Hood design-Week 2

DESIGN OF CAR HOOD/BUMPER

Theory

Hood

The hood or bonnet is the hinged cover over the engine of motor vehicles. Hoods can open to allow access to the engine compartment, or trunk on rear-engine and some mid-engine vehicles) for maintenance and repair. 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 race cars, 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 aluminium. Aftermarket car hoods may be constructed from various other materials, including carbon fibre, fiberglass, or dry carbon.

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Lifting Mechanism of a Hood

1. Hood Support Rod

The hood support rod holds up the hood of the vehicle. It allows to access the engine compartment without holding the hood up by hand or with a prop rod. 

2. Gas Stay

Gas Stays, provide direct support for safely lifting a car hood to allow access to the engine compartment. Gas strut is a sealed energy source containing pressurised inert gas and a small amount of oil. Gas struts work by forcing, under pressure, an inert gas (Nitrogen) into a cylinder. The internal pressure then greatly exceeds atmospheric pressure. This differential in pressure exists at any rod position and generates an outward force on the rod, making the gas spring extend.

When the rod is either compressed or extended, this movement of gas and oil within the cylinder from one side of the piston to the other, creates a damping effect, reducing sudden shocks on mountings, hinges and your application.

EURO NCAP Pedestrian safety

Euro NCAP, or the European New Car Assessment Programme to give it its full name, is an independent crash-test safety body, which rigorously assesses the safety of new cars, and helps to further the advancement of car safety technology.

The current Euro NCAP safety regime includes a dual rating, with cars tested both with and without various high-tech safety features. This means some cars may get five stars if they are configured with autonomous emergency braking, for example, but drop to four stars for the second rating if this feature is not standard.

To get a second optional star rating on a car, manufacturers just have to confirm that they expect the extra safety technology (defined as a 'safety pack') to be fitted on at least 25 per cent of models sold. 

Vulnerable Road User (VRU) Protection

As well as assessing how well cars protect their occupants, Euro NCAP tests how well they protect those vulnerable road users – pedestrians and cyclists – with whom they might collide.

In these tests, the potential risks of injuries to a pedestrian's head, pelvis, upper and lower leg are assessed. Cars which perform well can gain additional points if they have an autonomous emergency braking (AEB) system which recognises pedestrians and cyclists.

The tests include

1. Head Impact test
2. Upper Leg Impact test
3. Lower Leg Impact test
4. AEB Pedestrian test
5. AEB Cyclist

Manufacturing Process of the Hood

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 "deep" drawing when the depth of the drawn part exceeds its diameter. This is achieved by redrawing the part through a series of dies. The flange region (sheet metal in the die shoulder area) experiences a radial drawing stress and a tangential compressive stress due to the material retention property. These compressive stresses (hoop stresses) result in flange wrinkles (wrinkles of the first order). Wrinkles can be prevented by using a blank holder, the function of which is to facilitate controlled material flow into the die radius.

In the automotive industry, deep drawing is usually carried out using rigid tools.

HEMMING

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

There are various types of hemming operations:

1. Conventional die hemming
2. Roll hemming

3 Steps of Roller Hemming

Uses of Hem

Hems are commonly used to re-enforce, hide imperfections and provide a generally safer edge to handle.  When a design calls for a safe, even edge the added cost of material and processing of a hem is often preferable to other edge treating processes.  Designers should look beyond a single small flat hem to treat edges.  Doubling a hem can create an edge perfectly safe to be handled without almost regard for the initial edge quality.  Adding a hem in the ‘middle’ of a bend profile can open the doors to a variety of profiles not possible without fasteners or welding.  Even without sophisticated seaming machines a combination of two hems can create strong, tight joints with little or minimal fastening.  Hems can even be used to strategically double the thickness of metal in areas of a part which may require extra support. 

Design of the Hood

The design of the Hood takes place in various steps, which are described below

Design Approach

We take the Top Down approach for the Hood design and create an Assembly file and import the existing parts in it and create a new part file for the Inner Pannel.

The Inner Panel is then created by using the Geometry of the Outer Panel and following the Master Data provided for the Inner Panel.

Emboss Definations

For pedestrian safety reason the part of the Hood which would be the Adult Head Impact Zone is made comparatively weaker as compared to the other zone, due to this reason we create proper embosses and trim the sheet in those areas. Also, we create the emboss shapes at an angle, such that in a event of frontal crash or collision, the force gets dissipated to the Hinge region which is comparatively a stronger region of the Hood, thus allowing the Hood to crumble rather than to break.

Mastic Points

The Hood Inner Panel is attached to Outer Panel by means of Mastic (sealents) rather than spot welds to reduce NVH levels, so we create mastic points in the Inner Panel for it.

Latch Trajectory

The Latch Trajectory is created after the proper positioning of the Hinges. This trajectory defines the path the striker follows when the Hood is opened or closed.

Reinforcements

Reinforcements for both the Hinges and the Striker are created since the region where they are fitted loose strength. Thus, to compensiate the loss of strength, additional reinforcement is added.

Views of the Hood

Front View

Back View

Side View

Isometric View

Conclusion

Thus, the Car Hood was successfully designed accoring to the dimensional requirements and also alligning to the safety regulations EURO NCAP.