Hood design-Week 2

A car hood also referred to as a bonnet in some other countries is the hinged cover that rests over the engine of a front-engine vehicle. Its purpose is to provide access to the engine for repair and maintenance. A concealed latch is typically used to hold down the hood. On vehicles with an aftermarket hood and on racecars, hood pins may be used to hold down the car hood. Hoods sometimes also contain a hood scoop, wiper jets, power bulge, and/or hood ornament. Car hoods are typically constructed from steel and sometimes from aluminum. Aftermarket car hoods may be constructed from various other materials, including carbon fiber, fiberglass, or dry carbon. The hood design may vary from one make and model to another, including SUV, truck, minivan,, and passenger-car models.

Concerns regarding the severity of a potential pedestrian head injury during a collision with a motor vehicle have led to advanced hood designs on many models. Such designs may include a multi-cone inner hood panel design. An active structure that is capable of pushing the surface of the hood away from the vehicle's hard motor components in the event of a pedestrian crash is also in use. Pyrotechnic or spring force designs are often used in the the the active-hood design.

The hood of a car is considered the frontal BIW enclosure component. Bonnets are designed to reduce air resistance as the air flows 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 & the pedestrians in the event of an accident. It also acts as the cover for the engine.

The European New Car Assessment Programme (Euro NCAP) is a European car safety performance assessment program that has developed test specifications & rating systems for assessing pedestrian injuries during impacts with the front of vehicles such tests include a series of tests using impacts designed to bio-mimic a collision at speed of 40 km/h.

• They describe a situation in which a pedestrian is situated laterally & a car first hits one of his legs. Tests are conducted on the leg, upper leg & most importantly the head, as head trauma is most commonly responsible for casualties.
• So to avoid severe injuries to the head, the high impact area for adults & children is subjected to less thickness. Also, no reinforcement should be provided in that area as during head collision, the hood will absorb the head impact energy leading to the collapse of the hood & thus protecting the severe head injury.
• Another important criterion for the hood design is the protection of the occupants inside the car during a front collision. The design of the hood should be in such a way that during the collision, the force should travel in the upward direction through the embosses & directed towards the body of the car. Thus protecting the occupants inside.

Heming:

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. Material deformations, which occur during the hemming process, can lead to dimensional variations and other defects in parts. Typical hemming defects are splits or wrinkles in the flange, material overlaps in the corner areas,,, or material roll-in. This is why it is important to use simulation tools to, on the one hand, better understand the hemming process and, on the other hand, significantly reduce the number of “trial and error” loops during tryout and production.

Deep drawing:

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 "deep" drawing when the depth of the drawn part exceeds its diameter.

PROCEDURES OF DESIGN HOOD:

Hood is one of the major components of the Vehicle body. It mainly covers 2 major power transmitting components i.e. Engine and Radiator assembly. Along with covering them through its inner panel, the hood outer panel also serves a major role in the aesthetics and ergonomics of the vehicle.

It has 3 supportive components Viz.

1. A) Hinges- for easy opening and closing of the hood
2. B) Latch- to keep hood in correct closed position &
3. C) SupportRod- helps in keeping the hood in the open position.

We will be mainly discussing

1. Hood outer panel
2. Hood inner panel
3. Hinge Assembly
4. Latch assembly

Hood inner panel is designed as per the given Master Section sketch concerning the Hood outer skin provided.

HOOD INNER PANEL:

• In the inner panel only we can research & implement the safety of pedestrians & as well as the driver. In this panel only we can attach the reinforcement, embossed, mastic points, etc.
• It is an object of the present invention to provide a hood inner panel capable of reduction of a reaction force of the hood inner panel even if a level of deformation of a hood of a vehicle increases, & ensuring a sufficient energy absorption capacity.

First, we have to create a hood inner panel with the given master section and create embosses for forces impact in the hood.

Offsets the given outer panel which given master section 1.7mm,4.7mm,26.5mm and 21.5 mm. split the offsets surfaces by using a parallel curve, in first offset surface 15mm form edge, the second surface between 17mm and 30 mm from the edge, third surface 50mm from the outer edge. Use multi section tool from the split edges and join three surfaces with the surface fillet option. The fillet value is min 3mm and max 5mm in the sheet metal.

Then for the embossing surface draw a sketch excluding are for hinge and latch emboss position. extrude the sketch and split joined surface offset surface. Create sweep in split edges and trim with a 21.5mm offset surface.

The necessity of Emboss and its Designing:

After the basic master layout is done we have to provide emboss sections to dissipate the collision force to outer corners toward hinge portions. Thus impact force passing to passenger cabin be reduced majorly.

While creating these emboss we have to keep in mind the draft direction and draft angle so that the stamping tool will create proper embossed structures on the panel. Consider the following image for reference of embossed sections on the inner panel.

Create a sketch for embossing on the above-trimmed surface. As same extrude the sketch and split with surfaces, sweep emboss split edges with the angle of 7 degrees concerning the surface. Trim with 5.7 mm offset surface.

Mastic creation:

Mastic Data

To reduce NVH i.e. noise vibration and hardness between the hood inner panel and an outer one, mastic sealants are needed to be applied. Every mastic sealant improves the strength locally by around 80mm. It joins the inner panel with the outer panel also reducing the vibrating noise.

Thus while designing embosses on the inner panels we have to design mastic portions where sealant can be applied as shown in the image: Mastic Data.

Mastic regions are designed such that they will be closer to the outer panel. Rubber sealant is applied on the mastic regions of the Hood's inner panel which will glue both panels together.

Hinge Positioning:

Hinge positioning is needed to be properly designed so that the opening and closing of the hood will occur correctly. Also, it will decide if the latch and striker get closed correctly or not.

To have proper hinge location, align hinges on the inner panel such that the midpoint of an axis passing through both hinges with striker center point be radius which will create a trajectory.

Latch Emboss- Similar to hinge emboss we have to design latch emboss to improve the local strength of the inner panel providing a support structure for latch mounting.

Hinge Assembly:

Hinge is a component that fixes the hood to the fenders of the car. The latch must be fixed perpendicular to the hinge axis so that the hood opens or closes properly.

Latch Assembly:

Latch & striker is used for locking the hood with the car. It is usually placed on the space provided on the inner panel.

REINFORCEMENT:

To furthermore reduce the impact on the occupants inside the car, additional latch and hinge reinforcement to the hood. When latch and striker are placed, there is a drop in strength at that position thus reinforcement is required. Also, stiffness pads are placed to reduce NVH. Hinge and latch reinforcement thickness is kept at 1.5 mm but it may vary.