DESIGN OF A CAR HOOD

HOOD DESIGN

Aim:

To design and develop hood for a passenger as per NCAP safety standards and design guidelines using NX cad.

Basic concepts

Plastic deformation:

It is the deformation beyond the elastic range of the material. Sheet metal operations are worked by plastic deformation because of the beneficial effects of the imparted mechanical properties by it. The necessary deformation can be achieved by the application of a large amount of mechanical force or by heating the metal and applying a small amount of force.

Hot-working & cold working

The division of hot working & cold working is on the basis of the amount of heating applied to the metal before applying the mechanical forces. Those processes, working above the recrystallization temperature are termed as hot working processes whereas those below are termed as cold working.

Sheet metal operations:

The sheet metal operations are cold working operation that manufactures low-cost parts with very high volumes and at a fast rate. The following sheet metal operations are used for the manufacturing of hood

Stamping:

Metal stamping is a cold-forming process that makes use of dies and stamping presses to transform sheet metal into different shapes. Pieces of flat sheet metal, typically referred to as blanks, are fed into a sheet metal stamping press that uses a tool and die surface to form the metal into a new shape.

Metal stamping also referred to as pressing, is a low-cost high-speed manufacturing process that can produce a high volume of identical metal components. Stamping operations are suitable for both short or long production runs, and be conducted with other metal forming operations, and may consist of one or more of a series of more specific processes or techniques, such as:

• Punching
• Blanking
• Embossing
• Coining
• Bending
• Flanging

Punching and blanking refer to the use of a die to cut the material into specific forms. In punching operations, a scrap piece of material is removed as the punch enters the die, effectively leaving a hole in the workpiece. Blanking, on the other hand, removes a workpiece from the primary material, making that removed component the desired workpiece or blank.

Embossing:

Embossing is a process for creating either a raised or recessed design in sheet metal, by pressing the raw blank against a die that contains the desired shape, or by passing the material blank through a roller die. These embossing processes are used in increasing the stiffness of the component.

Materials used:

Most of the materials used in the BIW applications are as follows:

Stainless steels:

The prime reason for using steel in the body structure of an automotive is its inherent capability to absorb impact energy in a crash situation. This, in combination with the good formability and joining capability, makes these materials often a first choice for the designer of the body-in-white (BIW) structure. High strength steel is a type of alloy steel which are more commonly used in the BIW structure. These high strength steels have very high yield strength and cost 50% more than mild steels.

Aluminium:

Aluminium sheets are used for BIW structures as it offers significant reduction in weight by about 50% percent of the total vehicle weight. The stamped body panels are spot welded to steel fenders, door and hood.

Defects in Stampings:

Wrinkles:

Wrinkles occur when the sheet metal stamping process produces compressive strains that “push” material together, causing the material to overlap each other in the worst case. The thicker material resists compressive forces more than the thinner material and hence thinner material are prone to wrinkling.

Splits/Tearing:

Splits occur when strains cause the material to thin beyond the material’s safe limits. Although sheet metal has work-hardening characteristics that increase the material’s formability as strains increase, there is a finite limit at which splitting will occur. This also depends on the direction of strains in the sheet metal product being formed. The beginning of a split may be observed as a localized yield (or necking) before a full split is opened up. This occurs because the material has yielded and then stretched past its ultimate tensile strength, and then gone a little further along the stress-strain curve.

Spring back:

A spring back defect may occur when an unexpected shape change takes place after forming or stamping is completed. The final part dimensional shape does not match the desired nominal shape and falls outside of required tolerances.

Spring back defects are caused by the elastic region of the given material’s stress-strain curve, whereby the material is strained but then relaxes according to its elastic characteristics. High strength materials typically exhibit severe spring back problems usually caused by a much smaller difference between Yield Strength and Tensile Strength, compared to mild or low strength steels.

What is a hood?

Hood is a body panel which covers the engine compartment of the vehicle. The main function of the hood is to provide access to the engine compartment during the service of vehicles. Most of the materials used are steel and aluminium alloy which are less in weight compared to the steel sheets. In modern race cars like F-1 carbon fibre body is used which reduce the mass much less more than aluminium.

Design input:

• Hood head zone for lock striker
• Hood stay
• Crashworthiness
• Important regulations
• Hood hinge and its function
• Gap & flushness requirement
• Joining operation – hemming, spot welding, glueing & riveting
• Section modulus

Hood head zone:

The first important design consideration for hood design is a hood head zone for lock striker. This parameter of head zone solely depends on the opening angle of hood which is calculated based on the average height of the people in the country to which the product has been made.

Hood Stay:

Hood stay is the kind of support rod that is used to rest the hood while in the opening condition during service of the vehicle. The length of the support rod mainly depends on the hood opening angle which is nothing but the hood head zone. The hood stay is of two types support rod & gas stay.

Crashworthiness:

Crashworthiness is the ability of a structure to protect its occupants during an impact. Crashworthiness is also highly dependent on how the materials, construction and design of the vehicle work together. Another important factor in crashworthiness is lightweight structure.

Kinetic Energy relation with crashworthiness.

Kinetic energy is the energy of motion. Objects that are moving, such as a roller coaster, have kinetic energy (KE). If a car crashes into a wall at 5 mph, it shouldn't do much damage to the car. But if it hits the wall at 40 mph, the car will most likely be totalled. Kinetic energy is similar to potential energy. The more the object weighs, and the faster it is moving, the more kinetic energy it has

KE = 1/2*m*v2

Where m is the mass and v is the velocity.

One of the interesting things about kinetic energy is that it increases with the velocity squared. This means that if a car is going twice as fast, it has four times the energy.

Input data for design:

Master section:

It is the predefined sketch which is used for the design of hood it is one of the design inputs that are considered in design of hood as shown

Modelled parts Exploded view:

MODELLING DETAILS

STRIKER TRAJECTORY:

The striker trajectory determines the position of striker hook when rotated and contacted with the latch at the bottom.

MASTIC DETAILS:

The mastic sealant or anti-flutter sealants are the materials which are used in the localised reinforcement of both the inner and outer panel of the hood. The anti-flutter products provide optimal isolation of sheet metal components to reduce vibrations. These sealants are also used for sealing of hemming portion of sheet metal. There are various types of mastic sealant available such as

• PVC
• ANTI-FLUTTER
• NON PVC

These mastic sealants have the tendency of expansion of about 0-1000%. Mastic sealants are used in all the types of sheet metal panel enclosures of a car. The below-shown images show the mastic sealant position.

Reason for consideration of section modulus in structural design:

As bending equation which represents the bending stresses acting on a beam when subjected to bending is given by the image shown. The practical example of an increase in stiffness between sheet metal without embossing & sheet metal with embossing is shown in the below image.