2. Forces at WorkForces at Work  External forces can cause internal forces in structures  Each type of internal force can cause particular types of damage to the structure

3. Shear (Sliding)Shear (Sliding)  Solid materials nearly always have microscopic cracks or weaknesses  When a solid material is compressed, cracks can enlarge or break apart  One section may shear (slide over another section along a weakness

4. Shear FailuresShear Failures Notice diagonal cracks in the brickwork

5. Shear FailuresShear Failures Wrist shear fractures

6. Shear FailuresShear Failures Shear fractures in a bamboo bridge

7. Shear FailuresShear Failures Shear faults in dolomite rock

8. Bending or BucklingBending or Buckling  If you put pressure on a metal can, the thin metal folds and the can buckles under the compressive force  All thin panels tend to bend and buckle when they are compressed  Compression forces cause materials to bend on the inside of the curve and pull and snap on the outside of the curve.

9. Buckling FailuresBuckling Failures Beam bending rather than breaking

10. Buckling FailuresBuckling Failures Bridge support column buckle after earthquake

11. Buckling FailuresBuckling Failures Ladder truck buckled ladder

12. Buckling FailuresBuckling Failures Foundation wall buckling from lateral force of soil

13. Torsion (Twisting)Torsion (Twisting)  Twisting forces can cause material failure  Brittle structures often shear when they are twisted (plastic cutlery)  Very flexible structures (rubber bands, hoses) will fold up and twist into tangles and knots  Although these structures are unbroken, they have lost their shape, which is a form of failure

14. Torsion FailuresTorsion Failures Twisting damage in a key lock mechanism

15. Torsion FailuresTorsion Failures Twisting damage to a car axle

16. Torsion FailuresTorsion Failures I think there might be some twisting going on!!

17. Torsion FailuresTorsion Failures Twisting damage in steel drive cables

19. Making Use of StressMaking Use of Stress  Snapping, twisting, buckling, bending and shearing: that is what materials do when they fail  These same “problems” can be put to good use  Bending in cross country skis is obviously a good thing  Here are some more….

20. Buckle Car bumpers are designed to buckle so they absorb the force of the impact and there is less chance of the occupants being seriously injured!

21. Shear Shear pins are designed to snap in half if a motor or propeller encounters too much force. The shear pin breaks and saves the motor from burning out.

22. Shear Shear dowels are put in a bridge expansion joint, so the two pieces don’t slide past each other at the expansion joint.

23. Twist Rope is always twisted because the twisted fibres are much stronger than long, straight fibres.