Studies For Quantitatively Measuring Technique By Using Thermal Wave Imaging And Their Applications

The requirements of modern industry for non-destructive testing(NDT)technology no longer remain in the judgment of the presence of defects,but more accurate quantitative measurement is needed,so that to further evaluate the product damage level and serving life.Based on the internal information acquisition technology of infrared thermal wave,the thesis studies the quantitative measurements methods,including material thermal parameters and the depth of defects or material thickness.As an application example,the quantitative measurement methods was used on wind power blades.The study is as follows:1.For the limitations of infrared thermal wave technology applied to the measurement of thick materials,the analytical solution of heat transfer in a finite thickness plate model under the excitation of a step-heating source was deduced using the mathematical convolution method,and a step-heating measurement method was proposed.Both ANSYS finite element simulation results and experimental results of designed samples were applied to evaluate the thickness measurement accuracy of this method.The simulation results show that the measurement error is less than 0.5% for all tested models which thickness vary between 10mm-50mm;the experiment results for the designed samples demonstrate that the measurement error is less than 3% for the thick(>10mm)steps,and the error decreases with the thickness of the sample's steps.This is mainly result from that the heating source we can get in the experiment is not theoretical step-heating,2.A new method of measuring thermal diffusivity was proposed.Using non-linear fitting,the theoretical value was approximated by the loop iterations to the experimental data,and then the diffusivity of the material was obtained.Compared with the traditional method,the detection efficiency was improved by 5 times.ANSYS numerical evaluation results show that the measurement error of common materials is less than 0.15%.The thesis also evaluates the influence of the pulse width for the measurement result: the wider the pulse,the greater the error;the slower the heat is transmitted through the material,the less the pulse width affects it.3.The aspect ratio p of the defect is the ratio of the defect size to its depth,and is often used to evaluate the detection resolution in the thermal wave NDT.When p is small,due to three-dimensional thermal diffusion,using the logarithmic peak second derivative method(LPSD)to calculate the depth will bring a larger error.A correction term related to the p is introduced into the temperature-time function of the one-dimensional heat conduction model.The rectification curve of the depth measurement affected by the p is established,and the correction method is proposed to improve the precision of the depth prediction.This rectification method is independent with the material.4.Using the analytical solution of the pulsed heat conduction equation in the two-layer model,a method for measuring the heat effusivity was derived.In the logarithmic temperature-logarithmic time first derivative curve of the equation,the peak value is only related to the ratio of the two-layer medium heat storage coefficient e1/e2.Based on this,an electrical tape which has a pre-calibrated effusivity was selected as the first layer media,and the effusivity of six materials of aluminum,copper,carbon steel,stainless steel,black plastic,and plexiglass were measured.The measurement results show that this method has high accuracy on the tested samples.5.Quantitative measurement methods talked in this thesis are applied to predict thermal diffusivities of commonly used materials in wind turbine blades,and evaluate the bonding quality between the main beam and the shear web in wind turbine blades.A large number of adhesive structure were employed in the wind turbine blades,among which the adhesive of the beam and the shear web distributes in the whole length of the blade,from the root to the tip,and the main beam bears most of the blade's shear load,so the reliability of its bonding quality is very important.ANSYS numerical analysis was used to simulate the adhesive status.The specimens were designed and tested using the step-heating thickness measurement method.The program was programmed to automatically identify the adhesion edge,calculate the adhesive width and further evaluate the adhesive quality.Finally,this method was used to detect the overall adhesive quality of the 45.3m blade.