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MM 362 ASSIGNMENTOBJECTIVE :- STUDY THE FRACTURE SURFACESa) Tensile failureb) Fatigue failure c) Creep fracture d) Failure in corrosive atmosphere e) Explanation of fractographs experimentally examined in the lab with the help of mechanisms of different types of fracture.

SUBMITTED BY :-Group 3 (B.Tech)A fracture is the separation of an object or material into two or more pieces under the action of stress.The fracture of a solid usually occurs due to the development of certain displacement discontinuity surfaces within the solid. If a displacement develops perpendicular to the surface of displacement, it is called a normal tensile crack; if a displacement develops tangentially to the surface of displacement, it is called a shear crack, slip band, or dislocation.Example:-The failure of Mississippi River bridge in August 2007, USA has caused terrible losses in life and required high cost of reconstructions

What is Fracture ?

Fracture of gusset responsible for the collapse

Collapsed BridgeWhat is Fractography ?Fractography is the study of the fracture surfaces of materials. Fractographic methods are routinely used to determine the cause of failure in engineering structures, especially in product failure and the practice of forensic engineering or failure analysis.

ALUMINIUM

BRASS

EN8

MILD STEEL

STAINLESS STEELA) FRACTURE SURFACE OF TENSILE FAILUREDepending on the ability of material to undergo plastic deformation before the fracture two fracture modes can be defined - ductile or brittleDuctile fracture - most metals (not too cold):Extensive plastic deformation ahead of crackCrack is stable: resists further extension unless applied stress is increasedFracture surfaces of the tensile specimen failed in a ductile mode exhibit a so called cup and cone type fracture, showing gross plastic deformation on both half of the fracture surfacesTherefore requires higher energy to create two new fresh fracture surfaces in comparison to energy required to cleave flat brittle surfacesBrittle fracture - ceramics, ice, cold metals: Relatively little plastic deformationCrack is unstable: propagates rapidly without increase in applied stressTensile specimens that have been failed in a brittle failure mode have flat surfaces with limited plastic deformation Brittle materials normally exhibit flat fracture surfaces consisting of transgranular cleavage facets

Brittle vs. Ductile Fracture

Ductile materials - extensive plastic deformation and energy absorption (toughness) before fracture

Brittle materials - little plastic deformation and low energy absorption before fracture

Brittle vs. Ductile Fracture

ABCA) Very ductile, soft metals (e.g. Pb, Au) at roomtemperature, other metals, polymers, glasses at hightemperature.B) Moderately ductile fracture, typical for ductile metalsC)Brittle fracture, cold metals, ceramicsBrittle FractureDuctile Fracture

Crackgrows90o toappliedstressCup-and-cone fracture45O -maximumShear stress

(Cup-and-cone fracture in Al)

Tensile failure

Shear failure

Brittle fracture in a mild steel

Fracture surfaces showing crack initiation sites eventually leading to failures

B) FATIGUE FAILURE Fatigue fracture surfaces possess quite a unique characteristic of flat surfaces with limited plastic deformationSurface condition is a prime factor in controlling surface crack initiationRough surfaces are attractive to stress concentration, leading to easy fatigue crack initiationstudy of stage II fatigue fracture surface using SEM technique show striations orientated normal to the fatigue crack growth directionEach striation is due to plastic blunting processWhen the tensile loading progresses, the crack opens and allows the slips to operate at the top and bottom ends to produce local plastic deformationAt a higher tensile loading, plastic blunting occurs, which leads to an increase in the fatigue crack lengthDuring unloading, the fatigue crack is then closed and the slips are now operating in the opposite directionTherefore, after one cycle, the fatigue crack now arrives at the original stage of crack closing with an increase in one fatigue striation

the fatigue failure can be found to occur in conjunction with corrosive and high temperature environmentCorrosion and high temperature accelerate the rate of the fatigue crack propagation and promote severe fatigue failures

rust on the fatigue surface observed in an automobile shaft

a beach mark which indicates the fatigue crack initiation to have started at the top surface

C) CREEP FAILUREcreep failures will include the degraded microstructures of graphite or spheroidized carbides along with the grain-boundary voids and cracks characteristic of these high-temperature, long-time failures.While creep failures are expected for superheaters and reheaters operating at design conditions, deviations from these parameters will promote early failures. The steam temperature always varies some from individual tube to tube, and the design allows for this variability. However, when the range of temperatures is larger than accounted for, the hottest tubes fail sooner than expected. A more likely cause of premature failure is the slow increase in tube-metal temperatures due to the formation of the steam-side scaleThe microstructures themselves will show the grain-boundary sliding and the resultant creep cracks or voids. For stainless steels, the microstructures are similar in that the failure is by grain-boundary-sliding and crack formation.What is creep? Creep may be defined as a time-dependent deformation at elevated temperature and constant stress. It follows, then, that a failure from such a condition is referred to as a creep failure or, occasionally, a stress rupture. The temperature at which creep begins depends on the alloy composition. For the common materials used in superheater and reheater construction, Table I (see below) gives the approximate temperatures for the onset of creep. It should be pointed out that the actual operating stress will, in part, dictate or determine the temperature at which creep begins.Creep failures are characterized by: bulging or blisters in the tube thick-edged fractures often with very little obvious ductility longitudinal "stress cracks" in either or both ID and OD oxide scales external or internal oxide-scale thicknesses that suggest higher-than-expected temperatures intergranular voids and cracks in the microstructure

A SEM micrograph of the crack surface, at left, shows the intercrystalline fracture mode and voids typical for creep failures

The voids that form on the grain boundaries in the early stagesCreep and creep testing

D) Failure in corrosive atmospheredeterioration of fatigue properties due to corrosion The influence of corrosion cracks developing in material and thus facilitating the fatigue crack initiation The decrease of fatigue limit due to the development of surface roughening

The examination of the fracture surfaces of specimens with corroded layer by means of SEM demonstrates that the fatigue cracks initiate exclusively on corroded surfaces

An example of a dimple initiating the crack under loading with R = 0 examination of fatigue crack initiation sites in specimens with corrosion layer brought an evidence of initiation at corrosion dimples The arrow indicates the dimple, from which the fatal fatigue crack started to propagate

The fracture surface of a failed specimen at higher magnificationThe layer of rust contains cracks. They do not grow further into the base material.They are not responsible for initiation of fatigue cracks which determine the lifetime under cyclic loading. They very often turn to the rust/base material boundary and separate the rust from the un-corroded material

(E) Explanation of fractographs experimentally examined in the lab with the help of mechanisms of different types of fracture.

We can see the necking and reduction in cross section area.Mild Steel SampleThis was a ductile fracture as we can observe the voids

Brass Sample There is no necking and fracture surface is almost planer Number of voids are less compared to ductile fracture and more planer surface are present hence is was a brittle fracture

Aluminium Sample Necking is visible

Iron Sample We can see the fracture origin Large number of voids are visible which occurs in ductile fracture Thank You