Characterization of glass epoxy laminates and validation of laminate design software through experimentation.
Shankar, D.V. Ravi ; Reddy, P. Ram ; Hussain, Manzoor 等
Introduction
The present work is focused on preparing the laminates of balanced
symmetric orientation sequence with cross ply stacking in view achieving
the symmetric inter-laminar properties of the laminate. The theory used
by Whitney and Leissa [1], which is generally recognized as the
classical, linear, thin-plate theory for arbitrarily laminated
anisotropic plates. Thus, both geometric and material nonlinearities, as
well as thickness shear flexibility and thickness normal stress effects,
are neglected. These simplifications are considered to be sufficiently
accurate for most practical composite-material panels as pointed out
recently by Ashton [2]. Fiber reinforced composite materials are often
over designed to meet unexpected, unconsidered failure criteria. There
is large number of works in literature about vibration problems with
composite materials and structures, Koo and Lee [3], Khdeir [4], Rao and
Ganesan [5] reported. In the present work experiments were carried out
to In order to establish realistic approach in designing the composite
components. In view of achieving the same the composite laminates made
of glass epoxy were prepared and analyzed with help of analytical
experimentation to cross check the results obtained through experimental
results. Analyses were performed on carbon-epoxy composite laminates,
the details and specifications of carbon-epoxy laminates were reported
by Volnei Tita [6].and he also discussed that the Laminates properties
can be altered through a change in the stacking sequence. Tsai and Hahn
[7] Tsai [8] Vinson and Sierakowski [9] reported that the tailoring of
the material to achieve the desired natural frequencies and respective
mode shapes, without changing its geometry drastically or increasing its
weight. In the present work laminate design software is developed to
estimate the resultant elastic properties of balanced symmetric
laminates. This provides an approach to design composite structures
incorporating tailored properties. As the general fabricating procedures
have some limitations the theoretically estimated properties cannot be
arrived in the realistic structure. In this connection laminates of
different orientations which are mentioned in the following sections
were fabricated and subjected to tensile tests.
Experimentation
(1) Laminate design software
(2) Specimen preparation
(3) Volume fraction estimation
(4) Tensile testing
(5) Comparison of test results
(6) Results and discussion
Laminate Design Software
While working with laminated composite materials evaluation of the
mechanical properties through theoretical approach is essential before
experimenting with laminates. The mechanics of materials deal with the
stresses, strains, and deformations in engineering structures subjected
to mechanical and thermal loads. A common assumption in the mechanics of
conventional materials, such as steels and aluminum, is that they are
homogeneous and isotropic continua. For a homogeneous material,
properties do not depend on the location, and for an isotropic material,
properties do not depend on the orientation. Unless severely cold
worked, grains in metallic materials are randomly oriented so that, on a
statistical basis, the assumption of isotropy can be justified.
Fiber-reinforced composites are microscopically inhomogeneous and no
isotropic (orthotropic). As a result, the mechanics of fiber reinforced
composites are far more complex than that of conventional materials.
Classical laminate theory provides the solution for estimating the
elastic properties of laminated composite materials the fallowing flow
chart furnished in the Fig.No.1 is the basis for the laminate design
soft ware, the front end of the soft ware is furnished in the Fig.no.2.
[FIGURE 1 OMITTED]
[FIGURE 2 OMITTED]
Specimen preparation
Specimens were moulded by compression moulding technique. As per
ASTM specifications 2.5 mm thickness laminates of desired orientation
sequences are moulded. Each laminate (240 mm X 240 mm) consists of four
lamina with 0deg orientation made of UD glass fabric. In the same manner
five different laminates with cross-ply balanced symmetric laminates
with orientations of +/-20deg, +/-30deg, +/-45deg, +/-55deg and +/-60deg
were prepared. The tensile specimens with a width of 25 mm and 170 mm
long were cut by diamond disc saw, similarly specimens with 5.5 mm
thickness with eight layers were also prepared for deflection test in
cantilever mode the specimens were of 30 mm width and 140 mm long were
cut from the big laminate of 240 X 240 mm the matrix material used is
epoxy with K6 room temperature hardener the details of mould and
laminate preparation process are discuss in detail in the following
sections.
Design and fabrication of metallic mould
The mould is made of MS material. To prevent the leakage of resin,
four dams were fixed through nuts and bolts on a 10 mm thick MS plate
which was having machined by facing operation on lathe machine. The
mould cavity area is 300 x 300 mm2. The mould is fabricated to meet the
above specifications as shown in the fig4.1. The required pressure is
applied through pressure plate by tightening the nuts and bolts, the
arrangement of which is shown in figure.
[FIGURE 3 OMITTED]
In this particular mould to achieve uniform thickness of the
compressed laminate surface ground spacers made out of MS flat with 5 mm
thickness.
Pressure Plate
Pressure plate was also of the MS with flat turned surface finish
ensuring perfect flatness. In order to prevent crippling and flexing due
to compressive forces produced due to the top cover plate, a 20 mm
thickness is maintained.
Top cover plate
The purpose of top cover plate is to apply uniform pressure on the
laminates through this arrangement as shown in fig 2. As there is no
control over the applied pressure the spacer plates of desired thickness
i.e, 2.5 mm, 5.5 mm (depending on the requirement) and width 25 mm are
placed at the edges of four dams which restricts the movement of the
pressure plate which ensures a uniform thickness the is equal to the
thickness of spacer plate.
Precautions to fabricate a sound laminate
To ensure relay able test results, a defect free laminate
preparation is essential, in view of achieving the same the following
preventive measures were considered
* The prepared glass fiber should be kept carefully and avoid
moisture absorption.
* It should be placed at dry place.
* As all the metallic components and mould is machining finish. To
improve the surface finish paint putty has been applied on the contact
surfaces of the mould and ground to achieve required surface finish. The
molding procedure starts with applying wax polishing on the surface.
This ensures improvements in good surface finish of a component. After
that poly vinyl alcohol viscous liquid is applied on the surface of the
mould uniformly and left for drying about 15 minutes. This liquid
creates an invisible film which works as impervious layer preventing the
contact between resin and mould surface.
* While impregnating resin, rolling has to be performed ensuring no
air bubble is entrapped in the resin or in between layers to obtain a
sound laminate (Defect free).
* While applying pressure on the laminate through top cover
assembly simultaneous tightening of nuts should be done.
The above mentioned precautions a laminate of god quality can be
made as shown in the Fig. 4. From this laminate the test coupons are cut
with required specifications which have all ready been discoursed in the
above sections.
[FIGURE 4 OMITTED]
Volume fraction of glass loading in the laminate (Burn Test)
Specimens of laminates with specifications (25 x 25 x 2.5 mm) are
cut from the laminates of all orientation sequences are prepared. The
volume of the specimen and weight is assessed accurately. Then the
laminate test coupon is kept in the electric Furness maintained at
600[degrees]C to burn the matrix material. The Glass fiber residue is
weighed and volume of the glass fibre is assessed from the specific
gravity of glass. Calculations were made and the volume fraction of
glass loading is given as fallows.
Volume fraction of glass loading ([V.sub.f]) = Volume of
glass/Volume of the specimen
Volume fraction of matrix material ([V.sub.m]) = 1 - [V.sub.f]
The assed list of volume fractions of the reinforcement loading
were furnished in the comparative statement of the materiatial
properties of laminates Table.no.2
[FIGURE 5 OMITTED]
[FIGURE 6 OMITTED]
Tensile Tests
Tensile tests are performed on specimens made as per the ASTM
specifications. The specification of the specimens are the length is 150
mm 2.5 mm thickness and 25 mm width these are the fallowing figures
related to tensile tests conducted on various specimens. The Fig.7 (a)
represents the tensile test in progress and these figures
Fig.7 (b),(c),(d),(e),(f) furnished bellow are subjected to tensile
test.
[FIGURE 7a OMITTED]
[FIGURE 7b OMITTED]
[FIGURE 7c OMITTED]
[FIGURE 7d OMITTED]
[FIGURE 7e OMITTED]
[FIGURE 7f OMITTED]
[FIGURE 8 OMITTED]
[FIGURE 9 OMITTED]
Software validation
Software validation is essential as this soft ware program is
utilized for further experimental analysis. Tensile test specimens are
prepared and tested as per ASTM specifications. The test results were
published in table. No.2 and compared with the theoretical results of
the software program. It is clearly evident from these test results the
elastic properties of the laminate are 23.33% lower when compared to the
theoretically estimated elastic properties.
Results and discussion
The objective of the present experimental work is the validation of
laminate design software's developed by author to cross check
consistency with experimental tensile test results is proved. The
deviation of the elastic modulus is due to the constraints in the
manufacturing process adopted in the experimental work. The comparison
of the experimental test results at .5% strain is justifies keeping the
designer in safe limit. A. K. Kaw and G. Willenbring [10] reported about
the behavior of UD laminated composite's in biaxial state of stress
is well in agreement with the soft ware based estimated properties; the
tensile experimental test results were furnished the Fig. 9.clearly
indicating that the tensile strength of laminate is decreasing up to
45[degrees] then there a small increment in the strength due to
poisons' effect. The present experimental investigation provides a
path way to the composite material designer to arrive at worthy design
with considerable reliability.
References
[1] J. M. Whitney and A. W. Leissa, J. Appl. Mech., 36, Trans. ASME
91, Ser. E: 261-6 (1969).
[2] J. E. Ashton, Analysis and Design Methods for Composite
Structures-Overly Intimidating, AIAA Paper 75-825, AIAA, ASME & SAE
16th Structures, Struct. Dynamics & Mater. Conf. Denver, Co., May
1975.
[3] Koo, K.N.; Lee, I. Dynamic behavior of thick composite beams.
Journal of Reinforced Plastics and Composites, n. 14, p. 196-210, 1995.
[4] Khdeir, A.A. Dynamic response of ant symmetric cross-ply
laminated composite beams with arbitrary boundary conditions.
International Journal Engineering Science, v. 34, n. 1, p. 9-19, 1996.
[5] Rao, S.R.; Ganesan, N. Dynamic response of non-uniform
Composite beams. Journal of Sound and Vibration, v. 200, n. 5, p.
563-577, 1997
[6] Volnei Tita *, Jonas De Rvalho, Joao Lirani, A Procedure to
Estimate the Dynamic Damped Behavior of Fiber Reinforced Composite Beams
Submitted to Flexural Vibrations, Department of Mechanical Engineering,
Engineering School of S. Carlos.
[7] Tsai, S.W; Hahn, H.T. Introduction to composite Materials.
Westport, Technomic, 1980.
[8] Tsai, S.W. Composites Design. Think Composite, Dayton, 1986.
[9] Vinson, J.R.; Sierakowski, R.L. Behavior of Structures Composed
of Composite Materials. Dordrecht, Martins Nijhoff, 1986.
[10] A. K. Kaw and G. Willenbring. International Journal of
Engineering Education A Software Tool for Mechanics of Composite
Materials * Vol. 13, No. 6, p. 433 [+ or -] 441, 1997 0949-149X/91
Printed in Great Britain. # 1997 Tempus Publications.
D.V. Ravi Shankar (1), P. Ram Reddy (2) and Manzoor Hussain (3)
(1) Research scholar and associate professor Nizam institute of
engineering and technology, Deshmukhi, Nalgonda (Dt). E-mail:
[email protected]
(2) Professor In Mechanical Engineering Principal Mall Reddy
Engineering College for Women, Hyderabad, Former-registrar JNTU
Hyderabad.
(3) Associat Professer In Dept. of Mechanical Engineering JNTUCE
Hyderabad.
Table 1: INSTRON Tensile Test Results.
Sample orientation Max Tensile Tensile strain Modulus at 0.50%
sequence in deg strength (MPa) 0.50% Strain (GPa)
(CROSSPLY)
[0.sup.0] 358 189.8 37.96
[20.sup.0] 206.9 151.1 30.33
[30.sup.0] 117.6 94.52 18.9
[40.sup.0] 82.1 51.07 10.21
[45.sup.0] 64.8 52.7 10.54
[55.sup.0] 74.3 54.46 10.89
[60.sup.0] 101.3 54.28 10.86
Table 2: Comparative statement of tensile test results and volume
fraction of glass loading.
Seria Balanced Tensile Estimated Volume Percentage
l No. symmetric modulus Tensile fraction deviation in
Stacking By modulus Of Fiber elastic
sequence of tensile By in properties with
Orientation test In laminated composite reference to
GPa design Sample theoretical
software prepared estimation
program
In GPa
1 0 37.96 51.78 .68 26.33%
2 +/-20 30.22 38.41 .67 21.4%
3 +/-30 18.9 26.0945 .68 27.57%
4 +/-40 10.21 15.703 .67 34.9%
4 +/-45 10.54 15.26 .73 30.9%
6 +/-55 10.89 12.0387 .70 10%
7 +/-60 10.86 12.3711 .75 12.2%
Average variation of elastic properties 23.33%
