What Is Crack Growth?
The crack growth test measures how fast the crack in a pre-cracked specimen grows under repeated loading. The result is given as the relationship between crack advance per cycle (da/dN) and the stress intensity factor range (ΔK).
Purpose of Crack Growth
The purpose is to produce data for damage-tolerant design and residual life calculation. When a crack is found in a structure, this data is used to calculate how long it will take to reach a critical size; inspection intervals are set accordingly.
Which Products Is Crack Growth Applied To?
- Aerospace structural components
- Pressure vessels and pipelines
- Bridge and steel structure details
- Turbine and rotor components
- Welded joints
In Which Sectors Is Crack Growth Used?
Standards Related to Crack Growth
- ASTM E647 (measurement of fatigue crack growth rates)
- ISO 12108 (metallic materials — fatigue crack growth)
- ASTM E399 (plane-strain fracture toughness — related method)
- ASTM E1820 (elastic-plastic fracture toughness — related method)
Crack Growth Conditions
- Specimen type
- Compact tension (CT) or middle-tension (M(T)) specimen
- Pre-crack
- Initiated by controlled fatigue before the test
- Stress ratio (R)
- From the specification; the result depends strongly on R
- Crack measurement
- Optical tracking, potential drop, or the compliance method
Equipment for Crack Growth
- Servohydraulic fatigue test system
- Crack mouth opening displacement (CMOD) gauge
- Potential drop or optical crack-tracking system
- High-resolution data acquisition system
- Fracture surface examination microscope
Sample Requirements for Crack Growth
- Specimen geometry and thickness must conform to the ratios defined in the standard.
- The notch and pre-crack must be produced per the standard.
- The load applied during pre-cracking must not exceed the test load.
- Specimen orientation (e.g. L-T, T-L) must be reported.
- The surface must be prepared to allow optical tracking of the crack tip.
Duration of Crack Growth
The test is long because growth is very slow in the low-ΔK region; a study covering the threshold region can take days. Duration shortens if only the stable growth region is required.
Crack Growth Procedure
- 1The specimen is machined, notched, and pre-cracked.
- 2The specimen is mounted in the machine and the crack measurement system is set up.
- 3Stress ratio and initial load range are set.
- 4Cyclic loading is applied; crack length is continuously monitored.
- 5Crack length-cycle data is recorded.
- 6da/dN and ΔK are calculated and the curve is constructed.
- 7The fracture surface is examined to verify the measurement.
Crack Growth Acceptance Criteria
The method standard defines how the test is performed, not what the result should be. The acceptance limit comes from the material standard, the product specification or the customer's technical drawing; this page therefore gives no numerical limit.
Evaluation of Results
The result is given as a da/dN–ΔK curve on a log-log scale. The linear middle region of the curve is called the Paris regime; the threshold region and unstable growth region are interpreted separately. Results depend on the stress ratio and cannot be directly compared across different R values.
Crack Growth Report
- Specimen identification, material type and cast/batch number, if any
- Specimen geometry, dimensions and preparation method
- Standard and test conditions applied
- Test temperature and ambient conditions
- Measured values and calculated quantities
- Fracture surface or indentation photographs (where applicable)
- Measurement uncertainty (if requested)
- A statement that the result applies only to the specimen submitted for testing
Accreditation
EUROLAB holds internationally recognised accreditation obtained from accreditation bodies party to the IAF and ILAC multilateral recognition arrangements, under ISO/IEC 17025 for laboratory testing, ISO/IEC 17020 for inspection and surveillance, and ISO/IEC 17021-1 and ISO/IEC 17065 for system and product certification. This chain ensures the reports and certificates issued are accepted worldwide.
Whether this test can be offered within accredited scope depends on your product and the standard applied; since accreditation is method-based, the scope is confirmed in writing at the quotation stage.
Scope and authorisationsRelated Regulations
- EN 10204 (inspection documents for metallic products)
- EU 305/2011 Construction Products Regulation (for products used in construction)
- Harmonised standards applied according to the product family
Mekanik Testler — What Does This Testing Field Cover?
The section below describes the field that the Crack Growth test belongs to; the conditions and acceptance criteria of this particular test are given in the sections above. The scope specific to your product is confirmed in writing at the quotation stage.
Mechanical testing measures how a material or a part behaves under force. The family ranges from static loading (tension, compression, bending) to time-dependent behaviour (fatigue, creep), and from hardness and wear to fracture toughness.
What these tests have in common is that the result is reported as a MATERIAL PROPERTY. Yield strength or hardness is not a conformity decision; it is numerical data against which the design and the product standard will compare. Mechanical test reports are therefore usually an input to an engineering decision rather than to a product approval — material selection, safety factor, life prediction.
The distinction between static and dynamic behaviour is the key to understanding the field. A part may appear entirely unstressed under a static load yet break under repeated loading; fatigue fracture occurs at stresses well below the yield strength and gives no warning. For rotating, vibrating or cyclically loaded parts, a tensile test alone is therefore not sufficient evidence.
Test conditions determine the result, and this is critical for the readability of the report: specimen geometry, loading rate, temperature, conditioning and the gripping arrangement. The same material gives different values under different conditions. In plastics the rate effect is very pronounced, in metals the temperature effect; in composites the specimen orientation changes the result entirely.
Hardness is the most misused measurement in the field. Because it is fast, close to non-destructive and inexpensive, it is often used in place of strength. Conversion tables from hardness to strength exist, but they are APPROXIMATE and depend on the material group; using them as the sole evidence in material acceptance is wrong.
The fracture mechanics side requires a specific competence. Fracture toughness and fatigue crack growth rate measurements make it possible to calculate how long a cracked structure will last; they are the basis of damage-tolerant design in fields such as pressure equipment, aircraft structures and pipelines.
Method groups in this field
Static tests
Tension, compression, bending, shear and torsion. Yield and tensile strength, elongation and modulus of elasticity are obtained.
Impact tests
Charpy and Izod notch impact; the brittle-to-ductile transition behaviour of the material is determined as a function of temperature.
Hardness
Brinell, Vickers, Rockwell, Shore and micro hardness. Valuable for surface treatment depth and local differences in strength.
Fatigue
Force- and strain-controlled axial fatigue, fatigue in bending; S-N curves and the endurance limit.
Creep and stress relaxation
Behaviour under long-term load at high temperature; decisive for boiler, turbine and pressure equipment materials.
Fracture mechanics
Plane-strain fracture toughness, the J integral, CTOD and fatigue crack growth rate.
Wear and friction
Taber and rotating drum abrasion, coefficient of friction measurements.
Commonly confused in this field
- Calculating strength from hardness is approximate; it cannot be used as the sole evidence in material acceptance.
- Adequate static strength does not demonstrate fatigue strength; a cyclically loaded part requires a separate test.
- If test rate and temperature are not reported, the result cannot be compared — especially for polymers and composites.
- A test specimen and the actual part may not give the same value; moulding orientation, weld lines and differences in surface treatment all have an effect.
- A strength value given for a composite without stating the specimen orientation is meaningless.
- ASTM and ISO counterparts measure the same quantity but use different specimen geometries; the results are not treated as directly equivalent.
Which decisions these results support
- Design verification and material selection
- Incoming material control and supplier approval
- Meeting the mechanical requirements of the product standard
- Failure analysis, life prediction and damage tolerance calculation
- Verifying heat treatment and surface treatment processes
Related Tests
Frequently Asked Questions About Crack Growth
Does the result give life directly?
No, it is an input to the life calculation. Residual life is calculated by integrating it together with the structure's stress distribution and initial crack size.
Why does the threshold value (ΔKth) matter?
Below this value, the crack is considered not to grow in practice. The threshold is decisive when setting the inspection interval and the acceptable defect size.