Roof challenge

                                                                 ROOF DESIGN

Objective:

            A dumb solid body of roof CAD model is provided for which following points to be designed and analyzed are as follow,

1. Extracting the styling surface and for the given car roof styling and design the essential flanges and reinforcement, namely Ditch Area, Front Roof Rail, Rear Roof Rail, Centre Roof Rail, and Bow-Roofs.
2. Do a curvature study on the roof and perform calculations for Heat Distortion and Snow load criterion to determine and justify the position of the Bow-roofs.
3. Finding a moment of inertia and Section Modulus for Bow Roof Rails and Centre Roof Rail.
4. Perform a Draft Analysis of each reinforcement.

Introduction:

An automobile roof is the portion of an automobile that sits above the passenger compartment, protecting the vehicle occupants from sun, wind, rain, and other external elements. Under the styling surface that is seen from the outside, many other structures under the roof contribute to its strength and stability. The number of such reinforcement structures/supports differs concerning the size of the vehicle. They are primarily installed on flatter areas of the roof to enhance strength.

These are the Front Roof Rail, Rear Roof Rail, Centre Roof Rail, and Bow Roof Design which are spot-welded to roof flanges & ditch areas and improve NVH value by increasing the strength of the roof using mastic sealants.

Roof Crush Test for Passenger Car:

Safety point of view vehicle roof plays a very crucial role especially rollover kind of accidents, in which vehicle tips over onto its side of the roof. And vehicle rollover crashes are the causes of many fatalities and head, neck, and spine trauma around the world.

Therefore, passenger safety is also a very important parameter in front of OEMs. It is not only a marketing strategy but also it is an obligation stipulated by international standards that are now placed in several countries, as well as a governmental requirement. For this reason, strength is a very important criterion for designing a vehicle roof.

The strength of the roof structure to occupant protection in real-world rollover crashes, requiring the maximum moving distance of the roof structure should be less than 127 mm (5 in.) when the induced load is 1.5 times the vehicle weight.

For the test, the passenger car shall be rigidly placed or positioned on a horizontal surface with the doors locked and the window closed. Furthermore, a flat, rigid block with a lower surface a rectangle measuring 30 inches wide by 72 inches long (762mm X 1829mm) shall not move more than 5 inches(127mm), measured as the distance between the original location of the lower surface of the test device and its location as the specified force level is reached, when it is used to apply a force of 1.5 times the unloaded vehicle weight (UVW) or 5000 lbs(22224N), whichever is less, to either side or forward edge of the vehicle.

This test can be conducted on either side of the roof structure, the front left or the front right side but not on both sides in one single test and still meet the requirements of the test. The rigid plate shall not move faster than 0.5 inches/second and the force applied on the plate shall not exceed the lesser of 1.5 times the unloaded vehicle weight (UVW) expressed in kilograms multiplied by 9.8 or 5000lbs (22240N).

Also, the direction of the force shall be downward and perpendicular to the lower surface of the rigid plate and such that it moves in a straight line without rotation. The duration of the test shall not exceed 120 seconds. The longitudinal axis of the plate is pitched forward at a 5 angle as viewed from the side of the passenger car.

The lateral axis or the roll of the plate is such that it forms a 25 outboard angle with a horizontal surface as viewed from the front of the passenger car.

The lower surface of the plate shall always maintain a tangential contact with the surface of the roof. The initial point contact is 10 inches (254 mm) from the leading edge of the plate and in line with the longitudinal axis on the lower side of the plate. DESIGNED ROOF

Following these design considerations and thus manufacturing the parts helps the car to perform better in real-life conditions adhering to testing parameters that ensure avoiding or minimize the severity of injury during the mishaps of the car during fatal accidents.

Design consideration of roof:

1. Visibility criteria
2. Head clearance
3. Curvature study
4. Hear distortion and snow load criteria
5. Draft analysis.
6. Safety parameters

DESIGN METHODOLOGY:

The design methodology of the Roof is explained with the help of a flow chart for easy and better understanding

MASTER SECTION:

Front Roof Rail:

Rear Roof Rail:

Styling Surface to Extracted Surface:

                   The styling team has provided the solid dump outer panel from that we are extracting the surface to model the necessary reinforcements like Ditch area, Front Roof Rail, Bow Roof Rails, Centre Roof, and Rear Roof Rail.

Reinforcements and Design:

Ditch Area:

The Ditch Area is the extended portion of the main roof concealed by other BIW panels. The rail support of the roof is attached to the Ditch area using spot welds since it is not a styling surface and remains concealed. Headroom clearance, Visibility criteria, Clearance for Windshield and Back door are some of the factors to be considered while designing the Ditch Area. The material used for the ditch area is SP121AQ (Cold Rolled Mild Steel) and it is given a thickness of 0.75mm.

Front Roof Rail:

It is the support that is joined with the Windshield glass, Bodyside outer, and inner panels. It is usually made from Body side outer scrap and therefore the thickness of the rail is 0.75mm. A draft angle of 3-5 degrees is provided keeping in mind the deep-drawing operation. The Windshield end flange is the exact offset of the roof surface. The bottom-most flange is decided after taking into consideration the Visibility criteria and the Headroom clearance of the passenger. Spot welds are applied at the Windshield end of the rail and the other end is joined with the roof using Mastic Sealants which also contributes to increased strength. The material used for the roof rail is SP783AJQ (Terrace Zinc Galvanizing).

Rear Roof Rail:

It is the support that is joined to the Back door and the Body side outer and the inner panels. It is also usually made from Body side outer scrap and therefore the thickness of the rail is 0.75mm. Similar to the Front Roof rail design, we have to take into consideration the Rear Roof rail. Inputs from the backdoor design are also taken to accommodate the required clearance. Spot welds are applied at the Back door end of the rail and the other end is joined with the roof using Mastic Sealants. The material used for the roof rail is SP783AJQ (Terrace Zinc Galvanizing).

Centre Roof Rail and Bow Roofs:

Centre Roof Rail:

Bow Roof 1:

Bow Roof 2:

Bow Roof 3:

Similar to other Rails designs, we have to take into consideration the Headroom clearance and their visibility criteria while designing the center Roof Rail and Bow Roofs. But unlike Front and Rear Roof Rails, Spot welds are applied for joining these rails to the roof. Instead, Mastic sealants are introduced at both ends since Centre Roof Rail and Bow Roof are fully positioned under the styling surface of the roof. For increasing the strength of the rails, embossing is done. To reduce the weight of the material used, a different styling has been followed with a thickness of 0.75mm. The material used for the Centre Roof Rail is SP153 – 980 PQ (Cold Rolled JIS Steel) and the SP121BQ (Cold rolled steel coil).

Analysis:

Three types of analysis were carried out to get a successful design of the BIW roof of a car which satisfies the Manufacturing Standards.

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 colour ranges identifying zones on the analysed element where the deviation from the draft direction at any point, corresponds to specified values.

Roof with Ditch Area:

 

Front Roof Rail:

 

Rear Roof Rail:

 

Centre Roof Rail:

 

Bow Roofs -1:

 

Bow Roofs -2:

 

Bow Roofs -3:

 

Section modulus analysis:

Front roof rail:

 

Rear Roof Rail:

 

Centre Roof Rail and Bow Roofs -2:

 

Bow Roofs -1:

 

Bow Roofs -3:

 

Curvature Study on the Roof:

The heat distortion study plays an important role in sheet metal usage. Heat distortion temperature is a temperature limit above which the material cannot be used for 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 roof, based on heat distortion temperature, this study will predict the bow roof position on the roof to strengthen the roof.

Heat Distortion Criteria:

Heat Distortion is the temperature at which the metal softens and deforms under a specific load. This study is done to check the ability of the roof to withstand high temperature concerning the positioning of the reinforcements which contributes to increasing the strength.

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

Judgemental 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 Length for styling Surface (L):

For the L we want take the total of the styling surface don't include the ditch area.

Bow Roof Span (l):

Here want to take the distance from the Front Roof Rail to Bow Roof 1 to find the Bow Roof Span as shown in fig.., remaining all the span distance also want to measure.

Rx & Ry (X & Y Curvature):

From 0-Y axis, at -3Y about distance 300mm is drawn parallel on the perpendicular plane as shown in fig. after that 100 mm lines are offset to either side of -3Y line. Now centre lines are drawn between each adjacent roof rail edges respectively. Intersection points are marked on the sketch and exit the sketch. Make all geometry in construction lines except the points.

1. Project the intersection points in the sketch to roof surface as shown in fig.
2. Join the projected curves using arc curve and measure the minimum radius and linear dimension as follow & put into the formula to calculate “W”.
3. Now project the points on the top of the Roof and draw the arc with the three points intersecting area.
4. Find the Rx Curvature and Ry Curvature withe the tool Simple Radius.

Calculation Table:

Roof Rails	Rx	Ry	R	L	t	W	Result
FB-BR1	5741.2193	2420.1886	3405.00	2034.0733	0.75	1.55	Okay
BR1-(BR2, CR)	4971.8417	6199.2858	5518.13	2034.0733	0.75	2.09	Okay
(BR2, CR)-BR3	3985.1340	10542.1088	5783.85	2034.0733	0.75	2.17	Okay
BR3-RR	3240.2803	14852.8692	5319.96	2034.0733	0.75	1.96	Okay

Snow Load Prediction:

Snow Load Criteria study is done to check the withstand ability when the snow loads over the roof during winter in snowfall regions. This study will also analyze the roof that whether the permanent deformation occurs after removing the snow load. That it may bend but needs to be a bounce back to its original position.

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

Where,

α = My x Lx^2 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]

Lx = Distance between the Left and Right end of the roof on the Roof BOW [mm] i.e. (Width of the roof panel exposed on the surface)

Y = Distance 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

Rx & Ry (X & Y Curvature):

At 0-Y axis on perpendicular plane to roof as shown in fig. draw 100 mm lines offset to either side of 0Y axis line. Now centre lines are drawn between each adjacent roof rail edges respectively. Intersection points are marked on the sketch and exit the sketch. Make all geometry in construction lines except the points.

1. Project the intersection points in the sketch to roof surface as shown in fig.
2. Join the projected curves using arc curve and measure the minimum radius and linear dimension.
3. Project the intersection plane bow roof and centre roof rail on the horizontal plane to find the moment of inertia of the respective section to find Iy values.
4. Put all the linear and radial dimension in following formula to calculate Qr Value.
5. Now project the points on the top of the Roof and draw the arc with the three points intersecting area.
6. Find the Rx Curvature and Ry Curvature withe the tool Simple Radius.
7. To find the Lx and Ly values take the styling surface of the roof

Calculation Table:

Roof Rails	Iy		S	Rx	Ry	t	Qr	Result
FR-BR1	118.35	0.96	461.37	4611.1188	2489.0883	0.75	1.12	Fail
BR1-(BR2, CR)	942.67	0.91	509.96	3764.5932	6108.0676	0.75	4.68	Okay
(BR2, CR)-BR3	121.29	0.87	497.55	4272.0635	11033.3517	0.75	0.26	Fail

• From the above results, the two cases were failed to satisfy the judgemental condition.
• Thus, embosses are added on the Roof Outer Panel to add strength to the roof on the low strength area.

Views of Model:

Front View:

 

Side View:

 

Top View:

 

Isometric View:

 

Conclusion:

For the Given Roof styling, the essential flanges and reinforcements were developed and checked for Draft analysis. The curvature study and Section Modulus on the roof was carried out to determine the position of the Bow-roof the Heat Distortion and Snow Load Criteria calculations were also performed. Thus, the roof of a car was designed with the ditch area, front roof rail, enter roof rail, rear roof rail, and bow roofs.