MEE401 — Mechanical (Machine) Engineering Design II: Lecture Note

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This is the complete lecture note for MEE401 Mechanical (Machine) Engineering Design II. You can read it online below, open it in the full viewer, or download the PDF for offline study. It pairs with the course's video lectures, so you can read and watch together.

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Course code: MEE 401 Course title: Mechanical (Machine) Engineering Design II Course credit load: 3 Units Series 1: Principles of Design, Material Selection, and Failure Analysis • Part 1.1: Principles of Mechanical Design o Sub Part 1.1.1: The Design Process o Sub Part 1.1.2: Design Criteria and Constraints o Sub Part 1.1.3: Factor of Safety and Design Codes • Part 1.2: Simple Stress Analysis in Design o Sub Part 1.2.1: Stress and Strain Revisited o Sub Part 1.2.2: Combined Stresses o Sub Part 1.2.3: Stress Concentration • Part 1.3: Material Selection for Design o Sub Part 1.3.1: Mechanical Properties Relevant to Selection o Sub Part 1.3.2: Material Selection Criteria and Trade-offs o Sub Part 1.3.3: Common Engineering Materials Compared • Part 1.4: Failure Analysis o Sub Part 1.4.1: Modes of Mechanical Failure o Sub Part 1.4.2: Fatigue Failure o Sub Part 1.4.3: Fracture and Failure Investigation • Part 1.5: Machine Elements, System Design, and Joints o Sub Part 1.5.1: Designing Individual Machine Elements o Sub Part 1.5.2: System-Level Design Thinking o Sub Part 1.5.3: Types of Mechanical Joints

Introduction Every machine element examined throughout this course, from the bearings of the next Series to the gears and fluid power components of those that follow, rests on the same foundation: a sound design process, a correctly chosen material, and a clear understanding of how and why such elements actually fail. This Series builds that foundation, revisiting design principles and stress analysis at a more advanced level before turning to material selection and failure analysis in detail. The aim of this Series is to equip you with proficiency in the principles of design, proficiency in the selection of materials for design, and the ability to carry out simple stress analysis. We shall commence this Series with the principles of mechanical design, before turning to simple stress analysis and material selection. We will then examine failure analysis, and conclude this Series by considering machine elements, system design, and joints. Series Learning Outcomes Upon completing this Series, you should be able to: • SLO 1.1: describe the stages of the mechanical design process, • SLO 1.2: explain the role of factor of safety and design codes in mechanical design, • SLO 1.3: apply simple stress analysis to a loaded machine component, • SLO 1.4: explain the concept of stress concentration, • SLO 1.5: describe the mechanical properties relevant to material selection, • SLO 1.6: compare common engineering materials for a given design application, • SLO 1.7: describe the common modes of mechanical failure, • SLO 1.8: explain the mechanism of fatigue failure, • SLO 1.9: distinguish between designing an individual machine element and designing a complete system, • SLO 1.10: describe the common types of mechanical joints used in machine design, 1.1 Principles of Mechanical Design 1.1.1 The Design Process

The mechanical design process is a structured sequence typically comprising problem definition, conceptual design, in which candidate solutions are generated, detailed design, in which components are sized and specified, and design review, in which the completed design is validated before release. This process is inherently iterative rather than strictly sequential, since a problem discovered during detailed design frequently sends the designer back to reconsider an earlier conceptual decision, rather than allowing the process to proceed smoothly from one stage to the next. Worked example: A designer discovering during detailed sizing that a shaft's diameter would need to be impractically large returns to the conceptual stage to consider a different arrangement of components. Application: This design process, revisited here at a more advanced level, underlies every specific machine element examined throughout the remainder of this course. Fill in the blank: A design process typically comprises problem definition, conceptual design, detailed design, and design ________. 1.1.2 Design Criteria and Constraints Design criteria are the specific, measurable requirements a design must satisfy, such as strength, weight, cost, and manufacturability, while constraints are fixed limitations a design must respect, such as available space, standard component sizes, or budget, that are not themselves optimised but simply must not be violated. Design criteria often conflict with one another, minimising weight while maximising strength being a common example, requiring the designer to make deliberate trade-offs rather than attempting to optimise every criterion simultaneously. Worked example: A designer aiming to minimise a bracket's weight while also maximising its strength must deliberately trade off between these two conflicting criteria rather than optimising both simultaneously. Application: Recognising this distinction between criteria and constraints, introduced here, clarifies which aspects of a design a designer actively optimises and which they must simply satisfy. Fill in the blank: A fixed limitation a design must respect, rather than optimise, is called a design ________. 1.1.3 Factor of Safety and Design Codes Factor of safety is the ratio between a component's failure load, or stress, and its actual expected service load, chosen specifically to account for uncertainty in loading, material properties, and manufacturing.

A design code is a published standard specifying required factors of safety, material properties, and design procedures for a specific type of component or application, ensuring designs across an entire industry meet a consistent, agreed minimum standard of safety. Worked example: A pressure vessel designed according to a recognised design code is required to use a specific minimum factor of safety appropriate to that application, rather than one chosen arbitrarily by the individual designer. Application: Factor of safety and design codes, introduced here, will be applied directly whenever a specific machine element is sized later in this course. Fill in the blank: A published standard specifying required factors of safety and design procedures for a specific application is called a design ________. Figure 1.1: The mechanical design process. Pilot questions 1.1 1. Describe the stages of the mechanical design process. (Linked to SLO 1.1) 2. Explain why the design process is inherently iterative. (Linked to SLO 1.1) 3. Distinguish between a design criterion and a design constraint. (Linked to SLO 1.2) 4. Explain the purpose of factor of safety in design. (Linked to SLO 1.2) 5. Describe the purpose of a design code. (Linked to SLO 1.2)

1.2 Simple Stress Analysis in Design 1.2.1 Stress and Strain Revisited Stress, force per unit area within a loaded component, and strain, the resulting deformation per unit length, are revisited here specifically in the context of sizing a real machine component, rather than as a purely theoretical concept explored for its own sake. Normal stress acts perpendicular to a surface, as in tension or compression, while shear stress acts parallel to a surface, and a real machine component frequently experiences both simultaneously under a realistic combination of loads. Worked example: A bolt loaded in tension experiences normal stress along its length, while the same bolt, if also loaded sideways, simultaneously experiences shear stress across its cross-section. Application: This basic stress and strain foundation, revisited here, underlies the combined stress analysis examined next in this Part. Fill in the blank: Stress acting perpendicular to a surface, such as in tension or compression, is called ________ stress. 1.2.2 Combined Stresses Combined stress describes the situation where a component experiences more than one type of stress simultaneously, bending combined with torsion in a shaft being a common example, requiring these individual stresses to be combined mathematically to determine the actual maximum stress the component experiences. This calculation produces a single combined, or equivalent, stress, representing the worst-case combination of the individual stresses present, which can then be compared directly against a material's allowable stress to judge whether a proposed design is actually safe. Worked example: A rotating shaft transmitting power experiences bending and torsion simultaneously, and these must be combined mathematically to determine the shaft's actual maximum stress before it can be safely sized. Application: Combined stress analysis, introduced here, is essential whenever a real machine component, such as the shafts examined later in this course, experiences more than one type of loading at once. Fill in the blank: A single calculated value representing the worst-case combination of individual stresses present is called a combined, or equivalent, ________.

1.2.3 Stress Concentration Stress concentration is a localised increase in stress that occurs near a geometric discontinuity such as a hole, groove, or sharp corner, where the actual stress rises well above the average value calculated for the surrounding cross-section. A stress concentration factor is a numerical multiplier applied to the average calculated stress to estimate the actual peak stress at such a discontinuity, with sharper discontinuities generally producing higher stress concentration factors. Worked example: A small hole drilled through an otherwise uniformly loaded plate can locally raise the stress near the hole's edge to roughly three times the plate's average stress elsewhere. Application: Understanding stress concentration, introduced here, explains why a sharp internal corner is so often the specific location where a real machine component eventually fails. Fill in the blank: A localised increase in stress near a geometric discontinuity such as a hole or sharp corner is called stress ________. Figure 1.2: Stress concentration around a hole in a loaded plate. Pilot

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