# Difference between brittle fracture and fatigue crack growth

• ehabmozart
In summary, static loading can cause a crack to propagate only up to a certain point, and fatigue loading can cause a crack to propagate even further.

#### ehabmozart

My question here is regarding fracture of materials. I have problems in understanding some concepts. Now, for static loading, there is a property called KIC (fracture toughness). By equating our stress from the static load to this KIC, we get the maximum length of the crack which does not propagate. Above this length for this static loading, crack start propagating.

q1) What happens if our K becomes larger than K_IC? Does it rupture or the crack starts propagating?

Now, linking this to fatigue loading. In Paris relation, da/dN = C Δ Km. This is applicable for region II in the crack growth. As seen from the attached figure, region II it has two asymptotes along its sides. Stage I is the ΔKth. Here this threshold intensity factor is the point at which if we pass this threshold, the crack starts propagating.

q2) Can we link this figure to what is happening in the static failure. In other words, can we use a critical crack length which is common between them?

It would be better if I have a discussion group chat or something since this might save a lot of time and clear any confusion.

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My understanding is one parameter measures the failure of a steadily applied load, eg a notched-bar under increasing tension. The other is the resistance to failure when a lesser, 'working' load is repeatedly applied.

I suppose the classic example is the UK's Comet airliner where fatigue cracking at window corners destroyed several aircraft. This failure mode was proven by ground testing of a complete aircraft. After which, such corners were routinely rounded to prevent concentration of stress. Another engineering lesson bought in blood...

Nik_2213 said:
My understanding is one parameter measures the failure of a steadily applied load, eg a notched-bar under increasing tension. The other is the resistance to failure when a lesser, 'working' load is repeatedly applied.

I suppose the classic example is the UK's Comet airliner where fatigue cracking at window corners destroyed several aircraft. This failure mode was proven by ground testing of a complete aircraft. After which, such corners were routinely rounded to prevent concentration of stress. Another engineering lesson bought in blood...
And I believe an example of brittle fracture is the habit of the "liberty ships", which were welded construction, of breaking apart in low temperatures even when not highly loaded.

## What is the difference between brittle fracture and fatigue crack growth?

Brittle fracture is a sudden, catastrophic break in a material without any prior deformation or warning. It occurs when the stress on a material exceeds its strength, causing it to fracture. On the other hand, fatigue crack growth is a gradual process of crack initiation and propagation due to repeated loading and unloading cycles. It occurs in materials that are subjected to cyclic or fluctuating stresses, even if the magnitude of the stress is below their yield strength.

## What are the main causes of brittle fracture and fatigue crack growth?

Brittle fracture is mainly caused by high stress concentrations, such as notches or flaws in a material, and can also be triggered by sudden changes in temperature or loading conditions. Fatigue crack growth, on the other hand, is primarily caused by cyclic loading, which leads to the formation of microcracks that grow and eventually result in failure.

## How do brittle fracture and fatigue crack growth affect the lifespan of a material?

Brittle fracture can significantly reduce the lifespan of a material as it causes sudden, catastrophic failure. On the other hand, fatigue crack growth can also shorten the lifespan of a material, but it occurs gradually over time and can be predicted using fatigue testing and analysis methods. Materials that are susceptible to fatigue crack growth may have a shorter lifespan compared to those that are not.

## Are there any similarities between brittle fracture and fatigue crack growth?

Both brittle fracture and fatigue crack growth can result in material failure and can be affected by factors such as material properties, loading conditions, and environment. They can also both be prevented or mitigated by proper design and material selection, as well as regular inspection and maintenance.

## How can brittle fracture and fatigue crack growth be prevented or controlled?

To prevent brittle fracture, materials should be designed with minimal stress concentrations and flaws, and loading conditions should be carefully considered. For fatigue crack growth, materials should be selected based on their fatigue properties, and loading conditions should be kept below their fatigue limit. Regular inspection and maintenance can also help detect and address any potential issues before they lead to failure.