BT001

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Published on November 16, 2008

Author: charliechong

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BT001

Chapter 12 Brittle fracture and impact tests Subjects of interest • Objective • The brittle-fracture problem • Notch-bar impact tests • Ductile to metal transition temperature curve • Metallurgical factors affecting transition temperature. • Drop-weight test and other large scale tests • Embrittlement in metals Suranaree University of Technology Tapany Udomphol May-Aug 2007

Objectives •This chapter provides an understanding of characteristics and causes of brittle fracture and factors affecting brittle fracture will be indicated. • The awareness of brittle fracture under service conditions will be made by the use of ductile to brittle transition temperature curve. Suranaree University of Technology Tapany Udomphol May-Aug 2007

The brittle-fracture problems Three basic factors contribute to a brittle cleavage fracture. 1) Triaxial state of stress 2) Low temperature 3) High strain rate Impact test Failure of Liberty Ships during services in World War II. To determine the susceptibility The cause of failure was due to crack of materials to brittle behaviour. initiated from defects in the welded area and subjected to subzero temperature. Suranaree University of Technology Tapany Udomphol May-Aug 2007

Notch bar impact test • Notched bar test specimens are used in different sizes and designs. • The Charpy impact specimen is the most widely used. • The parameter (energy absorption) obtained are not readily expressed in terms of stress level, so it is difficult for design. • Can use the test result to indicate how brittle the materials are. The specimen is hit by a pendulum until fracture. Method of loading in Charpy V notch and Izod tests. Suranaree University of Technology Tapany Udomphol May-Aug 2006

The Charpy impact test The Charpy test measures the total energy absorbed during specimen fracture. • The standard Charpy specimen is hit by a pendulum at the opposite side of the notch and the energy required to break open is measured. • Standard specimen size is 10x10x55 mm3 with a V notch of 2 mm deep, 45o angle and 0.25 mm root radius. Charpy V Notch Geometry 45 o 2 mm www.twi.co.uk Notch root radius of 0.25 mm Charpy impact test diagram Absorbed energy Impact toughness Suranaree University of Technology Tapany Udomphol May-Aug 2007

Ductile to brittle transition temperature curve • The absorbed energy (Joule) is plotted against testing temperature, giving a ductile to brittle transition temperature curve (DBTT curve). Lower shelf • The curve represents a change in fracture behaviour from ductile at high temperature to brittle at lower temperature. DBTT curve Upper shelf Transition – mixed mode Transition Lower shelf Suranaree University of Technology Tapany Udomphol Upper shelf May-Aug 2006

Fracture surfaces of tested specimens Transition – Lower shelf mixed mode Upper shelf Shear lip Cleavage facets Cleavage facets Ductile tearing Microvoids Brittle fracture Mixed mode of brittle and ductile Microvoid coalescence in ductile failures failure Suranaree University of Technology Tapany Udomphol May-Aug 2007

Transition temperature Different criteria are used to determine 1) T1 transition temp is the the transition temperature, depending Temp at which fracture is on the purpose of the application. 100% ductile (fibrous). 2) T2 transition temp is the Temp at which fracture is 50% cleavage and 50% ductile. 3) T3 transition temp is the Temp at the average energy absorption of upper and lower shelves. 4) T4 transition temp is the Temp defined at Cv = 20J. 5) T5 transition temp is the Temp at which fracture is Various criteria of transition temperature 100% cleavage. obtained from Charpy test Note: FTP is fracture transition plastic Suranaree University of Technology Tapany Udomphol May-Aug 2007

Metallurgical factors affecting DBTT curves • The shape and position of the DBTT curve is important because it determines the transition temperature, which indicates where it is safe to use for the required application. • There are several factors affecting the DBTT curve. • Crystal structure • Interstitial atom • Grain size • Heat treatment • Specimen orientation • Specimen thickness Suranaree University of Technology Tapany Udomphol May-Aug 2007

Effect of crystal structure • Only BCC structure materials experience ductile to brittle transition temperature. be careful to select the service temperature. • This is due to limited active slip systems operating at low temperature. very low plastic deformation. • Increasing temperature allows more slip systems to operate more plastic deformation. • FCC and HCP metals do not experience ductile to brittle transition, therefore they give the same energy absorption at any temperatures. Relationship between energy absorption and test temperature Suranaree University of Technology Tapany Udomphol May-Aug 2007

Effect of interstitial atom • Carbon and manganese contents have been observed to change the DBTT curve. Carbon Smoother curve content Become ductile at Higher Transition temp higher temperature Ex: in steel • Mn: C ratio should be at least 3:1 to satisfy notch toughness. • P, Si, Mo, O raise the transition temperature while Ni is beneficial to notch toughness. Effects of carbon content on DBTT curves for steel Suranaree University of Technology Tapany Udomphol May-Aug 2007

Effect of grain size • Grain size has a strong effect on transition temperature. Grain size Transition temperature Small grain size Absorbed energy, J Large grain size T3 T3 Temperature • Reducing grain size shifts the DBTT curve to the left has a wider range of service temperatures. • Heat treatments that provide grain refinement such as air cooling, recrystallisation during hot working help to lower transition temperature. Suranaree University of Technology Tapany Udomphol May-Aug 2007

Effect of heat treatment • Tempered martensitic structure steel produces the best combination of strength and impact toughness. Tempering temperature Energy absorption DBTT curves of different alloy steel, having tempered martensitic structure Suranaree University of Technology Tapany Udomphol May-Aug 2007

Effect of specimen orientation • For impact test, anisotropic properties are also observed in rolled or forged products, giving different energy absorption according to specimen orientations. • Longitudinal (B) shows the best energy absorption because the crack propagation is across the fibre alignment. • Transverse (C) gives the worst energy absorption because the crack propagates parallel to the rolling direction. Effect of specimen orientation on DBTT curve Suranaree University of Technology Tapany Udomphol May-Aug 2007

Effect of specimen thickness • Larger specimen size (in-service components) provides higher constraint more brittle. If large size specimens are used, the transition temperature will increase. Large scale tests Effect of section thickness on transition temperature Suranaree University of Technology Tapany Udomphol May-Aug 2007

Drop-weight test and other large scale tests • Several techniques have been developed to test specimens with different sizes to suit the applications. • The specimen thickness is at least 25 mm. 1) Explosion-crack starter test 2) Drop-weight test (DWT) 3) Dynamic-tear test (DT) 4) Robertson crack-arrest test Suranaree University of Technology Tapany Udomphol May-Aug 2007

Explosion crack starter test • The plate was placed over a circular die and dynamically loaded with an explosive charge. • The brittle weld bead introduces a small natural crack in the test plate. • The test is carried out over a temperature range, giving different fracture appearance. NDT – nil ductility temperature FTE – Fracture transition elastic FTP – Fracture transition plastic Plate dimensions : 350x350x25 mm3 Fracture appearance vs temperature Suranaree University of Technology Tapany Udomphol May-Aug 2007

Drop weight test Robertson crack- arrest test. Dynamic tear test Suranaree University of Technology Tapany Udomphol May-Aug 2007

Embrittlement in metals • Temper embrittlement • Hydrogen embrittlement • Stress corrosion cracking • Liquid metal embrittlement • Neutron embrittlement Delayed fracture curve Suranaree University of Technology Tapany Udomphol May-Aug 2007

Reference • Dieter, G.E., Mechanical metallurgy, 1988, SI metric edition, McGraw-Hill, ISBN 0-07-100406-8. Suranaree University of Technology Tapany Udomphol May-Aug 2007

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