Fire Resistive Materials: Adhesion Performance Assessment and Optimization of Fire Resistive Materials NIST July 14, 2005

Microstructure Experimental 3-D Tomography 2-D optical, SEM Confocal microscopy Modeling 3-D Reconstruction Parameters Porosity Pore Sizes Contact Areas Properties (all as a function of T) Thermal Heat Capacity Conductivity Density Heats of Reaction Adhesion Pull-off strength Peel strength Adhesion energy Fracture toughness Equipment TGA/DSC/STA Slug calorimeter Dilatometer Blister apparatus Materials Science-Based Studies of Fire Resistive Materials Environmental Interior Temperature, RH, load Exterior Temperature, RH, UV, load Performance Prediction Lab scale testing ASTM E119 Test Real structures (WTC)

Adhesive Performance of FRMs Why should we care? Opportunity: Recent events have demonstrated the importance of in-service adhesive performance in the ability of FRM to protect steel. What can we learn about existing adhesive properties of FRM?

How do we measure adhesion? Want a geometry independent property. –Adhesive Fracture energy, Gc How do we measure Gc? energy to create a unit of surface area units: J/m 2 What is Gc

Experimental: Schematic and Theory P w 2a2a Assumptions: Thin, stretching membrane, loaded elastically and at a point Eh: Film Tensile Rigidity (modulus, E, · thickness, h)

Experimental: Schematic and Theory P w 2a2a Load-based equation ( P ) displacement-based equation ( w )

Experimental Set-Up: N kg Load suspended from center of specimen

WTC Material

Experimental: Shaft-Loaded Blister Test for WTC Material P h Kai Tak Wan and Yiu-Wing Mai, International Journal of Fracture, 74, (1995) E: Young’s Modulus coating Rigid substrate Mechanically driven shaft attached to Instron Shaft attached To instron Stand to hold sample Film is on the underside Of stand Mirror to view film w

WTC Sample P Fire Retardant Material (3/4”) Primer 2 mil steel For a bending plate: The mechanical properties of the film are estimated from: Steel E (Pa) = 2 * 10^11 h (m) = 0.05 *10^-3 Adhesion Promotor E (Pa) = 3 * 10^9 h (m) = 0.20 *10^-3 and: E composite = v 2 E 1 +v 2 E 2

54 % of the samples were entirely debonded when received Experimental: Shaft-Loaded Blister Test G = / J/m 2

Fire Retardant Coatings

Testing of Adhesive Joints: Introduction to Sub-Critical Adhesive Fracture Testing and the Wedge Test Wedge Test: a (t > 0) a (t = 0) v - G curves will tell you: -rank order of adhesive -failure mechanisms -engineering design parameters Log Crack Velocity, v (m/s) Crack Driving Energy,(J/m) v (m/s) (J/m v (m/s) G (J/m

v- G Curve Reveals Mechanisms of Adhesive Failure at the Crack- Tip: Regions I, II, III v*v* Region II: diffusion to crack tip Region III: stress controlled G Th Region I: stress-dependent chemical reaction Crack Driving Energy,(J/m) I Crack Driving Energy,(J/m) I II Log Crack Velocity, v (m/s) Crack Driving Energy, G (J/m 2 ) I III Increasing Aggressiveness of Environment Crack Velocity, v (m/s) Crack Driving Energy, G (J/m 2 ) Region II

Application of Sub-Critical Adhesion Testing: Residual Stress (σ r ) in Coatings σ r arise due to CTE mismatch or processing G Th Log Crack Velocity, v (m/s) Crack Driving Energy,(J/m) v (m/s) (J/m v (m/s) G (J/m 2 Data from wedge test

Constant-Load Subcritical Blister Test Measure debond easily with micrometer!

Fire Resistive Coating Cold Rolled Steel Substrate

Epoxy DCB Screening Tool for Different Coatings Bonded to Cold Rolled Steel at 100% r.h. & RT A SLBT B SLBT A DCB D C

“Soft” vs. “Hard” Coating, low humidity vs. high humidity “Hard” coating performs better than “Soft” coating! Coatings perform better at low humidity High (95%) RH Low (1%) RH

FRC DCB : Increase humidity and reduced residual stress Tensile Residual Stress is reduced by moisture absorption Reduction in residual stress leads to improved durability

FRM’s. Modern Adhesion testing methods can give LRFD parameters. These parameters are environmentally sensitive. –(increasing RH can either increase or decrease performance.) What about Temp and UV? Rate effects? What about the existing test methods? –Can we modify existing test methods?

Adhesion Tests: Ideal Ideal Adhesion Test: Simple, cheap, fast, easy to perform Grounded in fundamental mechanics and material science- LFRD guidance –Includes modes- opening, in-plane shear, torsion, mixed, Could include environmental and rate dependence. How far are the test from idea?

Current FRM Standards: ASTM E759 (Effect of Deflection) 12 Ft Deflect 1/120 or 1 inch. ASTM E760 (Effect of Impact) 12 Ft Concrete 60 lb from 4 ft. ASTM E736 (Cohesive/Adhesive)

Current Empirical Methods: Advantages: –Quick, easy, Cheap –Practical –Pass/Fail guidance Disadvantages: –Highly dependent on sample preparation –Specific to situation tested (geometry, speed, etc) –Little or no design guidance

Quantitative Adhesive Test Methods: Beam, JKR and Peel Measure G and E JKR Test At equilibrium G =W  P P a Beam Test, ex. Wedge Test Blister Test P w 2a2a P Peel Test(s)

Fundamental Mechanics Advantages: –Link to fundamental mechanics and material science properties G –Results are independent of geometry/sample preparation. –Gives LFRD guidance Disadvantages: –Expensive –Require equipment –Time consuming

Current Methods: Fundamental Mechanics –DCLB –Peel Tests –Blister Tests –JKR Empirical, Practical –Pull off –Lap Shear –Impact –Deflection  P P a P w

General Approach: Simple Test, –Fast, easy, inexpensive to perform Calibrate. –Can be calibrated against fundamental mechanics and material science –Rate, strain and environmental dependence Round robin

Prototype: Steel FRM Bending, twisting, stretching will produce known strains at the interface (different modes) Stress can be calculated from first principles and calibrated with known adhesion geometries. Visually evaluated, or calibrated. Can give both rate and environmental performance. Never have to touch the material

Summary Current methods for evaluating the adhesion performance are pass/fail. Modern adhesion testing methods present the ability to give design guidance for FRMs. It appears possible to build a close to “ideal” adhesion test for FRMs. –Simple, cheap, fast, based in mechanics, calibrated by NIST.