From:

http://www.geekinterview.com/question_details/62921

A block fracture occurs when a block is being read by the backup and bieng written to at the same time by the DBWR. This happens because OS reads blcoks at a different rate than Oracle. your OS copy will pull pieces of an Oracle block at a time. What if the OS copy pulls half a block and while that is happening the block is changed by DBWR? When the OS copy pulls the second half of the block it will result in mismatched halves which Oracle would not know how to reconcile. This is also why the SCN of the datafile header does not change when a tablespace enters hot backup mode. When receovery occurs the fractured data file block will be replaced with a complete block from redo.

The concept of fractured block occrus when backup is under process.
It means you are taking backup and some other session is updating it.
Then backup process can not get the consistent image of the block and it will be inconsistent right thats why it is called fractured block.

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What is fractured Block?

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