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, and includes the study of past structural failures in order to prevent failures in future designs.

Structural integrity is the ability of an item—either a structural component or a structure consisting of many components—to hold together under a load, including its own weight, without breaking or deforming excessively. It assures that the construction will perform its designed function during reasonable use, for as long as its intended life span. Items are constructed with structural integrity to prevent catastrophic failure, which can result in injuries, severe damage, death, and/or monetary losses.

Structural failure refers to the loss of structural integrity, or the loss of load -carrying capacity in either a structural component, or the structure itself. Structural failure is initiated when a material is stressed beyond its strengthlimit, causing fracture or excessive deformations; one limit state that must be accounted for in structural design is ultimate failure strength. In a well- designed system, a localized failure should not cause immediate or even progressive collapse of the entire structure.

_INTRODUCTION_

Structural integrity is the ability of a structure to withstand its intended loading without failing due to fracture, deformation, or fatigue. It is a concept often used in engineering to produce items that will serve their designed purposes and remain functional for a desired service life

To construct an item with structural integrity, an engineer must first consider a material’s mechanical properties, such as  toughness, strength, weight,  hardness,  and elasticity, and then determine the size and shape necessary for the material to withstand the desired load for a long life. Since members can neither break nor bend excessively, they must be both stiff and tough. A very stiff material may resist bending, but unless it is sufficiently tough, it may have to be very large to support a load without breaking. On the other hand, a highly elastic material will bend under a load even if its high toughness prevents fracture.

Furthermore, each component’s integrity must correspond to its individual application in any load-bearing structure. Bridge supports need a high yield strength, whereas the bolts that hold them need good shear and tens ile strength  Springs need good elasticity, but lathe tooling needs high rigidity. In addition, the entire structure must be able to support its load without its weakest links failing, as this can put more stress on other structural elements and lead to cascading failures

_HISTORY_

The need to build structures 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, the science of  fracture mechanics  as it exists today was not developed until the 1920s, when Alan Arnold Griffith  studied the  brittle fracture  of glass.

Starting in the 1940s, the infamous failures of several new technologies made a more scientific method for analyzing structural failures necessary. During World War II, over 200 welded-steel ships broke in half due to brittle fracture, caused by stresses created from the welding process, temperature changes, and by the  stress concentration  at the square corners of the bulkheads. In the 1950s, several De Havilland Comets  exploded in mid-flight due to stress concentrations at the corners of their squared windows, which caused cracks to form and the pressurized cabins to explode. Boiler explosions, caused by failures in pressurized boiler tanks, were another common problem during this era, and caused severe damage. The growing sizes of bridges and buildings led to even greater catastrophes and loss of life. This need to build constructions with structural integrity led to great advances in the fields of material sciences and fracture mechanics.

_TYPES OF FAILURE_

Structural failure can occur from many types of problems, most of which are unique to different industries and structural types. However, most can be traced to one of five main causes.

The first is that the structure is not strong and tough enough to support the load, due to either its size, shape, or choice of material. If the structure or component is not strong enough, catastrophic failure can occur when the structure is stressed beyond its critical stress level.The second type of failure is from fatigue or corrosion, caused by instability in the structure’s geometry, design or material properties. These failures usually begin when cracks form at stress points, such as squared corners or bolt holes too close to the material's edge. These cracks grow as the material is repeatedly stressed and unloaded (cyclic loading), eventually reaching a critical length and causing the structure to suddenly fail under normal loading conditions.The third type of failure is caused by manufacturing errors, including improper selection of materials, incorrect sizing, improper heat treating, failing to adhere to the design, or shoddy workmanship. This type of failure can occur at any time and is usually unpredictable.The fourth type of failure is from the use of defective materials. This type of failure is also unpredictable, since the material may have been improperly manufactured or damaged from prior use.The fifth cause of failure is from lack of consideration of unexpected problems. This type of failure can be caused by events such as vandalism, sabotage, or natural disasters. It can also occur if those who use and maintain the construction are not properly trained and overstress the structure.