Buckling-driven debonding in stiff block: compliant joint structural assemblies patched with composite materials

The initiation, growth, and stability of buckling driven debonding in structural assemblies of stiff blocks, compliant joints, and adhesively bonded composite layers are analytically investigated. The model is developed with focus on masonry walls externally strengthened with composite materials where static and, mainly, dynamic loads may induce compression in the strengthening layers triggering a buckling driven debonding near the joints. The model introduces the interfacial nonlinearity (debonding) through a cohesive interface approach. The geometrical nonlinearity is introduced through the kinematics of intermediate class of deformation (large deflections, moderate rotations, small strains), and the material nonlinearity of the masonry construction is introduced through the constitutive law for the mortar joints. A numerical study of the debonding process in strengthened masonry walls is presented. The study uses the periodicity of the wall for sub-structuring and examines configurations that include composite strips or sheets, strengthening on one face of the wall or on both faces, and compliant mortar materials. Emphasis is placed on the localized debonding near the joint, its stability characteristics, and the possibility to detect the debonding process before it reaches the point of instability.     

The effect of damage and creep interaction on the behaviour of masonry columns including interface debonding and cracking

Creep can produce significant effects on the structural behaviour of composite quasi-brittle systems, such as masonry, by altering the stress distribution between and within structural elements. The failure of a masonry element can be accelerated through damage incurred by weathering or degradation from creep effects. In this study, a three-dimensional finite element model of a grouted masonry column is used to evaluate the significance of the interaction of creep and damage on the structural behaviour of the column. The effects of Poisson??s ratio in producing differential out-of-plane constraint stresses can be simulated using this model. By utilizing a cracking criterion and incorporating a cohesive zone material (CZM) model for the brickwork-grout interface, the sequence and the patterns of cracking of the masonry column, debonding of the interface and local failure are examined. It is shown that debonding of the brickwork-grout interface occurs prior to cracking of the outer shell of brickwork. Case studies are presented to demonstrate the significance of the interaction of creep and damage on local failure, cracking and debonding. It is shown that cracking and debonding can result in a stability failure of a masonry column that was originally in a stable condition. Further work on local buckling and post-buckling analysis seems crucial to explain composite masonry behaviour.     

Out-of-plane behavior of unreinforced masonry walls strengthened with FRP strips

The out-of-plane behavior of unreinforced masonry walls strengthened with externally bonded fiber reinforced polymer (FRP) strips is analytically studied. The analytical model uses variational principles, equilibrium requirements, and compatibility conditions between the structural components (masonry units, mortar joints, FRP strips, and adhesive layers) and assumes one-way flexural action of the strengthened wall. The masonry units and the mortar joints are modeled as Timoshenko’s beams. The FRP strips are modeled using the lamination and the first-order shear deformation theories, and the adhesive layers are modeled as 2D linear elastic continua. The model accounts for cracking of the mortar joints and for the development of debonding zones near the cracked joints. Numerical and parametric studies that reveal the capabilities of the model, throw light on the interaction between the variables, and quantitatively explain some aspects of the behavior of the strengthened wall are also presented.     

The performance of unreinforced masonry walls subjected to low-velocity impacts: Finite element analysis

A simplified discrete-crack finite element modelling approach has been developed to model the performance of unreinforced brickwork and blockwork masonry walls subject to out-of-plane impacts. The approach involves the use of linear elastic solid elements for masonry units in conjunction with a specially formulated contact interface model for masonry joints. Key features of the latter include: (i) a Mohr–Coulomb failure criterion; (ii) a cohesive crack model for initial fracture; (iii) inclusion of dilatancy. The contact interface model has been implemented in LS-DYNA, a three-dimensional non-linear explicit finite element program. The modelling approach was used to simulate the behaviour of a series of unreinforced walls tested previously in the laboratory. It was found that the dynamic response of full-scale masonry walls could be predicted with reasonable accuracy. However, parametric studies showed that wall response was highly dependent on small changes in loading impulse, base friction, fracture energy, joint failure stress and angle of dilatancy.     

Experimental and analytical investigation on bond between Carbon-FRCM materials and masonry

Historical masonry constructions often need to be strengthened and upgraded to satisfy current seismic code requirements. Recently many interventions have been done bonding composite materials to the surface of existing masonry elements. The effectiveness of these interventions strongly depends on the bond between the strengthening material and the masonry and on the mechanical properties of the masonry substrate. In this paper the bond between fiber reinforced cementitious matrix (FRCM) materials made out of a Carbon net embedded in a cement based matrix and the masonry is experimentally and analytically investigated. Experimental results of double shear tests involving different bond lengths are presented. The results evidence that the debonding occurs at the fibers/matrix interface after a considerable fibers/matrix slip. They also confirms the effectiveness of the Carbon-FRCM materials as external reinforcements for masonry structures. The obtained experimental results are used to calibrate a local bond-slip relation that is essential in the modeling of the structural behavior of masonry elements strengthened with Carbon-FRCM.     

钢纤维水泥砂浆加固砌体墙的平面外受力性能

用钢纤维混凝土加固修复工程结构是一种提高其极限承载力和延性的有效方法。该文研究了钢纤维水泥砂浆加固砌体墙的平面外受力性能,试验了4个1000mm×1000mm×115mm的砌体墙,采用30mm厚、纤维体积含量分别为1.0%、1.5%、2.0%和2.5%的钢纤维水泥砂浆加固。试验结果表明:用体积含量为1.5%的钢纤维水泥砂浆加固效果最好;同时该文数值模拟了钢纤维水泥沙浆加固砌体墙的平面外极限承载力,数值模拟结果与试验结果吻合较好。     

Textile reinforced mortar (TRM) versus FRP as strengthening material of URM walls: out-of-plane cyclic loading

The application of a new structural material, namely textile reinforced mortar (TRM), as a means of increasing the load carrying capacity and deformability of unreinforced masonry walls subjected to cyclic out-of-plane loading is experimentally investigated in this study. The effectiveness of TRM overlays is evaluated in comparison to the one provided by fiber reinforced polymers (FRP) in the form of overlays or near-surface mounted (NSM) reinforcement. TRM systems may be considered as alternative to FRPs, tangling with some of the drawbacks associated with the application of the latter without compromising performance. Medium-scale tests were carried out on 12 masonry walls subjected to out-of-plane bending. The parameters under investigation comprised mortar-based versus resin-based matrix materials, the number of layers, the orientation of the moment vector with respect to the bed joints and the performance of TRM or FRP jackets in comparison to NSM strips. It is concluded that TRM jacketing provides substantial increase in strength and deformability. Compared with their epoxy-resin counterparts (FRP), TRM may result in generally higher effectiveness in terms of strength and deformability. NSM strips offer lower strength but higher deformability, due to controlled debonding. From the results obtained in this study it is believed that TRMs comprise an extremely promising solution for the structural upgrading of masonry structures under out-of-plane loading.     

A cohesive element model for mixed mode loading with frictional contact capability

We present a model that combines interface debonding and frictional contact. The onset of fracture is explicitly modeled using the well‐known cohesive approach. Whereas the debonding process is controlled by a new extrinsic traction separation law, which accounts for mode mixity, and yields two separate values for energy dissipation in mode I and mode II loading, the impenetrability condition is enforced with a contact algorithm. We resort to the classical law of unilateral contact and Coulomb friction. The contact algorithm is coupled together to the cohesive approach in order to have a continuous transition from crack nucleation to the pure frictional state after complete decohesion. We validate our model by simulating a shear test on a masonry wallette and by reproducing an experimental test on a masonry wall loaded in compression and shear. Copyright © 2012 John Wiley & Sons, Ltd.     

Debonding analysis of fiber-reinforced-polymer strengthened beams: Cohesive zone modeling versus a linear elastic fracture mechanics approach

A linear elastic fracture mechanics (LEFM) approach and a cohesive interface (cohesive zone) modeling approach to the debonding analysis of concrete beams strengthened with externally bonded fiber-reinforced-polymer (FRP) strips are studied and compared. The analytical models that are based on the two approaches are presented and discussed. The cohesive interface model is formulated using a potential function and it takes into account the shear effects, the effect of the peeling stresses, and the coupling of the shear and the peeling effects. This model takes the form of a set on nonlinear differential equations. The LEFM model combines stress analysis using the high order theory and fracture analysis using the concepts of the energy release rate and the J-integral. In addition, an algorithm that converts the results of the LEFM model into the equilibrium path of the debonding process is developed. The main advantages and disadvantages of the two approaches are also discussed. The two approaches are compared in terms of their applicability to quantify and describe the debonding process in various cases that include a single shear test, an edge peeling test, and a beam specimen strengthened with FRP.     

Constitutive behaviors of composites with interface debonding: the extended Mori–Tanaka method for uniaxial tension

Debonding of particle/matrix interfaces can significantly affect the macroscopic behavior of composite materials. We have used a nonlinear cohesive law for particle/matrix interfaces to study the effect of interface debonding on the macroscopic behavior of particle-reinforced composite materials subject to uniaxial tension. The Mori–Tanaka method, which is suitable for composites with high particle volume fraction, is extended to account for interface debonding. At a fixed particle volume fraction, small particles lead to the hardening behavior of the composite while large particles yield softening. The interface sliding may contribute significantly to the macroscopic behavior of the composite.     

Out-of-plane flexural behavior of unreinforced red brick walls strengthened with FRP composites

This paper presents the results of a study focused on evaluating the out-of-plane flexural behavior of two fiber reinforced polymer (FRP) composite systems for strengthening unreinforced red brick masonry walls. The full-scale tests followed the International Code Council Evaluation Service (ICC-ES) AC 125 procedure. In the experimental program, a total of four full-scale destructive tests were conducted on UMR red brick walls. One wall specimen was used as control (as-built) specimen without composites, and the remaining three wall specimens were strengthened with either E-glass/epoxy or carbon/epoxy composite systems with different fiber architecture. The effect of applying a cross-ply laminate on the ultimate failure mode has been investigated. Full-scale experimental results confirmed the effectiveness of the FRP composite strengthening systems in upgrading the out-of-plane flexural structural performance of URM walls. In addition, an analytical model was developed to predict the ultimate load capacity of the retrofitted walls. The analytical modeling is based on deformation compatibility and force equilibrium using simple section analysis procedure. A good agreement between the experimental and theoretical results was obtained.     

组合材料加固砖砌体的有限元模拟方法

基于ABAQUS有限元软件,提出了组合材料加固砌体数值建模方法。该方法是在未加固砌体整体式模型的基础上,结合分离式思想建立组合材料加固砌体模型。通过对8片采用粘钢-聚合物砂浆组合材料加固的砖砌体墙体（其中,4片采用粘贴正交钢片,4片采用粘贴斜撑钢片）的拟静力试验结果进行了数值模拟对比分析,结果显示:模拟所得墙体滞回、骨架及刚度退化曲线与试验曲线基本吻合;仿真破坏形态与试验现象一致;计算所得荷载、位移、延性和耗能等全部指标中有81%的误差在20%以内。     

Interfacial debonding behavior of composite beam/plates with PZT patch

This paper studies interfacial debonding behavior of composite beams which include piezoelectric materials, adhesive and host beam. The focus is put on crack initiation and growth of the piezoelectric adhesive interface. Closed-form solutions of interface stresses and energy release rates are obtained for adhesive layer in the piezoelectric composite beams. Finite element analyses have been carried out to study the initiation and growth of interfaces crack for piezoelectric beams with interface element by ANSYS, in which the interface element of FE model is based on the cohesive zone models to characterize the fracture behavior of the interfacial debonding. The results have been compared with analytical solution, and the influence of different geometry and material parameters on the interfacial behavior of piezoelectric composite beams have been discussed.     

Failure analysis of adhesively-bonded skin-to-stiffener joints: Metal–metal vs. composite–metal

The purpose of this research is to evaluate the performance of two adhesively bonded skin-to-stiffener connections: composite stiffener bonded to a Fiber Metal Laminate (FML) skin, representing a hybrid joint, and an Aluminium stiffener bonded to a FML skin, representative for a metal joint. The bonded joints were tested using Stiffener Pull-Off Tests (SPOT), which is a typical set-up used to simulate the structural behavior of full-scale components subject to out-of-plane loading, such as internal pressure of a fuselage or leading edge low pressure zone. In the hybrid joint, the damage initiates at the central noodle of the composite stiffener. Unstable delamination then propagates from the noodle to the tip of the stiffener foot, preferably through the stiffener foot plies (>90% of inter/intra-laminar failure) and, in limited areas, through the adhesive bond line (<10% of cohesive failure). In the metal joint, the failure starts at the tip of the stiffener foot at the adhesive bond line. Unstable debonding then propagates along the stiffeners foot. The complete failure occurs in the adhesive bond line (100% cohesive failure). The loads associated with >90% of inter/intra laminar failure of the composite stiffener (hybrid joint) are 40–60% lower than the ones associated with 100% cohesive failure (metal joint). This research identifies that in order to use the full capacity of adhesively bonded hybrid joints, the adhesion between carbon fibers of the composite laminate, ie intralaminar strength, must be improved. Otherwise, Aluminium stringers are still very competitive.     

碳纤维布加固RC梁中粘结性能的非线性有限元分析

碳纤维布加固钢筋混凝土(RC)梁中,碳纤维布与梁底混凝土的剥离破坏使碳纤维布的强度不能得到充分发挥。分析碳纤维布与梁底混凝土的粘结应力,是研究碳纤维布加固剥离破坏承载力的基础问题。根据4根碳纤维布加固RC梁的试验研究结果,采用商业有限元程序MSC.Marc建立有限元模型,进行了非线性计算分析。通过分离总粘结应力中的局部粘结应力,得到粘结延伸长度范围内的锚固粘结应力分布,并结合试验数据对其分布规律进行了研究。根据分析和试验结果,引入了“有效锚固粘结长度”和“锚固粘结应力”的概念,给出了极限荷载下锚固粘结应力的计算建议。     

Mechanical behaviour of FRP-confined masonry by testing of full-scale columns

The vulnerability of masonry constructions under seismic forces, or more generally under the mechanical actions during the centuries, has been highlighted in the last years by several events that caused the loss of significant heritage buildings. Faced with this difficulty, the use of composite materials, fiber reinforced polymers (FRP) may be a solution for mitigating the vulnerability of masonry buildings. This solution has been tested in the laboratory by researchers in the last decade. In particular, studies regarding elements such as walls, arches and vaults, strengthened with FRP materials are available. A few numbers of studies are known for columns, which have been tested only as small or middle scale samples. The current state of the art does not report studies on FRP-confined masonry columns tested in real scale. The research presents the results of an experimental program performed on full-scale masonry columns strengthened with different composite systems. The same kind of study had been previously performed by the authors on medium scale masonry columns, using the same materials for both the masonry core and for the FRP system. Prismatic columns with a square cross section were subjected to compression tests according to the following test schemes: two control unconfined columns; column with continuous wrapping by using unidirectional glass FRP (GFRP) sheets; column with discontinuous wrapping by using GFRP unidirectional sheets; column with continuous GFRP wrapping and internal carbon FRP bars bonded in the transverse directions; column wrapped with continuous alkali resistant GFRP grid and steel spikes bonded together in lime based matrix. The experimental results are presented and discussed in the paper along with the comparison with the results obtained from the experimental tests on medium scale specimens. The comparison between experimental data and theoretical predictions provided by the analytical model found in the guidelines of the CNR technical document is also illustrated.     

Nonlinear finite element analysis of masonry wall strengthened with CFRP strips

In many experimental studies, it has been proved that unreinforced masonry (URM) brick walls have high strength against lateral forces acting in plane. However, out-of-plane strength of URM brick walls against lateral forces has found to be quite low. According to the experiences that were obtained from the major earthquakes, the low out-of-plane performance of URM brick walls resulted in excessive loss of human lives during an earthquake, hence the strengthening of URM brick walls with CFRP strips has been appeared to be a very important subject. However, very limited literature has been found. Especially, the data obtained from experimental studies must be increased for the true understanding of the behavior of strengthened brick walls under out-of-plane lateral forces. However, in most cases, this procedure required large number of expensive experiments. At this stage, numerical analysis can be an appropriate choice, thus in this paper a finite element model is presented for modeling URM brick walls that are strengthened with CFRP strips. The numerical results are compared with the experimental ones and consistent results are obtained from the finite element model. General purpose finite element analysis software ANSYS is used throughout this study. Contact elements are used along the masonry wall–CFRP strip interfaces for the investigation of the stress distribution and load – strain behavior.     

Scattering of anti-plane shear waves by a partially debonded piezoelectric circular cylindrical inclusion

Summary This paper presents an analysis of the scattering of anti-plane shear waves by a single piezo-electric cylindrical inclusion partially bonded to an unbounded matrix. The anti-plane governing equations for piezoelectric materials are reduced to Helmholtz and Laplacian equations. The fields of scattered waves are obtained by means of the wave function expansion method when the bonded interface is perfect. When the interface is partially debonded, the region of the debonding is modeled as an interface crack with non-contacting faces. The electric permeable boundary conditions are adopted, i.e. the normal electric displacement and electric potential are continuous across the crack faces. The crack opening displacement is represented by Chebyshev polynomials and a system of equations is derived and solved for the unknown coefficients.     

Explicit modeling the progressive interface damage in fibrous composite: Analytical vs. numerical approach

Two micromechanical, representative unit cell type models of fiber reinforced composite (FRC) are applied to simulate explicitly onset and accumulation of scattered local damage in the form of interface debonding. The first model is based on the analytical, multipole expansion type solution of the multiple inclusion problem by means of complex potentials. The second, finite element model of FRC is based on the cohesive zone model of interface. Simulation of progressive debonding in FRC using the many-fiber models of composite has been performed. The advantageous features and applicability areas of both models are discussed. It has been shown that the developed models provide detailed analysis of the progressive debonding phenomena including the interface crack cluster formation, overall stiffness reduction and induced anisotropy of the effective elastic moduli of composite.