Effect of fiber orientation in uni-directional glass epoxy laminate using acoustic emission monitoring

Acoustic emission(AE) can be used for in situ structural health monitoring of the composite laminates. One of the main issues of AE is to characterize different dam-age mechanisms from the detected AE signals. In the present work,pure resin and GFRP composites laminates with different stacking sequences such as 0,90,angle ply[±45 ],cross-ply [0 /90 ] are used to trigger different failure mechanisms when subjected to tensile test with AE monitoring. The study of failure mechanisms is facilitated by the choic...     

Synthesis of tetrapod-shaped ZnO whiskers and microrods in one crucible by thermal evaporation of Zn/C mixtures

Tetrapod-shaped ZnO whiskers and microrods were synthesized in one crucible by thermal evaporation of Zn/C mixtures at 930 ℃ in air without any catalyst. The digital camera, optical microscopy, scanning electron microscopy, energy dispersive X-ray spectroscopy, and X-ray diffraction techniques were used to study the morphologies and crystal structures of these tetrapod-shaped ZnO microcrystals. The results show that these two types of ZnO tetrapods are grown at different heights within the same crucible. The legs of these tetrapod-shaped ZnO crystals are hexagonally faceted. Some tetrapod-shaped ZnO whiskers show hierarchical structures. A short button-like hexagonal ZnO microcrystal is observed at the triple junctions of some tetrapod-shaped ZnO whiskers. The tetrapod-shaped ZnO microrods are capped by two sets of hexagonal pyramids with two different groups of crystal planes for the surfaces. These two types of tetrapod-shaped ZnO microcrystals have different side faces and aspect ratio, which are believed to be the result of their different growth behaviors. The octa-twin model was used to discuss the different growth behaviors of these two types of ZnO tetrapods. The crystal planes of the legs and the pyramids were determined.     

Microstructure and fatigue crack growth behaviour of electron beam welding in 30CrMnSiNi2A steel

The effects of two post-weld heat treatment processes on the microstructure and fatigue properties of the electron beam welded joints of 30CrMnSiNi2A steel were studied. Electron beam local post-weld heat treatment ( EBLPWHT) , in a vacuum chamber, immediately after welding and a traditional furnace whole post-weld heat treatment (FWPWHT) were accepted. The experimental results show that, after EBLPWHT, the main microstructure of weld is changed from coarse acicular martensite into lath martensite, and base metal is changed from ferrite and perlite into upper bainite and residual austenite, however the microstructures of different zones of joints in FWPWHT conditions are tempered sorbite. The fatigue crack growth rate da/dN of welds and base metal are not obviously changed among EBLPWHT, FWPWHT test and as-welded (AW) test, as the mechanical properties of materials have a certain but not large effect on the da/dN of welded joints. The resistance to near threshold fatigue crack growth data of welded joints can be largely improved by EBLPWHT and it is related to microstructure and crack closure effect.     

EFFECTS OF SLIP MODE ON CYCLIC BEHAVIOR OF PRESTRAINED MATERIALS

The cyclic behaviours of two prestrained materials with different slip modes have been com-pared.From the analysis of hysterisis loops,it is suggested that the decrease in back stressdominates the cyclic process of Cu-Zn alloy with planar slip mode,while the decrease infrictional stress is the main factor affecting the behaviour of Cu with wavy slip mode.The re-lation between the dislocation configuration and internal stress has been also discussed.Bymeans of the relation just mentioned,the variations of dislocation configuration before andafter fatigue of the two materials,the differences in their softening mechanism and mechani-cal behaviour could be interpreted.     

FATIGUE LIVES FOR INDUCTION HARDENED SHAFTS MATERIALS ACCORDING TO THE ENVIRONMENTAL TEMPERATURES

Rotary bending fatigue tests were carried out with two kinds of materials, S43C and S50C, using the front engine and front drive shaft （FF shaft） of vehicle. The specimens were induction hardened about 1.0mm depth from the specimen surface, and the hardness value on the surface was about HRC56-60. The tested environment temperatures were -30, 25 and 80℃ in order to look over effect of the induction hardening and the environmental temperatures on the fatigue characteristics. The fatigue limit of induction hardened specimens increased more about 45% than non-hardened specimens showing that the endurances of S43C and S50C were 98.1 and 107.9MPa in non-hardened samples, 147.1 and 156.9MPa in hardened samplesrespectably. The maximum tensile and compressive stress on the small circular defect was about ＋250 and -450MPa respectively when circular defect is situated on top and bottom. The fatigue life increased 80, 25 and -30℃ in order regardless of hardening. In comparison of the fatigue lives on the basis of tested result at 25℃, the fatigue lives of non-hardened specimens decreased about 35%, but that of hardened specimens decreased about only 5% at 80℃ more than at 25℃. And fatigue life of non-hardened and hardened specimens were about 110% and 120% higher at -30℃ than that of 25℃. Based on the result of stress distribution near the defect, the tensile and compressive stress repeatedly generated by load direction were the largest on the small circular defect due to the stress concentration.     

FATIGUE SOFTENING BEHAVIOR AND ITS THRESHOLD OF COLD STRAINED POLYCRYSTALLINE Cu

The symmetrical push-pull fatigue tests in load control were carried out for prestrained polycrystalline Cu.The change in mechanical behaviour during cycling was recorded in detail and the effect of various degree of prestraining on saturated behavior and dislocation structure was compared.The results show that the cyclic saturation state is still history-dependent at low stress amplitude and the dislocation structure after fatigue exhibits an “inheritance” be- havior.The level of softening was calculated at various load amplitudes and a softening threshold of about 0.43 to 0.45 of the flow stress was obtained.This is the same as that of low carbon steel.Three stages of cyclic processes and their mechanism have been described.     

Quantitative microstructure and fatigue life of B319 casting alloys

In this study, the microstructure of B319 casting alloys and effects of five different casting conditions on microstructure were studied. Multi-sc ale microstructure was quantified in terms of secondary dendrite arm spacing(SDAS), and Si particle size and aspect ratio. The effects of SDAS, Si aspect ratio and size on fatigue life were analyzed. The results indicate that the size and aspect ratio of Si particles are a function of SDAS which is dependent on cooling rate during solidification. The fatigue life decreases with SDAS increasing as SDAS is smaller than 30 μm while it increases with SDAS increasing as SDAS is larger than 60 μm. In addition, the fatigue life decreases with Si aspect ratio and size increasing at the same SDAS. Moreover, SDAS and Si particles have also influence on fatigue fracture, such as the area of cracks propagation region and the roughness of fatigue fracture. The cracks propagation area is smaller, and the fatigue fracture is similar to tensile fracture with larger SDAS. Besides, the longitudinal section of fatigue fracture is rougher with large SDAS and elongated Si particles.     

Phase and its morphologies of Ti-45 % Al alloy directionally solidified at different growth rates

The microstructures of Ti-45%Al(molar fraction) alloy directionally solidified at different growth rates in alumina tube by electromagnetic heating zone melting were studied. The measured temperature gradient of the system is about 104K/m. The microstructures show that the primary solidified phase is β phase at different growth rates. The growth at low rates from 1.94 × 10-6 m/s to 4. 16 × 10-6 m/s results in a transient solid/liquid interface structure from planar to shallow cellular. This transient rate is larger than the theoretical value of vc = 6.94 × 10-7 m/s.Compared with vt = vc/k = 1.01 × 10-6 m/s, the cellular-dendritic transient rate of experiment is observed in the range of 1.67 × 10-5- 2.50 × 10-5 m/s. The primary arm spacing decreases with increasing growth rate.     

Effect of HIP Treatment on Fatigue Crack Growth Behavior of Ti–6Al–4V Alloy Fabricated by Electron Beam Melting

The effect of hot isostatic pressing treatment on the fatigue crack growth behavior of Ti–6Al–4V alloy fabricated by electron beam melting was investigated.The results indicate that the fatigue crack growth rate of the HIPed samples is higher than that of the as-fabricated one under certain stress intensity factor(DK\18 MPa m~(1/2)).With further increase in DK,the fatigue crack growth rates of the studied two samples become similar.The variation of a lamella thickness and the pore defects distribution have an effect on the fatigue crack growth rates in the studied samples,and the latter plays the dominant role.     

Effects of defect geometry on fatigue crack growth behavior of cross-ply Ti–6Al–4V/SCS-6 metal matrix composites

The present study describes the fatigue crack growth behavior of [0/90]_2s and [90/0]_2s laminates of Ti–6Al–4V/SCS-6 composite containing either part-through thickness or through thickness defects under a constant load range at room temperature in air. This study also compares the effect of composite architecture on the fatigue crack growth resistance of these laminates. Acoustic emission was used to detect the number and the location of in situ fiber failures. It was found that the specimens containing part-through thickness defects showed lower crack arrest/catastrophic failure (CA/CF) transitions compared to the specimens where cracks were grown from through thickness defects. Additionally, the higher CA/CF transition of [90/0]_2s laminate compared to [0/90]_2s laminate irrespective of defect geometry could be attributed to the composite architecture or lay up sequences in these laminates.     

含低速冲击损伤复合材料层合板的压缩失效

通过含低速冲击损伤的两种平面编织复合材料层合板（G803／5224、G827／5224层合板）的压缩试验,研究了低速冲击损伤对复合材料层合板失效行为的影响。试验后采用超声c扫描检测、外观检查与断口侧面宏观观察方法对损伤与失效特征进行了对比分析。结果表明,两种含低速冲击损伤层合板被压缩时,G803／5224层合板冲击背面首先发生子层微屈曲,微屈曲沿着垂直于压缩方向扩展,最后剪切分层失效;G827／5224层合板冲击背面首先发生子层屈曲分层,屈曲分层也是沿着垂直于压缩方向扩展,最后剪切屈曲失效。两种层合板低速冲击后压缩的失效模式与光滑板压缩失效模式基本相同。     

金属/金属层合板疲劳性能研究

以１４２０Ａｌ－Ｌｉ合金。ＬＹ１２爆炸层合板和ＬＦ２１热轧层合板为研究对象。考究了不同结合强度界面对Ｌ－Ｔ取向疲劳裂纹扩展的影响。结果表明：随界面结合强度的减弱分层越来越剧烈。其止裂韧化效果越来越明显。三种实验层合板材的疲劳裂纹扩展都出现了止裂现象。但其机制有所不同。Ａｌ－Ｌｉ合金的止裂主要来自事机制。爆炸层合板和热忆合层板的止裂主要来自分层机制。     

Ultrasonic detection and sizing of compressed cracks in glass- and carbon-fibre reinforced plastic composites

Nondestructive evaluation of compressed cracks is a major challenge. A quantitative study of the effect of crack-tip closure on the pulse-echo ultrasonic sizing of delaminations in fibre-reinforced polymer-matrix composites (FRP) is presented. In particular, this study focuses on the interaction of ultrasound with a closed crack or kissing disbond, and their effect on the ultrasonic inspectability of FRP laminates consisting of carbon and glass plies. The compression of laminar cracks in these two different laminate types is clearly detectable via both pulse-echo and through-transmission ultrasonic measurements, but the reflected ultrasonic pulses in the two material types exhibit markedly different behaviour. The glass-fibre laminates show a drop in the reflected signal for crack openings up to approximately half the crack growth load, whereas the corresponding carbon-fibre laminates show the expected increase in the reflected signal as the crack opens. The origins of the observed effect of crack closure on the reflection and transmission of ultrasound are analysed in detail to ascertain possible mechanisms responsible for these effects.     

The mechanical behavior of GLARE laminates for aircraft structures

GLARE (glass-reinforced aluminum laminate) is a new class of fiber metal laminates for advanced aerospace structural applications. It consists of thin aluminum sheets bonded together with unidirectional or biaxially reinforced adhesive prepreg of high-strength glass fibers. GLARE laminates offer a unique combination of properties such as outstanding fatigue resistance, high specific static properties, excellent impact resistance, good residual and blunt notch strength, flame resistance and corrosion properties, and ease of manufacture and repair. GLARE laminates can be tailored to suit a wide variety of applications by varying the fiber/resin system, the alloy type and thickness, stacking sequence, fiber orientation, surface pretreatment technique, etc. This article presents a comprehensive overview of the mechanical properties of various GLARE laminates under different loading conditions. For more information, contact J.-M. Yang, University of California, Department of Materials Science and Engineering, 6532 Boelter Hall, Los Angles, CA 90024; (310) 825-2758; fax (310) 206-7353; e-mail jyang@seas.ucla.edu.     

Toughness Enhancement in Roll-Bonded Al6061-15 vol.% SiC Laminates via Controlled Interfacial Delamination

Researchers have examined different approaches to improve damage tolerance of discontinuously reinforced aluminum (DRA). In this study, three-layer DRA laminates containing two exterior layers of Al6061-15 vol.% SiCp and an interlayer of Al1050 were fabricated by hot roll bonding. Interfacial adhesion between the layers was controlled by means of rolling stain. The results of shear test revealed that, the bonding strength of laminates was influenced by number of rolling passes. Considering this effect, the role of interfacial bonding on the toughness of laminates was studied under three-point bending in the crack divider orientation. The quasi-static toughness of the laminates was greater than that of the monolithic DRA. Plastic deformation of the ductile interlayer and interfacial delamination were found as the major sources of energy absorption in this fracture process. It was shown that interfacial adhesion in these laminate does not alter the initiation energy in quasi-static test. Propagation energy under same loading condition, however, illustrated significant sensitivity to the interfacial bonding. The results of the current study reveal that improving the interfacial adhesion by means of rolling strain eliminates the ease of plastic deformation of the ductile interlayer and thus reduces the contribution of this mechanism in quasi-static toughness of the laminate.     

基于单向拉伸的半固化GLARE层板成形性能分析

通过半固化工艺制备GLARE层板以提高其成形性能,成形后再固化以达到使用要求.为了了解半固化工艺制备的GLARE层板的成形性能,利用单向拉伸试验方法,分别对不同铺层厚度的GLARE2和GLARE3层板进行力学性能测试.试验结果表明,半固化层板的拉伸曲线依然符合传统层板的双线性特征.半固化工艺制备的GLARE2层板的成形...     

电液伺服双轴疲劳试验力控制系统及其幅值相位控制算法的研究

由于汽车排气管在实际运行环境中受到复杂交变载荷,单轴的疲劳试验不能准确预测试件的疲劳寿命。双轴的协同加载能更好地模拟实际工况,但需要对两个轴力传感器输出信号的幅值和相位差进行精确控制。因此,针对排气管双轴动态加载的需求,研发一种力控制的电液伺服式双轴疲劳试验加载装置,并设计基于PXI总线结构和多DSP并行处理技术的多轴伺服控制器,基于伺服闭环和外环驱动谱修正相结合的控制算法,实现了双通道正弦波的幅值相位控制。为验证算法的性能,两通道参考信号均采用频率为10 Hz,幅值为2. 363 k N的正弦波,并且相位差为90°,对排气管进行双轴疲劳试验。试验结果表明:该双轴力加载系统能精确地跟踪参考信号的幅值和相位差。     

IR thermography study of the effect of moulding parameters on impact resistance in E-glass/polypropylene commingled laminates

Infrared thermography has often been used to characterise impact damage on polymeric composite materials. In this study, the impact properties of commingled E-glass polypropylene composites (Twintex®) moulded using different tool temperatures (60, 80 and 100 °C) and different preheating time are compared. With this aim the material, a balanced weave laminate (1:1 Twintex) with 60% wt glass content, compression moulded at different tool temperatures and using different preheating time, is impacted with impact energies ranging from 15 to 55 J. After that, the impact-damaged areas on the laminates are observed and characterised using a long pulse active thermography approach. The positive effect of an increasing tool temperature and of an increased preheating time on impact damage tolerance is clear from the analysis of thermographic images.     

Tensile,Creep, and Fatigue Behaviors of 3D-Printed Acrylonitrile Butadiene Styrene

Acrylonitrile butadiene styrene (ABS) is a widely used thermoplastics in 3D printing. However, there is a lack of thorough investigation of the mechanical properties of 3D-printed ABS components, including orientation-dependent tensile strength and creep fatigue properties. In this work, a systematic characterization is conducted on the mechanical properties of 3D-printed ABS components. Specifically, the effect of printing orientation on the tensile and creep properties is investigated. The results show that, in tensile tests, the 0° printing orientation has the highest Young’s modulus of 1.81 GPa, and ultimate strength of 224 MPa. In the creep test, the 90° printing orientation has the lowest k value of 0.2 in the plastics creep model, suggesting 90° is the most creep resistant direction. In the fatigue test, the average cycle number under load of 30 N is 3796 cycles. The average cycle number decreases to 128 cycles when the load is 60 N. Using the Paris law, with an estimated crack size of 0.75 mm, and stress intensity factor is varied from 352 to 700 \(N\sqrt m\), the derived fatigue crack growth rate is 0.0341 mm/cycle. This study provides important mechanical property data that is useful for applying 3D-printed ABS in engineering applications.