Study On Cryogenic Adhesive-Bonded Performance Of GFRP

The non-magnetic Dewar can provide the cryogenic and non-magnetic environment that is necessary for SQUID (Superconducting Quantum Interference Device). The different parts of non-magnetic Dewar are usually adhesive bonding. The key of the low temperature bonding technology is not only to ensure the firmness of adhesive joints at the cryogenic temperature, but also to ensure vacuum tightness and duration. The vacuum performance of threaded-adhesive joints was investigated experimentally using the helium mass spectrometer leak detector. The test results showed that the triangular threaded-adhesive joints and the trapezoidal threaded-adhesive joints have good vacuum tightness under the atmospheric temperature and immersed into the LN2 and heating can maintain well vacuum tightness. However, after the threaded-adhesive joints experience repeatedly cooling and warming cycles, they may occur vacuum failure while they don't occur mechanical failure. To testify that the vacuum failure is caused the cyclic thermal stress during the cooling and warming process, the two aspects of the work were done in the paper. One is that the cyclic compression load was applied to the sample to simulate the thermal stress load caused by temperature change using the MTS810 material test system. The results showed that the vacuum failure of adhesive joints first occurred after experiencing a certain times of cyclic load. The other is that the thermal stress analysis of the sample for vacuum performance experiment was carried out. The analysis results showed that the stress level of the upper threaded-adhesive joint is higher than that of middle joint. In the experiment, the vacuum failure also occurred at the two endings of the sample. Therefore, the conclusion was reached that the vacuum failure of adhesive joints of non-magnetic Dewar at cryogenic temperature is caused the steady stress and transient thermal stress produced by repeatedly cooling and warming process. The vacuum failure is different from the mechanic failure of joints and is piror to the mechanic failure. The design of adhesive joints of non-magnetic Dewar should base on the vacuum failure.Because the thermal stress is main reason of the vacuum failure of adhesive joints, the geometrical non-linear steady thermal stress analysis and the transient thermal stress analysis of the threaded-adhesive joints at cryogenic temperature were conducted using ANSYS software. The results showed that the stress levels of geometrical non-linear thermal stress analysis are usually less by 6% than those of linear thermal stress analysis. Whether the geometrical non-linear should be consider or not deponds on the required precision of the computational problem. The peak transient thermal stress is far higher than the stress level at the steady state. It is up to about 2.5 times of the peak steady thermal stress. This is because the steady stress is caused the temperature change from room temperature to liquid temperature while the transient thermal stress is caused the temperature change and the large temperature gradient of the joint region. The peak steady stress occurred the free endings of the adhesive fillet while the maximum transient stress occurred at the outside edge of the adhesive fillet. Therefore, the adhesive fillet is the most critical region. The shape of the adhesive fillet should be concerned to avoid forming the sharp corner even crack during the adhesive bonding process of non-magnetic. In addtion, the effects of property parameters,the effects of the thread length, thread pitch and the adhesive thickness on the stress distribution and the stress level of adhesive joints region were discussed. The property parameters of the adhesive and GFRP should meet the certain requirement. When the liquid nitrogen is charged into the Dewar, the method should be adopted to decrease the peak transient thermal stress that the Dewar should be pre-cooled using the nitrogen gas, and then charge the liquid nitrogen.Using the surface treatment to improve the adhesion strength is the other feasible measure of the increase of the duration of cryogenic adhesion. The whole roughness---adhesion strength was established by the combination of the modified single roughness model and the statistic model of the whole rough surface. The model is made up of the shape model of the single roughness, the stress analysis of the single roughness, the effect of a single roughness on the adhesion and the statistic model of the relationship between the roughness and the adhesion strength. The stress analysis results of the single roughness surface was fitted using appropriate formula, then the fitted results and the statistic model of roughness were combined to establish the new model. The established model can make good effect on the predicting the relationship between the surface roughness and adhesion strength of the used the cryogenic adhesive and GFRP in the study. The progress is made in quantitative description of the relationship between the surface roughness and the adhesion strength. However, the cdeviation is too large when the surface roughness is too small or too large, the applicability of the established model will be awaited the further improvement.