This project advocates the design and use of composite bistable structures, a material-based mechanism which allows delayed and controlled energy absorption. Bistable structures are developed in order to have damage distributed over a large volume instead of one localized area, with resulting greatly improved crashworthiness. Composite bistable structures are particularly attractive because it is possible to tailor them in order to have the right trade-off of strength and elongation to failure. This allows the structure to behave in a fail-safe manner, without the brittleness that typically characterizes composites. Two-dimensional bistable composite structures with high impact resistance/weight will be investigated, manufactured, tested with a drop-weight tower, analyzed through the calculation of inelastic energy curves and impact performance maps, and modeled with finite elements using the commercial software ABAQUS. The first part of the proposed research uses therefore destructive testing as a key aspect of the design such structures. In the second part of the proposed research, structural health monitoring techniques will be carried out to identify impact damage in the bistable impact-resistant structures. Damage due to impact may be invisible in routine inspections of monolithic and sandwich composites, however it dramatically affects the strength of the structure, with possible catastrophic consequences. This issue has greatly influenced people's confidence in using composites, as well as damage assessment techniques of composite structures.
Increasing composite use in large structures in aerospace and civil engineering, ship construction, and automotive industries has the risk that not clearly understood failure mechanisms and undetected damage will have catastrophic consequences. Improvement of energy absorption, crashworthiness and detection of impact damage would promote and enhance the use of composites in applications where high damage tolerance and energy absorption properties are needed, for example in the design of road vehicles, rail vehicles, air and spacecraft structures, ships and submarines, on- and off-shore installations and for constructions in earthquake-prone and hazard-prone areas. Female students and students from underrepresented groups will be actively recruited. A national student competition will be held on crashworthy composites, where college students will design crashworthy structures, and students from local middle schools will compete for the design of a section of the test-rig. This latest activity will be carried out through a) the Student Development and Recruitment Office of the College of Engineering, which has a consistent track-record of recruiting underrepresented students, and b) the UC Davis Engineering Joint Council, a consortium of student representatives from the Engineering-related organizations, which is active in outreach activities.
CMMI-0642814 CAREER: Impact Resistance and Structural Health Monitoring of Composite Bistable Structures Composite materials, such as those with carbon or fiberglass reinforcement used in many engineering applications, have typically brittle failure and poor crashworthiness, which affect their adoption in applications requiring ductility and redundancy (particularly, in aerospace, civil, and transportation engineering). Bistable structures were proposed as a potential solution to mitigate this problem. They are a material-based mechanism that can be tuned to give higher energy absorption than conventional composite structures. This part of the project was successfully achieved with two designs based on a carefully tuned combination of ultra-high molecular weight polyethylene (tradename Spectra) and carbon reinforcements. The other scope of this project was to devise structural health monitoring methods for composite materials with complex material combinations and shapes. Effective structural health monitoring methods allow detection of ongoing damage in a structure before it reaches a dangerous level, and reduce the structure's off-line time for repairs. In this project, we have focused on configurations and testing conditions that attempt to be realistic and practical (with a ‘red flag/green flag’ ultimate goal), within the constraints of a laboratory setting. We have obtained successful results for damage sensing through changes of electrical resistance in multiscale composites (built with carbon nanotubes). We have also introduced a novel damage detection technique based on ultrasonic methods and off-the-shelf piezoelectric transducers. Through this grant, many collaborations have been established with researchers from the University of California, Davis, from other universities in US and outside US, and with researchers from a national laboratory and from a subcontractor of the Air Force. This has greatly broadened the research skills of all people involved, and has given exposure to diverse engineering practices. A relatively large number of junior personnel have been involved in and/or partially funded for several parts of the project (7 graduate students, 13 undergraduate students and 2 research engineers), many of whom are from minority/under-represented groups (broadening participation is part of the ‘Broader Impacts’ requirement for NSF funding). Outreach activities have been successfully carried out through laboratory demos, such as the so-called ‘Tower of Death’, where middle- and high-school students from the greater Sacramento area have been exposed to concepts of material crashworthiness, damage detection and bike safety. In summary, the main research goal of enhancing the durability of composites, through increased crashworthiness and improved structural health monitoring techniques, has been met (thus showing the intellectual merit of the work). The key educational goal of dissemination and outreach to K-12 students, particularly middle-school girls from the very diverse Sacramento area, has been met as well. We continue building research advances started through this grant, and we continue also our commitment to outreach.
