3.8 Limits of Stress
63
stress is sufficiently small, since there is a minimum quantum energy that must be
exceeded to excite a bond between atoms.
The stress at the transition from elastic to inelastic behavior is called the ‘elastic
limit’.
If a material does not return to its original shape when the stress is gone, but has
been left deformed, then it shows mechanical ‘hysteresis’. 39 (See Fig. 3.15.)
A material left in a deformation state, after the external forces on the material
are brought back to zero, may still contain internal stresses. These stresses can be
relieved by ‘tempering’, i.e. the application of heat until atoms or molecules in the
material are able to move in response to the stress. Some unusual materials show
hysteresis at low temperature, but will restore themselves to their original shape
when heated. When cold, the material can hold internal stress, and in this way retains
some ‘memory’ of its original shape.
If internal relative displacement occurs in a solid under shearing stress that carry
atoms past each other, the material is said to undergo ‘plastic flow’. ‘Creeping’
is plastic flow that occurs over relatively long times while the material suffers
stress. If there are internal bands of material slipping in relative position, we say
the material is yielding. The stress is likely to be dissipated into heat as atoms move
past each other. A microscopic slip band which opens into a microscopic gap is a
‘microfracture’.
Materials with a buildup of microfractures are called ‘fatigued’. Bone can
become fatigued, but bone can also repair itself, unlike the metal in airplane wings.
The time scale for measurable plastic flow under natural stress can be from
nanoseconds to geological ages, depending on the material.
The stress at which a material macroscopically separates is called the ‘rupture
strength’. Rupture can occur when microfractures join to form a ‘macrofracture’.
The limiting ‘stretching strength’ before rupture is called ‘tensile limit’; under
compression, ‘compressive limit’; and under shear, ‘shear limit’. When a material
under stress breaks after rupture, it is said to have undergone ‘material failure’.
A material which has relatively large stress-strain tensor-components is called
‘stiff’. If the material fractures after significant stress but relatively little strain, we
say it is ‘brittle’. Materials that yield easily but do not fracture when stressed are
said to be ‘malleable’. A material malleable under tension is call ‘ductile’. While
under tensile stress, a ductile material tends to ‘neck’, i.e. layers of material move
to make the cross-section smaller. A material which is easily bent is call ‘pliable’.
A brittle material yields to stress by fracturing instead of having atomic creep or
slippage. Glass at room temperature is an example. A material can be brittle at one
temperature and ductile at a higher temperature or at a lower pressure.
39 Hysteresis is Greek for deficiency.
63
stress is sufficiently small, since there is a minimum quantum energy that must be
exceeded to excite a bond between atoms.
The stress at the transition from elastic to inelastic behavior is called the ‘elastic
limit’.
If a material does not return to its original shape when the stress is gone, but has
been left deformed, then it shows mechanical ‘hysteresis’. 39 (See Fig. 3.15.)
A material left in a deformation state, after the external forces on the material
are brought back to zero, may still contain internal stresses. These stresses can be
relieved by ‘tempering’, i.e. the application of heat until atoms or molecules in the
material are able to move in response to the stress. Some unusual materials show
hysteresis at low temperature, but will restore themselves to their original shape
when heated. When cold, the material can hold internal stress, and in this way retains
some ‘memory’ of its original shape.
If internal relative displacement occurs in a solid under shearing stress that carry
atoms past each other, the material is said to undergo ‘plastic flow’. ‘Creeping’
is plastic flow that occurs over relatively long times while the material suffers
stress. If there are internal bands of material slipping in relative position, we say
the material is yielding. The stress is likely to be dissipated into heat as atoms move
past each other. A microscopic slip band which opens into a microscopic gap is a
‘microfracture’.
Materials with a buildup of microfractures are called ‘fatigued’. Bone can
become fatigued, but bone can also repair itself, unlike the metal in airplane wings.
The time scale for measurable plastic flow under natural stress can be from
nanoseconds to geological ages, depending on the material.
The stress at which a material macroscopically separates is called the ‘rupture
strength’. Rupture can occur when microfractures join to form a ‘macrofracture’.
The limiting ‘stretching strength’ before rupture is called ‘tensile limit’; under
compression, ‘compressive limit’; and under shear, ‘shear limit’. When a material
under stress breaks after rupture, it is said to have undergone ‘material failure’.
A material which has relatively large stress-strain tensor-components is called
‘stiff’. If the material fractures after significant stress but relatively little strain, we
say it is ‘brittle’. Materials that yield easily but do not fracture when stressed are
said to be ‘malleable’. A material malleable under tension is call ‘ductile’. While
under tensile stress, a ductile material tends to ‘neck’, i.e. layers of material move
to make the cross-section smaller. A material which is easily bent is call ‘pliable’.
A brittle material yields to stress by fracturing instead of having atomic creep or
slippage. Glass at room temperature is an example. A material can be brittle at one
temperature and ductile at a higher temperature or at a lower pressure.
39 Hysteresis is Greek for deficiency.