Research On Measurement Of Bolt Clamping Force At High Temperature Based On Fiber Bragg Grating Strain Sensors

Proper bolt clamping force is important to ensure the safety of high temperature flange joints.It is well known that the leakage is likely to occur when the bolted flange joints operate at high temperature with variation in bolt preloads due to the differential thermal expansion of each part,a reduction in gasket stress and creep relaxation in joint components.As many design variables affect joint sealing performance particularly at high temperature,it is difficult to ensure the safety of joints merely using a conventional design.Therefore,it is crucial to monitor the bolt clamping force during assembly process and service at high temperatures to ensure the safety of bolted flange joints.A new technique for monitoring the bolt clamping force is thus proposed using metal-packaged RFBG(MPRFBG)strain sensors spot-welded onto the bolt in the thesis.Main research work and conclusions have been summarized as follows:(1)A new type of mental-packaged RFBG(MPRFBG)sensors for measuring high temperature bolt clamping force are successfully developed by combining magnetron sputtering and electroplating.The interfacial bond has a sufficient strength between the optical fiber and the titanium layer,as well as the nickel layer and the steel substrate.The MPRFBG sensors was mounted on the surface of the bolt shank by spot welding,which can monitor bolt clamping force in high temperature flange joints,for convenience,the bolt with MPRFBG sensors were called "MPRFBG-bolt" in the thesis.It is demonstrated that the innovative designed steel substrate can reduce the influence of sensor installation on the intrinsic properties of the bolt.(2)Based on the sensing principle of FBG sensors,a new engineering method for measuring the bolt clamping force in high temperature flange joints is proposed,which can overcome the cross sensitivity of MPRFBG sensor to temperature and strain.Uniaxial tensile and thermal tests are carried out on MPRFBG-bolt at temperature up to 500? for calibration.The observed shifts in Bragg wavelength and tensile forces show linearity,and the relationship between thermally induced wavelength shift and the temperature can be fitted using a cubic polynomial equation.The experimental results demonstrate that the MPRFBG-bolt has good a stability and repeatability.(3)The measurement of the clamping force in a bolted steel-steel joint subjected to thermal loading is conducted.The experimental results show that the bolt clamping force can be monitored by the MPRFBG-bolt high temperature up to 400?.At the same time the experiment are compared with numerical simulation,results show that the simulated results agree to the experiment.This measuring technique provides a new possibility for structural health monitoring of bolted joints operated at high temperature.