Fracture mechanics is the field of mechanics concerned with the study of the formation of cracks in materials. It uses methods of analytical solid mechanics to calculate the driving force on a crack and those of experimental solid mechanics to characterize the material's resistance to fracture.
In modern materials science, fracture mechanics is an important tool in improving the mechanical performance of materials and components. It applies the physics of stress and strain, in particular the theories of elasticity and plasticity, to the microscopic crystallographic defects found in real materials in order to predict the macroscopic mechanical failure of bodies. Fractography is widely used with fracture mechanics to understand the causes of failures and also verify the theoretical failure predictions with real life failures.
Contents 
Fracture mechanics was invented during World War I by English aeronautical engineer, A. A. Griffith, to explain the failure of brittle materials.^{[1]} Griffith's work was motivated by two contradictory facts:
A theory was needed to reconcile these conflicting observations. Also, experiments on glass fibers that Griffith himself conducted suggested that the fracture stress increases as the fiber diameter decreases. Hence the uniaxial tensile strength, which had been used extensively to predict material failure before Griffith, could not be a specimenindependent material property. Griffith suggested that the low fracture strength observed in experiments, as well as the sizedependence of strength, was due to the presence of microscopic flaws in the bulk material.
To verify the flaw hypothesis, Griffith introduced an artificial flaw in his experimental specimens. The artificial flaw was in the form of a surface crack which was much larger than other flaws in a specimen. The experiments showed that the product of the square root of the flaw length (a) and the stress at fracture (σ_{f}) was nearly constant, which is expressed by the equation:
An explanation of this relation in terms of linear elasticity theory is problematic. Linear elasticity theory predicts that stress (and hence the strain) at the tip of a sharp flaw in a linear elastic material is infinite. To avoid that problem, Griffith developed a thermodynamic approach to explain the relation that he observed.
The growth of a crack requires the creation of two new surfaces and hence an increase in the surface energy. Griffith found an expression for the constant C in terms of the surface energy of the crack by solving the elasticity problem of a finite crack in an elastic plate. Briefly, the approach was:
where E is the Young's modulus of the material and γ is the surface energy density of the material. Assuming E = 62 GPa and γ = 1 J/m^{2} gives excellent agreement of Griffith's predicted fracture stress with experimental results for glass.
Griffith's work was largely ignored by the engineering community until the early 1950s. The reasons for this appear to be (a) in the actual structural materials the level of energy needed to cause fracture is orders of magnitude higher than the corresponding surface energy, and (b) in structural materials there are always some inelastic deformations around the crack front that would make the assumption of linear elastic medium with infinite stresses at the crack tip highly unrealistic. F. Erdogan (2000)^{[2]}
Griffith's theory provides excellent agreement with experimental data for brittle materials such as glass. For ductile materials such as steel, though the relation still holds, the surface energy (γ) predicted by Griffith's theory is usually unrealistically high. A group working under G. R. Irwin^{[3]} at the U.S. Naval Research Laboratory (NRL) during World War II realized that plasticity must play a significant role in the fracture of ductile materials.
In ductile materials (and even in materials that appear to be brittle^{[4]}), a plastic zone develops at the tip of the crack. As the applied load increases, the plastic zone increases in size until the crack grows and the material behind the crack tip unloads. The plastic loading and unloading cycle near the crack tip leads to the dissipation of energy as heat. Hence, a dissipative term has to be added to the energy balance relation devised by Griffith for brittle materials. In physical terms, additional energy is needed for crack growth in ductile materials when compared to brittle materials.
Irwin's strategy was to partition the energy into two parts:
where γ is the surface energy and G_{p} is the plastic dissipation (and dissipation from other sources) per unit area of crack growth.
The modified version of Griffith's energy criterion can then be written as
For brittle materials such as glass, the surface energy term dominates and . For ductile materials such as steel, the plastic dissipation term dominates and . For polymers close to the glass transition temperature, we have intermediate values of .
Another significant achievement of Irwin and his colleagues was to find a method of calculating the amount of energy available for fracture in terms of the asymptotic stress and displacement fields around a crack front in a linear elastic solid.^{[3]} This asymptotic expression for the stress field around a crack tip is
where σ_{ij} are the Cauchy stresses, r is the distance from the crack tip, θ is the angle with respect to the plane of the crack, and f_{ij} are functions that are independent of the crack geometry and loading conditions. Irwin called the quantity K the stress intensity factor. Since the quantity f_{ij} is dimensionless, the stress intensity factor can be expressed in units of .
Irwin was the first to observe that if the size of the plastic zone around a crack is small compared to the size of the crack, the energy required to grow the crack will not be critically dependent on the state of stress at the crack tip.^{[2]} In other words, a purely elastic solution may be used to calculate the amount of energy available for fracture.
The energy release rate for crack growth or strain energy release rate may then be calculated the change in elastic strain energy per unit area of crack growth, i.e.,
where U is the elastic energy of the system and a is the crack length. Either the load P or the displacement u can be kept fixed while evaluating the above expressions.
Irwin showed that for a mode I crack (opening mode) the strain energy release rate and the stress intensity factor are related by:
where E is the Young's modulus, ν is Poisson's ratio, and K_{I} is the stress intensity factor in mode I. Irwin also showed that the strain energy release rate of a planar crack in a linear elastic body can be expressed in terms of the mode I, mode II (sliding mode), and mode III (tearing mode) stress intensity factors for the most general loading conditions.
Next, Irwin adopted the additional assumption that the size and shape of the energy dissipation zone remains approximately constant during brittle fracture. This assumption suggests that the energy needed to create a unit fracture surface is a constant that depends only on the material. This new material property was given the name fracture toughness and designated G_{Ic}. Today, it is the related quantity K_{Ic} which is called the fracture toughness and is now universally accepted as the defining material property in linear elastic fracture mechanics.
But a problem arose for the NRL researchers because naval materials, e.g., shipplate steel, are not perfectly elastic but undergo significant plastic deformation at the tip of a crack. One basic assumption in Irwin's linear elastic fracture mechanics is that the size of the plastic zone is small compared to the crack length. However, this assumption is quite restrictive for certain types of failure in structural steels though such steels can be prone to brittle fracture, which has led to a number of catastrophic failures.
Linearelastic fracture mechanics is of limited practical use for structural steels for another more practical reason. Fracture toughness testing is very expensive and engineers believe that sufficient information for selection of steels can be obtained from the simpler and cheaper Charpy impact test.^{[citation needed]}
Most engineering materials show some inelastic behavior under operating conditions that involve large loads.^{[citation needed]} In such materials the assumptions of linear elastic fracture mechanics may not hold, that is,
Therefore a more general theory of crack growth is needed for elasticplastic materials that can account for:
An early attempt in the direction of elasticplastic fracture mechanics was Irwin's crack extension resistance curve or Rcurve. This curve acknowledges the fact that the resistance to fracture increases with growing crack size in elasticplastic materials. The Rcurve is a plot of the total energy dissipation rate as a function of the crack size and can be used to examine the processes of slow stable crack growth and unstable fracture. However, the Rcurve was not widely used in applications until the early 1970s. The main reasons appear to be that the Rcurve depends on the geometry of the specimen and the crack driving force may be difficult to calculate.^{[2]}
In the mid1960s James R. Rice (then at Brown University) and G. P. Cherepanov independently developed a new toughness measure to describe the case where there is sufficient cracktip deformation that the part no longer obeys the linearelastic approximation. Rice's analysis, which assumes nonlinear elastic (or monotonic deformationtheory plastic) deformation ahead of the crack tip, is designated the J integral.^{[5]} This analysis is limited to situations where plastic deformation at the crack tip does not extend to the furthest edge of the loaded part. It also demands that the assumed nonlinear elastic behavior of the material is a reasonable approximation in shape and magnitude to the real material's load response. The elasticplastic failure parameter is designated J_{Ic} and is conventionally converted to K_{Ic} using Equation (3.1) of the Appendix to this article. Also note that the J integral approach reduces to the Griffith theory for linearelastic behavior.
If the metal is so tough that the yielded region ahead of the crack extends to the far edge of the specimen before fracture, the crack is no longer an effective stress concentrator. Instead, the presence of the crack merely serves to reduce the loadbearing area. In this regime the failure stress is conventionally assumed to be the average of the yield and
The following information is needed for a fracture mechanics prediction of failure:
Usually not all of this information is available and conservative assumptions have to be made.
Occasionally postmortem fracturemechanics analyses are carried out. In the absence of an extreme overload, the causes are either insufficient toughness (K_{Ic}) or an excessively large crack that was not detected during routine inspection.
Arising from the manufacturing process, interior and surface flaws are found in all metal structures. Not all such flaws are unstable under service conditions. Fracture mechanics is the analysis of flaws to discover those that are safe (that is, do not grow) and those that are liable to propagate as cracks and so cause failure of the flawed structure. Fracture mechanics as a subject for critical study has barely been around for a century and thus is relatively new. There is a high demand for engineers with fracture mechanics expertise—particularly in this day and age where engineering failure is considered 'shocking' amongst the general public.
For the simple case of a thin rectangular plate with a crack perpendicular to the load Griffith’s theory becomes:
where G is the strain energy release rate, σ is the applied stress, a is half the crack length, and E is the Young’s modulus. The strain energy release rate can otherwise be understood as: the rate at which energy is absorbed by growth of the crack.
However, we also have that:
If G ≥ G_{c}, this is the criterion for which the crack will begin to propagate.
Eventually a modification of Griffith’s solids theory emerged from this work; a term called stress intensity replaced strain energy release rate and a term called fracture toughness replaced surface weakness energy. Both of these terms are simply related to the energy terms that Griffith used:
and
where K_{I} is the stress intensity, K_{c} the fracture toughness, and ν is Poisson’s ratio. It is important to recognize the fact that fracture parameter K_{c} has different values when measured under plane stress and plane strain
Fracture occurs when . For the special case of plane strain deformation, K_{c} becomes K_{Ic} and is considered a material property. The subscript I arises because of the different ways of loading a material to enable a crack to propagate. It refers to socalled "mode I" loading as opposed to mode II or III:
There are three ways of applying a force to enable a crack to propagate:
We must note that the expression for K_{I} in equation 2.1 will be different for geometries other than the centercracked infinite plate, as discussed in the article on stress intensity. Consequently, it is necessary to introduce a dimensionless correction factor, Y, in order to characterize the geometry. We thus have:
where Y is a function of the crack length and width of sheet given by:
for a sheet of finite width W containing a throughthickness crack of length 2a, or
for a sheet of finite width W containing a throughthickness edge crack of length a
Since engineers became accustomed to using K_{Ic} to characterise fracture toughness, a relation has been used to reduce J_{Ic} to it:
The remainder of the mathematics employed in this approach is interesting, but is probably better summarised in external pages due to its complex nature.
