No description available.

<p>Composite materials have become widely used for high-performance applications, particularly in the aerospace industry where annual production volumes are low and a higher part cost can be supported. During the last decades composite materials are beginning to see use in a broader range of applications, including the automotive and sports equipment industries. Simultaneously, there is increasing demand from consumers and regulatory bodies to make cars more fuel efficient and in the case of EV’s longer drive range, which can be accomplished by reducing vehicle weight. Composite materials have high specific stiffnesses and strengths, resulting in weight savings when they are used to replace traditionally metal components. However, in order for widespread adoption of composite parts to be viable for the automotive industry, high-rate manufacturing must be realized to reach the required production volumes and part costs.</p> <p>Toward this goal, advanced composite manufacturing techniques have been developed. These techniques typically combine high automation with careful material selection, which can include fast-curing resins and thermoplastics with adapted melt viscosities and thermomechanical properties. They also allow for complex part geometries to be produced in a single step, reducing the need for additional assembly time. Further, they can be used to easily create multi-material components, which can result in parts that benefit from the desirable mechanical properties of the constituent materials without sacrificing performance.</p> <p>This thesis develops a framework for the design and high-rate manufacture of multi-material components. First, a critical literature review is conducted to develop a clear understanding of existing research into combinations of dissimilar materials, including epoxy/polyamide, thermoplastic elastomer/polyamide, and aluminum/thermoplastic. It is shown that, for all material combinations studied, interfacial delamination and subsequent deformation are the primary energy absorption mechanisms and that manufacturing conditions may affect interfacial bond strength. Based on this foundation, adhesion testing is performed on devoted sample configurations fabricated under controlled molding conditions. For these material combinations, interfacial adhesion can be significantly improved with carefully selected processing temperatures, even to the extent that adhesive bond between dissimilar materials can be stronger than the cohesive bond in the constituent materials. Next, impact and quasi-static indentation testing were performed to determine the effects of interfacial adhesion and part design on crash performance. The materials tested all benefit from the placement of a more ductile material on the impacted side of the sample (top surface), indicating a more favorable dissipation of the contact stresses from the impactor, and a higher strength material on the bottom surface where it can withstand tensile stresses imposed by impact-induced bending. </p> <p> Finally, a complex part consisting of a unidirectional polyamide/carbon fiber preform and a thermoplastic overmold is manufactured via a hybrid overmolding process. Interfacial temperature during overmolding is varied to confirm if the same improvements in interfacial bond strength seen in the compression molding test samples are attainable under realistic high-rate manufacture conditions. Additionally, the preform volume is varied to examine the effect of the preform reinforcement on a part’s bending performance. For this system, varying the preform temperature had no effect on interfacial bond strength. A predictive technical cost model is also used to determine the effect of manufacturing changes on part costs. Increasing the tow volume three-fold increased the absorbed energy by more than 30% and requires an increased cost of only 3.8%. </p> <p>This thesis proves that a tough, multi-material part can be rapidly produced via hybrid overmolding. It was demonstrated that a complex shaped part could be produced at a complete line cycle time of approximately 90 secondsmaking it a viable method to produce high-performance, low-cost components. </p>

No description available.

No description available.

<div>The service life of aircraft structure, primarily composed of aluminum alloys, is markedly lower when galvanic corrosion is present due to early crack initiation at localized pitting, with the likelihood of cracking being higher at pits spanning several microns. To understand the joint effect that the mechanical and chemical behavior of AA7050-T7451 have on the evolution of corrosion prior and until its transition to cracking, the microstructure, constituent particles, mechanical strains, and the corrosion morphology were experimentally characterized using high-resolution methods and the mechanical stresses are computationally modeled at the micrometer level using a FFT-based crystal plasticity framework. </div><div><br></div><div>The material was corroded under both mechanically loaded and unloaded conditions under different corrosion intervals to properly capture the evolution of corrosion before, during, and after particle fallout. For the events prior to cracking, statistical cross-correlations between the mechanical state of the material and the corrosion morphology were performed to understand the mechanisms driving corrosion at its various stages. For the cracking event and its subsequent growth, the joint analysis of strains and stresses obtained from 3D crystal plasticity models were used to calculate Fatigue Indicator Parameters (FIPs) that can quantitatively give an insight of the major mechanisms driving crack initiation and growth in pre-corroded materials. The development of micromechanical models that account for both the environmental degradation and the microstructure in the material allowed to accurately predict the location of crack initiation arising from pits, which has been a longstanding problem in the field of corrosion. It is concluded that both corrosion growth and its transition to cracking are multivariable events, where corrosion growth is jointly driven by the local chemistry and the micromechanics, and crack initiation is driven by the coupled interaction between the corrosion geometry and the micromechanics.</div><div><br></div>

Platelets made of slit and chopped unidirectional, carbon-fiber prepreg are becoming a popular option for use as a high performance molding compound because of their high fiber volume fraction and increased ability to flow compared to continuous fiber systems. As this molding compound is newly introduced to industry, increasing amounts of research have gone into understanding how platelets flow during molding and how components perform mechanically based on the final orientation state of platelets. This work investigates the performance of prepreg platelet molding compound (PPMC) as a viable alternative to continuous fiber systems for use with geometrically complex structural members on vehicles subjected to collisions. In doing so, the crash performance, energy absorption, and failure morphology of crush tubes made with PPMC are investigated and quantified. Then, a simulation methodology is developed to obtain manufacturing-informed performance models to predict the effect of platelet orientation state on mechanical behavior of PPMC components. This methodology uses a building block approach where each block in modeling is verified against closed-form solution (when available) and validated against experimental results. Once confidence is developed in a modeling block, the complexity of the simulation is increased until a component with full platelet orientation distribution is captured. The result is PPMC component models that are capable of predicting mechanical performance in orientation regimes that are not investigated experimentally.

<p>Aeronautical products additively manufactured by Selective Laser Melting (SLM), are known to have fatigue properties which are negatively impacted by porosity defects, microstructural features and residual stresses. Little research is available studying these phenomena with respect to the short fatigue crack growth (FCG) inconsistency problem, the large focus being on the long FCG. This thesis seeks to add useful knowledge to the understanding of the mechanisms for short crack growth variability in SLM manufactured Ti-6Al-4V, with the two variables for the process conditions and build directions investigated. An in-situ FCG investigation using x-ray synchrotron computed micro-tomography (μXSCT) was used to visually observe and quantify the short crack path evolution. Crack growth, deflections and porosity interactions were noted and discussed in relation to microstructure, build layer thickness and build layer orientation. A novel use of in-situ energy dispersive x-ray diffraction (EDD) was able to show the lattice strains evolving as a propagating crack moved through a small region of interest. The results presented show the ability to reliably obtain all six elastic strain tensor components, and interpret useful knowledge from a small region of interest. </p> <p> </p> <p>There are conflicting views in literature with respect to the damage tolerance behavior of as built SLM manufactured Ti-6Al-4V. In the 2018 review by Agius et al., the more prominent studies were considered with Leuders et al. showing the highest long FCG rates for cracks parallel to the build layer and Cain et al. showing cracks propagating through successive build layers as highest [1]–[3]. Cain et al. and Vilaro et al. report significant anisotropy in long FCG for different build orientations whereas Edwards and Ramulu present similar FCG behavior for three different build directions [2]–[5]. Kruth et al. concluded that for optimized build parameters without any (detectable) pores, the building direction does not play a significant role in the fracture toughness results [6]. All of the mentioned literature reported martensitic microstructures and the presence of prior grain structures for as built SLM Ti-6Al-4V.</p> <p> </p> <p>No studies to the authors knowledge have considered the short FCG of SLM manufactured Ti‑6Al‑4V and its implications to the conflicting damage tolerance behaviors reported in literature [1]. In this work small cross-sectional area (1.5 x 1.5 ) samples in two different build conditions of as built SLM Ti-6Al‑4V are studied. The short FCG rate of three different build directions was considered with cracks parallel to the build layers shown to be the most damaging. The microstructure and build layer are shown to be the likely dominant factors in the short FCG rate of as built Ti-6Al-4V. In terms of porosity, little impact to the propagating short crack was seen although there is local elastoplastic behavior around these defects which could cause toughening in the non-optimized build parameter samples tested. The fracture surfaces were examined using a Scanning Electron Microscope (SEM) with the results showing significant differences in the behavior of the two build conditions. From the microindentation hardness testing undertaken, the smooth fracture surface of the optimized sample correlated with a higher Vickers Hardness (VH) result and therefore higher strength. The non-optimized samples had a ‘rough’ fracture surface, a lower VH result and therefore strength. Furthering the knowledge of short FCG in SLM manufactured Ti-6Al-4V will have positive implications to accurately life and therefore certify additive manufactured aeronautical products.</p>

Micro-mechanical Raman spectroscopy is an excellent tool for direct stress measurements in the structure. The presence of mechanical stress changes the Raman frequency of each Raman modes compared to the Raman frequencies in absence of stress. This difference in Raman frequency is linearly related to stress induced and can be calibrated to stress by uniaxial or biaxial tension/compression experiments. This relationship is not generally linear for non-linear behavior of the materials which limits its use to experimentally study flow stress and plastic deformation behavior of the material. In this work strontium titanate ceramic particles dispersed inside epoxy resin matrix were used to measure stress in epoxy resin matrix with non-linear material behavior around it. The stress concentration factor between stress induced inside ceramic particles and epoxy resin matrix was obtained by non-linear constitutive finite element model. The results of finite element model were used for training a machine learning model to predict stress in epoxy resin matrix based on stress inside ceramic particles. By measuring stress inside ceramic particles using micro-mechanical Raman spectroscopy, the stress inside epoxy matrix was obtained by pre-determined stress concentration factor.

Electrical Discharge Machining (EDM) is a non-traditional machining process that uses hundreds of thousands of minute electrical sparks per second to machine any electrically conductive material, no matter the hardness or how delicate it is. EDM allows a much greater range of design possibilities, unconstrained from the traditional machining processes, in which material is removed mechanically by either rotating the cutting tool or the work piece. Shapes that were impossible to machine by any other method, such as deep, precision, square holes and slots with sharp inside corners, are readily produced. It provides accurate geometries in high- aspect ratio holes and slots, blind undercuts, small holes adjacent to deep sidewalls, and complex cuts in thin, fragile parts. Micro-EDM is a growing form of manufacturing and will continue to expand within various production fields. Micro-EDM is especially attractive for the applications where the cutting time is minimal, but precision and accuracy are maximized. Micro- EDM is a non-traditional cutting process, which consistently produces ultra-precise holes with fine surface finishes and better roundness, while holding extremely close diameter tolerances. The process could be an excellent problem-solving tool for configurations that are difficult or impossible to produce using conventional machining processes. This study presents a comparative experimental investigation on the micro-EDM machinability of difficult-to-cut Ti-6Al-4V and soft brass materials. As both materials are electrically conductive, they were machinable using the micro-EDM process irrespective of their hardness. The machining performance of the two materials was evaluated based on the quality of the micro-features produced by the micro-EDM process. Both blind and through micro-holes and micro-slots were machined on brass and Ti-6Al-4V materials. The quality of micro-features was assessed based on the shape accuracy, surface finish and profile accuracy of the features. Finally, the arrays of micro-features were machined on both materials to compare the mass production capability of micro-EDM process on those materials.

Thesis (MScEng)--Stellenbosch University, 2011. / ENGLISH ABSTRACT: Fretting wear results in the loss of fit and tolerance at contact interfaces. The aerospace and aircraft industry is severely impacted by fretting wear and fretting fatigue that frequently occurs in turbo machinery and riveted structural connections. There have been numerous studies, investigating the fretting phenomenon for these aerospace applications. Literature available in regard to fretting wear encountered in these aerospace applications is limited. This study is therefore aimed at investigating the fretting wear encountered in aerospace application. An in-house fretting test apparatus was specially designed and developed in order to perform the fretting wear experiments. Ti-6Al-4V and Al7075-T6 are the two aerospace materials that were tested using the fretting test apparatus. An extensive experimental study was conducted in order to investigate the effect of the normal force on the fretting wear and friction behaviour of the two aerospace materials. The most severe of these experiments were identified and then repeated for up to 106 fretting cycles. Additional fretting wear experiments were also conducted between the two aerospace materials and cemented carbides, since the carbides are currently being utilized as coatings in some aerospace contacts that are prone to fretting induced damage. The experimental study revealed that a decrease in the normal force resulted in an increase in the severity of the fretting wear of both aerospace materials. The additional fretting wear experiments involving carbide-metal contact couples found that Ti-6Al-4V and Al7075-T6 were prone to adhesive wear. / AFRIKAANSE OPSOMMING: Knaagslytasie veroorsaak materiaalverlies by die kontakoppervlakke. Die lugvaart industrie is erg geraak deur knaagslytasie en knaaguitputting wat dikwels voorkom in turbo-enjin toepassings en strukturele verbindings. Daar was al talle studies gedoen oor die effek van knaag op lugvaart toepassings. Literatuur met betrekking tot knaagslytasie in lugvaart toepassings is egter beperk. Hierdie studie was dus gemik daarop om knaagslytasie in sekere lugvaart toepassings te ondersoek. Tydens die studie is ʼn toetsopstelling ontwerp en ontwikkel om knaagslytasie eksperimente uit te voer. Ti-6Al-4V en Al7075-T6 is die twee lugvaartmateriale wat ondersoek is met behulp van die toetsopstelling. ʼn Omvattende eksperimentele studie is gedoen om die effek van die normaal krag op knaagslytasie en die wrywings gedrag van die lugvaartmateriale te ondersoek. Die eksperimente wat die ergste slytasie en hoogste wrywing getoon het, is herhaal vir 106 siklusse. Bykomende knaag eksperimente was ook tussen die twee lugvaartmateriale en sekere karbiede gedoen, aangesien karbiede tans as deklae in sommige lugvaart kontakte gebruik word. Die eksperimentele studie het getoon dat 'n afname in die normale krag gelei het tot 'n toename in wrywing vir beide lugvaartmateriale. Die bykomende knaagslytasie eksperimente op karbied metaal pare het getoon dat Ti-6Al-4V en Al7075-T6twee lugvaart materiale nie in staat was om enige van die karbide te beskadig nie. Die lug-en Ruimte-materiaal aan die ander kant ervaar het kwaadaardige dra.