Scribe marks at fuselage joints : initiation and propagation of fatigue cracks from mechanical defects in aluminium alloys

摘要

Mechanical damages, like scratches, are commonly detected on the surfaces ofaircraft components and structures. They can be accidentally introduced duringmachining or maintenance operations or be the result of wear and impacts duringaircraft service. Under the action of service loads, such mechanical damage can generatefatigue cracks reducing the component fatigue life and compromising the aircraftstructural integrity. The evaluation of the effect of scratches and other small mechanicaldefects on the structure and component fatigue lives is therefore necessary to define aninspections programme and ensure the structural safety. Conventional fatigue lifeprediction methods generally consider scratches tents of microns deep too shallow toappreciably affect the fatigue performances of structural components. However thediscovery of the scribe marks on fuselage joints disproved that prediction. In factseveral commercial airlines discovered during inspections that aircraft which have beenrepainted showed multiple scratches on the fuselage skin along longitudinal andcircumferential joints. Those scratches, referred to scribe marks, appear to have beencaused by use of sharp tools during sealant removal process prior repainting. Scratchesless than 200 μm deep were capable of severely reducing the fatigue life performanceunder service load rendering some aircraft beyond economical repair.This thesis investigates the fatigue performances of 2024-T531 aluminium alloysheets weakened by mechanically machined scratches. 2 mm thick clad and uncladsamples were scribed at their gauge section using a diamond tipped tool. The scribingprocess produced very regular rounded V-shaped notches with an included angle of 60°across the sample width. Scratches from 25 μm to 185 μm deep, with 5 μm, 25 μm and50 μm root radii were cut on the sample surfaces. Scribed sample were subsequentlyfatigue tested under constant amplitude tensile and bending load with a stress ratio ofR=0.1 at a maximum stress of 200 MPa. Scribes were found to reduce the fatigue life oftension and bending samples up to 97% compared to that of smooth unscribedspecimens. Both scribe shape and size affected the fatigue life of tensile and bendingaluminium samples. The sharper and the larger the notch, the shorter the fatigue life.Post failure fractography investigations were performed on sample fracture surfacesby means of optical and scanning electron microscope. Crack nucleation sites, fracturemorphology and peculiar features left during crack propagation were analysed. Finallycrack propagation data under different loading conditions were obtained by striationcounting performed on fracture surfaces. Multiple crack initiation occurred at scriberoots usually from inclusions, defects or weak points along the root. The number anddensity of crack nucleation sites appeared to be determined by the scribe; increasing fornotches with larger stress concentrations. Scribe geometry did not affect the fatiguegrowth rate but the propagation life for cracks deeper than 50-100 μm was influenced.Cracks nucleated from scribe marks showed a typical short crack behaviour growingfaster than long cracks with the same linear elastic stress intensity factor.Finite element calculations were performed on scribed samples evaluating how thepresence of scribes altered the local stress and strain fields. Monotonic elastic and elastic-plastic and cyclic elastic-plastic analyses were carried out under tensile andbending loads. Local elastic-plastic stress and strain fields in the neighbourhood ofdifferent scribes were determined by the notch shape and size. According to theoccurrence of mechanical similitude conditions, scribes with the same shape butdifferent size showed similar plastic zone and stress and strain distributions. A stabilisedcyclic plastic zone was developed just at the root of scribes with a ratio between the rootradius and depth ρ/d≤0.2. No correlations were observed between the occurrence of astabilised cyclic plastic zone and the sample nucleation lives defined as the number ofcycles to obtain an initial crack 50 μm deep from the notch root.Traditional fatigue life prediction methods, based on the notch sensitivity factor,were not able to correctly characterise the effect of scratches few tens of microns deepon the fatigue life of 2024-T351 aluminium alloy components. An approach based onthe critical distance theory was developed to characterise the total fatigue life reductionproduced by the introduction of scribes relating the fatigue live to a critical stress rangeΔσlm. The critical stress range was capable of describing the effect of the elastic stressdistribution produced by dissimilar notches on the nucleation and propagation of fatiguecrack considering also the effect of the variation of the fatigue load nominal appliedstress.