Structure-sensitive properties

Most mechanical
STRUCTURE-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