Tham khảo tài liệu 'laser pulse phenomena and applications part 3', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | The Effect of the Time Structure of Laser Pulse on Temperature Distribution and Thermal Stresses in Homogeneous Body with Coating 51 For greater pulse rise times Tr the dimensionless time Tc connected with the change of stresses type from compressive to tensile one also increases Fig. 4 . This dependence is approximately described by the equation Tc . In thermal processing of brittle materials it is the sign change of superficial stresses what plays key role in controlled thermal splitting Dostanko et al. 2002 . The beginning of superficial cracks generation is accompanied with the monotonic increase of tensile lateral stresses and makes the controlled evolution of the crevices possible. By equating the relation 45 to 0 at T Tc Ts z 0 and taking into account the equations 42 48 49 one obtains Q00 rc - Q0o Tc -ts T -T-T 70 yjn where the function Qq0 t has the form 66 . With the absolute inaccuracy smaller than 3 the solution of nonlinear equation 70 can be approximated by the function Tc T Fig. 5 . Fig. 5. Dimensionless time Tc of the sign change of lateral stress z on the irradiated surface z 0 versus dimensionless duration Ts of the rectangular laser pulse Yevtushenko et al. 2007 . The forced cooling of the surface in the moment time t tc would cause the jump of temperature AT T 0 tc - 0 - T 0 tc 0 in thin superficial layer. From the equations 45 and 46 it follows that the dimensionless lateral deformation of the plate E y is determined by the integral characteristic of temperature only. For this reason the rapid cooling of the thin film practically does not change the surface deformation Ey 0 tc 0 0 tc - 0 but 52 Laser Pulse Phenomena and Applications at the same moment it produces the increase of the normal stresses ơy 0 tc 0 ac T. Finally the development of the superficial crack can be described as a series of the following phases 1. due to local short heating a surface of the sample in it the field of .
