Understanding metal fracture mechanisms is critical for non-destructive testing (NDT) professionals. This technical guide covers six primary fracture types with prevention strategies for industrial applications.
Stress corrosion fracture is a fracture that occurs under the combined action of tensile stress and specific corrosive media. The fracture process usually has no obvious warning signs and is sudden. The fracture surface generally shows brittle fracture characteristics but may sometimes be accompanied by slight plastic deformation.
In corrosive media, a corrosion product film forms on the metal surface. When metal is subjected to tensile stress, the corrosion product film ruptures, exposing fresh metal surface. The fresh metal surface is rapidly corroded, forming new corrosion product film. This cycle repeats, causing cracks to continuously propagate inside the metal, ultimately triggering fracture.
Stress state, corrosive media, and material sensitivity are the main factors affecting stress corrosion fracture. Tensile stress is a necessary condition for initiating stress corrosion fracture; different corrosive media have different corrosion effects on different metal materials; certain metal materials have high sensitivity to specific corrosive media.
Properly select materials, choosing materials insensitive to stress corrosion;
reduce component stress levels, using annealing and other processes to eliminate residual stress;
improve environmental conditions, such as reducing corrosive media concentration and controlling temperature;
use surface protection measures such as coatings and electroplating.
Liquid penetrant testing, ultrasonic crack detection
Creep fracture is the slow plastic deformation and fracture that occurs over time under high temperature and constant stress. The creep process usually consists of three stages: initial creep stage, steady-state creep stage, and accelerated creep stage. The creep fracture surface is generally rough with obvious oxidation color.
In high-temperature environments, atomic activity within the metal increases, and dislocations easily climb and glide. Under constant stress, dislocations continuously move, causing slow plastic deformation of the metal. Over time, deformation accumulates, and when reaching a certain level, it triggers crack formation and propagation, ultimately leading to fracture.
Temperature, stress, and time are the main factors affecting creep fracture. Higher temperatures increase metal creep rates; greater stresses result in more obvious creep deformation; longer times increase the possibility of creep fracture. Additionally, material chemical composition and microstructure also affect creep properties.
Select high-temperature resistant and creep-resistant materials;
rationally control working temperature and stress levels, avoiding long-term high-temperature and high-stress states;
optimize material microstructure to improve creep resistance.
Fatigue fracture is a fracture that occurs after a certain number of cycles under alternating stress. The fracture process usually consists of three stages: crack initiation, crack propagation, and final fracture. The fatigue fracture surface generally consists of smooth and rough zones, where the smooth zone is the area of slow crack propagation and the rough zone is the area of final rapid fracture.
Under alternating stress, some weak areas on the metal surface, such as grain boundaries and inclusion edges, produce tiny cracks - crack initiation. As cycle numbers increase, cracks continuously expand under stress, forming macroscopic cracks. When cracks propagate to a certain extent, the remaining cross-section cannot withstand external force, resulting in final fracture.
Stress amplitude, mean stress, cycle numbers, and material fatigue limit are the main factors affecting fatigue fracture. Higher stress amplitude and mean stress accelerate crack propagation and shorten fatigue life; more cycles increase the possibility of fatigue fracture; higher material fatigue limit indicates stronger resistance to fatigue fracture.
Rationally design component structures to reduce stress concentration; select materials with high fatigue limits;
perform surface strengthening treatments such as shot peening and rolling to improve surface fatigue strength;
control load magnitude and cycle numbers to avoid exceeding material fatigue limits.
Brittle fracture is a fracture mode where metal undergoes almost no obvious plastic deformation before fracture. The fracture process occurs suddenly, with a flat and smooth fracture surface, often showing crystalline or herringbone patterns, with metallic luster.
Brittle fracture is mainly caused by the presence of cracks or defects inside the metal. Under external force, stress concentration occurs at crack tips. When stress concentration reaches the material's fracture toughness, cracks rapidly propagate, leading to metal fracture. This fracture mode is usually related to material crystal structure, impurity content, and stress state.
Material brittleness is affected by various factors. Higher carbon content and impurity content reduce metal toughness and increase brittleness; low-temperature environments change metal crystal structure, reducing toughness; triaxial tensile stress states also promote brittle fracture.
Strictly control material chemical composition and reduce impurity content;
perform appropriate heat treatment to improve microstructure and increase toughness;
rationally design component structures to avoid triaxial tensile stress states;
implement preheating measures when used in low-temperature environments.
Ductile fracture is a fracture mode where metal undergoes obvious plastic deformation before fracture. During the fracture process, the metal material first experiences necking phenomenon, where the local cross-section significantly reduces, followed by fracture at the necking location. The fracture surface usually appears fibrous or cup-and-cone shaped, with a dull color and no obvious luster.
Ductile fracture is mainly caused by dislocation movement and multiplication within the metal. When metal is subjected to external force, dislocations slide on slip planes, causing plastic deformation of crystals. As deformation continues, dislocations become entangled and accumulate, forming dislocation walls and subgrain boundaries. When local stress concentration reaches a certain level, it triggers the formation and growth of microvoids. The interconnection of microvoids ultimately leads to metal fracture.
The chemical composition, microstructure, and temperature of materials have significant effects on ductile fracture. For example, steel containing appropriate alloying elements usually has better toughness;
fine grain structure can improve metal toughness;
while in low-temperature environments, metal toughness decreases significantly, making ductile fracture more likely.
Properly select materials to ensure good toughness;
optimize material microstructure and refine grains through heat treatment processes;
avoid using low-temperature-sensitive metal materials in low-temperature environments.
| Fracture Type | Characteristics | Formation Mechanism | Prevention Methods | NDT Testing Methods |
|---|---|---|---|---|
| Stress Corrosion Cracking (SCC) | Brittle appearance, environment-specific, unpredictable | Corrosion film rupture → localized attack → crack propagation | Material selection, stress relief, environmental control | Liquid penetrant testing, ultrasonic crack detection |
| Creep Fracture | Rough oxidized surface, time-dependent deformation | Dislocation climb → grain boundary sliding → void formation | High-temperature alloys, stress reduction, life assessment | Ultrasonic thickness measurement, metallographic analysis |
| Fatigue Fracture | Smooth + rough zones, beach marks, progressive failure | Crack initiation → stable growth → rapid fracture | Surface hardening, stress reduction, material selection | Eddy current testing, magnetic particle inspection |
| Brittle Fracture | Flat, crystalline surface, minimal plastic deformation, sudden failure | Crack propagation from stress concentration at defects | Impurity reduction, preheating, stress state optimization | Acoustic emission testing, phased array ultrasound |
| Ductile Fracture | Fibrous/cup-cone surface, visible necking, dark appearance | Dislocation motion → void nucleation → coalescence → failure | Grain refinement, alloy optimization, temperature control | Ultrasonic testing, radiographic inspection |
Zhou, Hongyu & Li, Jian & Liu, Jie & Yu, Peichen & Liu, Xinyang & Fan, Zhiyang & Hu, Anqing & He, Yinsheng. (2024). Significant reduction in creep life of P91 steam pipe elbow caused by an aberrant microstructure after short-term service. Scientific Reports. 14. 10.1038/s41598-024-55557-w.
https://nte.mines-albi.fr/SciMat/en/co/SM6uc1-4.html
https://www.nde-ed.org/Physics/Materials/Mechanical/NotchToughness.xhtml
https://eengineerkey.com/creep-and-creep-fracture
