Paris' law (also known as theParis–Erdogan equation) is acrack growth equation that gives the rate of growth of afatigue crack. Thestress intensity factor${\displaystyle K}$ characterises the load around a crack tip and the rate of crack growth is experimentally shown to be a function of the range of stress intensity${\displaystyle \mathrm{\Delta }K}$ seen in a loading cycle. The Paris equation is^[1]

where${\displaystyle a}$ is the crack length and${\displaystyle \mathrm{d}a/\mathrm{d}N}$ is the fatigue crack growth for a load cycle${\displaystyle N}$. The material coefficients${\displaystyle C}$ and${\displaystyle m}$ are obtained experimentally and also depend on environment, frequency, temperature and stress ratio.^[2] The stress intensity factor range has been found to correlate the rate of crack growth from a variety of different conditions and is the difference between the maximum and minimum stress intensity factors in a load cycle and is defined as

${\displaystyle \mathrm{\Delta }K=K_{\text{max}}-K_{\text{min}}.}$

Being apower law relationship between the crack growth rate during cyclic loading and the range of the stress intensity factor, the Paris–Erdogan equation can be visualized as a straight line on alog-log plot, where thex-axis is denoted by the range of the stress intensity factor and the y-axis is denoted by the crack growth rate.

The ability of ΔK to correlate crack growth rate data depends to a large extent on the fact that alternating stresses causing crack growth are small compared to the yield strength. Therefore crack tip plastic zones are small compared to crack length even in very ductile materials like stainless steels.^[3]

The equation gives the growth for a single cycle. Single cycles can be readily counted forconstant-amplitude loading. Additional cycle identification techniques such asrainflow-counting algorithm need to be used to extract the equivalent constant-amplitude cycles from avariable-amplitude loading sequence.

In a 1961 paper,P. C. Paris introduced the idea that the rate of crack growth may depend on the stress intensity factor.^[4] Then in their 1963 paper, Paris and Erdogan indirectly suggested the equation with the aside remark "The authors are hesitant but cannot resist the temptation to draw the straight line slope 1/4 through the data" after reviewing data on a log-log plot of crack growth versus stress intensity range.^[5] The Paris equation was then presented with the fixed exponent of 4.

Higher mean stress is known to increase the rate of crack growth and is known as themean stress effect. The mean stress of a cycle is expressed in terms of thestress ratio${\displaystyle R}$  which is defined as

${\displaystyle R=\frac{K_{\text{min}}}{K_{\text{max}}},}$ 

or ratio of minimum to maximum stress intensity factors. In the linear elastic fracture regime,${\displaystyle R}$  is also equivalent to the load ratio

${\displaystyle R\equiv \frac{P_{\text{min}}}{P_{\text{max}}}.}$ 

The Paris–Erdogan equation does not explicitly include the effect of stress ratio, although equation coefficients can be chosen for a specific stress ratio. Othercrack growth equations such as theForman equation do explicitly include the effect of stress ratio, as does theElber equation by modelling the effect ofcrack closure.

The Paris–Erdogan equation holds over the mid-range of growth rate regime, but does not apply for very low values of${\displaystyle \mathrm{\Delta }K}$ approaching the threshold value${\displaystyle \mathrm{\Delta }{K}_{\text{th}}}$ , or for very high values approaching the material'sfracture toughness,${\displaystyle K_{\text{Ic}}}$ . The alternating stress intensity at the critical limit is given by .^[6]

The slope of the crack growth rate curve on log-log scale denotes the value of the exponent${\displaystyle m}$  and is typically found to lie between${\displaystyle 2}$  and${\displaystyle 4}$ , although for materials with low static fracture toughness such as high-strength steels, the value of${\displaystyle m}$  can be as high as${\displaystyle 10}$ .

Because the size of the plastic zone${\displaystyle (r_{\text{p}}\approx K_I^2/{\sigma }_y^2)}$  is small in comparison to the crack length,${\displaystyle a}$  (here,${\displaystyle {\sigma }_y}$  is yield stress), the approximation of small-scale yielding applies, enabling the use of linear elastic fracture mechanics and thestress intensity factor. Thus, the Paris–Erdogan equation is only valid in the linear elastic fracture regime, under tensile loading and for long cracks.^[7]