Principles of Design

Introduction to Elements of Machine Dynamics & Design

Some important definitions are:

• Design  ⇔  requirement converted into meaningful and functional plan.
• Engineering Design  ⇔  applying techniques and scientific principles on a design system.
• Machine Design  ⇔  creation of machine using scientific design.
• Machine  ⇔  Consist of machine elements which convert input into output (modifies force or motion).

Mechanical Properties of Metals

• Strength  ⇔  Ability of material to resist external loads without breaking.
• Stiffness  ⇔  Ability of material to resist deformation under stress.
• Elasticity  ⇔  Property of regain its original shape (steel is more elastic than rubber).
• Plasticity  ⇔  Property of material which regains deformation permanently (forging/stamping).
• Ductility  ⇔  Property of material to be drawn into wires under tensile force. Measured by Percentage Elongation or Reduction in area. If  %El > 20% ⇔ Ductile material. If %El < 5% ⇔ Brittle material.
• Brittleness  ⇔  Property of material to break with little permanent distortion.
• Malleability  ⇔  Ability of material to be drawn into sheets under compressive loads.
• Toughness  ⇔  Property of material to resist fracture under high impact loads.
• Machineability  ⇔  Ability of material to cut.
• Resilience  ⇔  Energy absorb per unit volume during elastic limit.
• Creep  ⇔  If constant stress is applied for long time, it will undergo slow and permanent deformation.
• Hardness  ⇔  Ability of metal to cut other material (resistance to wear, scratching, deformation, machineability).
• Fatigue  ⇔  When material is subjected to repeated stresses, it fails at stresses below the yield point stresses. This failure is called Fatigue (consider in designing shafts, connecting rods, springs, gears).
• Fatigue Life  ⇔  The number of applied repeated stress cycles a material can endure before failure.
• Endurance or Fatigue Limit  ⇔  Maximum stress a material can endure for an infinite number of stress cycles without breaking.
• Factor of Safety  ⇔  Ratio of maximum value to applied value (actual).

Stress-Life Diagram (S-N Curve)

Some important points are:

• S-N curve is used to find out fracture (or failure) point. 
• If material bears 1e6 - 1e7 cycles of loading  ⇔  material life become infinite and fatigue loading does not break material.
• For steel and iron (ferrous related material)  ⇔  have knee called Endurance Limit (Horizontal Line). 
• For non-ferrous metal and alloy  ⇔  no endurance limit, have finite life. Fatigue life = 0.5 x Normal Loading.
• Under fatigue loading  ⇔  Failure point vary, use S-N curve.
• Under normal loading  ⇔  Failure point remains same, use σ-∊ curve.

Types of Variable Loading or Stresses

1. Completely Reversed or Cyclic Stresses  ⇔  Stresses which vary from one value of compressive to the same value of tensile.
   
2. Repeated Stresses  ⇔  Stresses which vary from zero to a certain max. (or min.) value. From mean to minimum (or maximum) is called Variable Stress.
   
3. Alternative Stresses  ⇔  Stresses which vary from compressive value to tensile one (have different values). 
4. Fluctuating Stresses  ⇔  Other than cyclic, alternative and repeated stress, all comes under the fluctuating stress.
   

Stress Concentration

Irregularity or discontinuity in machine alters stress distribution are called Stress Raisers or Areas of Stress Concentration. 

• It occurs at holes, grooves, fillet, notches, keyways, splines, surface roughness, scratches.
• The point at which line becomes horizontal of S-N curve is called Endurance Limit.

Notch Sensitivity

It is defined as, "How much a material is sensitive to notch".

• If q = 0  ⇔  no sensitive to notches which means soft or ductile material.
• If q = 1  ⇔  full notch sensitive which means hard or brittle material.
  
  

Combined Steady and Variable Stress

It is the actual scenario. If normal and fatigue loading are applied simultaneously,to find fracture/failure point, we different methods.

1. Gerber Method

• It is used to correlate ultimate stress and endurance limit.
• It is used to find failure point of ductile material.

2. Goodman Method

• It is used to correlate ultimate stress and endurance limit.
• It is used to find failure point of both ductile and brittle material.

3. Soderberg Method

• Used to correlate yield strength and endurance limit
• Used when design is based on yield strength.

References:

• Material from Class Lectures + Book named Mechanical Engineering Design by Shigley (8th Edition) + my knowledge. 
• Pics and GIF from Google Images.  
• Videos from YouTube.