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    Thorax Response in Young and Aged Small Females Under Side Impact, Isolated Ribs and Full-Body Computational Studies
    (University of Waterloo, 2026-03-02) Corrales Fabre, Miguel Angel
    Small-stature females and aged individuals face a disproportionate risk of thoracic injury in automotive crashes. Epidemiological studies indicate the heightened susceptibility observed in the aged small statured individuals compared to average statured males through a significantly increased number of rib fractures (NRF), especially in side impacts. The present thesis investigates the biomechanical factors contributing to the increased NRF observed in the aged small statured female individuals, focusing on the age-related changes in thoracic geometry and material properties. Advanced finite element human body models (HBMs) were employed to quantify these effects under side-impact loading conditions, with a specific emphasis on young and aged small female modelling. First, age and sex-specific isolated rib models were developed including a 26-year-old mid-stature male (M5026IR), 26-year-old small female (F0526IR) and a 75-year-old small female (F0575IR) 6th rib. Using morphing techniques, with targets based on regression models, the rib gross geometry, cortical bone thickness, and rib cross-sectional area were enhanced to reflect population-specific characteristics. The Cortical Bone Fracture and Continuum Damage Mechanics Constitutive Model (CFraC) was implemented to enhance the representation of rib cortical bone material behaviour, integrating age-specific mechanical properties. The isolated rib models were loaded in anterior-posterior compression and compared to available population-specific experimental data. The isolated rib models served as a stepping stone for full body model development and as foundational model validation exercise. Subsequently, three novel HBMs were developed by integrating the isolated rib developments and performing full-body morphing: a young adult small female (F0526), an aged adult small female based on statistically derived population data (F0575), and an aged adult small female based on subject-specific imaging data (F0584). These three models, alongside the baseline small female GHBMC model (F05B), were assessed in two environments: a simplified side impact sled using rigid plates, and an Advanced Side Impact System (ASIS) representing a realistic vehicle environment with a deformable seat, seatbelt with pretensioner, thorax airbag, and intruding door. Isolated rib results demonstrated good correlation to the age-, sex- and size-specific experimental data (i.e. young mid-size male, young small female and aged female) predicting the overall rib stiffness, force-displacement to fracture and fracture location. The rib models predicted the expected age and size/sex differences, that is, reduced stiffness and lower force and displacement to fracture with increasing age and decreasing size. Transition zones from thick to thin cortical bone led to stress concentrators that dictated the fracture location in the isolated rib models. The CFraC material model was shown to be a foundational improvement allowing for the prediction of cortical bone fracture. At the full body level, the baseline F05B model demonstrated limited sensitivity to impact severity, underestimating injury outcomes attributed to its simplified cortical bone model. The F0526 demonstrated sensitivity to impact severity. The F0575 model predicted NRF, chest compression maximum magnitude and timing, and AIS injury scores higher than the F0526 with values aligned closely to experimental Post-Mortem Human Subject (PMHS) data for aged small females. The subject-specific model (F0584), experienced higher rib torque owing to increased rib angles associated with this subject specific model, relative to the population average F0575 model. Increased rib torque, led to higher shear stresses in the cortical bone, resulting in the initiation of rib fracture earlier in the impact an increased NRF. These findings quantified and suggested that age-related reductions in cortical bone strain to failure, rib cortical bone thickness, and inter-subject variability can increase NRF. The present thesis demonstrates, for the first time, a methodology to develop predictive thorax models capable of capturing age-, sex-, and size-related differences considering rib fracture and thorax response. The presented results suggest that the development of population-specific tissue-level-predictive HBMs require a combination of appropriate material models coupled with population-specific material properties and geometry. Importantly, the HBMs developed in this work can be used to evaluate existing safety systems and aid in the development of future inclusive safety systems
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    Transient Development of a Laminar Separation Bubble Over a Low Reynolds Number Airfoil
    (American Society of Mechanical Engineers, 2026-02-05) Zilstra, Alison; Johnson, David A.
    Airfoils operating in low Reynolds number (Re) conditions frequently have a laminar separation bubble (LSB) form as a part of the natural boundary layer (BL) transition. A transient analysis of the Kelvin-Helmholtz (K-H) rolls in the LSB uncovered a new pressure feedback process that alters the development of the BL transition. The SD 7037 airfoil at a modest Re of 41,000 is studied using Large Eddy Simulation (LES) where a free-shear flow filter length criterion is applied to capture the K-H roll development. Multiple detailed experimental data sets are used to validate the chordwise positioning of the time averaged LSB and the dominant K-H roll frequency. The K-H rolls are shown to pinch-off from the reverse flow region of the LSB and the lift-off and subsequent touchdown of the K-H rolls are the source of the pressure feedback. The pressure feedback occurs consistently at 1 deg angle of attack (AOA) resulting in a dominance of the K-H roll frequency throughout the transitional BL. The dominant frequency is relevant to the aeroacoustic performance of low Re airfoils, where K-H rolls at a consistent frequency can generate tonal noise. The intermittent feedback at 5 deg AOA provides a clear distinction between the transition structures during a natural BL transition and feedback-initiated transition. The analysis of the K-H rolls and the discovery of a novel feedback mechanism provides invaluable information for the aerodynamic and aeroacoustic performance of low Re airfoils.
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    Determination of Liquid Metal Embrittlement Crack Propagation Kinetics During Resistance Spot Welding Using In-Situ Monitoring
    (University of Waterloo, 2026-03-02) Kim, JiUng
    Resistance spot welding (RSW) is a predominant joining technique for sheet metals in the automotive sector due to its efficiency and rapid processing capabilities. However, when applying RSW to zinc-coated advanced-high-strength steel (AHSS), surface cracking near the weldment is frequently observed. These cracks are primarily attributed to zinc infiltration along the grain boundaries of the substrate, which significantly decreases ductility. This type of cracking is classified as liquid metal embrittlement (LME) cracking. The presence of such cracks undermines weld integrity, prompting ongoing research efforts to develop effective prevention methods. Most studies exploring the underlying mechanism of LME, such as critical stress levels, LME-sensitivity temperature range, and zinc transportation method, used physical or numerical simulation to recreate conduction during welding using high-temperature tensile testing (HTTT) or finite element method (FEM) simulation that do not necessarily completely replicate the welding environment. Using research methods to simulate welding process is preferred because in-situ monitoring of LME cracking during the RSW process is challenging due to the cracking location in welding being hidden from the exterior. LME cracks appear as circumferential surface cracks with narrow widths that extend internally within the steel sheet. Although the half-sectioned RSW (H-RSW) process has been introduced for real-time monitoring of LME crack analysis, no substantial findings have been reported concerning quantitative crack behavior or the underlying mechanisms. One of the limitations of the alternative method is its inability to determine the mechanism of zinc transport into grain boundaries. The literature presents conflicting views on whether liquid zinc first infiltrates the grain boundaries or whether the crack initially forms due to weakened grain boundaries caused by stress-assisted zinc diffusion, followed by zinc infill. Additionally, recent FEM-based studies have identified thermal stress induced by thermal shock, as a consequence of an instantaneous temperature drop (ITD) due to mechanical collapse between electrodes and the steel sheet surface, as a key factor contributing to LME crack formation. However, it remains unclear whether the findings attained from the HTTT are applicable or relevant to the RSW process, as the stresses and constraints applied to the joint differ from those in the HTTT. Furthermore, the effect of thermal stress induced by ITD on LME crack initiation must be validated through experimental observation from an actual RSW process. The first stage of this research program consisted of parameter optimization for the H-RSW process of third-generation AHSS (3G-AHSS) through the development of a process map. Thermal cycles of the H-RSW and RSW processes were compared by examining nugget size and indentation depth. It was determined that the H-RSW process enables the identification of LME cracking, which is linked to the temperature gradient in the weld shoulder area. An issue was identified where the emissivity value fluctuates within the LME-sensitive temperature range from 700 °C to 900 °C, leading to inaccurate temperature measurements. To address this, a temperature correction model for an infrared (IR) camera was developed, resulting in an 85% improvement in measurement accuracy over non-calibrated data, particularly within the LME-sensitive temperature range. The optimized H-RSW setup enables a comprehensive understanding of the kinetics involved in LME cracking behavior. It has been observed that the crack growth rate diminishes as the crack length increases. Additionally, the cracking process can be divided into two stages based on the crack growth rate: an initiation stage characterized by rapid crack propagation, followed by a propagation stage with slower crack growth. In-situ monitoring analysis revealed the timing of crack propagation and zinc infiltration, showing that cracking preceded liquid zinc infiltration, providing experimental evidence that stress-assisted zinc diffusion facilitates LME crack formation. The impact of thermal stress induced by ITD on LME cracking behavior has been validated, and the comprehensive underlying mechanism of LME cracking has been identified. During the initiation stage, thermal stress is the primary factor influencing crack formation, with higher thermal stress resulting in longer cracks. In the propagation stage, LME crack growth rate is affected by four factors: the momentum of the crack aiding its propagation, thermal stress at the crack tip acting as a driving force, damping effects that oppose crack growth, and the reduction of thermal stress due to temperature increases at the crack tip during extension, which causes a ferrite to austenite phase transformation. A damage map has been developed to predict the initiation of LME cracks at sensitive temperatures and thermal stress levels. Additionally, a crack and crack growth rate prediction model has been established to elucidate the influencing factors during the welding process. These findings can be effectively utilized to develop in-process mitigation strategies and to manufacture LME-resistance AHSS.
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    VR Streaming as a New Frontier: Investigating Virtual Cameras as a Multifaceted Bridge Between Streamers and Viewers
    (University of Waterloo, 2026-02-27) Wu, Liwei
    Although live streaming and virtual reality (VR) have been widely studied, their combination, VR streaming as a mass media format, has received less attention. In VR streaming, the streamer uses a VR headset to share their experience in a virtual environment, while viewers watch on traditional 2D devices like computers or smartphones. As VR adoption increases in consumer markets, VR streaming is expected to become a key part of online content ecosystems. However, few studies have examined VR streaming in real-life settings, especially the critical role that virtual cameras can play during the streaming process. Just as physical cameras are essential for storytelling and viewer engagement in traditional media like film, virtual cameras are also crucial for creating effective communication and engaging interaction in VR streaming. Yet the common ways VR streamers currently use virtual cameras, the associated challenges they encounter, and the opportunities to enhance the virtual cameras and their overall streaming experience still remain largely unexplored. This thesis explores the essential role of the virtual camera in VR streaming for fostering connections and engagement between streamers and viewers through three projects. In the first project, I collected and analyzed VR streaming videos to observe VR streaming common practices and understand their associated challenges. In the second project, I interviewed media domain experts to investigate the connections between traditional media and VR streaming, as well as explored design considerations to enhance current virtual cameras. In the final project, I explored the structured representation of VR streaming with a design canvas and how to efficiently support streamers understand and explore current and future VR streaming formats.
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    The Spirit in the Machine: Romantic Resurgence in the Digital Age
    (University of Waterloo, 2026-02-26) Vines, Hannah
    Generation Z (Gen Z), often described as the “digital generation,” is the first cohort to experience childhood and adolescence extensively mediated by digital technologies. This has coincided with a widespread retreat from analog to digital spaces, frequently described as “digital escapism.” Existing sociological and psychological literature tends to explain this shift through sociopolitical stressors, individual pathology, or engineered addiction (Haidt, 2024; Jouhki et al., 2022; Kardefelt-Winther, 2014). This paper offers an alternative interpretation by theorizing digital escapism as a cultural phenomenon shaped by shared narratives, symbols, and meaning-making practices. Drawing on the strong program in cultural sociology (Alexander, 2003), this paper argues that contemporary narratives of techno-utopianism and posthumanism provide key symbolic frameworks through which Gen Z interprets their relationship with technology. These narratives idealize technological mediation, promote intimate human-computer interaction, and encourage transcendence of embodied and social limitations. Rather than viewing Gen Z’s digital retreat as a novel response to technological change, this paper situates their practices and discourse within broader cultural patterns that have roots in enduring historical traditions, challenging narratives of technological determinism. By foregrounding culture and meaning, this paper contributes to digital sociology by demonstrating how technological practices are embedded within symbolic systems that precede and exceed individual choice or technological design. The analysis shows that digital escapism is not simply a reaction to technological affordances, but a culturally mediated response shaped by powerful collective imaginaries. The paper concludes by suggesting that understanding digital engagement through cultural narratives transcends psychological or materialistic explanations, addressing a critical gap in the literature for analyzing the social implications of digital technologies and the evolving relationship between technology and contemporary social life.