RESIDUAL STRAIN MEASUREMENT IN Presentation by: Jason Cantrell - PowerPoint PPT Presentation

RESIDUAL STRAIN MEASUREMENT IN Presentation by: Jason Cantrell COMPOSITES USING CURE- COMPOSITES USING CURE EAS 6939 1/31/11 REFERENCING METHOD by: P.G Ifju, X. Niu, B.C Kilday, S.-C. Liu & S.M. Ettinger

Background g  Residual stress results from thermal mismatch between fiber and matrix  Also results from chemical shrinkage of matrix on polymerization l i i  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 significantly warp 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 - [0 16 ] T  Cross ply [0 /90 /0 /90 ]  Cross-ply [0 2 /90 2 /0 2 /90 2 ] S  Angle-ply [0 2 /45 2 /0 2 /45 2 ] S  18-ply woven panel 18 l l

Specimen Preparation p p  A diffraction grating is attached to the composite to view chemical shrinkage after solidification of the matrix t i  The grating in a stress-free state is known and serves as reference f  The grating is applied and cured with the composite then observed using moiré interferometry to view residual stresses.

Moiré Interferometry Measures orthogonal in-plane  di displacements, U and V l U d 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 lighting  The specimen grating and the Th d h fixed reference grating interact to form the moiré pattern

Experiments at Elevated Temperature p 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 Composite y p  Gradients in both fields are nearly equal.  As expected because of equal number of plies in 0 deg and 90 plies in 0-deg and 90- deg direction  No detectable residual  No detectable residual strain due to chemical shrinkage

Angle-Ply Composite g y p  U field gradient is small,  U field gradient is small, consistent with direction fiber dominance.  V gradient is inclined, indicating strong vertical and horizontal gradients. d h i l di  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 p New York: John Wiley & Sons, Inc., 1999.  Jenkins, C. H., ed. Manual on Experimental Methods for Mechanical Testing of Composites. 2nd. Lithburn: The g p Fairmont Press, Inc., 1998.  Dr. Peter Ifju