Mechanical and fracture properties of cellulose-fibre-reinforced epoxy laminates

Epoxy laminates reinforced with cellulose-fibre mats (CFM) have been synthesized and characterized. The influence of CFM dispersion on the mechanical and fracture properties of these laminates have been characterized in terms of elastic modulus, hardness, flexural strength, fracture toughness, indentation responses, impact-fracture, crack-growth resistance and in situ fracture. The reinforcement by the CFM resulted in a significant increase in the strain at break, indentation creep, fracture toughness and impact toughness but moderate increase in flexural strength and flexural modulus. A pronounced R-curve behaviour is exhibited by the CFM-reinforced epoxy sample, which failed in a graceful manner with slow and stable crack-growth. The micromechanisms of toughening and crack-tip failure processes are identified and discussed in the light of observed microstructures from in situ and ex situ fracture. The implications for new approaches in the ‘eco-design’ of environmentally friendly composite materials are addressed.     

Effect of mode III fracture on composite laminates

The digital speckle correlation method (DSCM) is preformed to study the inplane displacements along the x and y direction of laminated composite panels containing preset elliptical damage. The different principle axis of the ellipse and the different sequence of laminates are considered in the experiments. It is shown that the method is very useful to get the displacements on the laminate surface and between the adjacent plies. According to the experiment results, the deformations of x and y direction can be obtained. The conclusion that the mode III fracture may exist and may have an effect on the crack growth is formed. The total strain energy release rate is calculated by the finite element method. Using Mindlin plate theory and the virtual crack closure technique, the energy release rate of mode III fracture can be calculated by FEM. The results show that mode III fracture has an influence on the total energy release rate and also on the delamination growth. The energy release rate of mode III fracture cannot be ignored. The delamination growth also is influenced by the stacking sequence.     

The correlation of interfacial and macroscopic toughness in SiC laminates

Specimens have been manufactured by viscous processing of SiC powder into sheets, which were then laminated with thin carbon interlayers and sintered without pressure to give a > 98% dense product. These have been tested both in four-point bend delamination' of single interlayer sandwich specimens, to give interfacial critical strain energy release rate. G_IC data, and in three-point bending of multi-layer laminates. The value of G_ic was found to be ∼7.5 J m⁻², which compares with about 28 J m⁻² for G_bc of the bulk SiC itself produced by this route. During three-point bending of the laminates, crack deflection was consistently observed at the interfaces between laminae. This led to energy dissipation during the process, such that the apparent G_c value of the laminate was around 6 kJ m⁻². A numerical model was set up to describe the crack advance sequence. This is based on through-thickness cracks propagating when a critical stress is reached (using a Monte Carlo method to determine the strength of successive laminae, for a given Weibull modulus) and interfacial cracks then advancing a distance dictated by the available energy. This model allowed accurate prediction of both load/displacement curves and the absorption of energy within the specimen (by interfacial cracking). A distinction is drawn between the area under the load/displacement curve and the energy absorbed in the specimen. These differ if the energy available to drive the interfacial cracks is more than sufficient for them to reach the ends of the specimen. It is shown that there is scope for maximization of the energy absorbed in the specimen (i.e., for optimization of the toughness of the laminate when loaded in bending) by control over the G_ic value and the thickness of individual laminae.     

Investigations on stamping of C/PEEK laminates: Influence on meso-structure and macroscopic mechanical properties under severe environmental conditions

This study was aimed at addressing the influence of stamping on the mechanical performance (tensile, in-plane shear and inter-laminar shear) of fabric reinforced thermoplastic laminates under severe conditions. The effects of processing have been discussed at different levels: influence on the micro-structure (porosity and mean free path) and meso-structure (reinforcement and matrix distribution), changes in the matrix properties as well as in the fiber/matrix interface. The obtained results and the SEM observations suggest that these changes are closely associated with the macroscopic mechanical behavior of laminates. Stamping proved to be a re-consolidation process, and the high stamping pressure promotes two primary mechanisms: re-compaction of the fiber network and migration of melted matrix. These mechanisms significantly influence the meso-structure properties (better interlaminar adhesion and fiber/matrix bonding), resulting in the improvement of the material properties.     

Effect of interfacial mobility on flexural strength and fracture toughness of glass/epoxy laminates

Mechanical testing and surface fractography were used to characterize the fracture of E-glass fiber reinforced epoxy composites as a function of the silane coupling agent used. -Aminopropyltriethoxysilane (APS) and -aminobutyltriethoxysilane (ABS) were used because these have been shown to have different interfacial mobilities at multilayer coverage. The values of the properties studied generally increased from untreated c, as determined from a Mode I translaminar fracture toughness tests, for the untreated composites (10.5 ± 0.4 kJ/m²) was lower than that for the ABS-treated composites (14.3 ± 2.1 kJ/m²) which was lower than that for the APS-treated composites (17.1 ± 2.4 kJ/m²). Macroscopic observations showed that a larger fiber debonding area was formed in the crack tip region for the untreated glass composites, suggesting poorer bonding compared to those treated with coupling agents. Since these silanes have similar chemistry, the differences were attributed to differences in the interfacial mobility of the coupling agent layers.     

The combined effects of curing and environmental exposure on fracture properties of woven carbon/epoxy laminates

This paper concerns a study of the combined effects of curing conditions and environmental exposure on the ultimate properties of two commercial woven carbon/epoxy laminates. Curing parameters (heating rate and applied pressure) were varied so as to obtain six different conditions for each material. Moisture saturation was also achieved by exposing some of the cured samples to environmental conditions of 70°C and 95% relative humidity. Four different tests (tensile, impact, Mode I and Mode II interlaminar fracture resistance) were therefore performed, and the results obtained on the different materials before and after moisture saturation compared. Neither curing pressure nor heating rate nor moisture absorption were observed to have any practical effect on tensile and impact properties. On the contrary, one noticeable effect was the interlaminar fracture resistance of the laminates. The results are discussed and interpreted in terms of damage formation and stress intensification mechanisms.     

纤维分布均匀性对T300/BMP316复合材料层压板力学性能的影响

采用三点弯曲法和数字化冲击仪, 研究了纤维分布均匀性对T300/BMP316复合材料弯曲和冲击性能的影响, 用有限元方法分析了纤维分布不均匀复合材料弯曲变形时的应力分布, 提出了用纤维分布效应系数R来评价纤维分布不匀复合材料的性能。结果表明, 纤维分布不均使T300/BMP316复合材料的弯曲性能和冲击性能强烈地受到纤维分布方式和载荷方向的影响, 纤维分布效应系数R反映了纤维分布与性能的关系。      

Effect of z-pin surface treatment on delamination and debonding properties of z-pinned composite laminates

The effect of z-pin surface treatment on the delamination fracture properties of z-pinned unidirectional carbon fibre/epoxy prepreg laminate is presented in this paper. Cryogenic and plasma treatments were used to increase the pin/composite interface properties. Z-pin pullout tests were carried out to study the relations between the bridging force and the displacement. Mode-I double-cantilever beam tests were used to characterize the improvements in delamination toughness. It was pointed out that appropriate treatments could effectively increase the delamination fracture properties. Oxygen-containing functional groups could be induced on the pin surface through cold plasma treatment. An increasing surface energy is improving the wettability so that more chemical reactions can be generated between the epoxy group and z-pin surface. Furthermore, the surface roughness of z-pins can be extended with a plasma or cryogenic treatment. The pins obtained a larger surface area, which could wet by the epoxy matrix during the z-pin-insertion and curing process.     

Effect of temperature and strain rate on tensile mechanical properties of ARALL-1 laminates

The combined effect of temperature and strain rate of the mechanical properties for unidirectional 3/2 ARALL^®-1 laminates was studied. In this paper, the effect of strain rates from 0.00083–0.833 min^–1 on tensile behaviour at temperatures up to 250°F (121 °C) has been conducted. It is demonstrated that tensile strength, tensile modulus, and fracture strain are found to depend on temperature and strain rate. However, the effect of strain rates at 75 °F (24 °C) and 180 °F (82 °C) was found to be insignificant except the lowest strain rate at 180 °F. It was also observed that the tensile yield strength decreased as the strain rate decreased. The tensile properties were moderately reduced at high temperatures and were higher at high strain rates than at low strain rates. The temperature effect is more significant than that due to the strain rate. Scanning electron photomicrographs of the fracture surfaces observed in the aramid/epoxy layer of ARALL-1 laminates at the lowest strain rate are shown to be significantly different only at 250 °F (121 °C). But this phenomenon is not obvious when the highest strain rate is employed.     

Effect of carbon fibres on the static and fatigue mechanical properties of fibre metal laminates

It is crucial to understand the characteristic fatigue crack initiation and its growth mechanisms, as well as the relationship between the mechanical properties and the fatigue damage evolution in fibre metal laminates (FMLs). Two types of FML were studied in this work: a polyacrylonitrile‐based carbon fibre epoxy matrix composite sandwiched by Ti‐6Al‐4V (Ti‐alloy) sheets (IMS60‐Ti) and a pitch‐based carbon fibre epoxy matrix composite sandwiched by Ti‐alloy sheets (K13D‐Ti). The static and fatigue mechanical properties of IMS60‐Ti and K13D‐Ti were investigated. The increased failure strain of the FML was greater than that of carbon fibre‐reinforced polymer (CFRP) matrix composites. The fatigue life of IMS60‐Ti was much longer than that of K13D‐Ti. The fatigue damage process in IMS60‐Ti was related to the fatigue creep behaviour of the Ti‐alloy face sheet and mode II cracking at the CFRP/Ti‐alloy interface, and the damage in K13D‐Ti was related to the K13D CFRP laminate.     

Estimation of apparent fracture toughness of ceramic laminates

The paper presented deals with the fracture behaviour of ceramic laminates. The residual stresses in individual layers of Al₂O₃/5vol.%t-ZrO₂ (ATZ) and Al₂O₃/30vol.%m-ZrO₂ (AMZ) are determined. Assumptions concerning linear elastic fracture mechanics and small scale yielding are considered. In this frame the procedure based on a generalization of Sih’s strain energy density factor to the case of a crack touching the interfaces between two dissimilar materials is used for determination of effective values of the stress intensity factor on material interfaces. An important increase of fracture toughness at the AMZ/ATZ interface was predicted in comparison to the fracture toughness of individual material components. Predicted values were compared with data available in the literature and mutual good agreement was found. The procedure suggested can be used for estimation of resistance to crack propagation through multilayered structures and its design. The procedure can contribute to enhancing the reliability and safety of structural ceramics or, more generally, of layered composites with strong interfaces.     

阳极氧化工艺对纤维-铝合金层板力学性能的影响

通过改变铝合金表面阳极氧化工艺参数,研究了阳极氧化电压和时间对玻璃纤维-铝合金(GLARE)层板抗拉强度和层间剪切强度的影响.通过SEM观察了铝合金表面Al2O3多孔膜和层板断面形貌,分析了铝合金/树脂胶接界面对层板力学性能的影响.结果表明,阳极氧化电压为20 V时,GLARE层板抗拉强度和层间剪切强度随着阳极氧化时间延长而增大,在20 min时出现最大值,继续延长阳极氧化时间,层板强度随之下降;阳极氧化时间为20 min时,GLARE层板抗拉强度和层间剪切强度随着阳极氧化电压增大而增大,在20V时出现最大值,继续增大电压,强度随之下降.     

阳极氧化工艺对纤维-铝合金层板力学性能的影响

通过改变铝合金表面阳极氧化工艺参数, 研究了阳极氧化电压和时间对玻璃纤维-铝合金(GLARE)层板抗拉强度和层间剪切强度的影响。通过SEM观察了铝合金表面Al₂O₃多孔膜和层板断面形貌, 分析了铝合金/树脂胶接界面对层板力学性能的影响。结果表明, 阳极氧化电压为20 V时, GLARE层板抗拉强度和层间剪切强度随着阳极氧化时间延长而增大, 在20 min时出现最大值, 继续延长阳极氧化时间, 层板强度随之下降; 阳极氧化时间为20 min时, GLARE层板抗拉强度和层间剪切强度随着阳极氧化电压增大而增大, 在20 V时出现最大值, 继续增大电压, 强度随之下降。     

MWCNTs对碳纤维非褶皱无纺布/环氧层合板力学性能的影响

将羧基化多壁碳纳米管(MWCNTs)添加到TDE85环氧树脂中,然后与碳纤维非褶皱无纺布(C-NCF)复合,制备成[0°/90°/+45°/-45°]_S层合板。采用三点弯曲、短梁剪切和单边切口弯曲测试方法以及动态力学性能分析方法,研究了不同含量的MWCNTs对层合板弯曲性能、层间剪切强度(ILSS)、Ⅱ型层间断裂韧性(G_ⅡC,以及玻璃态转变温度(T_g)的影响。并采用SEM对Ⅱ型试样的断面进行分析。结果表明,MWCNTs的加入显著提高了NCF层合板的力学性能。与空白试样相比,当MWCNTs在树脂中的质量分数为2.0%时,弯曲强度和模量分别提高了约26%和6%;当MWCNTs的质量分数为0.5%时,ILSS、G_ⅡC、T_g分别提高约14%、27%和14%。     

Flexural properties of z-pinned laminates

This paper examines the effect of pinning on the flexural properties, fatigue life and failure mechanisms of carbon/epoxy laminates. Five-harness satin weave carbon/epoxy laminates were reinforced in the through-thickness direction with different volume fractions and sizes of fibrous composite pins. Microscopic examination of the laminates before flexural testing revealed that the pins caused considerable damage to the microstructure, including out-of-plane crimping, in-plane distortion and breakage of the fibres and the formation of resin-rich zones around each pin. The pins also caused swelling of the laminate that reduced the fibre volume content. Despite the damage, the pins did not affect the flexural modulus of the laminate. However, increasing the volume content or diameter of the pins caused a steady decline in the flexural strength and fatigue life, which appear to be governed by fiber rupture on the tensile side of the laminate. Property changes are discussed in terms of transitions in the dominant failure mechanisms due to the presence of pins.     

Intralaminar and interlaminar fracture in graphite/epoxy laminates

Various matrix failure modes (intralaminar and interlaminar) in T300/934 graphite/ epoxy laminates are studied. The intralaminar mode is considered by using centre-notchedtension. surface-notched-tension, three-point-bend and compact-tension specimens where transverse fracture toughness and 0° split initiation are investigated. The interlaminar fracture is studied by using double-cantilever-beam and cracked-lap-shear specimens for mode 1 and mode 2 respectively. A simple method for the prediction of split initiation is given and it is seen that the predicted and experimental results agree well. In addition to testing the dry specimens. a few hygrothermal conditions are also used to assess the influence of environment on various failure modes. The effect of environment shows a mixed trend on fracture toughness depending on damage mechanisms involved in the failure modes. The moisture and temperature show a deleterious effect on interface-controlled failure modes, but a beneficial effect on the modes controlled by matrix cleavage.     

Effect of resin system on the mechanical properties and water absorption of kenaf fibre reinforced laminates

The objective of this study is to compare the mechanical and water absorption properties of kenaf (Hibiscus cannabinus L.) fibre reinforced laminates made of three different resin systems. The use of different resin systems is considered so that potentially complex and expensive fibre treatments are avoided. The resin systems used include a polyester, a vinyl ester and an epoxy. Laminates of 15%, 22.5% and 30% fibre volume fraction were manufactured by resin transfer moulding. The laminates were tested for strength and modulus under tensile and flexural loading. Additionally, tests were carried out on laminates to determine the impact energy, impact strength and water absorption. The results revealed that properties were affected in markedly different ways by the resin system and the fibre volume fraction. Polyester laminates showed good modulus and impact properties, epoxy laminates displayed good strength values and vinyl ester laminates exhibited good water absorption characteristics. Scanning electron microscope studies show that epoxy laminates fail by fibre fracture, polyester laminates by fibre pull-out and vinyl ester laminates by a combination of the two. A comparison between kenaf and glass laminates revealed that the specific tensile and flexural moduli of both laminates are comparable at the volume fraction of 15%. However, glass laminates have much better specific properties than the kenaf laminates at high fibre volume fractions for all three resins used.