Fracture Mechanics Online Class

Overview > Crack propagation in structures submitted to cyclic loading

Picture I.71: Specimen under cyclic loading.
Picture I.72: Evolution of the crack size in terms of the number of cycles.

The last concepts of fracture mechanics we need to introduce are related to fatigue. Let us consider a specimen under cyclic loading as described on Picture I.71. The value of $P$ ranges from $P_\text{min}$ to $P_\text{max}$ so that $\Delta P = P_\text{max} - P_\text{min}$. Due to the cyclic loading a crack nucleates at the stress concentration and propagates until failure of the specimen, although at the beginning the SIF remains lower than $K_{IC}$, see Picture I.72. In this overview we will explain the phenomenological explanation of fatigue, and also introduce a way to evaluate the life of a structure with an initial crack subjected to a cyclic loading. We will finish by introducing different design approaches.

Phenomenological explanations

Crack nucleation

Picture I.73: Dislocation motion due to cyclic loading: formation of Persistent Slip Bands (PSBs) from which the crack initiates.

The crack nucleates in a stress concentration area. During the loading of the sample, dislocations move along slip planes until reaching a free surface (or a bulk defect), see Picture I.73, center left. Remember that dislocations are missing atoms, so when they reach the surface this creates a step. During the unloading phase, dislocations can move in the opposite direction but usually it happens on other slip planes, see Picture I.73, center right. After a few cycles one can observe the formation of Persistent Slip Bands (PSBs), see Picture I.73, right. These PSBs are the locations from which the crack can nucleate.

Fatigue crack growth: Stage I.

Once the crack has nucleated, under the cyclic loading condition, it starts to growth along a slip plane of the crystal. The crack thus growths in a direction allowed by the crystallographic orientation.

Fatigue crack growth: Stage II.

Picture I.74: Fatigue fracture in rubber brake seal showing striations at left (SEM). Picture by Peterlewis licensed under the Creative Commons Attribution-Share Alike 3.0 Unported license. Subject to disclaimers.

During the second stage the crack has passed the first grains layer from the surface. In each grain the crack has to follow their crystallographic orientations. However when the crack size is larger than the size of a few grains, the crack appears as propagating at the macroscopic level in a direction governed by the maximum hoop stress (straight for pure mode I).

This crack propagation stage is thus a macroscopic propagation stage. It can easily be obseved on fatigue fractured sample under the form of striations (one per cycle) on the failure surface, see Picture I.74.

Fatigue crack growth: Stage III.

Picture I.75: Fracture of an aluminum crank arm. Bright: brittle fracture. Dark: fatigue fracture. Picture by Lokilech licensed under the Creative Commons Attribution-Share Alike 3.0 Unported license.

As the crack propagates under the cyclic loading its size increases. While at the beginning of the cyclic loading the SIF remains small compared the material toughness, as $a$ increases the SIF also increases and, at maximum loading $P_{\text{max}}$, becomes close to the toughness $K_{IC}$. At this stage we are back to a classical fracture mechanics problem and the sample will break at once. A fatigue fracture surface thus exhibits two surfaces: striation by fatigue in Stage II and brittle (or ductile) fracture in Stage III, see Picture I.75.

Prediction of the structural life

Picture I.76: Model of the $f$ function.

The question is now how can we predict the evolution of the crack size, and the failure point, in terms of the cycles number. In other words, how can we predict the curves of Picture I.72.

As the crack loading remains small during fatigue problems, the SSY assumption usually holds during some intervals of the crack growth or even until failure, depending on the case. As at the macro-scale the life of the structure with an initial crack size $a$ is observed to depend on the loading $\Delta P$ and on $\frac{P_\text{max}}{P_\text{min}}$ only, the SSY assumption allows us to say that the conditioning parameters are $\Delta K$ and $R = \frac{K_\text{max}}{K_\text{min}}$. Indeed from the SIFs equations we observed a linearity in $\Delta P$ of $\Delta K$. Therefore the evolution of the crack size obeys to

\begin{equation} \frac{d a}{d N_f} = f\left(\Delta K, R\right). \label{eq:dadN}\end{equation}

With the knowledge of this curve it is possible to determine for the number of cycles a structure can sustain before being replaced and to schedule the inspection intervals. Note that the uncertainty on this curve can reach several (tens of) percent, so regular intervals are thus required to validate or modify the prediction.

What remains to be defined now is the form of $f\left(\Delta K, R\right)$ in (\ref{eq:dadN}). Experimentally it has been found that this function follows the curve shown in Picture I.76.

Crack propagation in Stage I

Experimentally, it has been observed that if $\Delta K < \Delta K_\text{th}$, where $\Delta K_\text{th}$ the crack growth is in average smaller than one atomic spacing per cycle. Such a crack is considered as dormant. The value of $\Delta K_\text{th}$ is the fatigue threshold and depends on the material but also on the loading ratio $R$.

If $\Delta K > \Delta K_\text{th}$, the crack will propagate until reaching the stage II.

For steel, $\Delta K_\text{th}$ is between 2 and 5 $\text{MPa}\cdot\sqrt{\text{m}}$, but for steel in sea water, $\Delta K_\text{th}$ is between 1 and 1.5 $\text{MPa}\cdot\sqrt{\text{m}}$. This means that the environmental conditions in which the material is considered are very important.

Crack propagation in Stage II

This corresponds to the stage during which we can observe the striations. As shown in Picture I.76, the crack rate curve is linear with $\Delta K$ in a logarithmic scale. This is the Paris-Erdogan (1963) law:

\begin{equation} \frac{d a}{d N_f} = C \Delta K ^m.\label{eq:Paris} \end{equation}

This law is characterized by two parameters $C$ and $m$ which depend on the material and on the loading ratio $R$. For steel, $C \approx 0.1\times 10^{-11}$ $\text{m}\cdot\left(\text{MPa}\cdot\sqrt{\text{m}}\right)^{-m}$ and $m \approx$ 4. For steel in sea water, these values change a lot: $C \approx 1.6 \times 10^{-11}$ $\text{m}\cdot\left(\text{MPa}\cdot\sqrt{\text{m}}\right)^{-m}$ and $m \approx$ 3.3. This means that the environmental conditions in which the material is considered are very important. Note that the units of $C$ are cumbersome as they depend on the coefficient $m$.

When applying the Paris law, it is important to remember that $K$ depends on the crack size. Thus $\Delta K$ should be replaced by its complete expression when integrating to get $a(N_f)$. For instance, for Mode I in an infinite plate, we have:

\begin{equation} K_I = \sigma_\infty \sqrt{\pi a}, \end{equation}

and $\Delta K$ is given by:

\begin{equation} \Delta K = \left(\sigma_{\infty,\text{max}}-\sigma_{\infty,\text{min}}\right) \sqrt{\pi a}. \end{equation}

Crack propagation in Stage III

In this zone the crack grows rapidly until failure of the structure. The failure is reached as soon as the crack size reaches $a_f$ such that $K(P_\text{max},\,a_f)$ is equal to $K_c$.

Design methods

Picture I.77: Illustration of the fail safe design: Aloha Airlines flight 243. This image is a work of a National Transportation Safety Board employee, taken or made as part of an employee's official duties. As a work of the U.S. federal government, all NTSB images are in the public domain.

When an engineer has to design a structure submitted to cyclic loading, there exist different design approaches, appeared chronlogically in time with the improvment of fracture mechanics theory:

This ends the overview of what will be seen during the different lectures. As you could see fracture mechanics uses many new concepts, which at the first glimps seem to be very complex, but will look simpler at the end of the class.