Why Crack Arrest is Critically Important?
Fatigue cracks and corrosion are the primary cause for the failure of ships, road tankers, and railway tank-cars. Bridges and other metal structures too are plagued by cracks. While fatigue cracks are induced by repeatedly applied small loads, yielding failure results from excessive tensile loads and buckling failure follows large compressive loads.
With container ships getting larger by the day, thicker and stronger steel plates are increasingly used for building their upper decks and hatch coamings. The Japanese steel industry registered a concern that such plates with large initiation toughness may not effectively arrest the propagation of brittle fractures.
Fatigue cracks can precipitate structural failure of the entire ship, flooding, cargo and ballast leakage, and hull degradation. Joints between the longitudinal and transverse load-bearing members are particularly vulnerable to fatigue failure.
In bridges, unrepaired or poorly repaired fatigue cracks could cause devastating failures with great loss of infrastructure, property, business activity, and precious life and limb. Arresting cracks therefore is serious business.
Arresting Cracks in Ships, Bridges, & Other Metallic Structures
Cracks typically originate in brittle, highly stressed, or low toughness zones. The Crack Initiation approach focuses on welding quality control and close monitoring of workmanship but it is impossible to guarantee that no cracks will form and spread.
The Crack Arrest approach is more practical. It is based on the principle that the material surrounding the crack is strong enough to stop crack propagation if the resistance to crack propagation is greater than the force that drives the spread of the crack.
Waves are the primary cause of ship fatigue. Loads that induce fatigue cracks in ships include:
- Longitudinal Loads affect the longitudinal members of the ship structure and are produced by longitudinal bending movement, shear force, and torsional moment. Local load asymmetries create Static Longitudinal Loads while waves cause Dynamic Longitudinal Loads
- Transverse Loads burden the transverse members of the ship structure. Cargo, ship weight, ballast water, hydrostatic and hydrodynamic loads exerted by surrounding water and waves, and impact stresses create these loads
In response to the aforementioned Japanese steel industry’s query, the International Association of Classification Societies (IACS) issued Unified Requirements S33 and W31 for steels with crack arrest properties. Such steels are:
- EH40 of over 85mm thickness
According to research conducted by Lloyd’s Register (LR), cracks are arrested if the Crack Arrest Temperature (CAT) for the steel is below the design temperature. Studies by Smedley (1989) and Wiesner (1996) have identified the following relationship between CAT and Nil Ductility Test Temperature:
NDTT is Nil Ductility Test Temperature in 0C
σ is stress equaling 2/3 of the minimum specified yield strength in N/mm2
B is plate thickness in mm
Crack arrest techniques in bridges and other structures:
Crack Arrest Hole (CAH) is the most common method. A hole is drilled at the crack tip to blunt the tip, reduce the tip’s stress intensity, and stop crack propagation
This is a temporary, not-very-effective measure because holes may:
- fail to arrest crack growth
- cause structural failure
- produce cracks on the other side
- be made only at accessible locations
- be disproportionately large
- miss the crack tip
- introduce residual stresses
Cutting the Crack and Welding a Repair Patch
- Installing Interference Bolts and Fasteners
- Surface Treatment viz. Ultrasonic Impact and Short Peening Around Hole
- Pre-Stressing Crack Tip with Hammer Blows
- Pre-Stressing Hole with Piezoelectric Method
Most of these methods require cumbersome follow-up inspections. Most importantly, they are cannot check crack growth when the driving stress is high or crack length nears the critical length.
Fatigue Technology’s (FTI’s) StopCrackEx System has effectively arrested cracks on New Jersey’s Manahawkin Bay Bridge, Margaretville’s NYDOT Bridge, New Jersey Turnpike, and many others.
A combination of the Hole Cold Expansion and the ForceMate Bushing techniques, the StopCrackEx System installs a bushing inside a carefully drilled, small-sized (typically ½” or 12.7mm) crack arrest hole (CAH):
- ForceMate Bushing Method reinforces the hole while lowering applied stress and stress amplitude
- Hole Cold Expansion creates a benevolent, residual compressive stress zone around the CAH and the bushing thereby shielding it from cyclic stresses
Cracks in pressure vessels, gas pipelines, and LPG storage tanks can be arrested when:
- temperature of crack tip is above the CAT
- intensity factor of applied stress is below crack arrest fracture toughness
- crack driving force is lower than the local arrest property
- energy available for crack propagation is less than the energy absorbed by the material during crack propagation
Considering the consequences of unrepaired or badly repaired cracks, it is no wonder researchers have devoted tons of time and effort for better crack arrest methods.
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Featured Image – Failure of Erika Tanker in 1999 (Source: http://www.timesofmalta.com/articles/view/20080116/local/updated-total-found-guilty-in-1999-erika-oilspill-case.192110)