Section Modulus calculation and optimization
SECTION MODULUS
Aim
To calculate the section modulus of the previously designed hood for analyzing its strength and also optimizing the design to see the difference in the bending strength of the hood. Higher the section modulus of a structure, the more the resistive it becomes to bending.
Section modulus
Section modulus is a geometric property for a given cross-section used in the design of beams or flexural members, it is also referred to as the Polar Modulus or the Torsional Sectional Modulus. Other geometric properties used in design include area for tension and shear, radius of gyration for compression, and moment of inertia and polar moment of inertia for stiffness. Any relationship between these properties is highly dependent on the shape in question. Equations for the section moduli of common shapes are given below. There are two types of section moduli, the elastic section modulus and the plastic section modulus.
S=I/Y
Here,
S = Section modulus
I= Moment of Inertia (Unit mm 4)
Y= Distance from Neutral axis to the extreme end (unit mm)
The Unit of a “section modulus” is (mm3)
Elastic section modulus
For general design, the elastic section modulus is used, applying up to the yield point for most metals and other common materials.
The elastic section modulus is defined as S = I / y, where I is the second moment of area (or area moment of inertia, not to be confused with moment of inertia) and y is the distance from the neutral axis to any given fibre. It is often reported using y = c, where c is the distance from the neutral axis to the most extreme fibre, as seen in the table below. It is also often used to determine the yield moment (My) such that My = S × σy, where σy is the yield strength of the material.
| Cross-sectional shape | Figure | Equation | Comment |
|---|---|---|---|
| Rectangle |  |
Solid arrow represents neutral axis | |
| doubly symmetric I-section (major axis) | 
|
NA indicates neutral axis | |
| doubly symmetric I-section (minor axis) |  |
NA indicates neutral axis | |
| Circle |  |
Solid arrow represents neutral axis | |
| Circular hollow section |  |
Solid arrow represents neutral axis | |
| Rectangular hollow section |  |
NA indicates neutral axis | |
| Diamond |  |
NA indicates neutral axis | |
| C-channel | |
NA indicates neutral axis |
,
Plastic section modulus
The plastic section modulus is used for materials where elastic yielding is acceptable and plastic behavior is assumed to be an acceptable limit. Designs generally strive to ultimately remain below the plastic limit to avoid permanent deformations, often comparing the plastic capacity against amplified forces or stresses.
The plastic section modulus depends on the location of the plastic neutral axis (PNA). The PNA is defined as the axis that splits the cross section such that the compression force from the area in compression equals the tension force from the area in tension. So, for sections with constant yielding stress, the area above and below the PNA will be equal, but for composite sections, this is not necessarily the case.
The plastic section modulus is the sum of the areas of the cross section on each side of the PNA (which may or may not be equal) multiplied by the distance from the local centroids of the two areas to the PNA:
the Plastic Section Modulus can also be called the 'First moment of area'
The plastic section modulus is used to calculate the plastic moment, Mp, or full capacity of a cross-section. The two terms are related by the yield strength of the material in question, Fy, by Mp=Fy*Z. Plastic section modulus and elastic section modulus are related by a shape factor which can be denoted by 'k', used for an indication of capacity beyond elastic limit of material. This could be shown mathematically with the formula :-
shape factor for a rectangular section is 1.5.
Here we calculate the elastic sectional modulus
Case 1
- Take the intersection of the hood with the YZ plane.
- Then the curve needs to be closed one so take the top surface to intersect the hood outer panel and lower surface of the hood inner panel. Then trim all the intersect and line to create a close curve. Finally, fill the curve.
As per the section modulus equation
S=I/Y
Section Modulus
Z = I/Y
Moment of inertia(M1) = 2.271 x 10^-6 M4
= 2.27 x 10^6 MM4
Moment of inertia(M2) = 1.612 x 10^-4 M4
= 1.61 x 10^4 MM4
Distance between the neutral axis and the extreme fiber
(Y)= 480.53/2
=240.265 mm
Z = (2.271x10^6) / (240.265) mm^3
= 9452.06 mm^3
Therefore Section modulus(S)=9452.06 mm^3
Case 2
- The hood inner panel curve is offset by 0.5 mm outwards and section inertia analysis is done the same as in case 1 for a close curve.
Moment of inertia(M1) = 2.353 x 10^-6 M4
= 2.353 x 10^6 MM4
Moment of inertia(M2) = 1.66 x 10^-4 M4
= 1.66 x 10^4 MM4
Distance between the neutral axis and the extreme fiber
(Y)= 480.53/2
=240.265 mm
Z = (2.353x10^6) / (240.265) mm^3
= 9793.35 mm^3
Therefore Section modulus(S)=9793.35 mm^3
Result
Case 1: S= 9452.06 mm^3
Case 1: S= 9793.35 mm^3
By changing the distance of the inner panel to 0.5 mm the section modulus value also increases. So we can by ching the distance to 0.5 mm the section modulus also increases.
Conclusion:-
Section modulus is successfully determined and improved by section area.