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 Residual stress can cause premature failures in structures and equipment. It is created by a variety of sources including welding (thermal), fabrication (force fit), manufacturing (cold forming), and structural loads (thermal ratcheting, pressure overload, and excessive force). Corrosive environments are particularly detrimental to metals in tension. In some situations, heat-treating reduces the impact of residual stress, but these procedures often are not practical or not specified.
Stress Engineering can measure residual or ambient stress using strain gages with the Blind Hole Drilling Method (ASTM E-837). This technique measures deformation as a small hole is drilled into the surface. As the new hole expands or contracts in response to the local stresses, the surface complies with the deformation. Principal stress is calculated from the measurements of a three-element strain gage oriented radially around the hole.
We have developed several enhancements to the conventional incremental method of measurement including:
 Magnetic attachment of the milling guide
Depth of hole is recorded along with strain gages
Real-time calculation of stress and plotting of measurements
Real-time evaluation reduces the likelihood that decisions are based on faulty measurements. This provides greater confidence in results during a turnaround inspection in a refinery.
Residual stress is associated with many problems:
Structural overloads and plastic deformation
Fabrication fit-up alignments
Differential cooling in manufacture:
? Welds
? Castings
? Forgings
? Bimetal joints
Reduces cyclic fatigue life and allowable amplitude
Encourages crack propagation (fracture)
Attracts stress corrosion
In refinery vessel applications, residual stress can:
Contribute to stress corrosion cracking and premature failure in equipment/piping
Require estimates when unknown
Produce overly conservative Fitness for Service assessments
Residual stress is an important indicator for the criticality of Sulfide Stress Corrosion cracking to the integrity of the vessel. Measurements are performed for a variety of applications:
Large rollers for Pulp and Paper making should have low stress after machining to assure
straightness and long life
Inside refinery vessels at:
? Areas of repair welding
? Areas of questionable stress relief
? Welds subjected to critical service environments such as SCC and SOHIC
- End of weld beads
- Juncture of two seams
- Areas with history of repair
- Discovery of new cracks
? Refinery piping welds
? Weldments of thick plates
? Fabricated ships and barges
 Acoustic Emission | Life Assessment | Failure Analysis
Field Measurements | Materials/Metallurgy | Residual Stress
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