276 computer methods in composite materials · 2014-05-14 · 276 computer methods in composite...
TRANSCRIPT
Experimental investigation of damage
progression of a graphite-epoxy in flexural
bending test
J. Echaabi*, F. TrocW & M. Quelled
B.f!
Route d'El Jadida, Casablanca, Maroc
^Center for Applied Research on Polymers, Mechanical
Engineering Department, Iicole Polytechnique de
Montreal
Abstract
The main objectives of this study are to monitor damage progression step bystep of a quasi-isotropic graphite-epoxy composite in flexural bending testand to compare the experimental results obtained with the theoreticalpredictions of various failure criteria. The failure sequence has beenmonitored by a C-Scan technique and a microscopic investigation.Initiation and growth of cracks which lead to delamination have beenidentified up to the first macroscopic failure. Comparison betweenexperimental and theoretical results show that the failure criteria used inthis study don't permit to predict the failure sequence nor the load at thefirst macroscopic failure.
Transactions on Engineering Sciences vol 21, © 1998 WIT Press, www.witpress.com, ISSN 1743-3533
276 Computer Methods in Composite Materials
1 Introduction
A great number of theoretical failure criteria have been proposed todescribe the failure of composite materials [1, 2]. There is also a highdegree of uncertainty associated with their use [3] and usually noinformation is available on the description of failure modes anddamage progression. The experimental validation of failure criteria is
also quite difficult. Experimental biaxial failure stresses are not easyto obtain, not reliable and present a large scatter. A simple three pointbending test can be used to validate failure criteria [4, 5]. Few authorshave described the successive failure of composite specimens usingthis test. Recently, some experimental and theoretical study has beenperformed on this topic [6, 7], Further investigations on damageprogression are presented in this paper. The failure of compositesamples has been monitored step by step in order to determine themechanism of crack growth. Two types of specimen geometries havebeen used, and experimental results will be compared with theoretical
predictions.
2 Experimental procedure
The laminate staking sequence used in this work is |(±45/90/0)3]s.Each specimen was originally cut from a 16 x 16 cm plate. The sideedge of the specimens was polished with 5 and 0.5 micro-polishingpowder to prevent damage induced during the cutting. The dimensionsof the specimens and the test parameters follow the ASTMspecification D790 [8]. The experiments are simple three point bendingtests carried out on the composite beams cut from the plate.
The number of specimens and their characteristics are shown in table
1.
Transactions on Engineering Sciences vol 21, © 1998 WIT Press, www.witpress.com, ISSN 1743-3533
Computer Methods in Composite Materials 277
Laminates
le,...,ln
2c, ..., 2n
Width(W)(mm)
25
10
Span (1)(mm)
57,5
57,5
Length(I)(mm)
75
75
Thickness(T)(mm)
3,6
3,6
\fl
16
16
Table 1: Specimen characteristics
Ultrasonic scanning is used before and after preparation of the samplesto ensure that they are free of cutting damage.
3 Tests and results
A specimen of each geometrical type has been used to obtain themacroscopic load vs displacement curves shown in figure 1.
1500
1000
500
DISPLACEMENTSpecimen width 25 mm
DISPLACEMENTSpecimen width 10 mm
This figure shows that the succession of failures depends strongly ongeometrical parameters of the specimens, particularly the width. Inorder to monitor exactly the succession of damage, three specimenswere loaded up to 1.9, 2.5 and 3 mm deflections and other specimenswith the same characteristics are loaded by an increment of 1 mmdeflection to determine the initiation and growth of cracks. The resultsobtained are shown respectively in table 2 and figure 2.
Transactions on Engineering Sciences vol 21, © 1998 WIT Press, www.witpress.com, ISSN 1743-3533
278 Computer Methods in Composite Materials
Localization Transverse
cracks
Longitudinal
cracks
90 1 0 1 0
45
90/0
II 45/90
90
0 || 0
0
0
0
45 || 0
90/0 1 0
0
0
0
0
0
1 45/90 1 0 1 0
1,9 1 90 I 3 I 0
2,0
45 I 0 I 0II II
90/0 1 0 1 0
45/90 || 0
90 || 5
|| 45 1 3
1 90/0 1 0
| 45/90°
1
0
0
0
1
Delamination
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
Transactions on Engineering Sciences vol 21, © 1998 WIT Press, www.witpress.com, ISSN 1743-3533
2,1
2,3
2,51
n
2,7
2,9
90
I
45
45/90
90
45
90/0
45/90
90
45
90/0
45/90
90
45
90/0
45/90
,0
45
90/0 j
45/90
10 1 0
5
0
0
15
10
0
0
15
15
0
0
20
20
[ W
1 0
20
20
°0
0
0
1
0
0
0
2
0
0
0
2
1
0
1 I3
1
° 12
3
«
0
0
0
0
0
0
0
0
0
1
1
0
0
'1
0
°1
1
Table 2: Specimen loaded with by increments of 1 mm deflection.Table 2 indicates the growth of cracks in the four bottom plies asindicated in figure 2. Also the same table shows the nature and numberof cracks up to a deflection of 2.9 mm.
Transactions on Engineering Sciences vol 21, © 1998 WIT Press, www.witpress.com, ISSN 1743-3533
280 Computer Methods in Composite Materials
3 mm deflection
Fig. 2: Photomicrographs of specimens loaded up to 1.5, 2.5 and 3 mmdeflections.
The progression of damage in the later case is shown in figure 3.
Transactions on Engineering Sciences vol 21, © 1998 WIT Press, www.witpress.com, ISSN 1743-3533
1.9 mm deflection
- ..;, - StkpfmrpCT-tff. '"• * " KS fI'-FT W
^ ' S ^ &" % \ -.
2.5 mm deflection
Transactions on Engineering Sciences vol 21, © 1998 WIT Press, www.witpress.com, ISSN 1743-3533
*
1.9 mm deflection
2.5 mm deflectionFig. 3: Crack growth before the first macroscopic failure.
Transactions on Engineering Sciences vol 21, © 1998 WIT Press, www.witpress.com, ISSN 1743-3533
Computer Methods in Composite Materials 283
This figure illustrates the progression of cracks and the initiation ofdelamination before the first macroscopic failure.
The theoretical results for this material are shown in table 3.
Maximum Stress Failure Tsai-Hill Failure Tsai-Wu Failure
Failure Load P Disp. Ply Load P Disp. Ply Load P Disp. PlySample # N (mm) angle (N) (mm) angle (N) (mm) angle
A
1)
12345
12345
1408.72104.21998.01221.41350.2
591.68837839.2512.9567.0
2.03.244.084.084.35
2.03.244.084.084.35
900090-45
900090-45
1351.72040.61759.31680.71603.9
5677857.0738.9705.9673.6
1.933.143.603.643.67
1.933.143.603.643.67
900-454590
900-454590
1204.01781.81739.61704.81702.8
505.7748.4730.6716.0715.2
1.722.742833.683.87
1.722.742833.683.87
90900-450
90900-450
Table 3: Maximum stress, Tsai-Hill and Tsai-Wu failure loads forspecimens 1 and 2 (from [7]).
The succession of failures before the first macroscopic failure is not
predicted for all the failure criteria used. Also, the experimental loadfor the first macroscopic failure, i.e., failure of the first 0° ply, ishigher than the predicted one and varies with the failure criterion used.The difference between theoretical and experimental results increaseswith damage progression.
4 Conclusion
The progression of damage for a graphite-epoxy composite has been
monitored precisely. The results obtained have been compared to thepredictions of various failure criteria.The difference between theoretical and experimental results increaseswith damage progression and is about 35% for the first macroscopicfailure. The failure criteria used don't permit to predict the failuresequence in this kind of composite.
Transactions on Engineering Sciences vol 21, © 1998 WIT Press, www.witpress.com, ISSN 1743-3533
284 Computer Methods in Composite Materials
References
[1] Nahas, M.N., Survey of failure and post-failure theories oflaminated fiber reinforced composites, J. Comp. Technology ofResearch, Vol. 8, pp. 138-153, 1986.
[2] Echaabi, J., Trochu, F and Gauvin, R., Review of failure criteriaof fibrous composite materials, Polymer Composites, Vol. 17, No. 6,December 1996.
[3] Burk, R.C., Standard failure criteria needed for advancedcomposites, Astro-Aeronautics, Vol. 21, pp. 5862.
[4] Whitney, J.M., Journal of composite materials, Vol. 4, pp.
135-137, January 1970.
[5] Greif, R and Chapon, E , Investigation of successive failure modesin graphite-epoxy laminated composite beams, Journal of Reinforced
Plastics and Composites, Vol. 19, 1993.
[6] Echaabi, J., Trochu, F, Ratle, A. and Gauvin, R, Failure modesand damage progression of a quasi-isotropic graphite-epoxycomposites in flexural bending tests, Proceedings of ICCM-10Whistler, British Columbia, Canada, August 14th-18th, 1995.
[7] Echaabi, J, Trochu, F, Pham, T and Ouellet, M, Theoretical andexperimental investigation of failure and damage progression ofgraphite-epoxy composites in flexural bending test. J. Reinf Plasticsand Composites, Vol. 15, No. 7, 1996.
[8] ASTM, Test D790, American Standards of Testing and Materials,
Vol. 8.01, 1992.
Transactions on Engineering Sciences vol 21, © 1998 WIT Press, www.witpress.com, ISSN 1743-3533