Roof challenge

CAR ROOF DESIGN ANS ANALYSIS OF REINFORCEMENTS

INTRODUCTION:-

An automobile roof is a portion of an automobile that sits above the passenger compartment, protecting the vehicle occupants from sun, wind, rain, and other external elements. Through it just like a styling surface that protects from sun, wind, and other elements, various environmental conditions are taken into account for designing the same.

The roof should be provided with the proper reinforcement for strengthening. Vehicle roofs can be subjected to high heat conditions or load during snow conditions. To make the roof withstand the condition, various calculations are done. This ensures the safety of the passengers in the vehicle.

OBJECTIVE:-

The main objective of the project is to design roof and roof reinforcements from the styling surface received. After the design is done, heat distortion and snow load calculation will be done on the OY section of the roof.

Design and Calculation:-

Roof and roof reinforcement are designed in NX CAD from the given styling surface. Sheet thickness is considered 0.75 mm for all components. Two bow roofs are provided for the strength of the roof.

Front Roof Rail:-

This is the reinforcement provided at the front. This part is joined to the BIW body and windshield. Visibility criteria are taken into account for joining it with the windshield. It is fixed to BIW with spot welds and the roof surface is attached with the help of mastic sealants. A draft angle is provided for deep-drawing operation. Draft analysis is also done on the section to check its manufacturing ability.

Bow Roof_1:-

It is an additional reimforecmentprovided to increase the strength of the roof. It is fixed with a roof body with mastic sealants.

Centre Roof Rail:-

Centre rail roof can be considered as reinforcement joining exact center to the outer roof. It is fixed to the roof by mastic sealants and BIW by spot welds.

Bow Roof_2:-

It is similar to the bow roof_1 provided for better strength.

Rear Roof Rail:-

It is the reinforcement provided at the rear side of the roof. Backdoor and body side panels are attached to this part. It is also attached with mastic sealant and spot welds.

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 structure application. This study is used to predict the heat distortion temperature at where the material is starting to soften when exposed to a fixed load at elevated temperature. To avoid bending or damages on the roof, based on heat distortion temperature, this study will predict the bow roof position on the roof to strengthen the roof.

Heat Distortion Criteria:-

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]

The thickness of the roof

Roof Outer Panel,t = 0.75 mm

Front Roof Rail, t = 0.75 mm

Rear Roof Rail, t = 0.75 mm

Bow Roof Rail_1, t = 0.75 mm

Centre Roof Rail, t = 0.75 mm

Bow Roof Rail_2, t = 0.75 mm

Roof Length for styling surface(L)

For the L we want to take the total length of the styling surface don't include the ditch area as shown below fig,.

Bow Roof Span(l):-

Here we want to find the distance from the front roof rail to bow roof_1 to find the bow roof span as shown in fig, Similarly, we can measure the remaining span distance also want to measure.

Finding Rx & Ry (X & Y Curvature):-

• From the 0-Y axis, at -3Y about the distance of 300 mm is drawn parallel on the perpendicular plane as shown in fig. After that 100 mm lines are offset to either side of the -3Y line. Now center lines are drawn between each adjacent roof rail edge 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 as follows & put them into the formula to calculate 'W"

Projects the points and make the arc

• Finding the Rx curvature and Ry curvature

Apply the values in the formula:-

To find the Heat distortion apply all the necessary values from the roof of the given formula.

HEAT DISTORTIONS CRITIRIA CALCULATIONS:

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

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

HEAT DISTORTION CRITERIA CALCULATIONS

ROOF RAILS	Rx(mm)	Ry(mm)	Rx*Ry	Rx+Ry	R(mm)	L(mm)	t(mm)	l(mm)	W	Result
FR-BOW1	4737.54	1862.90	8825563.266	6600.44	2674.23	2032.02	0.75	404.45	1.3158	OK
BOW1- CR	4895.78	4491.50	21989261.125	9387.28	4684.905	2032.02	0.75	391.77	1.3510	OK
CR- BOW2	3878.78	9413.70	36513671.286	13292.48	5493.88	2032.02	0.75	391.84	1.4015	OK
BOW2-RR	3902.63	10100.00	39416563	14002.63	5630.133	2032.02	0.75	419.78	1.7201	OK

From the above table, it can be concluded that all values of W<2.7 and also not W > 3.1 so thus infer that the current positioning of bow roofs are good in the state as per the 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.

Snow load Prediction formula

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

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]

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

To measure the Rx & Ry (X & Y curvature)

Previously we calculate the Rx & Ry in heat distortion criteria at this time we calculate the values at the center of the roof as shown below fig,.

Projects the points and make the arc

Find the Rx Curvature and Ry Curvature for example shown in fig below.

Rx curvature Ry curvature

• To find the Lx & Ly values take the styling surface of the roof.

Lx value Ly value

Moment of Inertia for the roof rails:

Fig; MOI of Bow Roof_1

Fig; MOI of the center roof rail

Fig; MOI of bow roof_2

Apply the values in the formula: To find the snow load applies all the necessary values from the roof to the given formula.

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)

s = L1/2+L2/2

SNOW LOAD CRITERIA CALCULATION

Roof Rails	Iy(mm)	Lx(mm)	Ly(mm)	L1(mm)	L2(mm)	Rx(mm)	Ry(mm)	t(mm)	Y(mm)	S	My	α	Qr	Result
FR-BOW_1	734.94	1179.33	2032.02	413.76	463.64	4343.02	2588.83	0.75	413.076	438.706	669571.25	0.69814	6.67130	OK
BOW_1-CR	1035.86	1132.02	2032.02	463.64	464.73	3578.18	6018.92	0.75	463.64	463.54	718894.74	0.75402	4.97254	OK
CR-BOW_2	756.25	1102.82	2032.02	464.73	440.82	4270.64	11015.21	0.75	464.73	458.72	748052.11	0.79274	1.58848	NG

It is observed from the values that obtained values of Qr > 3.1

From the calculation we found bow roof 2 has NG, So we have to ask the styling team to modify the surface with curvature in the y-axis (Ry) as 6425.48.

Section Modulus:

Section modulus is a geometrical property for a given cross-section used on the design of beams or flexural members. Other geometrical properties used in the design include an area for tension and shear, the radius of gyration for compression, 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 modules.

• Elastic section modulus
• Plastic section modulus

the elastic section modulus is defined as,

S=I/Y

S= section modulus

I= moment of inertia

Y is the distance between the neutral axis to the most extreme fiber.

FRONT ROOF RAIL:

Moment Of Inertia (Max), Imax= 1.606334971*10^4( mm^4)

Moment Of Inertia (Min), Imin= 862.18403515(mm^4)

Section modulus, S = I/Y,

Y= 61.45mm

by usibg this values we can find the section modulus (S)

For maximum ,S = 1.606334971*10^4/ 61..45

= 2453.126937 mm^3

For minmum, S = 862.18403515(mm^4)/ 61.45

= 124.321129341 mm^3

REAR ROOF RAIL:

Moment of inertia (max) Imax= 9.71165135*10^3[mm^4]

Moment of inertia (min) Imin = 447.301769090*[mm^4]

Section modulus (S)= I/Y

Smax = 4235.92 mm^3

Smin= 224.78mm^3

similarly, we can calculate the section modulus, bow roof_1, center roof rail, bow roof_2

ROOF MODEL VIEWS

CONCLUSION:

The design of the roof, front roof rail, rear roof rail, bow roof_1, bow roof_2, center roof rails by following the master section with the design parameters for the given class A surface is done.

• All reinforcement BIW parts of the roof are designr=ed according to the input data provided by the styling team. Designed parts will be submitted to the CAE team to carry out further FEM analysis to obtain numerical data to improve the design.
• Made the curvature study and analyzed 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 in the y-axis (Ry) as 6425.48.
• Moment of inertia, section modulus, and Draft analysis is carried out the check the feasibility of manufacturing are found Ok.