Dimitris Karalekas

Study of shrinkage strains in a stereolithography cured acrylic photopolymer resin

Abstract The shrinkage characteristics of stereolithography (SL) built square laminate plates using an acrylic-based photopolymer were studied after they had been post-cured under ultraviolet and thermal exposure. The specimen plates consisted of a resin layer laser cured on an identical layer of the same material that had already been cured and post-cured. The assembled laminate was then cured, and the resulting out-of-plane displacement (warpage) due to shrinkage was recorded by means of the shadow moire method. The exhibited warpage of the plates was related to the polymerisation shrinkage strains through the elastic lamination theory, which was implemented to calculate the magnitude of the resulting shrinkage strains. It was obtained that the acrylic-based resin plates exhibited considerable warpage, while thermal post-curing resulted to higher shrinkage in the y -axis of the plates.

Study of the mechanical properties of nonwoven fibre mat reinforced photopolymers used in rapid prototyping

The paper describes a series of preliminary experiments that were conducted to investigate the mechanical properties of two stereolithography resins reinforced using commercially available nonwoven fibre mats. Two commonly used photo-curable resin systems, an acrylic and an epoxy based one, were used to laser build the test specimens. Comparisons of the mechanical properties between pure-polymer specimens and their fibre-reinforced counterparts were made, by subjecting the parts to tensile tests. It was found that the fibre-reinforced specimens yielded higher measured values of elastic modulus and ultimate tensile strength.

On the Effects of the Lateral Strains on the Fiber Bragg Grating Response

In this paper, a combined experimental-numerical based work was undertaken to investigate the Bragg wavelength shift response of an embedded FBG sensor when subjected to different conditions of multi-axial loading (deformation). The following cases are examined: (a) when an isotropic host material with no constrains on planes normal to the embedded sensors axis is biaxially loaded, (b) when the same isotropic host material is subjected to hydrostatic pressure and (c) when the hydrostatically loaded host material is an anisotropic one, as in the case of a composite material, where the optical fiber is embedded along the reinforcing fibers. The comparison of the experimental results and the finite element simulations shows that, when the axial strain on the FBG sensor is the dominant component, the standard wavelength-shift strain relation can be used even if large lateral strains apply on the sensor. However when this is not the case, large errors may be introduced in the conversion of the wavelength to axial strains on the fiber. This situation arises when the FBG is placed parallel to high modulus reinforcing fibers of a polymer composite.

Mechanical characteristics of an Ormocomp(®) biocompatible hybrid photopolymer.

In this work, the mechanical behaviour of a photocured Ormocomp(®) hybrid material is investigated. Its biocompatible nature has attracted a growing interest for microfabrication applications in biomedicine and tissue engineering. Measurements of in situ solidification strain development and achieved degree of curing, as obtained using a fibre optic sensor, are presented. The results show that the solidification strains generated during UV-curing are significant at the maximum achieved degree of curing. The mechanical response (Youngs modulus) of the material was investigated by testing of thin-film and regular size specimens. It was found that the measured mean elastic modulus of the thin-film specimens was of the same order of magnitude as that of the larger specimens but noticeably smaller.

A Methodological Study for Optimizing Material Selection in Sustainable Product Design

A computational intelligence‐based identification of the properties of maximally sustainable materials for a given application, as derived from key properties of existing candidate materials, is put forward. The correlation surface between material properties (input) and environmental impact (EI) values (output) of the candidate materials is initially created using general regression (GR) artificial neural networks (ANNs). Genetic algorithms (GAs) are subsequently employed for swiftly identifying the minimum point of the correlation surface, thus exposing the properties of the maximally sustainable material. The ANN is compared to and found to be more accurate than classic polynomial regression (PR) interpolation/prediction, with sensitivity and multicriteria analyses further confirming the stability of the proposed methodology under variations in the properties of the materials as well as the relative importance values assigned to the input properties. A nominal demonstration concerning material selection for manufacturing maximally sustainable liquid containers is presented, showing that by appropriately picking the pertinent input properties and the desired material selection criteria, the proposed methodology can be applied to a wide range of material selection tasks.

Monitoring of hardening and hygroscopic induced strains in a calcium phosphate bone cement using FBG sensor.

This study initially deals with the investigation of the induced strains during hardening stage of a self-setting calcium phosphate bone cement using fiber-Bragg grating (FBG) optical sensors. A complementary Scanning Electron Microscopy (SEM) investigation was also conducted at different time intervals of the hardening period and its findings were related to the FBG recordings. From the obtained results, it is demonstrated that the FBG response is affected by the microstructural changes taking place when the bone cement is immersed into the hardening liquid media. Subsequently, the FBG sensor was used to monitor the absorption process and hygroscopic response of the hardened and dried biocement when exposed to a liquid/humid environment. From the FBG-based calculated hygric strains as a function of moisture concentration, the coefficient of moisture expansion (CME) of the examined bone cement was obtained, exhibiting two distinct linear regions.

Design, Fabrication and Computational Characterization of a 3D Micro-Valve Built by Multi-Photon Polymerization

We report on the design, modeling and fabrication by multi-photon polymerization of a complex medical fluidic device. The physical dimensions of the built micro-valve prototype are compared to those of its computer-designed model. Important fabrication issues such as achieving high dimensional resolution and ability to control distortion due to shrinkage are presented and discussed. The operational performance of both multi-photon and CAD-created models under steady blood flow conditions was evaluated and compared through computational fluid dynamics analysis.

Computational study of crack growth in SiC/Al composites

An elastic-plastic finite-element analysis has been carried out to study the fracture behaviour of a SiC/6061-Al filamentary composite. The composite was modelled as a two-material cylinder subjected to uniform displacement and consisting of an inner cylinder simulating the fibre and a surrounding shell simulating the matrix. Using the strain energy density criterion, the critical applied displacements for crack initiation and stable crack growth were determined for three studied cases: an intact fibre, a cracked fibre, and a fully broken fibre. The numerical results were compared, where possible, to experimentally obtained ones. Finally, results concerning the variation of strain energy density versus distance from the crack tip, for the determination of the critical value of applied displacement at crack initiation, are presented and discussed.

Temperature Mapping of 3D Printed Polymer Plates: Experimental and Numerical Study

In Fused Deposition Modeling (FDM), which is a common thermoplastic Additive Manufacturing (AM) method, the polymer model material that is in the form of a flexible filament is heated above its glass transition temperature (Tg) to a semi-molten state in the head’s liquefier. The heated material is extruded in a rastering configuration onto the building platform where it rapidly cools and solidifies with the adjoining material. The heating and rapid cooling cycles of the work materials exhibited during the FDM process provoke non-uniform thermal gradients and cause stress build-up that consequently result in part distortions, dimensional inaccuracy and even possible part fabrication failure. Within the purpose of optimizing the FDM technique by eliminating the presence of such undesirable effects, real-time monitoring is essential for the evaluation and control of the final parts’ quality. The present work investigates the temperature distributions developed during the FDM building process of multilayered thin plates and on this basis a numerical study is also presented. The recordings of temperature changes were achieved by embedding temperature measuring sensors at various locations into the middle-plane of the printed structures. The experimental results, mapping the temperature variations within the samples, were compared to the corresponding ones obtained by finite element modeling, exhibiting good correlation.

FBG Based In Situ Characterization of Residual Strains in FDM Process

Additive Layered Manufacturing has emerged as a popular manufacturing direction to accelerate product creation. Layered manufacturing can build parts that have traditionally been impossible to build because of their complex shapes or variety of materials. Layered manufacturing processes accumulate residual stresses and strains during material build up. Theses stresses may cause layer delamination and part distortion. This paper presents the work done on investigating residual strain accumulation in one of the most used additive processes, namely the Fused Deposition Modeling (FDM) and as a function of two typically selected process parameters. The developed residual strains at the end of the fabrication process were recorded using an optical sensor with a short fiber Bragg grating (FBG), embedded at the midplane of FDM built prismatic specimens. To assess the strain development without constraining effects from any adhesion to the building platform surface, measurements were taken at free-standing state at the end of the fabrication process. It is demonstrated that the magnitude of the solidification induced residual strains is significant and depends significantly on the selected material deposition direction (bead orientation) as a result of the formation of poor interbead interfaces and void (air gap) regions.