Research

NOTE: The Materials for Extreme Environments (MEE) team has multiple on-going hypervelocity research projects on various materials, structures, and environmental debris. The content below is a summary of all projects and is not meant to be comprehensive.

Principle Investigator(s) (PIs): Thomas Lacy, Jr.

Affiliated Researchers: Waruna Kulatilaka, Kalyan Raj Kota, Jacob Rogers, Aniket Mote, Paul Mead, Khari Harrison, Gavin Lukasik, Joseph Stricklin, Max Murtaugh, Cullen Miller, James Leaverton, Nathaniel Bass

Funding Agencies: NASA, NSF, ERDC

Summary

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The MEE team has expertise in performing hypervelocity impact (HVI) testing and hypervelocity-related research. For a brief read about our history with HVI research, see here. All current HVI experimental research is conducted at the recently-established Texas A&M Hypervelocity Impact Laboratory (HVIL). Our current research projects deal primary with developing materials and material structures for HVI mitigation. However, some projects focus directly on the hypervelocity projectile and its interactions with water droplets, air, and debris.

Characteristic HVI Experiments

NOTE: The Materials for Extreme Environments (MEE) team has multiple on-going high strain-rate computational modeling research projects on various materials, structures, and environmental debris. The content below is a summary of all projects and is not meant to be comprehensive.

Principle Investigator(s) (PIs): Thomas Lacy, Jr.

Affiliated Researchers: Jacob Rogers, Paul Mead

Funding Agencies: NSF, ERDC

Summary

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The Materials for Extreme Environments (MEE) team has unique access to state-of-the-art modeling capabilities using both university and Department of Defense (DoD) based supercomputing resources. The team has experience with high-fidelity finite element models, smooth particle hydrodynamics models, and even modern DoD codes that use a hybridization of finite element and hydrodynamic techniques. Accompanying the full-scale (macroscopic) modeling approaches, the team is employing molecular dynamics (MD) codes such as LAMMPS to simulate the effects of extreme environments on the nano and micro scales. The results of MD simulations can provide a more intuitive understanding of material response to ultra-high strain rate loading (e.g., consistent with a hypervelocity impact) and educate materials models used in full-scale methods. These unique computational capabilities allow the team to develop, validate, and implement material models that match hypervelocity impact (HVI) experimental data and rapidly develop materials and structures that effectively mitigate HVI threats.

 Microscopy

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Fractography assessment of the perforated target fracture surfaces can provide valuable insights into the fracture behavior and various failure modes involved during an extreme dynamic event. Optical and scanning electron microscopy are the main methods utilized for characterizing such target fracture surfaces. The MEE team has developed an expertise in operating

• (relatively) low magnification/resolution optical microscopes.
• high magnification, and ultra-high resolution scanning electron microscopes.

Polymer Material Characterization

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Lightweight polymers are being increasingly used in ballistic and micrometeoroid/orbital debris shielding concepts for space and military applications to protect against HVIs. The MEE team is actively pursuing the development of novel layered and architectured polymer protective structural concepts. One of the key challenges in applying polymers within such protective structures is understanding their complex behavior when subjected to high strain‑rate HVI conditions. In general, polymer chemistry and microstructure heavily influence their impact response. The MEE team has expertise in many polymeric material characterization methods, including

• differential scanning calorimetry (DSC).
• dynamic mechanical analysis (DMA).
• gel permeation chromatography (GPC).
• thermogravimetric analysis (TGA).

General Material Property Characterization

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In addition to the above techniques, the team has gained extensive experience and expertise in performing ASTM standard tests to determine material (e.g., metal, composite, polymer, concrete)

• hardness.
• density.
• impact strength.
• tensile strength.
• shear strength.
• flexural properties.

Forensic analysis

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As a supplement to the in situ analysis performed during HVI events such as debris could/velocity tracking, MEE also performs forensic examination of the HVI specimens to characterize mass loss, perforation radii, and damage morphology.

Principle Investigator(s) (PIs): Matthew Priddy, Thomas Lacy, Jr.

Collaborators: Charles Pittman, Jr., Santanu Kundu, Aj Madabhushi, Hasnaa Ouidadi, Hajar Rigui, Aniket Mote

Collaborating Universities: Mississippi State University

Funding Agencies: Federal Aviation Administration (FAA)

Program Monitor: Dave Stanley

Summary

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Due to their high strength-to-weight and stiffness-to-weight ratios, excellent corrosion resistance, and ease of manufacture/repair, fiber-reinforced composite materials are increasingly used in a wide range of applications including aircraft structures. However, due to their extensive use, composite materials are exposed to extreme mechanical and/or thermal stresses which can severely degrade aircraft flight safety, damage tolerance, and structural integrity. For example, aircraft fires are the fourth highest contributor to commercial aviation fatalities. In-flight aircraft fires may result in severe degradations in composite material performance and reductions in overall flight safety. Fire effects on fiber-reinforced composite materials include matrix and organic fibers decomposition/pyrolysis, porosity formation due to volatile outgassing from matrix decomposition, delamination, matrix cracking, char formation, etc. In addition, post-crash fire can drastically degrade the mechanical properties of composite materials and alter the features and morphology of their failed surfaces in ways that mask relevant aspects of the structural damage morphology and other evidence necessary to identify the underlying failure mechanisms, leading to catastrophic structural failures. Therefore, the analysis of the response of fiber-reinforced composite materials to fire and the understanding of their fire resistance is considered a key technical challenge to ensure flight safety.

For this purpose, the Federal Aviation Administration (FAA) has funded our research group to perform a “Post-Crash Fire Forensic Analysis on Aerospace Composites.” As part of this project, a series of experimental tests will be performed to investigate the effect of fire on carbon/epoxy composite laminates failed in Mode I Mode II or mixed-mode failure. Fire tests on the failed specimens will be performed using both Bunsen burner and cone-calorimeter. Furthermore, an investigation of various physical and chemical techniques will be used as a tentative for char and other fire by-products removal to identify post-fire failure surface characteristics. Finally, multi-scale models will be developed to predict the progressive degradation of the thermal and mechanical properties of carbon/epoxy composites as a function of increasing temperature, heat flux, and exposure time.

Figure 1: SEM micrographs of the flat surface after 30s of fire exposure.
(SEM performed at Mississippi State University)

Figure 2: SEM micrographs of the edge surface after 30s of fire exposure.
(SEM performed at Mississippi State University)

Figure 3: SEM micrographs of the progressive thermal damage on a single carbon-fiber due to a controlled open-flame at a standoff distance of 0.5 in
(SEM performed at Mississippi State University)

 

Principle Investigator(s) (PIs): Thomas Lacy, Jr.

Collaborators: Mike Mazzola, Charles Pittman, Jr., Dounia Boushab, Aniket Mote, Khari Harrison, Pedram Gharghabi, Juhyeong Lee

Funding Agencies: NASA

Summary

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The lightning damage resistance of Pultruded Rod Stitched Efficient Unitized Structure (PRSEUS) panels was investigated experimentally. Two PRSEUS panels were subjected to standard Society of Automotive Engineers impulse current waveforms (consistent with actual lightning strikes) with 50, 125, and 200 kA nominal peak currents at a variety of panel locations. The outer mold line skin of one panel was lightly sanded to probe the influence of surface finish on lightning damage development. Lightning damage characteristics of the sanded panel were compared with those of a painted panel. The size of lightning-damaged regions in each panel increased as the peak current increased. The severity and size of lightning damage in the painted panel were much greater than that for the sanded panel due to paint’s greater dielectric properties. Lightning damage to PRSEUS panels manifests itself as (1) severe local damage occurring in the vicinity of the lightning attachment point and (2) surrounding widespread surface damage. The damaged regions were generally elliptical or semi-circular in shape and elongated along the outer lamina’s fiber direction. This is consistent with observations from unstitched carbon/epoxy laminates. Distributed widespread small-scale fiber damage displayed the same periodicity as the polyester warp-knit thread spacing used in the non-crimp fabric. A third painted PRSEUS panel was slotted and subjected to 50 and 125 kA nominal peak currents at mid-bay locations to investigate the effects of magnetically induced currents and lightning arc-expansion on widespread surface damage formation. The slotted panel experiments suggested that the observed surface damage is likely a result of the lightning arc-expansion. All of the lightning experiments demonstrated that the presence of through-thickness Vectran™ stitching dramatically reduced the development and spread of lightning damage. Moreover, PRSEUS panels exhibited markedly less lightning damage than comparable pre-impregnated laminated composites subjected to similar lightning currents. Nondestructive and destructive evaluation techniques were employed to assess the type and degree of through-the-thickness damage in PRSEUS and traditional laminated specimens. Phased array C-scan imaging of painted PRSEUS panels revealed that lightning damage (matrix decomposition, fiber breakage, delamination, etc.) was restricted to the upper skin-stack and that the size of the interply delamination was smaller than the observed surface damage. These results were consistent with microscopy images obtained from destructively sectioned panels. A series of stitched warp-knitted composite flat panels with different exterior lightning protection layers (i.e., copper mesh, graphene paper, and pitch carbon fiber paper) were fabricated and tested at 150 kA to assess protection layers effectiveness. A finite element model is in development to analyze and predict the lightning damage in stitched composite laminates. This research demonstrated that the use of stitched composites can enhance the lightning damage resistance of integrated composite aircraft structures.

Figure 1. PRSEUS preform.

Figure 2. A typical lightning strike test on a PRSEUS panel subjected to a nominal peak current of 200 kA.