After its planned descent capsule failed its pressure test, the launch of the manned Soyuz TMA-04M spacecraft was delayed from 30 March to 15 May. The test failure occurred on 23 January during a vacuum chamber test of the descent module – the part that is used to return the crew safely via a re-entry and a parachute landing. The capsule reportedly popped along a welded seam and the interior of the craft was damaged as a result of its exposure to vacuum. The damaged decent module has now been replaced by the descent module of Soyuz TMA-05M which was planned to fly after the Soyuz TMA-04M mission. The failure was not due to a design fault but was caused because test procedures had not been followed correctly. During the faulty test, the internal pressure was allowed to be raised over twice the nominal test pressure of 1.5 atmospheres before capsule popped causing decompression.

Soyuz capsule 

  © NASA

Soyuz TMA-7 in orbit - the descent moduel is the conical module in the centre which is located just  ahead of the solar arrays  

Source: FlightGlobal.com