Challenge

Roof challenge

SShehbaz Malik20 Oct 2021

CAR ROOF DESIGN AND ANALYSIS OF REINFORCEMENTS

OBJECTIVE:

An automobile roof is the portion that placed above the the passenger compartment, protecting the vehicle occupants from sun, wind, rain and other external element. Because the earliest automobile were designed in an era of horse-drawn carriages early automobile roofs used similar material and designs and Improve the NVH value.

Ther are some point

Extracting the styling surfaces and for the given car roof styling and design the essential flanges and rienforcement, namely Ditch Area, Front Roof Rail, Rear Roof Rail, Center Roof Rail and Bow-Roofs.
Do a curvature study on the roof and perform calculation for Heat Distortion and Snow load criteria to determine and justify the position of bow-roof.
Find the Moment of Inertia and Section Modulus for bow roof rail and center roof rail.
Perform a draft analysis of each reinforcement.

SOFTWARE USED:

NX-CAD &MX Excel.

DESIGN CONSIDERING OF ROOF

Visibility Criteria
Head Clearance
Curvature Study
Head Distortion and Snow load criteria
Draft analysis
Safety Parameter

ROOF CRUSH TEST FOR PASSENGER CAR:

Safety point of view vehicle roof plays a very crucial role especialy roll over kind of accident, in which vehicle tips over onto its side ofthe roof.

Therfore passenger safety is also a very important parameter in front of OEMs. It is not only the marketing strategy but also it is an obligation stipulated by international standard that are now placed in several country as well as government requirement for this reason strength is very important factor for the design 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 then 127mm (5inch) when induced load1.5 times the vehicle weight.

for the test car must be place in horizontal position with the door locked and window closed. Further more a flat rigit block with a lower surface rectangle measure 30 inches wide by72 inches long (762mm X 1829mm) shell not move more then 5 inches (127mm) measured as the distance between the original location of the original surface of the test device and its location as the specified force level is reached, when it is used to apply a force 1.5 times the unloaded vehicle wieght (UVW) or 5000 ibs (22224N) whichever is less to either side or forward edge of the vehicle.

Also, direction of force shall be downward and perpendicular to the lower surface of the rigit plate such that it moves on straight line without rotation. The duration of the test shell not exceed 120 sec. The longitudnal axis of the plate is picthed forward at 5 degree angle as viewed from the cra passenger car as shown in below figure.

the lateral axis or the roll of plate 25 degree angle with horizontal surface as viewed from the front of the passenger car as shown in below figure.

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 reinforcement Ditch Area, Front Roof Rail, Bow Roof Rail, Center Roof Rail and Rear Roof Rail.

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 perameter of visibility criteria, head room clearance and the other important parameter like dimention, fitment of windshield, sealant, rubber etc. All these data are well calculated in the planning stage to design the peripheral component accordingly.

FRONT DITCH AREA:

From dumpsolid part class A surface is extracted and ditch area are developed considering parameter specifeid in 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. thickness is 0.75.

REAR DITCH AREA:

Surface is offset from the outer panel to form the ditch area and with law extension command, corner relief at rear flange and positive draft in required degree considering for the drowing operation process thickness is 0.75.

FRONT ROOF RAIL:

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

Thickness =1mm

Embosses hieght:2<3T

Radius: (3-5mm)>T

REAR ROOF RAIL:

it is support which is joined to the back door and the body side outer and the inner panel. it is also usually made from the body side outer scrap and therefore the thickness of the rail is 0.65mm. similarly to the front roof rail design we have to take into considering the rear head room clearance and the rear visibility criteria while designing the rear roof rail.

Headroom Clearance: 120mm

Embosses hieght:2<3T

Radius: (3-5mm)>T

BOW ROOF RAIL 1:

It improves the torsional, stiffnes and load bearing capacity of the roof structure. the no. of bow roof depend on the surface of the roof. they are attached the outer panel with help of sealant. each mastic point will have up to 80mm circular effective area. the positioning of bow roof is done based on the curvature study. the heat distirtion criteria and snow load criteria are checked and analysis the dta.

the thickness of bow roof is 1mm and also mastic area has been created.

BOW ROOF RAIL 2:

It improves the torsional, stiffnes and load bearing capacity of the roof structure. the no. of bow roof depend on the surface of the roof.

CENTER ROOF RAIL:

center roof rail is provided to effectively support the flat surface is is more chance to failure under the action of load. usually it placed at the center of the roof surface. the thickness is 1mm.

MASTIC SEALANTS:

bow roof rail and center roof rail are joint together with the help of mastic sealant. here we can not use weld spot it will damage the A class area of the roof.

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 can not be used structual application. this study is used to predict the heat distortion temperature at where temp start to soften when exposed to a fixed load at elevated temp. in order to avoid bending or damage on roof, based on this study we will pridict the bow rail position on the roof and strengthen the roof.

bow roof pridiction formula

HEAT DISTORTION CRITERIA:

judgement conditiopn :ok<2.7>3.1

Where

l=roof length in x-direction in mm (roof dimention on 0-Y)

R= roof curvature

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

Rx= x-curvature

Ry=y-curvature

t=Roof plate thickness

l=Bow roof span

Thicknes all Roof and Panel:

Roof ouetr panel=0.75mm

Front roof rail = 1mm

bow roof rial 1 & 2=1mm

center roof rail=1mm

rear roof rail=1mm

Roof Length for Styling Surface (L):

for the L we want to take total surfce lenth of the roof. do not include the ditch area given below in figure.

BOW ROOF SPAN (l):

Here we want to take distance from front roof rail to bow roof 1 to find the bow roof span as show in figure

Rx & Ry (X & Y) curvature

from O-Y, at -3Y about distance 300mm is drawn parallel on the perpendicular plane as shown in figure after that 100 mm lines are offset to either side of -3Y line. Now center line are drawn between each adjacent roof rail edge respectively. Intersection points are marked on the sketch and exit the sketch. make all line conruction line except ponts.
Project the intersection point in the sketch to roof surface as shown in figure.
Join the projected curves using arc curve and measure the minimum radius and linear dimention as follow and put the formula to calculate the W.

For the above figure now project the point on the top of the roof and drow the arc with the three point intersecting area as shown in figure.

Find thr Rx And Ry Curvature with the simple measure

Aplly the Rormula

to find the heat distorton criteria apply all necasaary value to find the value of W.\

table and calculation are performed in mx excel.

Fro the above dta we can conclude it all the value of W<2.7 and also not W>3.1 so thus infers that current position of bow roof arte good in state as per design 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 pridiction formula

Note: The Qr value given over here is correct.

where

judgement condition= Qr≥ 3.1

250≤s≤380

t=roof plate thickness in mm

Ly = distance between the front and rear roof rail on the vehicle along with OY in 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 = distnace between left and right end of the roof on the roof bow in mm. width of the roof panel exposed on the surface.

Y= distance from front roof rai to roof bow

s=distance for which roof bow bears divided load

s=L1/2 + L2/2

ly= geometric moment of inertia of roof bow (Y cross-section)

Rx=lateral distance curvature radius of roof panel (Y cross-section on roof bow)

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 and Ry (X and Y curvature)

at O-Y axis on perpindicular plane to roof as shown in figure drow 100mm lines offset to either side of OY axis line. Now center lines are drawn between each adjacent roof rail edge respectively. intersecting point are marked on the sketch and exitthe sketch. make all geometry in construction lines except the points.
project the intersection in the sketch to the roof surface as shown in figure.
join the projection curve using arc curve and measure the minimum radius and linear dimention.
project the intersection plane bow roof and center roof rail on the horizontol plane to find the moment of inirtia of the respective section to find ly value s.
put all the radial and linear dimention in following formula to calculate Qr value.

In this figure three point intersecting area as shown in fig

Find Rx And Ry curvature with the help of simple radius.

Apply the value in this formula

To find the snow load criteria aplly all necessary values

S=L1/2+L2/2

TABLE AND CALCULATION ARE PERFORMED IN MX EXCEL:

It observed from the value that obtained value of Qr>3.1

From the calculaion we found Center Roof Rail has NG. So we have to ask styling team to change the surface of roof with curvature.

Apply Embosess

We also apply embosess to increase the strength of the model.

SECTION MODULUS:

The section modulus of the cross section shape is of significant importance in designing of structure. it is direct measure the strength of structure.

S = l/y

S: is section modulus

l: is the area moment of inertia

y: it is distance from the neutral axis to the given figure

BOOW ROOF 1

Create an intersection curve with the help plane and select bow roof rail

When the intersection curve is created then model must be hide then you clrealy see the intersecion curve

Moment of inertia(max) lmax = 1.51533x10^4 (mm^4)

Moment of inertia(max) lmax = 337.35 (mm^4)

y = 22.0068 mm

Smax = 688.5735 mm^3

Smin = 15.3293 mm^3

BOOW ROOF 2

Moment of inertia(max) lmax = 3.74x10^4 (mm^4)

Moment of inertia(max) lmax = 434.8555 (mm^4)

y = 30.0256 mm

Smax = 1,247.5021 mm^3

Smin = 14.4828 mm^3

CENTER ROOF RAIL

Moment of inertia(max) lmax = 3.74488x10^4 (mm^4)

Moment of inertia(max) lmax = 433.7175 (mm^4)

y = 30.0243 mm

Smax = 1,247.283 mm^3

Smin = 14.4455 mm^3

FRONT ROOF RAIL

Moment of inertia(max) lmax = 2.4764x10^4 (mm^4)

Moment of inertia(max) lmax = 1.2563x10^3 (mm^4)

y = 66.065 mm

Smax = 374.8429 mm^3

Smin = 19.016120 mm^3

REAR ROOF RAIL

Moment of inertia(max) lmax = 1.0488x10^5 (mm^4)

Moment of inertia(max) lmax = 5.9438x10^3 (mm^4)

y = 61 mm

Smax = 97.4393 mm^3

Smin = 19.016120 mm^3

DRAFT ANALYSIS:

The draft analysis will be performed for the tooling direction created at first and checked that the model is suitable for manufacturing.
Then to get a complete green portion the necessary changes to the draft angle of the embosses were carried out by optmizing the model.
The draft analysis command enable us to detect that the part drafted will be easily removed or not.
This type of analysis is performed based on the color range identifying zones on the analysis element where the deviation from the draft direction at any point corresponding to specified value.