Gruppo Italiano Frattura (IGF)
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Crack of a helicopter main rotor actuator attachment: failure analysis and lessons learned
No full textA Light Utility Helicopter (LUH), in the course of a training flight, leaving the ground during thetaxi to take off, went into an uncontrolled rolling to the right; consequently the helicopter gradually laid downon the right side. The impact with the runway destroyed the rotating blades up to the hubs rotor. The accidentinvestigation focused on main rotor oscillatory plate servo actuators . These components, directly linked to thecloche movements, regulate main rotor blades plane tilt and pitch. Following the preliminary examination, onlyfront servo actuator attachment was found to be broken in two parts. In detail, the present paper deals with thefracture analysis results. The servo actuator attachment material is a 2014 Aluminum alloy extrudate, undergoneto T651 heat treatment. Fracture surfaces were examined by optical and electronic microscopy in order todetermine the main morphological features and consequently to trace the origin of failure mechanism andcauses. The accordance with the specification requirements about alloy composition was verified by quantitativeelementary analysis through inductive coupled plasma spectroscopy (ICP); furthermore, semi-quantitativeelementary analysis was locally verified by Energy dispersion spectroscopy X ray (EDS_RX). Finally, thehydrogen content of the material was evaluated by the total hydrogen analysis. Microstructural andtechnological alloy characteristics were verified as well by using metallographic microscopy and hardness testingof the material.Macroscopic fracture surfaces evidences were characterized by the lack of any significant plastic deformationsand by the presence of symmetry compared to the servo actuator axis. Microscopic fracture features of both theinvestigated surfaces were not coherent to the hypothesis of an impact of the main rotor to the soil. Furtherachieved evidences, such as grain boundary fracture propagation, the presence of corrosion products, were all inaccordance with a Stress Corrosion Cracking (SCC) progressive fracture mechanism.Finite Element Analysis (FEA) located the highest tensile stress value, when the servo actuator is in its nominalworking condition, at the same points where the corrosion products were more concentrated (i.e. in the part ofthe fracture exposed to oxidative air effect for the longest time). The good agreement between FEA andmorphological evidences allowed to determine the progressive fracture origin area, though it was not possible toindividuate the crack initiation point. In fact, in correspondence to the initiation area of both the fracturesurfaces, shining and flat morphology was found;. then there were evidence of plastic deformations, due to thedetachment of a servo actuator part.The ICP analysis and hardness testing results were in accordance with the material specification requirements.However, the hydrogen content was one order of magnitude greater than the required value and many andunexpected globular formations were observed on the fracture surface. Part of these were dendritic formations,while the others looked smooth and shining. Further, FESEM boundary grain observation gave evidences of ahigh presence of precipitates on the investigated surfaces. Hence, observed microstructural characteristics,boundary grain precipitates and globular formations allowed to hypothesize possible overheating/eutecticmelting phenomena, occurred during manufacturing processes.As widely reported in literature, the AA 2014 alloy is one of the aluminum-copper-magnesium-silicon type,employing copper aluminide (CuAl2 ) as the primary precipitation-hardening agent. The need for a maximumCu phase dispersion in solid solution requires a heat treatment range with an upper limit (507°C) that is near tothe melting of the eutectics (510°C). Moreover, since the 1960s, AA2014 has been defined as sensitive to SCC.This condition is mainly related to the presence of coarse-grained and aligned CuAl2 precipitates. Thisarrangement is due to an overheating (more than 507°C) or to a cooling process carried out too slowly.Microstructural analysis was carried out on three items: 1) a large portion of the broken actuator attachment; 2)on a servoactuator coming from the same production batch; 3) on a servo actuator coming from a differentproduction batch.The microstructure from the broken actuator attachment showed a great amount of precipitates (secondphases) lengthwise aligned to the boundary grain, pores, and also cavities and dendritic globular formations.Analysis results, morphology evidences and reference images available on scientific literature were found to bein excellent agreement and validated the embrittlement and subsequent SCC mechanism hypotesis(intergranular failure propagation).In conclusion, flight accident causes are attributable to main rotor actuator attachment failure.Failure mechanism is classifiable as SCC supported by microstructural anomalies of the material. Theinvestigation of the manufacturing process highlighted how one of the servo actuator batches was not properlyproduced due to poor control and accuracy of heat treatment temperature and/or cooling time. This led tohydrogen embrittlement and to a microstructural problem (globular formations and boundary grainprecipitates). The combination of those phenomena caused an increase of the SCC sensitivity and were thebasic progressive failure driving forces.Nevertheless, as above mentioned, alloy composition was found compliant with the material specificationrequirements and this just because none of the scheduled quality control tests is able to determine the peculiarmicrostructural anomalies reported
A review of using thermoelasticity for structural integrity assessment
No full textThe advances in the use of thermoelastic stress analysis (TSA) for fracture mechanics assessmentare reviewed. The development of techniques to determine stress intensity factor is presented followed by theapplication of these techniques to fatigue crack growth, crack closure and the study of mixed mode cracks
About the certification of railway rails
No full textWhen the compliance with the European Code of some rail steel has to be verified, the need ofcarrying out the experimental activities in accordance with several testing Standards forces the operator both tosolve the problems related to the choice of a suitable testing practice and often to interpret subjectivelyStandards guidelines. This does not facilitate the comparability and/or the quality of the results produced byseveral laboratories. With reference to a series of fatigue, fracture toughness and fatigue crack growth testscarried out by the authors on specimens extracted from rails, the main lacks in the current standards, related toboth the choice of the control parameters and the testing procedures, are pointed out. Regarding the crackgrowth testing, several procedures to compute the crack growth rates to be compared with the limits prescribedby the Code are proposed. These procedures have been applied to a data set produced during theaforementioned testing activity, in order to highlight, by comparison of the results obtained by them, thesignificant differences in the crack growth rate estimates and the magnitude of the errors that can be done dueto the lacks in the standard practices currently adopted
Modello numerico per la simulazione e l’ottimizzazione di controlli non distruttivi con ultrasuoni
No full textI controlli non distruttivi basati sull’impiego di ultrasuoni sono ampiamente usati per la loro efficacia e affidabilità nel rilevamento di difetti. La generazione di onde ultrasonore e la propagazione in strutture di forma non regolare sono difficili da analizzare, soprattutto se la sorgente impiegata è un laser. Le tecniche numeriche per la simulazione del fenomeno reperibili in letteratura mostrano limiti di applicabilità per frequenze nel campo dei MHz e lunghezze d’onda molto corte. In questo lavoro presentiamo un metodo numerico in grado di risolvere accuratamente ed efficientemente problemi di generazione di onde ultrasonore tramite laser, con frequenze nel range dei MHz, e di propagazione in corpi relativamente estesi. La ricezione viene simulata con la propagazione degli ultrasuoni in aria, al fine di poter ottimizzare la configurazione completa per controlli non distruttivi con ultrasuoni senza contatto. Diverse configurazioni di ispezione sono state prima simulate tramite l’analisi numerica e poi riprodotte sperimentalmente per confrontare i risultati
Effect of crack propagation on crack tip fields
No full textCrack closure influences fatigue crack growth rate and must be included in the design ofcomponents. Plasticity induced crack closure is intimately linked with the crack tip plastic deformation, whichbecomes residual as the crack propagates. The objective here is to study numerically the effect of crackpropagation on crack tip fields. The transient effect observed at the beginning of crack propagation is linked tothe hardening behavior of material. The effect of mesh refinement is studied, and a singular behavior is evident,which is explained by the sharp crack associated with mesh topology, composed of a regular pattern of squareelements. The plastic zone size measured perpendicularly to crack flank in the residual plastic wake is quantifiedand compared with literature models. Finally, the removal of material at the first node behind crack tip withload cycling was observed for plane strain state and some hardening models in plane stress state
Mode I Stress Intensity Factors for triangular corner crack nearby intersecting of cylindrical holes
No full textThe paper deals with the Stress Intensity Factor assessment of cracks at the intersection of holesloaded by internal pressure.Triangular flaws are considered at the intersection of two holes inside a specific specimen. The researchexamines the influence of hole diameter ratio D1/D2 and the angle between their axes ?. Numerical analysis isperformed to determine the Stress Intensity Factors (SIF) of mode I in many different geometricconfigurations.The actual shape of a real crack nucleated at the intersection of two cylindrical holes is subject to variableinternal pressure and is usually geometrically complex. The Stress Intensity Factor changes along the crackcontour and the crack shape development is controlled by its local value, e.g. during a fatigue loading. Ingeneral, the estimation of the Stress Intensity Factors of cracks with a complex shape is made by means ofnumerical methods since closed form solutions in literature are limited. However, in order to solve the problemof crack propagation more quickly, in the case of a crack corner at the intersection between two cylindricalholes, we can assume, in agreement with scientific literature, a symmetrical triangular crack shape and the StressIntensity Factor are only calculated at the middle of the crack. Obviously, this is a strong approximation, butthis allows a reduction in the computation effort for crack growth rate assessments and safety evaluation.In this paper, the weight function technique is used by integrating the actual stress field evaluated in the uncrackedmodel. The method of the weight function is of general validity and the weight function is related to thedisplacement components close to the crack front, as proposed by Bueckner and Rice. From a computationalpoint of view, the use of the three-dimensional weight function is complex and in scientific literature a weightfunction of general validity is not available. Nevertheless, thanks to the work conducted by Petroski andAchenbach, Shen and Glinka, an efficient generalised weight function has been adopted and then developed bySha and Yang [9], which considers a series expansion of non-singular terms. In this way, the integration of theweight function, multiplied by a nominal stress, is made along a line and not in a two-dimensional domain.In this preliminary work, according to Herz et al., we consider a weight function with three terms by assuming apriori the coefficients of the second and third non-singular terms. This contribution is essentially an extensionof a previous paper by Herz et al. They only considered the case of D1/D2=1 and ?=90° (Di are the diametersof the two cylindrical holes and a is the angle between their axis). Here, we extend the analysis to D1/D2 equalto 2, 4 and 8 with an ? of 60 and 45 degrees. With the aid of three-dimensional modelling, an accurate FE model of a triangular corner crack at differentcrack depths has been made. Subsequently, by using ANSYS finite element software, it is possible to employ thecommand KCAL that evaluates the Stress Intensity Factors in the middle of the crack. Subsequently, acomparison between numerical FE results and the analytical results, giving the values of the unknowncoefficients of the weight function (the unknown coefficient is indicated in the paper as M1).As reported in the tables, the accuracy of the weight functions in SIF predictions is about 5% despite the strongsimplification previously introduced in the model. This result is considerable because it is possible to determinethe Stress Intensity Factor of a triangular shaped crack by a line integral of a stress profile in a model withoutconsidering the crack
Sharp contact corners, fretting and cracks
No full textContacts with sharp edges subject to oscillatory loading are likely to nucleate cracks from thecorners, if the loading is sufficiently severe. To a first approximation, the corners behave like notches, where thelocal elastic behaviour is relieved by plasticity, and which in turn causes irreversibilities that give rise to cracknucleation, but also by frictional slip. One question we aim to answer here is; when is the frictional slipenveloped by plastic slip, so that the corner is effectively a notch in a monolithic material? We do this byemploying the classical Williams asymptotic solution to model the contact corner, and, in doing so, we renderthe solution completely general in the sense that it is independent of the overall geometry of the components.We then re-define the independent parameters describing the properties of the Williams solution by using theinherent length scale, a procedure that was described at the first IJFatigue and FFEMS joint workshop [1]. Byproceeding in this way, we can provide a self-contained solution that can be ‘pasted in’ to any complete contactproblem, and hence the likelihood of crack nucleation, and the circumstances under which it might occur, canbe classified. Further, this reformulation of Williams' solution provides a clear means of obtaining the strength(defined by crack nucleation conditions) of a material pair with a particular contact angle. This means that theresults from a test carried out using a laboratory specimen may easily be carried over to any complicated contactproblem found in engineering practice, and a mechanical test of the prototypical geometry, which may often bequite difficult, is avoided
Applicazione della meccanica della frattura viscoelastica alla previsione della vita di tubi in polibutene
No full textIsotactic polybutene-1 (i-PB1) is a polymer used for the manufacturing of pressurized pipes.
In this work two grades of i-PB1 with a different degree of isotacticity have been investigated; they have
been supplied by Basell Polyolefins.
Fracture tests have been performed at various temperatures and testing speeds. Two configurations have
been used, single edge notch bending (SENB) and double cantilever beam (DCB), the latter only to study
crack propagation. Optical methods have been used to detect crack initiation and measure propagation
speed.
From the phenomenological point of view, the formation of highly stretched material regions has been observed
during crack propagation. A continuous tearing of these regions as the crack advances has often been
interrupted by their sudden rupture, with the load decreasing accordingly. This partial instability has been
observed on both material grades, with both testing configurations.
Results of the tests have been interpreted using the fracture mechanics framework; a time-temperature superposition
scheme has been adopted to represent viscoelastic behavior over several decades. An analytical
model has been applied to predict the lifetime of pressurized pipes. A good agreement has been reported between
model predictions and experimental data obtained from tests on polybutene pipes
A new method for the experimental study of fatigue behaviour of thermoplastic materials
No full textNowadays most industrial realities undergo a strong push to improve cost-effectiveness, productivity and quality of manufactured products. In particular we focussed our attention in the area of design of plastic structural components, including both optimization of existing structures and design of new ones. In this case, but the following considerations have a more general value, these needs could be translated into demanding requirements of cost-effectiveness, weight reduction, reduced time-to-market with guarantee reliability. From a material perspective this means demanding mechanical performances, attention to safety margins and need of a better control of key design parameters. To obtain these results, we need to develop a new approach and effective tools in the design of plastic materials and components aimed at tailoring part behaviour to endurance and performance requirements.
The target of the project is to find effective tools for predicting life endurance and damage evolution of plastic materials and components under mechanical/thermal service loading, in order to support the development of new material formulations and the design and optimization of structural components.
In a particular way, we focussed our work in the characterization and modellization of materials durability and damage mechanisms.
One of the main problems related to materials durability is due to fatigue failure. Fatigue process is a progressive weakening of a component with increasing time under load such that loads to be supported satisfactorily for short duration produce failure after long durations [1, 2, 3]. Fatigue failure should not be thought only as the breaking of the specimen into two separated pieces, but as a progressive material damage accumulation [2]. Material damage during fatigue loading manifests as progressive reduction of stiffness and as creep [5].
As standard fatigue testing are expensive in terms of money and time, it is essential to develop new approaches less time consuming and simpler to be implemented. One of the most important goals of the present work is the setting of an investigation method (Accelerated Fatigue Test) very simple to be implemented that is able to differentiate damage accumulation and durability performances of various material formulations in reduced time
Modello di tenuta della flangia bullonata, senza guarnizione, mediante l’analogia della meccanica della frattura di una fessura parzialmente aperta
No full textI compressori centrifughi di elevate dimensioni non permettono l’utilizzo di guarnizioni
deformabili, per cui le due metà della flangia di connessione sono forzate mediante bullonatura e la tenuta è
affidata al contatto completo delle due superfici. La previsione della pressione di perdita è un aspetto di
progetto di notevole interesse per questa tecnologia. L’azione della pressione interna sollecita la separazione
delle superfici della flangia, che invece è contrastata dall’azione di serraggio dei bulloni. Il presente lavoro
propone un modello per prevedere la condizione di perdita, basato sulla meccanica della frattura. Dato che le
due superfici della flangia sono semplicemente a contatto, esse costituiscono una vera e propria fessura
parzialmente aperta. Come ben noto il fattore di intensificazione di una fessura parzialmente aperta è nullo.
Imponendo che le due superfici siano parzialmente separate ad una distanza fino al bordo del foro del bullone
(che offre un canale di fuoriuscita per il fluido in pressione), e imponendo la condizione di fattore di
intensificazione nullo, è possibile determinare la pressione di perdita, analiticamente, mediante la tecnica delle
“weight functions” (o “funzioni peso”). Il presente lavoro riporta una positiva validazione del modello proposto
mediante sia simulazione numerica sia risultati sperimentali in piena scala e in scala ridotta. Il modello analitico
proposto offre uno strumento di progetto di immediata implementazione per comparare diverse geometrie di
flangia bullonata