Presentation on theme: "Chapter 5 – Design for Different Types of Loading"— Presentation transcript:
1 Chapter 5 – Design for Different Types of Loading Part 1 – Types of stress and loading, stress ratio, endurance strength, design factorsPart 2 – Failure theoriesE. R. Evans, Jr./ R. MichaelMET 210W
2 Static Load Stress Stress Ratio, R = 1.0 F and P are applied and remain constantStressStress Ratio, R = 1.0Time
3 Dynamic Stress:Loads that vary during normal service of the product produce dynamic stress.Dynamic stress can be cyclic or random.High cycle fatigue – part subject to millions of stress cycles.Examples: Parts subject to dynamic stress?
4 Cyclic loads produce cyclic stress which can lead to mechanical fatigue failure: Mechanical Fatigue = The progressive and localized structural damage that occurs when a material is subjected to cyclic loading. The cyclic stress is well below tensile, Su and yield, Sy , strengths!
5 Types of Cyclic Stress: Repeated and Reversed (i.e. RR Moore, rotating shafts, etc.) – mean stress = 0.Fluctuating stress (mean stress not zero):Tensile mean stress (can cycle between tension and compression or all tension)Compressive mean stress (can cycle between tension and compression or all compression)Repeated, one-direction stress
6 Definitions: = Alternating stress = Mean stress = R value: R = 0, repeated and one direction, i.e. stress cycles from 0 to max value.R =-1, Fully reversed (R-R Moore)
7 1.Repeated & Reversed Stress an element subjected to a repeated and alternating tensile and compressive stresses.Continuous total load reversal over timeDemo: Switch to Excel
8 1.Repeated and Reversed Stress The average or mean stress is zero.
9 All stresses above are repeated and reversed (R = -1) Cyclic loading. (a) Very low amplitude acoustic vibration. (b) High-cycle fatigue: cycling wellbelow general yield, sy. (c) Low cycle fatigue: cycling abovegeneral yield (but below the tensile strength sts).All stresses above are repeated and reversed (R = -1)
10 Fatigue Testing Bending tests R-R More = Spinning bending elements – most common.Fast, cost effective, pure bending stressSee:
11 Fatigue Testing Bending tests Sontag = Constant stress cantilever beamsGood for flat stock (sheets)Get shear stress in addition to bending stress.Top ViewSpecimen
12 Fatigue Testing Test Data Stress, s (ksi) Number of Cycles to Failure, NData from R. B. Englund, 2/5/93
13 Endurance Strength = 0.50(Su) Endurance strength is the stress level that a material can survive for a given number of load cycles.Endurance limit is the stress level that a material can survive for an infinite number of load cycles.Estimate for Wrought Steel:Endurance Strength = 0.50(Su)Most nonferrous metals (aluminum) do not have an endurance limit.
14 Representative Endurance Strengths Estimated endurance strength of steel is about 0.50 * Su
24 Example of the Effect of Stress Ratio on Endurance Strength of a Material Reversed is the worst case and is the case used to determine the endurance strength of materials (RR Moore, etc)Fluctuating stress has less effect on endurance strength than reversedMott, Fig. 5-11, Pg. 180
25 Stages of Fatigue:Micro structural changes – nucleation of permanent damage (mm)Creation of microscopic cracks (mm)Growth and coalescence of cracks into dominant crack (striations).Stable crack growth (Beach marks)Instability and rapid failure (area goes down, stress goes up eventually exceeding tensile strength).
26 Stages of Fatigue:Micro structural changes – nucleation of permanent damage (mm)Creation of microscopic cracks (mm)These two steps = crack initiation = 99% of the total life!!!!!!!!!!!!!!!!!!!Key: prevent cracks from forming at surface!!!!!!!!!!
27 BInstantaneuos Fast Fracture!ACrack nucleation and Growth
28 fail in fatigue at stresses well below the tensile strength. The endurance limit plotted against the tensile strength. Almost all materialsfail in fatigue at stresses well below the tensile strength.
30 Factors Effecting Design Factor ApplicationEnvironmentLoadsTypes of StressesMaterialConfidence
31 Factors Effecting Design Factor ApplicationEnvironmentLoadsTypes of StressesMaterialConfidenceHow many will be produced?What manufacturing methods will be used?What are the consequences of failure?Danger to peopleCostSize and weight important?What is the life of the component?Justify design expense?
32 Factors Effecting Design Factor ApplicationEnvironmentLoadsTypes of StressesMaterialConfidenceTemperature range.Exposure to electrical voltage or current.Susceptible to corrosionIs noise control important?Is vibration control important?Will the component be protected?GuardHousing
33 Factors Effecting Design Factor ApplicationEnvironmentLoadsTypes of StressesMaterialConfidenceNature of the load considering all modes of operation:Startup, shutdown, normal operation, any foreseeable overloadsLoad characteristicStatic, repeated & reversed, fluctuating, shock or impactVariations of loads over time.MagnitudesMaximum, minimum, mean
34 Factors Effecting Design Factor ApplicationEnvironmentLoadsTypes of StressesMaterialConfidenceWhat kind of stress?Direct tension or compressionDirect shearBendingTorsional shearApplicationUniaxialBiaxialTriaxial
35 Factors Effecting Design Factor ApplicationEnvironmentLoadsTypes of StressesMaterialConfidenceMaterial propertiesUltimate strength, yield strength, endurance strength,DuctilityDuctile: %E 5%Brittle: %E < 5%Ductile materials are preferred for fatigue, shock or impact loads.
36 Factors Effecting Design Factor ApplicationEnvironmentLoadsTypes of StressesMaterialConfidenceReliability of data forLoadsMaterial propertiesStress calculationsHow good is manufacturing quality controlWill subsequent handling, use and environmental conditions affect the safety or life of the component?
37 Recommended Design Factors Confidence in material properties, analysis, loads, the environment, etc.See Mott, pages