There are many reasons why a component fails. They generally fall under the following areas, bad design, incorrect materials, poor manufacturing and finally abuse of the component in-service.
Every component is made from a material however it can lose up to 90% of its intended mechanical properties by poor processing. Incorrect heat treatments can lead to the rapid wear of a component or fast fracture in a component due to lack of toughness where poor tempering was carried out. Poor welding is a major cause of failure.
Often the incorrect material was specified following the design process. Often poor labelling and poor traceability of materials can lead to an incorrect material being used to manufacture a component.
Poor design is another aspect, often due to incorrect stress analysis or assumptions. Another issue is the designer not having enough knowledge of different processes and their inherent weakness, e.g. sharp corners in castings. Designers must be familiar with fracture and the causes of fracture, this is especially important where structural integrity is paramount or serious loss of capital equipment or more importantly loss of lives. Examples of this would be transportation systems or facilities where high temperature and pressure vessels are involved.
However, there are times when it is not clear why a failure happened. So the investigating forensic engineer must be opened minded and that there may be a bigger picture leading to the failure.
The photo of container ship shows quite a failure due to fast fracture of the hull, credit to photo to the link below.