Objective: The student will be acquainted with brittle fracture analysis based on linear elastic fracture mechanics.

Module 4.4: Design against brittle fracture

1

Scope: Fracture toughness, Critical stress intensity, Critical crack size, Temperature and material toughness Overview of calculation methods in a relevant design guidance document, e.g., EN 1993 Eurocode 3-part 1-10: Design of Steel Structures: Selection of materials for fracture toughness and through thickness properties

IWSD M4.4

Expected results: Review theory of fracture mechanics and brittle fracture. Explain relationship between material fracture toughness and temperature. Review calculation procedures in a relevant design guidance document. Compute critical crack size for structural element with typical material properties. Compute stress intensity factor for a welded connection.

Brittle fracture

2 IWSD M4.4

Characterized by the failure occurs without prior plastic deformation

Some form of fracture is required (e.g. micro-cracks in a weld) to start the crack

A material that is ductile have less risk of brittle fracture.

Risk for brittle fracture when The stress level is high Low temperature Multi axial stress state (= large plate thickness) Stress concentrations

Brittle fracture

3 IWSD M4.4

Brittle fracture in Liberty ships due to sudden decrease in temperature

4

Impact Toughness

The steel used in the Titanic hull had adequate strength but low toughness in cold temperatures. When the Titanic hit the iceberg, instead of the steel bending and causing small cracks (ductile failure), a crack grew very fast and very large (due to brittle failure).

IWSD M4.4

5

Tensile testing stress strain diagram ductile material

10 mm

SEM photo of a ductile metal failure surface

IWSD M4.4

6

Tensile testing stress strain diagram brittle material

Fig 2.1 Stress-strain diagram for a typical brittle material.

10 mm

SEM photo of a brittle metal failure surface

IWSD M4.4

7

Impact Toughness

Depending on: Temperature Material thickness Chemical composition of the material The Structures design If there are notches in the structure If impact is precent Then the failure could either be ductile of brittle

IWSD M4.4

8

Impact Toughness testing

The toughness for a material is characterized by a Charpy V test The specimen, 10x10 mm in cross section, is impacted by a hammer and

the applied energy is registered as function of temperature

IWSD M4.4

9

Impact Toughness testing

Charpy V test: Measurement of the energy absorption in a notched specimen Rule of thumb: If the energy at service temperature 27 J the risk for

brittle failure is small Defines at which temperature (ITT) the test speicmens is failed at an

energy of 27 J (alt. 40, 50...) ITT = Impact Transition Temperature

IWSD M4.4

10

Impact toughness

Relation between transition temperature for impact toughness and lowest service temperature without risk for brittle failure for C- and CMn steels according to British Standard 4741

IWSD M4.4

11

Toughness

With regards to the risk for brittle fracture the steels are divided in different quality classes (toughness classes) from A to E where the highest requirements are E

General structural steels

SS EN 10027 -1:2005

IWSD M4.4

12

Choosing toughness class (BSK07/SS-EN 10025-2)

Toughness class is varying based on the risk for brittle fracture. It is determined according to this:

IWSD M4.4

13

Execution class (utfrandeklass) (BSK07/SS-EN 10025-2)

IWSD M4.4

Cutting class (skrklass) (BSK07/SS-EN 10025-2)

14

Toughness

Besides the requirements for yield stress and ultimate strength, the material should fulfill three requirement regarding the thougness according to Eurocode 3

fuk/fyk 1,10

Elongnation 14%

u 15fyk/Ek

All the steels in the following tables fulfill these three requirements

IWSD M4.4

15

Excercise 4.4.1

IWSD M4.4

What toughness class is required for an un-alloyed steel S355 (SS-EN 10025-2) with the following conditions: Plate thickness 30 mm Lowest service temperature -30C Fatigue, but no impacts Joint class (frbandsklass) C = 50 Execution class (utfrande class) GB

16

Chooice of toughness class (Eurocode 3)

One reference temperature is determined

IWSD M4.4

= Lowest design temperature (EN 1991-1-5). Given for all Swedish cities

= Radiation loss (= 0, EN 1991-1-5).

= Adjustment for geometry, design and material (could be set to = 0 if table 2.1 is used)

= Extra safety margin (normally = 0)

= Adjustment if the strain rate is higher than , see next page

= Adjustment for cold forming, see next page

17

Chooice of toughness class (Eurocode 3)

Adjustment for strain rate other than:

which is base for table 2.1

This could for instance be impacts where the real strain rate is known

fy(t) = the materials yield strength as function of the plate thickness

When cold forming

where cf= is the cold forming grade in %

IWSD M4.4

18

Chooice of toughness class (Eurocode 3)

Ed= External design stress fy(t) = the materials yield strength as function of the plate thickness

IWSD M4.4

19

Crack growth of macro cracks

IWSD M4.4

20

Region 1: Threshold

Depends on the R-ration

For welds Kth = 2 MPam

Region 2: Linear stable crack growth

a = crack length N = number of cycles da/dN = crack growth/cycle K = SIF range C and n are material dependent constants

Paris law

IWSD M4.4

21

Region 3: Instability final failure

Fracture toughness Kc depends on: Material quality (increases with increase quality) Thickness (decrease with increased thickness) Temperature (decrease with lower temperature)

Threshold region Paris region

Instability region

IWSD M4.4

22

Region 3: Instability final failure

Fracture toughness Kc

Plane stress Transition area Plane strain

IWSD M4.4

23

Fracture toughness Kic testing

CT-specimen

3 point bending

IWSD M4.4

24

Fracture toughness Kic testing

The test is carried out in two stages: First stage is fatigue to develop a fatigue crack. The crack length is

determined after the fatigue test (0.45W < a < 0.55W) A slowly increased load P is applied. The Crack tip Opening

Displacement (COD) is measured and plotted vs. P.

Tensile test, CT (Compact Tension)

Bending test

IWSD M4.4

25

Fracture toughness Kic testing

Stress intensity factor - CT

IWSD M4.4

26

Fracture toughness Kic testing

Stress intensity factor 3 pt bending

IWSD M4.4

Stress Intensity Factor, SIF

27

aSK yC

Kc is a material parameter called the critical stress intensity factor or fracture toughness

IWSD M4.4

Apporximative relation between impact toughness and fracture toughness According to SSAB

28 IWSD M4.4

KV = Impact toughness according to charpy V testing in Joule

29

Problem 4.4.1

Change to a High Strength Steel Assume that a component in the shape of a large sheet is to be fabricated C-Mn Steel. It is required that the critical flaw size be greater than 2 mm, the resolution limit of available flaw detection procedures. A design stress of one half the tensile strength is indicated. To save weight, and increase in the tensile strength is suggested, from 1520 to 2070 MPa. Is such a strength increment allowable ? (assume plane-strain conditions in all computations)

IWSD M4.4