图像处理技术在板料激光弯曲中的应用

为了提高激光弯曲过程数值模拟结果的验证精度,用图像处理技术对铝合金板料AA6056进行了激光弯曲变形过程的实时测量.自行设计了硬件测量系统和软件的测试系统,使用MV21300UM CCD拍摄板料上测量点在激光照射过程中的动态变化过程,通过软件测试系统中实时采集软件记录动态变化过程、图像处理软件进行图像处理,得到了测量点动态变化的实测曲线.实测曲线表明,板料厚度对激光弯曲过程的位移变化影响较大,板料越薄,边界效应越明显,使得位移的变化曲线越易出现突变.实验结果证明该实测方案是可靠有效的.     

Investigation of thickness effects on the bending singularities of a notched plate

In this paper, the bending singularities of a notched plate are investigated through the finite element method. The computed singular parameters, including the singularity orders and associated corner functions, are compared with those of classical plate theory and first‐order shear deformation plate theory (FSDPT); the comparisons demonstrate the inaccuracy of the boundary conditions in classical plate theory. The thickness effects are then investigated through the finite element method. The results show that although the singularity parameters computed by finite element agree with those of the FSDPT in the interior of the plate, the FSDPT is not applicable at the notch tip on the top or bottom surface because of substantial free surface effects. For a very thin plate, the inconsistence in the comparison slightly increases because of the free surface effects.     

Four-point bending fatigue behaviour of an iron-based laser sintered material

New material processing methods such as laser sintering of metal powder necessitates new knowledge and characterization of the material to support its implementation in technical applications. Fatigue behaviour of a laser sintered FeNiCu-alloy was studied with emphasis on crack path, initiation and propagation. Fatigue crack growth was investigated by surface replication in four-point bending fatigue tests. The fatigue behaviour was controlled by the complex layered structure. Pores on or under the surface were preferable places for crack initiation. Crack linkage and deflection occurred due to crack tip interaction with microstructure and sinter layers where microcracks initiated at pores adjacent to the advancing crack tip. Crack growth rate and stress intensity factor were calculated from surface replicas and showed an oscillating behaviour.     

Selection of process parameters in a single-pass laser bending process

In this article, a strategy for the optimum selection of laser process parameters, namely, scan speed, power and beam diameter, is presented that considers not only the accuracy of laser bending, but also the residual stresses, the maximum temperature along with temperature-affected zone, radius of bend, energy consumption and production rate. Performance is predicted by means of a three-dimensional thermoelastic–plastic finite element model, incorporating certain features to make it computationally efficient. In addition, an empirical model is used for the quick prediction of the maximum temperature, which helps in reducing the number of time-consuming runs of the finite element model. As the optimization of the process is multi-objective in nature, a fuzzy-set based strategy is suggested for a higher level decision. The efficacy of the procedure is demonstrated through examples and in-house experiments.     

Nomex蜂窝夹层结构弯曲刚度温度相关性的力学建模

胶黏剂粘接性能下降导致的面板与蜂窝芯分离是Nomex蜂窝夹层结构在高温下力学性能发生退化的主要原因。为此定义了胶黏剂的等效脱粘系数为温度升高引起蜂窝夹层结构弯曲刚度下降的损伤变量,并引入面板和蜂窝芯弹性模量的温度保持系数,建立了蜂窝夹层结构弯曲刚度温度相关性的力学模型。经外伸梁三点弯曲法试验校验,所建力学模型计算值与试验值的误差在15%内,可以较好地实现蜂窝夹层结构在高温下的弯曲刚度预报。研究成果可以用于软夹心蜂窝夹层结构在高温下弯曲刚度的估算。      

Failure mechanisms and bending strength of Fuchsia magellanica var. gracilis stems

In the course of biological evolution, plant stems have evolved mechanical properties and an internal structure that makes them resistant to various types of failure. The mechanisms involved during damage development and failure in bending are complex and incompletely understood. The work presented builds on a theoretical framework outlined by Ennos and van Casteren, who applied engineering mechanics theory to explain why different woody stems fail in different ways. Our work has extended this approach, applying it to a detailed analysis of one particular species: Fuchsia magellanica var. gracilis. When subjected to three-point bending, stems of this species exhibited one of two failure mechanisms: a plastic hinge or a greenstick fracture. We developed a predictive model using a computer simulation and a mathematical analysis using the theory of plastic bending. Required material properties were obtained from tests, the literature and imaging techniques. We found that greenstick fractures are more likely to occur in more lignified stems with a higher density. We discovered a new failure mode: an internal crack caused by tensile transverse stress. This work helps in understanding how plants have evolved their bending resistance and may assist in the creation of novel engineering structures inspired by these principles.     

铜钛双金属复合管绕弯成形回弹的尺寸效应研究

为了精准地预测铜钛双金属复合管绕弯成形的回弹情况,本文基于ABAQUS/Explicit平台建立了铜钛双金属复合管绕弯-回弹有限元模型,并通过实验验证了模型的可靠性。本文首次采用两个全新的无量纲尺寸参数,研究回弹的尺寸效应特性,即在弯曲角度θ增大的条件下,回弹角Δθ随中空程度系数λ和弯曲曲率w的变化规律,提出了一种适用于多种管材、多尺寸研究的数学模型。将有限元的模拟结果与数学模型的计算结果综合分析得到:1)λ不同的复合管,Δθ均随着θ的增大而近似呈线性增大;当θ＞45°时,回弹角增速VΔθ随λ的增大先减小后略有增大;λ=0.565的复合管产生的回弹角最小。2) w不同的复合管,Δθ均随θ的增大而线性增大;当θ＞45°时,回弹角增速VΔθ随w的增大呈递减趋势;当w≤0.60,VΔθ值稳定在0.038,故对于λ/w接近于1的复合管,在实际生产中可以使用相同的回弹补偿模型,以提高生产效率。     

Experimental analysis of behavior and damage of sandwich composite materials in three-point bending. Part 2. Fatigue test results and damage mechanisms

The analysis of stiffness degradation and the identification of damage mechanisms during and after fatigue tests of sandwich panels with PVC foam cores have been performed. The sandwich panels with cross-ply laminates skins made of glass fiber and epoxy resin were manufactured by vacuum moulding and subjected to three-point bending tests. Two PVC cores of similar type but with differing densities were investigated. The effect of core density and thickness on the damage behavior was highlighted. Using the cyclic life criterion, fatigue curves were plotted according to two models and compared with those of the literature. It has been demonstrated that the sandwich SD 2, with the higher core density, withstands a higher load and possesses greater rigidity in static tests, combined with an enhanced fatigue resistance, when compared to sandwich SD 1 which has a lower core density. Translated from Problemy Prochnosti, No. 3, pp. 32–44, May–June, 2009.     

Formation mechanism and three-point bending behaviour of directly fabricated aluminium foam plates

New-style smooth-faced aluminium foam plates (AFPs) covered by dense skins have been fabricated directly. AFPs with porosity ranging from 79.3 to 85.2% and with an average diameter of 2.3 to 2.7?mm are obtained via the melt foaming method. The formation mechanism of external dense skins is studied. The dense skin is composed of a cell wall of non-ruptured bubbles, a coarse region formed from ruptured bubbles and plateau borders. The fracture mechanism of AFPs is studied based on static three-point bending tests. They show that the cracked region is only observed at the centre of the experimental samples and that brittle fracture is the main fracture mechanism of these AFPs.     

Device for determining the ultimate strength in bending for refractories at elevated temperature

A device is developed and prepared for determining the ultimate strength in bending of refractories at elevated temperature according to GOST R 50523-93 and ISO 5013-85, that in functional possibilities, metrological characteristics, accuracy of determining physicomechanical and thermomechanical properties of test objects and other parameters surpasses existing Russian analogs. Translated from Izmeritel’naya Tekhnika, No. 2, pp. 34–37, February, 2009.