# Browsing by Subject "Lattice structures"

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Item An open source DIC system to test additively manufactured parts(2020-06-29) Allan, Andrew Ogando dos Santos; Seepersad, CarolynShow more Additively manufactured (AM) parts often assume unconventional geometries. Testing them in a load frame is difficult because they lack standard features on which traditional measurement devices (e.g. extensometers) can be attached. To solve this problem, an open source Digital Image Correlation (DIC) testing system was designed and implemented to complement an Instron load frame. DIC is a non-contact measurement technique that measures full-field strains and displacements from image sequences captured during load tests. The first step in designing the system was to identify the hardware, software, and specimen features necessary for successful DIC analysis. Then, based on that information and the lab’s testing requirements, the system components were selected, and hardware was designed to install them on the load frame. The system build was finalized by establishing a procedure for accessing, processing, and plotting the DIC data. The resulting DIC system can perform stress-strain analysis on specimens ranging in size from 0.5mm x 3mm to 10cm x 10cm in size. The total cost of the system is approximately $1,300. The final section presents a study that was performed to demonstrate the efficacy of the DIC system. It examines the effective material properties of direct metal laser sintered (DMLS) lattice struts as a function of build orientation and size. The test specimens consist of miniature tensile bars with four struts of equal diameter in the gauge section. Two different diameters of struts were built in five orientations and tested with the DIC system to evaluate Young’s modulus, ultimate strength, and elongation at break. The results indicate that all three properties decrease with decreasing build angle and strut diameter. Varied amounts of surface roughness on the struts resulting from varied build conditions could explain the observed trends in material properties, but further analysis is necessary to fully contextualize the results.Show more Item Modeling and computing based on lattices(2010-12) Zhao, Haifeng, 1980-; Rodin, G. J. (Gregory J.); Mear, Mark E.; Ravi-Chandar, K.; Makarov, Dmitrii E.; Kovar, DesiderioShow more This dissertation presents three studies addressing various modeling and computational aspects of lattice structures. The first study is concerned with characterization of the threshold behavior for very slow (subcritical) crack growth. First, it is shown that this behavior requires the presence of a healing mechanism. Then thermodynamic analysis of brittle fracture specimens near the threshold developed by Rice (1978) is extended to specimens undergoing microstructural changes. This extension gives rise to a generalization of the threshold concept that mirrors the way the resistance R-curve generalizes the fracture toughness. In the absence of experimental data, the resistance curve near the threshold is constructed using a lattice model that includes healing and rupture mechanisms. The second study is concerned with transmission of various boundary conditions through irregular lattices. The boundary conditions are parameterized using trigonometric Fourier series, and it is shown that, under certain conditions, transmission through irregular lattices can be well approximated by that through classical continuum. It is determined that such transmission must involve the wavelength of at least 12 lattice spacings; for smaller wavelength classical continuum approximations become increasingly inaccurate. Also it is shown that this restriction is much more severe than that associated with identifying the minimum size for representative volume elements. The third study is concerned with extending the use of boundary algebraic equations to problems involving irregular rather than regular lattices. Such an extension would be indispensable for solving multiscale problems defined on irregular lattices, as boundary algebraic equations provide seamless bridging between discrete and continuum models. It is shown that, in contrast to regular lattices, boundary algebraic equations for irregular lattices require a statistical rather than deterministic treatment. Furthermore, boundary algebraic equations for irregular lattices contain certain terms that require the same amount of computational effort as the original problem.Show more