Relevant Degree Programs
Graduate Student Supervision
Doctoral Student Supervision (Jan 2008 - May 2019)
Out-of-autoclave (OOA) processing of prepregs represents a promising low cost alternative to expensive autoclave processing, which today is the standard method for producing high-quality CFRP parts such as aerospace structures; however, OOA processes have a greater potential to experience problems with voids than autoclave processes do. Understanding the evolution of voids during the prepreg process, and the OOA prepreg process in particular, is a key to robust and low cost processing that can produce high quality composite parts with low porosity. The evolution of voids in prepregs is best understood as the result of interacting transport mechanisms during the debulk and cure processing steps. The most important such mechanisms are the in-plane flow of gasses through an interconnected network of voids, the diffusion of dissolved moisture within the resin, the sorption and desorption of moisture into and out of the resin, and the flow of resin into the void spaces. A large experimental project has been carried out to experimentally measure the effect of varying the parameters of the OOA process on the porosity levels of cured parts. The results of the experiments show that factors like part size, moisture content, and debulk time influence the porosity levels of parts produced. The governing transport mechanisms have been investigated and characterized, and a numerical model capable of simulating the simultaneous action of these interacting mechanisms is proposed. The numerical model was validated by comparison with experimental measurements of moisture content during a debulk, and it was shown that the proposed model is able to predict the measured values with reasonable accuracy. A method of estimating part porosity using the numerical model was proposed and its results were compared with the experimental results. The knowledge presented is then used to explain observed experimental results published by other authors and to suggest a novel processing technique.
Out-of-Autoclave (OOA) prepreg processing is a promising candidate for replacement of autoclave (AC) processing, which is the current standard for manufacturing primary structural parts in the aerospace industry. However, its success is dependent on the ability to produce high quality parts with low porosity. This thesis develops an understanding of porosity in this process by studying the evolution of the voids during processing.Characterization of voids in partially cured laminates is challenging due to the soft nature of the prepreg matrix. A method for preparation of partially cured samples for optical microscopy and porosity measurement is developed and validated by comparison with results from the ASTM standard density method. It is also shown that thickness can be used to estimate porosity for the no-bleed prepreg system used in this study but that the accuracy is lower than microscopy and density methods.The evolution of voids during different processing cycles and process conditions was studied using the aforementioned optical microscopy on partially cured laminates made of MTM 45-1/5HS carbon/epoxy prepreg. Fiber tow geometry and gas permeability were also measured to determine fibre tow compaction and the gas transport capability throughout the cure process. It is shown that gas transport, fiber bed compaction and resin infiltration govern void evolution during the process. Porosity is governed by multiple chemical and transport phenomena, among which gas transport through vacuum evacuation plays a crucial role. An understanding of gas transport in OOA prepreg processing is developed by examining the time scales for gas transport by Darcy flow and molecular diffusion and comparing those to experimental gas permeability and porosity data. Darcy flow is shown to be the primary means of gas removal during the process. The study shows that the dominant direction of gas transport is dependent on the aspect ratio of the laminate, the prepreg material and the processing history as both in-plane and through-thickness permeability vary throughout the cure cycle. Based on these observations, a simple debulk map that gives the minimum recommended room temperature debulk time for OOA laminates as a function of in-plane and through-thickness dimensions is presented.
Revision total hip arthroplasty with femoral impaction allografting has an attractive potential for restoring bone stock in femurs with bone loss caused by the failure of hip implants. However, problematic implant subsidence is often reported after this procedure. A lack of understanding remains over the mechanisms that cause subsidence. The objectives of this study were to: a) explore the relationships between subsidence and morphometric features of the graft and bone cement regions after femoral impaction allografting in a cadaveric femur model; b) characterize mechanical properties of the graft bed as a function of impaction force, and explore new alternative graft compaction methods; and c) develop a finite element model to investigate the key mechanisms that contribute to initial implant subsidence. High levels of cement penetration into the graft bed were observed, resulting in extensive regions of cement contact with the host bone in a cadaveric femur model. The implant subsidence correlated negatively with the amount of cement-endosteum contact. The density, compression stiffness and shear strength of the graft were proportional to the impaction force. A slower alternative graft compaction method resulted in higher graft stiffness and shear strength than traditional graft impaction, but the benefit of this new compaction method was small compared to the effect of increasing the impaction force. In a finite element model, the relationship between graft density and subsidence was dependant on cement penetration profile. Without cement-endosteum contact, subsidence decreased with increasing graft density; however, graft density did not affect subsidence in constructs with cement-endosteum contact. Initial subsidence was primarily attributed to slippage at the stem-cement and endosteum interfaces, and the latter mechanism was greatly affected by changes in graft density and cement penetration profile. This study demonstrated that extensive cement penetration can occur in femoral impaction allografting, which may compromise the potential for new bone formation but may be important in preventing excessive subsidence. The endosteum interface was identified as a key factor in the development of subsidence. Finally, our results indicate that the potential benefit of achieving a denser graft bed depends upon the cement penetration profile.
Master's Student Supervision (2010 - 2018)
Carbon-epoxy prepreg C-channels were charge-formed by drape forming onto an aluminum tool. Flange lengths, forming temperatures, ply counts, ply sequences, forming methods, and curing bag types were varied and related to wrinkling, waviness, termination profiles, thickness profiles, microstructures, and mesostructures. Post-cure microscopy evaluation revealed wrinkling in parts formed at room temperature. Greater flange lengths and thinner laminates resulted in greater wrinkling severity. Fibre misalignments were more severe for the ply sequence whose 0° plies were located at the surface, taking the form of full-ply waviness after cure. A wrinkling conversion mechanism was proposed to explain the disappearance of externally visible defects after forming and the appearance of full-ply waviness after cure. The extent of conversion was attributed the curing bag tension and the post-formed wrinkled ply’s location within the charge with respect to the laminate’s outer surface. In all flanges, waviness misalignment angles were equal to or significantly greater than wrinkling misalignment angles. Missing length at the flange termination provided a good approximation of the excess length trapped in the form of wrinkling and full-ply waviness. Additionally, part thickness profiles provided further information on the occurrence of wrinkles. Furthermore, fibre misalignment types and locations were corroborated by non-destructive surface photography. Whether laminates were formed by hand lay-up or by drape forming, waviness gradients were found through the thickness of all 0° plies and attributed to the absence of sufficient intra-ply shear. The differences observed when forming the same ply sequence in alternate orientations may provide insight into the forming compound curvatures. The suggested post-form wrinkle conversion to waviness mechanism provides reason to develop waviness detection capabilities and to improve the understanding of mechanical performance knockdown effects from waviness. This study also proposes that post-forming part surface inspection for externally visible defects can be a critical step in identifying post-cure waviness sites. Defects detected in surface photographs after cure showed promising applicability for locating and identifying types of defects in mould-side surface plies. The occurrence of wrinkling and subsequent full-ply waviness is expected to be reduced if inter-ply slip characteristics are improved, though waviness gradients within individual plies will remain.
Primary load-bearing aerospace structures have traditionally been manufacturedusing autoclave processing. However, recent advances in material technologyhave led to the development of pre-impregnated (prepreg) composites that aredesigned to be cured out-of-autoclave (OOA) and can potentially reduce thecosts of processing.In OOA processing, voids are removed by vacuum evacuation through gaspathways in the prepreg. The availability and interconnectivity of these pathwaysdetermine the prepreg’s gas transport ability. Voids enter into the prepreg viamoisture absorption during storage and physical air entrapment duringmanufacturing and material handling. Environmental effects, such as relativehumidity, can alter the moisture content of prepregs and lead to significantvapour generation during cure. This study examines the gas transport and watervapourization characteristics of OOA prepreg CYCOM 5320/T650(epoxy/carbon). Gas permeabilities in the in-plane and through-thicknessdirections are measured, and the effects of processing history (debulk time,temperature etc.) on gas transport are examined. The relationships betweenrelative humidity, moisture content, and vapour generation are analyzed, and theuse of mass flow sensors for water vapour quantification is validated.Gas transport is shown to be highly anisotropic in CYCOM 5320/T650, with inplanegas permeability being three orders of magnitude greater than through thickness. Processing history has a significant effect on permeability, withextended debulking sessions reducing in-plane permeability by 50%. Increasingtemperature causes in-plane and through-thickness permeabilities to fall.Microscopy analyses reveal that permeability change is a result of collapsingvoids and resin flow during heating. Moisture content is relatively unaffected byrelative humidity until reaching 30% RH, after which increases in moisturecontent become more apparent. Vapourization of absorbed moisture between 0%RH and ambient conditioned vacuum bagged laminates are similar, suggestingthat under typical process conditions moisture vapourization mainly comes fromthe vacuum bag consumables. During vacuum bag processing, vapourization ofabsorbed moisture occurs immediately upon heating, peaks around 40 to 60oC,and then dissipates as heating reaches the hold temperature. The mass flowsensors are demonstrated to be capable of detecting the onset and terminationof moisture vapourization and quantifying the total mass of water vapourized towithin 10% error.
Out-of-autoclave (OOA) pre-impregnated (prepreg) materials are a prospective alternative to traditional autoclave processing, with the potential to reduce processing costs and build structures without size limitations imposed by the autoclave. Gas transport pathways in prepreg laminates play an important role in the removal of entrapped gases and volatiles during processing. Removal of gases by vacuum evacuation is essential in order to produce composite laminates with low final void content. Gas pathways are of particular importance in OOA prepregs where the maximum applied pressure during processing is 1 atm. In this study, the gas transport of OOA prepreg MTM45-1/CF2426A (epoxy/carbon) laminates is examined. The gas permeability of laminates is carefully measured in the in-plane and through-thickness directions. The study examines the effect of the number of layers, the effect of internal ply terminations, and the effects of heat on laminate gas transport. Supplemental experiments such as laminate compaction, microscopy, and water visualization are conducted to gain additional understanding of laminate gas transport.The study shows that gas transport is strongly directional for the studied prepreg with significantly higher permeability in-plane than in the through-thickness direction. Counter-intuitively, the permeability of MTM45-1/CF2426A is not found to be greater than autoclave prepreg when compared to carbon/epoxy Toray 3900-2 (plain weave). The physical nature of gas transport pathways in MTM45-1/CF2426A prepreg laminates is found to change with processing state. Debulking was found to decrease in-plane gas transport from its as-laid-up permeability. Laminate heating is found to affect laminate gas transport. In-plane permeability decreased with increasing temperature, while through-thickness permeability increased with increasing temperature. Correlations between gas transport and laminate compaction is also evident. During debulking, laminate compaction is found to correlate to decreasing in-plane permeability. Additionally, laminate compaction is found to relate to the quality of edge breathing.