Investigations of interlaminar fracture toughness of laminated polymeric composites

The behavior of interlaminar fracture of fiber reinforced laminated polymeric composites has been investigated in modes I, II, and different mixed mode I/II ratios. The experimental investigations were carried out by using conventional beam specimens and the compound version of the CTS (compact tension shear) specimen. In this study, a compound version of the CTS specimen is used for the first time to determine the interlaminar fracture toughness of composites. In order to verify the results obtained by the CTS tests, conventional beam tests were also carried out. In the beam tests, specimens of double cantilever beam (DCB) and end notched flexure (ENF) were used to obtain the critical rates of the energy release for failure modes I and II. The CTS specimen is used to obtain different mixed mode ratios, from pure mode I to pure mode II, by varying the loading conditions. The highest mixed mode ratio obtained in the experiment was G _I /G _II =60. The data obtained from these tests were analyzed by the finite element method. The separated critical rates G _I and G _II of the energy release were calculated by using the modified virtual crack closure integral (MVCCI) method. The experimental investigations were performed on a unidirectional glass/epoxy composite. The results obtained by the beam and CTS tests were compared. It was found that the interlaminar fracture toughness G _IC ^init of mode I at crack initiation and the corresponding value G _II ^Cinit of mode II obtained by the conventional beam and the CTS tests were in rather good agreement. The experimental results of interlaminar fracture of mixed mode were used to obtain the parameters required for the failure criterion. The two different failure criteria were compared. The best correlation with the experimental data was obtained by using the failure criterion proposed by Wu in 1967 containing linear and quadratic terms of the rates of the energy release.Presented at the 10th International Conference on the Mechanics of Composite Materials (Riga, April 20–23, 1998).Translated from Mekhanika Kompozitnykh Materialov, Vol. 34, No. 3, pp. 307–322, May–June, 1998.     

Nonlinear failure mechanisms of laminated composites

Two distinct analytical models are described representing geometrically nonlinear instabilities in layered composites under in-plane compression — kink-banding and delamination buckling. The utilized technique is based on of energy minimization principles in order to examine the underlying mechanics of the systems. It is demonstrated that using this approach enables investigations to be undertaken far into the postbuckling range whilst changing system parameters. Thereby a greater phenomenological understanding of the mechanics of the systems is achieved. (© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Electromechanical properties of laminated piezoelectric composites

The general expressions for the effective elastic, piezoelectric, and dielectric coefficients of a laminated piezocomposite composed of laminates in parallel connection are shown. By means of the asymptotic averaging method [12] these effective coefficients are calculated. The effective coefficients as well as physical properties such as electromechanical piezoelectric coupling coefficients, acoustic impedance, and wave velocity for a two-phase composite, where one phase is ceramic and the other one is polymer, are analyzed. A comparison with another results is shown.Presented at the Ninth International Conference on the Mechanics of Composite Materials, Riga, October, 1995.Published in Mekhanika Kompozitnykh Materialov, Vol. 32, No. 3, pp. 410–417, May–June, 1996.     

Interlaminar fracture toughness of graphite fiber-reinforced polyimide composites

Conclusion The present study has proved the effectiveness of the application of viscoelastic polymers with increased fracture toughness to graphite/polyimide composites interlaminar fracture toughness improvement. Thermoplastic polysulphone film and thermoresistant structural adhesive have proved to be inherently more effective for composites' delamination resistance growth than maleimide resin toughening and structural modification. The former inevitably results in increase of the honeycomb delamination resistance (Fig. 1) and its durability.Published in Mekhanika Kompozitnykh Materialov, Vol. 30, No. 6, pp. 848–852, November–December, 1994.     

Strength vs. toughness optimization of microstructured composites

Aim of this paper is to present a new fractal approach linking the macroscopic mechanical properties of micro- and nano-structured materials with the main parameters: composition, grain size and structural dimension, as well as contiguity and mean free path. Assuming the key role played by the interfaces, the proposed fractal energy approach unifies the influences of all the above parameters, through the introduction of a fractal structural parameter (FSP), which represents an extension of the Gurland’s structural parameter. This modeling approach is assessed through an extensive comparison with experimental data on poly crystalline diamond (PCD) and WC–Co alloys. The results clearly show that the theoretical fractal predictions are in a fairly good agreement with the experiments on both hardness and toughness. This new synthetic parameter is thus proposed to investigate, design and optimize new micro- and nano-grained materials. Eventually, FSP-based optimization maps are developed, that allow to design new materials with high hardness and toughness.     

Fracture toughness investigations on UHPC by spalling tests

Ultra High Performance Concrete (UHPC) is defined as a new generation of concrete which shows improved performance and higher strength than traditional concrete. This allows to realize slender and much more durable structures and in this way significantly reduces the required resources. Despite its huge potential in construction, technical information about this new type of material is still limited. This contribution presents investigations on the dynamic mechanical behavior and properties of UHPC specimens by spalling experiments. Two different recipes were used to compare the properties. Due to the special specimen geometry (slender cylindrical) a flowable consistency was required to enable a sufficient degassing of the mixtures. For the test, a Split Hopkinson Pressure Bar (SHPB) has been modified and used. A high speed photograph system was focused on the fragmentation process during the test. On the basis of these experiments the dynamic E-moduli as well as the dynamic tensile strength of the UHPC specimens were determined. By observation of the specific crack patterns on each tested specimen and corresponding times, the dynamic fracture energy is calculated. Numerical simulations also were performed and compared to the experimental result. It is concluded that the dynamic tensile strength of the UHPC increases at higher strain rates. The results of the current study provide technical information about fracture and dynamic behavior of UHPC and the obtained values could be used for future computational models. (© 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)     

A physically based failure criterion for laminated composites

The stress and strain fields in laminated composites can be described realistically with the help of a refined theory of elasticity for anisotropic materials. In contrast, the respective failure characteristics cannot be predicted satisfactorily based on the commonly used failure criteria. The main disadvantage of these generalized failure criteria, such as the quadratic failure criteria of Sakharov, Azzi/Tsai, Tsai/Wu, etc., is that they combine fundamentally different fracture mechanisms of the homogenized UD layer in one approximation by an interpolation polynomial. A completely different method for the formulation of realistic failure criteria, taking into account the heterogeneous anisotropic material structure relevant to the fracture, is based on the Mohr hypothesis for brittle materials that in fact only the stresses in the fracture plane induce failure. This physically based failure criterion not only considers the decisive eifference between the fiber fracture and the interfiber fracture, but also characterizes further fracture types in the plane parallel to the fibers.Institut für Leichtbau und Kunststoffetechnik (ILK) Technische Universität Dresden, D-01062 Dresden, Germany. Translated from Mekhanika Kompozitnykh Materialov, Vol. 35, No. 4, pp. 413–422, July–August, 1999.     

Internal instability of laminated composites with a metal matrix

Conclusions The constructed characteristic determinant for a three-dimensional nonaxisymmetric problem in the theory of stability of laminated composites with a metal matrix coincides with the characteristic determinant for the axisymmetric problem with the corresponding substitution of the wave parameters. The solution of the characteristic equation for a real material shows that loss of stability according to the different forms and for different values of deformation and the oscillation parameter as a function of the concentration of filler is possible.Translated from Mekhanika Kompozitnykh Materialov, No. 6, pp. 1051–1056, November–December, 1990.