预应力无含浸混杂纤维布的加固机制与界面本构模型

The large scattering of bond strength models, poor interfacial bond, and the low structural ductility are the key issues in structures strengthened by externally-bonded prestressed FRP. To overcome these problem, this proposed project aims at enhancing the bonding quality of prestressed FRP sheets by using the method of non- impregnation. To investigate the bonding failure mechanism of the FRP-concrete interface, a potential-based fictitious crack model will be developed using the Helmholtz free energy, strain-softening based damage constitutive model for the FRP-concrete interface, and the corresponding damage evolution law. Due to the high strain gradient resulting from the interfacial strain-softening and the strain localization, a transformed integration approach will be proposed to constructs the associated finite element and solve the mesh-dependency problem that is related to the constitutive model. Moreover, the transformed integration approach will also provide a theoretical solution to the prediction of the interfacial bonding strength. The ductility of the strengthened structure can be enhanced by using the hybrid effects of fiber sheets and the progressive failure through multilevel tension control of hybrid sheets, which will result in stress redistribution and thus reduction of the stress concentration. The results from the project can be applied to improve the overall structural performance including strength, rigidity and ductility at the service stage, as well as facilitate the cross-disciplinary research among material science, engineering and mechanics.

FRP-混凝土界面粘结强度模型的计算结果离散性大、预应力FRP片材的低粘贴质量、加固结构的低延性是预应力FRP片材加固中亟待解决的关键问题。本项目以解决上述问题为目标，采用无含浸方法提高预应力FRP片材的粘贴质量；依据内变量理论及不可逆热力学原理，研究Helmholtz自由能表达的"势相关"虚拟裂纹计算模型、界面应变软化损伤本构关系及损伤演化法则，以揭示界面粘结破坏机理；鉴于界面应变软化及其引起的变形局部化存在高应变梯度的特征，提出转换积分方法，构造一类单元，以从根本上解决本构模型的网格依赖性问题，并为提高界面粘结强度的预测提供理论解决；从材料和结构两个层次，根据混杂效应和多级张拉控制的混杂纤维布累进失稳破坏导致的应力重分布以缓解应力集中，從而提高加固结构的延性。项目的研究成果将有助于提升加固结构在使用阶段的综合力学性能（强度、刚度、延性），并促进材料、工程结构、力学这三个学科的交叉研究。

当前预应力FRP片材加固中亟需解决的关键科学与技术问题包括：FRP-混凝土界面粘结强度模型的低预测性、预应力FRP片材的低粘贴质量、加固结构的低延性。本项目以解决上述问题为目标，依据内变量理论及不可逆热力学原理，建立了以Helmholtz自由能表达的界面应变软化损伤本构模型及损伤演化法则，揭示了界面粘结破坏机理；鉴于界面应变软化及其引起的变形局部化存在高应变梯度的特征，提出了转换积分方法，构造了一类单元，并进一步应用非局部化方法，从根本上解决了本构模型的网格依赖性问题，更进一步的是，为提高界面粘结强度的预测提供了较好的理论解决；提出了用无含浸方法提高预应力FRP片材的粘贴质量；最后，从材料和结构两个层次，根据混杂效应和多级张拉控制的混杂纤维布累进失稳破坏导致的应力重分布以缓解应力集中的原理，提高了加固结构的延性。