Fracture measurement on materials with low yield stress and high toughness

by Owen, Robin Campbell

(Uncorrected OCR) ABSTRACT OF THESIS ENTITLED:

"FRACTURE MEASUREMENT ON MATERIALS WITH LOW YIELD STRESS AND HIGH TOUGHNESS II

submitted by:

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,

I y

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

elastic foundation.

3. Cracking is stable and quasistatic and all deforma-

tion other than that at the crack tip is completely

reversible.

4. The specimen is able to slide in the slots of the rig

during bending.

"

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 

. y

2. Determine the critical width and depth of the rig and

the grooving of the specimen to ensure cracking before

general yielding.

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.