specimen surface, and slight changes in the experimental environment. Furthermore, such a technique cannot be performed in real-time, and is thus inefficient. In addition, 3D scanning technology also has limitations [36] in terms of accuracy and scale, which can be time-consuming. The application of digital holography in the field of high-precision inspection of transparent materials has been well established [37]. As a micro and nano-level detection technique, DH has been used to nondestructively detect the shape of cracks on the material surface in real-time [38]. Both the phase and intensity of the object could be numerically reconstructed by only one hologram. Hence, DH has a fast computational process that is faster than nanoindentation and 3D scanning techniques [39]. Digital holographic microscopy for cracking in transparent materials has also been successfully conducted by our team [40], showing that digital holography is suitable for crack detection in transparent materials. The fundamental principle of DH to detect crack growth is explained in Section 2. Our research goal was to examine the cracks on the surface of oil paintings and their irradiation growth.