Everyday thousands of people travel through airplanes all around the globe but most people aren’t aware of why the windows are rounded. The current generation of airplane travelers think that the windows has always been the shape it is today, i.e. oval but this is not the case. Before 1950s aircraft did have square windows. The evolution from square to round/oval windows isn’t simply for aesthetics, there is also a science behind this. This is the physics of Fracture Mechanics.In 1903 wright brother made the first flight and it wasn’t until 1952 the first jet airliner made its first maiden flight. The De Havilland DH 106 Comet was a great success since it could fly up to 35000 ft because of its pressurized cabin. The Higher Flight altitude meant that the density of air is low. This results in less drag on the structure which leads to higher speed. Bad weather could be avoided at higher altitude. The aircraft was considered feat of British engineering at that time. Even queen Elizabeth made a special flight on 30 June 1953 but the fame did not last long when two DH 106 Comet broke apart in midair killing 56 people. BOAC Flight 781 after suffering explosive decompression crashed into the Mediterranean killing all 35 people on board and South African Airways Flight 201 disintegrated in the night sky at around 35,000 feet (11,000 m), killing everyone (21 people) on board.INVESTIGATION:After an extensive investigation, the main cause of disintegration of both aircraft in midair was found because of their square windows. De Havilland ran many tests in pre-production to prove the safety of the Comet: from pressure tests, to flight tests, to stress tests. The extensive proof testing of the fuselage was believed to be hard evidence that the Comet was safe. This experiential knowledge gained from actual testing bolstered de Havilland’s confidence in their analyses. Calculations had been made for an average stress “in the neighborhood of the corners” which found the stress to be less than half the ultimate strength of the material. De Havilland did not consider further stress calculations to be any more accurate than the one already done, and preferred to rely on testing as the main evidence for the adequacy of the Comet. The company missed one thing, it did not take account of stress concentration in the corners.Figure 1: De Havilland DH 106 Comet with square windowsWHAT IS STRESS CONCENTRATION?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.Figure 2: Stress concentration on the corners due to abrupt change in geometryINVESTIGATION OUTCOME:The stress concentrations were high specifically because of the squarish shape of the windows and window frames which is very different from the round/oval shapes of modern airplane windows. With modern windows, the stress flows freely around the curved edges with minimal build up. But with the Comets’ squarish windows, stress cannot smoothly flow around the abrupt corners. This creates stress concentrations.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.