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ASTM Selected Technical Papers
Composite Materials: Fatigue and Fracture (Third Volume)
By
TK O'Brien
TK O'Brien
1
U.S. Army Aerostructures Directorate (AVSCOM), NASA Langley Research Center
,
Hampton, Virginia
;
symposium chairman and editor
.
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ISBN-10:
0-8031-1419-2
ISBN:
978-0-8031-1419-7
No. of Pages:
838
Publisher:
ASTM International
Publication date:
1991

The effect of the stress-free edge on the growth of local delaminations initiating from a matrix crack in a composite laminate is investigated using a three-dimensional finite element analysis. Two glass epoxy layups, (02/904)s and (±45/904)s, were modeled with a matrix crack in the central group of eight 90° plies, and delaminations initiating from the matrix cracks in the 0/90 and -45/90 interfaces, respectively. The analysis indicated that high tensile interlaminar normal stresses were present at the intersection (corner) of the matrix crack with the stress-free edge, suggesting that an opening (Mode I) delamination may initiate at these intersections.

In order to analyze the strain energy release rates associated with delaminations that may form at the corners, three different configurations of the local delamination were assumed. One configuration was a uniform through-width strip growing normal to the matrix crack in the direction of the applied load. The other two configurations were triangular shaped delaminations, originating at the intersection of the matrix crack with the free edge, and growing away from the corner. The analysis of the uniform delamination indicated that the magnitude of both the total strain energy release rate, GT, and its components increased near the free edges. This edge effect was symmetric for the (02/904)s layup. However, for the (±45/904)s layup, the G distribution across the front was asymmetric, with the total G and its components having higher values near one free edge than the other. For both layups, the GI component was large at small delamination lengths, but vanished once the delamination had reached a length of one-ply thickness.

The second and third delamination configurations consisted of triangular-shaped delaminations with straight fronts inclined at angles of 10.6 and 45°, respectively, to the matrix crack. The total G along the delamination front decreased sharply near the matrix crack for both configurations and increased sharply near the free edge for the 10.6° configuration. However, the total G distribution was fairly uniform in the middle of the delamination front for the 10.6° configuration. These inclined models suggest that if the exact geometry of the delamination front could be modeled, a uniform G distribution may be obtained across the entire front. However, because the contour of the delamination front is unknown initially, it may only be practical to model the uniform through-width delamination and use the peak values of G calculated near the free edges to predict delamination onset. For the layups modeled in this study, the total G values near the free edge agreed fairly well with a previously derived closed-form solution. However, a convergence study may need to be conducted to have confidence that peak values of G calculated from three-dimensional finite element analyses near the free edges are quantitatively correct.

1.
Reifsnider
,
K. L.
and
Talug
,
A.
, “
Analysis of Fatigue Damage in Composite Laminates
,”
International Journal of Fatigue
 0142-1123, Vol.
3
, No.
1
,
01
1980
, pp. 3–11.
2.
Masters
,
J. E.
and
Reifsnider
,
K. L.
, “
An Investigation of Cumulative Damage Development in Quasi-Isotropic Graphite/Epoxy Laminates
,”
Damage in Composite Materials
, ASTM STP 775,
Reifsnider
K. L.
, Ed.,
American Society for Testing and Materials
,
Philadelphia
,
1982
, pp. 40–62.
3.
Reifsnider
,
K. L.
, “
Some Fundamental Aspects of the Fatigue and Fracture Response of Composite Materials
,”
Proceedings
, Fourteenth Annual Meeting of Society of Engineering Science,
Lehigh University
,
Bethlehem, PA
, 14–16 Nov. 1977.
4.
O'Brien
,
T. K.
, “
Mixed-Mode Strain Energy Release Rate Effects on Edge Delamination of Composites
,”
Effects of Defects in Composite Materials
, ASTM STP 836,
American Society for Testing and Materials
,
Philadelphia
,
1984
, pp. 125–142.
5.
Adams
,
D. F.
,
Zimmerman
,
R. S.
, and
Odom
,
E. M.
, “
Frequency and Load Ratio Effects on Critical Strain Energy Release Rate Gc Thresholds of Graphite/Epoxy Composites
,”
Toughened Composites
, ASTM STP 937,
Johnston
N. J.
, Ed.,
American Society for Testing and Materials
,
Philadelphia
,
1987
, pp. 242–259.
6.
Whitney
,
J. M.
and
Knight
,
M.
, “
A Modified Free-Edge Delamination Specimen
,”
Delamination and Debonding of Materials
, ASTM STP 876,
Johnson
W. S.
, Ed.,
American Society for Testing and Materials
,
Philadelphia
,
1985
, pp. 298–314.
7.
O'Brien
,
T. K.
, “
Analysis of Local Delaminations and Their Influence on Composite Laminate Behavior
,”
Delamination and Debonding of Materials
, ASTM STP 876,
Johnson
W. S.
, Ed.,
American Society for Testing and Materials
,
Philadelphia
,
1985
, pp. 282–297.
8.
O'Brien
,
T. K.
,
Rigamonti
,
M.
, and
Zanotti
,
C.
, “
Tension Fatigue Analysis and Life Prediction for Composite Laminates
,” NASA TM 100549,
NASA Langley Research Center
,
Hampton, VA
,
10
1988
.
9.
O'Brien
,
T. K.
, “
Towards a Damage Tolerance Philosophy for Composite Materials and Structures
,”
Composite Materials: Testing and Design
, ASTM STP 1059,
Garbo
S. P.
, Ed.,
American Society for Testing and Materials
,
Philadelphia
,
1990
, pp. 7–33.
10.
Crossman
,
F. W.
and
Wang
,
A. S. D.
, “
The Dependence of Transverse Cracking and Delamination on Ply Thickness in Graphite/Epoxy Laminates
,”
Damage in Composite Materials
, ASTM STP 775,
Reifsnider
K. L.
, Ed.,
American Society for Testing and Materials
,
Philadelphia
,
1982
, pp. 118–139.
11.
Murri
,
G. B.
,
Salpekar
,
S. A.
, and
O'Brien
,
T. K.
, “
Fatigue Delamination Onset in Tapered Composite Laminates
,” NASA TM 101673,
NASA Langley Research Center
,
Hampton, VA
,
1989
.
12.
Martin
,
R. H.
and
Murri
,
G. B.
, “
Characterization of Mode I and Mode II Delamination Growth and Thresholds in Graphite/PEEK Composites
,”
Composite Materials: Testing and Design
, ASTM STP 1059,
Garbo
S. P.
, Ed.,
American Society for Testing and Materials
,
Philadelphia
,
1990
, pp. 251–270.
13.
Fish
,
J. C.
and
Lee
,
S. W.
, “
Three-Dimensional Analysis of Combined Free-Edge and Transverse-Crack-Tip Delamination
,”
Composite Materials: Testing and Design
, ASTM STP 1059,
Garbo
S. P.
, Ed.,
American Society for Testing and Materials
,
Philadelphia
,
1990
, pp. 271–286.
14.
Chan
,
W. S.
,
Rogers
,
C.
, and
Aker
,
S.
, “
Improvement of Edge Delamination Strength of Composite Laminates Using Adhesive Layers
,”
Composite Materials: Testing and Design (Seventh Conference)
, ASTM STP 893,
Whitney
J. M.
, Ed.,
American Society for Testing and Materials
,
Philadelphia
,
1986
, p. 266.
15.
Shivakumar
,
K. N.
,
Tan
,
P. W.
, and
Newman
,
J. C.
, Jr.
, “
A Virtual Crack Closure Technique for Calculating Stress-Intensity Factors for Cracked Three-Dimensional Bodies
,”
International Journal of Fracture
, Vol.
36
,
1988
, pp. R43–R50.
16.
Raju
,
I. S.
,
Shivakumar
,
K. N.
, and
Crews
,
J. C.
, Jr.
, “
Three-Dimensional Elastic Analysis of a Composite Double Cantilever Beam Specimen
,”
AIAA Journal, American Institute of Aeronautics and Astronautics
 0001-1452, Vol.
26
, No.
12
,
12
1988
, pp. 1493–1498.
17.
Raju
,
I. S.
, “
Q3DG—A Computer Program for Strain Energy Release Rates for Delamination Growth in Composite Laminates
,” NASA CR-178205,
NASA Langley Research Center
, Hampton, VA,
11
1986
.
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