Role of Plasticity in Nitinol Fatigue

Disertace analyzuje vliv koncentrátorů napětí na průběh martensitické transformace, vznik plastické deformace a její vliv na přerozdělení napětí a vznik zbytkového pnutí a reziduálního martenzitu v okolí koncentrátorů v prvcích ze slitin s tvarovou pamětí NiTi. Vliv je analyzován v režimech superelastického isotermálního cyklování a aktuačního cyklování, t.j. teplotního cyklování pod vnějším napětím. Disertace využívá pro vyhodnocení vlivu experimentální přístup spolu s numerickými simulacemi metodou konečných prvků na modelových případech tenkých pásků ze slitin NiTi opatřených půlkruhovými vruby. V experimentální části je vyhodnocován vliv koncentrátorů pomocí termomechanických experimentů s využitím metod obrazové korelace a rentgenové mikrodifrakce pro lokální analýzu deformací a fázových objemových podílů v průběhu cyklování v okolí vrubů. Simulace metodou konečných prvků poskytují komplementární informace o průběhu napětí, deformací a martensitické transformaci, zejména o vývoji jednotlivých složek celkové deformace, tj. elastické a plastické, a vývoji zbytkového pnutí a s ním souvisejícím zbytkovým martensitem.Disertace je dále doplněna o numerickou analýzu vlivu konstrukce stentů na lokální cyklický průběh martensitické transformace a jeho vliv na únavové vlastnosti.This doctoral thesis deals with the analysis of the role of stress risers in NiTi elements on the local cyclic evolutions of martensitic transformation, plastic deformations, and mechanical gradients at the stress risers, under both iso-thermal cyclic loading in the superelastic regime and thermal cycling under external load in actuator regime. The methodology of investigation includes both experimental measurements and finite element simulations of notched thin NiTi superelastic/shape-memory ribbons. In the experimental part of the work, the DIC method is used to capture the spatial evolutions of strain gradients around the notch, and the X-ray microdiffraction is used to analyze the local strain fields around the notch and the plasticity induced remnant martensite at the notch-tip. Besides, the finite element results help assess the corresponding evolutions of stresses and strains, martensitic transformation, and the partitioning of total strains into elastic, plastic and residual strains around the notch. In addition, the thesis includes numerical analysis investigating the effect of the design of self-expanding NiTi stents on local cyclic evolution of martensitic transformation and its relation to fatigue performance.

Implant-Supported Overdentures: Current Status and Preclinical Testing of a Novel Attachment System

Numerous attachment systems exist for implant-supported overdentures, with each having specific limitations in terms of retention, cost, wear, maintenance and cleanability. A retrospective analysis of patients restored with implant-supported overdentures using bars, telescopic crowns and Locator-type attachments was performed and the patients were interviewed. An in vitro strain gauge study compared telescopic crowns, Locator-type attachments and a novel flexible attachment system employing a shape memory alloy (NiTi) with respect to peri-implant strain development during insertion, loading and removal of an overdenture. A significantly lower number of attachment-related complications was observed in bars as compared to telescopic crowns (p = 0.00007) and Locator-type attachments (p = 0.00000), respectively. Greater overall patient satisfaction was noted in bar-retained restorations while Locator-type attachments led to lower levels of satisfaction regarding prosthesis retention. In vitro, telescopic crowns caused maximum strain development during prosthesis insertion and loading, while during removal this was observed in Locators with white retentive inserts. NiTi attachments caused significantly lower strain development during insertion as compared to telescopic crowns (p = 0.027). During loading, NiTi attachments caused significantly lower strain development than Locators with blue retentive inserts (p = 0.039). During removal, NiTi attachments caused significantly less strain development as compared to Locators with white retentive inserts (p = 0.027). Positional discrepancies between male and female attachment parts affected the retention and reaction force between both components, which may be minimized by using the novel NiTi attachment system. This may be beneficial in terms of component wear and implant loading

Design and Numerical-Method-Aided Optimization of a Novel Attachment System for Implant-Retained Dental Prostheses Using NiTi Shape Memory Alloys

While nickel-titanium (NiTi) is the primary shape memory alloy (SMA) used in endodontic instruments, restorative dental components so far have not been fabricated from SMAs. The flexibility of these materials may solve problems in implant prosthodontics resulting from non-parallel implant positions and transfer inaccuracies. Based on a prototype of a novel attachment system for implant overdentures, a finite element model was created and used for studying different loading situations and design parameters followed by numerical analysis aided design optimization. The results revealed that the basic design of the attachment is capable of compensating misalignments of supporting implants as well as transfer inaccuracies of a clinically relevant magnitude by accommodating the large deformations induced under masticatory loading upon martensitic phase transformation at almost constant stress. The application of NiTi resulted in the reduction of the reaction forces recorded in the surrounding of the supporting implant, as well, the reaction forces between male and female parts of the attachment system could be reduced which will minimize wear phenomena and subsequent maintenance costs. These effects were seen to be enhanced in the optimized design

Implant-Supported Overdentures: Current Status and Preclinical Testing of a Novel Attachment System

Numerous attachment systems exist for implant-supported overdentures, with each having specific limitations in terms of retention, cost, wear, maintenance and cleanability. A retrospective analysis of patients restored with implant-supported overdentures using bars, telescopic crowns and Locator-type attachments was performed and the patients were interviewed. An in vitro strain gauge study compared telescopic crowns, Locator-type attachments and a novel flexible attachment system employing a shape memory alloy (NiTi) with respect to peri-implant strain development during insertion, loading and removal of an overdenture. A significantly lower number of attachment-related complications was observed in bars as compared to telescopic crowns (p = 0.00007) and Locator-type attachments (p = 0.00000), respectively. Greater overall patient satisfaction was noted in bar-retained restorations while Locator-type attachments led to lower levels of satisfaction regarding prosthesis retention. In vitro, telescopic crowns caused maximum strain development during prosthesis insertion and loading, while during removal this was observed in Locators with white retentive inserts. NiTi attachments caused significantly lower strain development during insertion as compared to telescopic crowns (p = 0.027). During loading, NiTi attachments caused significantly lower strain development than Locators with blue retentive inserts (p = 0.039). During removal, NiTi attachments caused significantly less strain development as compared to Locators with white retentive inserts (p = 0.027). Positional discrepancies between male and female attachment parts affected the retention and reaction force between both components, which may be minimized by using the novel NiTi attachment system. This may be beneficial in terms of component wear and implant loading

Experimental and Numerical Investigation of Thermomechanical Cycling of Notched NiTi Shape Memory Ribbon Using SMA Model Accounting for Plastic Deformation

Shape memory alloys (SMAs) are being increasingly applied as thermally driven actuators. The commercially available SMA elements, however, frequently contain stress risers, either due to internal or surface defects or due to the required component shape. Stress risers represent a potential danger for the preliminary fatigue failure of such actuators but its actual mechanism is not very clear. This paper presents a combined experimental (2D DIC analysis of surface strains) and numerical analysis (SMA model with plastic deformation) of the stress, strain and phase fraction fields evolving in a thin NiTi shape memory ribbon with an artificial notch subjected to cyclic cooling-heating through transformation range under constant external force. It appeared that, even if only very low tensile stress is externally applied upon thermal cycling, local tensile stress at the notch tip sharply increases during the first cooling due to the forward martensitic transformation (MT) proceeding heterogeneously in space. This heterogeneity gives rise to plastic deformation at the notch-tip, which gradually accumulates upon thermal cycling and shields the notch tip from tensile overloading. Hence, fatigue performance of thermal NiTi actuator with stress riser depends very much on the plastic deformability of the alloy

Design and Numerical-Method-Aided Optimization of a Novel Attachment System for Implant-Retained Dental Prostheses Using NiTi Shape Memory Alloys

While nickel-titanium (NiTi) is the primary shape memory alloy (SMA) used in endodontic instruments, restorative dental components so far have not been fabricated from SMAs. The flexibility of these materials may solve problems in implant prosthodontics resulting from non-parallel implant positions and transfer inaccuracies. Based on a prototype of a novel attachment system for implant overdentures, a finite element model was created and used for studying different loading situations and design parameters followed by numerical analysis aided design optimization. The results revealed that the basic design of the attachment is capable of compensating misalignments of supporting implants as well as transfer inaccuracies of a clinically relevant magnitude by accommodating the large deformations induced under masticatory loading upon martensitic phase transformation at almost constant stress. The application of NiTi resulted in the reduction of the reaction forces recorded in the surrounding of the supporting implant, as well, the reaction forces between male and female parts of the attachment system could be reduced which will minimize wear phenomena and subsequent maintenance costs. These effects were seen to be enhanced in the optimized design

Stress raisers and fracture in shape memory alloys: review and ongoing challenges

International audienceShape memory alloys (SMAs) are able to recover large inelastic strains due to their thermal-/ stress-induced phase transformation between austenite and martensite. Stress raisers can either initially exist in SMA components as the manufacturing-induced micro-defects, or may nucleate upon monotonic/cyclic loading, for instance, due to decohesion of the second particles or local cyclic plastic deformations. Furthermore, from a physical point of view, there is a problem why SMAs can withstand tens of millions of cycles if they deform elastically but only thousands of cycles if the martensitic transformation is involved in their cyclic deformation under the stress, even if the martensitic transformation is reversible. One of the possibilities is the nucleation and propagation of cracks from the stress raisers since the evolution of the transformation and local mechanical gradients are completely different at the high-stress zones at stress raisers than that being experienced within the elastic bulk. Thus, the successful implementation of SMA elements into engineering applications requires understanding and analysis of the role of the stress raisers in fracture and fatigue crack growth properties of shape memory alloys. The linear and non-linear Fracture Mechanics theories, commonly used to describe the fracture processes in typical structural alloys, need to be enhanced to capture the complex deformation mechanisms characterizing SMAs. In the present paper, first, the latest progress made in experimental, numerical, and theoretical analyses on the role of the stress raisers in the fracture parameters of SMAs are reviewed and discussed under both pure mechanical and thermomechanical loading conditions. Then, the state-of-arts in fatigue crack growth are addressed. In the end, summary and future topics are outlined