The influence of the shear deformations on the local stress state of pultruded composite profiles

Due to the relevance of shear deformability, the practical use of composite profiles still conflicts with the serviceability requirements related to the stiffness demand for civil applications. Moreover, when dealing with shear deformable beams, it is also well recognized that displacement-based 1-D models can lead to inaccurate stress predictions.Hence, a relatively simple beam model allowing to evaluate both strains and stresses accurately may represent a useful tool.The main motivation of the present paper is precisely to investigate these features by presenting relevant numerical results dealing with the mechanical response of pultruded composite profiles with thin-walled open cross-section made of both Glass (GFRP) and Carbon Fiber Reinforced Plastic (CFRP).Comparisons with solutions given via classical 1-D/2-D mechanical models are also provided, which highlight the accuracy of the proposed kinematics, especially with the aim of a reliable stress evaluation.     

Application of the NSCD method to analyse the dynamic behaviour of stone arched structures

In this paper, a masonry arch is simulated in order to assess both its structural and seismic vulnerability. The non-smooth contact dynamics (NSCD) computational method is used to simulate this type of structure as a collection of bodies under the hypothesis of unilateral constraints and frictional contact, with or without cohesion. Sinusoidal oscillations in three dimensions and real earthquake data have been applied to the supporting base element of the arch model.The primary aim of this study is to better understand the dynamic behaviour of the masonry arch, a typical component of historic unreinforced masonry (URM) structures. This study also assesses the influences of the input parameters on the mechanical and dynamic behaviour of the arch structure. Its collapse mechanism is studied for both cohesive and non-cohesive contact.In addition, we examine the behaviour under seismic loading of the Arles aqueduct, a real historical arched structure located in the south-east of France. Significant information can be obtained from the comparison of the results of advanced numerical analysis, taking into account the precise geometry of the model, the mechanical characteristics of the materials and the observation of the in situ monuments after their collapse.     

An experimental method for considering dispersion and attenuation in a viscoelastic Hopkinson bar

An experimental method is developed to perform Hopkinson tests by means of viscoelastic bars by considering the wave propagation attenuation and dispersion due to the material rheological properties and the bar radial inertia (geometric effect). A propagation coefficient, representative of the wave dispersion and attenuation, is evaluated experimentally. Thus, the Pochhammer and Chree frequency equation is not necessary. Any bar cross-section shapes can be employed, and the knowledge of the bar mechanical properties is useless. The propagation coefficients for two PMMA bars with different diameters and for an elastic aluminum alloy bar are evaluated. These coefficients are used to determine the normal forces at the free end of a bar and at the ends of two bars held in contact. As an application, the mechanical impedance of an accelerometer is evaluated. A part of this work has been performed in the Laboratoire Matériaux Endommagement Fiabilité of the Ecole Nationale Supérieure des Arts et Métiers de Bordeaux.     

THE FAILURE MODES OF CONCRETE BEAMS STRENGTHENED WITH NEAR SURFACE MOUNTED FRP BARS

为研究不同参数下表面内嵌纤维筋加固后T 形混凝土梁的破坏模式, 对5 根不同梁端锚固、FRP(fiber reinforced polymer) 筋表面特征和FRP 筋类型的T 形混凝土梁进行受弯性能试验. 结果表明, 无梁端锚固、光圆GFRP (glass fiber reinforced polymer) 筋和CFRP (carbon fiber reinforced polymer) 筋加固梁试件发生粘结破坏. 梁端锚固和FRP 筋表面特征影响加固梁试件的极限载荷, CFRP 筋加固梁试件的屈服载荷和极限载荷较大. 螺纹FRP 筋和有梁端锚固加固梁试件FRP 筋利用率较高. 因此, 有梁端锚固的表面内嵌螺纹GFRP 筋加固是最为有效的加固方式.     

THE DURABILITY OF GLASS FIBER REINFORCED COMPOSITE REBARS IN SALT LAKE HIGH AND COLD AREA1)

对玻璃纤维(glass fiber reinforced polymer, GFRP)筋和GFRP -钢筋夹芯复合筋在盐湖地区多重环境因素耦合作用下进行耐久性试验,分析环境类型及作用时间对极限抗拉强度、弹性模量、极限应变的影响。结果表明：在多重因素耦合作用下,随着盐湖卤水腐蚀周期、冻融次数、干湿循环次数的增加,GFRP筋和GFRP夹芯复合筋的抗拉强度逐渐减小,但是GFRP筋减小的幅度较小,而GFRP夹芯复合筋由于有钢筋的存在,抗拉强度减小比较大,特别是在盐湖卤水90 d以上、冻融150次以上时GFRP夹芯复合筋的极限抗拉强度实验不是很明显,屈服强度几乎不存在并且与抗拉极限强度相接近,表现出明显的脆性;在各种因素作用下,GFRP筋随着龄期的增加,弹性模量先减少后增加,而GFRP夹芯复合筋的弹性模量逐渐减小,相对来讲减小的幅度不是很大;各种耦合因素作用下GFRP筋和GFRP夹芯复合筋的极限抗拉强度、弹性模量均比单因素作用下小,且腐蚀性的大小关系是盐湖卤水+干湿循环+冻融耦合>盐湖卤水+干湿循环>冻融>盐湖卤水。     

Dynamic response of rocking cracked masonry walls

The behaviour of masonry constructions results to be very far from the one characterizing ductile structures. In masonry constructions, the seismic action activates a rocking motion rather than a dissipating mechanism. A strength resource of masonry structures, properly reinforced in order to avoid early local failures, consists in exhibiting rocking behaviour, until a failure condition is attained. Aim of the paper is to investigate the dynamic behaviour of masonry single storey walls, according to Housner’s studies and innovatively introducing the effect of diagonal cracks developing from the toes of the piers and shown by typical post-earthquake cracking patterns. The proposed procedure can be easily applied to the case of multi-storey regular masonry walls with openings representing the main resistant structural components of a masonry building. Starting from the evaluation of the incipient rocking acceleration of the system, the free and forced motions of the wall are examined. In the paper, according to the classical Housner’s approach, the energy dissipation occurring during the impact is modelled. Finally, a numerical application, considering a simple constant horizontal acceleration impulse of given duration has been carried out.

     

三维DIC在GFRP锚杆拉伸力学性能测试中的应用

随着玻璃纤维增强塑料(GFRP)广泛运用于轨道交通及边坡加固中,其力学性能的测试显得尤为重要。基于三维数字图像相关(DIC)方法,研究了三种不同加载速率下GFRP锚杆的拉伸力学性能,分别得到了应力-应变曲线及弹性模量、抗拉强度、最大力总延伸率等参数。实验结果表明,螺旋状变截面锚杆的轴向应变呈非均匀分布,由三维DIC得到的锚杆弹性模量结果一致性优于传统的接触式引伸计方法,GFRP锚杆几乎呈线性分布的应力-应变曲线表明该材料为典型的脆性材料。     

A micro-mechanical homogenisation model for masonry: Application to shear walls

An improved micro-mechanical model for masonry homogenisation in the non-linear domain, is proposed and validated by comparison with experimental and numerical results available in the literature. Suitably chosen deformation mechanisms, coupled with damage and plasticity models, can simulate the behaviour of a basic periodic cell up to complete degradation and failure. The micro-mechanical model can be implemented in any standard finite element program as a user supplied subroutine defining the mechanical behaviour of an equivalent homogenised material. This work shows that, with the proposed model, it is possible to capture and reproduce the fundamental features of a masonry shear wall up to collapse with a coarse finite element mesh. The main advantage of such homogenisation approach is obviously the possibility to simulate real complex structures while taking into consideration the arrangement of units and mortar, which would otherwise require impractical amount of finite elements and computer resources.     

后加混凝土-砌体组合结构简化计算模型研究

后加混凝土构件的加固方法是在原有砌体结构外部新增钢筋混凝土构件,形成一种新型的组合结构．本文提出了考虑连接节点的后加混凝土构件-砌体组合结构的简化理论计算模型,并推导了不同构件的等效计算公式．考虑均布分布荷载和倒三角分布荷载作用,对组合结构的变形、剪力分配进行计算分析．计算结果从定性方面验证了本文提出的简化计算模型的可行性和有效性．同时针对某实际工程进行理论计算,与三维空间模型在弹性阶段的计算结果进行对比,从定量方面验证了本文提出的简化计算模型的准确性．     

Planar mobility modes of 8-bar-jointed structures with a single degree of freedom

A range of modes of mobility for spatial structures can be achieved by selecting the rotational features of the joints. Similar mechanical characteristics are observed in structures ranging from molecules to buildings. The local rotational manner in a solid with an internal spatial structure performs a key part in dictating the nature of structural deformability. In this paper, we propose a two-dimensional repetitive motion structure assembled with straight rigid bars connected by 8-bar pivot joints. By determining the rotational modes of the 8-bar joint with certain geometrical constraints, we introduce some telescopic transformations of the proposed structure producing both well-known and novel motions. In addition, we verify one of the deployable mechanisms by manufacturing it and testing activation by a single rotary operation.     

PVDF压电薄膜用于Hopkinson压杆测量泡沫金属的动态性能

首先对PVDF(polyvinylidene fluoride)压电薄膜在不同温度不同压力作用下的响应进行了系统的试验研究。然后在Hopkinson压杆系统的透射杆之间夹上PVDF压电薄膜,对其动态响应进行了检验。最后应用这个镶嵌PVDF压电薄膜的Hopkinson压杆系统,测试了泡沫铜材料在不同应变率下的应力应变关系。结果表明:(1)PVDF压电薄膜的压电常数D33是随温度和压力而变,实际应用时应对其进行标定;(2)PVDF压电薄膜可有效的用于Hopkinson压杆系统来测试低强度泡沫材料或低阻抗材料的动态响应;(3)当应变率小于0.1/s时,泡沫铜的塑性流动应力对应变率不敏感,在约400/s到5000/s应变率范围,应变小于40%下泡沫铜对应变率也不敏感。但当应变大于约20%,应变率高于400/s时,与低应变率下的值比较,塑性流动应力的应变率敏感性增加。     

Equivalent nonlinear beam model for the 3-D analysis of shear-type buildings: Application to aeroelastic instability

Tower buildings can be very sensitive to dynamic actions and their dynamic analysis is usually carried out numerically through sophisticated finite element models. In this paper, an equivalent nonlinear one-dimensional shear–shear–torsional beam model immersed in a three-dimensional space is introduced to reproduce, in an approximate way, the dynamic behavior of tower buildings. It represents an extension of a linear beam model recently introduced by the authors, accounting for nonlinearities generated by the stretching of the columns. The constitutive law of the beam is identified from a discrete model of a 3D-frame, via a homogenization process, which accounts for the rotation of the floors around the tower axis. The macroscopic shear strain in the equivalent beam is produced by the bending of columns, accompanied by negligible rotation of the floors. A coupled nonlinear shear–torsional mechanical model is thus obtained. The coupling between shear and torsion is related to a non-symmetric layout of the columns, while mechanical nonlinearities are proportional to the slenderness of the columns. The model can be used for the analysis of the response of tower buildings to any kind of dynamic and static excitation. A first application is here presented to investigate the effect of mechanical and aerodynamic coupling on the critical galloping conditions and on the postcritical behavior of tower buildings, based on a quasi-steady model of aerodynamic forces.     

A kinematic FE limit analysis model for thick English bond masonry walls

Two-wythes masonry walls arranged in English bond texture were often used in the past as bearing panels in seismic area. On the other hand, earthquake surveys have demonstrated that masonry strength under horizontal actions is usually insufficient, causing premature collapses of masonry buildings, often ascribed to out-of-plane actions. Furthermore, many codes of practice impose for new brickwork walls a minimal slenderness, which for instance is fixed by the Italian O.P.C.M. 3431 equal to 12 for artificial bricks and 10 for natural blocks masonry.For the above reasons, the analysis at failure of English bond brickwork walls under out-of-plane actions is a topic that deserves consideration, despite the fact that almost the totality of the studies of masonry at failure is devoted to running bond arrangements. Furthermore, it must be noted that an approach based on the analysis of running bond texture – in comparison with English bond pattern – is not suitable for the investigation of the behavior of bearing panels.In this framework, in the present paper, a Reissner–Mindlin kinematic limit analysis approach is presented for the derivation of the macroscopic failure surfaces of two-wythes masonry arranged in English bond texture. In particular, the behavior of a 3D system constituted by infinitely resistant bricks connected by joints reduced to interfaces with frictional behavior and limited tensile/compressive strength is identified with a 2D Reissner–Mindlin plate. In this way, assuming both an associated flow rule for the constituent materials and a finite subclass of possible deformation modes, an upper bound approximation of macroscopic English bond masonry failure surfaces is obtained as a function of macroscopic bending moments, torsion and shear forces.Several examples of technical relevance are treated both at a cell level and at a structural level, addressing the differences in terms of collapse loads and failure surfaces due to different textures and constituent laws for joints. Finally, two meaningful structural examples consisting of a panel in cylindrical flexion and a masonry slab constrained at three edges and out-of-plane loaded are discussed. A detailed comparison in terms of deformed shapes at collapse and failure loads between a 2D FE Reissner–Mindlin limit analysis approach and a full 3D heterogeneous FE model shows the reliability of the results obtained using the kinematic identification approach proposed.     

A quadratic hierarchical Bayesian dynamic prediction model for infrastructure maintenance

The main objective of this study is the development of a correlation model in dynamic Bayesian belief networks (DBBNs) followed by an inverse economic analysis. This is based on a quadratic hierarchical Bayesian inference prediction method using Markov chain Monte Carlo simulations. The developed model is implemented to predict the future degradation and maintenance budget for a suspension bridge system. Bayesian inference is applied to find the posterior probability density function of the source parameters (damage indices and serviceability), given 10 years maintenance data. The simulated risk prediction under decreased serviceability conditions gives posterior distributions based on a prior distribution and likelihood data updated from annual maintenance tasks. Compared with a conventional linear prediction model, the proposed quadratic model provides highly improved convergence and closeness to the measured data. Finally, the developed inverse DBBN analysis method allows forecasts of future performance and the financial management of complex infrastructures by providing the sensitivity of serviceability and risky factors to the maintenance budgets of structural components and the overall system.     

Homogenization towards a mechanistic Rigid Body and Spring Model (HRBSM) for the non-linear dynamic analysis of 3D masonry structures

A mechanistic model with rigid elements and interfaces suitable for the non-linear dynamic analysis of full scale 3D masonry buildings is presented. The model relies into two steps: in the first step, a simplified homogenization is performed at the meso-scale to deduce the mechanical properties of a macroscopic material, to be used in structural applications; the second step relies into the implementation of a Rigid Body and Spring Model (RBSM) constituted by rigid elements linked with homogenized interfaces. In the homogenization step, a running bond elementary cell is discretized with 24 three-node plane-stress elastic triangular elements and non-linear interfaces representing mortar joints. It is shown how the mechanical problem in the unit cell is characterized by few displacement variables and how homogenized stress–strain curves can be evaluated by means of a semi-analytical approach. The second step relies on the implementation of the homogenized curves into a RBSM, where an entire masonry structure can be analyzed in the non-linear dynamic range through a discretization with rigid elements and inelastic interfaces. Non-linear structural analyses are conducted on a church façade interconnected with a portion of the perpendicular walls and on a small masonry building, for which experimental and numerical data are available in the literature, in order to show how quite reliable results may be obtained with a limited computational effort.     

On the coupling between a vibrating mechanical system and the external forces acting on it

A theoretical model is proposed in order to investigate the coupling effects between a mechanical system acted upon by periodic forces and the exciting device; application to the Bouasse-Sarda regulator is dealt with.     

砌体结构破坏倒塌过程的离散单元模拟

基于传统颗粒离散单元模型（DEM）,开发了一种绑定式离散单元模型（BTDEM）用于砌体结构崩塌机制的研究,并对一个典型砌体结构的破坏倒塌过程进行数值模拟。结果证实BTDEM方法能够详细地模拟出砌体建筑结构的崩塌过程,同时还可以模拟出砌块的破碎,是一种普适性较强的模拟砌体结构建筑的高效数值方法。BTDEM方法具有计算速度...     

Phase-field modeling of hydraulic fracture

In this work a theoretical framework implementing the phase-field approach to fracture is used to couple the physics of flow through porous media and cracks with the mechanics of fracture. The main modeling challenge addressed in this work, which is a challenge for all diffuse crack representations, is on how to allow for the flow of fluid and the action of fluid pressure on the aggregate within the diffuse damage zone of the cracks. The theory is constructed by presenting the general physical balance laws and conducting a consistent thermodynamic analysis to constrain the constitutive relationships. Constitutive equations that reproduce the desired responses at the various limits of the phase-field parameter are proposed in order to capture Darcy-type flow in the intact porous medium and Stokes-type flow within open cracks. A finite element formulation for the solution of the governing model equations is presented and discussed. Finally, the theoretical and numerical model is shown to compare favorably to several important analytical solutions. More complex and interesting calculations are also presented to illustrate some of the advantageous features of the approach.     

3D macro and micro-block models for limit analysis of out-of-plane loaded masonry walls with non-associative Coulomb friction

In this paper, a masonry system composed of a façade wall connected with two sidewalls and subjected to out-of-plane loading is investigated within the framework of three-dimensional limit analysis. Two different modeling approaches, namely macro and micro-block models, are adopted. A rigid-perfectly plastic model with dry contact interfaces governed by Coulomb failure criterion is assumed for masonry walls with regular units and staggering (non-standard limit analysis). Three classes of failure modes are investigated, involving rocking, sliding, twisting failure and combinations of them. The macro-block model is based on the assumption that the failure involves a number of cracks which separate the structure into a few macro-blocks and all the possible relative motions among micro-blocks are concentrated along the cracks. Two limiting conditions for the ultimate load factor are kinematically computed by use of minimization routines. The micro-block model is based on a concave contact formulation in which contact points are located at the corners of interfaces, allowing failure modes involving opening and sliding to be simulated. An iterative solution procedure is used to solve the non-associative friction problem, with second order cone programming (SOCP) used to allow the conic yield function to be solved directly. Both models are validated against experimental outcomes from the literature. A parametric analysis is carried out in order to highlight the influence of each geometrical and mechanical parameter on the prevalence of a mechanism over the other. The presence of an unrestrained horizontal floor system with different orientations is also analyzed.