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Definition: Crack growth refers to the progressive propagation of an existing crack under mechanical or corrosive loading. It is described in fracture-mechanics terms using parameters such as the stress intensity factor ΔK or the J-integral. Under cyclic loading, the growth often follows the Paris law.

Practical relevance: Crack growth largely determines the remaining service life of cracked components. Influencing factors are stress amplitude, mean stress, material toughness, temperature and medium. Quantitative assessment is carried out using da/dN-ΔK curves. Applications are found in pressure equipment construction, structural steelwork and aviation.

Decision-making perspectives:

• Technical decision-makers: Definition of permissible crack sizes and inspection intervals.
• Purchasing/project management: Assessment of repair or replacement strategies based on remaining-service-life analyses.
• Science: Investigation of fatigue crack propagation and environmental influences.
• Insurance/law: Demonstration of whether a crack would have been controllable with proper monitoring.

Typical testing or verification methods: Fatigue crack growth test, fracture-mechanics assessment to DIN EN ISO 12108, non-destructive crack testing.

FAQ:

• What does the Paris law describe?
• It describes the relationship between the crack growth rate (da/dN) and the stress intensity factor range (ΔK) under cyclic loading.

Definition: Crack initiation refers to the formation of an initial microcrack in a material as a result of mechanical, thermal or chemical loading. It represents the first phase of a potential failure process. Typical causes are fatigue, stress corrosion cracking or local overload.

Practical relevance: Cracks frequently form at notches, inclusions, weld seam transitions or microstructural inhomogeneities. Influencing variables are stress amplitude, mean stress, surface roughness and corrosive media. Early detection is decisive for integrity assessment and remaining service life estimation in accordance with fracture mechanics approaches.

Decision-making perspectives:

• Technical decision-makers: Optimisation of design, surface quality and material selection.
• Purchasing/project management: Specification of suitable testing intervals and quality requirements.
• Science: Analysis of microstructural influencing factors and crack nucleation mechanisms.
• Insurance/law: Assessment of causes in cases of premature component failure.

Typical testing or verification methods: Non-destructive testing (MT, PT, UT), SEM fractography, metallography, fatigue tests.

FAQ:

• Where do cracks typically start?
• Frequently at notches, surface defects or microstructural inhomogeneities with local stress concentration.