Frank and Ora Lee Marble Professor of Aeronautics and Mechanical Engineering


Surviving Contact: A Revolutionary Approach to Controlling the Energy Pathways in Armor Ceramics

Subconract: Johns Hopkins University Prime Sponsor: DARPA

Executive Summary

We propose a novel approach to the design and development of advanced lightweight armor ceramics that will use the internal defect distribution in the material to control its response to ballistic events by design and control of its energy pathways. Our very recent work has demonstrated that, unlike in the quasistatic domain where it is the largest defect that controls the behavior, the entire distribution of defects (Figure 1) controls the response and failure under high-strain-rate loading. We will use this energy pathways framework to provide armor ceramics manufacturers with specifications that they need to achieve within their product materials, and we will demonstrate that we can dramatically enhance the ballistic performance of an armor ceramic using designed energy pathways. We focus in this proposal on the ballistic regime defined in the BAA, although our framework provides some insight also into the blast regime.

Figure 1. Predicted influence of defect distribution at the same defect density, on the very high rate strength of silicon carbide. Note the observed range of HEL for SiC., at the same defect density, on the very high rate strength of silicon carbide [1]. Note the observed range of HEL for SiC.

The basis for this approach is grounded in the realization that the energy densities deposited into an armor package during a ballistic event are so large that every available energy pathway in the system will be exercised. With respect to “ballistic” threats, the dominant energy pathways in armor ceramics are at the micro-structural scale. The key technical challenges lie in (a) the design and control of these energy pathways and in (b) the integration of these within an armor system. The critical technologies needed to overcome this challenge are the ability to experimentally interrogate the energy pathways during the impact event, the ability to control the pathways through the physics-based design of the microstructure, and the ability to model and simulate the dynamic evolution of these pathways and their interactions with the system. We have assembled a multidisciplinary team of university researchers and industry experts with state-of-the-art capabilities in these critical technologies.

Our proposed effort uses recent results from separately funded work as a springboard, and brings together an experimental-theoretical-computational team of unparalleled capability to address this DARPA challenge. We are harvesting a key insight (that of the importance of defect distributions in the dynamic failure of ceramics) developed under an ARL-funded program at Johns Hopkins, a constitutive model developed with ONR support at UCSB and a predictive science computational capability developed with DOE support at Caltech, and combine and accelerate all three in a heretofore impossible way within this DARPA proposal.

Summary of Technical Approach

The development of dramatically improved lightweight armor material systems for ballistic threats requires two steps:

  1. the development of an innovative concept for materials systems design within the ballistic environment, and
  2. the incorporation of advanced armor ceramics that have dramatically improved performance over existing materials.

We present here a unique conceptual approach to the design of the response of material systems to ballistic loads, and then use this approach to design dramatically improved armor ceramics. Currently, armor ceramics OEMs design their manufacturing process flow around best engineering practices, raw material quality/availability, cost considerations, etc. However, their process flow has not been optimized based on first-principles material system design requirements appropriate for the intense dynamic nature of the problem, and hence key armor performance issues are not fully addressed. The revolution here is that we are going to turn the process around, i.e., we will tell the manufacturers (from a dynamic mechanics & materials perspective) what they need to achieve within their product materials so that they can re-engineer and optimize their process to produce it. Our team will demonstrate the performance gains achievable by the design and control of energy pathways, with the emphasis on the ballistic problem for vehicular armor. Our approach clarifies much of the apparent confusion in this field by providing a physics-based framework that allows one to understand the behavior, and not merely describe it.

Our key innovative concept grows out of an overarching scientific understanding that has very recently developed within the impact physics community for the response of materials to this extreme environment.

Figure 2. Predicted influence of defect density on very high rate strength of alumina [2]. Note the difference between quasistatic and high-rate behavior.

When a large amount of energy is deposited into a material in a very short time, the speeds at which the energy can be mechanically propagated through the system (e.g., crack speeds) are small in comparison to the rate at which the energy density in the material increases. Hence the energy in the material must find other places to go, and it will seek out internal dissipative pathways corresponding to the structure of the material (i.e., the structure of the material will be modified, as through fragmentation and comminution, phase transformation, twinning and plasticity). With respect to the specific ballistic threat levels identified in the BAA, the dominant energy pathways will be at the microstructural scale, and so this is the scale at which we must provide design and control (typically, higher intensity threats invoke smaller length scales).

Guided by this novel concept, we focus on using the internal defect distribution (inclusions, pores, crack-like flaws, grain boundaries) in armor ceramics to design and control the energy pathways within the materials during the ballistic event. Our very recent work has demonstrated that, unlike in the quasistatic domain where the largest defect controls the behavior, the entire distribution of defects controls the response The fundamental flaw in existing armor material systems design is an emphasis on seeking metrics based solely on material properties, rather than metrics based on dynamic failure processes (energy pathways). and failure under dynamic loading. Indeed, the largest defect has little influence (this is why so-called “improved” materials with increased quasistatic fracture toughness typically provide so little improvement in the ballistic response). We have demonstrated through analytical modeling (Figure 2) that an order of magnitude improvement in the high rate strength can be obtained through control of the full distribution of internal defects (a trend supported by data in the literature), and potentially the triggering of new mechanisms such as local plasticity. We will demonstrate that this can result in a significant enhancement of ballistic performance (in traditional ballistic terms, we will influence the dwell-penetration transition). Our team will use physics-based modeling, high- strain-rate experiments and explicit simulations to design the distribution to address the first three stages of the ballistic event. We will focus our efforts on the armor ceramic materials of silicon carbide and boron carbide.