Low cycle fatigue of a commercial pseudoelastic Ni-rich Nickel-Titanium alloy has been analysed in this investigation. Fatigue tests have been carried out within the stress-induced transformation regime, by using flat dog-bone shaped specimens obtained from as-received NiTi sheets. In particular, the tests have been executed in two subsequent steps: i) material stabilization and ii) fatigue life estimation. In the first step a variable strain ratio was adopted, in order to avoid compression stresses during unloading, and the strain ratcheting mechanisms have been recorded, up to a stable mechanical response of the alloy. Subsequently, the stabilized specimens have been subjected to strain controlled fatigue tests, under a fixed strain ratio, up to complete failure. Results on functional fatigue, i.e. in terms of stabilized pseudoelastic response, and on structural fatigue, in terms of cycles to failure, are reported and discussed. Furthermore, experimental data have been analysed within the framework of a recent phenomenological strain-life model, based on a modified Coffin-Manson approach. Finally, the fracture surfaces have been analysed by scanning electron microscopy (SEM) in order to analyse the stable and unstable crack growth mechanisms.
Nel presente lavoro è stato analizzato il comportamento a fatica a basso numero di cicli di una lega commerciale a base di nickel e titanio (NiTi) con comportamento pseudoelastico. Le prove sono state eseguite per valori di deformazione massima compresi nella zona di trasformazione di fase indotta meccanicamente, utilizzando campioni ottenuti da lamine commerciali in lega NiTi. In particolare, le prove sono state eseguite in due fasi successive: i) stabilizzazione funzionale del materiale e ii) rottura a fatica. Nella prima fase sono stati applicati carichi ciclici fino ad ottenere una risposta pseudoelastica stabile della lega; successivamente, i campioni stabilizzati sono stati sollecitati fino a rottura con rapporti di deformazione costante. I risultati ottenuti sono stati analizzati sia in termini di risposta pseudoelastica stabilizzata (fatica funzionale) che in termini di cicli a rottura (fatica strutturale). Inoltre, i dati sperimentali sono stati analizzati utilizzando un recente modello fenomenologico basato sulla legge di Coffin-Manson opportunamente modificata. Infine, sono state analizzate le superfici di frattura mediante microscopia elettronica a scansione (SEM) per valutare i meccanismi di evoluzione dei difetti.
Fatica oligociclica in una lega NiTi pseudoelastica
FURGIUELE, Franco;Maletta C;Sgambitterra E;
2012-01-01
Abstract
Low cycle fatigue of a commercial pseudoelastic Ni-rich Nickel-Titanium alloy has been analysed in this investigation. Fatigue tests have been carried out within the stress-induced transformation regime, by using flat dog-bone shaped specimens obtained from as-received NiTi sheets. In particular, the tests have been executed in two subsequent steps: i) material stabilization and ii) fatigue life estimation. In the first step a variable strain ratio was adopted, in order to avoid compression stresses during unloading, and the strain ratcheting mechanisms have been recorded, up to a stable mechanical response of the alloy. Subsequently, the stabilized specimens have been subjected to strain controlled fatigue tests, under a fixed strain ratio, up to complete failure. Results on functional fatigue, i.e. in terms of stabilized pseudoelastic response, and on structural fatigue, in terms of cycles to failure, are reported and discussed. Furthermore, experimental data have been analysed within the framework of a recent phenomenological strain-life model, based on a modified Coffin-Manson approach. Finally, the fracture surfaces have been analysed by scanning electron microscopy (SEM) in order to analyse the stable and unstable crack growth mechanisms.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
