A novel concept for the mitigation of blast and impact effects on people and structures with new materials that modify the transmission of stress waves in a controlled or tailored manner is proposed. The concept is to develop new materials by strategically reinforcing cellular solids (e.g., foams) at the micro-scale level by the controlled deposition of nanometer-thick coatings. Homogeneous and particulate-reinforced nano-layers will be deposited by the layer-by-layer assembly method to develop complex multiscale multilayer hybrid reinforcements with molecular-level control of its composition and microstructure. Material and structural component testing will be done to evaluate nano-scale morphology and mechanical properties under pseudo-static and high-rate loads. Multiscale computational models will be developed to study stress waves propagation through complex materials and to serve as design tools.
The research will lead to breakthrough knowledge for designing blast/impact absorbing materials in which their deformation and failure is tailored to guide the propagation of stress waves and thus control and optimize energy dissipation. It will also provide new insight to the problem of three-dimensional stress-wave propagation in complex materials. The concept has the potential of revolutionizing the design of automobiles, aircraft, body armor and protective devices for civil infrastructure. Two graduate students will be trained towards a Ph.D. degree under the multidisciplinary basis of the research. The project will be integrated with a new Residential Experience Program for undergraduate engineering students at MSU by developing a Theme Community in ?materials and security.? Summer research opportunities will be offered to traditionally under-represented undergraduate students.
The protection of people, vehicles and structures from the effects of impact and blast is a continuing challenge. A big hurdle is the nature of the load, which rapidly travels through the resisting materials initiating damage in multiple forms before ultimate failure. A path to improve this situation is to change the nature of the load propagation through the resisting material or elements. This research project was guided by the hypothesis that cellular materials, namely open-cell foams, can be strategically reinforced with nano-scale coatings to develop material systems with enhanced energy absorption characteristics and the ability to tailor the propagation of stress waves from blast and impact loads for improved performance of the material itself or a protected structure. Open-cell foams consist of an interconnected network of ligaments that form groups of repeating cells (defined by the edges of a closed volume). Metal foams are widely used as energy absorbers since they can dissipate considerable kinetic energy through the collapse of the foam’s cells. The behavior of open-cell foams depends on the properties solid material from which is made, and the geometry of the cells and the ligaments. Controlling the constitutive response of foams by modifying these three parameters has become of increasing interest. Yet, the open network of these foams makes them ideally suited for another approach to control their behavior: to reinforce the base material in a selective manner such as to create a composite. The result is a hybrid foam that has improved properties over those that can be gained by simple increase of the base material content. The material system that provided the framework for this study was an open-cell aluminum (Al) foam reinforced with copper (Cu) coatings (Image 1). An electro-deposition system was successfully developed in which the porous three-dimensional cellular structure was effectively coated at a neutral pH. Uniformity of the coating and the formation and aggregation of nano-sized crystalline metal grains (Image 2), which have superior mechanical properties compared to those displayed by the same material when in bulk, was confirmed through SEM (scanning electron microscopy) imaging and measurements. Aluminum foams coated with nano-crystalline copper were evaluated under quasi-static and high-rate compression testing and they were shown to have enhanced stiffness, strength, and energy absorption capacity compared to plain (or unreinforced) foams in which additional material was provided simply by a reduction of the foam’s relative density (Image 3). The performance gain is due to the composite material system in which the outer coating, due to its nano-scale crystalline structure, has higher strength and stiffness compared to the base substrate material. Therefore, the reinforcement of metal foams with nano-structured metals is an effective way to enhance the absolute and specific mechanical properties of metal foams. Micro- and macro-scale simulations were conducted to provided further insight into the mechanical behavior of hybrid foams. A unique feature of the compressive response of hybrid metal foams manufactured through electrodeposition is that the material suffers a rapid drop in capacity soon after the ligaments start buckling inelastically, and there is large fluctuation and hardening along the plastic collapse region (Image 3). Simulations demonstrated that the reason for this behavior is that the small crystalline size of the deposited metal coating has reduced ductility (Image 4), a feature associated with its increased strength and stiffness. The ductility of the nanocrystalline coating thus has a great influence on the overall energy absorption efficiency of hybrid foams. Experimental (Image 5) and numerical (Image 4) studies showed that the performance of metal/metal hybrid foams can be improved by proper annealing, which enhances the ductility of the electrodeposited copper coating. The annealing time should be sufficient for ductility increase the in coating material but limited to avoid the formation of excessive intermetallic compounds at the Al/Cu interface, which are brittle and again lead to fracture and reduced performance. The developed concept and methods for reinforcing metal foams is also an attractive pathway for the development of foam structures with optimized material layouts. The manufacturing of functionally graded hybrid foams (FGHF) was demonstrated and experimentally evaluated (Image 6). Enhanced stiffness, strength and ductility were obtained for FGHF structures under quasi-static and dynamic (from impact) flexural demands. Furthermore, the study confirmed that patterned designs can redirect stress distributions and avoid damage in protected regions. In conclusion, the study developed methods which demonstrated that nano-reinforced foams offer enhanced energy absorption capacity, and verified the research hypothesis that they can be designed in strategic layouts to manage the propagation of stress waves and the kinetic energy imparted to the material system from high-rate loads. Knowledge from this project was disseminated through peer-reviewed journal publications, conference publications and technical presentations. Through the participation in this project three students obtained a research-based MS degree, two students completed a Ph.D. degree and one undergraduate student gained research experience.
