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

RESIDUAL STRAIN MEASUREMENT IN COMPOSITES USING CURE-

Presentation by: Jason Cantrell EAS 6939

COMPOSITES USING CURE REFERENCING METHOD

1/31/11 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

l i i polymerization

 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 - [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 Preparation p p

 A diffraction grating is

attached to the composite to view chemical shrinkage after solidification of the t i matrix

 The grating in a stress-free

state is known and serves f as reference

 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 l U d V displacements, 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 lighting Th d h

 The specimen grating and the

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

• f 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 d h i l di and 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 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