Polymer Processing: Modeling and Correlations Finalized to Tailoring the Plastic Part Morphology and Properties

The analysis of polymer processing operations is a wide and complex subject; during polymer processing, viscoelastic fluids are forced to deform into desired geometries using non-homogeneous velocity and temperature fields down to solidification. The objective of analysis is the identification of processing conditions, which are finalized in the optimization of product final properties, which, in turn, are determined by the final part morphology. Depending on the operating conditions, the properties of the final part can change more than one order of magnitude. Properties of interest include the mechanical, optical, barrier, permeability, and biodegradability, and any other property of practical relevance including the characteristics of the surfaces as its finishing and wettability, which are connected to one another. The scope of this Special Issue is to select progress in or reviews of the understanding/description of the phenomena involved along the chain of processing–morphology–properties. Along this virtual chain, modeling may be a useful approach, and within the objective of understanding fundamental aspects, it may also be relevant to compare selected characteristics of the process and the material with the characteristics of the resulting morphology and then with the properties of the final part. This approach suggests the title: “Polymer Processing: Modeling and Correlations Finalized to Tailoring the Plastic Part Morphology and Properties”

Modeling-Driven Damage Tolerant Design of Graphene Nanoplatelet/Carbon Fiber/Epoxy Hybrid Composite Panels for Full-Scale Aerospace Structures

The objective of this study is to design a new nano graphenecarbon fiberpolymer hybrid composite that can be used for the NASA SLS Composite Exploration Upper Stage (CEUS) forward skirt structure. The new material will improve the resistance to open-hole compression failure of the structure relative to traditional polymer fiber composites. The material is designed rapidly and with little cost using the Integrated Computational Materials Engineering (ICME) approach. Multiscale modeling and experiments are used to synergistically optimize the material design to yield improved properties and performance by controlling key processing parameters for manufacturing nano-enhanced materials. Specifically, the nanocomposite panel showed a 22 reduction in mass relative to the traditional composite panel, while both designs are equal in terms of ease of manufacture. This potential mass savings corresponds to an estimated 45 savings in materials and manufacturing costs. The multiscale ICME workflow developed for this project can be readily applied to the development of nano-enhanced composite materials and large aerospace structures. In addition, all key aspects of ICME were employed to complete this project including multiscale modeling, experimental characterization and visualization, data management, visualization, error and uncertainty quantification, and education. The results presented herein indicate a dramatic level of success, as well as the power and potential of ICME approach and multiscale modeling for composite materials

Layered fabrication of tool steel and functionally graded materials with a Nd:YAG pulsed laser

Rapid Prototyping technologies have been developed to transform three-dimensional computer models into physical prototypes within a compressed period of time. The problem of many existing laser and metal powder based techniques is the insufficient strength of parts to meet the practical requirements due to incomplete sintering or melting of powders. One of the research objectives was to melt tool steel powder completely and form fully dense fused structures with a 550 W Nd: YAG pulsed laser. The other objective was to produce material structures with graded composition, so-called Functionally Graded Materials (FGM). It is believed that this process could eventually produce preforms with complex material structures. Tungsten carbide was selected to be mixed with tool steel powder for possible wear resistance applications. The investigation on laser fusing tool steel was first carried out. The optimal process settings were concluded by measuring the contact angle, the surface roughness, bead height and variance of bead width of each single bead produced under various conditions. Fusing overlapped beads and multiple layers was then followed by studying the effects of scan spacings, scanning patterns and layer thickness. A scanning pattern was developed to effectively reduce porosity. Dense cubes of tool steel were then successfully produced with porosity of less than 1 %. Based on the findings from processing tool steel powder, different ratios of WC and tool steel were mixed and processed under the same processing conditions to produce FGMs. Various analysis techniques, including scanning electron microscopy, energy and wavelength dispersive X-ray were applied to examine the microstructures. WC was found partially dissolved in the matrix and evidence of liquid phase sintering was found in powder densification. Hardness, microhardness and nano-indentation testing were performed to show the hardness values in accordance with compositional changes in macro, micro, and nano-scales. The FGM of 80wt% H 10 and 20wt%WC showed an increase in hardness of at least 5-10% from the samples of H10

Joining of Dissimilar Materials

Material manufacturers and engineering structure designers are currently focusing new ways to exploit the benefits of light-weight, hybrid materials with improved properties at a low cost. The ability to join dissimilar materials is enabling the design engineers to develop light-weight and efficient automobiles, aircraft and space vehicles. The objective of this PhD research study was to produce alternative and efficient joining solutions for automotive and aerospace applications. The joining of dissimilar material was experimented to obtain light-weight Fibre Reinforced Polymer (FRP) sandwich composites, Al-foam sandwich (AFS) composites, hybrid dynamic FRP epoxy/polyurethane composites and the joining of Ti6Al4V alloy with and without surface modification to Ceramic Matrix Composite (CMC) and itself. The joining of Al-foam and Al-honeycomb to FRP skins was performed. The experimental results show that higher flexural properties can be achieved by replacing Al-honeycomb with low-cost Al-foam as a core material in the sandwich structures. Compared to FRP-honeycomb sandwich panels, FRP-Al foam sandwich panels display ~25 % and ~65 % higher flexural strength in a long and short span three-point bending tests respectively. AFS composites with complete metallic character, to withstand high-temperature application conditions, were produced by soldering/brazing techniques using Zn-based and Al-based joining alloys. A post-brazing thermal treatment was designed to recover the mechanical properties of AFS composites, lost during the soldering/brazing process. The microstructural analysis of the Al-skin/Al-foam interface revealed that the diffusion of joining materials into the joining substrates (Al-sheet and Al-foam) was achieved. Around 80% higher bending load before failure was observed when the AFS specimens produced with Zn-based joining alloys were subjected to flexural load compared to those produced with Al-based joining alloys. Hybrid dynamic Carbon Fibre Reinforced Polymer (CFRP) composites with enhanced impact properties were produced by exploiting the reversible cross-linking functionalities of dynamic epoxy and dynamic PU resin systems. By joining dynamic CFRP-epoxy and dynamic CFR-PU laminates, hybrid dynamic composite in three different configurations and a non-hybrid composite were obtained. The four dynamic composites were characterised for structural, thermal, flexural and impact properties. The damage initiation upon impact was observed at around 95% higher energy level in the hybrid configuration (CFRP-4), compared to the non-hybrid configuration. The hybrid configuration CFRP-3 responded with around 55% higher perforation threshold energy compared the non-hybrid configuration. Preliminary work on Adhesive joining of the Ti6Al4V alloy to itself was performed to analyse the effect micro-machining on adhesion and the effect of shape/design of micro-slots on an adhesive joint strength. Three types of micro-slots: V, semi-circle and U-shaped micro-slots were produced on Ti6Al4V sheet surface by using an in-house developed Micro-Electro-Discharge Machining (Micro-EDM) setup. Ti6Al4V alloy specimens with and without micro-machined surfaces were bonded together using a commercial epoxy adhesive. The Single Lap Offset (SLO) shear test results revealed that the micro-slot oriented perpendicular to the applied load displayed ~23 % higher joining strength compared to when the micro-slots were oriented parallel to the applied load. U-shaped micro-slots configuration displayed ~30 % improvement in the joint shear strength compared to the specimens with un-modified surfaces. The fractured surfaces analysis revealed mix (adhesive-cohesive) with cohesive dominated failure in bonded specimens with micro-machined surfaces compared to the as-received where pure adhesive failure was observed. The joining of CMCs (C/SiC and SiC/SiC) to Ti6Al4V alloy was experimented using active brazing alloy (Cusil-ABA) and Zr-based brazing alloy (TiB590) in a pressure-less argon atmosphere. The CMC-Ti6Al4V joint strength was further improved by modifying the surface of Ti6Al4V alloy using an in-house built Micro-EDM setup. Around 40% higher joining strength was recorded when the Zr-based brazing alloy was used as a joining material compared to the conventional active brazing alloy, Cusil-ABA. Improvement in the joining strength was noticed when the Ti6Al4V surface was modified prior to joining

AN EXPERIMENTAL AND NUMERICAL STUDY OF NANOMECHANICAL BEHAVIOR OF HARD/SOFT MULTILAYERED COATINGS

Multilayer thin film composites, sometimes referred to as nanolaminates, have emerged as an important subset of materials with novel, and often tunable, properties such as high strength, high toughness, and resistance to wear or corrosion. Often fabricated using alternating layers of two or more materials, these multilayer thin film coatings are typically expensive and time intensive to fabricate and characterize and exhibit novel responses to nanomechanical testing such as plasticity during unloading. This thesis explores the nanoindentation response of hard/soft multilayer coatings through examination of the optical coating Al/SiC and similar coating Al/SiO2. Instrumented indentation was used to study single layer films of aluminum, silicon carbide, and silicon dioxide with thicknesses 40nm to 4μm. Results from individual and cyclic indentation load cycles provided insight into film mechanical properties. Additionally alternating 51 layers hard/soft multilayers on silicon and quartz substrates were studied with spherical and Berkovich indenters. These multilayer films were fabricated with bilayer thickness of 160nm but variable thickness ratio to achieve 25, 50, and 75% aluminum by volume. Further microstructural characterization is necessary to fully explain the indentation behavior, however an accurate prediction of indentation derived modulus for the nanolaminate based upon monolayer properties was found. Furthermore, cyclic indentation of the nanolaminates along with post-indentation TEM led to the conclusion that unloading plasticity was not occurring within the multilayer structure or the effect was not significantly altering the indentation response. Finite element simulations were created to model individual load cycles for each combination of indenter, thickness ratio, film material, and substrate using ABAQUS. Single layer and multilayer simulations exhibited plastic deformation increasing within the aluminum layers during the unloading phase of indentation for all cases. Further simulation was conducted focusing on the cyclic indentation of aluminum thin films and Al/SiC nanolaminates. It was concluded that the simulation adequately represented the single material film responses but were unable to predict the indentation-derived properties for the Al/SiC multilayer. Further investigation would benefit from knowledge of the ceramic microstructure and viscous properties

Review of advanced composite materials for spacecraft applications

Review, abstracts, and bibliography on advanced composite materials for spacecraft application

Embedded Sensors to Monitor Production of Composites : From Infusion to Curing of Resin

The need for using light-weight and high-strength fibre reinforced polymer in different applications has increased in the past few decades. The ideal product offers excellent mechanical and chemical properties with much lower weight compared to traditionally used metals. Initially, the fibre-reinforced polymers are being produced by trial and error iterations. This causes a very expensive product, with random quality and lack of reproducibility. There is a need to replace trial and error experiments with knowledge-based approaches. Using sensors for in-situ production to monitor the results in a reliable and repeatable way gives a high-quality composite product and optimizes the time and cost of the process. One of the common manufacturing processes of fibre-reinforced polymer composite is resin infusion in dry fabrics. The resin impregnates the fibrous textile through the existence of a pressure gradient in the fibrous mat, which is generated by a vacuum pump or by a resin injection at high pressure. The impregnation of the dry textile is a result of the pressure gradient between resin inlet and venting point in the mold. Therefore, the most relevant measurement to detect the resin front and the changes of resin hydrostatic pressure is measuring the pressure directly inside the laminate. In this study, pressure sensors provide real-time information about the resin front in laminate and the changes of resin hydrostatic pressure during the infusion. Different pressure sensors and interconnection techniques were examined to minimize the size of the sensing element in the composite. After complete impregnation of the fibres, the curing degree of the resin has to be measured. Microscale interdigital capacitive sensors with a perforated substrate of polyimide are designed and fabricated. The sensors are fabricated on polyimide substrate with a thickness of about 5 micrometers. The polyimide is thermally stable up to 450 degree celsius. Therefore, the sensor can be used for a variety of processes even with high-temperature curing requirements. They have a volume of around 0.1 mm3. The miniaturized dimensions of the sensor enables it to remain in the composite product with the negligible diminishing of mechanical properties. The metallization of the sensor is insulated with metal oxide built up from the metallization itself. This insulation layer enables measurement in electrically conductive carbon fibres. The sensors will remain inside the composite material for structural health monitoring during the life-time of composite. Ideally, the sensors for online process monitoring of composites should be made of the identical fibres or resin in that composite. This will eliminate the wound effect in the host material. To obtain sensorial material, a high-performance resin for aerospace application, type RTM6, is mixed with different plasticizers. The cured mixture of the resin is thin and flexible. An interdigital comb structure is screen-printed on the newly developed substrate. The curing degree of the RTM6 resin in glass and carbon fibres is measured by screen-printed planar interdigital sensor on flexible RTM6. Having sensors for online process monitoring is important for industry 4.0 to autonomously produce fibre reinforced composites in a so-called smart factory . Both, pressure sensors and interdigital capacitive sensors in this thesis can be used for online process monitoring. They will provide a knowledge-based approach for high-quality and low-cost products