Basics Flashcards
Define load, deflection, stiffness, torsional stiffness and compliance
Load - force that acts in a structure
Deflection - response of a structure due to a load (deformation)
Stiffness = load/deflection
Torsional stiffness = torque/angular deflection
Compliance - a structure with low stiffness
Explain poisons effect
As you pull a material it will extend in length and decrease in cross-area
When you compact a material it will decrease in length but increase in cross-area
AoLo = A1L1 (as volume is constant)
How is poisons effect removed?
Use stress and strain instead
Describe shear, torsion and torque
Stress acting in opposite directions can cause shear
Shear modulus = shear stress/shear strain
Torque - force acting away from centre, T = Fr
Torsion - twisting force on material (usually tube)
Draw cantilever bending point loaded and cantilever bending uniformally loaded
Point Loaded - One end is fixed, other has load applied
Uniformly - fixed at both ends, load through out = bending in middle
Define the area moment of inertia
Resistance to deflection from a shape
Rod - Ix = Iy = 0.25πr^4
Tube - Ix = Iy = 0.25π(r2^4-r1^4)
Column - Ix = Iy = (b.h^3)/12
How do asymmetric shapes affects inertia?
Stiffer in wider axis = higher inertia
Describe a tensile test (inc measurements needed)
Measure gauge length, and area of specimen, put on 50mm gauge markers, clamp one end into grip - move machine to grip other end in
Constant load increase - measure load vs extension then plot a stress vs strain graph - will be for engineering stress and strain (assumes area and length constant) as necking occurs
How do you reliably find Ym of a material?
Strain gauge - when conductors are strained their resistance increases (can be used to give strain), only five measurements for localised strain (2mm)
Extensometers - measure accurate extension of the material and general strain
Why is a tensile test not an accurate way of measuring the Young’s modulus?
Machine elastically deforms as well so not reliable values, must use extensometer or strain gauge
What values can be calculated from a tensile test?
UTS - highest stress material can withstand
Yield point - when plastic deformation begins
Young’s modulus - gradient of elastic deformation (only estimate)
Elongation to failure = Δx/x - how long material extends before failure
What’s the difference between 0.2% proof stress and yield stress?
0.2% proof stress is used when yield point is not obvious, and is found using a tangent at 0.2% strain on stress vs strain graph
Define torsional stiffness
Torque per angular displacement
Define poisons ratio and it’s significance
Poisons ratio = -Δεx/Δεy
Where x is strain normal to stress axis, y is strain in stress axis
A poison ratio of 0.5 will conserve volume under load (metals have a ratio of roughly 0.3)
Define the bulk modulus
The compressibility of a material
K = Young’s modulus/3 - 6.poisons ratio
What happens to the modulus of metallic alloys in impacts?
Modulus increases with strain rates - quick strain rates = higher modulus
What are the error sources for a tensile test?
Misshapen specimens Incorrect alignment (shear occurs) Poor surface finish (early onset necking) Poor gripping Internal defects (cause weaknesses)
How do you measure the Young’s modulus of brittle materials?
Can’t use tensile tests as grips damage material
Would use 3/4 point bending tests (more accurate)
Test gives load vs extension which can be converted to stress vs strain graph
Why is hardness testing done?
Most testing is destructive (can’t use material again) but some components need testing before they go into service - hardness testing is used
Describe the 3 types of hardness testing
Brunel test - small spherical indenter is pressed into material, indent size = Brunel hardness
Rockwell scale - pyramid diamond used instead as less damage to material
Vickers teat - same process as Rockwell but Hv = 1.854load/mean length^2
