Nick Stites

Quasi-active, minimal-sensing load and damage identification and quantification techniques for filament-wound rocket motor casings

Filament-wound rocket motor casings are being considered by the United States Army for use in future lightweight missile systems. As part of the design process, a real-time, minimal-sensing, quasi-active health-monitoring system is being investigated. The health-monitoring scheme is quasi-active because abnormal loads acting on the structure are identified passively, the input force is not measured directly, and the curve-fit estimate of the impact force is used to update the frequency response functions (FRFs) that are functions of the system properties. This task traditionally requires an active-interrogation technique for which the input force is known. The updated FRFs and the estimated impact force can then be used in model-based damage-quantification methods. The proposed quasi-active approach to health monitoring is validated both analytically with a lumped-parameter model and experimentally with a composite missile casing. Minimal sensing is used in both models in order to reduce the complexity and cost of the system, but the small number of measurement channels causes the system of equations used in the inverse problem for load identification to be under-determined. However, a novel algorithm locates and quantifies over 3000 impacts at various locations around the casing with over 98% success, and the FRF-correction process is successfully demonstrated.

Minimal-sensing, passive force identification techniques for a composite structural missile component

Structural health monitoring systems are often limited to the use of one sensor due to cost, complexity, and weight restrictions. Therefore, there is a need to develop load and damage identification techniques that utilize only one sensor. Two passive force estimation techniques are investigated in this work. The techniques focus on either the shape or the amplitude of the magnitude of the applied force in the frequency domain. Both techniques iteratively reduce an underdetermined set of equations of motion into many overdetermined systems of equations to solve for the force estimates. The techniques are shown to locate and quantify impulsive impacts with over 97% accuracy and non-impulsive impacts with at least 87% accuracy. A filament-wound rocket motor casing is used as a test structure. Impacts not acting at a specific input degree of freedom are also accurately located depending on the distance away from the modeled input degrees of freedom, and damaging impact forces are quantified by making assumptions about the impulsive nature of the applied force.

Perspectives on pedagogical change: instructor and student experiences of a newly implemented undergraduate engineering dynamics curriculum

ABSTRACT Engineering education has been slow to adopt research-based educational innovations. Few prior works on such adoption have investigated the combined classroom experiences of instructors and students when such innovations are being implemented. Therefore, this work focuses on the lived experiences of an instructor and her students when adopting an active, blended, and collaborative learning environment, known as Freeform, in a second-year dynamics course. Weekly reflections from the instructor were processed alongside student interviews using Thematic Analysis to discern prominent themes in their perspectives for comparison and discussion. The results indicate that the instructor navigated internal tensions between her previous instructional preferences and the philosophy and resources of Freeform. Similarly, students had to adapt to this new philosophy and suite of resources that were uncommon for their institution. Ultimately, this work highlights the contextual natures of teaching and learning, and how situational factors can influence educational innovation.

Semi-active Damage Identification for a Composite Structural Missile Component using Minimal Passive Sensing with Data-driven Models

Cost, complexity, and weight restrictions often constrain industrial structural health monitoring (SHM) systems to the use of one sensor. Often, this one sensor is passive, i.e., it only measures the structural response. Therefore, new SHM techniques that utilize only one passive sensor along with advanced data interrogation methods are needed. A semi-active damage detection technique is described in this study that utilizes passively estimated forces and response measurements to update data-driven frequency-response-function (FRF) models of a filament-wound missile casing that is used as a test structure. During a damaging impact, the structural response is influenced by both the healthy and damaged structure properties. As a result, the magnitudes of the estimated FRFs can exhibit a splitting phenomenon at the natural frequencies of the healthy and damaged structure, and this phenomenon is evident in the experimental data. The updated FRFs are used with an active damage detection technique to quantify the damage. The normalized differences in the damaged and healthy FRFs and the shift of the natural frequencies of the structure quantify damage in this study. The shift of the natural frequencies more accurately quantifies the damage caused by four experimental impacts than the normalized differences in the FRFs.