Damping characteristics of nanoclay filled hybrid laminates during medium velocity impact

The objective of this paper is to study the vibrational damping characteristics during medium velocity impact of nanoclay filled glass fiber reinforced epoxy hybrid laminates. A series of laminates with varying degree of nanoclay concentration (0–5 wt.%) and fiber weight fraction (25–75 wt.%) were prepared by vacuum assisted resin infusion molding (VARIM) method. The laminates were subjected to medium velocity projectile impact using in-house built gas gun set-up and the ballistic limit of laminates series was determined. The result indicated that during impact, the laminate undergoes vibrational damping. This damping property is a function of fiber weight fraction and orientation, nanoclay concentration and nanocomposite structure. A 42% increase of ballistic limit was observed for 5 wt.% nanoclay filled hybrid (50 wt.% fiber) when compared with unfilled composite. Structural and modal analysis of hybrids showed that the increased ballistic limit of nanoclay filled hybrids is due to the nanocomposite structure and improved damping and fracture properties.     

An experimental study on the long-term behavior of CFRP pultruded laminates suitable to concrete structures rehabilitation

The rheological behavior of structural materials has a significant role indeed in Civil Engineering, where concrete and FRP (Fiber Reinforced Polymer) materials undergo creep in normal environmental conditions, while steel exhibits a sizable creep only at high temperature (above 400 °C). With reference to RC structures strengthened by means of FRP laminates, FRP creep generally coexists with concrete cracking. The interaction between these phenomena should be taken into account in order to evaluate the structural durability. Here, the first results of a research program on creep in composite pultruded laminates used in Civil Engineering are presented, under various stress levels and in constant environmental conditions (many theoretical and experimental studies on creep have been performed so far in the aerospace and naval fields, but not as many in Civil Engineering). The specimens tested in this project are made of high modulus carbon fiber reinforced polymer – CFRP, whose mechanical properties are tailored for Civil Engineering applications. The tests are still in progress in the Material and Structures Testing Laboratory of the Civil Engineering Department of the University of Salerno (Italy).     

An experimental study of the influence of inclusions on the fatigue properties of steel

Thickness-direction specimens have been produced from a number of steel forgings differing mainly with respect to inclusion characteristics. After heat treatment specimens are fatigued and fracture surfaces are examined by means of scanning-electron microscopy in order to determine the site of fatigue-crack nucleation. Experimental observations are evaluated and discussed in the light of fracture mechanics.     

A micromechanics model for the prediction of fatigue characteristics of off-axis unidirectional laminates

In this paper, the concentric cylinders model is used to predict the fatigue characteristics of off-axis unidirectional laminates. A failure criterion that uses stresses averaged along the radial distance in each constituent phase is applied to predict the S-N curves of unidirectional laminates. The predicted S-N curves compare well with the experimental data for various off-axis Glass/Epoxy laminates.     

An experimental study on residual stress relaxation in low-cycle fatigue of Inconel 718Plus

Residual stress relaxation induced by the application of mechanical loads is determined by the nature of residual stress, the elasto-plastic material properties, and the type of applied load. Despite the importance of the first load cycle, analytical models available in the literature generally assumed residual stress relaxation as a continuous process. Residual stress induced by machining on Inconel 718Plus superalloy cylindrical specimens was measured before and after the application of load cycles under strain control. Low-cycle fatigue tests were carried out at room temperature for different strain amplitudes, and X-ray diffraction measurements were performed before and after 10 and 100 cycles. A comprehensive analytical model was derived to describe the relaxation process associated with the initial cycles and that associated with the continuous application of load cycle, which is based on the plastic strain energy per cycle W and requires the evaluation of parameters that are only dependent on the material and not on the strain amplitude.     

基于磁流变弹性体的缓冲装置设计及其冲击响应特性研究

利用聚氨酯基磁流变弹性体,结合ANSYS有限元仿真,设计制作了一种剪切式磁流变弹性体缓冲装置。搭建自由跌落冲击试验系统和冲击响应谱试验系统分别测试了该缓冲装置的冲击响应特性,研究了在不同磁场下缓冲装置的缓冲性能。实验结果表明在自由跌落冲击和冲击响应谱两种试验中,所设计的磁流变弹性体缓冲装置都具有较高的缓冲率,且装置的缓冲率随着磁场的增加而增大,表明磁流变弹性体缓冲装置起到了一定的缓冲作用,且可以实现外加电流对其缓冲效果的控制,该研究为磁流变弹性体在半主动/主动隔振缓冲领域的应用做出了有益探索。     

氨吸附制冷特性的实验研究

对以ＮＨ３为制冷剂,ＭｇＣｌ２、ＣａＣｌ２、ＳｒＣｌ２、ＢａＣｌ２和活性炭为吸附剂所组成的吸附式制冷工质对的基础吸附性能进行了实验研究,得到了吸附等温线。实验结果表明,ＣａＣｌ２－ＮＨ３、ＳｒＣｌ２－ＮＨ３、工质对性能较佳,具有商业开发价值。本还研究了活性炭浸渍ＳｒＣｌ２及加入导热促进剂石墨的混合吸附剂对ＮＨ３的吸附性能。     

An experimental study of fatigue crack initiation and growth from coldworked holes

The behavior of specimens with holes that were coldworked by radial expansion was studied. The initiation and growth of fatigue cracks was observed for a remote cyclic loading that generated stresses at the hole edge slightly above the yield stress. Coldworking affected the growth rate, not the initiation. Strains near the hole were measured with the moiré technique and used to predict the initiation of cracks in the noncoldworked specimens with reasonable accuracy. Stress intensity factors computed with a linear elastic superposition procedure compared favorably with those measured by crack growth and displacement techniques for cracks longer than 1 mm.     

金属橡胶的刚度特性和阻尼试验研究

基于金属橡胶内部微元螺旋卷结构,并以弹簧理论建立其力学模型,分析了在螺旋卷之间不同接触("未接触、滑动、压缩")形式下的刚度公式并解释载荷作用下刚度曲线不同阶段的特性。基于金属橡胶的非线性对阻尼进行计算,通过试验研究金属橡胶构件的密度、厚度对静态刚度曲线不同阶段的影响,及在不同振幅、频率下比阻尼随密度和厚度的变化规律,为金属橡胶的设计及工程应用有重要的指导意义。     

An experimental evaluation of fatigue crack growth

This paper examines the fatigue crack growth histories of a range of test specimens and service loaded components and concludes that in all cases, the crack growth shows, as a first approximation a linear relationship between the log of the crack length or depth and the service history (number of cycles). These cracks have grown from; semi- and quarter elliptical surface cuts, holes, pits and inherent material discontinuities in test specimens, fuselage lap joints, welded butt joints, and complex tubular jointed specimens and include cracks grown under uniaxial and biaxial loading. The implications of this exponential growth are discussed.     

High temperature hybrid titanium composite laminates: An early analytical assessment

This paper presents a new type of material system that shows great promise for aerospace applications. The concept consists of thin sheets of titanium bonded together with a polymer composite prepreg consisting of a high temperature resin reinforced with high or intermediate modulus fibers. The potential material performance is outstanding in terms of in-plane fatigue and fracture. As with all composite materials, the exact moduli and strength can be optimized for a given application by varying the constituents, their volume fractions and, in the case of the reinforcing fibers, their orientation. The paper includes analytical studies as well as experimental results.     

用应变能密度法分析纤维金属层板的微动疲劳特性

为研究纤维金属层板的微动疲劳特性,首先,基于三维坐标系下的临界平面法求解了纤维金属层板铝层临界平面上的应力和应变分量,并进一步求解了Smith-Watson-Topper （SWT）和I型Nita-Ogatta-Kuwabara （NOK）应变能密度参数;然后,建立了应变能密度参数-微动疲劳寿命关系式,并通过实验数据得到了寿命预测公式中的待定参数;最后,采用I型NOK应变能密度准则分析了铝层厚度、纤维层厚度、各层相对厚度和桥足圆角半径等对微动疲劳损伤位置和寿命的影响,并为纤维金属层板抗微动疲劳设计提出了一些合理化建议。结果表明:增加铝层厚度可以延长微动疲劳寿命,但增加纤维层厚度和桥足圆角半径不会改善微动疲劳特性。提出的方法可为分析纤维金属层板铆接和螺栓连接中的微动疲劳问题提供理论依据。      

Compression fatigue behaviour of notched composite laminates

The influence of compression fatigue on graphite/epoxy laminates containing circular holes of 0.635 cm diameter was investigated. The laminate stacking sequences were . Specifically this study examined the nature and extent of induced compression fatigue damage and determined the effects of this damage on the laminate residual failure mechanisms. Two modes of compression failure were found to occur: diagonal shear and net compression. Both failure modes were characterized by local instability of individual lamina or small lamina subgroupings, with diagonal shear predominant in the fibre-dominated laminates and net compression predominant in the quasi-isotropic laminates. The mode and direction of failure were dependent upon the nature of the specimen delamination. It was also found that the laminate stacking sequence influenced the intraply crack development in the laminates as well as the failure mode. The failure mechanisms were essentially the same for the two different material systems which were studied (Narmco 5208 and 5209).     

An experimental and analytical study of fatigue crack shape control by cold working

This paper presents an experimental and analytical study of crack shape evolution in steel specimens under cyclic loading. It is widely known that the introduction of compressive residual stresses by cold working the surface can be highly beneficial in improving the fatigue performance of structural components. Although it is recognised that relaxation of surface compressive residual stress can reduce the potential benefits, the effects of residual stress on crack shape evolution are often overlooked. A recently developed technique termed controlled stitch cold working, which applies differing intensities of compressive residual stress at specific regions in a structure, is shown in the paper to considerably influence fatigue crack propagation by containing crack propagation in one primary direction.     

Flexural fatigue of carbon fibre-reinforced PEEK laminates

Stress/number of cycles to failure relations were established for unidirectional, cross-plied and angle-plied laminates of APC-2 using three-point bend flexure tests. Results for constant stress ratio fatigue tests were modelled using both Weibull and fatigue modulus degradation analyses. Behaviour was compared with that of an established carbon/epoxy system.     

Impact properties of sisal-glass hybrid laminates

Work of fracture (WOF) of unidirectionally aligned sisal-glass hybrid laminates, especially glass core — sisal shell (SGS) laminates, have been studied using flat charpy test. Keeping the volume fraction of sisal at about 0.4, the WOF of SGS laminates linearly increased from 80.2 to 228 kJ m^–2 by varying the volume fraction of glass at the core (V _{g core}) from 0 to 0.2. However, further enhancement in WOF has been observed when the glass core is shifted from the midline towards the tensile side of the laminate with respect to impact. The specific work of fracture (165 kJ m^–2) of SGS laminate atV _{g core} 0.2 is identical to that of glass-reinforced polyester composites containing 0.6 volume fraction of glass. Addition of glass facings to SGS laminates increased its WOF by 20 to 30 kJ m^–2 when the volume fraction of glass in the facings (V _{g facings}) was about 0.04. The increase in resistance to compressive yielding that occurs by placing glass facings above a critical size (V _{g facings} 0.04 in the present case) adversely affected the WOF of SGS hybrid laminates.     

An experimental study on the effects of matrix cracking to the stiffness of glass/epoxy cross plied laminates

Three independent measurement techniques are applied to characterize glass fiber laminates. The effects of distributed fatigue damage to the stiffness related behavior of cross plied laminates are quantified. Tensile and flexural stiffness reduction is obtained from quasi-static testing. Vibration testing shows the degradation of flexural and in-plane shear stiffnesses. The reduction of the phase velocity of symmetric S₀ mode is observed from the experimental dispersion curves of Lamb waves. However, the mutual agreement of these results is less satisfactory than was earlier seen for virgin laminates. The phenomena causing the discrepancies are proposed and discussed.     

Analysis of fatigue damage in composite laminates

The degradation of laminated composite materials under service loading occurs by means of a complex combination of damage modes and damage mechanisms. These damage events combine to produce a damage state which controls the state of stress and the state of strength of the degraded laminate. Although data have been generated dealing with various details of this process, an understanding or philosophy of the precise nature of damage development has not, heretofore, been developed. This paper makes such an attempt for the general case of damage development in laminated plates under cyclic (fatigue) loading. The philosophy is based on the discovery of a ‘characteristic damage state’ in such plates which forms independently of load history, and is determined only by the properties of the laminae, their orientations, and their stacking sequence. The detailed nature of this characteristic damage state, the nature of its formation, and its influence on strength, life and stiffness is discussed. The general need for further work in this, and related fields is also assessed.     

An experimental investigation of carbon-fiber/aluminium laminates with double-edge cracks

The mechanical behavior in the damage zones around crack-tips in carbon-fiber/aluminium hybrid composite laminates (CALL) have been studied by moiré interferometry. The strain distributions in the zones around the double-edge crack-tips in longitudinal and transverse specimens were obtained under tensile loading. The morphology of the damaged sections was observed by the use of scanning electron microscopy and the damage characteristics were analyzed for these specimens.