An extensive investigation was conducted on both crashes.The findings of the investigation proved that the mid air disintegration was due to their square windows. De Havilland had infact conducted many tests during the production of the aircraft and the company had even proved the aircraft couldn’t fail under pressurized cabin. Everyone at that time believed the pre-production test to be enough for safety ,since de Havilland had conducted lots of experimental tests.The Calculations during the design had been made for an average stress which found the stress to be less than half the ultimate strength of the material. The calculation seemed enough for the flight safety and De Havilland did not calculated further stress values and relied on testing as the main evidence for the safety of the Comet. The company missed one thing, it did not take account of stress concentration in the corners.The investigation found that the stress concentration was high because of the square shape of the windows and window frame.The abrupt change is geometry on the corners disrupts the stress flow .This is not the case with modern rounded windows.Due to higher stress on the corners, cutouts in the structure there is always a chance of crack initiation on those areas. The aircraft structure is under various loads. All the loads are not of constant magnitude and are in cyclic nature. This results in fatigue load. Fatigue load results in crack growth over a large span of cycle. But since fracture mechanics was not understood in 1954, the crack propagation theory was not considered during design of the aircraft. The failure of the pressure cabin was due to fatigue crack growth from defects which were probably present from the construction of the aircraft and had not been a problem in earlier designs of aircraft, as the required cabin pressure had been lower. That this problem was not detected by the rigorous testing undertaken by de Havilland was probably due to an unfortunate set of circumstances with regard to the order in which the tests were performed, and could not easily be foreseen at the time.Stress concentration is the accumulation of stress in a body due to sudden change in its geometry. When there is a sudden change in the geometry of the body due to cracks sharp corners, holes and decrease in the cross section area, then there is an increase in the localized stress near these cracks, sharp corners, holes, and decreased cross section area. The body tends to fail from these places where the stress concentration is more. So to prevent a body from getting failed, the stress concentration should be avoided or reduced.