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Structure-sensitive properties
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lecture notes on structure-sensitive properties
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Most mechanicalSTRUCTURE-SENSITIVE PROPERTIES
structure
properties are sensitive properties which means
they are
very dependent
-
the microstructure of Most important
on a material structure sensitive properties are ;
yield strength
-
.
,
ductility and fracture strength .
"
"
How the material ? be expected for load ? " This
"
how much deformation
strong is ,
can a
given .
information be obtained the tensile test
can
Using .
The tensile test -
used to determine the strength of a material when it is stretched .
•
test specimens have standard dimensions
test specimens stretched
slowly at a constant rate
•
applied force and elongation of the specimen recorded
•
are
tensile
testing equipment
Image ( left) shows tensile
testing equipment .
bottom end of specimen connected load cell which measures applied
•
to a the
force
top end
•
connected to a movable cross beam that is raised
test specimens either thin pieces cut from sheet metal (rectangular
usually
•
cross
section) or machined from bulk metal (circular cross section )
Ao
d
I V
Important µ
I
dimensions : r
f- -7
- -
-
,
t
' '
- -
-
'
L -1
,
1 I × i
i
- - -
t
- -
•
to ( initial gauge length) -
part of test specimen where
increase in
length (elongation) is measured at
any point < >
to
test
during
to often -5
°
taken to be 5.65ft a) which is
•
A◦ (initial cross-sectional area) the
gauge length of 5 diameters for specimen
with circular cross section
Elongation ;
Elastic deformation initial deformation
•
-
reversible upon
unloading
-
Plastic deformation not reversible
unloading
•
upon
-
deformation is
permanent
-
, STRUCTURE-SENSITIVE PROPERTIES
"
I "'
It
all
' ' ' '
I. Elastic deformation 2. plastic deformation (elongation of specimen) 3. Failure
•
often linear .
i)
region of uniform ii ) non -
uniform region •
deformation process ends
Elongation produced deformation all parts where localised when specimen fractures
• -
under load is directly of gauge length deformation occurs .
in the necked
region
proportional to that load elongate to the same called
necking
( Hooke 's law) amount
Brittle or ductile ?
Ductile =
substantial plastic deformation before fracture
Brittle fracture in the elastic after small amount of plastic deformation
region only
:
occurs or a .
load
elongation curves converted to stress-strain curves
using the standard specimen dimensions
-
are .
For a
specimen with ; Nominal /engineering stress @n ) =
load @
any point during test
'
initial cross sectional area =
to divided
by original cross-sectional area
•
initial to on =
F-
gauge length
=
F
Ao
elongated under a load
•
,
to True stress (a) load divided by instantaneous cross-sectional
gauge length
•
L =
area
,
with Ot F-
corresponding cross-sectional
•
=
A
area A
,
Nominal /engineering strain ten) =
ratio of the change in
length to
the
original length
En =
l -
lo or I -
l
lo lo
In elastic where
region
stress ✗ strain (i. e. Hooke 's law ) ,
True strain (Et) =
incremental instantaneous strain integrated over
elastic modulus (i.e Young 's
. modulus
,
the whole of the elongation
E) is defined as :
'
E = I
G-
=/ 1- o
=
in
En )
÷ Et =
In ( It En
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