MATERIALS ENGINEERING – DAY 3
Quick Review of Hardness & Hardness Testing
 Big problem: when ductile materials go brittle…
 Impact Testing
 Ductile to Brittle Transition Temperature
 Introduction to Fracture. See how far we get.

REVIEW

What is the most common type of material test?
What does it measure?
STRESS CONCENTRATION
What is it?
Changes in geometry (holes, fillets, threads, notches) can cause
local increases in stress (stress raisers)
For example: Near a small hole in a large plate, the stress at the
edge of the hole is three times as high as the stress away from
the hole.
MORE ON STRESS CONCENTRATION
FACTOR
Importance:
1. high-strength, low ductility materials can crack
2. cyclic stress coupled with stress concentration is
typical for fatigue failures
 Quantifying:

Stress Concentration Factor, K=smax/snominal

where:
 K is from published charts
 snominal is the stress ignoring the stress
concentration
 smax is the highest local stress due to the
concentration

WHEN “GOOD” MATERIALS GO “BAD”
The Schectady. Here is the result.
 These points need mentioning:
1. Steel in the vessel had performed well in
tension and hardness testing in non-service
environment.
2. Many, many ship failures of this type took place
in WWII. Here’s another. The John. P Gaines.
3. The Navy had to change material specifications.
There was a major research effort. Here’s one
result.

TEST TO MEASURE NOTCH TOUGHNESS

1.
2.
3.

The following factors affect toughness
Tri-axial stress state
Presence of a notch or stress concentration
Cold Temperature
Therefore, a test was designed to probe the
materials performance under these more
demanding conditions.
CHARPY
TESTS
AND IZOD
Specimen
 Test Apparatus
 Result: Notch toughness

CHARPY TEST
Measures the energy to break, usually of a notched specimen
Energy
(ft-lb, J)
FCC
no transition
Hammmer
Upper Shelf
(ductile)
BCC
Supports
Lower Shelf
(brittle)
Temperature
Ductile-Brittle
Transition Temp.
Charpy Bar
DUCTILE TO BRITTLE TRANSITION
TEMPERATURE
There are several ways of specifying the actual
temperature at which transition occurs. One is
the highest temp on the lower shelf. Another the
temp ½ way between the two shelves. Where was
the SS Schnectady?
Effect of Radiation on Charpy Energy
www.msm.cam.ac.uk/.../Irradiated_Steel.html
Cup-and-cone fracture surface of
duralumin as a result of failure by
necking.
www.answers.com/topic/necking
The fracture surface of this Charpy
impact specimen show that the fracture
mode was a mixture of ductile (dull gray)
and brittle (shiny, salt and peppery
appearance).
www.matsci.ucdavis.edu/.../EMS-174L/EMS-174L.htm
metallurgist.com/.../Fractography.htm
http://pwatlas.mt.umist.ac.uk/internetmi
croscope/micrographs/failure/brittlesteel.html
IMPACT TESTING
Advantages
 Determine minimum
temperature of use
 Finds “notch sensitive”
materials
Disadvantages
 Difficult to apply
quantitatively in design
 Addresses temperature,
not stress or flaw size
FRACTURE MECHANICS
There is another problem, really more severe
than a stress concentrator. It is a sharp crack.
 Clearly, this can make a ductile material act like
a brittle material. i.e. sudden, unexpected
failure.
 Since WWII this has been a fertile field of study.
It is the science of fracture mechanics which
brings together
1. Analytical methods of solid mechanics to
calculate stress field around crack
2. Experimental methods to determine resistance
to crack propagation

INTRODUCTORY EQUATIONS
The f’s are functions of q. The variable r measures distance
from the crack tip. The variable K is called the stress
intensity factor.
K Ys  a
FRACTURE MECHANICS
For notched members, failure occurs when KI(applied)>KI(critical).
a
2a
Importance: Can quantify”strength” of flawed members.
Quantifying:
KI Ys a
Geometry
factor
stress
(nominal)
Advantage:
•can set inspection standards
•can use parts with subcritical flaws
Units: ksiin, MPam
flaw size
Stress Concentration Factor



multiplier of nominal stress
no units
can’t quantitatively predict
failure stress
Stress Intensity Factor



measure of local stress field
units of: ksiin, MPam
can predict failure stress for
discovered flaws
STRESS INTENSITY
The factor Y depends on crack length and
geometry.
 So, an alternate, and more useful way of talking
about crack stability is to frame the discussion in
terms of the stress intensity as opposed to the
critical strain energy release rate. A crack will
become unstable when K = Kc, a critical value
called fracture toughness.

a
Kc  Y  s c  a
W 

The specimen geometry comes in through Y. See
Figure 8.13 in the text.
EXAMPLE PROBLEM
A large, thick plate is fabricated from a steel
alloy that has a plane strain fracture toughness
of 55 MPA m1/2. If, during service, the plate is
exposed to a tensile stress of 200 MPA, determine
the minimum length of a surface crack that
would lead to failure?
 Ans. About 24 mm.
