1

Fracture Toughness Tests

CE671 – Lecture 13

Fracture MechanicsWhat is Fracture Mechanics?– Mathematical analysis of solids with

notches, cracks, or defects

Applied Stress Intensity Factor (SIF) ‘K’

Parameter used to assess or predict fracture due to applied stressCharacterizes applied stress at notches or cracks‘K’ has unusual units – ksi-√in or MPa-√m

Function of:– Size of crack or flaw– Nominal stress away from crack

2

‘K’ as a Material PropertyKc or Kd material fracture toughness– Measure of material resistance to fracture

Kc - static or slow loadingKd - dynamic loading

Similar to σyield, E, or any other material property Characterized in terms of applied K at onset of fractureKI, KII, KIII refer to mode of loading

Material Tests to Obtain K

Plane strain fracture toughness– Used to obtain Klc or Kld

– Specimens sized to approximate plane strain

Thick specimens often requiredSize required is dependent on Klc

– Expensive tests to conduct

Requirements on Thickness

in 0.004 CTOD inksi 100 K

valueslfor typica 5" B b, thus

CTOD 1500 ksi 50 σfor i.e. σK 2.5 B b,

K 399 E ASTM

y

2

y

IC

IC

==

≥

≥=⎟⎟⎠

⎞⎜⎜⎝

⎛≥

All dimensions, a, B, and W are dependent on KIC, which is what is to be measured??

3

Why is Thickness Important?

Must be thick enough so linear elastic behavior occurs so that effects of plastic zone ahead of crack can be neglectedNeed to ensure there is a triaxial tensile stress present so shear stress is very low compared to maximum normal stress– Ensures Mode I

Estimated from Charpy Tests (CVN)– Correlation gives dynamic fracture

toughness KId

Must convert to KIc for slower load ratesDrop the subscript “I” for plain strain except when K is from a valid fracture toughness test

Material Tests to Obtain K Cont’d

Compact Tension Test

See ASTM E399

4

Three-point Bend Test

2.1W (Min) 2.1W (Min)

W

W/2 See - ASTM E-399

Charpy Testing

Not a direct measure of fracture toughnessExcellent for material screeningExcellent for quality controlLow cost and easy

Will likely remain favored method

5

Since not a Direct Measure, we Need Correlation between ‘K’ and CVN

KId= [5(CVN)E]1/2

– KId = Dynamic fracture toughness (psi-in1/2)– CVN = Charpy energy (ft-lbs)– E = Modulus of elasticity (psi)

Valid for “lower shelf” and lower portion of transition regionNote that this yields Dynamic fracture toughness KId

Dynamic Fracture Toughness

Measure of material’s resistance to brittle fracture subjected to dynamic loading [(105 ksi√in)/sec] (Running Fracture)Bridge load rates are much slower– 0.2 to 1.0 sec

Since fracture toughness is affected by load rate, must account for slower loading rates in bridges– Interested in KIc – static to slow load rates

Impact Loading Rate Shift

Experimentally shown that slower load rates tend to shift toughness curves to left – i.e., increases toughness

Magnitude of the shift is function of yield strength of materialAccount for lower loading rate by applying “temperature shift”

6

Impact Loading Rate Shift

Energy level at which temp. shift is measured

Impact

Slow Bend

Temperature shift in transition temp. range

Ft-lb shift in upper-shelf level

Test Temperature

Abs

orbe

d En

ergy

in C

VN T

est

Impact Loading Rate Shift

Calculate temperature shift as: – Tshift = 0.75 (215 - 1.5 σys)– 36 ksi < σys < 140 ksi

For common steels:– 36 ksi » 120 deg F– 50 ksi » 105 deg F

Barsom & Rolfe includes equations which account for strain rate– Tshift = (150 - σys)(ε')0.17

Example

Given:– Set of CVN data– Steel with σys = 40 ksi

Develop:– KId curve– KIc curve

7

Charpy Testing

Charpy Testing

Example

Test Temperature (deg. F)-120 -100 -80 -60 -40 -20 0 20 40 60 80 100 120

CV

N E

nerg

y (f

t-lb)

0

20

40

60

80

100

120

140Test DataAASHTO Non-fracture CriticalAASHTO Fracture Critical

Step 1 – Develop Charpy curve from testing

8

ExampleStep 2 – Apply relationship - KId= [5(CVN)E]1/2

to convert from CVN to KId

Temperature (deg. F)-240 -210-180 -150-120 -90 -60 -30 0 30 60 90 120

KIc (

ksi -

in1/

2 )

0

20

40

60

80

100

120

140

160KId Dynamic Load RateKIc Bridge Load Rate

ExampleStep 3 – Adjust to intermediate load rate

Apply temperature shift - Tshift = 0.75 (215-1.5 σys)

Temperature (deg. F)-240 -210-180 -150-120 -90 -60 -30 0 30 60 90 120

KIc (

ksi -

in1/

2 )

0

20

40

60

80

100

120

140

160KId Dynamic Load RateKIc Bridge Load Rate

Tshift = 116 F

Effect of Temperature on Fracture Toughness

Fracture toughness decreases with decreasing temperatureBridges are subjected to large temperature variationsMinimum service temperature is critical– Related to geographic location

9

How does Specification address effect of temperature on fracture toughness?

Requires minimum CVN energy at various service temperaturesThree temperature “zones” defined as per AASHTOIndiana is Zone 2

Temperature Zone Designations

3-31 F to -60 F

2-1 F to -30 F

10 F and above

Temperature Zone

Min Service Temperature

Minimum Charpy requirements for bridges and buildings

Miminum Service Temperature

Material: -18oC -34oC -51oC

Joules@oC Joules@oC Joules@oC

Steel: non-fracture critical members*,** 20@21 20@4 20@-12

Steel: fracture critical members*,** 34@21 34@4 34@-12

Weld metal for non-fracture critical* 27@-18 27@-18 27@-29

Weld metal for fracture critical* 34@-29oC for all service temperatures

AISC: Jumbo sections and plates thicker than 50 mm** 27@21oC for all service temperatures

SAC: weld metal for seismic applications 27@16oC below service temperature

not to exceed -18oC

* These requirements are for welded steel with minimum specified yield strength up to 350 MPa up to 38 mm thick. Fracture critical members are defined as those which if fractured would result in collapse of the bridge.

** The requirements pertain only to members subjected to tension or tension due to bending.

10

Charpy Specifications

To assure ductile/brittle transition temperature is below minimum service temperatureTemperature shift (38 C or 70 F) allowed because Charpy test has much higher strain rate than service loads

Questions ?