We develop bimaterial composites with enhanced impact resistance by mimicking a unique hierarchical geometry inherent in nacre. Three-dimensional models of the nacre-like composites are developed using pattern-generating algorithms, and the corresponding experiment specimens are fabricated by means of an FDM-based 3D printer. Under drop weight impact tests, it is found that the impact resistance of the nacre-like composite is significantly improved compared with a monolithic stiff specimen. The performance enhancement is also verified through numerical simulation with the use of a commercial finite element code. Mimicking the natural hierarchical architecture can render a guideline toward the development of high-performance material systems.
We investigate the nonlinear wave propagation through micro-cracks that are compressed by external forces by means of nonlinear ultrasonic modulation technique. The nonlinear modulated waves are generated by the truncation of the waves passing through cracks due to the opening and closing of the cracks, and the nonlinear ultrasonic modulation technique has been known to be effective in detecting finer cracks in comparison with other linear ultrasonic methods since the technique utilizes the breathing of the cracks rather than wave reflections or refractions. However, if the cracks are strongly compressed, the crack opening is hindered due to the excessive initial stress and the nonlinearity does not show up.
In this study, the improvement of the nonlinear modulation wave technique for the detection of micro-cracks under compression is devised. By analyzing photomicrographs of the cracks with crack width measuring algorithm, a realistic crack model is generated, and a chirp signal is applied to find the resonant frequencies which are used as the excitation frequencies. Experimental tests are conducted to verify the numerical results. The aluminum plate is compressed in the direction normal to the cracks’ lateral surfaces and is excited using piezoelectric patches attached on the surface aluminum plate. The experimental and numerical results show good agreement for various excitation frequencies and different compressions.
We investigate the nonlinear wave caused by interaction of surfaces of a fatigue crack, and study the effect of the crack’s contact compression on the magnitudes of nonlinear waves. Nonlinear wave modulation is generated when two ultrasonic waves having different frequencies passing through a crack, and the so-called nonlinear ultrasonic wave modulation technique is developed using this nonlinear waves. However, the magnitude of the nonlinear wave decreases as the crack contact compression increases because the large compression prevents the cracks from opening in motion. Even if the nonlinear wave modulation occurs in the damaged structures under compression, the magnitude might be different with the magnitude without compression. Consequently, finding the range of contact compression and the excitation directions with which the nonlinear wave modulation might occur is essential to use the technique for structural components under constraint compression. In order to examine the relations between the constraint compression and nonlinear wave modulation, we conduct numerical simulations under various compression by changing the excitation directions. The numerical model consists of a thin aluminum plate with a fatigue crack under constant compression, and the crack surfaces are modeled mimicking the shapes of real cracks. The roughness of the crack is determined using the crack widths obtained from optical measurement of fatigue cracks. Effective range of contact compression to generate nonlinear wave and the requirements to make the magnitudes of nonlinear waves non-trivial are described.
We present a technique for microcrack modeling in the finite element framework, and numerically investigate the occurrence of nonlinear wave modulation. Typically, fatigue cracks are initiated and developed when structures are exposed to repeated loading; the crack widths of the fatigue cracks are extremely small in the early development stage. As the fatigue cracks grow by combining and coalescing, the overall size increases. Enlarged cracks undermine the safety of the structure. Therefore, fatigue crack detection is very important to ensure the integrity of structures. Although the nonlinear ultrasonic wave modulation technique has been widely used due to its high detecting sensitivity, the basic principle is not fully understood. To reveal the mechanism of nonlinear wave modulation, the movements of the crack surfaces are calculated through numerical simulation. The shape of the crack surface can determine the intensity of the wave modulation. In this study, we investigate the variation of the crack widths due to fatigue failure using microscopic imaging of real fatigue cracks, and use these images to create realistic models of the fatigue cracks.
This study presents nonlinear ultrasonic wave modulations that can be effectively used for crack detection in thin structural components. Fatigue cracks occur when structure is exposed to repeated load although the load causes the smaller stress than the yield stress. The existence of cracks deteriorates the integrity of structures and reduces the safety. To detect these cracks, several kinds of nonlinear ultrasonic wave modulation techniques have been proposed for many years. However, the fundamental reason of the nonlinearity has not been well explained theoretically yet. Mostly, the phenomenon has been investigated experimentally. In order to find the reasons of the observed modulation, numerical studies are performed considering a variety of sizes of crack widths and depths using a commercial FEA program.