Structural Fire Modeling: Ply-Level Thermal Mechanics of Composites

1. Structural Fire Modeling: Ply-Level Thermal Mechanics of Composites John Bausano, Theophanis Theophonous, Steven Boyd, Scott Case, John Lesko Materials Response Group Dept of Engineering Science & Mechanics Blacksburg, VA 24061 USA

2. Virginia Tech Structural Fire Modeling Philosophy • Goals: Develop an analysis tool for the deformation and failure of composite structures subjected to compressive mechanical loads and fire • Approach: Describe residual strength/stiffness using constituent property evolution, micromechanics, and ply level information • Why: In order to describe the failure of composite materials, it is necessary to account for composite behavior at each of the levels of analysis. Too much homogenization prevents this.

3. NIST FDS q, T s, T a, Eij, d Structural Response Under Fire Conditions Goal: Integrated Tool Representation of surface q’s and T’s for a given fire scenario Structural assessment & assessment of flash over Load Fire BC’s FEA First Principles/Phenomenological Thermo-mechanical property evolution (T,t) Residual Strength and Stiffness Calculation

4. List of Major Technical Accomplishments • Developed combined one sided simulated fire and load test • Extensively characterized the effects of load and heat flux on the time to failure, and axial strain profiles as a function of time • Developed composite micromechanics to describe compression strength loss and time to failure under combined gradient thermal conditions and mechanical loading • Emphasis placed on lower temperature effects in the absence of combustion (consistent with fire protection conditions) • Thermal property evolution • Characterization of thermal diffusivity, thermal conductivity, and specific heat as a function of temperature (30 to 500C) for as processed and thermally degraded glass/vinyl ester composites • Validated against Hughes Assoc. data • Extensive creep characterization • Shear creep and recovery characterized to 30 to 170C and stresses from 0.05 to 4 ksi • Developed the most complete data set characterizing the nonlinear viscoelastic -plastic behavior of glass/vinyl ester composites • Developed viscoelastic -plastic models to describe the complete range of response to temperature and load • Packaging of thermal mechanical property and strength evolution in FEA • Developed a direct connection between NIST Fire Dynamics Simulator (v. 3.2) and a commercial FEA package (ANSYS 8.0) for the specification of thermal boundary conditions • Incorporation of strength and viscoelastic stiffness changes into ANSYS 8.0 for structural-fire performance analysis

5. List of Major Deliverables • Developed combined one sided simulated fire and load test • Test procedure for combined one-sided heat flux and compression loading • Data set: Effects of load and heat flux on the time to failure, and axial strain profiles as a function of time • Micromechanics model for compression strength loss and time to failure under combined gradient thermal conditions and mechanical loading that would allow a designer to evaluate new composite systems (i.e. carbon/vinyl ester) • Thermal property evolution • Validated data set (against Hughes Assoc. data) of thermal diffusivity, thermal conductivity, and specific heat as a function of temperature (30° to 500°C) for as processed and thermally degraded glass/vinyl ester composites, using a commercial system (HotDisk) • Extensive creep characterization • Data set of shear creep and recovery (30 to 170°C and stresses from 0.05 to 4 ksi) • Viscoelastic -plastic models to describe the complete range of response to temperature and load • Packaging of thermal mechanical property and strength evolution in FEA • Integrated package incorporating composite property evolution and fire simulation

6. Creep Compression & One-Sided Heat Flux Exposure Test Times to failure are predicted using ply level mechanics and compression micromechanics under combined gradient thermal conditions that will allow a designer to evaluate new composite systems (i.e. carbon/vinyl ester) Runouts Predictions – solid lines System develop (Laser Extensometer) as part of ONR DURIP

7. Thermal Property Evolution w/Temp Validated method for accelerated characterization of composite thermal properties as function of temperature and degradation for future modeling I would like to have a better plot of this – larger font and data points. Low Temp Sensor to 250°C High Temp Sensor to 600°C Validated data set (against Hughes Assoc. data) of thermal diffusivity, thermal conductivity, for as processed and thermally degraded glass/vinyl ester composites, using a commercial system (HotDisk) HotDisk system procured under ONR DURIP

8. Shear Creep Compliance Master Curves Visco-elastic plastic properties model integrated into FEA Shear Laminate [±45]2s Vetrotex 324/Derakane 510A-40 Extensive creep and recovery data set (30° to 170°C & 0.05 to 4 ksi) and modeled using Zapas and Crissman visco-plastic model describing creep response to a wide range of temp & load

9. Structural Fire Analysis • Thermal Mechanical Property Evolution • Elastic property evolution • Compression micromechanics • Visco elastic/plastic effects NIST FDS Fire Simulation Thermal boundary conditions Click to play movie Load • Fire-Load Simulation of Structural Bulkhead • Packaging of mechanics and property evolution with user defined subroutines • Trial modeling efforts Fire Structural FEA Implementation

10. Next Steps • Well instrumented component and subcomponent structural fire testing validated against predictions • Packaging of a fully integrated simulation tool including FDS and thermal-mechanical FEA • Development of design guidelines considering fire as a load for composite structures