Roof challenge

Roof Design and Analysis of Reinforcements

Objectives:

Create a Car Roof model based on the styling surface, master sections given for various parts with mastic points and reinforcements.Determine whether the design is complying with Heat distortion criteria through curvature study on the roof.Judge the snowed condition criteria of the roof model using the prediction formula to conclude if the roof can withstand the load acting upon it.Prepare a section modulus study for roof parts Bow roof 1, Bow roof center and Bow roof 2 in the 0Y section.Draft analysis of all the parts to check for tooling feasibility.

Introduction:

The roof is an important part of an automobile as it protects the occupants from various environmental conditions like Heat, Snow, Rain, and winds. There are many types of the roofwhich differs based on the variants of the car and the region. The roof must be designed in such a way that it would able to withstand any load or impact due to accidents or objects fallingon it and other environmental factors. So, the design done is based on the standards and design intent which makes the roof to withstand the above-said conditions. Also, other factors Headroom, aesthetics play a role in the design.The roof is an assembly of many parts like Front roof rail, Bow roof, Bow roof center, and reinforcement center. The number of bow roofs is based on the length of the roof and sometimes even the bow roof center is avoided in cases like small cars. The roof is attached to the body of the car using mastic sealants provided between bow roofs and the front and rear rails.

SOFTWARE USED : SIEMENS NX12

AUTOMOTIVE ROOF

They act as a passenger cabin and protects from sun, rain, wind, dust, etc.

Types of Car roofs-

• Convertible roofs
• Hardtops
• Sunroofs
• T-tops
• Targa tops
• Vinyl roofs

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.

Rollover crashes, especially in the country, are usually very destructive events. Vehicle damage often includes deformation of the roof and its supporting structures. Head and neck injuries are common, and associated with roof deformation. Strengthening of the roof is suggested as an appropriate countermeasure for such injuries.

Types of Rollovers

1. Tripped-NHTSA data show that 95% of single-vehicle rollovers are tripped. This happens when a vehicle leaves the roadway and slides sideways, digging its tires into soft soil or striking an object such as a curb or guardrail. The high tripping force applied to the tires in these situations can cause the vehicle to roll over. E.g. Soft soil, Guardrail, Steep Slope
2. Un-Tripped -Un-tripped rollovers are less common than tripped rollovers, occurring less than 5% of the time, and mostly to top-heavy vehicles. Instead of an object serving as a tripping mechanism, un-tripped rollovers usually occur during high-speed collision avoidance manoeuvres.

3. For this reason, strength is very important criteria for designing vehicle roof.

   FMVSS 216 Roof crush standards-

   FMVSS 216, Roof Crush Resistance, establishes strength requirements for the passenger compartment roof of passenger cars, multipurpose passenger vehicles, trucks and buses with a GVWR of 2722 kilograms or less. The purpose of the standard is to reduce deaths and injuries due to the crushing of the roof into the passenger compartment in rollover accidents. The standard does not apply to school buses and passenger cars that conform to the dynamic rollover test requirements of FMVSS 208, Occupant Crash Protection, and S5.3 by means that require no action by passenger car occupants. It also does not apply to convertibles, except for optional compliance with the standard as an alternative to the rollover test requirements in S5.3 of FMVSS.

   The prescribed static loading device is a rigid unyielding rectangular block 762 millimetres by 1,829 millimetres. It shall not move more than 127 millimetres to achieve the specified resistive load when applied to the forward edge of a vehicle’s roof – see Figures 1 and 2. The resistive load to be achieved is 1½ times the unloaded vehicle weight (UVW) of the test vehicle or 22,240 Newtons, whichever is less.

   The force shall be applied in a downward direction perpendicular to the lower surface of the static loading device at a rate of not more than 13 millimetres per second until reaching the compliant resistive load. The test shall be completed within 120 seconds. The static loading device shall be guided throughout the test so that it moves, without rotation, in a straight line with its lower surface oriented as follows and as shown in Figures

   

   Following are the design considerations of the roof-

   1. Visibility criteria
   2. Head clearance
   3. Heat distortion and snow load criteria
   4. Draft analysis
   5. Safety parameters

   Input data from styling team-

   We will be having following data which is provided from styling team

   

   Surface of the roof along with railing rack

   Master section of rear roof rail

   Master section of bow roof

   Master section of front roof rail.

   Master section of reinforcement center

   
   

   Front roof Rail master section reference image
   
   
   
   

   Rear roof rail master section reference image

Design a Ditch Area for Outer Roof Panel:

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.

1. Front Ditch Area:

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 3 degree considering for the drawing operation process. Overall thickness of sheet metal consider 0.75mm throughout.

Front Ditch Area

2. Rear Ditch Area:

Surface is offset from the outer panel to form the ditch area and with the law extension command, corner relief at rear flanges and positive draft in required degree considering for the drawing operation process. Overall thickness of sheet metal for outer panel consider 0.75mm throughout.

Rear Ditch Area

Roof Panel with Front and 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.

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.

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.

The thickness of the Bow Roof is 1 mm and also mastic area has been created.

Bow Roof 1

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.

Bow Roof 2

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 mm thickness.

Centre Roof Rail

Bow Roof Centre

Roof Top View:

Bottom View:

Roof Rails in Transparent View :

Heat Distortion criteria:

Predicting Heat Distortion for the roof is important since it is exposed to heat for a long time and high temperatures will affect the roof structure if the design does notconform with the heat distortion criteria. This study helps to determine the positions of Bow roofs to provide strength to the roof to avoid bending or deformation of theroof due to heat. Bow roof curvature study using the below formula will help us to check whether the Bow roof positions impart required strength to the roof.

Prediction Formula: W = 1.73 X 10^-3XL+1.85X10^-8X(R^2/t)+1.10 X10^-3Xl-2.68

L = Roof Length in the X direction (Roof dimension with 0Y)

R = Roof curvature = 2(Rx X Ry) / (Rx + Ry) (Rx = X curvature , Ry =Y curvature )

t = Roof Thickness (mm)

I = Bow roof span (mm)

Snowed criteria :

The roof of an automobile must be able to withstand load especially snow which results in heavy load acting on the roof. If the roof is not able to withstand it, there will be deformation of the roof. To prevent that, Snowed criteria calculation helps us to determine whether the roof design will withstand the snow load.

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 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

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. There are two types of section moduli

• Elastic section modulus
• Plastic section modulus

The Elastic Section Modulus is defined as,

S = I/y

Where,

S: is the Section Modulus

I: is the second moment of area(or area moment of inertia, not to be confused with the moment of inertia)

y: is the distance from the neutral axis to any given fiber. It is often reported using y = c, where c is the distance from the neutral axis to the most extreme fiber

Bow roof 1 :

Bow roof 2 :

Bow roof center :

Roof Model Views:

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 surface is done by using NX CAD software.