Roof challenge

*ROOF DESIGN*

Introduction:- 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.

Aim:-

1. Extracting the styling surface for the given car roof styling and design the essential flanges and reinforcement, like- 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.

Crush Test of Roof for Passenger Car:-

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

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 º 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 º outboard angle with a horizontal surface as viewed from the front of the passenger car.

In below figure we can see the test angle visualization-

Components of roof:-

1. Front roof rail
2. Rear roof rail
3. Bow roof
4. Centre roof rail

Styling Surface-

Extracted Surface-

Ditch Area for Outer Roof Panel:-

1. Front Ditch Area-

Flange of around 15mm created with the law extension command, corner relief at rear flanges and positive draft more than 7 degree considering for the drawing operation process. Overall thickness of sheet metal consider 0.75mm throughout.

2. Rear Ditch Area-

Front Roof Rail:-

The front rail underpins the rooftop external shell at the front end. It is mounted with support rooftop rail fortification on the two closures. The rails are furnished with embosses to improve the strength over length and the rail is associated with the external board utilizing mastic sealants on one side and the other joinery is spot welded.

Master section-

Make sure maximum gap between outer surface and front roof rail should not grater than 32mm. Front flange of both should not have any gap inbetween, cause it will be spot welded.

Rear Roof Rail:-

It is the support which 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 0.65 mm. Similar to the Front Roof Rail design, we have to take into consideration the rear Headroom Clearance and the rear visibility criteria while designing the Rear Roof Rail. Inputs are taken from the rear master section shown in fig.

Make sure first negative draft angle fro roof outer surface should have a certain value that back door hinge side should move freely on its trajectory. And also make sure in spot welding area there is no gap between flanges.

Bow Roof 1:-

The Bow roofs are given to improve the torsional stiffness and load-bearing capacity of the roof structure. The number of bow roofs provided will depend upon the overall dimensions of the roof. Longer the roof more the bow roofs are integrated to BIW frame They are attached to the outer panel using mastic sealants. Each mastic point will have up to 80 mm circular effective area. The positioning of bow roofs is done based on curvature study. The heat distortion criteria and snow load criteria are checked to analyse the position of the bows in various conditions.

Bow Roof 2:-

This second bow roof will also be designed with referance of 1st bow roof master section.

Centre Roof Rail:-

Centre roof rail is provided to effectively support the flat area of the roof as it is more susceptible to failure under the action of the load. Usually, the centre reinforcement roof rail will be placed at the centre of the roof connecting the B pillar support structure which results in added support in the roll-over test. The thickness of the centre bow roof is 0.75 mm thickness.

Mastic area:-

Bow Roof Rails and Centre Roof Rail are joined with the help of Mastic Sealant. Here we cant's use the Spot welding because it damages the A class area of the Roof. So, the mastic sealant is used in this area.

Curvature Study:-

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-

The equation for heat distortion is-

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

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]

Finally if value of W is less than 2.7 (W<2.7) thaen it is ok.

L(Roof Length for styling Surface):-

t(Thickness):-

I(Bow Roof Span):-

Rx & Ry(X curvature and 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. Project the intersection points in the sketch to roof surface as shown in fig. Join the projected curves using arc curve and measure the minimum radius and linear dimension as follow.

Putting all the values in equation we got the below table-

As we can see from the table that all the values of W<2.7, so current positioning of Bow roofs are in right place and roof have passed Heat Distortion Criteria.

Snow Load criteria-

Snow load criteria study is done to check the durability of the roof to withstand snowfall and reflex back to its original shape without any permanent deformation once the load is removed.

The equation for calculating snow load criteria is given below-

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)

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]

The width of the roof panel is 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

t(Thickness):-

Thickness is same for all part that is 0.75 MM.

Ly(length of roof along 0-Y axis):-

Lx(Distance between the Left and Right end of the roof on the Roof):-

Y:-

s(Distance for which Roof BOW bears divided load):-

Rx & Ry:-

Iy(Geometrical moment of inertia of Roof BOW (Y cross-section )):-

If we put all the values in the equation we will get the below table-

As we can see from the above table that for 1st bow roof and center bow roof position roof snow load criteria is OK. But on 2nd bow roof position curvature of roof is NOT OK for snow load criteria.

So we have to ask the styling team to modify the surface with curvature(at 2nd bow roof rail position) in y axis (Ry) not more than 9890 MM. If team is rejected to do modification on curvature, we can make some emboss on roof outer surface to give strength.

Section Modulus:-

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 the design include an area for tension and shear, the radius of gyration for compression, and the moment of inertia, and the polar moment of inertia for stiffness. Any relationship between these properties is highly dependent on the shape.

The Section Modulus is defined as,

S = I/y

Where,

S - the Section Modulus

I - Area moment of inertia

y - distance from the neutral axis to the most extreme fiber

FRONT ROOF RAIL:-

1ST BOW ROOF:-

CENTER BOW ROOF:-

2ND BOW ROOF:-

REAR ROOF RAIL:-

ROOF OUTER SURFACE:-

If we put all the I & y values in equation S=I/y, then we will get the reasult as below table-

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.

Minimum draft angle of 7° is considered for analysis . Green colour infer on the parts that that all face along the tooling direction has positive draft angle greater than 7° and passed in analysis.

FRONT ROOF RAIL:-

1ST BOW ROOF:-

CENTER BOW ROOF:-

2ND BOW ROOF:-

REAR ROOF RAIL:-

ROOF OUTER SURFACE:-

From above figure we can see a red and a blue zone, that have negative draft. For that case roof outer have to go through a 2nd drawing process.

Some figure of model:-

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Conclusion:-

The Design of Roof, Front roof rail, Rear Roof Rail, Bow Roofs, and reinforcement roof center by following the Master section with the design parameter for the given class A surface is done.

All reinforcement BIW parts of roof are designed according to the input data provided by the styling team. Designed parts will be submitted to CAE team to carry out further FEM analysis to obtain numerical data to improve the design.

Made the curvature study and analysis the design for Heat distortion are found OK.

Made the curvature study and analysis the design for Snow Load are found OK. But we found Bow roof 2 has NG, So we have to ask the styling team to modify the surface with curvature(at 2nd bow roof rail position) in y axis (Ry) not more than 9890 MM.

Also we can provide emboss in the roof show in the below fig. The Embosses are radically increased the strength of the roof.

Moment of inertia, Section Modulus and Draft analysis are carried out to check the feasibility of manufacturing are found OK.