Long-term strength of reinforced composites

Conclusion A criterion of long-term strength was proposed for composite materials. The criterion can be used to calculate time to failure for arbitrary loading programs. It was shown that the criterion provides for good agreement with the experimental data not only in the cases of instantaneous and long-term static loadings, but also for fatigue loading in tension, in compression, and in mixed regimes with different asymmetry coefficients.Translated from Mekhanika Kompozitnykh Materialov, No. 1, pp. 16–22, January–February, 1989.     

Approximate methods for analyzing the growth of large cracks in fatigue damaged aircraft sheet material and lap joints

Several levels of approximation are investigated to account for the effect of small fatigue cracks on the residual strength of aircraft sheet materials and fuselage lap joints containing major cracks. A version of the Dugdale model is proposed which accounts for strain hardening of the sheet in an approximate way and which incorporates a criterion for crack advance leading to crack growth resistance. This model builds upon the model proposed by Nilsson and Hutchinson [1] and accounts for the detailed interaction between the major crack and the small damage cracks. A simpler version of the model uses the damage-reduced local strength of the sheet or joint in assessing the effect of the major crack on residual strength. The simpler approach thus bypasses the necessity of a direct determination of the highly complicated details of the interaction of the small cracks in a lap joint with a major crack.     

On a new stopping rule for stochastic approximation

Summary A new stopping rule for the Robbins-Monro process, based on an F-statistic criterion is proposed and its asymptotic behavior established. On the basis of evidence obtained through experimental sampling, the procedure seems to work well over a wide variety of situations. A two-stage procedure, coupling the new rule with an earlier one proposed by Sielken [1973] is recommended for practical use.     

Choice of strength criterion for rigid thermosets

On the basis of experimental data obtained by testing rigid thermosetting plastics under plane stress conditions the authors analyze the applicability to these plastics of the first and second classical theories of strength as well as the theories of Mohr, Yagn, Balandin, and Mirolyubov. A new strength criterion, a special case of the criterion proposed previously by one of the authors, is recommended as being less complicated, but still accurate enough for practical computations.Mekhanika Polimerov, Vol. 2, No. 4, pp. 543–549, 1966     

A New Static Failure Criterion for Isotropic Polymers

A new static failure criterion for isotropic polymers with different strengths in tension and compression based on exponential dependence between the mean stress and the von Mises equivalent stress is proposed. The two material parameters introduced can be determined by two simple tests - the uniaxial tension and compression. The locus of the criterion is nearly conical for low hydrostatic pressures and tends to a cylindrical form if an increased hydrostatic pressure is applied. The validity of the criterion is demonstrated by experimental strength data taken from the literature for several polymers in the case of superimposed hydrostatic pressure.     

Three-dimensionally reinforced woven materials

The article presents experimental data on the deformation and strength characteristics of a number of materials reinforced with three-dimensionally woven multilayer glass fabric, which are compared with the corresponding data for traditional glass-reinforced plastics. The accuracy of the formulas proposed [1] is evaluated. The effect of prestressing the warp and weft fibers on the mechanical properties of the material was investigated. The effectiveness of simultaneous prestressing of the warp and weft was evaluated; the magnitude of the optimal force was established.See [1] for report 1.Institute of Polymer Mechanics, Academy of Sciences of the Latvian SSR, Riga. Translated from Mekhanika Polimerov, No. 3, pp. 471–476, May–June, 1970.     

Strength of glass-reinforced plastics in the complex stress state

A new criterion of strength is proposed for anisotropic materials of the glass-reinforced plastic type. This criterion takes into account not only the different ultimate strengths in tension and compression in each direction but also the dependence of the ultimate shear strengths on the sign (direction) of the shear stresses. The criterion is given in tensor invariant form, so that it can be rewritten for any direction of the glass fibers. The criteria proposed by other authors, in particular, R. Hill's condition of plasticity [1], follow from the new generalized criterion as special cases. The results of experiments conducted both by the authors of the present article and by other investigators provide good confirmation of the proposed criterion.Mekhanika Polimerov, Vol. 1, No. 2, pp. 70–78, 1965     

Failure Analysis on Shell-like Composite Laminates Using the Puck Criteria

Due to their high specific strength and stiffness, carbon fibre-reinforced polymers (CFRP) are widely used in several fields of engineering, e.g. aerospace engineering. A new design scheme aims for the exploitation of material reserves. Since design is always linked to expensive experimental investigations, reliable simulation procedures are required to lower the related costs. In terms of detecting ply failure several criteria have been proposed. In this work those of Hashin [1] and Puck [2] are applied to several numerical models and compared to experimental results [3]. (© 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)     

An investigation of the impact and penetration of solids of revolution into soft earth

The numerical method proposed earlier in [1] is developed to solve problems of the impact and penetration of rigid and deformable bodies of revolution into soft soil, which are described by Grigoryan's model [2]. The effect of the surface Coulomb friction, the bulk compressibility and the shear strength of soft soil on the forces of resistance and contact pressures in the contact zone is analysed. The results of numerical solutions of problems in a coherent formulation are compared with analytical relations and experimental data on the determination of the forces and coefficients of resistance to the penetration of impactors of different shapes into soft soil.     

Selecting mixed-effects models based on a generalized information criterion

The generalized information criterion (GIC) proposed by Rao and Wu [A strongly consistent procedure for model selection in a regression problem, Biometrika 76 (1989) 369-374] is a generalization of Akaike's information criterion (AIC) and the Bayesian information criterion (BIC). In this paper, we extend the GIC to select linear mixed-effects models that are widely applied in analyzing longitudinal data. The procedure for selecting fixed effects and random effects based on the extended GIC is provided. The asymptotic behavior of the extended GIC method for selecting fixed effects is studied. We prove that, under mild conditions, the selection procedure is asymptotically loss efficient regardless of the existence of a true model and consistent if a true model exists. A simulation study is carried out to empirically evaluate the performance of the extended GIC procedure. The results from the simulation show that if the signal-to-noise ratio is moderate or high, the percentages of choosing the correct fixed effects by the GIC procedure are close to one for finite samples, while the procedure performs relatively poorly when it is used to select random effects.     

An overview of some recent results on the Euler system of isentropic gas dynamics

This overview is concerned with the well-posedness problem for the isentropic compressible Euler equations of gas dynamics. The results we present are in line with the programof investigatingthe efficiency of different selection criteria proposed in the literature in order to weed out non-physical solutions to more-dimensional systems of conservation laws and they build upon the method of convex integration developed by De Lellis and Székelyhidi for the incompressible Euler equations. Mainly following [5], we investigate the role of the maximal dissipation criterion proposed by Dafermos in [6]: we prove how, for specific pressure laws, some non-standard (i.e. constructed via convex integration methods) solutions to the Riemann problem for the isentropic Euler system in two space dimensions have greater energy dissipation rate than the classical self-similar solution emanating from the same Riemann data. We therefore show that the maximal dissipation criterion proposed by Dafermos does not favour in general the self-similar solutions.     

Numerical study of the primary breakup of a plane liquid sheet with co-flowing air streams

The objective of this study is to model the primary breakup of a plane liquid sheet emerging from an air-blast nozzle. In the present work the interface compression scheme proposed by OpenCFD Ltd. [1] has been used to capture the interface between the liquid and gas. A One-equation subgrid scale (sgs) turbulent energy transport model attributed to Yoshizawa [2] is used for modeling the effects of turbulence. The set up case selected for this study is based on the experiments carried out by Mitra [3]. The 2D simulations performed in this study predict the breakup length of the plane liquid sheet in good agreement with the experimental data. Future work will involve, performing 3D simulations of the plane liquid sheet generated by the air-blast nozzle and performing comparisons of the resulting droplet characteristics with the experimental data. (© 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Creep and long-time strength of oriented glass-reinforced plastics in interlaminar shear

The creep and long-time strength in interlaminar shear and the creep in compression in the direction of the reinforcement have been experimentally investigated for certain types of oriented glass-reinforced plastics. The specimens in the interlaminar creep tests took the form of short beams loaded in bending. The experimental creep data for shear and compression are well described by the hereditary theory with a kernel of the Abel type (shear) or in the form of a Rabotnov function (compression). If the stresses are constant in time, good agreement with experiment is also given by Findley's form of the aging theory. A deformation criterion of interlaminar shear strength is also obtained. The experimental curves and values of the creep and long-time strength constants are presented.Translated from Mekhanika Polimerov, No. 6, pp. 1003–1012, November–December, 1971.     

Numerical modeling of brittle fracture in porous CFC-materials

Both energy and stress criteria are necessary conditions for fracture but neither the one nor the other is sufficient. A combination of these criteria is proposed in [1]. This combined criterion is used for numerical simulation of crack propagation by the 4-point bending test in porous materials. Examples of such materials are carbon-carbon composites (CFC) [2, 3]. Micrographs of the cross-sections of these materials are used for FEM modeling of the crack propagation on the basis of the proposed criterion. Results of the numerical modeling are compared with experimental results. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)     

A comprehensive review and evaluation of permutation flowshop heuristics

In this work we present a review and comparative evaluation of heuristics and metaheuristics for the well-known permutation flowshop problem with the makespan criterion. A number of reviews and evaluations have already been proposed. However, the evaluations do not include the latest heuristics available and there is still no comparison of metaheuristics. Furthermore, since no common benchmarks and computing platforms are used, the results cannot be generalised. We propose a comparison of 25 methods, ranging from the classical Johnson's algorithm or dispatching rules to the most recent metaheuristics, including tabu search, simulated annealing, genetic algorithms, iterated local search and hybrid techniques. For the evaluation we use the standard test of Taillard [Eur. J. Operation. Res. 64 (1993) 278] composed of 120 instances of different sizes. In the evaluations we use the experimental design approach to obtain valid conclusions on the effectiveness and efficiency of the different methods tested.     

Analysis of the effect of impact damage on the strength characteristics of a composite

Theoretical and experimental investigations into the loss of the static tensile strength of a KMKU-2M.120. E01 composite after low-speed impacts of different energy have been carried out. All calculations are performed by the method of finite elements. The drop in the strength as a function of impact energy is estimated according to two criteria: the criterion of an equivalent hole (the upper limit of loss of strength) and the criterion of concentration (the lower limit of loss of strength). The results obtained agree closely with experimental data and therefore can be recognized as reliable.     

Estimation of the long-term strength of polymer materials under arbitrary loading histories

A nonlinear version of the phenomenological theory of long-term strength of polymer materials (viscoelastic bodies) is proposed. It is based on the introduction of a function accounting for the damage accumulation connected with changes in the load intensity. The form of this function may be determined from the results of testing the material with a load changing with time in a certain way, for instance, periodically. As a parameter, the function contains the rate of the changing load or the frequency for periodic loads. For a quasi-isotropic material, the basic relationships of the theory proposed are generalized to the case of combined stresses. The durability (failure time) calculations of the material based on this theory are compared with experimental data for a number of polymer and composite materials in a wide range of loading modes. Translated from Mekhanika Kompozitnykh Materialov, Vol. 35, No. 5, pp. 585–594, September–October, 1999.     

Problems of the anisotropy of strength

The concept of a strength tensor, analogous to the tensor of elastic constants, has been introduced with a view to using its components for approximate determination of laws governing the variation of strength of anisotropic materials in uniaxial tension or pure shear as a function of the orientation of the stress state. It has been shown that the plasticity condition postulated by Mises can be used as a criterion of strength of many "weakly" anisotropic materials, if one rejects the hypothesis according to which the condition is invariant in respect to hydrostatic pressure. In the case of "strongly" anisotropic materials, wood in particular, the Mises condition is at variance with the results obtained for variously oriented specimens tested in uniaxial tension. A strength condition in the form of a fourth-degree polynomial, based on the assumed existence of a strength tensor analogous to the tensor of elastic constants, has been postulated for these materials. The validity of this criterion has been demonstrated by constructing surfaces of equicritical plane stress states from experimental data obtained for pine wood, plywood, and glass-reinforced plastic laminates.Mekhanika Polimerov, Vol 1, No. 2, pp. 79–92, 1965