Interlaminar shear test by hybrid sandwich specimen under distributed load

A new method is proposed for the determination of the interlaminar shear strength of composites. The method is particularly pertinent to composites of high interlaminar shear strengths, where the ratio of tensile (compressive) strength to shear strength is relatively low. In such materials, including unidirectional composites with improved fiber/matrix bond strength and angle-ply laminates, an analysis based on a short beam interlaminar shear test is highly problematic and may, in fact, be erroneous. The test method is based on the use of a sandwich composite structure with a core made of layers of the tested composite and skins made of an elastic, strong unidirectional composite. A proper design procedure determines the choice of the skin material and of the relative thicknesses, so that flexural testing under distributed load leads to the intended core failure in shear. Calculations of the stress profile in a hybrid sandwich beam in bending and of the stress ratios under distributed load are presented. Also presented are experimental results recorded with sandwich hybrids made of unidirectional carbon-fiber-reinforced epoxy skins and a ±θ aramid-fiber-reinforced epoxy angle-ply core.     

Studies on the flexural modulus of structural foams

An investigation based on I-Beam models was undertaken in this paper for extending knowledge regarding the flexural modulus of structural foam. The applicability of five I-Beams models (I-Beam A, B, C, D and E) including a newly developed one (I-Beam E) were investigated in this work. The square law model was used to predict Young's modulus of uniform density foam, which was subsequently utilized for the calculation of the I-Beam models. I-Beam A, B and E were observed from the configuration analysls of each I-Beam to be the more suitable models for predicting the flexural modulus of the structural foams having either an integral skin or a skin with limited residual bubbles, among which I-Beam E is considered to be better than I-Beam B and A. The comparison of the experimental and theoretical values of the flexural modulus of the structural foams molded with gas counter pressure structural foam (CPSF) and low pressure structural foam (LPSF) molding methods also confirmed that the newly developed I-Beam E is the most adequate model for predicting the flexural modulus of structural foams having either an integral skin or a skin with few residual bubbles. I-Beam B and A were also demonstrated to be in good agreement with the experimental data.Nomenclature B width of I-Beam A, B, C and E - Bc core width of I-Beam A - Bc₁ center core width of I-Beam - Bc₂ half of center core width of I-Beam - C adjustable parameter for density distri-iion of structural foam - CPSF gas counter pressure structural foam injection molding - D thickness of I-Beam in foamed core sec tion or thickness of structural foam in foamed core section - D_s thickness of unfoamed beam - e ratio of skin thickness to half of the thickness of a specimen (reduced skin thickness) - E¹ flexural modulus of structural foam - E_c average Young's modulus for foamed core of structural foam - E_s Young's modulus of unfoamed solid - FLBF flexural load bearing factor - GASF gas assisted structural foam injection molding - HPSF high pressure structural foam injection molding - I_c equivalent moment of inertia of I-Beam - I_s moment of inertia of unfoamed beam - LPSF low pressure structural foam injection molding - R reduced density for center core of structural foam - SCSF sandwich coinjection structural foam molding - T thickness of I-Beam in skin section or skin thickness of structural foam - Y half of the thickness of a specimen - Z dimensionless distance from neutral axis of a specimen subjected to pure bending Greek symbols local density of structural foam - ¹ average density of structural foam - _c average density of foamed core of structural foam - ^f density of uniform density foam - _s density of unfoamed solid     

Flexural properties of moldings of rigid polyurethane made by reaction injection molding

Flexural properties of moldings made by Reaction Injection Molding (RIM), which are structural foams consisting of high density skin and low density core, were investigated by three-point bending tests. Two failure modes were observed in bending tests of the moldings made by RIM, and they are classified as follows according to the density ratio of skin layer to core layer: the opposite side of the skin layer to which load was subjected failed by tensile stress: and the same side of the skin layer to which load was subjected failed by compressive stress, causing wrinkling or buckling. Then the conventional composite beam theory was applied to the former failure mode and Hoff s buckling theory to the latter, and equations were derived to predict the flexural properties of the structural foams, which involved buckling from the flexural properties of solid construction. In addition, it has been shown that there exists a density distribution that maximizes the flexural strength of the moldings made by RIM with a given overall density. The results obtained here should be useful to the optimum structural design of moldings made by RIM.     

High density polyethylene foams. IV. Flexural and tensile moduli of structural foams

High density closed‐cell polyethylene foams (450–950 kg/m³) were prepared by compression molding, and their flexural and tensile moduli were measured in order to study (1) the normalized modulus as a function of the normalized density, and (2) the effect of thin skins on flexural and tensile moduli. For the flexural data, it was found that the model of Gonzalez and the I‐beam model of Hobbs predicted the data very well in the range of void volume fractions under study (0–55%). For the tensile data, it was found that a combination of the differential scheme or the square power‐law model with the sandwich structure gave the best predictions. Finally, we found that thin skins have an important effect on the flexural properties of polymer foams, while they seem to have a negligible effect on the tensile properties. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 90: 2139–2149, 2003     

Flexural behavior of polyester polymer concrete

The effect of temperature, strain rate, resin content, void content and methods of preparation (vibration and compaction) on the overall flexural behavior of a polyester resin based polymer concrete is studied under three-point bending. The strength and modulus of polyester polymer concrete are relatively independent of strain rate but decrease at varying rates with increase in temperature. Compaction of polymer concrete during preparation reduces the void content and enhances both the flexural strength and modulus. Modifications to composite stiffness models have been proposed to include excess polymer and excess sand phases for systems other than the optimal system. Using a combination of parallel and series models, it is possible to predict the flexural modular ratio and flexural modulus of polymer concrete. Modified tensile strength models are effective in predicting the flexural strength ratio and flexural strength of polymer concrete.     

The effect of density profile on the flexural properties of structural foams

In this study, a comparison is made on the models available to predict the flexural modulus of structural polymer foams. Skin thickness and density profile in the core zone are taken into account to better relate foam morphology with mechanical response. It was found that including skin thickness is not sufficient to predict flexural moduli with high levels of precision and thus a transition zone between the skin and the core must be included. To this end, a single continuous equation is proposed to represent the complete density profile, which enabled us to predict our experimental data within 3%. POLYM. ENG. SCI., 47:1459–1468, 2007. © 2007 Society of Plastics Engineers     

Coextruded polyethylene and wood‐flour composite: Effect of shell thickness,wood loading,and core quality

Coextruded recycled polyethylene and wood‐flour composites with core–shell structure were manufactured using a pilot‐scale coextrusion line. The influence of wood loadings and thickness of the shell layer and core quality on mechanical and water absorption properties of the composites were investigated. Core–shell structured profile can significantly improve flexural and impact strengths of composites especially when a relatively weak core was used. However, the coextruded profile with unreinforced shell may have a reduced modulus when a strong core was used. The shell layer also protected coextruded composites from long‐term moisture uptaking, leading to improved dimensional stability compared with the corresponding un‐coextruded controls. When the shell thickness was fixed, less wood loading in the shell layer did not cause obvious flexural modulus and dimension change but improved impact strength and water resistance of the coextruded composites. When wood loading in the shell layer was fixed, increased shell thickness improved impact strength but affected modulus negatively. Thickened shell layer helped reduce water uptaking but did not change dimensional stability of coextruded composites remarkably. Overall enhancement of composite strength was more pronounced for the weaker core system. Thus, the coextrusion technology can be used to achieve acceptable composite properties even with a relatively weak core system—offering an approach to use recycled, low quality plastic‐fiber blends in the core layer. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

Mechanical properties of commingled plastic from recycled polyethylene and polystyrene

The mechanical properties of commingled plastic in the form of thick beams prepared by the ET-1 process have been examined in flexure and compression. The mechanical properties were evaluated in relationship to the hierarchical morphology described in a previous study. It was found that the flexural modulus was dominated by the properties of the skin and was satisfactorily modeled by approaches based on the observed micro-morphology, such as the Nielsen and Davis models. It was not necessary to consider the skin–core macromorphology because the flexural modulus was dominated by the void-free skin. The compressive modulus was lower than the flexural modulus and was strongly affected by the skin–core macro-morphology. From the differences between the flexural and compressive moduli, it was determined that the core was essentially nonload-bearing in compression. Flexural fracture initiated on the tension side of the beam and propagated rapidly through the thickness, whereas compressive failure occurred by longitudinal splitting of the skin. © 1994 John Wiley & Sons, Inc.     

Some mechanical properties of rigid polyurethane structural foam

The mechanical response of integral-skin rigid polyurethane foam, with an average density of 300 to 700 kg/m³, to constant rate and creep loading was determined. Sandwich specimens were modeled by layers of a core material and two skins, whose secant moduli had been determined experimentally by separate tests and approximated by linear functions of the density. The effective rigidities of the sandwich in tension and flexure were calculated and compared favorably to experimental measurements. The sandwich structure improved the flexural rigidity of homogeneous foam by a factor of more than 2.20. Tensile creep tests of sandwich specimens at relatively low stress levels (up to about 38 percent of their strength) showed that the creep was nonlinear, but a single creep curve could represent creep of specimens of various densities, provided the relative load on them was the same. A limited number of flexural creep tests led to similar conclusions, but the creep rate was smaller than in tension. Results from torsion tests of core material, compressive tests of sandwich specimens, and tension and compression tests of nonskin rigid foam are included in this article.     

A study of the mechanical properties of randomly oriented short banana and sisal hybrid fiber reinforced polyester composites

The mechanical performance of short randomly oriented banana and sisal hybrid fiber reinforced polyester composites was investigated with reference to the relative volume fraction of the two fibers at a constant total fiber loading of 0.40 volume fraction (V_f), keeping banana as the skin material and sisal as the core material. A positive hybrid effect is observed in the flexural strength and flexural modulus of the hybrid composites. The tensile strength of the composites showed a positive hybrid effect when the relative volume fraction of the two fibers was varied, and maximum tensile strength was found to be in the hybrid composite having a ratio of banana and sisal 4 : 1. The impact strength of the composites was increased with increasing volume fraction of sisal. However, a negative hybrid effect is observed when the impact strength of the composites is considered. Keeping the relative volume fraction of the two fibers constant, that is, banana : sisal = 0.32 : 0.08 (i.e., 4 : 1), the fiber loading was optimized and different layering patterns were investigated. The impact strength of the composites was increased with fiber loading. Tensile and flexural properties were found to be better at 0.40 V_f. In the case of different layering patterns, the highest flexural strength was observed for the bilayer composites. Compared to other composites, the tensile properties were slightly higher for the composite having banana as the skin material and sisal as the core material. Scanning electron micrographs of the tensile and impact fracture surfaces of the hybrid composites having volume fraction 0.20 and 0.40 V_f were studied. The experimental tensile strength and tensile modulus of hybrid composites were compared with those of theoretical predictions. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 96: 1699–1709, 2005     

Effect of microcellular foaming on the fracture behavior of ABS polymer

In this work, the properties of microcellular ABS were studied. Foamed samples exhibited a solid skin/foamed core structure, with some elongated cells in the flow direction, while spherical cells were mostly observed in the transversal direction. The flexural modulus, flexural strength, and fracture toughness K_Ic decreased with the density. However, the Crack Tip Opening Displacement (CTOD) was found to increase with the foaming ratio. The evolution of the mechanical properties and fracture toughness was well described by prediction models considering the skin/core morphology of these microcellular materials. Foaming increased the anisotropic behavior of the material, due to the cell elongation caused by the fountain flow during injection. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 43010.     

Physical and mechanical properties of pultruded composites containing fillers and low profile additives

This article deals with the effect of fillers and additives content on the physical and mechanical properties of unidirectional pultruded glass/polyester composites. The physical characterization consisted of determining the void volume fraction (Vv), density, shrinkage ratio, coefficient of thermal expansion (CTE), and dynamic mechanical properties. The mechanical tests consisted of three‐point‐bending tests under static, impact, and fatigue loading. The low profile additive (LPA) has been found to compensate the cure shrinkage by microvoid formation. Dynamic mechanical analysis measurements show that the LPA slightly lowers the glass transition temperature Tg and increases the internal damping Tan δ. The transverse coefficient of thermal expansion was found to be sensitive to the LPA content. Three‐point bending tests show that the interlaminar shear strength (ILSS) is slightly sensitive to the fillers and LPA content, but the flexural strength is not affected. Impact test results on short beam shear and flexural specimens show the same behavior as in the static tests except that the LPA content has a detrimental effect on the flexural impact properties. The fatigue tests performed show that the effect of fillers is not significant, while the LPA effect is mixed. It seems that there is an LPA content for which the fatigue resistance is maximized. Finally, the wide range of behaviors and span‐to‐depth ratios investigated suggest that the ILSS as measured according to the ASTM 2344 recommendations can be largely misleading because of the unavoidable compressive yielding under the loading nose. For the materials investigated, higher span‐to‐depth ratio are more representative of the ILSS. POLYM. COMPOS., 27:71–81, 2006. © 2005 Society of Plastics Engineers     

Study on mechanical properties and material distribution of sandwich plaques molded by co‐injection

In the sandwich injection molding process (co‐injection), two different polymer melts are sequentially injected into a mold to form a part with a skin/core structure. Sandwich molding can be used for recycling, improving barrier and electrical properties, or producing parts with tailored mechanical properties. In this study the evaluation of flexural modulus and impact strength of co‐injected plaques have been investigated. Virgin and short glass fiber reinforced (10 and 40%) polypropylene were used in six different combinations of sandwiched layers. The skin and core thicknesses were measured by optical microscopy and used to calculate the theoretical flexural modulus, which was compared to the experimentally measured modulus. Fiber orientation states were also observed by scanning electronic microscopy (SEM) at some specific locations and their effect on mechanical properties discussed. The experimental results indicate that an important improvement in transverse modulus, near the gate, is obtained when the virgin polypropylene (PP) is used as a skin and 40% short glass fiber polypropylene (PP40) as core. When both skin and core are made of PP40, the flexural moduli are slightly higher than conventionally injected PP40. POLYM. COMPOS. 26:265–275, 2005. © 2005 Society of Plastics Engineers.     

Studies on a fiber reinforced plastics honeycomb structure

Based on an adhesive and fabric screening program, a rubber-modified resole adhesive and a flexible grade plain weave glass fabric were found appropriate for the fabrication of a glass-reinforced plastics honeycomb core material. Sandwich structures of different densities were fabricated. A linear regression analysis was performed to correlate the mechanical properties (S) with the density (p) for a wide range of sandwich structures based on honeycomb and cellular plastic core materials. An analytical model of the form S = Kρⁿ was derived, with two empirical constants K and n. The density exponent n was between 1 ≤ n ≤ 2 in all cases. An equation was also derived to relate density with thermal conductivity. The dynamic mechanical analysis (DMA) results revealed that the glass transition temperature of the resin matrix associated with the sandwich structure was higher than that of the corresponding facing laminate or near resin casting.     

Mechanical properties of blocked polyurethane/epoxy interpenetrating polymer networks

The mechanical properties of blocked polyurethane(PU)/epoxy interpenetrating polymer networks (IPNs) were studied by means of their static and damping properties. The studies of static mechanical properties of IPNs are based on tensile properties, flexural properties, hardness, and impact method. Results show that the tensile strength, flexural strength, tensile modulus, flexural modulus, and hardness of IPNs decreased with increase in blocked PU content. The impact strength of IPNs increased with increase in blocked PU content. It shows that the tensile strength, flexural strength, tensile modulus, and flexural modulus of IPNs increased with filler (CaCO₃) content to a maximum value at 5, 10, 20, and 25 phr, respectively, and then decreased. The higher the filler content, the greater the hardness of IPNs and the lower the notched Izod impact strength of IPNs. The glass transition temperatures (T_g) of IPNs were shifted inwardly compared with those of blocked PU and epoxy, which indicated that the blocked PU/epoxy IPNs showed excellent compatibility. Meanwhile, the T_g was shifted to a higher temperature with increasing filler (CaCO₃) content. The dynamic storage modulus (E′) of IPNs increased with increase in epoxy and filler content. The higher the blocked PU content, the greater the swelling ratio of IPNs and the lower the density of IPNs. The higher the filler (CaCO₃) content, the greater the density of IPNs, and the lower the swelling ratio of IPNs. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 101: 1826–1832, 2006     

Application of rigid polyurethane integral-skin foams to thermal insulating unit cases for car air conditioners

Integral-skin foams of rigid polyurethane are sandwich structures consisting of a core layer of closed cells enclosed in rigid surface layers on both sides. We examined the layer composition of integral-skin foam with the objective of maximum flexural strength, and then studied possibilities of reconciling the strength and thermal insulating properties in housings for evaporators in car air conditioners; i.e., unit cases. This examination showed that the most practical density range (250 ≦ ρ_pall ≦ 500 kg/m³) provides vibratile resistance and thermal insulating properties. In actual car-running tests, a maximum 0.1 MPa stress was generated on unit cases with overall densities of 350 kg/m³, We found this to be 0.4% of the flexural strength of an integral-skin foam and 2% of the fatigue strength. In the forcible vibratile test, a stress of 0.5 to 1.0 MPa was generated at the resonance point of a unit case with 250 to 500 kg/m³ overall density. We found that these values are 2 to 5% of integral-skin foam's flexural strength and 10 to 25% of its fatigue strength. These values are of the same level as the conventional unit case made of polypropylene blended with talc. An integral-skin foam with an overall density of 250 kg/m³, nearly equal to half the weight of polypropylene, has the same level of resistance to vibration.     

Cell morphology,bubbles migration,and flexural properties of non‐uniform epoxy foams using chemical foaming agent

Epoxy foams with different densities and microstructures were prepared by changing the process parameters including the foaming temperature, chemical foaming agent (CFA) content and precuring extent. The microstructure of foams reveals a smaller cell size, higher cell density, and more homogeneous distribution of cells at higher precuring extent. However, the cell size and distribution are not affected by the foaming temperature and CFA content without precuring process. In addition, the bubbles migration, which resulted in non‐uniform cell density distribution, was promoted by increasing the foaming temperature and depressed by increasing the CFA content and precuring extent. The flexural properties of the non‐uniform epoxy foams were also studied. Results showed that the flexural modulus was related to the cell morphology, while the flexural strength was affected by both the cell morphology and the position of the specimens during test. It was also found that the relative flexural modulus and strength exhibited a power‐law dependence with respect to the relative density. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 41175.     

Mechanical properties of a particle‐strengthened polyurethane foam

Quasi‐static compression tests have been performed on polyurethane foam specimens. The modulus of the foam exhibited a power‐law dependence with respect to density of the form: E* ∝ (ρ*)ⁿ, where n = 1.7. The modulus data are described well by a simple geometric model (based on the work of Gibson and Ashby) for a closed‐cell foam in which the stiffness of the foam is governed by the flexure of the cell struts and cell walls. The compressive strength of the foam is also found to follow a power‐law behavior with respect to foam density. In this instance, Euler buckling is used to explain the density dependence. The modulus of the foam was modified by addition of gas‐atomized, spherical, aluminum powder. Additions of 30 and 50 wt % Al measurably increased the foam modulus, but without a change in the density dependence. However, there was no observable increase in modulus with 5 and 10 wt % additions of the metal powder. Strength was also increased at high loading fractions of powder. The increase in modulus and strength could be predicted by combining the Gibson–Ashby model, referred to above, with a well‐known model describing the effect on modulus of a rigid dispersoid in a compliant matrix. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 74: 2724–2736, 1999     

纳米碳酸钙填充PLLA和PCL复合材料弯曲性能的研究

研究了纳米碳酸钙(nano-CaCO3)含量对填充聚左旋乳酸(PLLA)、聚己内酯(PCL)复合材料弯曲性能的影响及不同比例PLLA、PCL共混后的弯曲性能变化规律。结果表明:随着nano-CaCO3含量的增加,PLLA复合体系弯曲强度下降,而弯曲模量则先增大后减小;PLLA/PCL/nano-CaCO3复合体系的弯曲模量符合经典混合法则,弯曲强度随共混比的变化呈现出高度的线性规律。     

熔融沉积成型木塑复合夹层板的弯曲性能

采用熔融沉积成型(FDM)制造方法,以木塑复合线材为原料,利用3D打印软件Ultimaker Cura的“填充结构”功能设计网格、直线、三角形等13种芯层结构(二维6种、立体7种),并将其与纸板粘接得到木塑复合夹层板。利用三点弯曲测试,研究不同夹层板的破坏失效形式与弯曲性能。结果表明:木塑夹层板的失效模式主要有弹性变形、面板起皱、芯子剪切和芯子压溃。在13种芯层结构中,立体的同心3D芯层结构夹层板弯曲性能最佳,弯曲模量和静曲强度分别为159.56 MPa和4.85 MPa,分别是网格芯层结构夹层板的5.4倍和2.3倍,具有较强的抗弯曲变形能力,适合于设计制造轻质高强度制品。