Fabrication and residual stresses characterization of novel non-oxide multilayer ceramics

A multilayer composite was produced by the tape-casting technique in the AlN–SiC–MoSi₂ system, coupling insulating and electrically conductive layers. The composite was highly dense and delamination-free. The values of residual stresses due to the coupling of layers of different composition were assessed using the indentation technique and Raman piezo-spectroscopy. The experimental techniques agreed in respect to the sign and magnitude of the residual stresses, revealing compression in the insulating layers and tension in the conductive layers. The experimental results were lower than the expected values calculated by lamination theory. The choice of the parameter values needed for the theoretical calculations appears to be critical from this point of view.     

Effects of interlaminar stresses on damping of 0-degree unidirectional laminated composites

The principal aim of this paper is to formulate a general model for predicting damping in composites on the basis of the concept of strain energy-weighted dissipation. In this model, the effects of interlaminar stresses on damping have been included in addition to the effects of in-plane extension/compression and in-plane shear. Validation of the model was confirmed by performing damping measurements on 0° unidirectional composite beams with varying length and thickness. The results of theoretical predictions of damping in laminated composites were found to compare favorably with experimental data. The transverse shear (σ_xz) reveals to have a considerable effect on the damping mechanisms in 0° unidirectional polymer composites. However, the other interlaminar stresses (σ_yz, σ_z) were shown to have little influence on damping in composite beam.     

Thermoelastic analysis of residual stresses in unidirectional,high-performance composites

We present a thermoelastic analysis of the composite cylinder model for a undirectional composite including anisotropic fibers and an interphase region. We have found the magnitude of the residual thermal stresses on the micromechanics level induced by differential shrinkage between the anisotropic fibers and the matrix. For typical composites the largest residual stress is tension along the fiber direction, and a simple lower bound expression for this stress is given. Prediction of the magnitude of the thermal stresses requires knowledge of the thermal and physical properties of the matrix. The relevant properties for epoxy and thermoplastic matrices are discussed. The magnitude of the residual stresses can be reduced by tailoring the interphase region, but only if the interphase region serves to reduce the temperature for the onset of stress buildup. The volume fraction dependence of the longitudinal and transverse thermal expansion coefficients of the composite is compared to analogous expressions in the literature which do not include anisotropy of the fibers.     

Effects of residual stresses on the tensile fatigue behavior of concrete

The influence of Eigenstresses due to drying shrinkage on the development of residual deformations characterizing the tensile fatigue behavior of concrete is analyzed. During the loading phase the Eigenstresses are locally released around the cracks inducing a mismatch between the crack surfaces which inhibits a perfect crack re-closure. The analysis is performed by means of a 2D mesoscale implicit finite-element model. The shrinkage strain is first applied determining the development of a diffused micro-damage and then quasi-static loading–unloading tests are simulated. Different microstructures and different values of shrinkage strain are considered. The results show that the presence of residual stresses increases the amount of total dissipated energy and naturally leads to the development of residual deformations. However, the obtained values are only a portion of the residual deformations experimentally measured. The possible concomitant effect of another mechanism, namely the formation of debris at a small scale, is therefore discussed.     

The effect of residual stresses on the user properties of glass tubes

Glass and Ceramics -     

Mode I fracture resistance characteristics of graphite/epoxy laminated composites

Mode I crack resistance behavior of fiber‐reinforced (graphite/epoxy) composites laminated unidirectionally and anti‐symmetrically was studied. Double cantilever beam (DCB) specimens of stacking sequences, [O₁₂//O₁₂] and [(O/90)_3s//(90/O)_3s] were used where // represents the initial crack location. Resistance curves (R‐curves) were constructed for three initial crack lengths in order to determine the effects of initial crack length on the resistance behavior. The resistance force, G_R, for a crack increment was determined from the compliance calibration method. The results show that for the case of [(O/90)_3s//(90/O)_3s], the initial crack deviated from the midplane and propagated in a zigzag fashion within 13^th(90‐deg), while the crack propagated along the midplane for a [O₁₂//O₁₂] case. The results also show that for both cases, G_R was affected by the initial crack length before G_R was stabilized. However, G_R was not affected by initial crack length when G_R was stabilized for each case.     

Simulation of substrate clamping on the properties of piezoelectric/piezomagnetic laminated composites

A theoretical model was established to calculate the magnetoelectric voltage coefficient of piezoelectric (PMN-PT)/piezomagnetic (CFO) laminated composites on Si substrate. Using this model, the transverse and longitudinal magnetoelectric voltage coefficient formula of laminated composites was derived in ideal interface coupling state simply. The magnetoelectric voltage coefficient (α_E) is calculated as function of the volume fraction of piezoelectrics and the thickness of Si substrate. It was found that the magnetoelectric voltage coefficient of PMN-PT/CFO laminated composites decreases with increase of Si substrate thickness, which shows strong clamping effect of the substrate. Meanwhile, with increase of substrate thickness, the compressive stress of substrate to PMN-PT enhances, which change the lattice constant of PMN-PT and lead to low optimum PMN-PT volume fraction v.     

非氧化物复合耐火材料的热力学性能

为了在耐火材料中能动地应用非氧化物,系统了解非氧化物的热力学性质是非常必要的。为此,首分析了非氧化物在氧化气氛下的不稳定性及顺序,并就如何转变这个不利的性质,以实现在氧化气氛下烧结非氧化物复合材料而提出了“逆反应烧结”工艺。在深入研究Si、Al系氮化物的氧化机理后发现,当氧分压低于“转换氧分压”时,可显著生成其气态的亚氧化物。亚氧化物可以在邻近表面层沉积,形成致密层。致密层的形成使材料具有“自阻碍氧化”的性能。Si、Al的加入可增加亚氧化物的含量,从而加厚、加宽致密层,使材料的抗氧化和抗侵蚀能力得到提高。文中详述了Si3N4 -Al2O3、Si3N4 -MgO和Si3N4 -SiC三个体系的研究情况。结果表明:逆反应烧结工艺可制备出性能良好的复合物;Si、Al除可促进烧结外,还能提高致密层的密度和宽度。     

The influence of residual stresses and orientation on the properties of amorphous polymers

The effect of cold rolling on impact strength of poly(vinyl chloride) (PVC) acrylonitrile-butadiene-styrene (ABS), and an alloy of ABS and polycarbonate (Cycoloy) was studied. The results were presented in terms of Izod impact strength and present oil thickness reduction. The residual stresses molecule orientation and modification of mechanical properties due cold rolling were measured. It was shown that the mechanism of toughness enhancement in PVC, ABS, and Cycoloy different than the one observed in rolled polycarbonates. In the case of a highly localized failure initiation event (a single eraxe), which appears to be present in polycarbonate, the influence of residual stresses, is great, whereas in rubber modified polymers having multiple erasing over a large volume, orientation is more important than residual stress. Hot rolling of poly(methyl methaerylate) holds promise for a continuous process to achieve orientation and toughness enhancement.     

Measurement of thermal residual stresses in ZrB2-SiC composites

Neutron diffraction, Raman spectroscopy, and x-ray diffraction were employed to measure the stresses generated in the ZrB₂ matrix and SiC dispersed particulate phase in ZrB₂-30 vol% SiC composites produced by hot pressing at 1900 °C. Neutron diffraction measurements indicated that stresses begin to accumulate at ∼1400 °C during cooling from the processing temperature and increased to 880 MPa compressive in the SiC phase and 450 MPa tensile in the ZrB₂ phase at room temperature. Stresses measured via Raman spectroscopy revealed the stress in SiC particles on the surface of the composite was ∼390 MPa compressive, which is ∼40% of that measured in the bulk by neutron diffraction. Grazing incidence x-ray diffraction was performed to further characterize the stress state in SiC particles near the surface. Using this technique, an average compressive stress of 350 MPa was measured in the SiC phase, which is in good agreement with that measured by Raman spectroscopy.     

Thermal shock behaviour of laminated multilayer refractories for steel casting applications reinforced by residual stresses

Ceramic multilayer structures based on tape cast alumina and zirconia substrates have been manufactured for use as carbon-free refractory materials. The laminates were reinforced via residual stresses due to shrinkage mismatch or differences in thermal expansion in order to achieve an improved thermal cycling capability. Thermal shock tests have been carried out using a plasma test stand. The impact of layer sequence and residual stresses has been demonstrated via measurement of Young's modulus and microstructure images of the layered structures. Hasselman parameters as well as the crack propagation behaviour at interfaces have been analysed via wedge splitting test.     

Degradation of mechanical and electrical properties after long-term oxidation and corrosion of non-oxide structural ceramic composites

This work summarizes the results related to the influence of the starting composition and of microstructure on properties degradation, due to oxidation and corrosion, relatively to the following structural ceramics: Si₃N₄TiN, Si₃N₄MoSi₂, AlNSiCMoSi₂, AlNSiC.The effects of: (i) long-term oxidation in air (100 h), in the temperature range 600–1500 °C and (ii) of long-term corrosion (400 h) in acid or basic aqueous solution at RT, 40 and 70 °C, on the electrical resistivity and mechanical strength of the composites are analysed and compared. The degradation of the properties are related to the characteristics of the surface and sub-surface damage after oxidation and corrosion treatments.     

Multiferroic properties of (1-x)BCZT-xLCMO laminated composites

The electrical, ferromagnetic and magnetoelectric properties of the (1-x) Ba_0.85Ca_0.15Zr_0.10Ti_0.90O₃-xLa_0.67Ca_0.33MnO₃ [(1-x)BCZT-xLCMO] laminated composites were investigated. The ferroelectric and ferromagnetic characteristics demonstrated that the laminated composites are multiferroic in nature. Our results indicated that the performances of the laminated composites showed strong dependence on the thickness ratio between the ferroelectric BCZT layer and ferromagnetic LCMO layer. Enhanced dielectric properties were obtained for the laminated composites compared with that of the pure BCZT. The piezoelectric coefficient (d₃₃) and coercive electric fields (E_C) of the laminated composites declined with the increasing LCMO nonferroelectric layer, while the saturated magnetization (M_S) increased and the coercive magnetic field (M_C) kept constant. The 0.3BCZT-0.7LCMO composites obtained the largest magnetoelectric coefficient of α_E31 = 1.66 mV/cm·Oe.     

The determination of residual stresses in plastic pipe and their role in fracture

A method for the determination of residual stresses in plastic pipe by layer removal and subsequent slitting is described. Results are analyzed from deflections measured on a high density polyethylene pipe. The use of these data in the calculation of K_I around a semi-elliptical flaw at the pipe wall is then discussed.     

Preparation and anisotropic tribological properties of MoAlB/Al laminated composites

The unique microstructure of biomaterials provides inspiration in materials structure design and performance breakthroughs. Here we prepared a MoAlB reinforced Al matrix composites with laminated structure and strong interfacial bonding. This work mainly focuses on the relationship between tribological properties of the MoAlB/Al laminated composites and the MoAlB layer orientation. The results show that the tribological properties of the composites deeply depend on the relation of sliding direction VS. the MoAlB layer orientation, while the dependency gradually decreases with the MoAlB content increasing. The friction coefficient and wear rate of the composites can be decreased by 55.5% and 95.1%, lower than those of pure Al, when 15 vol% MoAlB reinforcement is added. The friction and wear mechanisms of the MoAlB/Al laminated composites are proposed, and the outstanding tribological properties are ascribed to the formation of tribochemical worn film and different stress states caused by the laminated structure.     

新型微纳叠层功能复合材料制备装置及其性能研究

介绍了新型微纳叠层共挤出技术,即通过对高分子材料熔体进行分层-扭转-合并来增加复合材料层数的技术。验证了该新型微纳叠层共挤出技术在高分子叠层复合材料加工上的可行性,在增加高分子叠层复合材料的层数方面具有明显的优势。利用该新型微纳叠层复合材料制备装置制备了PP/PP、LLDPE/LLDPE多层复合材料,并着重研究了螺杆转速对叠层复合材料层厚的影响,结果表明,层厚的变化与螺杆转速改变有很好的一致性。     

航空层合定向PMMA疲劳断裂性能的频闪法研究

座舱盖透明件的疲劳断裂力学性能对飞机的安全飞行起到了至关重要的作用。运用频闪法结合数字图像相关法进行疲劳测试,此方法能够将动态过程转化为准静态过程,以此研究疲劳断裂性能。运用此方法对带应力集中缺口的层合定向有机玻璃的疲劳断裂性能进行研究。对实验结果进行分析得出得出在应力集中区域,疲劳裂纹有沿倾斜方向开裂的趋势,而并非沿竖直方向开裂。并对其实际破坏形式进行分析,裂纹实际扩展方向与根据频闪疲劳测试系统测试的应变场预测结果相符,进而验证了实验结果的准确性。     

Determination of mechanical properties of Al2O3/Y-TZP ceramic composites: Influence of testing method and residual stresses

A series of Al₂O₃/Y₂O₃-stabilized zirconia (Y-TZP) ceramic composites with different zirconia contents (5 and 40 vol% Y-TZP) and fabricated by different green processing techniques (a novel tape casting and conventional slip casting) were studied. The microstructure and mechanical properties of the composites were investigated systematically, by means of scanning electron microscopy, Vickers indentation, depth-sensing nanoindentation, and single-edge laser-notched beam (SELNB) techniques. The indentation fracture method was found to be unsuitable for fracture toughness determination in this work. Reliable values of fracture toughness were obtained by the SELNB method with an almost atomically sharp laser-machined initial notch. The microstructure and mechanical properties of the ceramic composites mainly depended on the Y-TZP content. No significant differences were induced by the choice of green processing technique. The contribution of residual stresses to fracture toughness in Al₂O₃/Y-TZP ceramic composites was investigated. To this end, a theoretical model was applied to estimate the increase in fracture toughness due to the measured residual stresses in the samples. It was found that in this case, residual stresses were not the main factor responsible for the toughening in Al₂O₃/Y-TZP composites.     

Enhancement of fracture toughness of carbon fiber laminated composites using multi wall carbon nanotubes

In this paper, the effect of Multi Wall Carbon Nanotubes (MWNT) as a toughening agent of laminated composites is experimentally investigated. Carbon fiber laminates were manufactured by resin film infusion technique in which the resin flows in the through-the-thickness direction. The modified polymer systems showed 17% improvement in the stress intensity factor (K_Ic), whereas the laminated composites showed up to 48% improvement in Mode I and 143% improvement in Mode II fracture toughness. Scanning Electron Microscope (SEM) was then used to study the fractured surface and to explain the contrasting behavior of the MWNT-modified polymers when compared to the laminates.     

高延性水泥基复合材料的断裂性能分析

实验测试了不同纤维掺量下的高延性水泥基复合材料的载荷—挠度曲线,并对试件加载过程中的声发射信号进行收集,分析了试件断裂后的裂纹分布及纤维在基材中的破坏形式。结果发现:(1)当纤维体积掺量2.0%时,水泥基材的极限挠度、极限抗弯承载力分别可达20.16mm、19.47MPa;(2)高延性水泥基复合材料破坏主要来自于微裂纹的萌生、扩展以及损伤积累过程,试件从加载至完全破坏的时间为素水泥基复合材料破坏持续时间的2~5倍;(3)高延性水泥基复合材料中纤维的破坏以纤维被拔出和纤维被拉断两种模式,试件表现出明显的多缝开裂特征。