Mechanical behavior of glass/epoxy tubes under combined static loading. Part II: Validation of FEA progressive damage model

Experimental results from a series of biaxial static tests of E-Glass/Epoxy tubular specimens [±45]₂, were compared successfully with numerical predictions from thick shell FE calculations. Stress analysis was performed in a progressive damage sense consisting of layer piece-wise linear elastic behavior, simulating lamina anisotropic non-linear constitutive equations, failure mode-dependent criteria and property degradation strategies. The effect of accurate modeling of non-linear shear stress–strain response, dependent on the plane stress field developed, was proved of great importance for the numerical FEA predictions, concerning macroscopic stress–strain response. Ultimate load prediction was influenced more decisively when degradation strategies for the compressive strength along the fiber direction were considered.     

Failure prediction for a glass/epoxy cruciform specimen under static biaxial loading

Material models were developed to predict the mechanical behavior of glass/epoxy multidirectional laminates under complex stress states. An incremental plane stress analysis was performed, taking into account the anisotropic material non-linearity, separate damage onset conditions and distinct post-failure stiffness degradation rules. Theoretical formulations were implemented in a shell element of the 1st order shear deformation theory. Numerical results were validated via comparison with test data from cruciform specimens subjected to static biaxial tensile loading. Local strain gauge and full-field strain measurements, obtained using the Digital Image Correlation (DIC) technique, corroborated numerical predictions. Improved strength and failure mode results were derived when, in addition to stiffness reduction, compressive strength degradation in the fiber direction was also considered.     

Development of viscoelastic/rate-sensitive-plastic constitutive law for fiber-reinforced composites and its applications. Part I: Theory and material characterization

The viscoelastic/rate-sensitive plastic constitutive law to describe the nonlinear, anisotropic/asymmetric and time/rate-dependent mechanical behavior of fiber-reinforced (sheet) composites was developed under the plane stress condition. In addition to the theoretical aspect of the developed constitutive law, experiments to obtain the material parameters were also carried out for the woven fabric composite based on uni-axial tension and compression tests as well as stress relaxation tests, while the numerical formulation and verifications with experiments are discussed in Part II.     

Behaviour of ±45° glass/epoxy filament wound composite tubes under quasi-static equal biaxial tension–compression loading: experimental results

The primary aim of this paper is to present results describing in detail the behaviour of ±45° E-glass/MY750 (GRP) tubes, of various wall thicknesses, subjected to equal biaxial tension–compression loading, generated under combined internal pressure and axial compression. The role played by the non-linear lamina shear has also been assessed by comparing various shear stress–strain curves for embedded laminae (extracted from tests on ±45° tubes subjected to circumferential: axial stress ratios SR=1:0, 1:−1 and 2.3:−1) with that of an ‘isolated’ lamina (measured from torsion of 90° tubes). Extracted shear failure strains, for embedded laminae, were more than four fold larger than those measured at ultimate failure for an ‘isolated’ lamina. Soft characteristics were observed in the embedded lamina and these were believed to be due to interaction between early matrix damage initiation (and propagation) and shear. Factors affecting the behaviour of the tubes, such as bulging, scissoring, thermal stresses and stress variation through the thickness are discussed.     

The effects of geometric parameters on the failure strength for pin-loaded multi-directional fiber-glass reinforced epoxy laminate

A numerical and experimental study was carried out to determine the failure of mechanically fastened fiber-reinforced laminated composite joints. E/glass–epoxy composites were manufactured to fabricate the specimens. Mechanical properties and strengths of the composite were obtained experimentally. Tests have been carried out on single pinned joints in [0/90/0]_s and [90/0/90]_s laminated composites. A parametric study considering geometries was performed to identify the failure characteristics of the pin-loaded laminated composite. Data obtained from pin-loaded laminate tests were compared with the ones calculated from a finite element model (PDNLPIN computer code). Damage accumulations in the laminates were evaluated by using Hashin's failure criteria combined with the proposed property degradation model. Based on the results, ply orientation and geometries of composites could be crucial for pinned laminated composite joints.     

Strengths of C/C composites under tensile, shear, and compressive loading: Role of interfacial shear strength

Various strengths of carbon–carbon composites (C/Cs) are comprehensively reviewed. The topics reviewed include tensile, shear, compressive, and fatigue strength as well as fiber/matrix interfacial strength of C/Cs. When data are available, high temperature properties, including creep behavior, are presented. Since C/Cs have extremely low fiber/matrix interfacial strength τ_d, the interfacial fracture plays important roles in all of the fracture processes dealt in this review. The low τ_d was found to divide tensile fracture units into small bundles, to seriously degrade both shear and compressive strength, and to improve fatigue performance. In spite of the importance of the interfacial strength of C/Cs, techniques for its evaluation and analysis are still in a primitive stage.     

Platelet-reinforced polymer matrix composites by combined gel-casting and hot-pressing. Part I: Polypropylene matrix composites

Platelet-reinforced polymer matrix composites were fabricated by a combined gel-casting and hot-pressing method. Submicrometer-thin alumina platelets were dispersed in a highly diluted grafted maleic anhydride polypropylene solution. Upon cooling, the polymer formed a gel which trapped the platelets in their well separated positions. During subsequent solvent evaporation, the polymer–platelet gel densified and the platelets were oriented horizontally. The dried composites were hot-pressed to further improve the platelet orientation and increase the density of entanglements in the polymer. This method combines several advantages of large scale and lab-scale fabrication methods in that it is fast, simple but also versatile. Composites with platelet volume fractions up to 0.5 were easily fabricated. The maximal achieved yield strength and elastic modulus of the composites were 82% and 13 times higher, respectively, than the values of the polymer alone. The enhancement in the composites mechanical properties was caused by classical load transfer into the platelets as the crystallinity of the polymeric matrix was not affected by the platelets. Alumina platelets with an aspect ratio below the critical value allowed for the ductile platelet pull-out fracture mode enabling large plastic deformation of the composites prior to fracture. At high concentrations of platelets, the strength and stiffness decreased again and the ductility was almost lost due to out-of-plane misalignment of platelets and the increasing number and size of voids incorporated during the fabrication. The designing principles and fabrication method described in this work can potentially be extended to other types of polymers and platelets to create new composites with tailored properties.     

Micromechanics of hemp strands in polypropylene composites

The present paper investigates micromechanics of hemp strands. The main objective of the present work has been the determination of the intrinsic strength of hemp strands. Hemp strands have been used as reinforcement of Polypropylene composites. Different percentages of hemp strands and coupling agents (MAPP) have been tested to obtain a map of the mechanical properties of that kind of composites and the effect of the components on the final properties. Mechanical properties of the different specimens have been tested using standard experimental methods and equipment. Micromechanics of the strands have been obtained using Hirsch model, Bowyer–Bader methodology and Kelly-Tyson model.     

Fatigue crack growth in thin notched woven glass composites under tensile loading. Part I: Experimental

Helicopter blades are made of composite materials mainly loaded in fatigue and have normally relatively thin skins. A through-the-thickness crack could appear in these skins. The aim of this study is to characterize the through-the-thickness crack propagation due to fatigue in thin woven glass fabric laminates. A technological test specimen is developed to get closer to the real loading conditions acting on these structures. An experimental campaign is undertaken which allows evaluating crack growth rates in several laminates. The crack path is linked through microscopic investigations to specify damage in woven plies. Crack initiation duration influence on experimental results is also underlined.     

Cryogenic delamination growth in woven glass/epoxy composite laminates under mixed-mode I/II fatigue loading

This paper investigates the fatigue delamination growth behavior in woven glass fiber reinforced polymer (GFRP) composite laminates under mixed-mode I/II conditions at cryogenic temperatures. Fatigue delamination tests were performed with the mixed-mode bending (MMB) test apparatus at room temperature, liquid nitrogen temperature (77 K) and liquid helium temperature (4 K), in order to obtain the delamination growth rate as a function of the range of the energy release rate, and the dependence of the delamination growth behavior on the temperature and the mixed-mode ratio of mode I and mode II was examined. The energy release rate was evaluated using three-dimensional finite element analysis. The fractographic examinations by scanning electron microscopy (SEM) were also carried out to assess the mixed-mode fatigue delamination growth mechanisms in the woven GFRP laminates at cryogenic temperatures.     

Damage modes in 3D glass fiber epoxy woven composites under high rate of impact loading

A qualitative analysis of experimental results from small caliber ballistic impact and dynamic indentation on a 3D glass fiber reinforced composite are presented. Microscopic analysis of the damaged specimens revealed that the current 3D weaving scheme creates inherently two weak planes which act as potential sites for delamination in the above experiments. It is concluded that while the z-yarns may be effective in limiting the delamination damage at low loads and at low rates of impact, at high loads and high loading rates delamination continues to be the dominant failure mode in 3D woven composites. It is shown that dynamic indentation can be used to capture the progression of damage during impact of 3D woven composites.     

Mechanical and thermal behavior of non-crimp glass fiber reinforced layered clay/epoxy nanocomposites

Mechanical and thermal properties of non-crimp glass fiber reinforced clay/epoxy nanocomposites were investigated. Clay/epoxy nanocomposite systems were prepared to use as the matrix material for composite laminates. X-ray diffraction results obtained from natural and modified clays indicated that intergallery spacing of the layered clay increases with surface treatment. Tensile tests indicated that clay loading has minor effect on the tensile properties. Flexural properties of laminates were improved by clay addition due to the improved interface between glass fibers and epoxy. Differential scanning calorimetry (DSC) results showed that the modified clay particles affected the glass transition temperatures (T_g) of the nanocomposites. Incorporation of surface treated clay particles increased the dynamic mechanical properties of nanocomposite laminates. It was found that the flame resistance of composites was improved significantly by clay addition into the epoxy matrix.     

Experimental analyses of SFRP material under static and fatigue loading by means of thermographic and DIC techniques

The mechanical properties of injection moulded short glass fibre-reinforced polyamide composites was investigated using experimental techniques. The Digital Image Correlation technique was applied during the tensile tests in order to obtain the stress–strain curves and to identify the fracture location at the early stages of the tests. Moreover, the thermographic technique was used during the static tests in order to identify the fracture zone and also during the fatigue tests, carried out at different frequencies, to study the temperature evolution of the specimen. A theoretical model was developed to analyze the temperature evolution during the fatigue test and the effect of the test frequency. The aim of this study is the application of the Thermographic Method for the fatigue assessment of short glass fibre-reinforced polyamide composites.     

Platelet-reinforced polymer matrix composites by combined gel-casting and hot-pressing. Part II: Thermoplastic polyurethane matrix composites

A combined gel-casting and hot-pressing method was used to fabricate platelet-reinforced polymer matrix composites. Submicrometer thin alumina platelets were dispersed in a highly diluted polymer solution. A thermoplastic polyurethane elastomer was used as matrix for its high elasticity and excellent adhesion to the platelets. After dissolution of the polymer and casting, quick evaporation of the solvent triggered the formation of a polymer gel trapping the platelets in their well dispersed positions. The polymer–platelet gel densified during drying and the platelets were oriented horizontally due to the capillary forces and the large decrease in the thickness of the gel. The dried composites were hot-pressed to further improve the platelet orientation along the shear flow and close potential pores in the polymer. While the ultimate tensile strength of the composites gradually decreased with increasing platelet volume fractions, the increase in the elastic modulus and the stress necessary to deform the composite 10% was more than 100 and 18 times higher than the respective values of the pure polymer. The use of alumina platelets with an aspect ratio below the critical value allowed for the ductile platelet pull-out fracture mode. Since the polymer had to deform more to achieve identical deformation of the composite at higher platelet volume fraction, the strain at rupture steadily decreased. The incorporation of voids towards high platelet concentrations and the thereby triggered crack initiation and growth during straining lead to an additional decrease in the elasticity of composites with increasing platelet volume fractions. However, the extremely high extensibility of a polymer matrix allowed for the fabrication of composites that still deformed up to 162 ± 19% at platelet volume fractions as high as 0.33. When compared to other platelet-reinforced elastomers, the achieved platelet volume fraction is much higher and the relative increase in elastic modulus and stress at low strains is therefore much larger at the expense of a decrease in the strain at rupture. The fabrication method and designing principles employed in this study are transferable to other types of polymers and platelets and potentially allow the creation of new composites with tailored properties.     

The static and high strain rate behaviour of a commingled E-glass/polypropylene woven fabric composite

In this paper the effect of strain rate on the tensile, shear and compression behaviour of a commingled E-glass/polypropylene woven fabric composite over a strain rate range of 10⁻³–10² s⁻¹ is reported. The quasi-static tests were conducted on an electro-mechanical universal test machine and a modified instrumented falling weight drop tower was used for high strain rate characterisation. The tensile and compression modulus and strength increased with increasing strain rate. However, the shear modulus and strength were seen to decrease with increasing strain rate. Strain rate constants for use in finite element analyses are derived from the data. The observed failure mechanisms deduced from a microscopic study of the fractured specimens are presented.     

Mechanical and photocatalytic properties of hydroxyapatite/titania nanocomposites prepared by combined high gravity and hydrothermal process

Hydroxyapatite/titania nanocomposites of different ratios have been successfully synthesized by combined high gravity and hydrothermal methods. SEM and TEM observations showed that small spheres of TiO₂, identified as anatase crystals of 10–15 nm, were deposited on HAp rod-like crystals. EDAX analysis confirmed the presence of Ca, P, Ti and O. X-ray diffraction patterns indicated the presence of hydroxyapatite and anatase phase. More number of anatase peaks appeared in the XRD patterns with higher colloidal concentration of TiO₂ in the HAp/TiO₂ compound. Mechanical stability of the HAp/TiO₂ nanocomposites was determined by reinforcing them with high molecular weight polyethylene (HMWPE) and the tensile strength of the samples was analyzed. Photocatalytic activity of the HAp/TiO₂ particles was examined by decomposition of methyl orange (MO). The results showed that photocatalytic properties of HAp/TiO₂ composites are more effective than that of individual HAp and TiO₂ which implied that the HAp improved the photocatalytic activity of well known photocatalyst TiO₂.     

Impact surface fractures of glass-fiber/epoxy lamina-coated glass plates by small steel-ball

Fiber orientation effects on the impact surface fracture of glass plates coated with a glass-fiber/epoxy lamina layer were investigated using a small-diameter steel-ball impact experiment. Four kinds of materials were used: soda-lime glass plates, unidirectional glass-fiber/epoxy layer (one ply, two plies) coated glass plates, crossed glass-fiber/epoxy layer (only two plies) coated glass plates. The maximum stress and absorbed fracture energy of these plates were measured by a single-grid strain gage bonded to the back surface of the glass plates during the impact of the steel ball. With increasing impact velocity, various surface cracks, such as ring, cone, radial and lateral cracks, occurred near the impact sites of the uncoated glass plates. Plates with glass-fiber coating had a plastic deformation zone between the fiber layer and the glass plate that formed around the impact site while the surface cracks in the plates drastically diminished. The principal direction of this plastic deformation and delamination followed the fiber orientation. The impact surface-fracture index expressed in terms of the maximum stress and the absorbed energy could be used as an effective evaluation parameter for surface resistance.     

The rate-dependent behaviour of ± 55 ° filament-wound glass-fibre/epoxy tubes under biaxial loading

This paper presents the results of an experimental investigation into the behaviour of glass-fibre-reinforced epoxy tubes subjected to monotonic biaxial loading. Commercially available tubes with a filament winding pattern of ± 55 ° were tested in a biaxial testing machine with various ratios of axial stress to hoop stress. In addition, the tubes were tested at three rates of monotonic loading. The resulting stress/strain curves were analyzed and biaxial failure envelopes in terms of stress and strain were constructed demonstrating the complexity of the behaviour of the tubes. It is shown that the rate and ratio of biaxial loading affect the monotonic failure strength, damage accumulation and the mode of failure. In addition, these results are discussed based on macro and micro observations of damage and failure modes.     

Influence of fiber orientation and thickness on the response of glass/epoxy composites subjected to impact loading

Composite laminates, made of glass/epoxy using compression molding technique, were subjected to impact loading. The ballistic limit and energy absorption capacity of the laminates were obtained. Experiments were carried out to study the effect of fiber orientation and thicknesses on ballistic limit and energy absorption of the laminates, by using a rigid conical bullet having 9.5 mm diameter and mass of 7.5 g in an air gun. Analytical expressions were obtained to find the ballistic limit, residual velocity and energy absorption capacity of the laminates. The expressions obtained by considering the various damage modes, which were involved in penetration, when laminates subjected to impact loading. The values obtained from analysis were compared with experimental results and good agreement was found. The strain rate sensitivity of the glass/epoxy composites was considered for analysis.     

A novel predictive model for mechanical behavior of single-lap GFRP composite bolted joint under static and dynamic loading

Mechanical connection of composite is critical due to its complicated meso-structure and failure mode, which has become a bottleneck on reliability of composite material and structure. Although many researches on composite bolted joints have been carried out, the theory and experiment on mechanical behavior of such a joint structure under dynamic loading were rarely reported. Here, we propose a novel predictive model for quasi-static and dynamic stiffness of composite bolted joint by introducing the strain rate dependent elastic modulus into the mass spring model. Combined with the composite laminate theory and Tsai-Hill theory, the present model was capable of predicting the strain rate dependent stiffness and strength of the composite bolted joint. Quasi-static and impact loading experiments were carried out by Zwick universal hydraulic testing machine and split Hopkinson tension bar, respectively. The stiffness and strength predicted by our model showed good accordance with the experiment data with errors below 12% under quasi-static loading and below 30% under impact loading. The results indicated that under impact loading, stiffness and strength of the composite bolted joint were significantly higher than their quasi-static counterparts, while the failure mode of the joint structure trended towards localization which was mainly bearing failure. Among various lay-up ratios studied, the optimal lay-up ratio for quasi-static and dynamic stiffness was 0:±45:90 = 3:1:1.