Mechanical Properties Flashcards
what are mechanical properties?
properties that involve a reaction to an applied load
what do mechanical properties do?
determine the range of usefulness of a material
establish the service life that can be expected from a material
List 6 mechanical properties
strength
ductility/malleability
hardness
elasticity
impact resistance
toughness
what is tension?
two pulling forces directly opposing each other that stretch out an object and try to pull it apart/elongate it.
what happens to molecules under tensile stress?
the molecules pull back, trying to stay together and keep from being ripped away
what is compression?
two pushing forces directly opposing each other which squeeze an object, trying to squash/compress it.
what happens to molecules under compressive stress?
the molecules push back, trying to stay apart and not get crushed
what is shear?
two misaligned pushing or pulling forces that are close together but do not directly oppose each other.
what happens to molecules under shear?
the molecules hold onto each other to resist being slid apart
what is bending?
when a moment or ‘turning force’ is applied to a structural member, making it deflect (sag or arch) by bending from its original position
what happens to molecules under bending?
the top shelf of molecules is in compression; therefore, the molecules get squeezed together, therefore the molecules push back trying to stay apart.
the bottom of the shelf is in tension; therefore, it gets stretched apart, therefore the molecules pull back trying to stay together.
when a beam is bending, what is it said to do?
‘smile’
what is a moment?
a ‘turning force’ caused by a force acting on an object at some distance from a fixed point
what is torsion?
created when a moment (turning force) is applied to a structural member or piece of material, making it deflect by rotating (twist)
what is a moment which causes twisting called?
a twisting/torsional moment
what is the moment of a force?
a measure of its tendency to cause a body to rotate about a specific point or axis
how can the magnitude of a moment be increased?
by increasing the force
by increasing the distance from the pivot point
what do levers use moments as?
‘force multipliers’
what are examples of levers?
spanner
tap head
door handle
wheelbarrow
claw hammer
shovel
what is a tensile test?
a fundamental test in materials mechanics
it is performed by pulling a sample of material apart until failure, while measuring the force and displacement
what are pascals?
the metric unit for pressure
what does strain describe?
how much deformation has occurred with an applied force
what does engineering stress describe?
the amount of force per unit area
what is on the y axis of a stress stain curve?
stress
what is on the x axis of a stress stain curve?
strain
what does tensile stress result in?
elongation
what does compressive stress result in?
shortening
what is cross-sectional area?
the area of a shape that you get if you make a cut that is perpendicular to an applied force
what is engineering strain?
the change in dimension as a result of an applied stress
what is change in length also known as?
displacement
what is the initial linear region of a stress strain curve called?
the elastic region
what type of deformation does the elastic region undergo?
elastic deformation
in the elastic region, what will happen when the force is removed?
the material will regain it’s original shape
what is the end of the linear/elastic region marked by?
the yield point
any stress larger than the yield stress will cause what?
permanent deformation
what is another name for permanent deformation?
plastic deformation
What is the UTS of a material?
the most stress it can handle without fracturing
After UTS, why is less stress needed to cause further deformation?
because the cross-sectional area of the material begins to thin in a process called necking.
What is Young’s modulus?
a value that measures a materials resistance to being deformed elastically.
how can Young’s modulus be found?
by finding the gradient of the elastic (linear) region of the stress strain curve
what does a steeper elastic/linear region mean?
that the material is stiffer
what can Youngs modulus be used to predict?
deflection
What does Yield Strength or UTS divided by the safety factor give?
max allowable load/stress
What is load?
the amount of force applied to an object
Engineers aim to keep the max possible stress below what?
the max allowable stress (failure)
what does tough mean?
that the material can absorb a lot of energy without breaking
what does ductile mean?
that the material deforms under pressure
Springs need to:
absorb and release energy without permanently deforming
what does brittle mean?
that a material breaks with very little deformation
you can tell if a material is brittle or ductile by examining what?
the fracture surface
what does the fracture surface of a ductile material look like?
a characteristic cup and cone
what does the fracture surface of a brittle material look like?
granular and flat
some materials can go from ductile to brittle when:
their temperature is lowered
what is hardness?
a measure of how resistant solid matter is to various kinds of permanent shape change when a compressive force is applied
what is hardness directly related to?
the stiffness and yield strength of a material
young’s modulus gives what?
stiffness/elasticity
what is the yield stress?
the minimum stress required to cause permanent deformation
what is the ultimate tensile stress?
the maximum stress a material can withstand
what is a force?
an influence that can change the motion (velocity) of matter (an object)
what is a pivot?
a point or axis from which an object is rotated (or provided a force aiming to rotate)
what does a lever consist of?
a simple machine consisting of a beam or rigid rrd pivoted at a fixed hinge or fulcrum.
what is a lever?
A lever is a rigid body capable of rotating on a point on itself
what is the definition of strain?
a dimensionless quantity that depends on the length of the material and how much it is deformed (compressed or stretched)
a less massive object can still have a greater moment provided that:
it has a greater distance (leverage)
what is stress relaxation?
the observed decrease in stress in response to strain
what variables can impact stress relaxation?
magnitude of load, the speed of loading and temperature
What is stress relaxation due to?
keeping the structure in strained conditions (hence causing some plastic strain)
what is creep?
the tendency of a solid material to move slowly or deform permanently under the influence of persistent mechanical stresses.
What can creep be due to?
long-term exposure to high levels of stress that are still below the yield strength of the material
What are two similarities of creep and stress relaxation?
both are time dependent
both are properties of viscoelastic materials
what does annular mean?
ring shaped
what is it called when stress + strain change if you repeat the stress many times?
fatigue
what factors can affect elastic modulus?
change of length
elongation
temperature
impurities
