Hood design-Week 2

Aim:

Hood design using the CATIA V5.
Objective:

1. To design and model the car hood including the inner and outer panels.
2. Create the embossment for the hinge and striker with a proper trajectory for hood movement.

Introduction:

The hood or the Bonnet consists of the following parts.

• Hood outer panel
• Hood inner panel
• Latch and Striker re-enforcement
• Hinge re-enforcement
• Gas stays 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 aluminum.

The Stainless steel bonnet uses a technique called deep drawing to manufacture the component. The Aluminum bonnet employs a procedure called casting.

The hood is designed in accordance with the Euro NCAP (European New Car Assessment Programme) standards. The Adult and Child impact zone is taken into consideration and calculated by plotting the regions from the model data.

The model, in this case, the outer panel is offset to generate the deformation space in the Engine compartment below the styling surface and thereby determine the interference.

Package components that penetrate the generated deformation are considered critical and need to be relocated or tuned to fill.

This relocation or tuning leads to the decrease in the impact force exerted on the pedestrian from the compartment during a collision( since there are no critical components under the hood in this case).

This is done to reduce the impact on the pedestrian's head in particular.

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 are shown in Figure 1.

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.

   Figure 1. 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

`alpha`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

As shown in Figure 2, 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.

Figure 2 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 with respect to 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

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 legform to bumper tests, upper leg form to hood leading-edge tests, and child/adult headform to 19 hoods/windshield tests. Multiple tests are conducted at different test zones at the bumper, hood leading edge, and headform contact areas.

The headform test area is defined based on the pedestrian wrap-around distance (WAD) as shown in Figure 12, in which child and adult headform 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 headform test zone, respectively, leading to a total of 36 points available in the pedestrian protection assessment. 

Procedure :

Inner hood :

The inner panel hood creation:

• Initially, the A surface or the skin is given as the base for creating the hood. So the hood is split into half using the plane as the splitting element since both sides will be identical.
• Now here for the inner panel, we will be following the master sketch given for its creation.

• The top surface for the inner panel is offset to 2mm below as shown in the sketch.
• Then the boundary is extracted and move along the surface to 20mm using the parallel curve.
• Now, these surface is extruded along Y direction followed of the fillet at the corner.
• Next, the extruded surface is used to split the horizontal surface.

• Again following the sketch another surface is offset and following the above process. The two surfaces are joined using the multi sections followed by fillet at the sharp edges.
• Just like that, all the procedure is followed mentioned above using the master sketch.

Embossment :

Embossment is created to provide additional stiffness to the inner panel also the design of embosses is created in such a way that energy dissipation in a collision is from front to the hinge and from there to the A-pillar. 

• 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 below

     Embosses are created in a way to avoid any sharp corners having smooth flow for the force to propagate towards the hinge.

     So wherever possible the fillets are applied at the corners in creating the embossment.

    Certain cutouts are made to bring down the overall weight of the hood panel also to strengthened the same.

Mastics and mastic sealants :

Mastics are provided in the inner panel to improve the strength locally around the 80mm diameter.

Since we are reducing the overall weight of the hood, this mastic providing the strength also serves the purpose of joining the inner and outer panels using the mastic sealants.

Generally, spot welding is not preferred in joining the outer and inner panels as it may hamper the aesthetic look of the hood. So mastic points are included in the inner panel to maintain the overall look of the hood.  

The mastic sealants help in reducing the NVH(noise, vibration, and harshness )levels of the hood.

Mastic Sealant:

The Mastic sealant or the Mastic point is an elastic compound that behaves like 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. The mastic has an influence area of 80mm.

Hinges and striker :

Hinge is used to connect the hood to the BIW also enables the opening and closing of the hood.

So the embosses are created of the hinge along with reinforcements to provide additional support to the inner panel.

Both the hinges are mounted in line and are parallel to the X-axis 

Now, the latch and striker are fixed following the trajectory as follows 

• The midpoint between two hinges as a center 
• A midline between the striker surface.
• Drawing the circle by keeping the center and hinges midpoint and tangent to the midline of the striker.

Outer hood :

Hemming is a technology used by the automotive industry to join inner and outer closure panels together (hoods, doors, tailgates, etc.). It is the process of bending/folding the flange of the outer panel over the inner one.

The accuracy of the operation affects significantly the appearance of the car’s outer surfaces and is, therefore, a critical factor in the final quality of a finished vehicle.

Hemming is performed in order to:

• Achieve a better final geometry in the vehicle´s part
• Add resistance and rigidity to the part
• Allow safe handling of the part

There are various types of hemming operations:

• Conventional die hemming
• Roll hemming

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

 Roll hemming:

 Roll hemming is carried out incrementally 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.

Roll hemming is very flexible to use and tool costs are significantly lower as compared to those of conventional die hemming. However, the cycle times are much higher since the hemming is realized using a hemming roller that follows a defined path.

The outer hood is prepared by extrapolating the surface and then the offset surface is joined with the upper surface by using the multisection followed by providing the fillet.

After the hemming process is done, now we need to provide corner relief on the corners of the hood so that the part is manufactural

Conclusion:

The car hood consisting of the inner and outer panel were modeled using CATIA V5 and later hinge and striker were assembled with the proper trajectory for opening and closing.