Fracture measurement on materials with low yield stress and high toughness
"FRACTURE MEASUREMENT ON MATERIALS WITH LOW YIELD STRESS AND HIGH TOUGHNESS II
R. C. OWEN B.Sc. (Eng.)., A.R.S.M., Cert. Ed. A. I.M.
For the degree of:
Master of Philosophy.
At the University of:
HONG KONG in MAY 1974.
The measurement of fracture toughness of materials with low yield
stress, such as mild steel, has proved difficult in the past because
general yielding takes place in the specimen during testing. General
yielding invalidates results based on elasticity theory, and only small
scale yielding very near the crack tip which occurs under conditions of
plane strain, can be taken into account. The specimen thickness required
for materials of low yield stress to achieve these conditions is so large
that testing would be impracticable. As the thickness is reduced, plane
stress conditions become more dominant and the fracture toughness and the
size of the yield zone are increased. An increase in toughness implies
an increase in load before crack propagation which can produce general
yielding in low yield stress materials under plane stress conditions before
fracture takes place. The aim of this research was to devise a new testing
method which would fracture a small specimen of high toughness before general
yielding occurs and this is achieved by using a special testing rig. The
specimen width necessary to produce only local yielding at the crack tip
is shown to depend on the parameter EG/a 2, where E is the Young'S Modulus,
G is the fracture toughness and a is the yield stress. This parameter y
gives a certain size of specimen necessary to prevent general yielding and
explains the scale effect known in the past to prevail on ships and other
large structures. It was found that these large structures can fail in a
brittle manner and have an apparent low toughness, and a small specimen
made from the same material will be ductile and have a high toughness. It
is shown that a small grooved specimen of a low yield stress material can
be fractured without general yielding if the specimen is reinforced by
placing it in the special testing rig. A bending moment is applied to
the flanges of the rig and the toughness is measured by the compliance
method in a graphical form by measuring the areas representing the quantity
GdA where G is the fracture toughness and dA is the increment in crack area.
The research results showed that valid fracture toughness measure-
ments can be done under the following conditions:
1. The width of the rig was equal or greater than the
critical width to prevent yielding before cracking.
This was achieved by using a high strength rig and
a grooved specimen.
2. The length of the uncracked portion of the specimen
was greater than 3/A where A is the strain energy per
unit length of the specimen treated as a beam on an
3. Cracking is stable and quasistatic and all deforma-
tion other than that at the crack tip is completely
4. The specimen is able to slide in the slots of the rig
I The steps for designing a test rig for a low yield stress material
were found to be the following:
1. Find the value of EG/cr 2 for the material
2. Determine the critical width and depth of the rig and
the grooving of the specimen to ensure cracking before
3. Determine the specimen dimensions and find the modulus
of the elastic foundation.
4. Find the value of A and the critical rig length 3/A.
These steps were taken for designing a rig and specimen for mild
steel, aluminium and zinc. The test results confirmed the theory and
showed that the compliance method was suitable for measuring the fracture toughness. It was found that the toughness of mild steel depended on the grain size and average values of 223 kN/m and 177 kN/m were found for 'as received' and annealed mild steel bar.
An aluminium rig was used to test commercial aluminium, 2024-T3 alloy, 7075-T6 alloy and zinc. Consistent results were achieved giving average fracture toughness values of 188, 180, 107 and 13 kN/m respectively. These results were validated by other tests and previous research. The results show that using a test rig which satisfied the conditions specified, a measure of the fracture toughness of low yield stress materials under plane stress conditions can be done successfully without general yielding taking place.
School:The University of Hong Kong
School Location:China - Hong Kong SAR
Source Type:Master's Thesis
Keywords:metals fracture testing
Date of Publication:01/01/1976