Research Of Low Temperature Mechanical Property Of Nanocomposite Films Based On Digital Image Correlation Method

Polyimide and its nano hybrid films have lots of advantages in dielectric property,thermal stability and radiation resistance.The films have been widely used in the field of cryogenic engineering and aerospace engineering.Then higher demands of the mechanical properties of films at low temperature have been presented because of its severe service environment at low temperature in cryogenic engineering.Therefore accurately measuring the mechanical properties of Polyimide and its nano hybrid films at low temperature is of great significance in the design and protection of the structure at low temperature.As a new type of full field optical measurement mechanics method,the digital image correlation method has remarkable advantages:non-contact,full-field measurement,good adaptability to environment etc,especially suitable for the deformation measurement of low-dimension material?film and thread?under high and low temperature.In this paper,low-temperature mechanical properties of titanium dioxide nano hybrid polyimide films were measured by the digital image correlation method.According to the inadequacy of existing experimental study on films'mechanical properties at low temperature,this paper mainly completed the following work.Firstly,a cryogenic full-field deformation measurement system was built by combining DIC method with a cryogenic tensile test device.The test system is suitable for measurement of thin film materials'deformation under low temperature environment.And the thermal deformation of pure copper films was measured in the temperature range of-100°C²0°C,and the results match well with the existing data.It demonstrated that our system can be employed to measure low temperature thin film deformation with high accuracy and stability.Secondly,The system was then applied to measure the low-temperature mechanical properties of different components of TiO₂/PI films.The uniaxial tensile tests under-60°C¹8°C were carried out,with the stress-strain curves and elastic mechanics properties obtained.Experimental results show that the average values of the films'Young's Modulus increase when temperature decreases from 18°C to-60°C,as well as the stress-strain curves strengthen dramatically in linear trend and Young's Modulus increase with the addition of TiO₂ nanoparticles content.And the average values of films'poisson's ratio reduce with the drop of the temperature as well as the addition of TiO₂ nanoparticles,but the average values of pure PI films'poisson's ratio reduce unsignificantly with the temperature drops.Finally,according to the inadequacy of existing study on the measuring low-temperature coefficient of thermal expansion of thin films,especially to the phenomenon of serious crispation and warping caused by uneven PI film,a new method using small prestress and fixed boundary was applid to the measuring low-temperature coefficient of thermal expansion of TiO₂/PI films and some satisfactory results were obtained.The problem was worked out to a certain degree.