Papers by Author: Jun Yan Liu

Abstract: This paper provides the theory, mathematics analysis and experiments in support of the Infrared thermal-wave inspection on the subsurface defects in a solid using linear frequency modulated light excitation (LFMTWI). The specimen is heated by the heat flux of linear frequency modulation for launching thermal-wave into the sample in a desired range of frequency. The more thermal wave responses characters are obtained, and the IR thermal-wave imaging shows much more advantages for subsurface defects detection. The simulation and experimental results from steel sample are presented in support of this technique.

Abstract: This paper describes a set of theoretical and experimental results based on thermoelastic effect measurement from Glass-Fiber-Reinforced-Polymer (GFRP) composite structure for full-filed stress analysis. The sum of the principle stress (The first stress invariant) can be measured by means of thermoelastic stress analysis (TSA), and this method is used to determine the stress concentration of the composite structure. A finite element analysis is proposed to predict the stress distribution for GFRP gluing structure. The lock-in thermography has been applied to measure the structure stress distribution by its high thermal resolution. The experiment was carried out with different GFRP composite structures by lock in thermography. The experimental results show the stress distribution can be measured and evaluated with good accuracies by using of lock-in thermography

Abstract: The application of thermoelastic stress analysis in compound structure is particularly complicated because of the different material components, which determines the different thermoelastic effect to be depended on the different material property and mechanical performance. This paper describes a theoretical and experimental analysis on full-filed stress distribution from thermoelastic measurements and its application to determination of stress concentration for compound Steel-Foam-GFRP structure. A finite element modeling is proposed to calculate the sum of the principal stress under the condition of dynamic cyclic load. The sum of the principal stress can be measured by means of thermal stress analysis (TSA). Lock-in thermography has been applied to measure the sum of principal stress distribution of component by its high thermal resolution. In this study, Experiments were carried out with Steel-Foam-GFRP compound structure under dynamic periodic load. The thermoelastic constant is calibrated for different component of compound structure, respectively. An artificial neural network (ANN) is proposed to identify the different component stress distribution on whole compound structure. The experimental result shows that the stress distribution of compound structure can be measured and analyzed using lock-in thermography. It is found that the stress distribution of compound structure can be evaluated with good accuracies by lock-in thermography.

Abstract: This paper describes the quantitative analysis of the shape, boundary, and depth of subsurface defects by ultrasound lock-in thermography. The phase difference between defective areas and non-defective areas illustrates the qualitative analysis of the shape and the boundary of the subsurface defect. In order to accurately estimate the shape, boundary and depth of the defects, the optimal threshold value method is proposed to identify the shape and boundary of the subsurface defects based on the canny operator of image processing. A self–adaption artificial neural network (ANN) with Takagi-Sugeno modeling is proposed to determine the depth of the subsurface defect. Experimental results for a steel plate with artificial subsurface defects show good agreement with actual values.

Abstract: This paper describes a theoretical and experimental analysis on full-filed stress distribution from thermoelastic measurements and its application to determination of stress concentration. The sum of the principle stress can be measured by Thermal Stress Analysis (TSA). Lock-in Thermography is very effective tool to measure the structure stress distribution by its high thermal resolving. In this study, the thermoelastic effect theory is described and the relationship between the temperature and the applied stress is developed in an elastic material. Experiments were carried out with 2A12 aluminium alloys plate and ones with hole structure under cyclic load. The thermoelastic effect coefficient is obtained for 2A12 aluminium alloys materials, and the effect law is analyzed that the stress value measured was affected by load frequencies. The optional load frequency is obtained, and that is, the load frequency is selected greater than 3.5Hz for 2Al12 materilas, and it was found that the structure stress can be evaluated with good accuracies by the lock in thermography. The experiment was carried out for aircraft components stress distribution measurement and structure stress analysis. The experimental results show the stress concentration position is easy found from stress distribution by lock-in thermography.

Abstract: The application of thermoelastic stress analysis in composite materials is particularly complicated because of the anisotropy of the material, which determines the thermoelastic effect to be depended on the material property and mechanical performance. This paper describes a theoretical and experimental analysis on full-filed stress distribution from thermoelastic measurements and its application to determination of stress concentration. The sum of the principal stress can be measured by Thermal Stress Analysis (TSA). Lock-in Thermography has been applied to measure the sum of principal stress distribution of component structure by its high thermal resolving. In this study, Experiments were carried out with GFRP composite ply and foam materials under cyclic load. The thermoelastic constant is obtained for GFRP and foam composite materials. The stress concentration is analyzed for a specimen with a hole. The experimental results show the stress distribution can be measured and analyzed using Lock-in thermography. It is found that the composite material structure stress can be evaluated with good accuracies by lock in thermography.

Abstract: This paper describes a theoretical and experimental analysis on full-filed stress distribution from thermoelastic measurements and its application to determination of stress concentration. The sum of the principal stress can be measured by Thermal Stress Analysis (TSA). Lock-in Thermography has been applied to measure the sum of principal stress distribution of component structure by its high thermal resolving. In this study, Finite element method is used to calculate the sum of principal stress distribution, and the thermoelastic effect model is developed to study the relationship between the temperature deviation and the applied stress in an elastic material. Experiments were carried out with ANSI 7071 high strength aluminum alloys ply and ones with a crack under cyclic load. The thermoelastic constant is obtained for ANSI 7071 high strength aluminum alloys materials. The stress concentration factor is calculated for a ply with modeling crack under the condition of different loads. The experiment was carried out with high strength aluminum alloys component structure with rivet joints. The experimental results show the stress distribution can be measured and analyzed the contact stress distribution between ply and rivet by using Lock-in thermography. It was found that the structure stress can be evaluated with good accuracies by the lock in thermography.

Abstract: Lock-in thermography (LT), that is active infrared testing technology, mainly includes optical lock-in thermography (OLT) and ultrasound lock-in thermography (ULT). LT can be used to detect unbonds between honeycomb core and face sheet of sandwich structures. However, modulation frequency is an important influencing factor. In this paper, the principles of LT are represented, in experimental detections of simulated unbonds in honeycomb sandwich structures with Al-face sheet and CFRP-face sheet using OLT and ULT, detectability of OLT and ULT is compared and analyzed, effect of modulation frequency is researched and the optimal frequencies are obtained.

Abstract: In machining, coolants and lubricants improve machinability, increase productivity by reducing the tool wear and extend the tool life. However, the use of cutting fluid in metal working may seriously degrade the quality of environment. Green cutting is becoming increasingly more popular due to concern regarding the safety of the environment and operator health. The experimental investigations were carried out with cemented carbide tool in turning ANSI 304 stainless steel and applications of water vapor, CO2, O2 as coolants and lubricants. The application of water vapor as coolants and lubricants allowed extending tool life. The catalysis chemical reaction have been generated between water molecules (H2O), oxygen atoms (O) of water vapor and fresh metal surface of tool-chip contact region, and multi-dimension metal oxidations which has been formed in tool-chip contact zone weakened mutual action between tool bulk material and chip.

Abstract: Water vapor is a good, pollution-free and economical coolants and lubricants in green machining. In order to research the cutting temperature distributions with water vapor as coolants and lubricants in machining, the experiments conducted under the conditions of water vapor as coolants and lubricants and dry cutting. The cutting temperatures are studied by metal cutting theory, and then by employing the finite element method (FEM), the temperature distribution of cutting region is simulated with application of water vapor as coolants and lubricants and dry cutting conditions. The results show that the water vapor jet flow has high force-convection heat transfer coefficient and directly cooling action to reduce cutting temperature, and the stress, the length of tool-chip interface are reduced with application of water vapor lubrication. So that the cutting thermal is decreased and water vapor has indirect cooling action. Under the conditions of indirect and direct cooling action, the cutting temperature is reduced obviously with application of water vapor as coolants and lubricants.