Numerical analysis of the effect of membrane preloads on the low-speed impact response of composite laminates

This article presents a comprehensive study on the mechanical behaviour of composite laminated plates undergoing a low-speed impact of an external body while they are subjected to in-plane preloads. The effect of such preloading was investigated by means of finite-element analysis of several impact events on laminates with three different span-to-thickness ratios. Tensile and compressive preloads, both uniaxial and biaxial, were considered; in the case of compression, the impact on buckled specimens was also studied. The results obtained show that the span-to-thickness ratio is a fundamental parameter in determining the effect of initial strains. Under a tensile preload, the impact-caused peak stresses were higher than in the case of no preload, and their increment was higher in thicker laminates. Under compression, the most dangerous influence of initial stresses was found at medium span-to-thickness ratios for preloads comparable with the buckling load, whereas, in other cases, negligible or even beneficial effects were observed. These results can justify some experimental findings from the existing literature, even if they were obtained without modelling the material degradation due to damage. Also, they allow us to conclude that the explanation of other phenomena strictly related to damage, as well as an accurate prediction of the extent of damage, requires a failure model.     

Damage behavior of laminated E-glass/epoxy beams with an initial delamination subjected to an axial impact

The damage behavior of laminated E-glass/epoxy beams, with and without an initial delamination, subjected to an axial impact by a moving bullet has been investigated experimentally and numerically. The specimens were made from a unidirectional fiber fabric stacked in the sequences [0₆] _s , [±45₃] _s , and [90₆] _s , and a delamination was created in them by locating a copper foil at a specified position. The data on the bullet speed and strain history were recorded by a laser setup, a high-speed dynamic strain indicator, and a TDS420A oscilloscope. It is shown that the delamination and the ply stacking sequence play a significant role in the dynamic response and damage behavior of laminated beams. A numerical simulation is performed by using the commercial finite-element software ABAQUS/Explicit, and the results obtained are in a good agreement with experimental observations. Russian translation published in Mekhanika Kompozitnykh Materialov, Vol. 45, No. 1, pp. 49–64, January–February, 2009.     

Multiple cracking in carbon/epoxy laminates and pre-stressing of fibers

The multiple cracking of laminates with brittle plies can be suppressed by prestraining in tension of the fibers in the ductile plies. Experiments with tensile loading, acoustic emission, and crack counting demonstrate that such multiple cracking can be delayed and thus higher working strains can be allowed (without multiple cracking) for such laminates. In [0°/±65°/0°] laminates the strain for crack initiation can be increased from about 9.9% (for no prestrain) to above 1.4% (for 0.8% prestrain).Presented at the Ninth International Conference on the Mechanics of Composite Materials, Riga. October, 1995.Materials Department, Risøe National Laboratory, Roskilde, Denmark. Published in Mekhanika Kompozitnykh Materialov, Vol. 32, No. 3, pp. 350–362, May–June, 1996.     

Fatigue strength of glass fiber/epoxy angle-ply laminates with different matrix ductility

Conclusion Angle ply laminates made up of glass fiber/epoxy plies have a good static strength for laminate angles up to about 45 °. This is due to the limitation of transverse strain in the plies due to constraints exerted by neighboring plies. At laminate angles of 50 ° and above the transverse and shear strains in the plies are not sufficiently constrained and cause failure in the matrix material. The constraint effect is present in fatigue but since the polymeric matrix material is sensitive to fatigue loading fatigue failure will occur at much lower strain levels than in static loading. This effect cannot be offset by increasing the ductility of the matrix material. Thus, strain to failure under fatigue loading will be only a small fraction of the static strain to failure for angle ply laminates with ply angles up to around 45 ° where the static strength is due to the constraint effect.Published in Mekhanika Kompozitnykh Materialov, No. 5, pp. 632–638, September–October, 1992.     

Isogeometric vibration and buckling analyses of curvilinearly stiffened composite laminates

Isogeometric analysis (IGA) with the polynomial splines over hierarchical T-meshes (PHT-splines) is used to provide an efficient tool capable of carrying out the vibration and buckling analyses of the stiffened laminates. IGA offers increased accuracy and efficiency using the PHT-splines, which represent exact geometry of the stiffeners and make the refinement more flexible near the areas where the stiffeners and composite plate are connected. Numerical examples are given to validate the correctness and superiority of the present method, comparing with the results from existing literatures and commercial softwares. Besides, the influences of the orientation, curvature, location and cross-section size of the stiffeners on the natural frequencies and buckling loads are also studied. The results show that the optimization of the shape and size of the stiffeners has an important effect on the vibration and buckling characteristics of stiffened laminates.     

Interlaminar fracture of multidirectional glass/epoxy laminates under mixed-mode I + II loading

The mixed-mode I + II interlaminar fracture of multidirectional glass/epoxy laminates is investigated. Mixed-mode bending (MMB) tests were performed on specimens with delaminations in 0/θ-type interfaces, with θ varying from 0 to 90°. Preliminary three-dimensional finite-element analyses validated the beam theory model (BTM) used for analysing experimental data. The compliances measured are in a good agreement with BTM predictions. The total critical energy release rate G_c varies linearly with the mode II ratio G_II/G, although some discrepancies are observed in the high-mode II results for the 0/45 and 0/90 specimens. Russian translation published in Mekhanika Kompozitnykh Materialov, Vol. 43, No. 3, pp. 349–366, May–June, 2007.     

Effect of the properties of the material of the buckling of a plate

In [2] it was shown with reference to the cylindrical bending of a plate that the Kirchhoff-Love hypothesis is asympototically accurate for determining the critical load irrespective of the properties of the material. However, the question of the dependence of the critical load on the material properties was left unexplored. This question is now numerically investigated. Curves characterizing the variation of the critical load with relative shear modulus are presented.Institute of Mechanics, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Mekhanika Polimerov, Vol. 5, No. 2, pp. 355–357. March–April, 1969.     

Electric field effect on properties of PT/epoxy nanocomposites

The structure and optic properties of PbTiO₃ nanocrystalline powder/epoxy composites are studied under different electric fields during solidification. X-ray diffraction (XRD), scattered light, ultraviolet optic absorption and second harmonic generation (SHG) of the composites are measured. Results show that the electric field has a great effect on the properties of nanocomposites. The mechanism of electric field production is discussed. Project supported by the National Advanced Materials Research Project and the Natural Science Foundation of Guangdong Province of China.     

Effective plastic properties of laminates

In this contribution, we make use of the fact homogenization of elastic properties of laminates [1] can be extended relatively easy to the plastic case [2] to derive exact expressions for the effective yield limit and the ultimate loading in monotonic tension test. (© 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Mechanical properties of henequen fibre/epoxy resin composites

By using surface-treated and untreated henequen fibres and an epoxy resin, composites were made by compression moulding, and their mechanical properties and failure modes were determined experimentally in tension, bending, and impact loading. The results obtained show that the treatment of fibre surface does not improve the bond between the fibres and the resin matrix, and the general mechanical properties of the composites are similar.     

Interlaminar fracture and low-velocity impact of carbon/epoxy composite materials

The interlaminar fracture and the low-velocity impact behavior of carbon/epoxy composite materials have been studied using width-tapered double cantilever beam (WTDCB), end-notched flexure (ENF), and Boeing impact specimens. The objectives of this research are to determine the essential parameters governing interlaminar fracture and damage of realistic laminated composites and to characterize a correlation between the critical strain energy release rates measured by interlaminar fracture and by low-velocity impact tests. The geometry and the lay-up sequence of specimens are designed to probe various conditions such as the skewness parameter, beam volume, and test fixture. The effect of interfacial ply orientations and crack propagation directions on interlaminar fracture toughness and the effect of ply orientations and thickness on impact behavior are examined. The critical strain energy release rate was calculated from the respective tests: in the interlaminar fracture test, the compliance method and linear beam theory are used; the residual energy calculated from the impact test and the total delamination area estimated by ultrasonic inspection are used in the low-velocity impact test. Results show that the critical strain energy release rate is affected mainly by ply orientations. The critical strain energy release rate measured by the low-velocity impact test lies between the mode I and mode II critical strain energy release rates obtained by the interlaminar fracture test. Submitted to the 11th International Conference on Mechanics of Composite Materials (Riga, June 11–15, 2000). Published in Mekhanika Kompozitnykh Materialov, Vol. 36, No. 2, pp. 195–214, March–April, 2000.     

Effect of heat treatment on the distribution of residual stresses and the properties of epoxy polymers

Conclusions The analysis of the experimental data showed that heat treatment results in a reduction in the stressed state of a polymer, which affects the tensile strength of the material to a greater degree than the compressive strength. During heat treatment, significant changes take place in sections of the polymer damaged by the effect of residual compressive stresses and sections in which the residual stresses change sign. The quantitative relations in the change in the tensile strength of the material in these two sections make it possible to explain the mechanism of the effect of heat treatment and to predict its character.Translated from Mekhanika Kompozitnykh Materialov, No. 5, pp. 787–790, September–October, 1986.     

Effect of a deicing fluid on the durability of carbon/epoxy composites

The degradation of mechanical properties and the moisture absorption of a carbon/epoxy composite is studied through immersion of specimens in two common types of glycol-based (corn-based and synthetic) deicing fluids at two temperatures. Test results indicate that the effects of the fluids are similar at room temperature, while at elevated temperatures, the specimens immersed in the corn-based deicing fluid show a greater drop in the tensile strength. Moreover, gravimetric analysis results show a mass gain in all specimens, with a significantly lower gain from the corn-based deicing fluid than from the synthetic one at the two temperatures selected in this study.     

Effect of quartz sand filler on the mechanical and thermophysical properties of alkyd and epoxy coatings

The effect of a filler on the mechanical and thermophysical properties of polymer coatings has been investigated. It is shown that quartz sand particles are centers of structure formation in alkyd and epoxy coatings. It has been found that a denser, oriented, and stressed polymer structure, much exceeding the size of the individual macromolecules, is formed around the filler particles. The size of the spheres around the filler particles depends on the nature of the film-former, on the filler surface, and on the filler content. It is postulated that secondary supermolecular structures, and not individual polymer macromolecules, are adsorbed on the surface of the filler particles.Mekhanika Polimerov, Vol. 3, No. 3, pp. 507–510, 1967     

Fatigue of glass laminates subjected to repetitive impact loading

Fatigue characteristics obtained for repetitive impact tensile loading in a DSO impact tester are presented for a glass-reinforced Textolite. There are differences in the fatigue life, temperature rise, and the variation of the elastic properties as compared with the case of repetitive sinusoidal stress cycles. The effect of the geometry of the test piece gage length on the measured parameters is examined and methods of predicting fatigue life are discussed.Institute of Polymer Mechanics, Academy of Sciences of the Latvian SSR, Riga. Translated from Mekhanika Polimerov, No. 4, pp. 751–754, July–August, 1969.