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VI. The phenomena of rupture and flow in solids

Why this mattered

Griffith’s paper changed fracture from a problem of nominal strength into an energy-balance problem. Earlier engineering criteria asked whether the maximum tensile stress or strain exceeded a material limit; Griffith showed that a solid containing a crack could fail at far lower applied stresses because elastic strain energy released by crack extension could pay the energetic cost of creating new surfaces. This explained why real glass and other brittle solids were much weaker than estimates based on intermolecular cohesion, and why surface scratches could dominate rupture. The key shift was to treat cracks as physical defects with calculable consequences, not as incidental imperfections outside the theory.

What became newly possible was a quantitative theory of brittle fracture. Griffith’s relation between crack size, elastic modulus, surface energy, and breaking stress made strength depend on flaw population and geometry, giving engineers a way to reason about polishing, scratches, notches, and scale effects. It also connected macroscopic rupture to microscopic cohesion without requiring the whole solid to reach a theoretical cohesive stress. In that sense the paper supplied the conceptual foundation for fracture mechanics: failure could be predicted from crack growth conditions rather than only from bulk stress limits.

The later breakthroughs in the field largely extended Griffith’s idea rather than replacing it. Irwin’s stress-intensity factor and energy-release-rate formulations recast Griffith’s criterion into tools usable for engineering cracks in structural materials, while Orowan and Irwin added plastic dissipation to explain fracture in metals where surface energy alone was insufficient. Modern damage tolerance, fatigue-crack growth analysis, and fracture toughness testing all descend from this reframing: structures are not assumed flawless, and safety is assessed by whether existing or detectable cracks can grow under service loads.

Abstract

Abstract In the course of an investigation of the effect of surface scratches on the mechanical strength of solids, some general conclusions were reached which appear to have a direct bearing on the problem of rupture, from an engineering standpoint, and also on the larger question of the nature of intermolecular cohesion. The original object of the work, which was carried out at the Royal Aircraft Estab­lishment, was the discovery of the effect of surface treatment—such as, for instance, filing, grinding or polishing—on the strength of metallic machine parts subjected to alternating or repeated loads. In the case of steel, and some other metals in common use, the results of fatigue tests indicated that the range of alternating stress which could be permanently sustained by the material was smaller than the range within which it was sensibly elastic, after being subjected to a great number of reversals. Hence it was inferred that the safe range of loading of a part, having a scratched or grooved surface of a given type, should be capable of estimation with the help of one of the two hypotheses of rupture commonly used for solids which are elastic to fracture. According to these hypotheses rupture may be expected if (a) the maximum tensile stress, (b) the maximum extension, exceeds a certain critical value. Moreover, as the behaviour of the materials under consideration, within the safe range of alternating stress, shows very little departure from Hooke’s law, it was thought that the necessary stress and strain calculations could be performed by means of the mathematical theory of elasticity.

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