Tensile properties of glass microballoon‐epoxy resin syntactic foams

The effect of hollow glass particle (microballoon) volume fraction in the range of 0.3–0.6 on the tensile properties and fracture mode of syntactic foams is characterized in the present research. Sixteen types of syntactic foams have been fabricated and tested. Four types of glass microballoons, having 220, 320, 380, and 460 kg/m³ density, are used with epoxy resin matrix for making the syntactic foam samples. These foams contain 30, 40, 50 and 60% microballoons by volume. All types of microballoons have the same size but different wall thickness, which reflects as a difference in their density. It is observed that the tensile strength increases with a decrease in the volume fraction of microballoons. All types of syntactic foams showed 60–80% decrease in the tensile strength compared with that of the neat resin. The foams containing low strength microballoons showed lower tensile modulus compared with that of the neat resin, but the presence of high strength microballoons led to an increase in the tensile modulus of the composites. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 102: 1254–1261, 2006     

Rheokinetic studies and compressive response of high performance polybenzoxazine syntactic foams

Polybenzoxazines are finding increasing usage in demanding applications where high temperature stability is required, especially in the field of aerospace. In this work, thermally stable bisphenol F-based polybenzoxazine [poly(BF-a)] syntactic foams containing varying volume fractions (30–60%) of hollow glass microballoons (HGMs) were prepared and their mechanical response in the quasi-static regime was established. The effect of introducing glass microballoons on the curing profile of benzoxazine resin was studied using both nonisothermal differential scanning calorimetry and rheometry. Temperature-sweep experiments were performed to arrive at the optimal processing window of the benzoxazine-glass microballoons formulations, particularly in terms of viscosity, gelation temperature, and time. Thermally accelerated ring-opening polymerization of the benzoxazine resin led to complete curing of the syntactic foam formulations, as assessed by calorimetric studies. The thermal degradation behavior of the poly(BF-a)/HGM was studied using thermogravimetric analysis. As expected, the density of the syntactic foam specimens decreased with increasing microballoon content. Maximal increase in the specific compressive properties of the poly(BF-a)/HGM samples was observed in formulations containing 40% volume fraction of glass microballoons. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 47234.     

Mechanical properties of polybenzoxazine syntactic foams

Syntactic foams of polybenzoxazine, containing moderately high volume percentage of glass microballoons, were prepared. The specific gravity decreased with increase in microballoon content. The disproportionate decrease in specific gravity was ascribed to entrapment of air voids during compaction. The high content of microballoon increased the possibility for air voids that tended to get accumulated. The effect of microballoon concentration on tensile, compressive, and flexural strengths of the foams was studied. Tensile and compressive properties were optimized at about 68% by volume of microballoon while flexural strength decreased marginally on increasing the microballoon content. Althought the specific tensile and compressive strength showed a maximum followed by a decrease, the specific flexural strength systematically increased with microballoon content. The increased packing density of syntactic foam of a given constituent composition increased the compressive strength. The property variation was corroborated by morphological features, as evidenced in scanning electron micrographs. The syntactic foams showed “multiple resin‐neck formation” and “disc‐shaped microballoon regions.” The crushing of microballoons during molding was inevitable when compaction was effected to achieve a density beyond the theoretical one. Low‐density syntactic foams tend to fail at lower loads because of fracturing of microballoons. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Short‐beam three‐point bend test study in syntactic foam. Part III: Effects of interface modification on strength and fractographic features

Syntactic foam slabs having uncoated microballoons and paraffin oil surface‐treated microballoons were fabricated and tested for short‐beam three‐point bend test. The work points to the role of paraffin oil coating first weakening the interface between the microballoons and the matrix and hence lowering the efficiency of load transfer from matrix to the fillers (i.e., microballoons). This led to an overall decrement of 71% in the experimentally measured strength value compared to the deduced value for uncoated microballoons' specimens. The large strengths for uncoated microballoons specimens can be traced to the presence of the curvilinear marks in the matrix that, incidentally, are absent in the case of paraffin oil coated specimens. These observations are revealed distinctly in the microscopy of test‐failed specimens. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 98: 687–693, 2005     

A micromechanical model for quasi-brittle compressive failure of glass-microballoons/thermoset-matrix syntactic foams

We propose a micromechanical model for the quasi-brittle failure of syntactic foams subject to uniaxial compression. We focus on a failure characterised by shear bands inclined of about 45° with respect to the loading axis, often observed in thermoset polymers filled with glass microballoons. Our objective is to develop a three-dimensional Finite Element (FE) model for the effective compressive strength. Towards this aim, we extend our previous FE models, which include fifty randomly placed balloons and were developed to assess the accuracy of linear elastic homogenisation procedures for syntactic foams. Here, we account for the filler polydispersion and introduce a novel structural failure criterion for the glass microballoons. The proposed models are shown to be macroscopically isotropic with respect to the effective strength. We find good agreement with experimental results from the literature on syntactic foams with filler volume fraction of 60%, for which we assume the matrix to be linear elastic.     

Influence of chopped strand fibres on the flexural behaviour of a syntactic foam core system

The comparative performance in a three point bending test of syntactic foam comprising epoxy resin and glass microballoons with and without the inclusion of glass fibre in the form of chopped strands is reported. Test samples having a span‐to‐depth ratio of 16:1 were used. The data show that the glass fibre reinforced foam system had a higher strength compared to the unreinforced system. Resorting to light macroscopic and scanning electron microscopic examinations on mechanically tested samples expanded the scope of the work for a structure–property correlation to emerge. © 2000 Society of Chemical Industry     

连续玻璃纤维增强聚氨酯复合材料的力学性能

为了代替传统的钢制鱼尾板与绝缘部件组成的"机械绝缘接头",通过拉挤成型制备了连续玻璃纤维(GF)质量分数高达70.5%的聚氨酯/玻璃纤维(PUR/GF)复合材料。分别对复合材料在0°和90°方向进行了拉伸、弯曲和压缩性能测试;同时,结合扫描电子显微镜(SEM)观察了拉伸断口,分析了GF在PUR基体中的分布情况及复合材料在拉伸试验中的断裂机理。研究结果表明,0°方向的拉伸强度、弯曲强度以及压缩强度都有很明显的提高,且均符合钢轨鱼尾板的强度标准。     

Tensile properties and damage behaviors of glass‐bead‐filled modified polyphenylene oxide under large strain

Based on Continuum Damage Mechanics (CDM), a damage model for glass‐bead‐filled modified polyphenylene oxide (GB/PPO) has been proposed to describe its damage behavior at various levels of tensile strain by considering the reduction of effective loading area. Hence, an equation for prediction of effective elastic modulus of the damaged GB/PPO composites in terms of the three principal true strains was derived. The tensile properties and damage behaviors of the GB/PPO composites with different volume percentages of glass beads were investigated using standard tensile tests and load‐unload tests, respectively. The addition of glass beads increases Young's modulus of PPO but has a weakening effect on its tensile strength. A maximum value of tensile work to break and tensile strain at break was found when 5 vol% of glass beads with a mean diameter of 11 μm was blended with PPO. These results were justified through microscopic examination of the fracture surfaces of the tensile specimens by using a scanning electron microscope (SEM). In‐situ observations of the strain damage processes were made through the SEM equipped with a tensile stage to determine the strain at fully debonding of glass beads. The volumetric strain of GB/PPO composites increases because of microcavitation during strain damage. In general, the prediction for the effective elastic modulus of the damaged GB/PPO composites at different true strains is slightly higher than the experimental results. The damage evolution rates after fully debonding of glass beads from the matrix are close to those predicted by the proposed damage model.     

Hot water resistance of glass‐fiber and glass‐bead reinforced thermoplastics

The hot water resistance of three kinds of short glass fiber or glass bead‐reinforced plastics [polyphenyleneether (PPE), polyphenylenesulfide (PPS), and polyoxymethylene (POM)] was studied by hot water immersion testing, tensile testing and water‐hammer fatigue testing. It was found that the degradation of the strength was observed only for the reinforced plastics under hot water immersion and that the change of the tensile strength was most drastic in glass fiber‐reinforced PPS (GFPPS). Scanning electron microscope (SEM) observations of the tensile fracture surface revealed that the change in tensile strength was attributable to the deterioration of the interface between the glass fiber and the matrix resin. The results of acoustic emission analysis also supported the conclusion that the change in strength was due to the deterioration of the interface. Although the change in the tensile strength of glass fiber‐reinforced PPE (GFPPE) was small compared with that of GFPPS, debonding between the glass fiber and the matrix resin and surface cracks was observed on the surface of the GFPPE specimens.     

环氧树脂基复合泡沫材料组成与应用

介绍了环氧树脂基复合泡沫材料的组成,概括其组成成分空心微珠与基体的种类及发展概况。分析了空心微珠表面改性的方法,微珠在基体中的分布对材料性能的影响。综述了复合泡沫材料在不同领域的应用,并对其应用前景进行了展望。     

Poly(dimethylsiloxane)‐toughened syntactic foams

The potential of preformed elastomers as a toughening agent for epoxy–glass syntactic foam has been explored. Poly(dimethylsiloxane) microspheres were prepared by suspension polymerization. The microsphere dimensions could be varied from 58 to 255 µm by tuning the reaction parameters, particularly the stirring speed and feed concentration. Rheological studies indicated that the introduction of microballoons led to an increase in the viscosity of the resin, with the extent being proportional to the microballoon content. The zero shear viscosity increased from ～10³ mPa s at 30 °C to 10⁵ mPa s as the microballoon loading was increased to 40%. Syntactic foams containing varying amounts of microballoons (40–60% v/v) were prepared, and an analogous set of toughened foams were also prepared, where a fraction of the microballoons was replaced with poly(dimethylsiloxane) microspheres (3–7%). The effect of increasing dimensions of the elastomeric microspheres on the mechanical properties was also studied. The improvement in properties was more pronounced when the microsphere size was equivalent to that of the constituent microballoons. An improvement of 40% and 185% in flexural strength and flexural toughness was observed upon the introduction of poly(dimethylsiloxane) microspheres of optimal dimensions (diameter ～63 µm, 5% loading), without any undesirable increase in foam density. However, the compressive properties remained practically unaltered. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018 , 135, 45882.     

Study on the performance of syntactic foam reinforced by hybrid functionalized carbon nanotubes

Hollow glass microspheres (HGMs)/epoxy syntactic foam were reinforced by hybrid functionalized carbon nanotubes that were synthesized by simultaneous covalent and noncovalent functionalization of carbon nanotubes. The effect of hybrid functionalized carbon nanotubes on density, mechanical properties, and water absorption of HGMs/epoxy syntactic foam was studied. The study indicated that the dispersion of carbon nanotubes in epoxy resin can be improved by hybrid functionalization. The compression strength of syntactic foam reinforced by hybrid functionalized carbon nanotubes was significantly enhanced. The maximum compressive strength of syntactic foam corresponding to chitosan modified carbon nanotubes approached 60 MPa. Hybrid functionalized carbon nanotubes had little effect on the water absorption ability of syntactic foam, and was less than 1%. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2020 , 137, 48586.     

Interpreting acoustic energy emission in SiC/SiC minicomposites through modeling of fracture surface areas

The relationship between acoustic emission (AE) and damage source areas in SiC/SiC minicomposites was modeled using insights from tensile testing in-scanning electron microscope (SEM). Damage up to matrix crack saturation was bounded by: (1) AE generated by matrix cracking (lower bound) and (2) AE generated by matrix cracking, and fiber debonding and sliding in crack wakes (upper bound). While fiber debonding and sliding exhibit lower strain energy release rates than matrix cracking and fiber breakage, they contribute significant damage area and likely produce AE. Fiber breaks beyond matrix crack saturation were modeled by two conditions: (i) only fiber breaks generated AE; and (ii) fiber breaks occurred simultaneously with fiber sliding to generate AE. While fiber breaks are considered the dominant late-stage mechanism, our modeling indicates that other mechanisms are active, a finding that is supported by experimental in-SEM observations of matrix cracking in conjunction with fiber failure at rupture.     

Numerical simulation of the mechanical properties of a carbon-fiber-reinforced hollow glass microsphere–epoxy syntactic foam

The addition of carbon fibers has a great influence on the mechanical properties of hollow glass microsphere (HGM)–epoxy syntactic foam. Thus, to elucidate the reinforcement mechanism, the numerical simulation of HGM- and carbon-fiber-filled epoxy matrixes was carried out. The effect of the orientation of carbon fibers on the elastic modulus and stress distribution was studied. The effect of the elastic modulus of the matrix on the change of force was also studied. We noted that the orientation of carbon fibers affected the elastic modulus of the matrix, and when the carbon fibers were distributed in the direction of force, the elastic modulus of the matrix reached its maximum. The maximum stress of HGMs decreased with increasing matrix elastic modulus, and the mechanical properties of the syntactic foam increased with increasing elastic modulus of the matrix. When the carbon fibers were distributed in the direction of the force, the enhancement effect was the best. Because the carbon fibers had a higher elastic modulus than the matrix, the degree of compressive deformation of the carbon fibers was smaller than that of the matrix. During compression, carbon fibers were pulled out and consumed a lot of energy. Thus, the mechanical properties of the syntactic foam were improved. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 47083.     

Processing and properties of syntactic foams reinforced with carbon nanotubes

This article presents synthesis and mechanical characterization of carbon nanotube (CNT)‐reinforced syntactic foams. Following a dispersion approach (comprising ultrasonic, calendering, and vacuum centrifugal mixing), single‐ and multi‐walled functionalized CNTs (FCNTs) were incorporated into two foam composites containing various commercially available microballoon grades (S38HS, S60HS, and H50 from 3M). The FCNT‐reinforced composites were tested for compressive strength and apparent shear strength before and after hot/wet conditioning. The results showed that the FCNT‐reinforced composites' mechanical properties depended on the vacuum pressure used during processing. Compared with pristine and commercially available syntactic foam (EC‐3500 from 3M), the FCNT‐reinforced composites processed at high vacuum (0.2 kPa) showed significant increase in compressive strength and apparent shear strength before and after hot/wet conditioning. Dynamic mechanical analysis showed an increase of about 22°C in glass transition temperature for composites processed at high vacuum with 0.5 wt % FCNT and 45 wt % S38HS–5 wt % S60HS microballoons. Thermogravimetric analysis indicated water absorption and lower decomposition temperature for the FCNT‐reinforced composite mixed at atmospheric pressure, whereas no significant change was observed for the compound processed at high vacuum. Fracture analysis showed matrix failure for the composite processed at high vacuum and microballoon crushing for the composite mixed at atmospheric pressure. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Experimental study on the mechanical behavior and failure mechanism of 3d MWK carbon/epoxy composites under quasi‐static loading

Quasi‐static tensile, out‐of compression, in‐plane compression, three‐point‐bending and shear tests were conducted to reveal the mechanical behavior and failure mechanisms of three‐dimensional (3D) multiaxial warp‐knitted (MWK) carbon/epoxy composites. The characterization of the failure process and deformation analysis is supported by high‐speed camera system and Digital Image Correlation. The results show that tensile, bending, out‐of‐plane compression, in‐plane compression stress–strain response exhibit obvious linear elastic feature and brittle fracture characteristics, whereas the shear response exhibits a distinct nonlinear behavior and gradual damage process. Meanwhile, 3D MWK carbon/epoxy composites have good mechanical properties, which can be widely used in the fields of engineering. In addition, the failure for tension behaves as interlayer delaminating, 90/+45/−45° interface debonding and tensile breakage of 0° fibers; the damage for out‐of‐plane compression is mainly interlaminar shear dislocation together with local buckling and shear fracture of fibers; the failure pattern for in‐plane compression is 90° fiber separating along fiber/matrix interface as well as 0/+45/−45° fiber shear fracture in the shear plane. The main failure for bending is fiber/matrix interface debonding and fibers tearing on the compression surface, 0° fibers breakage on the tension surface as well as fiber layers delaminating. Although the shear behavior is characterized by a gradually growing shear matrix damage, 90/+45/−45° interface debonding, +45/−45° fibers shear fracture, and final 0° fiber compression failure. POLYM. COMPOS., 37:3486–3498, 2016. © 2015 Society of Plastics Engineers     

The 3D failure process in polymeric syntactic foams with different cenosphere volume fractions

The previous work (Huang and Li, Compos. Part B, 2015) proposed the failure mechanism in syntactic foams with low and high hollow microsphere volume fractions, based on the finite element simulation of localized stresses in the foam. In this work, in situ X-ray microtomography of uniaxial compression tests was performed to provide the direct experimental evidence to the proposed mechanism by tracking the internal three-dimensional failure process in epoxy syntactic foams with different cenosphere volume fractions (V). It was found that for both the low and high V, microcracks initiate in the matrix in the top and bottom of crushed cenospheres where the tensile stress concentrates, and then propagate longitudinally to become macrocracks. Increasing the cenosphere volume fraction also leads to the formation of matrix microcracks in the connection zone where the stress concentrates significantly; the matrix microcracks thus propagate diagonally and longitudinally in the high V foam. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 47491.     

Nanoclay‐reinforced syntactic foams: Flexure and thermal behavior

Syntactic foams containing 60 vol% of hollow glass microballoons in epoxy matrix are modified with untreated nanoclays using combined mechanical and ultrasonication methods. Effects of nanoclays on flexure and thermal behavior of syntactic foams are investigated by adding different amount of nanoclays in the range of 1–3% by weight. Microscopic examinations and physical property characterization are performed to determine the interactions among constituent materials and the void formation during fabrication. It is found that the syntactic foams with 2 wt% nanoclays show the highest improvement in flexural properties (∼42% strength and ∼18% modulus) and dynamic mechanical properties (∼30% storage modulus and ∼28% loss modulus) properties. Thermal decomposition temperature is found to be unaffected by the addition of nanoclays, whereas a continuous reduction in the coefficient of thermal expansion (CTE) is observed. An examination of failure surface indicates that the failure is initiated on the tension side of the flexure sample due to fracturing of microballoons. POLYM. COMPOS., 31:1332–1342, 2010. © 2009 Society of Plastics Engineers     

Viscoelastic behavior of flexible slabstock polyurethane foams: Dependence on temperature and relative humidity. I. Tensile and compression stress (load) relaxation

The relaxation behavior of the load in compression and the stress in tension was monitored at constant temperature and/or relatively humidity for a set of four slabstock foams with varying hard-segment content as well as two of the compression molded plaques of these foams. The majority of the compression relaxation tests were done at a 65% strain level in order to be consistent with the common ILD test. The tensile stress relaxation tests were performed at a 25% strain level. Over the 3-h testing period, a linear relationship between the log of compressive load or the log of tensile stress versus log time is observed for most testing conditions. For linear behavior, the values of the slope or the load/stress decay rate are comparable in both the tension and compression modes with the values being slightly higher in magnitude for the compression mode. These rates of decay are in the range of ?2.2 × 10 ^?2 to ?1.7 × 10 ^?2 for a 21 wt % hard-segment foam and ?3.2 × 10^?2 to ?2.4 × 10^?2 for a 34 wt % hard-segment foam. Increasing %RH at a given temperature does bring about a steady decrease in the initial load or initial stress as well as a slight increase in the rate of relaxation. The effect of temperature on the relaxation behavior is most significant at temperatures near 125°C and above. The FTIR thermal analysis of the plaques indicates that this significant increase is due to additional hydrogen bond disruption and possible chain scission taking place in the urea and urethane linkages that are principally present in the hard segment regions. The relaxation behavior in both tension and compression is believed to be mostly independent of the cellular texture of the foam at the strain levels given above. This conclusion is based on the similar relaxation behavior between the plaques and the foams. © 1994 John Wiley & Sons, Inc.     

Tensile fatigue behavior of plain-weave reinforced Cf/C–SiC composites

The fatigue behavior of plain-weave C_f/C–SiC composites prepared by liquid silicon infiltration (LSI) was studied under cyclic tensile stress at room temperature. The specimens were loaded with stress levels of 83% and 90% of the mean static tensile strength for 10⁵ cycles. The cross-sections and fracture surfaces of the fatigued specimens were examined by optical microscopy (OM) and scanning electron microscopy (SEM), respectively. The results show that the specimens can withstand 10⁵ fatigue cycles with a stress level of 90% of the static tensile strength. The retained strengths after fatigue for 10⁵ cycles with stress levels of 83% and 90% are about 19% and 11% higher than the static tensile strength. Due to the observation of the microstructures a relief of the thermal residual stress (TRS) caused by stress-induced cracking is probably responsible for the enhancement. Furthermore, the fracture surfaces indicate that the fatigue stress results in interfacial debonding between the carbon fiber and matrix. Additionally, more single-fiber pull out was observed within the bundle segments of fatigued specimens.