CHAPTER 3 BODY DESIGN & CONSTRUCTION
3.1 Chassis Design
3.1.1 Materials Selection and Design
The decision was taken to fabricate the body of the robot from composite materials to save weight and to control the thermal transmission gradient between the operating environment and that of the inner electronic components. The alternatives (constrained by availability), namely metals, have high thermal transmission coefficients and high density and would therefore be less suitable.
The reason for considering composite materials was determined by their strength properties. The stiffness K = ES/l characteristic is given to the mechanical performance of a beam. The ratio of bending stiffness of two arbitrary materials 1 and 2 is calculated from [39]:
K1
K2=E1S1 E2S2⋅l2
l1
{8}
where Ex = Young's modulus Sx = Cross-sectional area
lx = Length of the material
The ratio of the weights of the beams is:
m1
m2=E1l1 E2l2⋅℘1
℘2
{9}
where ρx = Density of the material mx = Mass of the material Combining equations 8 and 9,
K1
K2=E1/℘1 E2℘2⋅m1
m2⋅l2 l1
2
{10}
With the comparison of mechanical performance of two materials, the same length and weights must be used. Therefore;
K1
K2=E1/℘1 E2℘2
{11}
From equation 11 it was determined that the best material would be the one with the highest specific modulus (E/ρ). During comparisons it was established that Kevlar had a specific modulus value of 87 MN m/kg, which was higher than that for steel, aluminum alloys and tungsten [39].
Phenolic resin is a thermosetting resin and was used as it had the following characteristics [39]:
● excellent dimensional stability
● good thermal stability
● good chemical resistance
● low shrinkage
● good mechanical characteristics
● low cost
● dark colors of the resin
Kevlar has the properties of high impact resistance and damage tolerance as it causes the absorption of energy by widespread delamination and splitting [39].
A stress and deformation analysis of the robot was performed with an initial robot
body weight of 25 kg. The complete weight of CAESAR is 56 kg, but only the weight of the body and internal components is considered in this situation. An indication of the results are shown in figure 3-7.
Figure 3-7: Stress (above) and Deformation (below) analysis
As observed from the stress analysis, the maximum stress is 0.604 MPa (indicated with the red arrow), which occurs mostly along the axis areas. Taking into account that the maximum shear stress for mild steel is 210 MPa, the shaft design is safe from shearing. The maximum deformation of the robot body is 0.001494 mm, which is an acceptable value.
Bending and tensile tests were performed to determine whether the composite material that the body was manufactured from was sufficiently strong for the environments it would be exposed to [37].
The bending test were performed with an Instron 5500R. These tests were performed according to ASTM D790 standard, which required a three point loading test. The Instron 5500R was set to a constant cross speed of 12 mm per minute at
Load
Load
room temperature. The pitch was set at 176.5 mm. The test specimen's dimensions were captured by the Instron software and were the following:
Specimen 1: 35 mm x 290 mm x 5 mm (syntactic soric foam core thickness: 1 mm) Specimen 2: 30 mm x 290 mm x 8 mm (syntactic soric foam core thickness: 4 mm) Specimen 3: 40 mm x 290 mm x 3 mm
Specimen 4: 35 mm x 290 mm x 3 mm Specimen 5: 30 mm x 290 mm x 3 mm
Figure 3-8 shows the results that were obtained.
Figure 3-8: Bending test results
Specimen 1 and specimen 2 each consisted of a different thickness syntactic soric foam core. As seen from the graphs, specimen 2 which had a thicker syntactic soric foam core, was able to withstand a greater force and an extension of about 6 mm.
Specimens 3, 4 and 5 had no syntactic soric foam core and these appeared to be less able to withstand force. Results indicated that the substrate could absorb more energy with the syntactic soric foam core than without it. This is a result of the load distribution across the substrate. The three modes in which failure occurred is matrix failure, fiber failure and delamination. These results shown are only that of a straight piece of specimen. The composite body consists of curves and of two halves, therefore increasing the strength and enabling it to resist a greater force before disintegrating.
The bending stress can be calculated from equation 12.
f=My I =
F. L 2 x t
2 w.t3
12
=3FL wt2
=3x150x176.5
30x8 =330.938MPa
{12}
where σf = bending stress (MPa) M = Moment (Nmm)
y = Distance from cross-sectional neutral point to point of maximum tensional stress (mm)
I = Moment of Area (mm4) F = Force (N)
L = pitch length (mm) t = thickness (mm) w = width (mm)
Tensile testing were also performed on a specimen of which the dimensions were 17 mm x 290 mm x 5 mm. This allowed for investigation of the composite materials performance in a situation should bending have occurred. The results from this test is recorded in figure 3-9.
Figure 3-9: Tensile test results
These test results indicate that the specimens extended linearly as the load increased. The point where the linearity deviated is about 3000 N. Failure occurred at about 4000 N.
From this the shear stress can be calculated as shown in equation 13.
=F A= F
wt {13}
where:σ = shear stress (MPa) F = Force (N)
A = Area (mm2) w = width (mm) t = thickness (mm)
therefore, =3000
17x5=35.294MPa
After the materials selection and design had been confirmed, it was possible to initialize the construction phase.
Tests were performed to verify whether the shell of CAESAR is able to withstand temperatures of 200 °C. The composite material test pieces were inserted into an oven and left for a time period of an hour. The composite material did not disintegrate. The phenolic resin changed to a red color as it cured. This strengthened the composite structure.