Lack of Requirements in Product Development - C104

 

Lack of Requirements in Product Development
Case study
Document Type	Case study
Document Identifier	104
Tags	Development to production Integrated product development Functional Requirements
Objective functions	Cost Maintain Rate Maintain Quality Maintain
MSTE workflow	Development
Prerequisites	Development Selecting Functional Requirements

Summary

This case study recounts a development project that started without fully defined requirements and demonstrates the effects and outcome of this. The process and procedures used in this case study are outlined, along with links to pages in the KPC for further reading.

The purpose of this project was to design and verify the performance of a composite material layup for a vehicle seat. The results from physical testing of the laminate were used in a finite element analysis to develop and analyze the layup of the part including the part thickness and material orientation. The requirements were not fully defined at the start of the project resulting in a need to perform additional testing and rerun the analysis. This led to schedule delays and cost overruns.

Read about the iterative process of developing a composite product and the processes surrounding it: Integrated Product Development.

Challenge

A project of this scope had a lot of competing requirements including weight and cost targets, as well as minimum safety factors. These requirements were established at the launch of the project and were considered during the design process. This included 11 static and 11 fatigue load cases, each of which needed to be analysed, reviewed, and compared to the requirements. Unfortunately, the requirements related to flammability were not defined at the start of the project. The flammability requirements weren’t defined until after the physical testing and finite element analysis were completed. It was determined that the existing materials did not meet the flammability requirements and new materials needed to be selected.

Approach

Test Panel. Credit: Alastair Komus.

Stress Results from Finite Element Analysis. Credit: Alastair Komus.

At the start of the project, a fibre and resin were selected based on the initially defined requirements, which did not include flammability considerations (read more about Selecting Functional Requirements and Material Selection). The materials were selected to meet performance, weight, and cost targets. Test panels were manufactured and sent for tensile, compressive, flexural, and shear testing to define the mechanical properties of the laminate. Read about different types of testing in Foundational method documents.

A finite element model of the seat and its supporting structure was created and meshed. The material properties that were determined from the physical testing were added to the model. An initial layup that included material thickness and orientation was defined.

The requirements included 11 static and 11 fatigue load cases. Each load was applied to the model along with appropriate constraints. The analysis was run, and the results reviewed and compared to the required safety factors and weight target. In areas where the safety factor was too low, the material thickness was increased. In areas where the safety factor was higher than required, the material thickness was decreased. Adjustments to the material orientation of each ply were also made depending on the critical stresses. Once the final layup was selected manufacturing drawings were created (read more about Composite Production Part Drawing).

At this point in the project, the need for flammability requirements was identified. These were not specified until after the manufacturing drawings were created. The original materials did not meet the new requirement resulting in the requirement to select an alternative material. The manufacturing process was already selected, and the tooling had been built so it was important that the new materials were compatible.

Two alternate resins and three alternate gel coats were identified as candidates (read more about Fire Retardant Resins/Additives). Test panels were created for each combination and tested for tensile, compressive, flexural, and shear properties. Laminates produced with one of the alternate resins had equivalent mechanical properties to the original laminate, however it was determined that the resin was not suitable for the selected manufacturing process. The laminate with the other alternate resin had mechanical properties that were approximately 20% lower than the laminate with the original resin, but this resin was suitable for the manufacturing process.

Due to the lower mechanical properties, the finite element model was updated and all load cases were rerun. The updated results identified two areas that no longer met the required safety factors. The layup was adjusted and the analysis rerun to verify the new layup met all the requirements. The manufacturing drawings were then updated to reflect the new layup.

Outcome

The new materials and updated layup met the required safety factors and weight target. However, the delay in defining the flammability requirement resulted in a cost increase of approximately 30% for testing and analysis. It also resulted in a schedule delay of six months. Fully defining all the requirements at the start of the project could have prevented these overruns.

Related pages

Page type	Links
Introduction to Composites Articles
Foundational Knowledge Articles
Foundational Knowledge Method Documents
Foundational Knowledge Worked Examples
Systems Knowledge Articles
Systems Knowledge Method Documents
Systems Knowledge Worked Examples
Systems Catalogue Articles
Systems Catalogue Objects – Material
Systems Catalogue Objects – Shape
Systems Catalogue Objects – Tooling and consumables
Systems Catalogue Objects – Equipment
Practice Documents	Composite Production Part Drawing - P169 Material Selection - P153 Practice for Developing a Deposition Step - P157 Practice for Developing a Product - P111 Practice for Identifying Process Steps - P102 Process selection for production scale-up - P171 Selecting Functional Requirements - P152.0 Shape Development - P151.0
Case Studies	Developing for Future Production Scale Up - C113 Lack of Requirements in Product Development - C104 Use of Right Sized Simulation to Aid Decision Making in Thermoplastic Composite Manufacturing Systems - C114
Perspectives Articles	CKN’s Approach to Developing Products with Composite Materials - A332

About	Help

Engineered materials (designed to have specific properties) made from two or more constituent materials with different physical or chemical properties. The constituents remain separate and distinct on a macroscopic level within the finished structure.

For polymer matrix composites (PMCs), resin refers to the matrix; the continuous material phase that binds the reinforcement together, maintains shape, and transfers load. Resins are divided into two main groups: thermosets and thermoplastics.