Laser Sintering, a method of additive manufacturing, is used in the production of concept models, functional prototypes, and end-use production parts. As the technology has transitioned from a product development tool to an accepted production technique, functional qualities have become increasingly important. Tension properties reported for popular polyamide sintering materials are comparable to the molded properties with the exception of elongation. Reported strains for laser sintered polyamide are in the 15-30% range with 200-400% strains reported for molding. (CES Edupack n.d.) The primary contributors to poor mechanical properties in polyamide materials used during Selective Laser Sintering® are studied. Methods to quantify decreased mechanical properties are compared against each other and against mechanical properties of components fabricated using multiple process parameters. Of primary interest are Ultimate Tensile Strength (UTS) and Elongation at Break (EOB) of tensile specimens fabricated under conditions that produce varying degrees of ductile and brittle fracture. / text

Constrained negative stiffness elements in volume concentrations (1% to 2%) embedded within viscoelastic materials have been shown to provide greater energy absorption than conventional materials [Lakes et al., Nature (London) 410, 565–567 (2001)]. This class of composite materials, called meta-materials, could be utilized in a variety of applications including noise reduction, anechoic coatings and transducer backings. The mechanism underlying the meta-material's behavior relies on the ability of the negative stiffness element to locally deform the viscoelastic material, dissipating energy in the process. The work presented here focuses specifically on the design of the negative stiffness elements, which take the form of buckled beams. By constraining the beam in an unstable, S-shaped configuration, the strain energy density of the beam will be at a maximum and the beam will accordingly display negative stiffness. To date, physical realization of these structures has been limited due to geometries that are difficult to construct and refine with conventional manufacturing materials and methods. By utilizing the geometric freedoms allowed by the Selective Laser Sintering (SLS) machines, these structures can be built and tuned for specific dynamic properties. The objective of this research was to investigate the dynamic behavior of SLS-constructed meso-scale negative stiffness elements with the future intention of miniaturizing the elements to create highly absorptive meta-materials. This objective was accomplished first through the development and analysis of a mathematical model of the buckled beam system. A characterization of the Nylon 11 material was performed to obtain the material properties for the parts that were created using SLS. Applying the mathematical model and material properties, a tuned meso-scale negative stiffness structure was fabricated. Transmissibility tests of the meso-scale structure revealed that the constrained negative stiffness system was able to achieve overall higher damping and vibration isolation than an unconstrained system. Quasistatic behavior of the system indicated that these elements would be ideal for implementation within meta-materials. Based on the results of the meso-scale system, a method to test a representative volume element for a negative stiffness meta-material was developed for future completion. / text

The focus of this dissertation is on the development of a solid freeform fabrication (SFF) process for the design and manufacture of porous biodegradable orthopaedic implants from calcium polyphosphate (CPP). Porous CPP structures are used as bone substitutes for regenerating bone defects and/or as substrates in formation of so-called “biphasic” implants for repair of damaged osteochondral tissues. The CPP implants can be utilized in the treatment of many musculoskeletal diseases, osteochondral defects, and bone tumours while replacement of the defect site is required. In this study, the fabrication of CPP structures was developed through a powder-based SFF technique known as adhesive bonding 3D-printing. SFF is an advanced alternative to the “conventional” fabrication method consisting of gravity sintering of CPP pre-forms followed by machining to final form, as SFF enables rapid manufacturing of complex-shaped bio-structures with controlled internal architecture. To address the physical and structural properties of the porous SFF-made components, they were characterized using scanning electron microscopy, micro-CT scanning and mercury intrusion porosimetry. Specific surface area and permeability of the porous structures were also determined. Additionally, the chemical properties (crystallinity) of the specimens were identified by X-ray diffraction. The mechanical properties of the crystalline CPP material were also measured by micro- and nano-indentation. Moreover, the porous structures were tested by uniaxial and diametral mechanical compression to determine the compressive and tensile strengths, respectively. Furthermore, the effect of the stacked-layer orientation on the mechanical properties of the SFF-made constructs was investigated through the production of samples with horizontal or vertical stacked-layers. The properties of the SFF-made samples were compared with those of the conventionally-made CPP constructs. The SFF-made implants showed drastically higher compressive mechanical strength compared to the conventionally-formed samples with identical porosity. It was also shown that the orientation of the stacked-layer has substantial influence on the mechanical strengths. Moreover, this thesis examined the ability of in vitro forming of cartilaginous tissue on the SFF-made substrates where the chondrocytes cellular response to the CPP implants was evaluated histologically and biochemically. In addition, an initial in vivo assessment of the CPP structures as bone substitutes was conducted using a rabbit medial femoral site model. Significant amount of new-bone was formed within the CPP porous constructs during the 6-week implantation period demonstrating appropriate biological response of SFF-made CPP structures for bone substitute applications. Another accomplishment of this thesis was the development of a mathematical model which predicts the compact density of powder layers spread by a counter-rotating roller in the SFF technique. The results may be used in the control of the apparent density of the final implant. The potential of the developed SFF method as an efficient and reproducible technique for the production of porous CPP structures for use in orthopaedics and musculoskeletal tissue regenerative applications was concluded.

The focus of this dissertation is on the development of a solid freeform fabrication (SFF) process for the design and manufacture of porous biodegradable orthopaedic implants from calcium polyphosphate (CPP). Porous CPP structures are used as bone substitutes for regenerating bone defects and/or as substrates in formation of so-called “biphasic” implants for repair of damaged osteochondral tissues. The CPP implants can be utilized in the treatment of many musculoskeletal diseases, osteochondral defects, and bone tumours while replacement of the defect site is required. In this study, the fabrication of CPP structures was developed through a powder-based SFF technique known as adhesive bonding 3D-printing. SFF is an advanced alternative to the “conventional” fabrication method consisting of gravity sintering of CPP pre-forms followed by machining to final form, as SFF enables rapid manufacturing of complex-shaped bio-structures with controlled internal architecture. To address the physical and structural properties of the porous SFF-made components, they were characterized using scanning electron microscopy, micro-CT scanning and mercury intrusion porosimetry. Specific surface area and permeability of the porous structures were also determined. Additionally, the chemical properties (crystallinity) of the specimens were identified by X-ray diffraction. The mechanical properties of the crystalline CPP material were also measured by micro- and nano-indentation. Moreover, the porous structures were tested by uniaxial and diametral mechanical compression to determine the compressive and tensile strengths, respectively. Furthermore, the effect of the stacked-layer orientation on the mechanical properties of the SFF-made constructs was investigated through the production of samples with horizontal or vertical stacked-layers. The properties of the SFF-made samples were compared with those of the conventionally-made CPP constructs. The SFF-made implants showed drastically higher compressive mechanical strength compared to the conventionally-formed samples with identical porosity. It was also shown that the orientation of the stacked-layer has substantial influence on the mechanical strengths. Moreover, this thesis examined the ability of in vitro forming of cartilaginous tissue on the SFF-made substrates where the chondrocytes cellular response to the CPP implants was evaluated histologically and biochemically. In addition, an initial in vivo assessment of the CPP structures as bone substitutes was conducted using a rabbit medial femoral site model. Significant amount of new-bone was formed within the CPP porous constructs during the 6-week implantation period demonstrating appropriate biological response of SFF-made CPP structures for bone substitute applications. Another accomplishment of this thesis was the development of a mathematical model which predicts the compact density of powder layers spread by a counter-rotating roller in the SFF technique. The results may be used in the control of the apparent density of the final implant. The potential of the developed SFF method as an efficient and reproducible technique for the production of porous CPP structures for use in orthopaedics and musculoskeletal tissue regenerative applications was concluded.

Today's commercial rapid prototyping systems (i.e., solid freeform fabrication, layered manufacturing) rely on human intervention to load and unload build jobs. Hence, jobs are processed subject to both the machine's and the operator's schedules. In particular, first-in-first-out (FIFO) queuing of such systems will result in machine idle time whenever a build job has been completed and an operator is not available to unload that build job and start up the next one. These machine idle times can significantly affect the system throughput, and, hence, the effective cost rate. This thesis addresses this problem by rearranging the job queue to minimizing the machine idle time, subject to the machine's and operator's schedules. This is achieved by employing a general branch-and-bound search method, that, for efficiency, reduces the search space by identifying contiguous sequences and avoiding reshuffling of those sequences during the branching procedure. The effectiveness of this job scheduling optimization has been demonstrated using a sequence of 30 jobs extracted from the usage log for the FDM 1600 rapid prototyping system in the Department of Mechanical Engineering at Virginia Tech. / Master of Science