| Soft substrate film structure has been widely used in MEMS,aerospace,biomaterials,soft electronics and other fields because of its high ductility,high flexibility,high fabrication efficiency and good electrical properties.The soft substrate film structure is often subjected to biaxial tension and is in a complex stress state.As a result,damage and destruction behaviors such as film fracture,debonding and buckling are east to occur,which will affect the life of the structure and brings economic losses.Among them,fracture damage is the most common.Therefore,it is of great significance to study the fracture damage of soft substrate film structure and explore the mechanism of crack initiation and evolution.This paper focuses on this problem in following several aspects:The crack density is used to characterize the damage evolution of soft substrate film structures during tensile fracture.Two methods for defining the crack density of films under biaxial loading are proposed.Based on the shear lag theory of uniaxial tension,the stress distribution of thin films under biaxial tension is deduced.According to the fracture strength criterion and energy criterion,the relationship between crack density and strain of thin films under uniaxial and biaxial loads is obtained.By comparing the uniaxial tensile test with the theoretical results,it is found that the fracture strength criterion agrees better with the experimental results than the energy criterion.A force sensor is developed on the drive shaft to measure the force load and a digital image correlation method based on the surface defect of the thin film is used to measure the strain.Thus,an in-situ biaxial tension test platform for soft-based thin film structure is improved.According to the experimental phenomena,two definitions of crack density are proposed,and high-throughput crack density measurement is realized by program.The graphical user interface based on MATLAB is designed and all the data processing programs are integrated into one whole to realize a fast and efficient data processing.The crack morphology was observed by laser confocal microscopy,and the biaxial tensile damage experiment of Soft substrate film structure was carried out to study the effect of different loading ratios on crack initiation and evolution behavior.It is found that for non-equal biaxial loading,the crack initiation of brittle films is determined by the first principal stress,and the crack direction is always perpendicular to the first principal stress direction.For the equal biaxial loading,the first principal stress direction can be any direction in the plane of the film,which causes the direction of the crack to be controlled by defect.So that the crack initiation direction can be any direction in the film.In addition,the crack density was found to be proportional to the tensile force load.The modulation effect of precast crack and biaxial loading ratio on the new crack in thin film was studied.A theoretical model is proposed to analyze the influence of precast crack and loading ratio on the angle of new crack in thin film during elastic stage,and the angle of new crack is predicted.A special precast crack specimen is designed and the finite element simulation is carried out to determine the uniform strain zone.The biaxial loading experiment of precast crack specimens was carried out,and the experimental results were in good agreement with the theory.At the same time,the influence of precast crack density on crack morphology was found and explained.However,the specific modulation mechanism still needs further study. |
PDF Full Text Request