Hood design-Week 2

Automotive Hood Design – Detailed Report

1. Introduction to Hood Design

The automotive hood, also referred to as the bonnet (in some regions), serves as a protective cover for the engine compartment. It plays a crucial role in vehicle aerodynamics, pedestrian safety, and overall structural integrity. From a design perspective, the hood must balance multiple factors such as styling, safety standards, manufacturability, and weight.

A well-engineered hood contributes to:

• Engine compartment accessibility

• Crash safety (both for passengers and pedestrians)

• NVH (Noise, Vibration, and Harshness) performance

• Aerodynamic efficiency

• Thermal management

2. Hood Components Overview

The typical hood structure is composed of the following primary elements:

• Outer Panel: This is the visible surface, designed mainly for aesthetic appeal and aerodynamic function. It is usually manufactured from sheet metal or aluminum for lightweighting.

• Inner Panel: Acts as a reinforcement structure, providing stiffness and support to the outer skin. It includes features like embosses and mastic channels.

• Latch and Striker: Mechanism used to secure the hood to the body of the vehicle. The striker is mounted on the hood while the latch assembly is positioned on the body side.

• Hinges: Connect the hood to the vehicle body, allowing for smooth opening and closing.

• Mastic Patches: Adhesive materials applied between the inner and outer panels to improve local stiffness and reduce NVH.

3. Latch Trajectory Design

The latch trajectory defines the movement path of the hood latch when opening or closing the hood. It must ensure proper alignment and engagement of the latch and striker without any interference.

Key considerations:

• The trajectory should follow a natural arc based on hinge geometry.

• Simulations are often run to ensure the striker enters the latch correctly during closure.

• Misalignment can lead to latch malfunction or increased wear over time.

• The latch position is strategically chosen to minimize loading on the hood during frontal impacts.

4. Mastic Sealant Application (Mastic Data)

Mastic data refers to the application pattern, quantity, and placement of mastic material used between the outer and inner panels.

Objectives:

• Dampen vibrations between the two panels.

• Enhance structural stiffness, especially in large flat areas.

• Maintain panel gap consistency and reduce drumming noise.

Design Considerations:

• The placement is optimized using modal and stiffness analysis.

• Mastic dots are typically 25–30 mm in diameter and spaced 150–200 mm apart.

• Overuse can lead to excessive weight; underuse may reduce effectiveness.

5. Emboss Feature Definition

Embossing involves adding localized forms or indentations on the inner panel, which improves the hood's stiffness and structural integrity.

Benefits:

• Reduces panel vibration and oil canning.

• Directs deformation during crash impact.

• Supports functional elements like hinges, latches, and bump stops.

Design Guidelines:

• Emboss depth: Typically 2–4 mm.

• Shapes: Circular, trapezoidal, or elongated depending on location and function.

• Positioned using FEA to ensure proper load distribution and to avoid stress concentration.

6. Manufacturing Processes

a. Deep Drawing

A core forming process for hood outer and inner panels, deep drawing transforms flat sheet metal into a contoured shape using a die and punch.

Parameters:

• Blank size and shape are determined through simulation to minimize wrinkling and thinning.

• Draw depth and punch radius are controlled to avoid material failure.

• Lubrication and draw beads are used to control material flow.

Challenges:

• Risk of wrinkling, tearing, or springback if parameters aren't optimized.

• Material choice (like aluminum) adds complexity due to lower formability compared to steel.

b. Hemming

Hemming is the process of folding the outer panel over the inner panel, typically along the edges, to form a tight and secure assembly.

Steps:

• Pre-hemming: The panel edge is bent partially.

• Final hemming: The edge is fully folded and compressed.

Tools Used:

• Roll hemmers or press hemming dies.

Critical Points:

• Consistent hemming ensures panel fit and aesthetic quality.

• Hemming radius is usually 0.8–1.5 mm depending on material thickness.

• Adhesives may be added in the hem for structural and anti-corrosion purposes.

7. Design Parameters and Standards Followed

Key parameters considered during hood design include:

• Pedestrian Safety: Space between the hood and rigid engine components is maintained (typically 65–85 mm) to allow deformation during impact.

• Gap & Flushness: Gaps between hood and adjacent panels (fenders, bumper) are maintained between 3–5 mm with consistent flushness for aesthetics.

• Weight Targets: Hood assembly should meet weight reduction goals (often <10 kg for aluminum hoods).

• Corrosion Resistance: Coating and sealing processes are incorporated to enhance longevity.

• Crashworthiness: Emboss direction and hood deformation paths are optimized for crash performance.