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Procedia Engineering
ELSEVIER
Procedía Engineering 38 (2012) 2207 - 2213
www.elsevier.com/loeate/procedia
ICMOC 2012
Mode-I fracture and Impact analysis on Stitched and Unstitched Glass/Epoxy Composite Laminate
A number of approaches have been used to improve the impact damage resistance and tolerance of composite materials. These include control of fibre/matrix interfacial adhesion, laminate design and through the thickness reinforcement. One of the techniques of arresting delamination is providing through-the-thickness reinforcement like stitching. The objective of this work is to understand the influence of fibre orientation and stitching on the mechanical response of the laminate. In this paper, the drop weight impact properties and mode I interlaminar fracture toughness of stitched and unstitched glass fibre epoxy laminate are were studied experimentally. The ultrasonic C-Scan image of the damaged impact specimen is used to find the damage area. A detailed comparison between the stitched and unstitched laminates experimentally confirmed that the stitching effectively suppressed the out of plane impact damage and increased the interlaminar fracture toughness.
© 2012 Published by Elsevier Ltd. Selection and/or peer-review under responsibility of Noorul Islam Centre for Higher Education
Key words: Stitched laminate; drop weight impact; mode I interlaminar fracture toughness;
1. Introduction
In terms of structure, materials can be divided into four basic categories: metals, polymers, ceramics, and composite materials. A composite structure is a material composed of two or more phases combined in a macroscopic scale, whose properties are superior constituent materials, acting in an independent manner. In other words, a composite is a combination of at least two different materials both chemically and geometrically.
* Corresponding author. Tel.: +91 44 22357657; fax: +91 44 22203255. E-mail address: muruganandhan@annauniv.edu.
Muraganandhana, Vela Murali a*
Department of Mechanical Engineering, Anna University, Chennai,600 025, India.
Abstract
1877-7058 © 2012 Published by Elsevier Ltd. doi:10.1016/j.proeng.2012.06.265
Nomenclature
b Width of the specimen
Ô Crack opening displacement
p applied load
a Initial delamination length
A Effective delamination extension correction factor
C Compliance= ratio of the load point displacement to the applied load, i.e., C = 5/P
I Moment of inertia
E Young's modules.
U Strain energy stored
Wext External work done
Aymerich. F [1] examined the impact response of stitched graphite/epoxy laminates with the aim of evaluating the efficiency of stitching as a reinforcing mechanism able to improve the delamination resistance of laminates. The investigation showed that the role of stitches in controlling damage progression of laminates and their capability to reduce the impact sensitivity of specimens are greatly dependent on the impact behaviour of base laminates. Mehmet Aktas, et al. [2] investigated the impact response of unidirectional glass/ epoxy laminates by considering energy profile diagrams and associated load deflection curves. Damage modes and the damage process of laminates under varied impact energies are discussed. Velmurugan R and Solaimurugan S [3] have worked on effect of plain stitching. The effect of plain stitching by untwisted fiber roving in the in-plane mechanical properties and mode-I interlaminar fracture toughness of glass/polyster composite is examined. Uniform distribution of untwisted stitch fibers causes higher crack closure force and result in increased mode I interlaminar fracture toughness up to 20 times higher than that of unstitched specimen. It is understood that, stitching of untwisted roving, in plain stitch, results in higher mode I interlaminar fracture toughness without losing the in plane mechanical properties much better than the glass and carbon roving.
2. Experimental Procedures
2.1. Materials
The material used as fibre is unidirectional E glass fibre and the resin used is epoxy LY 556 in the fabrication of the composite material. The hardener used is HY 951. Epoxy and hardener are mixed in the ratio 10:1.
2.2. Preparation of specimen
The specimen dimension for drop weight impact testing as per the standard ASTM D5628 - 96 is 89x89 mm and 5mm thickness. The specimen dimension for DCB testing as per the standard ASTM D5528-01 is 125 x 25 with 5 mm thickness and 50 mm as the initial crack length. The orientations tested by varying the mid layers are (0/45/-45/0/0/-45/45/0), (0/45/-45/25/-25/-45/45/0), (0/45/-45/45/-45/-45/45/0), (0/45/-45/55/-55/-45/45/0) and (0/45/-45/90/90/-45/45/0).
The unstitched specimens are fabricated for the above stacking sequences. It is done by hand layup process first and then it is compressed at 5 bar pressure and 70°c temperature for 5 minutes in the compression molding machine.
Stitch thread
Fig. 1. Cross section of the laminate along the stitch thread
In this process, a stack of plies consisting of nominally in-plane fibres is penetrated and bounded together by the stitch thread and selvage thread. Stitching is done by twisted polyester yarn. Figure. 1 illustrates the cross section of laminate after stitching. Stitch direction is perpendicular to the fibre direction 0° surface layers of the laminate. The volume fraction of stitch thread material is about 0.4%. In order to reduce fibre distortion in the interior of the laminate, a modified lock stitch is adopted for stitching by easing the tension of the stitch thread. The stitch space is 10 mm and the stitch pitch is both 4 mm. The stitched plies were impregnated and molded by vacuum bag Molding method. Finally it is compressed 5 minutes in the compression molding machine for curing the wet layer laminate.
2.3. Drop weight Impact testing
Standard test Method for Impact Resistance of a composite laminate by means of a drop weight is ASTM D5628 - 96 [4]. A drop weight impact test is performed on the fabricated laminate. Damage is imparted through out-of-plane, concentrated impact using a drop weight with a hemispherical striker tip.
The impactor nose is hemispherical with a diameter of 12.5 mm. The weight of impactor is 12.967 kg. The boundary condition is clamped. The impactor is dropped under gravity assisted free fall condition from the height of 800 mm.
2.4. Mode I Interlaminar Fracture Toughness Testing
Standard test Method for Mode I Interlaminar Fracture Toughness Testing of a composite laminate is ASTM D5528-01. The double cantilever beam specimen is widely used to measure mode I interlaminar fracture toughness of composites. The specimen is a made with a non-adhesive insert placed at the mid-plane, at one end prior to curing or consolidation, to simulate a delamination.
The loads are applied to the specimen via hinges adhesively bonded to the surface of the specimen.During test, the specimen is subjected to displacement controlled loading and usually experiences stable delamination growth allowing several values of interlaminar fracture toughness to be determined along the specimen's length. As the delamination grows, fibre bridging usually occurs increasing the energy required to propagate further delamination. Therefore, only the first value of interlaminar fracture toughness obtained from delamination growth from the insert is unaffected by fibre bridging and can be considered a generic interlaminar fracture toughness.
3. Results and Discussion
3.1 Drop weight impact Test
It is observed that the impact force by unstitched laminate is always greater than the stitched plate. It could be due to high stress induced in the stitched points, which absorbs some amount of impact force. Table. 1 shows the comparison of both stitched and unstitched laminate by experimental results for various test conditions. Table. 1 Impact response of stitched and unstitched laminate
Specimen Orientation Peak Peak Peak Total
deformat energy (J) force (N) energy ion (mm) absorbed
Unstitched
Stitched
(0/45/-45/0/0/-45/45/0) 10.7 96.2 13976.6 24.4
(0/45/-45/25/-25/-45/45/0) 10.9 95.5 14330.7 28.8
(0/45/-45/45/-45/-45/45/0) 10.6 96.7 14481.8 31.9
(0/45/-45/55/-55/-45/45/0) 10.3 95.5 14122.2 27.6
(0/45/-45/90/90/-45/45/0) 10.1 95.6 13201.3 22.4
(0/45/-45/0/0/-45/45/0) 10.6 96.8 16460.7 43.8
(0/45/-45/25/-25/-45/45/0) 10.2 97.9 16292.3 49.8
(0/45/-45/45/-45/-45/45/0) 10.1 99.3 17248.4 56.1
(0/45/-45/55/-55/-45/45/0) 10.1 97.5 15291.1 43.8
(0/45/-45/90/90/-45/45/0) 10.2 97.3 15289.2 40.3
Damage Area
Fig.2. C-Scan image of the Unstitched Specimen
Damage Area
Fig. 3. C-Scan image of the stitched Specimen
In the current study, the impact phenomenon is characterized in terms of peak load and absorbed energy. Energy absorption in composites is mainly through two modes ©elastic strain energy and (ii)damage modes. As
composites are brittle in nature, there is no associated plastic deformation. The absorbed energy is calculated as the difference of total energy absorbed by the specimen at the end of the impact and the energy at peak load. For unstitched laminate, it was observed that inplane damage area was more compared to stitched ones from the C -Scan image shown in Figure. 2 and Figure. 3. The damage area of the Unstitched laminate is 320 mm2 which is 31.25 % higher than stitched laminate. On the other hand, in the case of stitched laminates, stitching contains the damage within the grid location. Visual observation of the failed stitched laminate indicates that damage area is highly localized.
For unstitched laminates, damage spread is conical, with the maximum damage on the back surface. However, for the stitched laminates, propagation of delamination front is arrested at the stitched location and the damage spread through the thickness in cylindrical fashion. However, it should be noted that the damage is very much localised due to the interlocking of the fabric, resists the splitting of the lamina. It increases delamination strength.
3.2 Mode I interlaminar Fracture Toughness
The critical strain energy release rate in the DCB specimen is represented as
G - ^n bda
Jt = (U - W^)
The crack opening displacement in the DCB specimen is
(1) (2)
In the actual case, crack front is identified by the point at which separation of (layers) arms starts. So, the virtual fixed point is away from the actual crack front. In order to find the virtual fixed point, delamination extension correction factor A is added to the visual crack length (a). The compliance C is the ratio of the load point displacement to the applied load, i.e., C =S/P . The A is determined experimentally by generating a least square plot of the cube root of i
compliance, C 3 , as a function of delamination length (a) as shown in figure 4, So the crack length, a is modified into {a + |A|). Therefore, according to modified beam theory, strain energy release rate of double cantilever beam is
2 b(a + |A|)
Double cantilever beam (DCB) tests were performed and the mode I interlaminar fracture toughness, Gic was determined with modified beam theory (МВТ) as per ASTM D5528-01.
s Ь ОД*
И Ia to 20 SO 40 50 150 70 SO 80 traHencili (mm]
Fig. 4. Plot for determination of (A) effective delamination extension
Figure 5. Effect of the midlayer fibre interface orientation on Gic.
3« QO 1590 JOQO > 15D0
Figure 6. Plot for Gic and crack length for stitched and unstitched laminates
3.2.1 Influence of fibre orientation
The Gic of unstitched and stitched specimens' increases with increase in orientation angle, 9 of 0/0 interface upto 45; further increase in 0 leads to decrease of Gic. The crack propagation in both plies adjacent to the 0/0 delamination interface is directed along the specimen length, and hence the crack propagates freely without deviation. Therefore, the 0/0 interface has the lowest value of Gic. In the case of 25/-25 interface, crack propagation is directed along the fibres i.e., +25 in one layer and along -25 in the adjacent layer of delamination interface .So the crack propagation in one path is opposed by another path. These path diversions of crack propagation in the adjacent plies offer more resistance to crack propagation and leads to higher Gic. Similarly, in the 45/-45 interface, crack path is diverted along +45 in one layer and along -45 in the adjacent layer with relative angle of 90, which leads to maximum of Gic. The influence of fibre orientation is shown in the Figure 5.
3.2.2 Effect of stitching on mode I interlaminar fracture toughness, Gic
It is observed from Figure 6 that stitching improves the mode I interlaminar fracture toughness, Gic. In the stitched specimens, in addition to matrix material, stitch threads offer resistance to delamination crack propagation.This is because during interlaminar mode I fracture propagation, the stitch threads undergo tensile
breakage and thus additional energy is required to break the stitch threads. Therefore the GIc for the stitched laminate is higher.
4. Conclusion
The stitched and unstitched specimens were fabricated with the different orientations at the interface. They were subjected to drop weight impact test and double cantilever beam test. The drop weight impacted specimen is analyzed using Ultrasonic C-Scan machine. The strain energy release rate Glc of stitched and unstitched specimen were found using the DCB test.
It is clear from the ultrasonic C-Scan image that the damage area is 33.33% high for unstitched laminate than the stitched laminate. The stitched laminate absorbs more energy than unstitched laminate in the drop weight impact test. Stitching of the laminate will further improve the delamination resistance, as the stitching helps in containing the damage within the grid location. Spread of damage is cylindrical in the case of stitched laminates whereas it is more likely to be conical in unstitched laminates.
The mode I interlaminar fracture toughness, Glc of laminate depends on the fibre orientation in the layers adjacent to the fracture plane in addition to the property of matrix material. Stitching increases the Glc by 51.41%. The Glc of both unstitched and stitched specimens increases with increase in orientation angle, 9 of 0/-9 interface upto 45°; further increase in 0 leads to decrease of GIc.
References:
[1] Aymerich F.Damage response of stitched cross-ply laminates under impact loadings. Elsevier Engineering Fracture Mechanics ;2007,p. 500-514.
[2] Mehmet Aktas, Cesim Atas, Bulent Murat Icten, Ramazan Karakuzu.An experimental investigation of the impact response of composite laminate. Composite Structures, Volume 87, Issue 4,2009, p. 307-313.
[3] Velmurugan R., Solaimurugan S .Improvements in Mode I interlaminar fracture toughness and in-plane mechanical properties of stitched glass/polyester composite. ELSEVIER publications, Composites science and Technology;2007, Vol. 67,p. 61-69.
[4] ASTM D 5628 -96. Standard Test Method for Impact Resistance of Fait, Rigid Plastic Specimen By Means of a Falling Dart (2001). Annual Book of ASTM Standards, 630-639.
[5] ASTM D5528-01. Standard test method for mode I interlaminar fracture toughness of unidirectional fiber-reinforced polymer matrix composites. Annual book of ASTM standards, vol. 15.03.
