C楞瓦楞纸板动态缓冲模型及应用

利用万能试验机和跌落试验台分别得到C楞瓦楞纸板静态、动态应力-应变数据,建立相应的理论模型,并用最小二乘法识别模型中的参数。给出了瓦楞纸板在缓冲动力学中应用的例子,结果表明所建立的缓冲模型,可以直接用于缓冲包装设计,克服了用最大加速度-静应力曲线来设计缓冲包装需要大量实验和较多的数据的弊端。     

E-GFRP单向板疲劳性能及失效机制的实验研究

对铺设角分别为0°、30°、45°、90°的玻璃纤维增强不饱和聚酯树脂单向板进行拉-拉疲劳试验, 得到不同铺设角玻璃纤维增强不饱和聚酯树脂(GFRP)单向板疲劳数据, 通过分析不同铺设角单向板中值S-N曲线及正则应力-寿命曲线特点, 得到了单向板疲劳性能随铺设角变化的关系。并且基于对不同铺设角单向板疲劳断口的宏观及微观形貌的分析, 研究了铺设角对单向板的疲劳损伤机制、失效模式的影响。分析表明, 对应于不同的铺设角, 单向板存在不同的失效机制, 从而导致不同形式的损伤模式。     

Bending stiffness loss of paperboard at conversion — predicting the bending ability of paperboard

The ability of paperboard to resist bending has been investigated. Paperboard is often bent in converting and packaging machines. The paperboard is bent over rolls and thus formed to certain curvatures. If the roll diameter is small the paperboard will be highly curved. This means that high tensile stresses occur on the convex side and high compression stresses on the concave side. If these stresses are too high the paperboard will be damaged by fractures and wrinkles on the surfaces. The bending stiffness of the board will also be reduced. The importance of certain parameters, such as roll diameter, angle of wrap, board thickness, board compression strength and others, have been investigated. In this study seven qualities of paperboard were investigated. It has been shown that the bending force–bending angle curve can be used to obtain information about the ability of different board qualities to be bent to certain curvatures without being damaged. © 1998 John Wiley & Sons, Ltd.     

A theoretical and experimental study on corrugated paperboard crushing under quasi‐static loadings

This paper presents 2 mathematical models to predict the initial peak stress and the plateau stress of corrugated paperboard which is simplified and regarded as the orthotropic plate under longitudinal compression. The resultant stress‐strain curve exhibits initial stiffening stage and a long plateau stage, where the initial peak stress determining the edgewise crush resistance of the board and most impact energy will be absorbed by the long plateau stage. By analyzing the elastic buckling of the board wall, a model of predicting the initial peak stress was obtained, while the plateau stress model was developed based on the gradual process of compression for corrugated board and energy conservation principle. Moreover, experiments were carried out to corroborate the presented model by comparing the predicted value with that by experiments, showing overall good agreement. It can be concluded that the proposed models can be applied for design of corrugated containers and cushioning packaging by corrugated paperboard.     

Effect of initial pre‐compression of corrugated paperboard cushions on shock attenuation characteristics in repetitive impacts

The use of paper as a cushioning material is motivated by its environmental advantage over polymeric materials such as expanded polystyrene. Various studies have been conducted to investigate the capability of corrugated paperboard (CPB) to act as cushioning materials for protective packaging. The studies have normally focused on performance of cushions and effects of climatic conditions on the shock attenuation in the first drop. However, no information is available to show that CPB can be engineered to produce a cushioning element with good shock attenuation in repetitive impacts. This study investigates the deterioration of the shock attenuation characteristics of CPB cushions in consecutive impacts. A series of cushion‐test style experiments on multilayer cushion pads made entirely from CPB were conducted. Each cushion was first subjected to 20 impacts, followed by simulated warehousing in a climatic chamber, and then the additional 15 impacts. The effect of initial pre‐compression strain introduced in the process of making cushion pads is discussed and demonstrated. The performance of cushions was judged not only by the maximum acceleration of shocks. Entire recorded shock pulses and calculated corresponding shock response spectra were analysed and are presented as 3D maps in order to reveal trends that occur when a cushion is subjected to multiple drops. The paper shows that the level of pre‐compression has an important effect on the degree of deterioration of cushion performance after multiple drops. As an example, the maximum acceleration for a pad pre‐compressed to 95% strain increased by less than 20% between the 1st and the 35th drop. In contrast, for 80% pre‐compression, the increase was 300%. It was found that the selection of optimum static stress should be made in conjunction with the expected number of impacts in order to optimize the cushion performance. Copyright © 2009 John Wiley & Sons, Ltd.     

An investigation on rotatory bending fretting fatigue damage of railway axles

Fretting damage failure analysis of a Chinese carbon railway axle RD₂ was carried out. The wheel hub was in situ cut to expose the damaged surface of the wheel seat to avoid additional damage. A small‐scale axle test rig was developed, and simulation tests were performed at different rotator speeds of 1800 and 2100 rpm. The wear mechanism of fretting damage areas was a combination of abrasive wear, oxidative wear and delamination. The fracture surfaces exhibited characterization of multisource and step‐profile. The fretting fatigue crack initiated at the subsurface and propagated along an inclined angle at the first stage. The fretting damage at the higher speed was more severe compared with the lower speed, which lead to a relatively shorter fatigue life. The damage morphologies of the axle in the simulation tests were in good agreement with that observed in the failure analysis on real axle.     

Stress‐life prediction of 25°C polypropylene materials based on calibration of Zhurkov fatigue life model

In this study, to evaluate the chemical and mechanical properties of polypropylene (PP), activation‐energy and tensile tests were performed at room temperature (25°C) on pure PP and PP reinforced with glass fibre (GF). To improve the prediction accuracy of the fatigue life, three models based on the calibration of the Zhurkov model were proposed: a regression model, modified strain‐rate model and lethargy coefficient‐based model. Based on the experimental data analysis and statistical assessment results, we proposed a modified strain‐rate model that satisfies the dependency of the physical parameters and is congruent with the predicted fatigue life data. The experimental data and modified strain‐rate model were compared with the direct cyclic analysis results. The tendency of the frequency factor as a correction parameter in the modified strain‐rate model corresponded to the experimental activation energy and the increasing GF content.     

Experimental investigation into the cushioning properties of honeycomb paperboard

The cushioning properties of honeycomb paperboards are studied by means of experimental analysis. Experimental results indicate that the cushioning behaviour of paper honeycombs with and without liners are similar, but the cushioning capability is significantly larger for the material with liners. Experiment and theory show that the relative density of paper honeycomb core has a significant effect on the cushioning properties of honeycomb paperboards; however, the basis weight of the liner has only little effect. Experimental results also show that the energy absorption properties of multilayer honeycomb paperboards are not always higher than those of monolayer honeycomb paperboards, and that the height of honeycomb paperboard has an effect on its cushioning properties. These results can be used to characterize and improve the cushioning properties of honeycomb paperboard pads efficiently. Copyright © 2008 John Wiley & Sons, Ltd.     

Experimental quantification of differences in damage due to in-plane tensile test and bending of paperboard

Creasing is an essential process to convert paperboards into packages since it enables folding along well-defined lines. The creasing process relies on purpose-made damage that is initiated in the paperboard structure: delamination. However, creasing might also cause in-plane cracks, which must be avoided. In this laboratory study, three paperboards were creased at six different depths, respectively. Two mechanical tests were performed to characterize the creases at standard climate (23°C and 50% RH): 2-point folding, to examine the bending force and short-span in-plane tensile test to evaluate the strength. The results were normalized with the values for the uncreased boards, which gave the relative strength ratios: relative creasing strength (RCS) and relative tensile strength (RTS). When the relative strengths were evaluated against the normative shear strains, a creasing window was formed. This window has an upper limit given by the RTS values, corresponding to the in-plane cracks, and a lower limit given by the RCS values, corresponding to the delamination damage initiated in the paperboard during creasing. It was observed that both the RCS and RTS values exhibit a linear relation against normative shear strain. From this, it was concluded that performing tests at two creasing depths might be sufficient to estimate the lower, and upper, limits for the creasing window in future studies. Finally, the effect of moisture was investigated by creasing, folding and tensile testing at 23°C and 90% RH, which showed that moisture had no clear effect on the RCS or the RTS values.     

Overload effect on the fatigue crack propagation in large‐scale tubular joints

This paper examines the overloading effect on the fatigue crack propagations monitored in a large‐scale tubular X‐joint specimen under two separate cyclic tests. The first cyclic test applies a constant‐amplitude brace in‐plane bending to the joint, with a single cycle of 150% overload before the crack depth reaches the mid‐thickness of the chord. The second fatigue test applies two batches of cyclic loads, with the amplitude of the second batch at 66% of the former. The X‐joint specimen experiences a 150% overload cycle during the first batch of loading, followed by the second batch after it has recovered from the overload effect. The experimental results reveal that deep surface cracks experience more significant overload retardation than does a shallow fatigue crack. The Paris law estimation indicates that the single overload cycle applied in the first specimen leads to a 7% increase in the fatigue life of the X‐joint.     

Experimental study on fatigue failure and damage of sandwich structure with PMI foam core

Sandwich structures consisting of aluminium skin sheets and polymethacrylimide foam core have been gradually used in the high‐speed trains. The static mechanical properties and fatigue damage of the sandwich structures with polymethacrylimide foam core were experimented in three‐point bending and were discussed. The failure mode is identified as local indentation. The static strength was obtained, and it showed good consistency with the forecasting formula. The fatigue property and damage evolution were also researched under cyclic loading. The fatigue life curve and the fitting formula were submitted. The fatigue damage evolution started from the skin sheet fracture and then the foam core indentation. The displacement at the midpoint as the damage parameter was discussed, and the evolution prediction formula was submitted, which showed great agreement with the experimental results.     

High‐cycle fatigue and fracture behaviours of Cu‐Be alloy with a wide strength range

The high‐cycle fatigue and fracture behaviours of Cu‐Be alloy with tensile strength ranging from 500 to 1300 MPa acquired by different treatments were studied. Fatigue crack initiation, fracture surface morphologies, S‐N curves and fatigue strength show obvious differences due to the change of microstructure. At relatively low‐strength level, some fatigue cracks originated from defects; while at high‐strength level, all the fatigue cracks initiated from cleavage facets. It was found that the fatigue ratio increases linearly and fatigue strength changes quadratically with increasing tensile strength, only considering one strengthening mechanism. Finally, the fatigue strengths of various Cu‐Be alloys were summarized.     

K型钢管混凝土节点疲劳性能试验研究

以湘潭四大桥桁架拱为工程背景,进行了三种规格的K型钢管混凝土节点的疲劳试验,基于热点应力计算了K型钢管混凝土节点的疲劳荷载幅值,表明热点应力法能够较好地评估钢管混凝土节点的疲劳寿命;试验表明K型钢管混凝土节点的疲劳性能明显优于K型钢管节点。在此基础上初步提出了K型钢管混凝土节点的S—N曲线,对建立K型钢管混凝土节点疲劳寿命设计准则具有一定的参考意义。     

Torsion fatigue failure of bus drive shafts

The mystery surrounding high failure rates in the drive shafts of a large municipal transit agency's fleet of 40 newly acquired articulated buses is investigated. The drive shafts were fabricated from a low-carbon (0.45%) steel such as AISI 5046. An examination of the drive shafts on all 40 buses is conducted, and 6 different drive shaft designs are identified among the fleet, but all of the failures, 14 in all, are limited to just one of the identified designs. Microscopic examination of the fracture surface of one of the failed drive shafts under a scanning electron microscope is conducted to determine the failure mode. Evidence of high-cycle fatigue is found, and a finite-element analysis is conducted to compare the maximum stress of the design exhibiting failures with the most common of the other designs that exhibits no failures. A fatigue life prediction is performed to determine just how much longer the expected fatigue life of the surviving design is compared to the design that suffered the early failures.     

疲劳振动效应对瓦楞纸板承载能力与缓冲性能的影响

瓦楞纸板作为产品运输包装的承载与缓冲基础材料,其使用过程中的性能保持性至关重要。运输振动将引起纸板疲劳破损,导致其相应性能的变化。对单瓦楞纸板进行加速度为0.5－2.0g、振动频率为20Hz的疲劳振动试验;进行瓦楞纸板的准静态压缩、跌落冲击试验,测定其承载能力与冲击加速度。结果表明,随着振动次数、振动强度的增加,纸板的承载能力、剩余屈服应力显著降低,缓冲性能下降。在此基础上,基于瓦楞纸板冲击试验数据,分析不同振动次数对瓦楞纸板的冲击最大加速度和缓冲系数的影响,获得了瓦楞纸板冲击最大加速度—静应力—振动次数、缓冲系数—最大应力—振动次数三维关系。研究表明瓦楞纸板存在疲劳破坏效应,研究结果为瓦楞纸板的疲劳缓冲包装设计提供技术依据     

Stepwise S-N curve and fish-eye failure in gigacycle fatigue

Fatigue failure is normally initiated at the surface of a material. For some materials, failure can be initiated both at the surface and the interior. This twofold materials behaviour in fatigue is represented by a stepwise shape in the S – N curve. An internal failure mode is especially important for fatigue life in the gigacycle range, as this mode is predominant at low stress ranges.
Materials with a hardened surface fail from the surface only at high stresses, and at low stresses from the inside, forming a fish-eye facet on the fracture surface. Exactly the same behaviour can be observed for materials without a hard surface, even at elevated temperatures. This paper displays some of the results obtained at NRIM and discusses possible interpretations.     

碳/碳复合材料疲劳损伤失效试验研究

对单向碳/碳复合材料纵向拉-拉疲劳特性及面内剪切拉-拉疲劳特性进行了试验研究; 对三维四向编织碳/碳复合材料的纵向拉-拉疲劳特性及纤维束-基体界面剩余强度进行了试验研究。使用最小二乘法拟合得到了单向碳/碳复合材料纵向及面内剪切拉-拉疲劳加载下的剩余刚度退化模型及剩余强度退化模型, 建立了纤维束-基体界面剩余强度模型。结果显示: 单向碳/碳复合材料在87.5%应力水平的疲劳载荷下刚度退化最大只有8.8%左右, 在70.0%应力水平的疲劳载荷下, 面内剪切刚度退化最大可达30%左右; 三维四向编织碳/碳复合材料疲劳加载后强度及刚度均得到了提高; 随着疲劳循环加载数的增加, 三维四向编织碳/碳复合材料中纤维束-基体界面强度逐渐减弱。      

Experimental study and modeling of fatigue life prediction of plain weave carbon/polymer composite under constant amplitude loading

For fiber-reinforced composites, the anisotropy and the tension-compression asymmetry have very important influence on fatigue performance. The quasi-static and fatigue mechanical behavior of plain weave laminates of carbon/polymer composites were experimentally investigated in this paper. The quasi-static and stress-controlled fatigue tests were carried out on the servo-hydraulic material testing machine. Fracture failure surfaces were observed in micro-mechanism with scanning electron microscopy. A phenomenological and nonlinear constant life diagram (CLD) model was developed based on quasi-static strength and S–N curve data-sets of different stress ratios. The relationship between six parameters of the proposed model and the failure cycles was studied. The fatigue experimental results showed that the fatigue failure type changed from tensile mode to compressive mode at nearby stress ratio R = ?0.2. A satisfactory agreement between the predicted value of cycle life and experimental data was observed. The results indicated that the fatigue performance was adequately described by the Basquin S–N formulation and proposed CLD.     

Evaluation of fatigue life of aluminium alloy wheels under bending loads

This paper presents the details of S–N curve for aluminium alloy (Al) A356.2‐T6 and fatigue life of alloy wheels under bending load of cornering fatigue test (CFT). Development of S–N curve has been carried out by conducting rotary bending fatigue test at different stress levels as per Standards IS 5075. The rotary bending fatigue test has been performed under constant amplitude fatigue loading. The CFT of the wheel in normal driving mode has been carried out as per the procedure given in Japanese Industrial Standard Disc Wheels (JIS D_4103). It has been observed from the test that the cracks are initiated at the spoke and hub joining area closer to spanner hole on the front face of the wheel. Fatigue life of the alloy wheel has been predicted by finite element analysis (FEA), simulating the realistic test conditions. From finite element analysis, it has been observed that the maximum stress occurs at the mounting face of the wheel. Further, it has been observed that there is significant difference between the computed fatigue life and experimental value. Parametric study has been carried out for reliable fatigue life estimation and proposed an appropriate safety factor for fatigue life estimation under rotary bending test.