PhD Oral Exam - Masih Moore, Mechanical Engineering

When studying for a doctoral degree (PhD), candidates submit a thesis that provides a critical review of the current state of knowledge of the thesis subject as well as the student’s own contributions to the subject. The distinguishing criterion of doctoral graduate research is a significant and original contribution to knowledge.

Once accepted, the candidate presents the thesis orally. This oral exam is open to the public.

Abstract

Prolonged occupational exposure to whole-body vibration (WBV) has been recognized as an important risk factor leading to numerous short- and long-term health effects among exposed human bodies. Reported studies have invariably shown that exposure to WBV is most critical in two distinct situations: (i) when the magnitude of vibration is high, or the exposure consists of repeated shocks, such as that encountered by off-road vehicle drivers and coast guards operating in a relatively rough sea environment; and (ii) when the WBV is predominant in the vicinity of the fundamental resonant frequency of the seated body, such as that encountered by the pilots and crew members of rotorcrafts. Helicopters exhibit a distinctive WBV attributed to the periodic aerodynamic loading generated by the main rotor blades. Such vibration is transmitted through the cabin structure to the seating system, resulting in pronounced low-frequency WBV, typically within the 4–12 Hz frequency range, to which the human body is particularly sensitive (ISO-2631). This level of WBV exposure is associated with an array of occupational health and safety issues such as musculoskeletal and spinal disorders, visual and vestibular impairments, reduced operational efficiency, and elevated error rates. Epidemiological studies have mostly focused on the vehicle driver population, with nearly negligible efforts on the effects of rotorcraft vibration and environment. Reported studies have shown substantial evidence of neck pain, fatigue of the head-neck complex (HNC), and impaired vision among the exposed pilot and crew members. Head-pitch oscillations, in particular, are known to cause HNC fatigue, apart from discomfort and considerable tracking errors that may compromise mission accuracy.

Furthermore, the safety protocols require the rotorcraft occupants to wear a helmet together with other head-mounted devices such as night vision goggles (NVG), balancing counterweights (CWs), head-up displays (HUDs), and the communication package. Moreover, the operational tasks frequently require the pilots and crew members to adopt non-neutral sitting and HNC postures. The non-neutral postures, together with wearing head-mounted equipment, substantially contribute to accentuated loading of the spinal column. Characterizing the biodynamic responses of the seated human body is thus fundamental to enhancing our understanding of potential vibration-induced injury mechanisms among the rotorcraft pilot/crew and design of effective intervention strategies.

Accordingly, this thesis research aimed at evaluating the biodynamic responses of a seated occupant considering the rotorcraft environment, through simultaneous measurements of seat-to-head vibration transmissibility (STHT) and driving-point apparent mass (APMS). For this purpose, a fully factorial experiment was designed considering five different head-worn scenarios, four distinct non-neutral HNC postures, two different sitting conditions, and three different levels of vertical vibration excitation. The study is conducted in four systematic stages. STHT measurement techniques are evaluated in the first stage, considering sensor design, measurement location, and data variability, apart from the effects of head-mounted devices. It should be considered that reported STHT data are not reliable due to excessive variabilities, suggesting the lack of reliable measurement methods. Therefore, this study proposed the simultaneous measurements of STHT at three different locations of the head, namely bit-bar measurement system (mouth level), Forehead-band measurement system (forehead level), and Headband measurement system (top of the skull). Effects of typical rotorcraft sitting and postural factors, namely: non-neutral HNC postures coupled with head-mounted devices on multi-axis STHT responses are subsequently evaluated to establish statistical significance of important sitting factors in a rotorcraft setting. The effects of rotorcraft operational conditions are also evaluated in terms of the force-motion relationship, characterized in terms of apparent mass.

Multi-factor statistical analysis was also performed in each stage of the research to establish the statistical significance of important sitting factors in a rotorcraft setting. The single-factor and factorial analysis of variance (ANOVA) indicated the significant effects (P-value < 0.05) of head-worn gears, non-neutral HNC posture, measurement location, and back support condition on both the peak STHT magnitude and corresponding frequencies, while the excitation magnitude mostly influenced the resonant frequencies significantly. The statistical analysis of the APMS response also revealed that the effects of the sitting condition > head-worn equipment > HNC postures > excitation magnitudes on the peak APMS magnitudes. Similarly, the sitting condition indicated the largest effect on the peak corresponding frequencies, followed by the excitation magnitudes, HNC postures, and head-worn scenarios.

The intra- and inter-subject variability analysis revealed that, Forehead-band measurement system exhibited the least variability in the vicinity of the dominant STHT magnitude responses. The sensor misalignment (i.e., due to the anatomical differences among the participants) affected the measured STHT responses at different locations, although the sensor orientation compensation method is proposed on the basis of low-frequency responses (≈ 0.5 Hz), which helped reduce the STHT coefficient of variation (CoV) considerably. The results also suggested that the pitch STHT responses at different locations of the head were almost identical, which showed the rigid-body rotation of the HNC around a unique pitch center, most probably the C7 vertebra. It was also shown that under the vertical excitation, the sagittal plane STHT responses (fore-aft, vertical, and pitch) were more pronounced compared to the non-sagittal plane responses (lateral, roll, and yaw).

Results of the studies also suggested that the currently used seats tend to amplify seat vibration transmitted to the head. Alternate relatively low natural frequency seats are strongly recommended to control the head vibration. In this regard, a simplified multi-body dynamic model of the head and cervical spine (C7 to C1 vertebrae) capable of predicting STHT responses at different locations of the head is proposed, which may serve as an effective design tool to evaluate appropriate pilot/crew seat design.