Brittle fracture in metals

Brittle fracture in metals is characterized by a rapid rate of crack propagation, with no gross deformation and very little microdeformation. This is demonstrated by cleavage. Brittle fracture can occur without warning. Cleavage fracture exhibits little or no plastic deformation and occurs along well-defined crystallographic planes. 

Orowan, E. (1952) Fundamentals of brittle behavior in metals, in Fatigue and Fracture of Metals, Symposium at MIT, ed. Murray, W.M. (MIT and Wiley, New York). 

Note that the service stress in any loaded machine or structure is the algebraic sum of the applied stress, due to the service load, and any residual stress that may have existed before the service load was applied. If the residual stress is not known, neither is the service stress. When the service stress reaches dangerous levels, failure occurs. Interest in residual stress stems mainly from the role it plays in three kinds of metal failure fatigue failure, brittle fractures in general, and stress-corrosion cracking. 

Hydrogen will dissolve easily in metals, and, once in solution, it can cause brittle fracture. In general. 

Abrahams, M.S. and Ekstrom, L. (1960) Dislocations and brittle fracture in elemental and compound semiconductors. Acta Metall., 8 (9), 654-662. 

Cottrell AH (1958) Theory of brittle fracture in steel and similar metals. Trans Metall Soc AIME 212 192... 

To model the failure of pressure vessels, we must first differentiate between brittle fracture and ductile failure. The easiest way to do this is to think of the child s toy called Potty Putty or Silly Putty. If this material is pulled slowly, it will stretch to tens of times its original length before it breaks (ductile failure). However, if it is pulled sharply, it snaps with hardly any stretching (brittle fracture). In the right circumstances, metal components can also fail in either a plastic or brittle manner. 

The analysis was first carried out by Griffith in a treatment of the brittle fracture of metals. Actually, the considerations are of general nature and can also be applied to polymers, after introducing some physically important but formally simple modifications. Griffith s approach is based on linear elasticity theory and its utility for polymers may look questionable at first, as those are neither elastic nor linear under the conditions near to failure. However, as we will see, the theory is indeed applicable and provides also here a satisfactory description of crack growth. 

SERVICE FACTORS. There are three service conditions which can promote brittle fracture in a metal that is ordinarily ductile low temperatures, high rates of loading, and stress concentrations. In evaluating the ability of a metal to resist brittle fracture, therefore, tests are used involving one or more of these three variables. The most severe tests are those involving all three variables—impact tests of notched specimens at low temperatures. 

Overpressme which may occur at normal or below normal pressures, as a result of reduced allowable stresses at higher than design temperatures, are also evaluated and appropriate protective features applied in the design. For example, such conditions may result from chemical reactions, startup or upset conditions. Likewise, low metal temperature must be considered, such as from autorefrigeration, to make sure that brittle fracture conditions do not develop. 

Whether the adsorbed hydrogen is produced from the gas phase or from aqueous solution, it appears that the presence of hydrogen atoms distorts the crystal structure of the metal surface, and this results in a surface solubility which is higher than that of the bulk. The depth of this distortion is not clear, but it seems possible that the distorted zone may play an important part in initiating brittle-fracture processes. 

---