Stress Analysis 2 Flashcards
How are torsional stresses caused?
twisting moments
What kind of fractures are caused by torsional stresses
spiral fractures
Why is it okay to use hollow bars and tubes when material/space saving is an issue?
the strength to weight ratio of a hollow bar is higher - as most of the stress is on the outer surface anyway. The ability to resist torsion of a hollow bar is the same as a solid bar of similar dimensions
Why do most fractures occur in the distal tibia?
smaller cross-sectional area and therefore lesser ability to resist torsion
How does muscle activity influence stress distribution?
e.g. tibial bending - calf muscles contract to spread the load and prevent fracture but inducing compression - bones are stronger in compression
In reciprocal gait, how does gluteus medius contraction affect load?
gluteus medius contraction eliminates the tensile force form the neck of femur and exerts a compressive force - thus allowing the neck of femur to withstand far greater force than it would have
What is a strain gauge?
a very thin metal foil which is located between two pieces of insulating film
it is cemented onto the surface of the material and as it changes shape, its resistance changes and this can be measured and its strain calculated
What is an advantage and a disadvantage of a strain gauge?
it can be applied to the structure under investigation
not practical when an investigation of a complete structure is needed
What is photoelasticity?
an experimental technique for stress analysis where polarised light is passed through a transparent material. the material becomes bifringent and a colourful interference pattern emerges. by close analysis of this, stresses can be worked out
What is the finite element method?
the use of complex mathematical models to work out stresses at different points in a material. This can be used for the whole material by splitting it up into ELEMENTS in which there are multiple NODES. Colours on the resulting graph represent different stresses
