Transcript DESIGNING WITH HIGH-STRENGTH LOW

DESIGNING WITH HIGH-STRENGTH LOWTOUGHNESS MATERIAL
High strength materials are being increasingly used in
designing critical components to save weight or meet
difficult service conditions.
Unfortunately these materials tend to be less tolerant of
defects coz of limited local plastic deformation to relieve
stress.
While a crack-like defect can safely exist in a part made of
lower-strength ductile material, it can cause catastrophic
failure if the same part is made of a high-strength, lowtoughness material.
This has led to more demand for the accurate calculation of
acceptable defect levels, and to increase use of
nondestructive testing (NDT) in manufacture.
DESIGNING WITH HIGH-STRENGTH LOWTOUGHNESS MATERIAL
Defects can be the result of
Initial flaws in the material, e.g inclusion & cavities
Introduced by production method, e.g welding.
Service conditions, e.g fatigue cracks, stress corrosion
cracks.
DESIGNING WITH HIGH-STRENGTH LOWTOUGHNESS MATERIAL
Fail-safe design
Require a structure to be sufficiently damage to allow
defect be detected before they develop to a dangerous
size.
Inspection has to be conducted before the structure is put
into service to ensure that none of the defects exceeds the
critical size.
Structure has to be inspected periodically during its
service life to ensure that none of the defects grows to a
dangerous size.
Principles involved in fail safe designs
DESIGNING WITH HIGH-STRENGTH LOWTOUGHNESS MATERIAL
The figure shows that it is not strictly necessary to select a
material with low crack propagation rate.
In principle the structure can be made to fail safe when
cracks propagate fast if the inspection interval is short
enough.
However, short inspection periods are not always possible
or cost effective.
A better alternative is to use a more sensitive inspection
method to reduce the minimum detectable defect size.
DESIGNING WITH HIGH-STRENGTH LOWTOUGHNESS MATERIAL
Guidelines for design
Interaction between fracture toughness, allowable crack
size & design stress need to be considered.
Toughness – A qualitative measure of the energy require
to cause fracture of material.
A material that resist failure by impact is said to be tough
Fracture toughness
The ability of materials containing flaws to withstand load.
Measured using :
Impact testing apparatus – Charpy and Izod test
Another is the area under the true stress-strain curve.
The impact test (a) the Charpy & Izod test (b) dimensions of typical
specimens
DESIGNING WITH HIGH-STRENGTH LOWTOUGHNESS MATERIAL
Designing with ductile unflawed parts, as the load increase
the nominal stress increase until it reaches the yield stress
and plastic deformation occurs.
In the case of high-strength, low toughness material, as the
design stress increases (or as the size of the flaw increase)
the stress concentration at edge of crack, the stress intensity
KI, increase until reaches KIC and fracture occurs.
Thus the value of KI in a structure design should always be
kept below the value of KIC in the same manner that the
nominal stress is kept below the yield strength.
DESIGNING WITH HIGH-STRENGTH LOWTOUGHNESS MATERIAL
The plane-strain conditions at thickness, t, occurs related to
the fracture toughness, KIC, and yield strength of the
material, YS, according to the relationship :
t  2.5KIC / YS  ............2.1
Condition of failure under plane-strain conditions, where a
crack of length 2a exists in a thick, infinitely large plate,
2
KI  KIC  Y a ............2.2
1/ 2
KIC = property of material/Fracture toughness,  = design
parameter, controlled by applied load & shape, a = ½ crack
length, controlled by manufacturing method & NDT, Y is a
dimensionless shape factor, a function of crack geometry.
DESIGNING WITH HIGH-STRENGTH LOWTOUGHNESS MATERIAL
Equation must be used in several ways to design against
failure.
For example, selecting a material to resist other service
requirements automatically fixes KIC.
In addition, if the minimum crack size that can be detectable
by the available NDT methods is known equation 2.2 is
used to calculate the allowable design stress.
Which must be less then K IC /Y a 1/ 2
Alternatively, if the space, weight and operating
stress have a limitation, the max allowable crack
size can be calculated to check whether it can be
detected using routine inspection methods
Define function &
service conditions
Preliminary
construction
1.Preliminary stress analysis
2. Preliminary material selection
Working stress
KIC
Determine critical crack size
No
Can the critical crack size be achieved
by manufacture and can it be detected
by available NDT
Yes
Are existing subcritical
cracks likely to grow ?
Yes
Determine crack
growth rate
No
Set Q.C and inspection limits
Flow chart giving the steps which can be followed in designing fracture-resistant
structures.
Example – Design of pressure vessel.
Internal pressure, P = 35 MN/m2
Internal diameter, D = 800 mm
Manufactured by welding of sheets & welded joints
inspected by NDT, capable of detecting surface cracks
> 15 mm.
 Consider use AISI 4340, tempered at 260°C, Yield
strength = 1640 MPa & KIC = 50 MPa m1/2. Treat as thinwalled cylinder, wall thickness, t ;
t = PD / 2w where w = working stress.
 n = 2, working stress = 820 MPa & wall thickness is
calculated as t = 17.1 mm.
Example
Pressure vessel thinwalled cylinder with
thickness t
 Critical crack size calculated from,
KIC = [1.1/Q] πa
 Taking conservative case of semicircular crack, a/2C = 0.5,
value of Q can be estimated from Table for /YS = 0.5 as Q
= 2.35.
 Critical crack length, a = 2.3 mm. Too small to detect.
 Use same steel but tempering at 425°C gives yield strength
= 1420 MPa & KIC = 87.4 MPa m1/2. Follow same
procedure gives t = 19.7 & a = 18.74 mm. Can be detected
by NDT & suitable monitoring technique can be arranged.
 Steel 4340 tempered at this condition is suitable for
making the pressure vessel.
 Other factors like availability of material, weldability,
weight of vessel and cost have to be taken into
consideration before this steel is finally selected
DESIGNING WITH HIGH-STRENGTH LOWTOUGHNESS MATERIAL
Leak-before-burst
Based on concept of if vessel containing pressurized
gas/liquid contains growing crack, toughness should be
sufficiently high to tolerate a defect size which will allow
the contents to leak out before failure occurs.
For leakage, crack must grow through wall thickness, t.
Design stress – assumed that failure is due to tensile
stresses occurring tangentially in the walls.
Should take into account the material, operating
environment, & P constant or varying in cyclic fashion.