Structural integrity and failure is an aspect of engineering which deals with the ability of a structure to support a designed load (weight, force, etc...) without breaking, tearing apart, or collapsing, and includes the study of breakage that has previously occurred in order to prevent failures in future designs.
Structural integrity is the term used for the performance characteristic applied to a component, a single structure, or a structure consisting of different components. Structural integrity is the ability of an item to hold together under a load, including its own weight, resisting breakage or bending. It assures that the construction will perform its designed function, during reasonable use, for as long as the designed life of the structure. Items are constructed with structural integrity to ensure that catastrophic failure does not occur, which can result in injuries, severe damage, death, and/or monetary losses.
Structural failure refers to the loss of structural integrity, which is the loss of the load-carrying capacity of a component or member within a structure, or of the structure itself. Structural failure is initiated when the material is stressed beyond its strength limit, thus causing fracture or excessive deformations. In a well-designed system, a localized failure should not cause immediate or even progressive collapse of the entire structure. Ultimate failure strength is one of the limit states that must be accounted for in structural engineering and structural design.
Structural integrity is the ability of a structure or a component to withstand a designed service load, resisting structural failure due to fracture, deformation, or fatigue. Structural integrity is a concept often used in engineering, to produce items that will not only function adequately for their designed purposes, but also to function for a desired service life.
To construct an item with structural integrity, an engineer must first consider the mechanical properties of a material, such as toughness, strength, weight, hardness, and elasticity, and then determine a suitable size, thickness, or shape that will withstand the desired load for a long life. A material with high strength may resist bending, but, without adequate toughness, it may have to be very large to support a load and prevent breaking. However, a material with low strength will likely bend under a load even though its high toughness prevents fracture. A material with low elasticity may be able to support a load with minimum deflection (flexing), but can be prone to fracture from fatigue, while a material with high elasitcity may be more resistant to fatigue, but may produce too much deflection unless the object is drastically oversized.
Structural integrity must always be considered in engineering when designing buildings, gears or transmissions, support structures, mechanical components, or any other item that may bear a load. The engineer must carefully balance the properties of a material with its size and the load it is intended to support. Bridge supports, for instance, need good yield strength, whereas the bolts that hold them need good shear and tensile strength. Springs need good elasticity, but lathe tooling needs high rigidity and minimal deflection. When applied to a structure, the integrity of each component must be carefully matched to its individual application, so that the entire structure can support its load without failure due to weak links. When a weak link breaks, it can put more stress on other parts of the structure, leading to cascading failures.
The need to build structure with integrity goes back as far as recorded history. Houses needed to be able to support their own weight, plus the weight of the inhabitants. Castles needed to be fortified to withstand assaults from invaders. Tools needed to be strong and tough enough to do their jobs. However, it was not until the 1920s that the science of fracture mechanics, namely the brittleness of glass, was described by Alan Arnold Griffith. Even so, a real need for the science did not present itself until World War II, when over 200 welded-steel ships broke in half due to brittle fracture, caused by a combination of the stresses created from the welding process, temperature changes, and the stress points created at the square corners of the bulkheads. The squared windows in the De Havilland Comet aircraft of the 1950s caused stress points which allowed cracks to form, causing the pressurized cabins to explode in mid-flight. Failures in pressurized boiler tanks were a common problem during this era, causing severe damage. The growing sizes of bridges and buildings began to lead to even greater catastrophes and loss of life. The need to build constructions with structural integrity led to great advances in the fields of material sciences and fracture mechanics.