Hood design-Week 2

DESIGN OF HOOD

OBJECTIVE

The main purpose of this project is to design a hood of a car with the given design data and by following the necessary safety precautions.

WHAT IS A 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. The hood is also considered as another type of door in a vehicle as it consists of an outer panel and inner panel. The inner panel provides strength while the outer panel functions as the metal cover.

PURPOSE OF HOOD DESIGN

• Safety of the passenger.
• Covers all engine, radiator, and other necessary components.
• Reduce air effect due to its aerodynamic shape design.
• Reduce engine noise which helps for a quieter drive.
• Provide access to engine maintenance.
• Absorbs most of the impact forces when a frontal crash occurs.

HOOD STAY

A hood stay is an apparatus for an engine room of a car, which can safely keep an engine hood in an open state even if an external force is abruptly generated due to vibration, wind and can facilitate an opening/closing operation of the engine hood.

Purpose

An automotive engine hood is arranged to be opened for the purposes of inspecting the engine room, replenishing cooling water or engine oil, changing parts, making repairs etc.

TYPES OF HOOD SUPPORT

• Gas stay
• Support Rod

1.Gas Stay

These are used to lift the car hood. Its purpose to just give the lifting force to the hood, then it automatically opens the car hood by means of gas pressure. This is used where the car bonnet size is heavy to lift. Spring stays are designed to be used on upward lifting doors, once you have opened the door slightly the spring will push the door to the fully open position. Struts contain nitrogen gas under pressure and some oil to damp movement and to lubricate the seals.

Support rod

This is widely used in many cars as the rate is cheap. In this type we have to lift the hood by ourselves and place the support rod in the space provided either in the hood or engine component. This is mostly used for where the car bonnet weight is small. Support Rod can be mounted with grommet side on the engine compartment of body and rod locking hole on hood or vice versa. Grommet and the clip of the rod are standard parts. Support rod is used to increase the support force, automatic buffering, stable and reliable, strong load-bearing capacity, safe use, not easy to fall, avoid hot hands, support the engine more easily, and balance the force without shaking.

DESIGN CONSIDERATIONS IN HOOD FOR PEDESTRIAN SAFETY

The hood in a majority of vehicles is generally fabricated from sheet metal, which is capable of absorbing energy posing a comparatively lesser threat. However, most fatal head injuries occur when there isn’t a sufficient clearance between the hood sheet and the underlying engine components. By creating a gap of approximately 10 cm pedestrian’s head can achieve a controlled deceleration and a reduced risk of death. But creating this room is a challenging task because there are other design constraints, such as aerodynamics and styling. Thus engineers attempt to overcome this problem by using deformable mounts, and by developing more ambitious solutions such as airbags that cover the stiff regions of the hood when activated.

Pedestrain Head Impact Zone

1.Wrap Around Distance

When a car crashes on the pedestrian, the whole human body wraps around the front shape of the car and the head impacts on the bonnet or the windscreen. The distance on which the head impacts on the car from the ground is called as the Wrap Around Distance (WAD).

To be specific the Wrap Around Distance is a measurement of distance from the ground to the head impact zone over the outer surface of the car. The wrap around distance is measured longitudinally in the center of the vehicle from the ground.

The severity of the injury caused by the frontal crash depends on the type and shape of the vehicle, speed of the vehicle and the movement of the pedestrian relative to the vehicle. In addition to these parameters, the wrap around distance plays a major role in the safety measurement of a pedestrian.

During the crash analysis, based on the Wrap Around Distance(WAD), two test areas will be created namely Child head impact zone and the Adult head impact zone. The child head impact zone is between 1000 to 1700 mm WAD and the adult head impact zone ranges between 1700 to 2100 mm WAD.

2.Bonnet Side Reference Line

The Bonnet Side Reference Line is defined as the geometric trace of the highest points of contact between a straight edge 700mm long and the side of a bonnet, as defined in Section 3.3.1 and A-Pillar, when the straight edge, held parallel to the lateral vertical plane of the car and inclined inwards by 45 deg is traversed down the side of the bonnet top and A-Pillar, while remaining in contact with the surface of the body shell, any contact with door mirrors is ignored. Where multiple or continuous contacts occur the most outboard contact shall form the bonnet side reference line.

3.Bonnet Leading Edge Reference Line

The Bonnet Leading Edge Reference Line is defined as the geometric trace of the points of contact between a straight edge 1000mm long and the front surface of the bonnet, when the straight edge, held parallel to the vertical longitudinal plane of the car and inclined rearwards by 50 deg and with the lower end 600mm above the ground, is traversed across and in contact with the bonnet leading edge. For vehicles having the bonnet top surface inclined at essentially 50 deg, so that the straight edge makes a continuous contact or multiple contacts rather than a point contact, determine the reference line with the straight edge inclined rearwards at an angle of 40 deg. For vehicles of such shape that the bottom end of the straight edge makes first contact then that contact is taken to be the bonnet leading edge reference line, at that lateral position. For vehicles of such shape that the top end of the straight edge makes first contact then the geometric trace of 1000mm wrap around distance will be used as the Bonnet Leading Edge reference line at that lateral position. The top edge of the bumper shall also be regarded as the bonnet leading edge, if it is contacted by the straight edge during this procedure.

• Child Impact Zone: 1000-1500 mm
• Adult Impact Zone: 1500-2100 mm

Impact Point Location

Manufacturer has to provide the Euro NCAP Secretariat with colour data at all grid points. Data must be supplied by the manufacturer before, the testing of the vehicle begins. The predicted level of protection offered by the vehicle is verified by EURO NCAP. Alternatively test points will be selected on the basis of worst case performance basis.

Pedestrain Head Injury Criteria & Rating

The head injury criterion (HIC) is a measure of the likelihood of head injury arising from an impact. The HIC can be used to assess safety related to vehicles, personal protective gear, and sport equipment. Normally the variable is derived from the measurements of an accelerometer mounted at the centre of mass of a crash test dummy’s head, when the dummy is exposed to crash forces.

where t₁ and t₂ are the initial and final times (in seconds) chosen to maximize HIC, and acceleration a is measured in gs. The time duration, t₂ – t₁, is limited to a maximum value of 36 ms, usually 15 ms. This means that the HIC includes the effects of head acceleration and the duration of the acceleration. Large accelerations may be tolerated for very short times. At a HIC of 1000, there is an 18% probability of a severe head injury, a 55% probability of a serious injury and a 90% probability of a moderate head injury to the average adult.

Rating

HIC15 < 650 = Green

650 ≤ HIC15 < 1000 = Yellow

1000 ≤ HIC15 < 1350 = Orange

1350 ≤ HIC15 < 1700 = Brown

1700 ≤ HIC15 = Red

DATA TO BE USED

Hood Thickness Information

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

MASTER SKETCH

COMPONENTS OF HOOD DESIGNED

Inner Panel

Inner panel is the main component of the hood as it is designed for dissipating the force of impact to the side of the automobile body. The inner panel consists of the reinforcements as well as the mastic points.

Outer Panel

The outer panel serves as an aesthetic component while also providing extra strength to the inner panel. The curvature of inner panel is based on the design of the outer panel.

DESIGN CONSIDERATIONS FOR DESIGNING THE HOOD

1.Latch Trajectory

The latch trajectory shows the path of the striker when the hood is opened and closed on a vehicle. The radius of the circular trajectory is determined by the set standard based on the average height of a person. The radius is measured from the hinge axis up-to the latch.

2.Fixing the Hinge Axis

The hinge axis are positioned in such a way that the axis lie in the same line so that it is parallel to the “x” axis plane. This is done in order to position the latch and sticker trajectory motions exactly perpendicular to the hinge axis.

3.Mastic Points

The mastic points are areas where the mastic is placed to attach the outer panel to the inner panel. We cannot use spot welding as it would give a less aesthetically pleasing result. Hence mastics are used to join the panels together as it does not melt the metal itself.

4.Emboss Definitions

The inner panel consists of various embosses which help dissipate the force during impact. The width and depth of the emboss is determined by simulations and analysis to check for failure. These emboss definitions are created at an angle so that they point away from the centre of the hood when viewed from the front of the vehicle.

5.Reinforcements

The area near hinge and latch has more chances of failure in the hood panel. Hence to avoid any failure, the hinge and latch areas are reinforced with extra material whose thickness is usually determined by the type of vehicle.

6.Dissipating Shock Waves

To dissipate the shock waves on to the sides of the hood. This is a major reason for designing the hood in the shape of “Y” or “V” to dissipate the forces on the sides. This is one of the primary reasons for designing it in such a way that the force is not carried on to the passengers inside so that the passengers are saved from possible damage. In this design it is found that the embosses are designed such that it forms the shape of “Y”.

7.Relief on the Corners

A relief on the corners is provided for the hemming process in order to avoid spring back issues. When the rollers are moving on the panels the force that is used to bend the sheet metal has to be maintained within the limit to avoid spring back issue. Due to which a relief is given on the corners to avoid the red zones on the hood.

MANUFACTURING PROCESSES INVOLVED IN HOOD DESIGN

1.Hemming In Sheet Metals

What is Sheet Metals?

Sheet metal is metal, in sheets. It’s thinner than plate metal (.25 in and thicker) but thicker than foil (.006 in and thinner). It’s most commonly available in steel and aluminum, in various gauges (thicknesses). It’s available with various coatings for corrosion resistance or surface finish. Sheet metal is made by taking a large cast ingot and rolling it into a long ribbon of the desired thickness. This long, flat piece of metal is then rolled into a coil and sent directly or cut into sheets before being sent to a machine shop.

What are Hems?

Hems are folds to the edge of a part to create a rounded, safe edge. Hems may be open, flat, or tear-dropped, and tolerances depend on the hem’s radius, material thickness, and features near the hem. The process of creating hems is called hemming.

Hems are created by bending a flange of your part back on itself. This increases the strength of the hemmed edge and also gets rid of any sharp edges, which your technicians (and possibly your future self) will thank you for. The downside (drawback) of hems is that they’re expensive (they usually take three bend operations to form) and add weight to your part.

Types of Hems & their design Constraints

Generally a hem has a round feature and a return flange. Based upon the nature of these two there are various types of hems.

• Open Hem

As said earlier a Hem has a full round feature and a return flange. In the Open type Hem, bend angle is equal to 180 degrees. It is recommended that the ratio of the open hem radius to the sheet metal thickness should be greater than or equal 0.5. Also, the ratio of return flange height to the sheet metal thickness should be greater than or equal to 4. For open hems, the minimum inside diameter is equal to the material thickness (larger diameters tend to lose circular shape), and the return length is at least 4 times the material’s thickness.

Where as,

H = Return flange height

D = Open hem diameter

T = Sheet metal thickness

Rolled Hem

In the case of Rolled Hems there is no return flange available. Here the only design criterias are available for bend radius of hems. It is recommended that the ratio of the external radius of a rolled hem [R] to material thickness[t] should be greater than or equal to 2. Also, the ratio of the rolled hem opening [D] to material thickness [t] should be greater than or equal to 1.

Where as,

R = Rolled hem's external radius

D = Rolled hem's opening

Tear Drop Hem

In tear drop hem the bend angle is greater than 180 degrees. It is recommended to consider the following guidelines while designing a tear drop hem -

i) The ratio of the radius of a tear drop hem to the sheet metal thickness should be greater than or equal to 0.5
ii) The ratio of the return flange height to the sheet metal thickness should be greater than or equal to 4.0
iii) The ratio of the hem opening (spacing between the hem edge and the part) to the sheet metal thickness should be greater than or equal to 0.25

Where as,

d = Diameter of a tear drop hem.

H = Return flange height of the tear drop hem.

D= Hem Opening

T = Sheet Metal Thickness

Closed Hem

Hems without any inside radius are called Closed Hem. Closed hems are not recommended if the hem geometry is to be painted or if the material used is SST or Aluminum. In closed hems, return flange length from outside the bend should be equal to or greater than four times the material thickness. Generally Closed hems tend to fracture at the bend and cause entrapment of solutions during the finishing process. So one should try to avoid using those in their design until its design requirement.

Where as,

H = Return flange height of the tear drop hem

T = Sheet Metal Thickness

Generally hemming is performed in sheet metals to avoid the Knife and sharp edges.And in all types of hems the minimum distance from a hole to a hem is two times the material thickness plus the radius of the hem.

The hem is created by first creating a perpendicular flange. The flange is then bent beyond 90 degrees in pre-hemming process. Finally the hem is created by pressing the flange back on the sheet completely. Hems can be open type or closed type. To lock two sheets together, generally open flanges are used.

2.Emboss Process

Embossing is a metal forming process for producing raised and sunken design or relief in sheet material by means of matched male and female roller dies, theoretically with no change in metal thickness, or bypassing sheet or a strip of metal between rolls of the desired pattern. Embossing is generally used to increase the stiffness of the sheet metal part and to transfer the shock to towards the hinges using Embosses.

3.Deep Drawing

Deep Drawing is a forming process that occurs under a combination of both tensile and compressive conditions. A flat sheet metal blank is formed into a hollow body open on one side or a hollow body with smaller cross section.

Deep Drawing Processes are divided into three types:

• Deep drawing with tools
• Deep drawing with active means
• Deep drawing with active energy

HOOD ASSEMBLY IMAGES

CONCLUSION

Thus the hood was designed in NX by following the desgin considerations required and mentioned by the euro NCAP. The main aim was to improve the pesdestrain safety rating by reducing the risk of injuries.