Histroy of Dutile Fractures

        Well, Hope everyone is preparing well for the Summer 2011 exams. Here are some of the cool facts about Ductile Fracture. The crack extension energy side of the Griffith equation applied only to "ideally brittle" materials. Believe me, it was not for lack of research that the materials research community failed to extend fracture theory into the very important field of ductile fracture. Some of the problems faced by Humanity due to Ductile fracture are given below:

Ships Break In Two!

This was an extremely serious problem in World War II, when over 250 ships fractured or cracked. Nineteen of these broke completely in two! Luckily, in some cases, fractures occurred in ships that were being outfitted and had never put to sea. All of the ship fractures and the two other examples that follow were in metals that were ductile, but just not tough enough.

The Great Boston Molasses Tank Disaster

One of the most famous brittle fractures was the Great Boston Molasses Tank Disaster in 1919. There was a tank of molasses, 90 ft in diameter and 50 feet high whose contents were supposed to have become rum. When the tank split, a wall of molasses advanced down the street. Many of the deaths and casualties occurred among people who were engulfed in their flats below the level of the street. There were 12 deaths and 40 injuries. Half a century later it was determined that the tank's steel was below its ductile/brittle transition temperature; the same problem as with the WWII merchant ships.

The Silver Bridge Collapse

A more recent brittle fracture disaster was the collapse of the Silver Bridge in West Virginia, in December 1967 in which 46 people perished as their cars plunged into the icy Ohio River. The National Bureau of Standards' metallurgists judged the bridge accident to be caused by stress-corrosion cracking resulting from long exposure to hydrogen sulfide vapor, H2S, from nearby paper mill digesters. The bridge failure is an example where the energy required to extend the fracture was reduced while the metal was in service.

With the benefit of 20/20 hindsight, the ship hull and molasses tank accidents occurred when the steel's energy required to extend the fracture at service temperatures was too low starting when the metal left the steel mills

The above information is taken from http://www.nhml.com/ Please do refer to them for more info. 

Ductile Fracture

In ductile fracture, extensive plastic deformation takes place before fracture. The terms rupture or ductile rupture describe the ultimate failure of tough ductile materials loaded in tension. Rather than cracking, the material "pulls apart," generally leaving a rough surface. In this case there is slow propagation and an absorption of a large amount energy before fracture.

Many ductile metals, especially materials with high purity, can sustain very large deformation of 50–100% or more strain before fracture under favorable loading condition and environmental condition. The strain at which the fracture happens is controlled by the purity of the materials. At room temperature, pure iron can undergo deformation up to 100% strain before breaking, while cast iron or high-carbon steels can barely sustain 3% of strain.

Because ductile rupture involves a high degree of plastic deformation, the fracture behavior of a propagating crack as modeled above changes fundamentally. Some of the energy from stress concentrations at the crack tips is dissipated by plastic deformation before the crack actually propagates.

The basic steps are: void formation, void coalescence (also known as crack formation), crack propagation, and failure, often resulting in a cup-and-cone shaped failure surface.

The steps are clearly shown in the figure given here. The above info is taken from http://www.websters-online-dictionary.org/. Please do refer to them for further info. 

Don't forget to grab a copy of Material Science and Engineering book, which is essential for preparing for AMIE, Material Science.

with warm regards

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Notes on Various Fractures

Hello Everyone,

Have a blessed day. Hoping your preparation is going cool unlike mine. I just thought I will share brief notes on various fractures to give a brief overview.

Brittle Fracture

     A fracture which takes place by rapid propagation of crack with a negligible deformation. In amorphous materials, the fracture is completely brittle. In crystalline materials, it occurs after small deformation.

Ductile Fracture:

     A fracture which takes place by a slow propagation of crack with appreciable plastic deformation. This type of fracture comes into play in materials which don't work harden much. 

Creep Fracture

     A fracture which takes place due to excessive creeping of materials, under steady load. Creep is exhibited in iron, nickel, copper and alloys at higher temperature.  Creep resistance may be increased by addition of certain elements such as cobalt, nickel , manganese, tungsten, ...

Fatigue Fracture:

    A fracture that occurs when a material is subjected to cyclic loading. If the loads are above a certain threshold, microscopic cracks will begin to form at the surface. Eventually a crack will reach a critical size, and the structure will suddenly fracture.

Don't forget to grab a copy of Material Science and Engineering book, which is essential for preparing for AMIE, Material Science.

with warm regards

AllMyPosts