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Modes of Failure

Modes of Failure. solids held together by bonds between their atoms. these bonds can be compressed. or extended. 1/22. Modes of Failure. tension . compression. buckling. shear. bending. stress pattens may be complex but consist of only 3 basic states of stress

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Modes of Failure

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  1. Modes of Failure • solids held together by bonds between their atoms • these bonds can be compressed or extended 1/22

  2. Modes of Failure • tension • compression • buckling • shear • bending • stress pattens may be complex but consist of only 3 basic states of stress tension - compression - shear 2/22

  3. Tension • state of stress where material • tends to be pulled apart • cable with weight becomes longer under pull • lengthening depends on X-section, length & load • larger the diameter - smaller the elongation 3/22

  4. a steel column under compression shortens as much as a rod of same steel lengthens in tension under same stress Compression • state of stress in which particles • pushed against the others • column under load shortens • squashes • shortening of material 4/22

  5. Compression (cont.) • compression elements very common must channel loads to ground • can have no tension elements but must have compression elements • materials weak in tension often strong in compression • with modern materials of high compressive strength, e.g. steel can build columns much slimmer • but slenderness introduces new problem 5/22

  6. Buckling • as compressive load increases, reach value whereslender elementsinstead of shorteningbuckle& usually break • bucklingis a basic design factor for slender elements in compression • buckling occurs even if load perfectly central • buckling load depends on: material, length, shape of X-section, restraints at ends 6/22

  7. Shear • state of stress in which particles of material slide relative to each other • rivets tend to shear • a hole puncher uses shear to punch • out holes in paper • load on short cantilever tends to • shear beam from wall 7/22

  8. Bending • consider plank loaded as shown • plank ends move down • section between stones deflects up • curve is arc of circle • upper fibres lengthen • lower fibres shorten • middle fibres remain original length - Neutral Axis 8/22

  9. Bending (cont.) • concrete beam fails intension due tobending • may fail indiagonal tension due to shear due to bending 9/22

  10. Behaviour of Materials • stress • strain • elasticity - plasticity - brittleness • safety factors • selecting appropriate materials 10/22

  11. Modes Of Failure - Under Stress • tension • compression • buckling • shear • bending stress patterns complex but consist only of three basic states of stress tension - compression - shear 11/22

  12. elastic or plastic General Load-Deformation Properties Of Materials • application of load produces dimensional changes in a member • member undergoes change in size or shape or both • deformation may be reversible or irreversible 12/22

  13. Stress • internal forcesdeveloped within a structure due to action ofexternal forces • stress is force intensity - force per unit area 13/22

  14. Fe Fe Fe Stress = Force / Area f = F / A Fi = Fe X X Fe Fe Fe Stress (cont1.) • consider member in tension • stress is force intensity -force per unit area 14/22

  15. A load of 1 N on each square metre represents an average stress of 1 N/m2, or 1 Pa 1N 1N 1m 1N 1m 1N 1m 1m 1m 1m 1m Stress (cont2.) • stress is force per unit area 1 pascal = 1 newton per square metre 15/22

  16. 1 MPa = 106 N/m2 = 1 N/mm2 Stress (cont3.) • we use stress in megapascals (MPa) for most materials ( remember 1 m2 = 106 mm2) • we use stress in kilopascals (kPa) for floor loads and foundation pressures (loads distributed over an area) 16/22

  17. stress developed DOES NOT DEPEND ON MATERIAL OF MEMBER Stress (cont4.) • internal force not concentrated at single spot • distributed over entire cross-section • stress in a member depends only on force applied and cross-sectionf = F / A 17/22

  18. Strength of Members • strength depends on many factors • in tension, failure will occur by pulling apart at weakest location • weak spot (point of reduced X-section) determines capacity of whole member • f = F/A higher because of smaller A • if material can sustain stress member will carry load • as load increases stress increases eventually material fails (pulls apart) 18/22

  19. Strain • response to stress • have stress --> get strain • strain to do with change in size or shape ratio of change in size or shape of element to original size or shape 19/22

  20. DL increase in length strain = e = original length L STRAIN (cont.1) • for member subject to simple tensile force • dimensionless - millimetre / millimetre 20/22

  21. STRAIN (cont2.) • determined by: • taking member of known length • subjecting it to a known load • measuring elongation • except for rubber bands, strains very small usually not visible • more a material strains under load - more the structure deflects 21/22

  22. causes stress puts material under strain results in deformation STRESS & STRAIN SUMMARY force 22/22

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