What Experts Say About Brittle Cleavage Fractures

In materials science, brittle cleavage fractures are an important consideration when evaluating the performance and structural integrity of crystalline and polycrystalline materials. This is because cleavage fractures (a brittle type of fracture) can ultimately result in catastrophic failures, posing major safety risks and leading to significant financial losses.

Understanding and addressing brittle cleavage fractures is therefore vital to ensuring the integrity and reliability of a wide range of critical parts in many sectors/markets. Some examples are given in the table below.

The number of applicable situations in which brittle cleavage fractures could occur is hardly exhaustive. In consideration of that, research and expertise in this area are not only useful but also necessary.

This article draws on both recent and long-established scientific research to explain the mechanisms behind such fractures. It also explores the strategies for mitigating their risk, from material selection and manufacturing practices to microstructural control and operational considerations. 

What Is a Cleavage Fracture?

A cleavage fracture is a common fracture mechanism that occurs in crystalline and polycrystalline kinds of materials. It occurs when grains fracture and fail from cleavage formation.

The breakage of atomic bonds results in separation or cleavage formation across crystallographic planes. The cleavage formation rate increases through two factors: lower temperatures and/or higher strain rates (due to rapidly rising loading rates). Many fractures could, in fact, occur through more than one fracture mode.

A cleavage fracture is usually a brittle type of fracture, occurring due to cleavage crack propagation throughout the component. This fracture is also described by researchers as a “brittle transgranular fracture,” caused by separation across crystallographic planes of metals.

How Brittle Cleavage Fractures Occur

Brittle cleavage fractures could initiate at low stress levels and low temperatures—with low strain energy—but this would involve the rapid rise of loading rates (rise in strain rates). Such fractures can occur without any warning and originate from tiny, neglected cracks and defects. These include welding microcracks and inclusions.

Grains then fracture due to cleavage generation, causing the formation of reflective facets. Tensile stresses are usually generated on the crystallographic planes (cleavage planes) of low bonds. 

In the case of ductile fractures, considerable plastic deformation occurs before fracturing. However, this doesn’t happen with cleavage fractures.

In steels, crack nucleation could start from carbide particles, while crack progression starts from ferrite/carbide microstructure interfaces (for example, when a ferritic matrix’s surrounding carbides become deformed and thus prone to cleavage fracture). Titanium nitrides, manganese sulfides, and other nonmetal inclusions can also be sites of cleavage initiation.

Brittle cleavage fractures could cause the catastrophic failure of components in critical or strategic assets. In bridges, for example, a brittle cleavage fracture results in severe accidents.

Fracture Toughness

Researchers have further concluded that in mild steel, the fracture toughness and cleavage fracture stress depend on grain size. Fracture toughness is related to cleavage fracture stress by a characteristic distance based on microstructure (grain size). 

“Tensile residual stresses can decrease the cleavage fracture toughness significantly,” concluded other researchers.  

One of the keys to cleavage fracture prevention is superior fracture toughness of the material. According to studies, while rolling steel plates (used for low-temperature offshore/maritime applications), the fracture toughness was observed to be lower in the middle portion of the plates. This reduction in fracture toughness was due to niobium (Nb) inclusions in the middle of the plates. Reduced fracture toughness would enhance the risk of brittle cleavage fracture.

In general, with fractures, the way in which the crack keeps progressing is indicative of the fracture mode.

How Exactly Does a Brittle Cleavage Fracture Start?

A flaw in microstructure is the first factor in the sequence that finally results in a brittle cleavage fracture. A flaw could be generated either before or after the component material is put in the intended operation/service. Crack propagation could be slow and steady until the crack length reaches a critical value or if the loading is continuous or recurring. Sudden fracture may result when the crack length exceeds the critical value.

The Probability of Brittle Cleavage Fractures

Two modes of cleavage initiation were explained by experts in a research study related to cleavage fracture probability in low-alloy bainitic steel. In this study, cracked-particle-induced cleavage was noted at lower temperatures, whereas cleavage induced by plasticity was recorded at higher temperatures. The threshold level of cleavage stress was significant, causing the skewness of cleavage fracture probability distribution.

In a study on the reduction of cleavage resistance, researchers investigating inclusion-induced cleavage fractures in a micro-alloyed steel reported that titanium nitride (TiN) inclusions bonded to ferritic microstructure are crucial cleavage initiators.

Researchers also reported that in certain conditions, for low-toughness welded structures, fatigue crack growth due to cyclic loading could cause the transition into unstable cleavage fractures. Therefore, controlling the rise in loading rate is key to preventing or minimizing cleavage fractures. Other factors leading to the transition into cleavage fracture include low toughness and the level of critical stress intensity.

Strategies to Prevent or Minimize Brittle Cleavage Fractures

The strategies to control brittle cleavage fractures should be based on the key factors influencing the following:

1. A very rapid increase in loading rate (high strain rate)
2. Low temperatures and triaxial stresses (normal stresses act on the component without generating shear stresses)

Materials

Chemical composition and microstructure influence the nil ductility transition temperature (NDTT) of the metal. In steels, small changes in carbon and manganese result in considerable changes in NDTT. For example, a 0.1% increase in manganese reduces NDTT by 10ºF. NDTT could also decrease by increasing carbon content, especially with the carbon/manganese ratio kept at 1:3. Changes in chromium do not influence NDTT, and an increase in molybdenum content decreases NDTT.

An increase in oxygen content in steels decreases ductility and increases the content of brittle oxides in the microstructure and at grain boundaries, increasing the risk of intergranular fracture. Toughness and ductility are related to the grain size in the steel microstructure.

Materials that remain tough and ductile through the ranges of operating (service) temperatures and ambient temperatures are suitable for minimizing the risk of cleavage brittle fractures.

Selecting materials on the basis of NDTT for high cleavage fracture resistance is a strategy that proved effective, as did the fracture arrest and initiation approaches.

Materials with an NDTT that is low enough to stop cracks through the entire thickness at low ends of service temperatures should be selected. Furthermore, materials with sufficiently high fracture toughness are suitable for minimizing the risks of brittle cleavage fracture crack initiation.

Manufacturing

Structural parts that are joined and welded at acute angles (below 90 degrees) and rigidly connected could increase the risk of brittle cleavage fractures. Other aspects that increase the risks of brittle cleavage fractures are notches that result from production and construction (fabrication) processes. Machining scratches, welding, and gauging could make notches appear on the surface. Threading holes and drilling for inserting bolts/connectors result in stress concentration. Casting and other manufacturing processes must ensure uniform mechanical characteristics and the minimization of flaws. Compressive residual stresses could be induced by impact treatment.

Microstructure

The distribution of carbides and inclusions could influence ductility and toughness in steel. An increase in carbide fractions could decrease the strain (which varies with the load) needed to cause voids to enlarge. Rolling, casting, and welding processes cause changes in the microstructure, or increase the oxidation and generation of inclusions that act as initiators of cleavages. 

A study pointed out that auto-tempered martensitic steels have higher toughness. In heat-treated steels, the distribution of carbide size in the microstructure was observed to be the most critical feature in controlling the risk of brittle cleavage fractures. Carbides were found to be the most debilitating for cleavage fracture toughness. Thus, controlling carbide distribution is key in minimizing cleavage fracture risk in pressure vessel steels.

Section Thickness

According to experts, the section thickness of steel can influence the ductile versus brittle behavior of a failure. In thin sections, higher toughness is obtained by increasing section thickness. However, after a certain increase in thickness, a further increase in thickness doesn’t lead to an increase in toughness.

Cleavage Fracture Resistance

The cleavage fracture resistance of structural steels can improve by decreasing cementite and other impurities, as well as carbon content. However, heat-affected zones in welded metals remain weak spots compared to the parent structural steel.

Conclusion

Brittle cleavage fractures are a significant challenge in the design, manufacturing, and maintenance of materials across many diverse industries. By understanding the mechanisms and conditions that lead to such fractures, engineers and material scientists can devise and implement targeted strategies to minimize their occurrence and mitigate the associated risks. From optimizing material selection and microstructural control to refining manufacturing processes and addressing operational factors, each strategy enhances the fracture resistance and reliability of critical components.

Scientific research continues to shed light on the interplay between microstructural features, environmental conditions, and mechanical stresses that influence cleavage fracture behavior. The knowledge shared by experts empowers professionals and industries to not only improve safety but also achieve cost efficiency by preventing catastrophic failures. With the ongoing evolution of materials science, further innovations in fracture toughness and resistance will play a pivotal role in ensuring the durability and integrity of critical infrastructure and systems across sectors.