numero26

L. Allegrucci et alii, Frattura ed Integrità Strutturale, 26 (2013) 104-122; DOI: 10.3221/IGF-ESIS.26.11 105 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 and unexpected 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 a high presence of precipitates on the investigated surfaces. Hence, observed microstructural characteristics, boundary grain precipitates and globular formations allowed to hypothesize possible overheating/eutectic melting 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 maximum Cu phase dispersion in solid solution requires a heat treatment range with an upper limit (507°C) that is near to the 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. This arrangement 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 different production batch. The microstructure from the broken actuator attachment showed a great amount of precipitates (second phases) 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 be in 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. The investigation of the manufacturing process highlighted how one of the servo actuator batches was not properly produced due to poor control and accuracy of heat treatment temperature and/or cooling time. This led to hydrogen embrittlement and to a microstructural problem (globular formations and boundary grain precipitates). The combination of those phenomena caused an increase of the SCC sensitivity and were the basic progressive failure driving forces. Nevertheless, as above mentioned, alloy composition was found compliant with the material specification requirements and this just because none of the scheduled quality control tests is able to determine the peculiar microstructural anomalies reported. S OMMARIO . Un elicottero della classe LUH (Light Utility Helicopter), nel corso di una missione di addestramento, effettuava un decollo rullato e, al momento del distacco da terra, subiva un rollio incontrollabile a destra; negli attimi successivi l’aeromobile si adagiava gradualmente sul fianco destro, con distruzione delle pale fino ai mozzi del rotore per effetto della loro rotazione e interferenza con la pista. L’investigazione è stata focalizzata sui servocomandi del piatto oscillante del rotore principale. Tali comandi regolano l’inclinazione del piano delle pale del rotore principale ed il loro passo in funzione dei comandi di volo. In particolare è risultato che solo uno dei tre attacchi dei servocomandi è stato trovato rotto in due parti. Le evidenze emerse nell’indagine di tale rottura sono oggetto del presente lavoro. L’attacco del servocomando è costituito da un estruso in lega di alluminio AA 2014 con trattamento termico T651. Le superfici di rottura sono state esaminate mediante microscopia ottica ed elettronica per individuarne le principali caratteristiche morfologiche, al fine di risalire al meccanismo della rottura ed alle sue cause. La rispondenza a specifica della lega è stata effettuata mediante analisi quantitativa elementare ICP (spettroscopia al plasma), analisi semiquantitativa puntuale EDS (spettroscopia a dispersione di energia) ed analisi del contenuto di idrogeno. Le caratteristiche microstrutturali e tecnologiche della lega sono state infine accertate attraverso microscopia metallografica e determinazioni di durezza. Le evidenze macroscopiche della rottura, caratterizzate dall’assenza di significative deformazioni plastiche e da superfici di rottura simmetriche rispetto all’asse del servocomando, e le evidenze microscopiche rilevate non sono risultate compatibili con l’ipotesi che la rottura sia avvenuta a seguito dell’urto con il terreno da parte del