Behavior of Materials in Service Lecture 4

The bend test for brittle materials In a ductile material, the stress-strain curve shows a maximum at the tensile strength, not at the point of failure (or, the breaking strength). In a brittle or moderately ductile material, the maximum load occurs at the point of failure. In extremely brittle materials (e.g., cast irons, ceramics), the yield strength, tensile strength and breaking strength are all have about the same value. stress-strain behavior of brittle materials compared with that of more ductile materials

The bend test for brittle materials  In many brittle materials, the normal tensile test cannot easily be performed due to the  presence of flaws  difficulties in gripping the test sample  high cost in preparing tensile test bar  One approach used to minimize these problems is the bend test. Bend test - Application of a force to the center of a bar that is supported on each end to determine the resistance of the material to a static or slowly applied load. Flexural strength or modulus of rupture (MOR) - The stress required to fracture a specimen in a bend test. Flexural modulus - The modulus of elasticity calculated from the results of a bend test, giving the slope of the stress-deflection curve.

(a) Three point and (b) four-point bend test setup Since cracks and flaws tend to remain closed in compression, brittle materials are mostly tested in a compression test, not a tensile test. (a) Three point and (b) four-point bend test setup

(a) the set-up of 3-point bend test for brittle materials Stress-deflection curve for MgO obtained from a bend test (b) the deflection δ obtained after bending

Ductile vs. Brittle Materials • Ductile materials - extensive plastic deformation and energy absorption (“toughness”) before fracture • Brittle materials - little plastic deformation and low energy absorption before fracture

Fracture Separation of a body into pieces due to stress, at temperatures below the melting point. Steps in fracture: crack formation crack propagation Depending on the ability of material to undergo plastic deformation before the fracture two fracture modes can be defined - ductile or brittle Ductile fracture - most metals (not too cold): Extensive plastic deformation ahead of crack Crack is “stable”: resists further extension unless applied stress is increased Brittle fracture - ceramics, ice, cold metals: Relatively little plastic deformation Crack is “unstable”: propagates rapidly without increase in applied stress Ductile fracture is preferred in most applications (why???)

Ductile vs. Brittle Fracture Very ductile, soft metals (e.g. Pb, Au) at room temperature, other metals, polymers, glasses at high temperature. B. Moderately ductile fracture, typical for ductile metals C. Brittle fracture, cold metals, ceramics.

Ductile Failure vs. Brittle Failure Extensive plastic deformation ahead of advancing crack Very little plastic deformation at the crack front High energy absorption before failure (high toughness) Little energy absorption before failure (low toughness) Process proceeds relatively slowly as the crack length extended Crack advances extremely rapidly Such crack is stable (i.e., it resists any further deformation unless an increased stress is applied) Such crack is unstable and crack propagation, once started, continues spontaneously

Ductile Fracture (Dislocation Mediated) Steps in Ductile Fracture: Necking (b) Formation of micro-voids (cavities) (c) Coalescence of micro-voids to form elliptical crack (d) Crack propagation by shear deformation (e) Fracture

Dislocation (Brief Idea) The regular lattice in which atoms in a metal are arranged can contain line-like defects called dislocations. The dynamics of dislocations is the underlying mechanism for the plastic deformation of metals.

Brittle Fracture (Limited Dislocation Mobility) No appreciable plastic deformation Crack propagation is very fast Crack propagates nearly perpendicular to the direction of the applied stress and yields relatively flat fracture surfaces. Crack often propagates by cleavage – breaking of atomic bonds along specific crystallographic planes (cleavage planes).

Fractographic Study of Brittle Fracture Origin of cracks V shaped “chevron” markings Origin of cracks Radial fan shaped ridges

Brittle Fracture Transgranular fracture: Fracture cracks pass through grains. Fracture surface have faceted texture because of different orientation of cleavage planes in grains. Intergranular fracture: Fracture crack propagation is along grain boundaries (grain boundaries are weakened or embrittled by impurities segregation etc.)

Brittle Fracture For most brittle crystalline materials, crack propagation corresponds to the successive and repeated breaking of atomic bonds along specific crystallographic planes; such a process is termed cleavage. This type of fracture is said to be transgranular (or transcrystalline), because the fracture cracks pass through the grains. Macroscopically, the fracture surface may have a grainy or faceted texture, as a result of changes in orientation of the cleavage planes from grain to grain. This cleavage feature is shown at a higher magnification in the scanning electron micrograph of Figure A. In some alloys, crack propagation is along grain boundaries; this fracture is termed intergranular. Figure B is a scanning electron micrograph showing a typical intergranular fracture, in which the three-dimensional nature of the grains may be seen.

Brittle Fracture The tendency for brittle fracture is increased with decreasing temperature Increasing strain rate Tri-axial stress conditions (usually produced by a notch) Brittle fracture is to be avoided at all cost because it occurs suddenly without any warning and usually produces disastrous consequences.

Impact Fracture Testing (testing fracture characteristics under high strain rates) Two standard tests, the Charpy and Izod, measure the impact energy (the energy required to fracture a test piece under an impact load), also called the notch toughness.

Ductile to Brittle Transition Temperature (DBTT) • Pre-WWII: The Titanic • WWII: Liberty ships Disastrous consequences for a welded transport ship, suddenly split across the entire girth of the ship (40˚F). The vessels were constructed from steel alloys that exhibit a DBTT  room temp 18 18

impact energy drops suddenly over a narrow temperatures range Ductile-to-brittle transition As temperature decreases a ductile material can become brittle . Ductile-to-brittle transition: Alloying usually increases the ductile-to-brittle transition temperature. FCC metals remain ductile down to very low temperatures. For ceramics, this type of transition occurs at much higher temperatures than for metals. The ductile-to-brittle transition can be measured by impact testing: the impact energy needed for fracture drops suddenly over a relatively narrow temperature range – temperature of the ductile-to-brittle transition. impact energy drops suddenly over a narrow temperatures range

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 Metallurgical Factors

Relationship between energy absorption 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

Effect of interstitial atom Carbon and manganese contents have been observed to change the DBTT curve. Carbon content Become ductile at higher temperature Higher Transition temp 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

Effect of grain size • Grain size has a strong effect on Ductile-Brittle transition temperature. Grain size DBTT DBTT for ferritic steels • 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, recrystallization during hot working help to lower transition temperature.