Petrini Lorenza, Bertini Alessandro, Berti Francesca, Pennati Giancarlo, Migliavacca Francesco
1 Laboratory of Biological Structure Mechanics, Department of Civil and Environmental Engineering, Politecnico di Milano, Milan, Italy.
2 Laboratory of Biological Structure Mechanics, Department of Chemistry, Materials and Chemical Engineering "Giulio Natta", Politecnico di Milano, Milan, Italy.
Proc Inst Mech Eng H. 2017 May;231(5):391-404. doi: 10.1177/0954411917696336.
Nickel-titanium alloys are commonly adopted for producing cardiovascular minimally invasive devices such as self-expandable stents, aortic valves and stent-grafts. These devices are subjected to cyclic loads (due to blood pulsatility, leg or heart movements), that can induce fatigue fracture, and may also be subjected to very large deformations (due to crimping procedure, a tortuous physiological path or overloads), that can induce material yield. Recently, the authors developed a new constitutive model that considers inelastic strains due to not-completed reverse phase transformation (not all the stress-induced martensite turns back to austenite) or/and plasticity and their accumulation during cyclic loads. In this article, the model is implemented in the finite element code ABAQUS/Standard and it is used to investigate the effects of inelastic strain accumulation on endovascular nickel-titanium devices. In particular, the behavior of a transcatheter aortic valve is studied considering the following steps: (1) crimping, (2) expansion in a tube resembling a durability test chamber and (3) cyclic loads due to pressure variation applied on the inner surface of the tube. The analyses are performed twice, activating and not activating that part of the new model which describes the development of irreversible strain. From the results, it is interesting to note that plasticity has a very significant effect on the local material response, inducing stress modification from compression to tension. However, permanent deformations are concentrated in few zones of the stent frame and their presence does not affect the global behavior of the device that maintains its capability of recovering the original shape. In conclusion, this work suggests that at least for cardiovascular devices where the crimping is high (local strain may reach values of 8%-9%), taking into account inelastic effects due to plasticity and not-completed reverse phase transformation can be important, and hence using a suitable constitutive model is recommended.
镍钛合金通常用于制造心血管微创设备，如自膨胀支架、主动脉瓣和覆膜支架。这些设备会受到循环载荷（由于血液脉动、腿部或心脏运动）的作用，这可能会导致疲劳断裂，并且还可能承受非常大的变形（由于压接过程、曲折的生理路径或过载），这可能会导致材料屈服。最近，作者开发了一种新的本构模型，该模型考虑了由于未完全逆相变（并非所有应力诱发马氏体都变回奥氏体）或/和塑性及其在循环载荷下的累积而产生的非弹性应变。在本文中，该模型在有限元软件ABAQUS/Standard中实现，并用于研究非弹性应变累积对血管内镍钛设备的影响。具体而言，考虑以下步骤研究了经导管主动脉瓣的行为：（1）压接，（2）在类似于耐久性测试腔的管中展开，以及（3）由于施加在管内表面上的压力变化而产生的循环载荷。分析进行了两次，一次激活新模型中描述不可逆应变发展的部分，另一次不激活。从结果中可以有趣地注意到，塑性对局部材料响应有非常显著的影响，导致应力从压缩变为拉伸。然而，永久变形集中在支架框架的少数区域，并且它们的存在不会影响设备恢复原始形状的整体性能。总之，这项工作表明，至少对于压接程度较高（局部应变可能达到8%-9%的值）的心血管设备，考虑由于塑性和未完全逆相变引起的非弹性效应可能很重要，因此建议使用合适的本构模型。
