Hood design-Week 2

Design Hood Outer Panel, Inner Panel and the necessary reinforcements by following the Master Section mentioned with the Design parameters

Data to be used:

Hood Thickness Information 

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

Introduction:

The Hood or the Bonnet of a car is considered as the frontal BIW enclosure component. Bonnets are designed to reduce air resistance as the air flows around the car. The design of the bonnet is vital since it affects the aerodynamic flow around the car. Apart from this, bonnets are also designed to meet several other requirements. They serve a great deal of purpose in protecting the passengers inside the car and the pedestrians in the event of an accident. It also acts as the engine cover.

 Theory:

• A perfect hood design means it should have a great impact distribution angled outside the car.
• In Japan and Europe, regulations have come into effect that places a limit on the severity of pedestrian head injury when struck by a motor vehicle. This is leading to more advanced hood designs, as evidenced by multicore hood inner panel designs.
• European Enhanced Vehicle-Safety Committee (EEVC), European New Car Assessment Program (euro NCAP)

 Pedestrian -Vehicle Collision

• India, however, ranks 1 in the number of road accident deaths across the 199 countries reported in the World Road Statistics, 2018 followed by China and the US. As per the WHO Global Report on Road Safety 2018, India accounts for almost 11% of the accident-related deaths in the World.
• Road transport remains the most favoured mode of transport for both freight and passenger movement in India. The fast-growing population, exceptional rate of motorization coupled with the ever-growing urbanization has made people vulnerable to frequent road accidents resulting in fatalities, injuries/disabilities.
• The sequence of injuries and events in a pedestrian collision images are mentioned below.

 The hood or the Bonnet consists of the following parts.

1. Hood outer panel
2. Hood inner panel
3. Latch and Striker re-enforcement
4. Hinge re-enforcement
5. Gas stay re-enforcement

The hood or the bonnet is commonly manufactured using Stainless steel of various grades. For vehicles that weigh less, the bonnet is made up of aluminium. The Stainless steel bonnet uses a technique called deep drawing to manufacture the component. The Aluminum bonnet employs a procedure called as casting.

Manufacturing Requirements:

While designing the inner panel hood that will be manufactured using the deep drawing process, there are some minimum requirements that we need to fulfil:

1. Minimum fillet radius = 3mm+Thickness
2. No sharp corners inside the component
3. No curvatures lesser than 5mm
4. Mounting areas and welding areas need to be flat
5. Provide appropriate corner relief on the area of curvature.

Functional and Safety Requirements:

The 1st and most important safety requirement to be considered while designing the Bonnet is the frontal collision requirements.

In this image, you can see the deformation on a hood that underwent a frontal collision:

It is clear that the design intention of the hood according to the frontal collision requirement is that the hood crushes and dissipates forces in the upward direction and to the hard points on the hood thereby deflecting the force on to the body and not the passengers.

Regarding the safety of the pedestrians, the above image from Euro NCAP shows the area of impact for a child and an adult. It is of utmost importance that we make sure not to place re-enforcement on the areas prone to high head impact. Also, the child impact area is designed with lesser thickness than the adult impact area so that in the event of a frontal collision where a child is involved, the bonnet will absorb the impact of the crash and crumble to protect the child’s head from absorbing the force.

The other major functional of the hood is the opening and closing of the hood. This is done with the help of the latch and striker. To position the latch and striker effectively, we position the hinge and draw the hinge axis. Now, using the hinge axis as the center we draw a big circle to the position where we need the latch to be placed. Then, we draw a tangent line to the circle at that point and place the latch in that region.

Master section creation:

 Below are the images of the master section used for the design.

Master Section:

Geometry and emboss creation:

The geometry is created based on the master section that was designed above. After performing the initial stages of design, we will provide local embosses to strengthen the component. The component will be strengthened based on the master section. When a force impacts the hood from the front, it travels in an upward direction. The component is designed to divert the force away from the passengers into the hinges, as shown in the image above-

In this image, you can see the force as it branches out on the hood, avoiding the passengers.The embosses in the inner panel are made according to the diversion of the load path

Fine-tuning the emboss: 

To make the component easy to manufacture, all the sharp corners from the embosses are removed. The curvature intersections from different nearby embosses are blended out to produce a smooth transition.

Creating the landing area:

The landing area of maximum of 6-8mm is maintained throughout the product. To mount the component effectively, the regions where mountings are placed are made flat using a small emboss.

 

When we apply the fillet option, the minimum radius of 3.75mm needs to be maintained in thickness throughout the whole corner. The main criteria is to have fillet more than 3mm but because our thickness is 0.75, the fillet given is 3.75mm.

Building re-enforcements and crash bead:

After the development of the entire inner panel for the hood, we need to develop some additional necessary re-enforcements. The Latch striker and the hinges will need a re-enforcement that needs to be developed. The re-enforcements are attached to the hood inner panel using the spot welding method. The positions to be spot-welded are provided by the design team. 

Mastic Sealant: 

The Mastic sealant or the Mastic point is an elastic compound that behaves like a rubber. The mastic points are defined on the hood inner panel where the hood can easily deform. They help the hood outer panel to come back to its original shape after it gets deformed.

Hemming:

Hemming is the process of joining both the outer and inner panel together. The hemming process roles and wraps the hood outer panel around the inner panel and holds it tightly together. After the hemming process is done, now we need to provide corner relief on the corners of the hood so that the part is manufacturable.