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In-depth | SHIPBUILDING TECHNOLOGY


Figure 6: Phased array screen shot showing detail available to operator.


Figure 5: Standard ultrasonic screen showing defective weld.


their severity of stress concentration, crack-like imperfections are always classed as defects and require repair. b. Lack of solid metal


• Porosity


• Worm holes • Crater pipe


• Root concavity • Under fill


• Slag inclusions


• Inter-run imperfections. These imperfections are formed when


• Incomplete root penetration


there is insufficient weld metal to completely fill the cross-section between the parent metal plates. Tey are volumetric (blunt) in shape, and as such are usually only associated with a reduction in the load bearing capacity of a weld. c. Lack of fusion


• Lack of sidewall fusion. Tese imperfections occur when there is


• Surface porosity


incomplete fusion between the parent metal and weld metal or between weld runs. Tey are essentially two-dimensional in shape and so are effective stress raisers within the material. Terefore it is important to control them as they can lead to cracking within the weld. d. Lack of smoothly blended surfaces


• Excess weld metal (reinforcement) • Excessive penetration • Undercut


• Misalignment • Arc strikes


This can especially lead to the formation of fatigue cracks (most commonly at weld toes). e. Miscellaneous


• Spatter. Several miscellaneous imperfections do


not conform to any particular category. Misalignment has a major effect on the


fatigue life of the weld. Figure 7 shows the combined effect of membrane stress and bending stress induced by the misalignment. Te stress magnification factor km (by


which the applied stress is increased as a result of the axial misalignment) is given below:


k Pm s


m = σ =1 3+ e t where σS is the bending stress; Pm the


membrane stress; e the misalignment inmm; and t the thickness inmm. Terefore, a 20mm plate with a 3mm axial alignment gives a stress multiplication factor of 1.45. Similarly, angular misalignment creates


additional bending stresses dependent on the angle of misalignment. Recent research


Figure 7: Axial misalignment.


carried out by Lloyd’s Register [7] showed: σ


k P m =


m s


=1 0.7+ 1 θ Te term θ is the angle of distortion given


in degrees. Tis relationship was confirmed recently when some fatigue specimens were found to have distortion and finite element modelling of the specimens confirmed the relationship. Te additional bending stresses arising from both axial and angular distortion will have the greatest effect on surface anomalies such as undercut and excess weld reinforcement. However, little or no effect will arise with internal defects unless they lie near to the surface. NA


References 2. Lloyd’s Register, ‘Rules for the Manufacture, Testing and Certification of Materials’, July 2009. 6. IACS, ‘Recommendation No.20, Non-destructive testing of ship hull steel welds’, December 2007. 7. H.Polezhayeva, ‘Effect of weld gap and penetration on fatigue strength of transverse fillet welded specimens’ Lloyd’s Register. Research Report No: 04/01/R.1, April 2004.


• Overlap. It is not immediately obvious that


irregularities on the surface of the weld are serious imperfections. However, any sudden changes in the contours of the surface produce local stress concentrations.


140 The Naval Architect September 2010


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