Associate Professor Jae-Hwang Lee of the University of Massachusetts Mechanical and Industrial Engineering Department and his graduate students currently have simultaneous cover articles appearing in two major scientific journals. One journal is the American Chemical Society’s Applied Nano Materials and the second is Soft Matter, published by the Royal Society of Chemistry. As Lee says about the research described in both cover papers, “We were funded by the Department of Defense (the Army for one paper and the Navy for the other) to understand mechanical properties of materials under extreme loading conditions from projectiles and blast waves.”
The first of the two cover-story papers, “Intrinsic Dynamics and Toughening Mechanism of Multilayer Graphene upon Microbullet Impact,” was published in Applied Nano Materials.
In this paper, according to Lee, “We investigated a 2D nanomaterial (multilayer graphene) for future lightweight bulletproof materials which can immediately redistribute a concentrated impact energy of a projectile to wider areas. Using microscopic ballistic testing, we demonstrated how multilayer graphene can be further improved in armor performance by reducing its brittleness.”
As the authors explained in their cover story, graphene is a very promising material platform for the mitigation of supersonic and hypersonic impacts due to its capability to delocalize impact energy at an exceptionally rapid rate. However, the intrinsic dynamic characteristics of graphene have not been quantified due to the coupling of aerodynamic effects to its dynamic deformation process.
“Here,” wrote the authors, “we present the comprehensive dynamic behavior of free-standing multilayer graphene using microscopic ballistic testing in vacuum with micro-bullets, at speeds ranging from 280 to 900 meters per second, and multilayer graphene thicknesses ranging from 15 to 95 nanometers.”
As the authors noted, compared to the previously known specific penetration energies, “…approximately 300 percent higher energy delocalization performance was demonstrated due to unrestricted fast deformation of multilayer graphene in vacuum. Moreover, a bimodal distribution of the penetration energies suggests that quasi-plastic behavior exists and enhances impact energy delocalization by suppressing graphene’s brittle nature.”
In the second paper, “Seeded laser-induced cavitation for studying high-strain-rate irreversible deformation of soft materials,” the researchers developed an extremely dynamic mechanical characterization method to quantify mechanical properties of soft materials, including brain tissues.
“In this method,” explained Lee, “a small black particle (one tenth of a human hair diameter) causes a micro-explosion within a soft material by absorbing an intense laser pulse.”
Through the observation of the rapidly expanding explosion within one-millionth of a second, said the authors, they were able to quantify mechanical properties of the soft materials under extreme loading conditions.
Characterizing the high-strain-rate and high-strain mechanics of soft materials is critical to understanding the complex behavior of polymers and various dynamic injury mechanisms, including traumatic brain injury. However, their dynamic mechanical deformation under extreme conditions is technically difficult to quantify and often includes irreversible damage.
“To address such challenges,” said the authors, “we investigate an experimental method, which allows quantification of the extreme mechanical properties of soft materials using ultrafast stroboscopic imaging of highly reproducible laser-induced cavitation events.”
According to the cover paper, “The presented method offers significant advantages with regard to quantifying high-strain rate, irreversible mechanical properties of soft materials and tissues, compared to other methods that rely upon the cyclic dynamics of cavitation. These advances are anticipated to aid in the understanding of how damage and injury develop in soft materials and tissues.” (November 2020)