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EGA324 – Stress Concentrations
Overview
❑ Stress Concentrations & Raisers
❑ Industrial Examples
❑ Infinite Circular Plate
❑ Stress Concentration Factors (SCF)
❑ Neuber’s rule to calculate SCF from stress-
strain curves
❑ Tensile Test Experiment
❑ Assignment
EGA324 – Stress Concentrations
Stress Concentrations & Raisers
❑ What is a Stress Concentration?
▪ A stress concentration (often called a stress
raiser) is a location in an object where stress in
concentrated
▪ An object is strongest when load is evenly
distributed over its areas so any reduction in area
results in a localised increase in stress
▪ Stress concentrations can arise from notches,
grooves, holes, threads, or any other abrupt
changes in geometry or in poor designs of sharp
corners and even from inspections/stamping
▪ These discontinuities are known as stress raisers
EGA324 – Stress Concentrations
Stress Concentrations
=
– stress concentration
– maximum stress
– reference (nominal) stress
?
EGA324 – Stress Concentrations
Examples of Stress
Concentrations
❑ Can anyone name an example?
❑ Pressure Vessels
❑ Poor weld profile
❑Alexander Kielland drilling rig disaster
❑ Bolt/Rivet holes
❑de Havilland DH.106 Comet (Commercial Airliner)
EGA324 – Stress Concentrations
Alexander Kielland drilling rig
disaster
EGA324 – Stress Concentrations
de Havilland Comet Accident
EGA324 – Stress Concentrations
Infinite Plate with Small Hole
❑ Kirsch Equations (1898)
=
∞
2
1 −
2
2
+
∞
2
1 −
2
2
1 −
32
2
cos 2
=
∞
2
1 +
2
2
−
∞
2
1 +
34
4
cos 2
= −
∞
2
1 −
2
2
1 +
32
2
cos 2
r – radial co-ordinate; a – hole radius
– loading direction; – stress
EGA324 – Stress Concentrations
Infinite Plate with Small Hole
❑ at the hole: r = a
= 0; = 0
= ∞ 1 − 2 cos 2 ;
Maximum stress at = ± 90° ( = 3∞)
=
=
EGA324 – Stress Concentrations
Notch Sensitivity Index
❑ Notch sensitivity (q) of material is a measure of
how sensitive a material is to notches or geometric
continuities
=
(ℎ )
(ℎ)
= 1 + − 1 1 ≤ ≤
=
− 1
− 1
0 ≤ ≤ 1
q is normally between 0-0.1 for ductile materials, 0.5
for cast irons and 0.5-1.0 for brittle materials
EGA324 – Stress Concentrations
SCF (SCC) Tables
W. C. Young and R. G. Budynas, Roark’s Formulas for Stress and Strain, 7th ed. New York: McGraw-Hill, 2002
EGA324 – Stress Concentrations
SCF (SCC) look-up graphs for stress-
raisers
EGA324 – Stress Concentrations
Neuber’s Rule
❑ Neuber’s rule states that the product of nominal
(elastic) stresses and strains is proportional to the
product of the local (elastic plastic) stresses and
strains
=
2 =
& – the nominal stress based on
specimen geometry
– theoretical stress concentration factor
(tables/look-up)
& – stress/strain value beyond yield point
of the material
EGA324 – Stress Concentrations
Neuber’s Rule
❑Typically know &
❑Need to determine the
effective stress concentration
EGA324 – Stress Concentrations
Example 1 – SCE for single
hole
❑ or determined by tables/equations:
= 3 − 3.140
20
100
+ 3.667
20
100
2
− 1.527
20
100
3
= 2.51
❑ found using net cross sectional area
=
−
=
64
100 − 20 8
∙
103
10−6
= 100 MPa
❑ Based on elastic behaviour peak stress
at edge of hole would be
= = 2.51 ∙ 100 = 251 MPa
EGA324 – Stress Concentrations
Example 1 – SCE for single
hole
❑ Nominal strain is read from the stress-strain
curve at = 100 MPa (A: = 5 ∙ 10
−4)
❑ Applying Neuber’s Rule:
=
2 = 2.51
2 100 5 × 10−4 = 0.315
❑ Intersection of = 0.315 (Point B)
yields = 243 MPa & = 13 ∙ 10−4. Effective
/ experimental stress concentration is:
or =
=
243
100
= .
❑ Results show 251 MPa exceeds the yield point of the material,
so actual peak stress at hole edge found using Neuber’s Rule
EGA324 – Stress Concentrations
Example 2 – SCE for two notches
❑ or determined by look up graph
,
=
5
60
= 0.08 ሶ3 → = 2.45
❑ found using net cross sectional area
=
∙
=
110
60∙12.5
∙
103
10−6
= 147 MPa
❑ Based on elastic behaviour peak stress at edge of notch would be
= = 2.45 ∙ 147 = 360 MPa
d=60mm; ρ=5mm;
t=12.5mm; P=110kN
EGA324 – Stress Concentrations
Example 2 – SCE for two
notches
❑ Results show 360 MPa exceeds the yield point of the material, so
actual peak stress at hole edge found using Neuber’s Rule
❑ A: = 147 MPa; = 7.2 ∙ 10
−4 –
can also be done using = when
E is known
❑ Applying Neuber’s Rule:
=
2 = 2.45
2 147 7.2 × 10−4 = 0.635
EGA324 – Stress Concentrations
Example 2 – SCE for two
notches
❑ B: Intersection of =
0.635 gives = 275 MPa & =
23 ∙ 10−4.
❑ The effective/experimental stress
concentration is:
or =
=
275
147
= .
= 0.635
EGA324 – Stress Concentrations
The Experiment…please watch in your own time
https://canvas.swansea.ac.uk/courses/36971/modules/items/1968427
EGA324 – Stress Concentrations
Experimental Aim
❑ The aim of this experimental investigation is to assess the
effects of introducing a stress concentration feature, such as
a hole or notch, to a component or structure
❑ A number of mild steel tensile specimens are provided with
and without stress raisers and the load/displacement
(stress/strain) characteristics of these specimens under
tensile loading need to be compared and contrasted
❑ An extensometer will be demonstrated on the plain
samples for more accurate strain measurements that would
typically be best practice
EGA324 – Stress Concentrations
Specimens
Plain
Sample
2.5 mm
single
notch
5 mm
Hole
2 mm
Hole
2x2.5 mm
double
notch
❑A total of 10 mild steel tensile test specimens will be provided
with five different geometries:
❑Key dimensions: thickness – 1mm, length – 180 mm, gauge length –
70mm, width of gauge in plain specimen – 15mm
EGA324 – Stress Concentrations
Specimens – Manufacture Route
2.5 mm
single
notch
5 mm
Hole
2 mm
Hole
2x2.5 mm
double
notch
❑ Tensile specimens have been manufactured through water
jet cutting
EGA324 – Stress Concentrations
Equipment
❑ The tests are performed
using a Hounsfield 25kN
electrically operated
tensile test machine
EGA324 – Stress Concentrations
Experimental Procedure
❑ Specimens are located between the jaws (wedge grips on the
Hounsfield machine)
❑ Setting the instrumentation to zero, tensile test is started with the
cross-head of the machine being driven at a displacement rate of
3 mm/min
❑ Force is measured using a load cell within the Hounsfield. Time,
load and displacement are recorded continuously during the test
❑ Once the specimen fractures, yield load and ultimate tensile load
should be noted to initially gauge any differences
❑ Start with the plain specimens and repeat for all – additional
details given in video & in the lab
EGA324 – Stress Concentrations
Table of Results
Yield Load [N] Ultimate Tensile Load [N]
Specimen 1 Specimen 2 Specimen 1 Specimen 2
Plain
2 mm central hole
5 mm central hole
Single-sided notch
Double-sided notch
❑ Initial table of results gives an insight to the effect of
the different stress concentration features
❑ Would expect to see a more complete results table in your
final reports, that may not include loads – with stress a better
comparator
EGA324 – Stress Concentrations
Analysing Tensile Data
❑ Data comes from the machine as
▪ time (s), displacement (µm), force (N)
❑ Needs to be converted to engineering stress, and
engineering strain, via:
=
0
& =
∆
0
Where is the applied force, 0 is the cross-sectional
area of the test piece, 0 is the gauge length and ∆ is
the extension
EGA324 – Stress Concentrations
Analysing Tensile Data
❑ This needs to be converted to true stress, and true
strain, which is approximated by:
= ( + 1) ∙ & = ln(1 + )
❑ Use Neuber’s rule as described above and in the
examples together with the calculated stress-strain to
compare the experimental SCFs with the empirical SCFs
and how do they compare?
❑ Nominal stress () values for the Neubers rule analysis for
the experimental data are to be taken from the elastic region
of the curve – using a consistent approach
EGA324 – Stress Concentrations
Typical Experimental Results
0
50
100
150
200
250
300
350
0 0.1 0.2 0.3 0.4 0.5 0.6
St
re
ss
(
M
P
a)
Strain
Eng Stress/Strain True Stress/Strain
0
50
100
150
200
250
300
350
0 0.1 0.2 0.3 0.4 0.5 0.6
Use Scatter with Smooth Lines, way too many data points to include every point!
EGA324 – Stress Concentrations
Fracture/Failure Surfaces
Available on Canvas:
❑What do they look like?
❑ What do the angles at which the surface
orientated tell you?
❑ Ductile or Brittle type fracture?
EGA324 – Stress Concentrations
Some Open Questions
❑ How does the plain sample properties compare to that of mild steel seen in
the literature?
❑How does the size of the hole affect the tensile characteristics of the
components and why ?
❑ How does the number of notches affect the tensile characteristics and why ?
❑ How do the tensile characteristics for a 5 mm hole compare with those for
two R2.5 mm notches and why ?
❑ Rank the five stress raisers in terms of their impact on the components’
behavior under load.
❑ Compare and contrast the degree and form of the deformation (strain) at
failure for the different stress raisers and how do they compare with the plain
specimen ?
❑ How do the shapes of the fracture surfaces compare and why might they be
different ?
❑ Comment on the repeatability of the tests and how could accuracy be
improved ?
❑ In addition – Computational Modelling aspects to be considered…
EGA324 – Stress Concentrations
Examples
Format:
❑ Review experimental protocol & raw data sets
❑ Individual tensile test data analysis
❑ SCFs – empirical & experimental (Neuber’s)
❑ Going further…
EGA324 – Stress Concentrations
Experimental Procedure
❑ Important points:
• Linear measurement – callipers/micrometer
• How would this influence your stress/strain results?
• Displacement rate – strain rate effects on materials
• Data acquisition rate
• Consistent jaw separation (returning jaws to same
starting position between tests)
• Testing standards:
• e.g. BS, ISO
EGA324 – Stress Concentrations
Experimental Practice
❑ Nominal Dimensions
– we are using these…