Origin of Fracture Mechanics
The Three-Age
Humans in the Stone Age mainly used stone for toolmaking. Flint axehead makers realised that flint was a relatively strong but under special conditions hopelessly brittle material. Flint axeheads could be fractured to produce the rough outline of the shape required and then accomplished by hammering against another stone to cause the axehead to fracture along the planes of weakness.
Although they were effective for crushing the heads of the tiger, flint made axeheads have a nasty tendency to shatter when struck against the ground - not a useful property...
In the Bronze (Copper & Tin alloys) age, men have developed a material that was capable of being moulded to almost anyform and possess a ductile property. However, bronze is too soft to be made as cutting tools or weapons.
Not until the Iron age, humans finally had a material which possessed a good balance between strength and hardness, but still a material with a propensity to fracture unexpectedly.
Since the beginning of the 21st century, engineers started to question themselves "So how can I really stop it from fracturing?"
The first approach was the one that is still in common usage today - To 'overdesign' the object. This has led to the common practice to proof test structures and components by subjecting them to a much larger load/ stress than they would ever see in service, in modern words 'a greater safety factor value'.
Despite this design philosophy and the generally low stress applied, catastrophic fractures continued to occur from time to time in a huge range of components and structures, from water boilers to railway equipments. But it was not until the 1940's, approximately a century after the first introduction of steel, steam engines and railways, when a series of disastous failures of steel structures gave a huge amount of driving force for the attention to be turned to answer the truly fundamental questions - "Why and How do they break?"
A Real Breakthrough
It was found that in 1920, a far more fundamental piece of research had already been carried out by a British physicist, A.A.Griffiths who had addressed why glass fibres fracture in practice at stress levels that are approximately 2 orders of magnitude lower than their theorectical strength. Griffiths realised that the fracture of glass is inevitable if the extenssion of an existing crack lowers the overall energy of the system, which apparently is an example of a thermodynamic approach to fracture mechanics.
Later in the 1940's an American scientist G.R.Irwin had modified Griffith's theory (considering only the energy balance between the strain energy of the body and the surface energy) by pointing out that Griffith's criterion could only be applied to materials that are truly brittle like glass, but for a metallic material, the work done in plastic deformation should also be taken into account.
Thus the basis for fracture mechanics came about with the definition of a material property G (the total energgy absorbed during a unit increment of crack length per unit thickness) which is referred to as the 'strain energy release rate". Only a few years later, Irwin had made an infremental step by showing that it was possible to reconcile the concept of a critical stress intensity (KC) causing fracture, with the idea of the critical strain energy release rate (GC).
The realisation that the stress intensity and strain energy approaches to the prediction of fracture has led to a rapid development in the disipline of Linear Fracture Mechanics (LEFM), allowing engineers to predict the tolerable defects in a given component under certain loading conditions - the fundamental aim of Fracture Mechanics.
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