Roof challenge

Roof Design

OBJECTIVE :

• The objective of this project is to design car roof with essential flanges and reinforcements like ditch area , front roof rail , bow roofs, central roof rail and rear roof rail for the the given styling surface.
• Perform a curvature study on the roof and perform the calculations for the heat distortion and snow load criteria to determine whether the bow roofs are at the correct positions.
• Finding moment of inertia and section modulus for Bow roofs , central roof rail . front roof rail and rear roof rail .
• Perform the draft analysis for each reinforcements.

INTRODUCTION :

Design of any product starts from an requirement . The design engineer sketches the product based on the the requirement . Then the 2D sketch with the proper dimensions is converted into the 3D model using the advanced CAD software's . The another important thing in the design is that , every product that is designed should be able to manufactured . so the role of the design engineer is to design the product that can be easily manufactured and with less cost effective . This is the place where the Design for manufacturing [DFM] and Design of assembly [DFA] takes a role while developing any product . The process of development of design carries on until the design satisfies the customer needs .
Here in this project the product that we are going to develop is a car roof.

CAR ROOF :
An automobile roof or car top is the portion of an automobile that sits above the passenger compartment, protecting the vehicle occupants from sun, wind, rain, and other external elements. Roof is the structure above the wind shield glass having the reinforcements to provide the strength to the overall structure and to with stand the over head forces like snow.

WHY REINFORCEMENTS :
If the roof is flat it is very weak and cannot able to with stand the forces like snow , so in order to increase the strength of the roof these reinforcements like front roof rail, central roof rail, rear roof rail and the bow roofs are introduced and joined to the flat surface of the roof with the different joining process.

Different types of reinforcements:

1. Front roof rail.
2. Bow roofs.
3. Central roof rail.
4. Rear roof rail.

FRONT ROOF RAIL :
Front roof rail is the one which joins the wind shield glass , body side outer and the inner panel also. Here the front roof rail is designed based up on the master section given.

REAR ROOF RAIL :
Rear roof rail is the one which joins the back door and the body side outer

CENTRAL ROOF RAIL :
Central roof rail helps in providing effectively support to the flat area of the roof as it is more susceptible to failure under the action of load . generally central roof rail is placed at the center of the roof which is connected to the B-pillar support structure . This central roof rail helps in adding the strength to the roof during the rool- over test.

BOW ROOFS :
The bow roofs are give to improve the torsional stiffness and load bearing capacity of the roof structure . The number of bow roof present is depends on the overall size of the roof . Presently in this project two bow roofs are added.

JOINING PROCESS :
Mastics are introduced between the roof outer panel and the reinforcements for joining . where ever the mastic are introduced there is remarkable improve of the strength.
Spot welding is also introduced to join the reinforcements and the outer panel.

DESIGN CONSIDERATIONS FOR A ROOF :

• Visibility criteria
• Head clearance
• Curvature study
• Heat distortion
• Snow load criteria
• Draft analysis

ROOF CRUSH TEST :
Roof crush test is the design given by the IIHS ( insurance institute of highway safety ) . After analyzing the many accidents IIHS has then fixed some regulations , that how to perform the crush test .
IIHS gives the rating based up on this test . For this they consider two points while giving the rating.

• How fast the car can reforms to its original shape after the roll over.
• How deep is the protrusion after the roll over.
  
  

GOAL OF ROOF CRUSH TEST :
To minimize the protrusion into the occupant compartment , so that the seat belts and air bags can do there job in protecting the occupant.

CONDITIONS TO FOLLOW FOR ROOF CRUSH TEST :
There are two conditions that should be followed for the roof crush test , they are

INITIAL CONDITION :

• Close and lock all the doors.
• Close all the windows.

SECOND CONDITION :
Fix the body sill or chassis frame on a hard lever surface horizontally.

METHOD OF APPLYING THE FORCE :
The load is applied in such a way that 50 is tilted from the front end to the rear end and the load will be in 1800 mm in length and 750 mm in width . The load is applied at the position of 250 from the flatness of the roof . The load is applied on the roof for almost 120 sec and the crush speed at which the load applied is 13 mm/sec or less than that.

RESULTS :
If the displacements goes above the requirements level then we have to increase the bow roof thickness or add more mastics points to increase the strength . the stronger the roof the better it can protect the occupants in the roll over crashes.

HEAT DISTORTION STUDY ON THE ROOF :
The heat distortion study plays a major role in sheet metal usage . heat distortion temperature is a temperature limit above which the material cannot be used for the structural applications . This study is used to predict the heat distortion temperature at where the material starts to soften when exposed to a fixed load at elevated temperature . In order to avoid bending or damages on the roof, based on the heat distortion temperature , this study will predict the bow roof position which helps to strengthen the roof .

Bow–roof prediction Formula

W = [1.73 x 10^(-3) x L] + [1.85 x 10^(-8) x (R^2)/t] + [ 1.10x10^(-3) x l] - 2.68

Judgement Condition : OK< 2.7> 3.1

Where

L = Roof Length in X-Direction[mm](Roof dimension in 0-Y)

R = Roof curvature

R = 2(Rx*Ry)/(Rx+Ry)

Rx = X curvature

Ry = Y curvature

t = Roof plate thickness [mm]

l = Bow Roof Span [mm]

Roof Rails	Rx	Ry	R	L	I	t	W	Result
Front-Bow1	5741.1617	2445.4192	3429.892704	2124.4	379.14	0.75	1.70	Pass
Bow1-Central	4963.3158	6244.6288	5530.731261	2124.4	410.44	0.75	2.20	Pass
Central-Bow2	3963.6513	10651.7646	5777.44979	2124.4	440.26	0.75	2.30	Pass
Bow2-Rear	3243.5947	14199.2927	5280.863138	2124.4	270.67	0.75	1.98	Pass

SNOW LOAD CRITERIA :
This test is done to know how is the roof behaving when there is a snow over . normally due to the snow weight the dent will happen. But the roof should be designed in such a way that when the snow is removal the roof should go it its original position . This is the basic requirements for snow load criteria .

Snow load Prediction formula

Qr = [Iy x t²] / [α x s x [(Rx + Ry)/2]² x 10^-8]

Note: The Qr value given over here is correct. There was a typo error in the formula given in the video.

Where

α = My x Lx² x 10^-12 , My = Y(Ly-Y)

Judgement condition = Qr ≥ 3.1

250 ≤ s ≤ 380

t = Roof plate thickness [mm]

Ly = Distance between the front and rear roof Rails on the Vehicle along with 0Y[mm]

Length of Roof panel with the center point between Roof rail Front /Rear as the front and rear reference point.

Lx = Distance between the Left and Right end of the roof on the Roof BOW [mm]

Width of the roof panel exposed on the surface.

Y = Distance front Front Roof Rail to Roof BOW[mm]

s = Distance for which Roof BOW bears divided load [mm]

s = L1/2 + L2/2

Iy = Geometrical moment of inertia of Roof BOW (Y cross-section )[mm4]

Rx = Lateral direction curvature radius of roof panel Y cross-section on Roof BOW [mm]

Roof panel curvature Radius of the Length Lx in Front view

Ry = Longitudinal Direction curvature radius of the Roof panel X cross-section on Roof BOW [mm]

Roof panel X curvature radius of length s in Side view

Roof Rails	Iy	t	Lx	Ly	Y	My	α	S	Rx	Ry	Qr	Result
Front-Bow1	71.915293262	. 0.75	1187.5	2117.8	429.33	724910.8251	1.022	747.885	4594.3232	2511.7241	0.419	Fail
Bow1-Center	2403.755809	0.75	1134.1	2117.8	533.22	844929.7476	1.087	518.225	6143.5488	3766.1470	9.776	Pass
Center-Bow2	74.492569818	0.75	1105.2	2117.8	503.23	812500.0611	0.992	488.195	4281.8487	11168.1509	0.145	Fail
Bow2-Rear		0.75	1090.8	2117.8	473.16	778177.8624	0.925	236.58	3949.5105	16738.3364

Section modulus is a geometric property for a given cross-section used in the design of beams or flexural members.Other geometric properties used in design include area for tension, radius of gyration for compression, and momentof 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 (S) and the plastic section modulus (Z).

• Section modulus is a direct measure of strength of any sectional area of an object about an reference axis(neutral axis).
• Strength is nothing but, capable of supporting greater loads, so higher the strength it has a high capable of supporting greater loads.
• This strength can measured directly from Section modulus formula, and defined by below formula.
• Section modulus represented with a symbol of 'S'.
  S=I/Y
  where 'I'moment of inertia '
  Y' is the distance from the reference line(neutral axis) to one of the extreme end of an object.
• Moment of inertia is nothing but offering resistance against of moment, bending or twisting, i.e if we applied any external load on the body, how much resistance its offering to tend to deform.
• So, higher the value of moment of inertia , have higher resistance to deform under certain load, hence we can say that moment of inertia directly proportional to the strength of the material , and strength of the material can be calculated with the help of section modulus.
• If we vary the neutral axis towards positive or negative side the overall strength of the body will decrease , because when nuetral axis line lies on the middle of the section only have higher strength at particular point, so, here 'Y' value was inversely proportional to section modulus.
• So Newton's first law also explains that ,if higher the mass , the force required to move the object is also high due to offering more resistance in order to move the object, This resistance we called it as a MOMENT OF INERTIA.

Bow Roof Rail 1:

IMAX:- 2.544462266X10^4 mm^4
Imin:- 71.915293262 mm^4
Y=73.9454 mm

1)Section modulus(S):-I/Y=2.544462266X10^4mm^4/73.9454mm
=344.1001423 mm^3(Maximum strength).

2) Section modulus(S):-I/Y=71.915293262 mm^4/73.9454mm
=0.9725458685 mm^3(minimum strength).

Center Bow Roof Rail :

IMAX:- 3.627762182X10^5 mm^4
Imin:- 2.403755809X10^3 mm^4
Y=111.9216 mm

1)Section modulus(S):-I/Y=3.627762182X10^5mm^4/111.9216mm
=3241.342316 mm^3(Maximum strength).

2) Section modulus(S):-I/Y=2.403755809X10^3mm^4/111.9216mm
=21.47713944 mm^3(minimum strength).

Bow Roof Rail 2:

IMAX:- 2.547995280X10^4 mm^4
Imin:- 74.4925698 mm^4
Y=73.9848 mm

1)Section modulus(S):-I/Y= 2.547995280X10^4mm^4/73.9848mm
=344.394427 mm^3(Maximum strength).

2) Section modulus(S):-I/Y=74.4925698 mm^4/73.9848mm
=1.006863164 mm^3(minimum strength).

Front Roof Rail:

IMAX:- 1.564893787X10^5 mm^4
Imin:- 8.464615463X10^3mm^4
Y=123.8301 mm

1)Section modulus(S):-I/Y=1.564893787X10^5mm^4/123.8301mm
=1263.742644mm^3(Maximum strength).

2) Section modulus(S):-I/Y=8.464615463X10^3 mm^4/123.8301mm
=68.356684761 mm^3(minimum strength).

Rear Roof Rail:

IMAX:- 1.305327951X10^6 mm^4
Imin:- 8.834404079X10^4mm^4
Y=262.3866 mm

1)Section modulus(S):-I/Y=1.305327951X10^6mm^4/262.3866mm
=4974.827034 mm^3(Maximum strength).

2) Section modulus(S):-I/Y=8.834404079X10^4 mm^4/262.3866mm
=336.6942058 mm^3(minimum strength).

If we modifiy the shape Bow Roof 1 & 2 as shown in the image below , we can get better moment of inertia at these roof rails.
Which leads good snow load result.

DRAFT ANALYSIS :
The Draft Analysis command enables you to detect if the part you drafted will be easily removed. This type of analysis is performed based on color ranges identifying zones on the analyzed element where the deviation from the draft direction at any point, corresponds to specified values.
Minimum draft angle of 5° is considered for analysis.

Front Roof Rail:

Bow Roof Rail 1:

Center Bow Roof Rail :

Bow Roof Rail 2:

Rear Roof Rail:

Roof Images:

Conclusion:
Design of roof was done and checked for heat load and snow load criteria. Designed passed the heat load criteria but failed at some sections in snow load criteria.
Need to increase the strength of the Bow Roofs to get better snow load.