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      MSE 5090: Case Studies in Material Selection

      Week 5 - Performance Indices

      Read this entire document before starting the assignments.
       
      1. Objectives
      2. A Look Ahead
      3. Reading assignment
      4. Discussion questions
      5. Week 5 assignments:  Ashby ProblemsCS Indices  CS Background
      6. Reference Material


      Objectives:
      Determine performance indices for materials selection without shape
      Use the CMS software to screen materials for selection of materials without shape
      Use the tables in the text to develop performance indices
      Use the CMS software to screen or select between processing options
      This week we will be studying how to develop the performance indices that were given in the examples from last week. In the examples from last week the performance indices were given. This week we will be deriving them. This week's homework includes two stage selections and performance indices both  without shape and  also indices taking shape into account using "shape factors". Reading both the ASM text page 281-290 and the Ashby book provides complementary treatments of performance indices. In essence, figure one on page 281 summarizes the way performance indices are derived. There are three main steps:
      1.  Identify the function(s) the material or component must serve (what does the component do?);
      2. Write down an equation for the function to be maximized or minimized (weight , cost etc. thus , what must be maximized or minimized in this application);
      3.  Identify the constraints (called design requirements) on the materials properties such as stiffness, pressure , temperature etc. (what is required, i.e. non-negotiable and what is desirable, i.e. negotiable in the application);
      4. Based on (2) and (3) , write the performance indices. Usually a  common variable is eliminated to reduce the dimensionality of the problem, but this will be discussed in class this week.


      A Look Ahead:
      This case is taken from the notes that accompanied the Cambridge Materials selector and discusses selection of process. We will  discussing process selection briefly this week. However,  we will be mainly discussing materials selection without shape this week. The purpose of the example  is to get everyone started on how to use the CMS software for materials selection  and  process selection. It is included as a look ahead for those students including process selection in their case study as well as a benefit to all by providing a first look at the other application of CMS. We will be discussing process selection in two weeks. Also, we will be delaying the study of the Aerospace Corporation case study until we get far enough into CMS, namely about two-to-three weeks from  now.  Based on the experience of earlier courses, it  seems better to  push a little harder earlier on  rather than have an end term crunch.
       

       Case study 7: Hair Brush Handle
       A hair brush handle is shown in Figure 8.1. It is made of nylon - a thermoplastic -and weighs about 200 gm. A large batch size (100,000) is planned, and the product must be cheap. The surface of the handle is very smooth to make it appealing to buyers, and this must be achieved without resorting to secondary machining or polishing operations. The minimum section thickness is about 2 mm. What process could he used to make it?
       
      Figure 8.1 A Hair Brush Handle

      Table 8.1 Hair Brush Handle: design requirements
      Material Class: nylon (thermoplastic polymer)
      Process Class: primary, discrete
      Shape Class: 3D-solid
      Mass:  0.1-0.2 kg
      Minimum Section:  2 mm
      Surface Finish: 0.5 pm
      Batch Size: 100,000

      The Selection

      The selection has three stages. Figure 8.2 shows the first: a chart of mass range against material class. A selection box for the brush handle is shown on it. It identifies the processes which satisfy the first set of design requirements. Only processes which can shape thermoplastic polymers are plotted. Almost all of them are capable of the desired mass range.
       
      Figure 8.2 A chart of mass range against material class. Processes capable of forming thermoplastic polymers
      are plotted. The box specifies the ones which can handle a mass range of 0.1 -0.2 kg.
      Material Class

      Figure 8.3 shows the second selection stage: a bar-chart of section thickness against process class, selecting "primary" from the process class menu. The selection box spans the thickness range 1.5 - 2.5 mm. Again, many processes pass.
       
      Figure 8.3 A chart of section thickness range against process class. The box isolates processes which can produce section thicknesses in the range 1.5 2.5 mm.
      Process Class

      A third stage is required as shown in Figure 8.4: a chart of surface roughness against shape class. Only processes which can handle "3D-solid" shapes (selected on the x-axis) are plotted. The selection box specifies processes which can produce a surface finishof 0.5 mm or better. Such a demanding requirement causes many processes to fail thisstage. Some polymer processes pass - however, except for injection molding of thermoplastics, the others are for thermosets and therefore have previously failed the material stage.
       
      Figure 8.4 A chars of surface roughness against shape class. The box isolates processes which can shape 3D-solid shapes and produce surfaces of high smoothness.
      Shape Class

      Consideration of the three selection stages reveals that only one process - injection molding- has passed them all, as indicated in Table 8.2.

      Table 8.2: Processes for the brush handle

       
      Injection Molding - thermoplastics
      Conclusions and Postscript:

      Injection molding emerges as the only process which satisfies all the design requirements. Because of the high tooling cost associated with injection molding, the process is only economical for large batch sizes (Figure 8.5). The large number of handles required means that the cost will be amortized over a large number of components and, therefore, the cost of one handle can be kept low. In practice, brush handles are made commercially using injection molding.
      Figure 8.5 Economic batch size against process class. The selection box specifies the desired batch size of 100,000.
      The option of relaxing the surface roughness requirement (the main discriminating feature) and forming a process chain would not be attractive in this particular case, since net-shape processes would produce the handle more cheaply..



      Reading Assignment:
       
       
      ASM pages 281 - 295
      Ashby pages 56 - 69


      Discussion Questions:
      How can you determine performance indices for different problems where shape isn't an issue?
      How does process considerations enter into the selection process?
      How can  sequential selection criteria be adjusted to provide more or fewer materials


      Week 5 Assignments:

      Ashby problems B 3.1, B 3.3, B 3.6, and B 3.9 (Due 10/7/98)
      Subject: Week 5 - Ashby B 3.1, 3.3, 3.6 & 3.9

      Outline Case Study Functional Requirements and Indices  (Due 10/16/98)
      Subject: Week 5 - Case Study Functional Requirements and Indices
      The Functional Requirements and Indices section of the report is the third element in the case study. As discussed in class, the elements of the case study , as you will assemble them over the course of the quarter are:
       

      1.          Case Introduction- This was handed in earlier. What is the case , what is your position, what are the stakes in this case-- submitted 9/18;
      2.          Case Background- Historical background plus theoretical background. Identification of the  materials and processing parameters and identification of the non-materials and process variables that will factor into your ultimate selection decision-- due 10/2;
      3.          Case Study Functional Requirements and Indices- A qualitative mechanistic model of the component or product that is the subject of your case study. Thus , for example , if you identified modulus and strength as two parameters in 2, then  you would , for this part , determine the quantitative functional requirements for various parameters such as, as an example,  E2 and (strength)1/2 or whatever functional requirement the model dictates. Thus for this part of the study you will be expected to to identify the parameters to optimize and the functions of the materials properties that will be needed in the optimization.--due 10/16/98
      4.          Selection Criteria, Constraints , and Enumerations of performance. In this part of the case study you will perform an Ashby- type analysis of the functional requirements and optimized performance indices for the  materials and processes involved.--due 11/2/98
      5.           Selection of Material and Process- incorporate all the non-material and non-process criteria such as cost , time , appearance, environmental impact, technical risk , reliability , maintainability etc. Set these up using a spread sheet format.--due 11/13/98
      6.           Combine technical and non-technical critical issues into a selection using weightings based on the literature and what industrial contacts indicate is important.--due 11/25/98
      7.           Compare the selection the student made with that made in industry , where applicable. In essence , did your selection agree with what the industry selected , or not. If not , was the selection in practice a success or not. If not, what is your analysis of the causes of failure and what lessons were learnt.--due 12/4/98
      8.          Submit case study.--due 12/14/98


      Presentation of  Case Study Detailed Background (Class period 10/2/98)



      Reference Material:
      This is a link to the resources page of the creators of the CSM software
      http://www.granta.co.uk/resources.links.html

      This page gives background and an introduction to process selection software, the other part of the materials selection process. You would be well advised to bookmark this page for use when we get to the processing part of the course.
      http://www-materials.eng.cam.ac.uk/research/process_selection/
       
       
       

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      Last update 8-14-98