FRACTURE FRACTURE TOUGHNESS & TOUGHNESS & its MEASUREMENT its MEASUREMENT

JAVED ALAM10MT10015

What is fracture toughness?It is a property which describes the ability of a material with a crack to resist fracture.

The linear-elastic fracture toughness of a material is determined by Stress intensity factor(K) at which a thin crack in the material begins to grow, denoted by KIC

Plastic-elastic fracture toughness is denoted byJIC

Fracture toughness is a quantitative way of expressing a material’s resistance to brittle fracture when a crack is present.

*If a material has much fracture toughness it will probably undergo ductile fracture.

*Brittle fracture is very characteristic of materials with less fracture toughness.

Stress intensity factor (K):

It is used to predict the stress state near the tip of a crack caused by a remote load or residual stresses.

It depends on nominal stress, and crack geometry: K = f(, a); It also depends on the location of crack, magnitude and distribution of stresses

Fracture Toughness Fracture Toughness TestsTests

A test sample is fatigue loaded to extend the machined notch by a prescribed amount.A clip gage extensometer is placed at the mouth of crack to monitor displacement.

If a lower toughness level is achieved with a thicker sample, then the value obtained initially is not valid.

Type of Specimens for Fracture Toughness Tests

Clip Gage Clip Gage ExtensometerExtensometer

Experimental setup

The sample was Perspex

Group-2

Max. load= 85.11 N

Extension at break=6.98mm

Flexural Tests with Pre-cracked Specimens

For three point bending: (D = S/2) = KIC = Y.(3PS/2W2t).a

Y = 1.93-3.07(a/w)+13.66(a/w)2-23.98(a/w)3+25.22(a/w)4

Calculation of KIC

Width=10.88 mm, t=5.7mm , a(crack length)=4.46mm , S=44 mm

(a/w)=0.41Y=2.027

Now P= Pmax=85.11 N

Substituting the value of P:

KIC = 1.143 MPa · m1/2

Group-1

a= 4.7166

a/w = 0.414

Y=2.039

KIC = 1.086 MPa · m1/2

Group-3 Crack length=5.066mm

Group -4

KIC =1.231 MPa · m1/2

Crack length=6.05mm

Thank you