Hood design-Week 2

Aim: 

To design Hood’s outer panel, inner panel, and the necessary reinforcements by following the design strategy and consideration that goes under the designing and developing of Hood to achieve maximum safety for pedestrians and the Master section mentioned with the design parameter.

 

Data to be used:

Hood Thickness Information

• Outer Panel Thickness = 0.75mm
• Inner Panel Thickness = 0.75mm
• Reinforcement Thickness = 1.5mm

Master sketch:

Objective: 

 

1. To design a model of car hood with inner and outer panels.
2. Create the embossment for the Hinge and Striker considering proper Latch trajectory for movement. 

 

Detailed report: 

The flow of information of the report will be as follows,

1. Introduction 
2. Design consideration 
3. Design/ Procedure 
4. Result and discussion 
5. The body 
6. Reference

 

Introduction:

 

The Hood consists of the following parts as given below,

 

• Hood inner panel
• Hood upper panel
• Hinge reinforcement
• Latch and striker reinforcement
• Gas stay reinforcement

 

Bonnet is mostly called in commonwealth countries, it essentially is a type of cover for engine compartment and powertrain. It is a critical component because of safety for pedestrian & frontal crashes which dissipates the energy of the crash on the body rather than inside the car body. It is also the car's styling surface, which provides a better look to the vehicle.

 

The Bonnet or upper hood is generally made of aluminum which is lighter in weight and some cases it is made of stainless steel in various grades to give better strength.

 

The aluminum hood is manufactured by the casting process and the stainless steel hood is formed by the Deep drawing process.

 

The selected lightweight material properties, which realized the integrated performance improved to different levels and the weight of the inner panel reduced by 30.4%. The case study of lightweight design based on lightweight material selection and structural optimization shows the feasibility and potential of the method, and it provides a reference for the lightweight design of the car body.

The deep drawing process: 

 

Deep drawing is a sheet metal forming process in which a sheet metal blank is radially drawn into a forming die by the mechanical action of a punch. It is thus a shape transformation process with material retention. The process is considered a "deep" drawing when the depth of the drawn part exceeds its diameter.

 

The most commonly used duplex grade is 0.02% C – 22% Cr – 5.5% Ni – 3% Mo – 0.15% N alloy.

 

The hood is designed by the EURO NCAP ( European New Car Assessment Programme) standard. The adult and child impact zones are taken through calculated by plotting the region from the standards.

 

NCAP: NCAP (Figure1) provides measures for the assessment of pedestrian protection performance of a passenger car experimentally by firing subsystem impactors representing a child head, adult head, upper leg, and lower leg at a specified angle and speed to the front end of a stationary vehicle. The resulting injury measures from these physical tests are assessed against the bounds specified by the protocols as shown in Figure

 

This study focuses only on child pedestrian head impact to the outer hood panel and does not include an inner hood, upper, or lower leg impacts defined in these protocols.

 

Standards of NCAP pedestrian protection head impact requirements As a result of the implementation of these regulations, vehicle manufacturers face technical challenges associated with the investigation of optimal hood panel configuration to meet the requirements of ANCAP while retaining or maximizing styling flexibility with minimal modifications to the general architecture of the design.

 

HIC values are used to predict the risk of engine hood to the pedestrian and the level of severity of engine hood design when the collision occurs. 

The value of HIC depends on the engine hood design, materials, impactor type, and structure. 

HIC is calculated according to Eq 

 

α is the resultant acceleration in g 

t1, t2 is two-time instants in seconds which defines the start and end of the recording when HIC is at maximum. Values of HIC in the time interval t1–t2 is greater than 15 ms are ignored for the aim of calculating the maximum value. 

 

Injury Distribution: 

 

Even though the injury percentages per body region vary among different countries, they all show a consistent U-shape, in which the head and lower extremities are the most common injured body regions in pedestrians; the body regions in between are less susceptible to the risk of injuries. 

 

Provides a general view of pedestrian injuries using the Abbreviated Injury Scale (AIS) 2+. 

The AIS is an injury-measurement system that classifies an individual injury by body region according to its relative severity on the following 6-point: 

scale 1 (minor), 2 (moderate), 3 (serious), 4 (severe), 5 (critical), and 6 (maximum) (AAAM 2008). 

The AIS ranks injuries for their potential threat to life but does not consider disability or cost factors.

 

The test procedure for Pedestrian safety : 

Pedestrian safety tests have been proposed by a variety of different organizations, including the working groups of the European Enhanced Vehicle Safety Committee (EEVC), the International Organization for Standardization (ISO), and the IHRA.

However, the test procedures are all very similar.

All of these tests are designed to replicate vehicle-to-pedestrian crashes at 40 km/h and use individual component tests representing impacts to different body regions instead of full-scale dummy tests.

 

Design consideration:

 

Hood is designed by considering by  Euro NCAP safety parameter explained below 

EURO NCAP test: 

Euro NCAP Pedestrian Protection Test Procedure Based on pedestrian injury data, the head and lower extremities is the most commonly injured body regions in vehicle-to pedestrian crashes. 

As a result, Euro NCAP (and all the other pedestrian impact-test procedures) focuses only on these two body regions. 

As shown in Figure 11, Euro NCAP pedestrian tests include leg form to bumper tests, upper leg form to hood leading-edge tests, and child/adult head form to 19 hoods/windshield tests. Multiple tests are conducted at different test zones at the bumper, hood leading edge, and head form contact areas.

The head form test area is defined based on the pedestrian wrap-around distance (WAD) as shown in Figure 12, in which child and adult head form test zones are separated. The test zones cover almost the full width of the vehicle so that the overall pedestrian protection can be evaluated throughout the vehicle front-end structures. 

Once the tests are conducted, the impact responses are then assessed by points and rated in color. The impact responses measured in the pedestrian impact tests and their associated injury criteria are shown in Table 4. 

Maximums of 6, 6, and 24 points are available for the bumper, hood leading edge, and head form test zone, respectively, leading to a total of 36 points available in the pedestrian protection assessment. 

 

The opening angle situation is an important consideration as per the region. The average height of humans in European countries is 6 feet. It is controlled by either the gas stay or support rod.

 

Gas Stay: 

Gas stay always is not straight, it always has some deflection to provide strength to the rod.

Striker position should be above the head of the human by adjusting it so that it will not hit the head.

There is always some gap between the hood and engine compartment, it will not produce a rattling noise while moving and it won't collide with each other.

Design the support rod by considering some deflection in an outward direction between 35mm to avoid bending by the weight of the bonnet.  

Support rod assembly should be after placing the battery system so it is easier to place the battery component. It is fixed by reinforcement because of its different components and placed on the center of gravity of the hood.

 

Procedure:

 

Inner hood creation:

 

Class A is given to create the hood, 

First, we split that class A with the YZ plane so that we get a half surface to work.

Next, the surface is offset by 2mm  following the given master section

Then we extracted the boundary and made a parallel curve by 20mm on the surface.

After that split the curve to give a cut section.

 

Again, the same operation followed by above to make another surface and joint than using a Multi-section surface.

As per the above method, I made a complete surface following the master section.

 

Embossment: 

Embossing is provided for giving strength and stiffness to the hood and it also helps to distribute force impact during a crash.

After creating the above surface we create an embossed master section to give strength to the component. As shown in the below image, the impact force dissipates equally and goes towards hinges following the given path and away from the passenger.

Embossed should be always in v direction and does not have sharp edges.

For that, we provided a fillet on sketches and cut it out, and helped to reduce the weight of the hood for lighter vehicles with better strength.

Mastic data: 

After creating the inner and upper hood we need to joint it together.

Mastic provides strength to the hood and joining purpose. It is provided on the inner panel with a distance of 80mm diameter from each mastic point.

Generally, in the automotive industry spot welding is not preferred due to its damaging surface finishing. 

Mastic sealant is like a rubber behaving as an elastic component.

Mastic sealant helps to reduce the NVH(Noise, Vibrations, and Harshness) to the hood.

 

Hinges:

 

Hinges are used to connect the hood and car chassis; it provides the opening and closing movement to the hood.

 

Hinges are fixed in a straight line along the X-axis. It requires reinforcement to assemble it with a hood to give additional support.

Latch and striker:

 

The latch and striker position is adjusted by following the center of the hinges to get accurate trajectory movement.

Made a center point on the hinge line 

The midline of the striker is to check movement.

The circle is drawn from the centerline of the hinges to move along a trajectory.

Upper Hood:

 

For the hemming process, the outer hood is extrapolated and offset the surface, and cut it out then, used Multi-section solids to join it providing fillets.

 

Hemming:

 

Hemming is a forming operation in which the edges of the sheet are folded or folded over another part to achieve a tight fit. Normally hemming operations are used to connect parts, improve the appearance of a part, and reinforce part edges.

In-car part production, hemming is used in assembly as a secondary operation after deep drawing, trimming, and flanging operations to join two sheet metal parts (outer and inner) together. Typical parts for this type of assembly are hoods, doors, trunk lids, and fenders.

The accuracy of the hemming operation is very important since it affects the appearance of the surface and surface quality.

There are various types of hemming operations:

• Conventional die hemming
• Roll hemming

In conventional die hemming, the flange is folded over the entire length with a hemming tool.

In roll hemming, the hemming roller is guided by an industrial robot to form the flange.

Roll Hemming:

 

Roll hemming is carried out steps with a hemming roller. An industrial robot guides the hemming roller and forms the flange. Roll hemming operation can also be divided into several pre-hemming and final hemming process steps.

 

After the Hemming process, we give corner reliefs it helps to control sheet metal material behavior and to prevent unwanted deformation during bending operations.

 

Results: 

We have created a Hood design following the master section and safety parameters.

 

Hood design: 

Reference:

 

1. Design case study of Bonnet:https://ieeexplore.ieee.org/document/5572684
2. Material selection for sheet metal with grades of Hood design:http://article.sapub.org/10.5923.j.ijme.20180803.02.html