A wide range of components failure in all type of engineering applications is due to the poor selection of proper materials to withstand applied forces, heat, electricity, etc. The number of engineering materials is vast, more than 150,000. In a competitive industry, it would be quite vital to employ a process that prevents possible failure that can occur right at the early stage of the design. The question is how should the engineer choose the best material for a specific application from this vast menu?

So, when engineers choose a material for an application, the main question that they should be able to answer is why that material and not any other one?

Mechanical parts have mass; they carry loads; they are exposed to wear and to corrosive environments; they conduct or isolate heat, they have shape, and they must be manufactured by different processes. However, the achievement of satisfactory properties in chosen materials is only one part of the design task. It is necessary also that they be achieved at acceptable cost.

Therefore, cost should also be incorporated into property parameters to facilitate comparison.

There is a systematic approach to materials selection that will allow the selection of the best cost-effective material out of all available materials. This is the only way to avoid unexpected failures that may have catastrophic consequences.

The formalized approach to materials selection was developed several years ago at the Cambridge University by Professor Ashby. This method was adopted in 1995 by Dr. Parvin as a graduate course. He taught it first at the University of Southern California (USC) and later at several campuses of California State University as an elective course.  He has used this approach at various industries to select materials for a wide range of applications. 

Our well-established methodology would guarantee the selection of safest, lightest, and least expensive material for any application. Our failure analysis of the failed components would determine whether the failure can be prevented by only changing the material without any design change. 

Our consultants who mostly have Ph.D. degree have expertise and experience in materials, machine design, and stress analysis using hand calculations and Finite Elements Methods (FEM) at various industries.  They are capable of doing precise load, stress, thermal, and electrical analysis on a component used in the most complex assemblies and derive the selection indices for each and then combine them to select the best material that satisfy the selection indices using material data basis without the need for arbitrary judgment. 

Description of the Role of Materials Selection Methodology in Design

The need for clear recognition of the service requirements of a component or structure in order to provide the most technically advanced and economic means of meeting those requirements points to the benefits that can follow from better communication between design engineers on the one hand and materials engineers, analysts, and scientists on the other.

This is most effectively achieved by the inclusion of materials selection as an integral part of the design process. There are two important principles that should apply to materials selection in engineering:

1. Materials selection should be an integral part of the design process;
2. Materials selection should be systematic and numerate.

Our service is about safe design, and the role of the materials in it. Mechanical parts have mass; they carry loads; they are exposed to wear and to corrosive environments; they conduct or isolate heat, they have shape, and they must be manufactured by different processes. Often the choice of material is dictated by the design and all related aspects indicated above. But sometimes it is the other way around.

At the beginning the design is fluid and all materials must be considered. As the design process becomes more focused, the selection criteria results in a short list of materials. In the final stages of design, precise data would result in a few materials. The procedure must consider all the available materials but should be able to provide a systematic method to narrow it down to a small subset on which the design calculations can be based.

The achievement of satisfactory properties in chosen materials is only one part of the design task. It is necessary also that they be achieved at acceptable cost. Therefore, cost should also be incorporated into property parameters to facilitate comparison.

The steps taken to choose materials for a safe design includes:

 ·      define the functions of the parts within the assembly

·      Define the circumstances under which the component should perform

·      Perform analytical load, stress, fatigue, fracture, vibration, thermal, and electrical analysis to formulate the constraints

·      Develop materials selection indices for the lightest, lowest cost material that satisfy all the constraints

·      Use the computer software to explore data bases and rank the candidate materials based on the derived selection indices

·      Refine the selection for process and shape.

The following short case study is based on an actual design process exercised at Eaton Aerospace Division in Costa Mesa.

The aluminum drive shaft assembly used on CASA Butterfly valve failed during the sinusoidal linear frequency sweep from 10 to 250 Hz.

 

 

Failure Analysis:

To avoid the failure during the vibration test, the necessary condition is that the natural frequency of the part to be outside the frequency range (10 – 250 Hz). The resonant frequencies of the shaft was determined by calculation and finite elements analysis, indicated that natural frequency of the assembly falls within the sweep range that explains the failure of the shaft assembly.

Through a material selection process outlined below, a proper material was selected so that the tube would have a natural frequency over 250 Hz as well as being stiff and strong at the minimum mass.

Materials Selection Process:

Material selection criteria were developed for maximizing natural frequency, stiffness, and strength as well as minimizing the mass. As described in the following, the selection criteria require maximizing the following indices:

E/ρ                  for natural frequency

E^1/3/ρ               for stiffness

σ^1/2/ρ               for strength  

The materials selection software (Granta CES) and its comprehensive materials database were utilized to select the materials that satisfy the above requirements. It was found that 2.5” diameter shafts made of certain class of carbon composite materials would satisfy the above criteria. Carbon Fiber K63712 with a density of .06 lb/in and Elastic Modulus of 34 MPsi would yield a natural frequency over the 250 Hz for lengths 47”and less.

The shaft assembly made from Carbon Fiber K63712, passed the qualification tests.