Stability of the vertical bearing structures of the Syracuse Cathedral: experimental and numerical evaluation

In the present paper the results from a recent monitoring campaign and numerical analysis performed on the ancient Cathedral of Syracuse in Sicily are presented. The acoustic emission (AE) technique is adopted to assess the damage pattern evolution. The localization of the propagating cracks is obtained using six synchronized AE sensors. A clear correlation between the regional seismic activity and the AE acquisition data is shown. In fact the AE count rate presents peaks corresponding to the main seismic events. In addition, a numerical analysis of the vertical bearing structures of the Cathedral is presented. The nonlinear Finite Element model is particularly refined to account for the cracking in the most damaged pillar. Some recent seismic events in the area acted as crack propagators. The crack occurrence obtained from the numerical analysis agrees quite well with the crack localization provided by the AE monitoring.     

Longitudinal vibration of a continuous cracked bar

A continuous cracked bar vibration theory is developed for longitudinal vibration of rods with an edge crack. The Hu–Washizu–Barr variational formulation was used to develop the differential equation and the boundary conditions of the cracked bar as a one-dimensional continuum. The crack was modelled as a continuous flexibility using the displacement field in the vicinity of the crack found with fracture mechanics methods.

The results of three independent evaluations of the lowest natural frequency of longitudinal vibrations of a bar with a single edge crack are presented: the continuous cracked bar vibration theory, the lumped crack bar vibration analysis, and experimental results obtained on aluminum bars with fatigue cracks. Experimental results fall between the values predicted by the two analytical methods. Moreover, the continuous bar theory agrees better with the experimental results than the lumped crack flexibility theory for small cracks. For larger cracks, a/h>0.4, experimentation was difficult due to the co-existence of several coupled modes and no reliable results could be obtained.     

Failure modes and serviceability of high strength self compacting concrete deep beams

The behaviour of deep beams is significantly different from shallow beams. In deep beams, the plane section does not remain plane after deformation. The main purpose of this study is to facilitate the prediction of deep beam failure related to tensile bar and web reinforcement percentage variations. Six high strength self compacting concrete (HSSCC) deep beams were tested until failure. Strains were measured on concrete surface along mid span, tensile bar and compression strut trajectory. The load was incrementally applied and at each load increment new cracks, their widths and propagation were monitored. The results clearly show that, at ultimate limit condition, the strain distribution on concrete surface along mid-span is no longer parabolic. In deep beams several neutral axes were obtained before ultimate failure is reached. As the load increases, the number of neutral axis decreases and at failure load it reduces to one. The failure of deep beams with longitudinal tensile steel reinforcement less than that suggested by ACI codes is flexural and is accompanied by large deflections without any inclined cracks. As the longitudinal tensile steel reinforcement increases, the failure due to crushing of concrete at nodal zones was clearly observed. The first flexural crack at mid-span region was always vertical. It appeared at 25–42% of peak load. The crack length was in the range of 0.24–0.6 times the height of section. As the tensile bar percentage increases number of cracks increases with reduced crack length and crack width. The appearance of first inclined crack in compression strut trajectory is independent of tensile and web bar percentage variations.     

Tension of a cylindrical bar having an infinite row of circumferential cracks

In this paper, the crack problems in the case of a cylindrical bar having a circumferential crack and a cylindrical bar having an infinite row of circumferential cracks under tension are analyzed by the body force method. The stress field for a periodic array of ring forces in an infinite body is used to solve the problems. The solution is obtained by superposing the stress fields of ring forces in order to satisfy a given boundary condition. The stress intensity factors are calculated for various geometrical conditions. The obtained values of stress intensity factor of a single circumferential crack are considered to be more reliable than the results of other paper's. As the crack becomes very shallow, the stress intensity factor of a row of circumferential cracks approaches the value corresponding to that of a row of edge cracks in a semi-infinite plate under tension. As the crack becomes very deep, it approaches the values corresponding to that of a single deep circumferential crack.     

Fatigue crack paths for inclined cracks and surface flaws under biaxial loading

Two approaches are developed for geometrical modeling of crack growth trajectories for the inclined through thickness central cracks and the part-through surface flaw respectively. The principal feature of such modeling is the determination of crack growth direction and the definition of crack length increment in this direction. The damage process zone size concept is employed for calculations of mixed-mode crack growth trajectories and surface flaw shape and positions. Crack front behavior for a straight-fronted edge crack in an elastic bar of circular cross-section is studied through experiments and computations under axial tension loading. The elaborated theoretical model is applied for fatigue crack shape simulation of part-through cracks in a hollow thick- and thin-walled cylinders under different biaxial loading conditions. Suggested approach of crack paths modeling is used for an analysis and prevention of operation failures of existing in-service aircraft gas-turbine engine rotating components.     

铁道货车转8A铸钢侧架B部位裂纹分析

在生产铁道货车铸钢转8A侧架时，近30％的侧架B部位产生横向裂纹。采用化学成分分析及裂纹的宏、微观检验等方法对产生横向裂纹的侧架B部位进行了分析，并对其铸造工艺进行了探讨。结果表明侧架B部位产生横向裂纹的主要原因在于防裂拉筋设计不合理，导致铸件凝固时出现热裂纹。并提出了有效的改进措施。     

The microstructure as crack initiation point and barrier against fatigue damaging

From the emission of dislocations till short crack propagation fatigue is a local process determined by the microstructure. We present experiments based on electron channelling contrast imaging (ECCI) as refined application of the scanning electron microscope (SEM) and new focused ion beam (FIB) technique like FIB crack initiation and FIB tomography which give detailed information about crack initiation and the interaction of short fatigue cracks with precipitates and grain boundaries as microstructural barriers. As main result the characteristic fluctuation in the propagation rate of short fatigue cracks in front of grain boundaries that has so far defied calculation can now be calculated analytically from the BCS-model and Tanaka model by using three constants measured in a single crystal.     

坡口类焊道的裂纹分析与防止措施

对于贴板与主筋板采用不同的焊接方式分析,阐明了焊接裂纹的形成主要原因是熔深较小,焊丝角度是导致熔深大小的重要工艺参数,强调了焊丝角度的重要意义。通过分析对比,确定了坡口在上的焊接方式是解决贴板与主筋板焊接裂纹的合理方式。     

Longitudinal vibration of a bar with a breathing crack

The dynamics of a cracked fixed-free bar with a breathing crack in longitudinal vibration is investigated. The Hu–Washizu–Barr variational formulation was used to develop the equation of motion and the boundary conditions of the cracked bar as a one-dimensional continuum. The crack was modelled as a continuous flexibility using the displacement field in the vicinity of the crack found with fracture mechanics methods. The eigenfrequency changes due to a single open-edge breathing crack, are shown to depend on the bilinear character of the system. The associated linear problems are solved over their respective domain of definition and then the solutions are matched through the initial conditions. These changes are smaller than the ones caused by open cracks. The method has been tested for different bar configurations corresponding to crack location, crack depths, cross-section dimensions, and Poisson’s ratio. The natural frequencies obtained from this model agree well with experimental results.     

复合材料层合板基体裂纹的协同损伤演化模型

首先,为研究复合材料层合板在准静态载荷下的基体裂纹演化特征,提出了一个基于能量的协同损伤演化模型。然后,通过模型对损伤进行了多尺度分析:从微观角度,采用三维有限元方法求得裂纹表面位移;从宏观角度,结合裂纹表面位移,推导了萌生基体裂纹的能量释放率。最后,根据裂纹萌生准则对基体裂纹的演化过程进行预测。模型考虑了演化过程中损伤的相互影响、残余应力、基体材料非线性、材料初始损伤分布及损伤演化的不均匀性。根据演化分析流程计算了[±θ/904]s铺层玻璃纤维复合材料的基体裂纹演化过程。结果表明:这一模型能够准确地预测准静态载荷下复合材料层合板基体裂纹的损伤演化规律。     

Causes of the formation of longitudinal surface cracks on the DC-cast 7039 aluminium alloy

The longitudinal surface cracks occurring on the DC-cast 7039 aluminium alloy were investigated. The cracks were characterized by a grooved appearance on the exterior surface and a branching and porous morphology in the interior area. Microscopic analysis showed that the crack opening was of an abnormal chilled structure, while the interior region was of slowly solidified structures. EPMA and DTA indicated that Mg-Zn intermetallic compounds were formed in the crack interior boundaries. The results suggested that the cracks originated from the uneven chill conditions of the mould. The liquid melt in front of the hot spot was enclosed by the rapidly growing solidified shell and the bridged liquid melt then suffered from a slow cooling rate, forming shrinkage cracks and pores. Formation of Mg-Zn intermetallic compound arising from the slow solidification may enhance the crack occurrence. The cracks were completely eliminated by the improvement in the mould cooling efficiency.     

Stress intensity factors for a circumferential crack emanating from a notch in a round tensile bar

A semi-empirical formula for the stress intensity factors for short cracks is presented. Once the crack has reached a critical depth, designated as a_o, the notch has no effect on the crack tip stress field and the stress intensity factor for a smooth bar with crack depth (a + d) is applicable, where dis the notch depth. The solution for shallow cracks less than a_o, which are influenced by the proximity of the notch, are determined from the maximum stress and stress gradient at the root of the notch and the smooth bar solution at a_o. The results are applicable for a wide variety of notch geometries ranging from semi-circular notches to deep hyperbolic shapes.     

Analysis of fatigue crack behaviour based on dynamic response simulations and experiments for tensile-shear spot-welded joints

Fatigue crack initiation and propagation behaviours were studied based on the dynamic response simulation by the three‐dimensional finite‐element analysis (FEA) and dynamic response experiments for tensile‐shear spot‐welded joints. The entire fatigue propagation behaviour from the surface elliptical cracks at the initiation stage to the through thickness cracks at the final stage was taken into consideration during the three‐dimensional FEA dynamic response simulations. The results of the simulations and experiments found that the fatigue cracks of spot‐welded joint from initial detectable crack sizes to crack propagation behaviour could be described by three stages. Approximately one‐half of the total fatigue life was taken in stage I, which includes micro‐crack nucleation and the small crack growth process; 20% of the total fatigue life in stage II, in which the existing surface crack propagates through the thickness of sheet and 30% of the total fatigue life in stage III, during which the through thickness crack propagates along the direction of plate width to the final failure. According to the relationship between the crack length and depth and the dynamic response frequency during the simulated fatigue damage process, the definition of fatigue crack initiation and propagation stages was proposed. The analysis will provide some information for the fatigue life prediction of the spot‐welded structures.     

Flaking failure originating from a single surface crack in silicon nitride under rolling contact fatigue

Flaking failure caused by surface cracks of silicon nitride ceramic bearings has been investigated from the viewpoint of the ring crack model. However, the relation between surface and subsurface cracks under rolling contact fatigue is not fully understood. In this investigation subsurface cracks branching from an initial surface crack were observed in detail, and the process of flaking failure was investigated. The specimens were observed prior to the separation of the surface layers and it was found that the initial surface cracks grew vertically to the surfaces and did not curve as predicted by the ring crack model. Subsurface cracks branched from the single surface cracks and grew in a direction parallel to the surface. They grew in both the same and the opposite directions to the ball movement, with small upward and downward branches. These subsurface cracks grew prior to the semi‐circular surface cracks. From these observations it was concluded that the flaking failures are not caused directly by the surface cracks, but by the subsurface cracks that branch from them.     

Q460C钢组织特性对表面裂纹成因的影响分析

通过光学显微组织观察,测定显微组织硬度和能谱分析等方法对Q460C中厚板表面裂纹成因进行了分析研究,分析了热送热装工艺对裂纹形成的影响,分析了轧制过程中裂纹的形成机理.利用热膨胀法结合金相法在Gleeble-1500热模拟实验机上测定了实验钢连续冷却转变曲线,分析了材料组织转变与裂纹形成规律的联系.分析结果表明：热送热...     

PRIZE FOR BEST PAPER VOLUME 28

Flaking failure caused by surface cracks of silicon nitride ceramic bearings has been investigated from the viewpoint of the ring crack model. However, the relation between surface and subsurface cracks under rolling contact fatigue is not fully understood. In this investigation subsurface cracks branching from an initial surface crack were observed in detail, and the process of flaking failure was investigated. The specimens were observed prior to the separation of the surface layers and it was found that the initial surface cracks grew vertically to the surfaces and did not curve as predicted by the ring crack model. Subsurface cracks branched from the single surface cracks and grew in a direction parallel to the surface. They grew in both the same and opposite directions to the ball movement, with small upward and downward branches. These subsurface cracks grew prior to the semi‐circular surface cracks. From these observations it was concluded that the flaking failures are not caused directly by surface cracks, but by subsurface cracks that branch from them.     

New testing method for assessing the cracking sensibility of stressed tendon rods in aggressive environments

The paper presents a new testing method for assessing the cracking sensibility in aggressive environments of tendon rods for prestressed concrete structures, on the basis of Fracture Mechanics concepts. First, it approaches the fundamentals of the designed test in the context of existing fracture specimens, regarding the geometrical limitations introduced by the environmental assisted cracking of the rods, perpendicularly to the load direction. The analysis showed that fatigue precracked chevron-notched short bar specimen (SBS) is providing the largest measurement range of stress intensity factor to be explored in stress-corrosion cracking (SCC) tests. Then, the equation relating the elastic stiffness of SBSs to crack size was experimentally validated for specimens with true cracks produced by fatigue. SCC verification tests were made with an innovative horizontal loading device and the crack mouth opening displacement (CMOD) was acquired and numerically analyzed with a video digital image correlation system. The tests showed that SCC is fully governed by the small scale-yielding regime at the crack tip. Hence, the main process parameters as crack extension and environment-assisted stress intensity factor were empirically obtained for each time sequence of the SCC tests. To simplify further SCC tests instrumentation, an empirical correlation was stated between CMOD and crack opening displacement (COD) as measured by a conventional extensometer mounted on the loading grips of the specimen. This correlation was also validated in the SCC tests.     

Stress intensity factors for fatigue cracking of round bars

The stress intensity factor for a single edge crack of either straight or circular front in a round bar has been determined using both the degenerated quarter-point isoparametric finite element and experimental fatigue crack growth data, and compared with values found by earlier investigators.The results of this study confirm that the stress intensity factors for straight edged surface cracks are lower in round bars than in square bars and a comparison of finite element and experimental results indicates that the effective stress intensity factor at the centre of the fatigue crack front in a round bar is 17% greater than its theoretical value.A correction function is proposed to account for the effect on the stress intensity factor of the circular boundary of a round bar.     

Fractographic and Metallographic Study of Spalling Failure of Steel Straightener Rolls

Spalled fragments from work rolls of a steel bar straightening machine were received for failure analysis. Visual inspection coupled with optical and scanning electron microscopy (SEM) were used as the principal analytical techniques for the investigation. Fractographic observations clearly revealed the presence of a characteristic fatigue crack propagation pattern (beach marks) and radial chevron marks indicating the occurrence of final overload through a brittle intergranular fracture mode. The collected evidence suggests strongly that surface-initiated cracks propagated by fatigue mechanism led to spalling, resulting therefore in severe work roll damage and subsequently high machine downtime and maintenance costs.     

Comparison of numerical behaviors of FRP reinforced concrete beams using three smeared crack models

The numerical behaviors of fiber reinforced polymer (FRP) bar reinforced concrete beams using three non-linear finite element models are compared with the recorded data. First approach is based on strain decomposition into elastic and crack strain and is capable of simulating multiple non-orthogonal cracks. The remaining two approaches are based on the total strain crack model and include a rotating crack model (RCM) and an orthogonal fixed crack model (FCM). The analysis is carried out with the help of 2D-isoparametric plane-stress elements. Compression softening and tension stiffening effects of cracked concrete are considered. Tension reinforcement consists of either steel or FRP bars. The accuracy of the models has been discussed with reference to the authors?? tests as well as various studies reported in the literature. Both RCM and orthogonal FCM models showed good agreement with the recorded data which was also found consistent with every type of FRP bar.