Lianxiang Yang

This paper presents a relevant and timely study on the application of thermal loaded digital shearography for crack classification in glass fiber reinforced plastic (GFRP) structures, particularly air-cooled condenser (ACC) fan blades. A thermal loaded digital shearography system was applied to measure strain concentration caused by the cracks at different fatigue cycles. A thermomechanical model was introduced to estimate the heating temperature and the time to ensure heat can reach to the desired depth and that both shallow and deep cracks can be detected. In order to correlate the information of strain concentration in the shearograms to the different stages of cracks, fatigue testing with dynamic three-point bending was conducted. The fatigue tests demonstrated how the strain concentration evolved in the shearograms, while the crack developed from the early (no noticeable strain concentration), to the middle (strain concentration is forming), to the late stage (significant strain concentration is found). The relationships between the degrees of strain concentration in the shearograms and the different stages of cracks can be obtained from testing of the artificial cracks. Using the rules and experimental results obtained from artificial samples, digital shearography was applied to classify the crack stages in parts of ACC fan blades from industry. The combination of artificial crack testing, fatigue loading experiments, and validation with CT scans demonstrates a comprehensive approach and provides potential guidance for industry to determine criticality and maintenance criteria.

This research paper is focused on the development of a digital holography system for simultaneous dual-resolution measurements. Digital holography has been widely used for deformation measurements and non-destructive testing (NDT) due to its advantages of high sensitivity, high accuracy, and whole-field, non-touch measurements. A traditional test only has one spatial resolution, which can cause a big deformation to be indistinguishable or minor defects to be ignored. Both large and small fields of view should be observed to reach a multi-spatial resolution measurement. Usually, multiple separate tests are used to observe the different sized fields of view, resulting in higher costs and longer required testing times. Furthermore, these tests may not be repeatable in some cases. This paper presents research on a novel digital holography system that achieves dual spatial resolution measurements simultaneously by testing different-sized fields of view with a single camera. The novel system has two optical channels with two optical layouts of holography to measure deformation. By changing the combined focus length, the two holographic setups have different fields of view, i.e., one has a large and the other has a small field of view. To realize a simultaneous test, the polarization technique is used to avoid cross-interference between the two optical layouts. Finally, spatial carrier fringes with different orientations are introduced into the two holographic setups by appropriately adjusting the reference beam of each setup. The different oriented spatial carrier fringes enable the spectrums of the two interferograms to be separated after a FT (Fourier transform) and the phase distributions of the two interferograms can be extracted and separated by windowing the spectrum to perform an IFT (inverse Fourier transform). The phase distributions can then be used to analyze and calculate the deformations. The experiment using this system is described in this paper and the practicability of this method is verified by the obtained experimental results.

Composite materials are widely used in engineering because of their high strength, high modulus, and corrosion resistance. Accurately obtaining the mechanical properties of composites is helpful for their further development. However, in a uniaxial tensile test, the strain distribution between the front and back of the composite sheet sample may differ due to its heterogeneity, and its stress-strain curve depends on which side the strain is measured. To report the stress-strain curve more accurately, a multicamera digital image correlation (DIC) system was employed to measure the double-sided strain in this work. Two sets of 3D digital image correlation (3D-DIC) systems were placed in front of and behind the specimen to record the tensile process of the specimen, and a double-sided calibration technique was implemented to connect the two sets of 3D-DIC systems into a unified coordinate system. Young’s modulus, percent bending, and local strain history are analyzed based on the measurements of strain on both sides. The results show that the strain on both sides of the composite is not exactly same, especially in the strain concentration area. As a comparison, the strain distribution on both sides of the metal material is almost identical.

Shearography is a coherent optical technique that allows the identification of the first derivative of deformation in the shearing direction. Due to direct measuring strain information, shearography is suited for non-destructive testing and evaluation (NDT/NDE). However, if there is a small defect parallel to the shearing direction, the first derivative of deformation in the direction has no noticeable change, and the defect is not visible. Therefore, the development of a shearography system with dual-directional simultaneous measurement of the first derivatives of deformation both in x- and y-directions is highly demanded in the field of NDT/NDE. It is suited to inspect complicated defects, such as long and narrow slots, microcracks, etc. This paper presents a review of shearography for different dual-directional systems developed in the last two decades. After a brief overview of shearography, the paper will display two dual-directional shearographic techniques—temporal phase-shift (TPS) and spatial phase-shift (SPS) methods. TPS dual-shearing systems are suited for static measurements, while the SPS dual-shearing systems are useful for dynamic measurements. The basic theories, optical layouts, and comparisons are presented. The advantages and disadvantages of practical applications are discussed.

More and more attention has been given in the field of mechanical engineering to a material’s R-value, a parameter that characterizes the ability of sheet metal to resist thickness strain. Conventional methods used to determine R-value are based on experiments and an assumption of constant volume. Because the thickness strain cannot be directly measured, the R-value is currently determined using experimentally measured strains in the width, and loading directions in combination with the constant volume assumption, to determine the thickness strain indirectly. This paper provides an alternative method for determining the R-value without any assumptions. This method is based on the use of a multi-camera DIC system to measure strains in three directions simultaneously. Two sets of stereo-vision DIC measurement systems, each comprised of two GigE cameras, are placed on the front and back sides of the sample. Use of the double-sided calibration strategy unifies the world coordinate system of the front and back DIC measurement systems to one coordinate system, allowing for the measurement of thickness strain and direct calculation of R-value. The Random Sample Consensus (RANSAC) algorithm is used to eliminate noise in the thickness strain data, resulting in a more accurate R-value measurement.

International Learning Experience in China for Engineering Students at Oakland University Introduction The U.S. has long been the worldwide leader in science and technology and continues to be so. However, in a growing global world and with an increasingly diverse U.S. workforce, it is becoming paramount for U.S. engineers and researchers to develop the skills and background necessary to effectively work, communicate and innovate on an international scale and to be able to collaborate on complex engineering and research projects with colleagues and collaborators across the world. Countries such as China and India, by virtue of their size, are graduating ever increasing numbers of engineers and scientists each year and are making great strides in scientific research. The effects of the globalization of science, engineering and manufacturing have been particularly felt in rust-belt states such as Michigan and Ohio where the loss of market share by the big three U.S. automakers has resulted in large job losses and a migration of a growing number of qualified engineering students and professionals to other states in search of job and educational opportunities [1-2]. In Michigan in particular, where Oakland University is located, there is a growing awareness for the need to maintain a high quality workforce that is able to innovate in the areas of manufacturing engineering. Much has been made in the literature about the cultural differences between the U.S., European, Asian and other educational systems [3-11]. Although exchange between U.S. scholars and researchers and counterparts from developing or emerging countries has been taking place for a long time, much of that exchange has been unidirectional in the sense that typically, foreign scientists come to U.S. institutions to conduct research and make use of state-of-the-art laboratory facilities, rather than the other way around. The number of foreign students at U.S. universities is significantly higher than that of U.S. students at foreign universities. However, if the U.S. is to remain competitive in a global economy, it is important that future engineering leaders and researchers be given the opportunity to experience firsthand what it is like to work in a foreign engineering environment and to establish a network of contacts that could lead to future international collaborations. With that in mind, the department of Mechanical Engineering and the School of Engineering and Computer Science (SECS) at Oakland University (OU) have organized a number of short-term exchange programs with Chinese Universities for OU engineering students. This paper will focus on some details of the organization of one of these programs, including pre-departure preparation, on-site activities and post-program assessment. Emphasis will be placed on the lessons learned: what worked, what didn’t work and what improvements can be made in the following years to ensure the sustainability of the program. About the Program Sponsored by the Chrysler Foundation and Oakland University, the primary purpose of these exchange programs was to provide OU engineering students with the opportunity to work in China with Chinese engineering students on design projects and to learn about Chinese culture and language. A total of 44 OU students have so far taken part in the exchange programs that

Springback caused by elastic recovery is known to be a major disadvantage in sheet metal forming. Many studies focused on the prediction and compensation of springback in the bending process have used simulation and numerical methods, with experimental tests rarely being reported. A new approach, aimed at decreasing springback, is presented in this paper to solve the springback problem and provide a reference for other researchers. A new edge bending method called “incremental bending” has been developed and implemented on aluminum sheet at room temperature. Using this method, bending strain was lowered, leading to a reduction in springback. 3D Digital Image Correlation, an optical measurement technique, was adopted simultaneously to provide a high accuracy measurement of the full field strain distribution and springback curvature, making it very suitable for springback measurement. The experimental results show an obvious reduction of springback, which agrees well with the theoretical analysis. This work could help to minimize springback in sheet metal bending dramatically and improve formability and productivity, as well as provide a reference for researchers studying cold-bending. Details of the basic bending theory, experimental method, experimental results, and analysis will be demonstrated in the following content.