CH1 - CH 1 Introduction Science explains what is...

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Shigley’s Mechanical Engineering Design, 8 th Ed. Class Notes by: Dr. Ala Hijazi Introduction Page 1 of 16 CH 1: Introduction Science explains what is; Engineering creates what never was. To find From Science Laws System, Input, Output Engineering System Input, Output, Laws Analysis Output System, Input, Laws Inverse Analysis Input System, Output, Laws The designed product must be : functional, safe, reliable, usable, competitive, manufacturable, and marketable. Codes and Standards Codes and standards are made to organize and unify the engineering work. Imagine; what if there was no standard for bolts? A code is a set of specifications for the analysis, design, manufacture, and construction of something. A standard is a set of specifications for parts, materials, or processes intended to achieve uniformity, efficiency and specific quality. Examples of organizations that established standards and design codes: AISI, AGMA, SAE, ASTM, ISO. Stress and Strength When designing a component, the designer need to make sure that the maximum stress is less than the strength of the material used for making that component. Stress ( σ ) depends on geometry and load . Strength ( S ) is a material property ; strength is the stress level at which something occurs (such as yield strength or ultimate strength). System or Componen t Input Output Laws of nature
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Shigley’s Mechanical Engineering Design, 8 th Ed. Class Notes by: Dr. Ala Hijazi Introduction Page 2 of 16 When designing mechanical components, the stress in the component should never reach the yield strength of the material Yielding is considered failure; why? According to AISC, the maximum allowable stress ( σ all , τ all ) is a reduced value of the yield strength: Tension: y all y S S 6 . 0 45 . 0 σ Shear: y all S 4 . 0 = τ Bending: y all y S S 75 . 0 6 . 0 σ Bearing: y all S 9 . 0 = σ Design Factor and Factor of Safety A design factor is used to: Account for uncertainty (material properties, load variability, validity of mathematical models, etc.). Ensure safety. By definition: Maximum load = Loss-of-function load / n d Example: The maximum load on a structure is known with an uncertainty of ±20%, and the load causing failure is known with an uncertainty of ±15%. Find the value of the design factor to account for the uncertainty. Solution : Critical situation when: Load is maximum & Loss-of-function load is minimum 708 . 0 2 . 1 85 . 0 = To make them equal, we take the inverse 41 . 1 708 . 0 1 = = d n 1 + 0.2 1 - 0.15
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Shigley’s Mechanical Engineering Design, 8 th Ed. Class Notes by: Dr. Ala Hijazi Introduction Page 3 of 16 Using the stress notation, the design factor is defined as: Stress Strength n d = However when the load is not linearly related to stress, the design factor can be defined as: 2 = Stress Strength n d for cylinders in contact 3 = Stress Strength n d for spheres in contact The factor of safety ( n s ) has the same definition as the design factor, but it is numerically different due to the rounding (usually up) caused by using standard sizes.
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