RESIDUAL STRAIN MEASUREMENT IN COMPOSITES USING CURE-

Presentation by:Jason CantrellEAS 6939 COMPOSITES USING CURE

REFERENCING METHOD1/31/11

by: P.G Ifju, X. Niu, B.C Kilday, S.-C. Liu & S.M. Ettinger

Backgroundg

Residual stress results from thermal mismatch between fiber and matrix

Also results from chemical shrinkage of matrix on l i ipolymerization

Thermal characteristics of composites are directional and depend upon fiber orientation.and depend upon fiber orientation.

These behaviors influence the fiber and matrix differently and are based upon fiber orientation angle.

If thermal and chemical characteristics are not taken into consideration during stacking the laminate may warp significantlywarp significantly.

Materials Tested

AS4/3501-6 graphite/epoxy and plain weave AS4/3501 6 graphite/epoxy and plain weave fabric

4 cases tested 4 cases tested Unidirectional - [016]T Cross ply [0 /90 /0 /90 ] Cross-ply [02/902/02/902]S Angle-ply [02/452/02/452]S

18 l l 18-ply woven panel

Specimen Preparationp p

A diffraction grating is attached to the composite to view chemical shrinkage after solidification of the

t imatrix The grating in a stress-free

state is known and serves fas reference

The grating is applied and cured with the composite then observed using moiré interferometry to view residual stresses.

Moiré Interferometryy

Measures orthogonal in-plane di l U d Vdisplacements, U and V

Two beams of light are impinged upon specimen and react forming an interference react forming an interference pattern consisting of light and dark bands

This is known as reference grating or virtual reference grating because it is just lightingTh d h The specimen grating and the fixed reference grating interact to form the moiré pattern

Experiments at Elevated Temperaturep p

Specimen must be heated to cure temperature to Specimen must be heated to cure temperature to remove residual strains that are due to contraction.

Residual strains left are due to chemical shrinkage and stress relaxation.

Unidirectional Fiber

U field pattern (Fiber/ U field pattern (Fiber/ x-direction) is nearly zero

V field pattern (Transverse/ y-/direction) is quite large

A large compressive strain was found in y-direction

Unidirectional Fiber

Strain transverse to Strain transverse to fiber direction at the cure temperature was not zero.

Demonstrates that a significant portion of residual strain in related to chemical shrinkage in matrix

Cross-Ply Compositey p

Gradients in both fields are nearly equal.

As expected because of equal number of plies in 0 deg and 90plies in 0-deg and 90-deg direction

No detectable residual No detectable residual strain due to chemical shrinkage

Angle-Ply Compositeg y p

U field gradient is small, U field gradient is small, consistent with direction fiber dominance.

V gradient is inclined, indicating strong vertical

d h i l diand horizontal gradients. Both normal and shear

strains did not return to strains did not return to zero.

Conclusion

This paper covers the experimental techniques used This paper covers the experimental techniques used to measure strain due to residual stresses

Subsequent papers describe how to calculate Subsequent papers describe how to calculate stresses with Laminate theory

Residual Strains and Thermal Analysis are covered Residual Strains and Thermal Analysis are covered in Chapter 3 of “Design and Optimization of Laminated Composite Materials”

References

Ifju, P. G., et al. "Residual Strain Measurements in "Composites Using Cure-referencing Method." Experimental

Mechanics Vol. 40.No. 1 (2000): 22-30. Key phases- composite materials, residual stress, cure-y p p

referencing method, moiré interferometry Gurdal, Zafer, Raphael T. Haftka and Prabhat Hajela.

Design and Optimization of Laminated Composite Materials.g p pNew York: John Wiley & Sons, Inc., 1999.

Jenkins, C. H., ed. Manual on Experimental Methods for Mechanical Testing of Composites. 2nd. Lithburn: The g pFairmont Press, Inc., 1998.

Dr. Peter Ifju