Publications

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Quasi-static experimental techniques for fracture toughness have been well developed and end notched flexure (ENF) technique has become a typical method to determined mode-II fracture toughness. ENF technique also has been implemented to high-rate testing using SHPB (Split Hopkinson Pressure Bar) technique for dynamic fracture characterization of composites. In general, the loading condition in dynamic characterization needs to be carefully verified that forces are balanced if same equations are used to calculate the fracture toughness. In this study, we employed highly sensitive polyvinylidene fluoride (PVDF) force transducers to measure the forces on the front wedge and back spans of the three-point bending setup. High rate digital image correlation (DIC) was also conducted to investigate the stress wave propagation during the dynamic loading. After careful calibration, the PVDF film transducer was made into small square pieces that are embedded on the front loading wedge and back supporting spans. Outputs from the three PVDF transducers as well as the strain gage on the transmission bar are recorded. The DIC result shows the transverse wave front propagates from the wedge towards the supports. If the crack starts to propagate before reaching force balance, numerical simulation, such as finite element analysis, should be implemented together with the dynamic experimental data to determine the mode-II fracture toughness.

Quasi-static experimental techniques for fracture toughness have been well developed and end notched flexure (ENF) technique has become a typical method to determined mode-II fracture toughness. ENF technique also has been implemented to high-rate testing using SHPB (Split Hopkinson Pressure Bar) technique for dynamic fracture characterization of composites. In general, the loading condition in dynamic characterization needs to be carefully verified that forces are balanced if same equations are used to calculate the fracture toughness. In this study, we employed highly sensitive polyvinylidene fluoride (PVDF) force transducers to measure the forces on the front wedge and back spans of the three-point bending setup. High rate digital image correlation (DIC) was also conducted to investigate the stress wave propagation during the dynamic loading. After careful calibration, the PVDF film transducer was made into small square pieces that are embedded on the front loading wedge and back supporting spans. Outputs from the three PVDF transducers as well as the strain gage on the transmission bar are recorded. The DIC result shows the transverse wave front propagates from the wedge towards the supports. If the crack starts to propagate before reaching force balance, numerical simulation, such as finite element analysis, should be implemented together with the dynamic experimental data to determine the mode-II fracture toughness.

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As the specimen gets smaller and thinner, traditional strain measurement method using the strain gage is impossible. In this paper, the strain is measured using non-contact laser interferometry method. Two markers are placed on the LIGA specimens along the loading direction to reflect the laser beams to generate the interferometric fringe patterns. The markers are generated using micro-hardness indentation for the LIGA specimens. A pair of CCD cameras is used to capture the interferometric fringes during each step of the loading along the longitudinal direction. Fast Fourier Transform (FFT) is then applied to calculate the frequency and phase shift of the fringes. The displacement and strain can be obtained from the phase shift of the fringe pattern. This ISDG strain measurement technique is further developed by using multi markers to obtain fringes during the whole loading when the specimen undergoes larger motion. Biaxial strain measurement using ISDG is also developed to obtain both Young's modulus and Poisson's ratio simultaneously. A third marker is located orthogonal to the first pair of markers along the loading direction. Two pairs of CCD cameras are used to acquire the digital images of the interferometric fringes patterns along both longitudinal and transverse directions. The stress-strain curves as well as the material properties are very consistent from the different tests using ISDG. Copyright © 2006 by ASME.

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It is essential to characterize the nonlinearity in scanning probe microscopes (SPMs) in order to acquire spatial measurements with high levels of accuracy. In this paper, a new characterization method is presented that combines a high-resolution image processing technique used by the experimental mechanics community known as Digital Image Correlation (DIC) with digital images from a standard type of SPM known as an atomic force microscope (AFM). The characterization results using this new method match those from the conventional method using micromachined calibration gratings. However, the new method uses the texture of a specimen surface and not a precisely micromachined calibration grating. As a consequence, the new characterization technique is a more direct method for measuring scanning errors that can be conducted in situ when imaging a specimen surface at any scale within the scanning range of the SPM. It also has the advantage of reconstructing the position error curve more continuously with less noise than the conventional method.