BASIC PRINCIPLES FOR
MATERIAL SELECTION
AND DESIGN
ผู้สอน รองศาสตราจารย์ ดร.ยุพาพร รักสกุลพิวัฒน์
From Textbook: Ashby, Michael F.. (2005).
Materials Selection in Mechanical Design (3rd
Edition). Elsevier.
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BASIC PRINCIPLES FOR
MATERIAL SELECTION
AND DESIGN
ผู้สอน รองศาสตราจารย์ ดร.ยุพาพร รักสกุลพิวัฒน์ From Textbook: Ashby, Michael F.. ( 2005 ). Materials Selection in Mechanical Design ( 3 rd Edition). Elsevier.
Materials Selection—The Basics
1. การคัดเลือกและการจัดลาดับ (screening and
ranking) วัสดุต่างๆเป็นรายการ (shortlist)
โดยทั่วไป Performance จะจ ากัดโดยสมบัติหลายอย่างรวมกัน ไม่ใช่ สมบัติอย่างเดียว ตัวอย่างเช่น
- วัสดุที่เหมะสมที่สุดสาหรับการออกแบบเป็น light stiff tie-rod ได้แก่ วัสดุที่มีค่า specific stiffness สูงที่สุด specific stiffness = E/ โดยที่ E = Young's modulus = density
- วัสดุที่เหมะสมที่สุดส าหรับการออกแบบเป็น spring โดย
s
f 2
/E สูงที่สุด
โดยที่ s
f
= the failure stress
- วัสดุที่เหมะสมที่สุดส าหรับการออกแบบเป็น best resist
thermal shock ได้แก่ วัสดุที่มีค่า s
f
/Ea สูงที่สุด
โดยที่ a = thermal coefficient of
expansion;
The selection strategy
Material attributes The Kingdom of Materials can be subdivided into families, classes, subclasses and members. Each member is characterized by a set of attrributes: its properties.
Material attributes
The selection strategy Selection involves seeking the best match between the property profiles of the materials in the universe and the property profile required by the design.
The selection strategy
You need a new car. To meet your needs it must be a mid-sized four- door family sedan with a gas engine delivering at least 150 horsepower—enough to tow your power boat. Given all of these, you wish it to cost as little to own and run as possible
The selection strategy
The requirements of four-door family sedan and gas power are simple constraints; a car must have these to be a candidate. The requirement of at least 150 hp places a lower limit but no upper limit on power it is a limit constraint; any car with 150 hp or more is acceptable.
The selection strategy
The wish for minimum cost of ownership is an objective, a criterion of excellence. The most desirable cars, from among those that meet the constraints, are those that minimize this objective.
The selection strategy
That is what the objective is for: It allows you to rank the surviving candidates by cost of ownership—those with the lowest values are ranked most highly. Rather than just choosing the one that is cheapest, it is better to keep the top three or four and seek further documentation, exploring their other features in depth (delivery time, size of trunk, comfort of seats, guarantee period, and so on) and weighing
Selecting materials involves seeking the best match between design requirementsand the properties of the materials that might be used to make the design. Figure 5 .4 shows the strategy of the last section applied to selecting materials for the protective visor of a safety helmet. On the left are requirements that the material must meet, expressed as constraints and objectives.