Hood design-Week 2

BIW Design- Hood

 

Hood Outer and Inner panel designing using nx-cad

 

Objective

To design 3D model of Hood of a car from given Class-A surface to a part model considering the design aspects, latch and hinge mounting parameters of an existing system using NX-CAD software.

 

Abstract

Body-in-white (BiW) is the name given to a car body’s sheet when all its components—barring moving parts (e.g., hoods, fenders, etc.), trims (e.g., glass, seats, etc.), or chassis subassemblies—have been welded together. Usually, BiW constitutes about 27% of a car’s curb weight and remains the key determinant of how the car will perform. BiW can be made into two structures: the more common monocoque structure where all body members are carrying load with chassis in-built with BiW and are integrated with each other, and the body-on-frame structure where the frame is the main load-carrying member. Example of BiW in automobile and material used for manufacturing of following part shown in fig. 1

 

 

1.1 Hood

The hood/bonnet system is an access panel to the engine compartment to enable maintenance of power train, drive belts, battery, fluid levels and lamp units. It is fundamentally a reinforced skin panel with many safety and quality requirements. Reduce damage of pedestrian considering head impact, associated with low-speed crash with pedestrian. Protect the underlying parts and the motor from theft. Protect parts under the hood from environmental damage, like corrosion and water. Give the car better aerodynamics and a good look with styling.

 

 

The sub system hood consists of a hood, striker, opening system, latch and in some cases pedestrian safety system. The main functions for the hood are to protect the under-hood system and work for pedestrian safety. The system should allow inspection of the engine and service of the under-hood systems, for example filling fluids. Together with the latch system and opening system, the hood should fulfil the demands in ergonomics concerning opening-closing forces and opening geometry. It should be robust enough to not be damaged by normal daily usage of the car.

The hood is a design element and is the part of the body that borders to the front fenders, front lights, bumper and the plenum cover. The complete hood consists of an inner and an outer plate ( fig.2) joined together by hemming around the edges of the hood. Rubber based glue is used in the flange joint as glue between outer and inner hood and on points as well in the inner plate pattern as anti-flutter.

Today a certain pattern is used in order to fulfil the requirements for pedestrian safety and total stiffness in the hood. Front reinforcement (lock striker reinforcement) and hinge reinforcements (steel) are clinched to the inner plate. Spot welding is used in part assembly of front reinforcement.

 

1.2Types of Hood

 

 

1.3 Types of Support stay for Hood

A hood stay is an apparatus for an engine room of a car, which can safely keep an engine hood in an open state if an external force is abruptly generated due to vibration, wind & can facilitate an opening/closing operation of the engine hood. The angle of opening varies from country to country as per the average height people in the respective country. In India average height consider around 5.5 feet and in USA its around 6 feet (fig.5). From below figure it’s clear that hood opening angle for car sold in Japan car will be smaller than the car sold USA. So accordingly, the support stay design is considered for manufacturing.

2. Design Consideration for Hood:

As per the European Aluminium Association following parameters are considered while designing the hood and material selection parameter are shown in fig.6

 

2.1 Pedestrian Safety:

According to the requirements of the Australian New Car Assessment Program (ANCAP) for pedestrian protection, it needs to define zones of car hood for analysis, as shown in Figure 4. It can be noted that when the collision projection point locates between (Wrap Around Distance) WAD 1000 and WAD 1500, the head type will use the children head type. The adult head type will be used while the collision projection point locates between WAD 1700 and WAD 2100. In the collision projection point located at WAD 1000 when the children head type will be used (fig.7).

 

 

The Head Injury Criterion (HIC) indicates a measure of head injury arising from an impact, which is evaluated by the impactor in terms of the simulation of child head. HIC includes the effects of head acceleration and the duration of the acceleration. ANCAP (Fig. 8a) 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(fig.8b) 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 .

 

 

2.2 Vehicle Quality:

Material selection followed with its manufacturing feasibility giving optimum results to mechanical properties benchmarking is the key factor for vehicle quality. Hood material are manufactured generally using steel, aluminium alloy or composite material like fiberglass/carbon fiber. Various factor are considered like analysis of torsional stiffness, dent resistance, corrosion resistance, bending stiffness, noise attenuation etc.

 

2.3 Occupant safety:

The hood panel is designed such a way that, during crash the hood section deforms completely so that maximum impact of crash is absorbed by hood to get deform and divert collision force away from the cabin. This help to properly damp the force to avoid or reduce the risk of injuries and death.

The deformation is achieved by designing the hood in two piece that is outer panel and inner panel separate by mastic sealants. Thus the cavity between the panel and the blanking in inner panel act as weakest section to get deform while crashing. The impact of crash is diverted to hinges which are reinforcement plates

 

2.4 Styling:

Styling team design the outer skin or class A surface of the car hood with considering major factor that often influence the vehicle sales in the market. The outer panel is the main consideration here as it is the only component that is visible outside. The factors such as surface quality, tight radii on hemmed panel edges aesthetic design etc. are crucial factors. Other than styling, aerodynamic factors are important while designing the outer panel.

 

2.5 Manufacturing:

In manufacturing of car hood various sheet metal operation are carried out like profile cutting of sheet metal, blanking, piercing followed by deep drawing and hemming process. In manufacturing of inner panel embossed surface are achieved by deep drawing where minimum 7mm degree drat angle is maintained respective to the tool direction.

Hemming is widely used in the automotive manufacturing industry to assemble an outer closure panel and an inner closure panel by folding the flange of the outer panel over the edge of the inner panel. This process is performed by material suffered a plastic deformation. The neat and compact combination of these methods that do not show up to join the resistance welding. Sometimes used special adhesives to improve strength.

The purpose of Hemming projects is to fold and fold the outer sheet metal panels on the inner sheet metal panels in various parts, such as the vehicle's door, engine hood, trunk lid, usually composed of two sheet metal panels, and to join the two panels together( Fig.9).

 

 

 

 

3.1 Step followed:

 

1. Importing Class -A Surface of outer hood.
2. Surface design of Inner hood section.
3. Assembly alignment inner hood with of hinges.
4. Thickening of inner hood panel to 0.75mm.
5. Assembly of Stricker to inner hood embossed profile to position open and close trajectory with respect to hinge axis of rotation.
6. Design of reinforcement pad for hinges.
7. Surface design of outer hood section with hemming parameters.
8. Thickening outer hood panel to 0.75mm.
9. Checking of inner hood for draft analysis.
10. Calculating section modulus and yield moment of designed hood section.

 

We, first import the class- A surface received from styling team and offset it according to the dimension shown in the master section diagram. Various profiles cutting and drafted embossed surface are created followed by edge blend radius to achieve smooth section. After verifying all dimensional values thickness is applied to final sewed surface by 0.75mm  (Fig.15)

Embossed section between the panel is designed in such a way that during the crash maximum impact of force is diverted toward hinge side and minimize the impact force traveling toward cabin frame by reducing the injury caused to occupant inside car. Sufficient cut-outs are provided on surface in order to reduce the overall weight of hood and increase vacant space, so that maximum deformation is achieved to the hood body. Here in fig.16 red arrow represent the impact forces direction during frontal crash and its being diverted toward hinges mounting emboss.

 

3.2 Assembly alignment of Hinges and lock Stricker plate on inner hood with open close trajectory:

Axis of left and right side hinges are colinear and back face hinge is parallel to the coordinate plane of hood inner panel on embossed surface. The trajectory of lock Stricker plate is aligned in such a way that the it perfectly follow the 250mm radius tangentially maintaining 90 degree centre line with the axis of hinges during open and closing of hood, fig front view of assembly and fig. 17 shows the trajectory path during operation.

 

3.3 Hinge reinforcement pad:

Hinge reinforcement pad is assembled on the hood inner panel to increase the local torsional stiffness and providing provision for permenant reverting of hinges to the inner panel. Thickness of 1.5mm gauge sheetmetal used to stamp it according to the embosed cavity profile of hood inner panel hinges mounting area. These pad are assembled to inner panel via spot welding/laser welding technique for better surface finish (Fig.18).

 

 

3.4 Mastic data:

It is basically a Noise, vibration and hardness NVH parameter where mastics are provided inside the embossed surface between gap of outer panel and inner panel of hood to achieve best NVH value. Lower the value better the NVH rating. So, mastic sealant are filling adhesive material used to occupy vacant space between inner and outer panel to reduce the NVH value. One spot of mastic sealant cover and provide strength to around 80mm diameter. Here red circle in fig.19 represents surface area covered by mastic sealant inside hood body. Semicircle profile are provided on the edges of cut out to place the mastic sealant.

 

 

3.4 Design of Hood Outer Panel

Design of hood outer panel is next step as per the master section criteria, here also class A surface of outer panel is used to create the basic surface of outer panel conseidering the hemming flange details. Outer panel is considered with thickness of 0.75mm and hemming flange having 6mm width and clerance of 0.2mm for structural adhesive for joining outer panel and inner panel using hemming process. Necessary corner relief are provided on hemming flange to achieve smoother surface and wrinklfree edges during manufacturing.

 

 

 

 

 

3.6 Draft analysis of hood inner panel:

The Draft Analysis command enables us to detect if the part we drafted will be easily removed during deep drawing process along tool direction. This type of analysis is performed based on colour ranges identifying zones on the analysed element where the deviation from the draft direction at any point, corresponds to specified values. Green colour on the surface(fig.25) represents that part satisfying the minimum 7 degree of positive draft parameter throughout in tooling direction. If red or blue colour appear it represent that correction is required at specific section to achieve positive draft.

 

 

3.7 Calculating section modulus and yield moment of designed hood section:

Section modulus is the direct measure of the strength of the steel. Bending a steel section that has a larger section modulus than another will be stronger and harder to bend. Section modulus is a geometric property for a given cross-section used in the design of flexural members. In simple terms, the section modulus is the ratio of bending moment to bending stress for steel. If your steel has a high section modulus it will be harder to bend and can withstand a high moment without having high bending stress.

Yield moment defined as the moment at which the entire cross section has reached its yield stress. This is theoretically the maximum bending moment that the section can resist - when this point is reached a plastic hinge is formed and any load beyond this point will result in theoretically infinite plastic deformation

Section modulus, S = I/y

Where,

                S - Section modulus

 I - Area moment of inertia

 y - Distance between neutral axis and extreme end of the fibre

Yield moment, My = S × σy

where,

σy - Yield strength of material

 

 

Case I: Calculating the values for designed section:

 

 

Therefore,

 (MOI) I  = 2.764 X 106 mm⁴

                                        (distance)y  = 878.46 /2

               = 439.23 mm

So,

Section modulus,                       S= (2.764 X 10⁶ mm⁴  ) ÷ 439.23 mm

                                                         S= 6292.830 mm³

Yield moment, My = S × σy

where,

                σy - Yield strength of material – i) considering medium carbon Steel AISI 1045
                                                                            i.e. σy= 505 N/ mm²

1. ii) Considering Aluminium Al 6061

                  i.e. σy= 275 N/ mm²

So, yield moment,  

                                     My = 6292.830 x 505 = 3.177 x 10⁶ N-mm ( Steel AISI 1045)

                                     My = 6292.830 x 275 = 1.730 x 10⁶ N-mm ( Al 6061)

 

                                         

Case II : When the distance between outer panel and inner panel is increase by 5mm:

 

Therefore,

 (MOI) I  = 3.228 X 10⁶ mm⁴

                                        (distance) y = 878.46 /2

               = 439.23 mm

So, Section modulus,                  S= (3.228 X 10⁶ mm⁴  ) ÷ 439.23 mm

                                                          S= 7349.224 mm³

Yield moment, My = S × σy

where,

                σy - Yield strength of material – i) considering medium carbon Steel AISI 1045
                                                                            i.e. σy= 505 N/ mm²

1. ii) Considering Aluminium Al 6061

                  i.e. σy= 275 N/ mm²

 

So, yield moment, 

                                     My =7349.224  x 505 = 3.711 x 10⁶ N-mm ( Steel AISI 1045)

                                     My = 7349.224 x 275 = 2.021 x 10⁶ N-mm ( Al 6061)

Comparing the values of section modulus of case I and case II it is clear that increasing the distance between the outer panel and inner panel by 5mm, increase the section modulus by 14%, thus increased the strength.

In Case II yield moment of aluminium AL 6061is reduced by 45% compare to steel AISI1045, hence using aluminium as base material for manufacturing of hood panel reduce the overall weight hood and early deformation of section during frontal crash of car.

 

4. Conclusion:

 

• Thus, inner panel and outer panel are designed as per reference of class A surface.
• Inner panel is analysed for draft analysis and found ok
• Section modulus for different case is calculated and its found that increase 5mm gap between panel increase the section modulus by 14%.
• CAD design will be passed to CAE team for further kinematic analysis under various condition mimicking the actual crash impact tests.

 

5. References:

• Design – Case study: Bonnet and boot lid Version 2011 © European Aluminium Association (auto@eaa.be)
• EUROPEAN NEW CAR ASSESSMENT PROGRAMME (Euro NCAP) PEDESTRIAN TESTING PROTOCOL Version 8.4 November 2017
• Composite material in car hood-Johan Karlsson
• Skill lync reports on hood design
• Design and Analysis of a Car Bonnet by N. Bhaskar† and P. Rayudu
• Research paper Design of Outer Hood Panel of Local Compact SUV to Achieve Better Pedestrian Safety using Finite Element Modeling by Binyamin et al 2019 J. Phys.: Conf. Ser. 1150 012049

 

 

Project Report By-

Abhijith Alolickal

(Mechanical Engineer)

              Profile Link: https://www.linkedin.com/in/abhijithalolickal/