Fatigue Life Prediction of Hybrid FRP Composite Beams

1
Fatigue Life Prediction of Hybrid FRP Composite
Beams

• Jolyn L. Senne1
• J. Lesko1, S. Case1, T. Cousins2
• Materials Response Group
• 1Engineering Science and Mechanics
• 2Via Department of Civil and Environmental
  Engineering
• SAMPE 2000
• 24 May 2000

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Toms Creek Bridge Blacksburg, VA

• Rehabilitation project in 1997 using pultruded
  FRP composite beams to replace the superstructrue
  
• 17.5 Span meeting AASHTO HS20-44
• Removable Beams
• Instrumented to monitor strain, deflection and
  temperature

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Outline

• FRP Composites in Infrastructure Applications
• Toms Creek Bridge Beam Characteristics
• Research Objective
• Beam Analysis
• Stiffness Predictions
• In-Plane Stresses
• Free Edge Problem
• Comparison to Experimental Results

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Outline (cont.)

• Fatigue Analysis
• Use of Coupon Fatigue Data
• Residual Strength Methodology
• Delamination Initiation and Crack Growth
• Fatigue Testing
• Summary / Conclusions
• Future Work

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Research Motivation

• Advantages of FRP Composite Materials in
  Infrastructure Applications
• High Stiffness to Weight Ratio
• Resistance to Fatigue
• Corrosion Resistant
• Speed of Installation

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Research Motivation

• Challenges To Composites in the Civil Engineering
  Community
• Cost compared to conventional materials
• Lack of Long Term Durability Data
• Absence of Design Standards
• Complicated Analysis Techniques
• Connections

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Bridge Beam Characteristics

• Pultruded by Strongwell, Corp
• Hybrid, Non-Symmetric Layup
• Glass 0, ?45 90 plies
• Carbon in top and bottom flanges
• Out-of-Plane Failure Mode Delamination
• Prototype for 36 beam for larger scale
  applications

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Bridge Beam Characteristics

• Cross-Section Properties
• Area 13.7 in2
• Iyy 129 in4
• Izz 31.7 in4

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Quasi-Static Failure Tests
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Quasi-Static Test Results

• Four-point bend quasi-static failure tests
  resulted in an average ultimate moment of Mult
  100 kip-ft (15 kN-m)
• Delamination of the top (compression) flange was
  the failure mode at this load
• Failure occurred at the Glass-Carbon interface
  closest to the midplane
• Shear contribution to deflection is about 5

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Delamination Failure
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Fatigue Life Prediction of Hybrid FRP Composite
Beams

• Objective
• Create a model which predicts fatigue life of the
  Toms Creek Bridge beams based on failure by
  delamination
• Experimentally validate the analytical
  techniques used

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Four Point Bend FRP Beam Stress Analysis
Energy Minimization
In-Plane Stresses sx, sy, txy
Out of Plane Stresses sz, tyz
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Beam Lamination Theory

• Analysis considers 10 Sublaminates

• Constant Curvature

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Laminated Beam Theory Agreement with
Experimental Results

• Predicted EIeff 8.41 (108) psi in4
• Measured EIeff 7.94 (108) psi in4

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In-Plane Stress Calculations

• Classical Lamination Theory is then used in each
  sublaminate to determine sx, sy, sxy, ex, ey,
  exy
• Bottom Flange Strain Predicted and Experimental

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Free Edge Stresses

• At the free edge, assumptions of CLT are not
  valid and interlaminar stresses must be
  considered
• The stress gradients exist over a distance h from
  the free edge

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Free Edge Stresses

• Stress distributions are found using Minimization
  of Complementary Energy 1
• Assume ply-stresses are separable in y and z
• sz g(y) f(z)
• Impose boundary conditions of sy sxy syz 0
  at free edge and matching conditions at
  interfaces
• The in-plane stresses must match the CLT values
  away from the free edge

1 Kassapoglou, C., Determination of
Interlaminar Stresses in Composite Laminates
under Combined Loads, Journal of Reinforced
Plastics and Composites, Vol. 9, Jan 1990.
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Free Edge Stresses

• Smear the properties of the webs and internal
  flanges into a ply to account for their energy
  and allow the symmetrical problem to be solved

Equivalent EIeff
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Free Edge Stresses

• Out-of-plane stresses have the form
• Variational calculus is used to determine an
  appropriate function for g(y) which minimizes the
  complementary energy of the laminate

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Stress Predictions at Carbon-Glass Interface

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Out-of-Plane Strength Tests

Zt 276 psi
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Top Flange Stress Distribution
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Top Flange Stress Distribution
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Life Prediction Assumptions

• Stiffness reduction is dominated by the tensile
  (bottom) flange until the redistribution of
  strains and neutral axis shift result in top
  flange delamination
• Reduction of the bottom flange modulus can be
  done based on tensile coupon fatigue data
• Matrix strength degradation is uniform
• Crack growth is symmetric from the free edges
• Ultimate failure occurs when the crack propagates
  across the entire beam width

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Life Prediction MethodologyPrior to Delamination
Initiation
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Life Prediction MethodologyPost - Delamination
Y
N
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Coupon Fatigue Testing

• Extensive characterization of pultruded glass
  laminates
• Cross-ply (0/90)5T
• Quasi-isotropic (0/90/45/-45/90/0)2T

Cross Ply
Quasi-Isotropic
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Correlation to Coupon Data

• The bottom flange is divided into sublaminates
  which mimic the coupon test layups
• For My 50 Mult the strains in the flanges are
  14 of ex, max which is lower than test
  conditions
• Coupon fatigue data is extrapolated for these low
  load levels
• The highest Fa ex / ex, max is used to degrade
  the strength of the beam

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Delamination Initiation Crack Growth

• Delamination Initiation
• Quadratic Delamination Criteria
• Crack Propagation

2
3
2 Brewer, J.C. and P.A. Lagace. Quadratic
Stress Criterion for Initiation of Delamination,
Journal of Composite Materials, Vol 22,
p1141-1155 (Dec 1988) . 3 OBrien, T.K.,
Generic Aspects of Delamination in Fatigue of
Composite Materials, Journal of the American
Helicopter Society, Vol 32, pp 13-18 (Jan 1987).
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Neutral Axis Shift
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Stiffness Reduction
Delamination Initiation 4,000,000 cycles
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Life Prediction
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Four Point Bend Fatigue Test

• 14 ft Span
• Triple Point Loading
• R-ratio .1
• Frequency ? 1 Hz
• Axial Strain
• Shear Strain
• Torsional Strain
• Mid-Span Deflection
• 1/4 pt Deflection

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Four Point Bend Fatigue Test
Delamination failure from fatigue loading in the
compression flange
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Fatigue Test Data
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Fatigue Test Data
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S-N Curve
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Summary

• A life prediction model for the Toms Creek
  Bridge Beams under four-point-bend fatigue
  loading has been developed
• The model predicts delamination as the failure
  mode, as seen from quasi-static and fatigue tests
• The beam stiffness properties and in-plane strain
  predictions have been verified experimentally
• The out-of-plane stresses, calculated using
  variational calculus and minimization of
  complementary energy appear to correlate with
  prior test results

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Future Work

• Experimentally determine compression strength
  characteristics
• Attain coupon compression fatigue data
• Validate out-of-plane stress calculations using
  FEM
• Non-linear fit for tensile coupon data in the
  initial portion
• Consider strain energy release rate methods for
  crack propagation problem

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Acknowledgements

• Strongwell, Corp.
• NSF Career Award Funding - CMS
• Stephen Pfifer
• David Haeberle and Doug Neely