Composites laboratory report

Composites Laboratory Report 1
COMPOSITES LABORATORY REPORT
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Composites Laboratory Report 2
Composites Laboratory Report
Introduction
General Contexts
The reinforcement of polymers by addition of carbon fibers has resulted in a new creation of
polymeric structural materials termed as carbon plastics. Due to the unified action of the joined
materials and interaction of the atoms of the materials and the interfaces at the joining plane,
composites have beneficial, superior synergetic, mechanical properties that have become
preferred in most industrial applications. In terms of specific strength and rigidity carbon plastics
have a wide range of structural applications (Callister, W. 2012, 170). Additional properties of
carbon plastics include; low thermal expansion, abrasive resistance and high fatigue strength
when statically and dynamically loaded.
Background Theory
The flexural strength of a material is not an elemental property that can be used in the analysis of
the material when subjected to a load (Callister, W. 2012, 170). This is because of the
combination of shear, tensile and compressive properties when the material is loaded under
flexure. When a flexural loading is imposed on to a material that is analyzed, the three basic
stresses, tension, shear, and compression are exerted on the material specimen (Callister, W.
2012, 170). Normally in a flexural test, the material specimen is loaded in a state where the
specimen is horizontal to its neutral axis. In this case, the stresses induced by compression occurs
in the top portion of the neutral axis. On the contrary, the tensile forces are present in the lower
portion of the neutral axis of the cross-section of the material specimen (Callister, W. 2012, 170).
For most composites usually, the compressive strength because of the combination of the datum
Composites Laboratory Report 3
materials is lower, hence the specimen when flexurally loaded will fail in compression (Callister,
W. 2012, 170). This failure is associated with the microbuckling of the fibers in the
microstructure. To determine the flexural modulus of a material, the applied force and the
deflection are determined. It is then calculated using the following equation: (Callister, W. 2012,
170).



Aim and Objectives
The aim of the experiment is to determine Young’s modulus using the rule of mixtures of carbon
of the four samples of fiber laminate composites using three-point loading. Additionally, the
objectives will be to compare the results with similar well-established investigations for control
purposes. Assumptions will also be made, and their implications based on the sources of errors in
the experiment.
Composites Laboratory Report 4
Method
Apparatus
Fig1. Flexural test labeled apparatus
Experimental procedure
A tape of the carbon-fibre reinforced composite was split to testing shape and dimension. The
sample was preimpegrenated with a polymer resin forming thickness through four different
stacking sequences, and then cured by heat and an application of pressure according to BS EN
2565:2013. The material samples, having three tangential contacts with the testing rig were
loaded by three-point loading that consisted of a support point near both ends of the beam
(Callister, W. 2012, 171). Additionally, one-point loading was induced at the midspan of the
beam with a Tinius Olsen 255T automatic flexural testing machine consistsisting of 220-240V at
50-60Hz exerting one load point at the midspan. The dimension of the specimens and span
between supports were obtained. Subsequently, the load and the subsequent corresponding
deflection were noted down to calculate the flexural modulus.
Composites Laboratory Report 5
Results
Graph of four specimen
Case1 is for specimen with stacking sequence 

, case 2 is for stacking sequence 

,
case 3 is for specimen with stacking sequence 

, case 4 is for specimen with stacking
sequence 

. It was noted that case 3 and 4 were on the same line due to
constant force and deflection values recorded up to a deflection of 10 mm where case 4
progresses. From the graphs plotted, the force is directly proportional to the deflection when all
the specimen is point loaded.
Sample calculations for Case 1

     



        


0
2
4
6
8
10
12
14
16
18
20
0 5 10 15 20
F(N)
W(mm)
Graph of four tested specimen
Case 1
Case 2
Case 3&4
Composites Laboratory Report 6
From the graph the gradient is 1.5845*10^-6





 

130 Gpa
From the graph,

  
 

  

Table
Stacking
sequence
Average
Dimension
Experiment
calculation
Theore
tical
Calcul
ation
Units
L
mm
b
mm
h
mm
Gpa

Gpa
Gpa
Gpa
Vm


123
25.132
1.89
6
130
2.0354
0.52
125
3
0.39


123
25.568
1.95
124
2.0061
0.54
125
3
0.39


123
25.84
1.94
126
2.1324
0.58
125
3
0.39






.
123
25.68
1.96
132
2.8313
0.59
125
3
0.39
Composites Laboratory Report 7
Discussion
Interpretation of results
From the graph drawn, it was visible that the curve for each case revealed that the force is
directly proportional to the deflection of the specimen when loaded. The intercept of the curve
revealed the youngs modulus that was used to determine the Poisson's ratio. From the table
above, it is evident that the flexural strength of the specimen stacked in sequence of all angles


. was the highest at 132Gpa. On the contrary the material that was
stacked

had the smallest strength with 124Gpa. The determination of the young’s
modulus also revealed that the stacked at 

revealed the lowest at 2.0061Gpa while the
young’s modulus of material stacked at 

revealed the highest young’s
modulus at 2.8313. Similarly, poisons ratio while the material stacked at 

revealed the highest poison’s ratio at 0.54. This is because of minimal interface between the
layers of stacking therefore, the micro-buckling of the fibers in the microstructure was minimal.
Assumptions
The assumption made in the experiment is that when loading each specimen, the axis of the pin
is parallel to the specimen neutral axis (Callister, W. 2012, 171). Additionally, the assumption is
made that the entire load is acting at the center of the span and the maximum stress is present at
only the central axis of the beam.
Composites Laboratory Report 8
Sources of Uncertainty and Errors.
In uncertainty, since no microscope was used it was uncertain that when stacking the specimen,
the laminated composites atomically bonded with each with the exertion of pressure and
temperature. Additionally, it was uncertain that direction of the load applied is exerted at the
midpoint of the span (Callister, W. 2012, 170). The sources of error in the experiment included
measurement errors as the resolution of the ruler had an accuracy of 0.05mm while that of the
Vernier had an error of 0.01mm. The difference in accuracy during measurement resulted in the
variations between theoretical values and experimental values.
Conclusion
To sum up, Young’s modulus using the rule of mixtures in carbon of the four samples of fiber
laminate composites were determined using three-point loading and a comparison of the results
with theoretical values determined. From the comparison, it was evident that the Young's
modulus of composite stacked in 

revealed the highest value of 2.8313.
This is because of minimal interface between the layers of stacking therefore, the micro-buckling
of the fibers in the microstructure was minimal. The young’s modulus of composite stacked at


revealed the lowest youngs modulus of 2.0061Gpa. This is because of the interface
between the stacked layer that resulted in micro-bulking of the carbon fibers (Callister, W. 2012,
171). The noted differences between the theoretical and experimental values was due to the
sources of errors and uncertainty in the experiment.
Composites Laboratory Report 9
References
Callister, W, 2012. Fundamentals of Materials Science and Engineering. New York: John Wiley
&Sons

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