Engineering plastics – The Manual - F.wood-supply.dk

Mechanical properties
σ R
0
σ B
σ R
σ S
σ R
Where plastic components are designed to withstand
stress, the mechanical characteristics of a material have a
particularly important role to play. The fundamental mechanical
material properties include
Δ σ
Δ ε
ˌˌStrength: dimension for the resistance
of a material to external stress
ˌˌFormability: the capacity of a material
to become deformed under external stress
ˌˌRigidity: ε R
ε S dimension for the resistance ε B
ε R
ε R
of a material to deformation
ˌˌToughness: dimension for the energy absorption
capacity of a material under external stress
ˌˌTensile strength at yield σ S is the tensile stress at which
the slope of the change of force versus length curve (see
graph) equals zero for the first time.
ε
ˌˌElongation ε is the change in length Δ L in relation to the
σ B original length L
σ 0 of the specimen at any point during ε el(0)
R
σ testing. The elongation at maximum force is described
S
as ε B , elongation at break as ε R , tensile strength εat el(t)
yield
as ε S . ε = ε el(0)
+ε el(t)
+ε pl
ˌˌModulus of elasticity E: A linear relationship can εonly pl be
σ R
observed in the lower range of the stress-strain diagram
for plastics. In this range Hooke’s law applies, which
says that the ratio of the stress and strain (modulus of
elasticity) is constant. E = σ / ε in [MPa].
ε
σ
S Strength
F FormabilityF
R Rigidity
T Toughness
V
σ
S
F
σ R
0
Δ σ
ε S
ε B
ε R
ε R
Δ ε
ε R
Based on the bending, compression and impact toughness
test, additional test methods are available for characterizing
materials and different load cases.
σ
S
R
Z
T
ε
0
ε
These material characteristics are generally determined by
briefly applying tensile load in one direction with a tensile
test (for example in accordance with DIN EN ISO 527):
σ
brittle plastics
Source: J. Kunz, FHNW
However, for the sound design of a component, the rele-
application conditions F must also σ be taken into consid-
S
σvant
eration: Because of their macromolecular structure, the
V
F
mechanical properties of plastics depend heavily on the
ambient conditions such as temperature, exposure period,
S
R
type and velocity Z of loading and moisture content. T
ε
0
ε
σ B
σ R
Influences on forming behaviour
σ S
t
ε
ϑ
tough hard plastics
ε
Time
σ
ε
Temperature
σ
σ R
ε el(0)
σ
σ
soft, elastic
έ
plastics
t
ϑ
ε el(t)
Δ σ
ε S
ε B
ε R
ε R
φ
Δ ε ε R
ε
σ B Ultra-high tension
σ R Tensile strength at break
σ S Tensile strength at yield
ε
ε B Elongation under ultra-high tension
ε R Elongation at break
ε S Elongation at yield
Moisture content
σ
ε
ε = ε el(0)
+ε el(t)
+ε pl
Velocity of load
σ
έ
ε
ε pl
ˌˌTensile stress σ is the tensile force at the smallest
σmeasured starting F cross-section of σthe test specimen S
at any optional point in time during the test.
V
ˌˌTensile strength σ B is the tensile stress at maximum force.
ˌˌTensile stress S at break σ R is the tensile stress Rat the
Z
T
moment of break.
F
φ
ε
ε
σ R
0
50
ε
0
ε