Effects of graphene nanoplatelets and graphene nanosheets on fracture toughness of epoxy nanocomposites

The effects of graphene nanoplatelets (GPLs) and graphene nanosheets (GNSs) on fracture toughness and tensile properties of epoxy resin have been studied. A new technique for synthesis of GPLs based on changing magnetic field is developed. The transmission‐electron microscopy and the Raman spectroscopy were employed to characterize the size and chemical structure of the synthesized graphene platelets. The critical stress intensity factor and tensile properties of epoxy matrix filled with GPL and GNS particles were measured. Influence of filler content, filler size and dispersion state was examined. It was found that the GPLs have greater impact on both fracture toughness and tensile strength of nanocomposites compared with the GNSs. For instance, fracture toughness increased by 39% using 0.5 wt% GPLs and 16% for 0.5 wt% GNSs.     

基于纳米压痕技术的碳纤维/环氧树脂复合材料各组分原位力学性能测试

基于纳米压痕技术对碳纤维/环氧树脂复合材料各组分的原位硬度、 弹性模量和蠕变性能进行了测试, 实验得到了基体、 纤维和微小厚度界面层的力学性能。结果表明, 从环氧树脂基体到碳纤维过渡过程中, 硬度和弹性模量有明显的梯度变化, 并且纤维和树脂基体的原位弹性模量平均值与其非原位性能有一定的变化, 实验得到纤维的原位弹性模量有所下降, 环氧树脂的弹性模量有所增加。试件制备过程中的机械研磨对其表面产生的残余应力和复合后两种材料的相互影响是组分材料原位性能变化的主要原因。各组分的蠕变性能呈现出明显的差异。     

Creep behaviour of injection moulded polyamide 6/organoclay nanocomposites by nanoindentation and cantilever-bending

The creep behaviour of injection moulded PA 6/organoclay nanocomposites was studied by depth-sensing nanoindentation and DMA cantilever-bending. The glass transitions of PA 6 and its nanocomposites were decreased below room temperature upon saturation with water so that the materials could be tested in the rubbery regime. For nanoindentation creep on the skin and core regions of injection moulded samples, whilst organoclay improves the creep resistance of PA 6, the enhancement is due to the decrease of the initial compliance at zero time but the time-dependent creep is actually increased. In contrast, for cantilever-bending creep, organoclay reduces the creep compliance and the time-dependent creep in PA 6. It is suggested that the organoclay imparts a constraint effect on the PA 6 molecular chains, restricting their mobility in the bulk compared to the surface and hence improving their resistance to creep. A modified Halpin-Tsai equation was used to model their creep behaviour under these two loading configurations and compared to experimental data.     

Investigation of the nanomechanical properties of nylon 6 and nylon 6/clay nanocomposites at sub-ambient temperatures

The nanomechanical properties of nylon 6, nylon 6/exfoliated clay and nylon 6/non-exfoliated clay nanocomposites have been investigated from room temperature to ?10 °C in a controlled environment with humidity less than 1% RH. The hardness, elastic modulus and creep resistance of nylon 6 were improved in the nanocomposites across the temperature range. However, the effective reinforcement of the clay depended on the temperature due to the change between the glassy and transition states in the nylon. The exfoliated clay nanocomposite showed the greater improvements than in the non-exfoliated clay nanocomposite at all testing temperatures due to the improved constraint of the polymer chains by the clay platelets in the exfoliated structure. The surface mechanical properties of nylon 6 and the nanocomposites were also found to be highly sensitive to the moisture level during the tests; increasing the humidity in the room temperature tests resulted in a dramatic decrease in hardness and stiffness due to plasticisation by water molecules. The kinetics of the re-humidification process on nylon 6 were studied by monitoring the change in nanoindentation response. Analysis of the indentation creep revealed a significant change in the strain rate sensitivity when the humidity of the near-surface region probed by nanoindentation was in the vicinity of the glass transition.     

Nanoindentation creep deformation behaviour of high nitrogen nickel-free austenitic stainless steel

Nanoindentation tests of the high nitrogen nickel-free austenitic stainless steel (HNS) were performed with peak load in a wide range of 100–600?mN to investigate the nanoindentation creep deformation behaviours. The results of the nanoindentation creep tests have demonstrated that the load plateaus, creep strain rate and creep stress of the cold-rolled HNS are larger and its creep stress exponent is smaller than the solution-treated HNS. The analysis reveals that the obvious creep deformation behaviour in the cold-rolled HNS arises from the rapidly relaxed dislocation structures in the initial transition regime, while the small creep deformation behaviour of the solution-treatedHNS is mainly attributed to that the stable dislocation structures for the intensive interactions between dislocations.     

Thermo-mechanical properties of randomly oriented carbon/epoxy nanocomposites

The thermo-mechanical properties of epoxy-based nanocomposites based on low weight fractions (from 0.01 to 0.5 wt%) of randomly oriented single- and multi-walled carbon nanotubes were examined. Preparation methods for the nanocomposites, using two types of epoxy resins, were developed and good dispersion was generally achieved. The mechanical properties examined were the tensile Young's modulus by Dynamic Mechanical Thermal Analysis and the toughness under tensile impact using notched specimens. Moderate Young's modulus improvements of nanocomposites were observed with respect to the pure matrix material. A particularly significant enhancement of the tensile impact toughness was obtained for specific nanocomposites, using only minute nanotube weight fractions. No significant change in the glass transition temperature of SWCNT/epoxy nanocomposites was observed, compared to that of the epoxy matrix. The elastic modulus of the SWNT-based nanocomposites was found to be slightly higher than the value predicted by the Krenchel model for short-fiber composites with random orientation.     

Effect of severe plastic deformation on creep behaviour of a Ti–6Al–4V alloy

This paper examines the effect of severe plastic deformation on creep behaviour of a Ti–6Al–4V alloy. The processed material with an ultrafine-grained (UFG) structure (d ≈ 150 nm) was prepared by multiaxial forging. Uniaxial constant stress compression and constant load tensile creep tests were performed at 648–698 K and at stresses ranging between 300 and 600 MPa on the UFG processed alloy and, for comparison purposes, on its coarse-grained (CG) state. The values of the stress exponents of the minimum creep rate n and creep activation energy Q _c were determined. Creep behaviour was also investigated by nanoindentation method at room temperature under constant load. The microstructure was examined by transmission electron microscopy and scanning electron microscope equipped with an electron back scatter diffraction unit. The results of the uniaxial creep tests showed that the minimum creep rates of the UFG specimens are significantly higher in comparison with those of the CG state. However, the differences in the minimum creep rates of both states of alloy strongly decrease with increasing values of applied stress. The CG alloy exhibits better creep resistance than the UFG one over the stress range used; the minimum creep rate for the UFG alloy is about one to two orders of magnitude higher than that of the CG alloy. The indentation creep tests showed that annealing had little effect on the creep behaviour in UFG Ti alloy at room temperature.     

Monitoring the mechanical behaviour of electrically conductive polymer nanocomposites under ramp and creep conditions

Various amounts of carbon black (CB) and carbon nanofibres (CNF) were dispersed in an epoxy resin to prepare nanocomposites whose mechanical behaviour, under ramp and creep conditions, was monitored by electrical measurements. The electrical resistivity of the epoxy resin was dramatically reduced by both nanofillers after the percolation threshold (1 wt% for CB and 0.5 wt% for CNF), reaching values in the range of 10(3)-10(4) omega . cm for filler loadings higher than 2 wt%. Due to the synergistic effects between the nanofillers, an epoxy system containing a total nanofiller amount of 2 wt%, with a relative CB/CNF ratio of 90/10 was selected for the specific applications. A direct correlation between the tensile strain and the increase of the electrical resistance was observed over the whole experimental range, and also the final failure of the samples was clearly detected. Creep tests confirmed the possibility to monitor the various deformational stages under constant loads, with a strong dependency from the temperature and the applied stress. The obtained results are encouraging for a possible application of nanomodified epoxy resin as a matrix for the preparation of structural composites with sensing (i.e., damage-monitoring) capabilities.     

Enhanced Mechanical Properties of Epoxy Nanocomposites by Mixing Noncovalently Functionalized Boron Nitride Nanoflakes

The influence of surface modifications on the mechanical properties of epoxy‐hexagonal boron nitride nanoflake (BNNF) nanocomposites is investigated. Homogeneous distributions of boron nitride nanoflakes in a polymer matrix, preserving intrinsic material properties of boron nitride nanoflakes, is the key to successful composite applications. Here, a method is suggested to obtain noncovalently functionalized BNNFs with 1‐pyrenebutyric acid (PBA) molecules and to synthesize epoxy–BNNF nanocomposites with enhanced mechanical properties. The incorporation of noncovalently functionalized BNNFs into epoxy resin yields an elastic modulus of 3.34 GPa, and 71.9 MPa ultimate tensile strength at 0.3 wt%. The toughening enhancement is as high as 107% compared to the value of neat epoxy. The creep strain and the creep compliance of the noncovalently functionalized BNNF nanocomposite is significantly less than the neat epoxy and the nonfunctionalized BNNF nanocomposite. Noncovalent functionalization of BNNFs is effective to increase mechanical properties by strong affinity between the fillers and the matrix.     

基于纳米压痕法的埃洛石纳米管/环氧复合材料力学性能表征

采用球磨法在环氧树脂中分散了不同质量分数(0、5wt%和10wt%)的埃洛石纳米管(HNTs),通过哌啶固化剂固化,制备了HNTs/环氧树脂复合材料,并利用纳米压痕法测试了HNTs/环氧树脂复合材料的弹性模量、硬度和蠕变性能。SEM和TEM观测表明:HNTs在环氧树脂中分散情况较好。纳米压痕实验结果表明:在不牺牲HNTs/环氧树脂复合材料弹性模量、硬度以及玻璃化转变温度的基础上,HNTs明显提高了环氧树脂基复合材料的抗蠕变性能,这主要是由于HNTs和环氧基分子链形成了新的交联结构,增加了材料的交联密度,刚性纳米粒子限制了环氧基分子链的活动性。     

Room temperature nanoindentation creep of nanoscale Ag/Fe multilayers

This paper deals with room temperature indentation creep behavior of nanoscale Ag/Fe multilayers. The constant-load nanoindentation test results reveal that all the multilayers exhibit steady-state creep after transient creep occurring at first 150 s and decreasing periodicity leads to a decrease in the stress exponent and an increase in creep rate. The dependence of the stress exponent and creep rate on the periodicity indicates that the creep process is dominated by dislocation glide-climb mechanism and the increasing fraction of grain boundaries and interfaces provide effective diffusion paths for the creep climb that determines the whole creep rate.     

Nanoindentation investigation of asphalt binder and mastic viscoelasticity

An exploratory nanoindentation technique for creep testing of two neat asphalt binders and one mastic at room temperature is developed, tested and verified. This work presents a new approach to obtain viscoelastic properties from low-load spherical (blunt) nanoindentation. Interconverted shear relaxation modulus mastercurves are determined from nanoindentation data. The magnitudes and trends of these mastercurves are found to be in reasonable agreement with Dynamic Shear Rheometer (DSR) results in a stiffness range associated with the range of time and temperature used in nanoindentation testing. Nanoindentation creep data is transformed to develop a mastercurve of dynamic modulus. The portion of this mastercurve corresponding to the frequency and temperature range included in nanoindentation testing demonstrates reasonable agreement with DSR results. These initial results suggest the potential to expand nanoindentation testing to forensic investigations involving testing of preserved asphalt binder and mastic components within field-extracted asphalt concrete composites.     

Measurement of Young’s relaxation modulus using nanoindentation

In a previous paper (Lu et al., Mechanics of Time-Dependent Materials, 7, 2003, 189–207), we described methods to measure the creep compliance of polymers using Berkovich and spherical indenters by nanoindentation. However, the relaxation modulus is often needed in stress and deformation analysis. It has been well known that the interconversion between creep compliance and relaxation function presents an ill-posed problem, so that converting the creep compliance function to the relaxation function cannot always give accurate results, especially considering that the creep data at short times in nanoindentation are often not reliable, and the overall nanoindentation time is short, typically a few hundred seconds. In this paper, we present methods to measure Young’s relaxation functions directly using nanoindentation. A constant-rate displacement loading history is usually used in nanoindentations. Using viscoelastic contact mechanics, Young’s relaxation modulus is extracted using nanoindentation load-displacement data. Three bulk polymers, Polymethyl Methacrylate (PMMA), Polycarbonate (PC) and Polyurethane (PU), are used in this study. The Young’s relaxation functions measured from the nanoindentation are compared with data measured from conventional tensile and shear tests to evaluate the precision of the methods. A reasonably good agreement has been reached for all these materials for indentation depth higher than a certain value, providing reassurance for these methods for measuring relaxation functions.     

Investigation into the room temperature creep-deformation of potassium dihydrogen phosphate crystals using nanoindentation

It has been a tremendous challenge to manufacture damage-free and smooth surfaces of potassium dihydrogen phosphate (KDP) crystals to meet the requirements of high-energy laser systems. The intrinsic issue is whether a KDP crystal can be plastically deformed so that the material can be removed in a ductile mode during the machining of KDP. This study investigates the room temperature creep-deformation of KDP crystals with the aid of nanoindentation. A stress analysis was carried out to identify the creep mechanism. The results showed that KDP crystals could be plastically deformed at the nanoscale. Dislocation motion is responsible for creep-deformation. Both creep rate and creep depth decrease with decrease in peak force and loading rate. Dislocation nucleation and propagation bring about pop-ins in the loaddisplacement curves during nanoindentation.The full text can be downloaded at https://link.springer.com/article/10.1007/s40436-018-0234-9     

A systematic methodology for creep master curve construction using the stepped isostress method (SSM): a numerical assessment

Long-term creep of viscoelastic materials is experimentally inferred through accelerating techniques based on the time–temperature superposition principle (TTSP) or on the time–stress superposition principle (TSSP). According to these principles, a given property measured for short times at a higher temperature or higher stress level remains the same as that obtained for longer times at a lower temperature or lower stress level, except that the curves are shifted parallel to the horizontal axis, matching a master curve. These procedures enable the construction of creep master curves with short-term experimental tests.The Stepped Isostress Method (SSM) is an evolution of the classical TSSP method. Higher reduction of the required number of test specimens to obtain the master curve is achieved by the SSM technique, since only one specimen is necessary. The classical approach, using creep tests, demands at least one specimen per each stress level to produce a set of creep curves upon which TSSP is applied to obtain the master curve.This work proposes an analytical method to process the SSM raw data. The method is validated using numerical simulations to reproduce the SSM tests based on two different viscoelastic models. One model represents the viscoelastic behavior of a graphite/epoxy laminate and the other represents an adhesive based on epoxy resin.     

Effects of SWCNTs on mechanical and thermal performance of epoxy at elevated temperatures

A property which limits the breadth of application of thermoset polymers and their composites is their relatively low maximum operating temperatures. This work investigates the potential application of both functionalized single-walled carbon nanotubes (f-SWCNTs) based on negative charging, and unfunctionalized SWCNTs (u-SWCNTs) to increase the mechanical and thermal performance of a high-temperature aerospace-grade epoxy with a glass transition temperature of approximately 270 °C. Thermal and mechanical properties of the baseline epoxy and nanocomposites containing a low content of SWCNTs (0.2 % by weight) were characterized through thermogravimetric analyses, tensile tests, and dynamic mechanical analyses. Tensile tests were performed both at room temperature and at 80 °C. Further, room temperature tensile tests were performed on untreated and heat-treated specimens. The heat treatment was performed at 300 °C, slightly above the resin glass transition temperature. Results demonstrate that f-SWCNTs are effective in improving the mechanical and thermal performance of the epoxy. No significant improvement was observed for u-SWCNT nanocomposites. For the nanocomposite with f-SWCNTs, the ultimate tensile strength and strain to failure at room temperature (80 °C) increased by 20 % (8 %) and 71 % (77 %), respectively, as compared to the baseline epoxy. The f-SWCNT nanocomposite, unlike other examined materials, exhibited a stress–strain necking behavior at 80 °C, an indication of increased ductility. After heat treatment, these properties further improved relative to the neat epoxy (160 % increase in ultimate tensile strength and 270 % increase in strain to failure). This work suggests the potential to utilize f-SWCNTs based on negative charging to enhance high-temperature thermoset performance.     

Creep of polymer composites

A model incorporating a modified thermal activation theory is presented to model and predict creep of polymer composites. Results are presented of the successful application of this model to predict creep of a unidirectional, continuous-carbon-fiber-reinforced polymer composite (AS4/3501-6) and its epoxy matrix, over a wide range of stress (10–80% of ultimate tensile strength) and temperature (295–433 K). From an analysis of model parameters, it is concluded that the reinforcing carbon fibers do not alter the creep mechanism but do alter the creep behavior of the epoxy matrix, resulting in reductions in creep rate and in the magnitude of creep.     

Evaluation of time-dependent change in fiber stress profiles during long-term pull-out tests at constant loads using Raman spectroscopy

Constant-load pull-out tests were carried out on single-fiber model composite specimens for 500 to 1,000 hours in order to investigate the time-dependent change in fiber axial stress profiles resulting from matrix creep in unidirectional continuous fiber-reinforced composites. Three resins used as the matrix materials, in which single carbon fibers were embedded, were normal epoxy, a blend with a more flexible epoxy, and UV-curable acrylic. The time-dependent change in fiber stress profiles in the constant-load pull-out tests was measured using Raman spectroscopy, and creep and relaxation tests for the matrix resins themselves were performed. It was observed that the normal epoxy matrix composite exhibited only a negligible change in the fiber stress profile with time whereas the flexible epoxy and UV-curable acrylic matrices allowed, respectively, considerable and significant changes. These observations were shown to be consistent with the creep and stress relaxation test results of the matrix resins. It was also found that the time-dependent change in fiber stress was much slower in the experiment than in the prediction based on perfect bonding at the fiber/matrix interface. The interfacial slip that occurred in the composites tested could be responsible for the gradual variation in fiber stress profiles.     

Indenter geometrical effects on sub-micro/nano indentation and scratch behaviors of polymeric surfaces

In this work, we study the indenter geometrical effects on the nanoindentation and scratch behaviors of polymers (epoxy resin and its based nanocomposites) through both experimental and numerical approaches. Two different types of indenter commonly used in nanoindentation tests, Berkovich (i.e., three-sided pyramid-shaped) and conical-shaped, are studied. An indenter (tip) geometry reconstruction procedure is presented and the results are input into finite element models to better understand the mechanisms of the geometrical effects on nanoindentation. The effect of indenter geometry on nanoscratch behavior is also investigated experimentally. Overall, our results show that the corrected Oliver and Pharr's method is applicable to nanoindentation of polymeric surfaces and the indenter geometry plays significant roles in both the nanoindentation and nanoscratch tests. The conical-shaped indenter provides slightly overestimated mechanical parameters for the polymers tested due to the material pile-up. Therefore, for nanoindentation and scratch tests, the type of indenter should always be mentioned and comparisons between tests using different types of indenter should be taken with caution.     

Cyclic fatigue of polymer nanocomposites

This paper presents an experimental study on cyclic fatigue of two polymer nanocomposites in two common failure modes: mechanical failure in epoxy nanocomposites and thermal softening in polyamide (PA, nylon) 6 nanocomposites. For epoxy nanocomposites, the effects of hard (silica) and soft (rubber) nano-particles on un-notched samples under constant cyclic stress amplitude fatigue were studied. Hard particles were shown to increase but soft particles decrease the fatigue life of nanocomposites compared to unmodified epoxy. At the same stress amplitude, the extent of fatigue crack growth prior to fast fracture was largest in rubber nanocomposites and least in pure epoxy, reflecting the differences in their fracture toughness values. Ternary nanocomposites with both hard and soft (silica and rubber) particles were also investigated and their fatigue performances were compared to the binary nanocomposites. Further, the stress (σ_a) versus life (N_r) test data of pure epoxy and its binary and ternary nanocomposites are well described by Basquin’s law.PA6 nanocomposites exhibited fatigue failure due to thermal softening when the maximum local temperature of the specimens subjected to cyclic loading reached the glass transition temperature, T_g, of the material. Critical stress (σ_a) versus frequency (ω) envelopes for design against thermal failure were obtained for PA6/organoclay, PA6/POE-g-MA and PA6/pristine clay. Experimental results compared favorably with theoretical predictions.