Roof challenge

Design of Roof and Analysis of Reinforcement

Objective:

A dumb solid body of roof CAD model is provided for which following points to be designed and analysed 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 moment of inertia for Bow Roof Rails and Centre Roof Rail.
4. Perform Draft Analysis of each reinforcement.

Software used

NX-CAD & MS Excel

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, there are many other structures under the roof which contribute to its strength and stability. The number of such reinforcement structures/supports differ with respect to the size of the vehicle. They are primarily installed on flatter areas of the roof to enhance the strength.

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

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

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

Therefore, passenger safety is also very important parameter in front of OEM’s. It is not only a marketing strategy but also it is a obligation stipulated by international standards that are now place in several countries, as well as a governmental requirement. For this reason, strength is very important criteria for designing 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 of 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 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. 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 as shown in fig.1.

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 front of the passenger car as shown in fig 2.

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 condition adhering to testing parameter which ensure to avoid or minimize the severity of injury during the mishaps of car during fatal accidents.

Design Methodology

Styling team provided the master section for the roof ditch area, front roof rail and rear roof rail as per the parameter of visibility criteria, head room clearance and other important parameter dimension like fitment of windshield, sealant rubber etc. All these data are well calculated in the planning stage to design the peripheral component accordingly. Thus, here designing is done just to replicate those input data considering engineering parameters which should be feasible for manufacturing. Master section of front and rear section is given in fig. 3 & 4 respectively.

From dump solid part class A surface is extracted and ditch area are developed considering the parameter specified in the following master section.

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.

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.

Following points are considered

Headroom clearance: 80 mm minimum

Front visibility criteria: 21.50mm projection for 15° vertical viewing angle as per master section.

Sheet metal thickness: T= 0.75mm

Emboss height: 2 < 3T

Flange length: 13.2 > 3T+R

Radius: (R=3mm and 5mmm )> T

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.

Following points are considered

Headroom clearance: 120mm minimum

Sheet metal thickness: T= 0.75mm

Emboss height: 2 < 3T

Radius: (R=3mm and 5mmm )> T

Bow roof 1 and Bow roof 2

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.

Sheet metal thickness of 1.25mm used with W cross section pattern. Dart pattern are used to increase the stiffness and strength in the structure over the length across the roof. Special notches on the flanges are provided for the application of mastic sealant. Either end flanges of the bow roof are flat offset to the roof flanges in order to join together by spot welding. Both bow roof 1 and bow roof 2 are similar in dimension and pattern only curvature profile is different at respective positions.

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 1.5mm thickness. Central rib type design is adopted here to take maximum load of roof reinforced with darts and convex curvature at flange end are considered to increase the surface area and load distribution to main BIW frame body during ideal and impact condition. In fig this is design is compared to actual BIW frame.

Assembly View

Heat Distortion 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.

Curvature study on the roof:

Bow – roof prediction Formula is given as

W = 1.73x10^-3 x L+1.85x10^-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 ( minimum radius in mm)

Ry = Y curvature ( minimum radius in mm)

t = Roof plate thickness (mm)

l = Bow Roof Span (mm)

Judgement condition: ok < 2.7mm < NG

Here following abbreviations are used for  ,

FR- Front roof rail

BOW-1 - Bow roof  1

CR - Centre roof rail

Bow 2  - Bow roof 2

RR- Rear roof rail

Steps followed:

1. 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.
2. Project the intersection points in the sketch to roof surface as shown in fig.
3. Join the projected curves using arc curve and measure the minimum radius and linear dimension as follow  & put into  the formula to calculate “W” .

From the above table it can be concluded that all values of  W< 2.7 so thus infers that current positioning  of Bow roof are optimum as per design and found OK.

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.

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

Where,

α = My x Lx² x 10^-12 ,

My = Y(Ly-Y)

Judgement condition = Qr ≥ 3.1

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 reference point of the front and the rear.

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.

Steps followed:

1. 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.
2. Project the intersection points in the sketch to roof surface as shown in fig.
3. Join the projected curves using arc curve and measure the minimum radius and linear dimension.
4. 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.
5. Put all the linear and radial dimension in following formula to calculate Qr Value.

It is observed from the values that obtained values of QR> 3.1, thus it infers that the designed roof rails and bow roof are safe for snow load condition and found ok.

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.

Conclusion

• 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
• Did curvature study and analysis the design for heat distortion and snow load criteria and found OK.
• Moment of inertia and draft analysis are carried out to check the feasibility of manufacturing and found OK.