Milan Růžička

Fatigue life prediction for broad-band multiaxial loading with various PSD curve shapes

The main purpose of this study is to determine, via a three dimensions Finite Element analysis (FE), the stress and strain fields at the inner surface of a tubular specimen submitted to thermo-mechanical fatigue. To investigate the surface finish effect on fatigue behaviour at this inner surface, mechanical tests were carried out on real size tubular specimens under various thermal loadings. X ray measurements, Transmission Electron Microscopy observations and micro-hardness tests performed at and under the inner surface of the specimen before testing, revealed residual internal stresses and a large dislocation microstructure gradient in correlation with hardening gradients due to machining. A memory effect, bound to the pre-hardening gradient, was introduced into an elasto-visco-plastic model in order to determine the stress and strain fields at the inner surface. The temperature evolution on the inner surface of the tubular specimen was first computed via a thermo-elastic model and then used for our thermo-mechanical simulations. Identification of the thermo-mechanical model parameters was based on the experimental stabilized cyclic tension-compression tests performed at 20°C and 300°C. A good agreement was obtained between numerical stabilized tractioncompression cycle curves (with and without pre-straining) and experimental ones. This 3 dimensional simulation gave access to the evolution of the axial and tangential internal stresses and local strains during the tests. Numerical results showed: a decreasing of the tangential stress and stabilization after 40 cycles, whereas the axial stress showed weaker decreasing with the number of cycles. The results also pointed out a ratcheting and a slightly non proportional loading at the inner surface. The computed mean stress and strain values of the stabilized cycle being far from the initial ones, they could be used to get the safety margins of standard design related to fatigue, as well as to get accurate loading conditions needed for the use of more advanced fatigue analysis and criteria.Abstract The fatigue calculation procedure analysed here applies the power spectral density (PSD) function of the equivalent stress together with the known spectral method for estimating the probability density function of stress amplitudes included in random loading. Here, the narrow-band approximation, Wirsching–Light, Benasciutti–Tovo and Dirlik models are used, together with the SWT parameter. The prediction capability of these four methods was verified on a set of 107 tests results obtained under random axial, torsion and combined axial and torsion loading applied to a tubular specimen with a one-sided hole. Several PSD shapes and combinations of loading were applied. It is shown that the results for fatigue life calculated using the Benasciutti–Tovo and Dirlik methods are well correlated with the results of experiments under this type of loading.

Effect of layered silicates and reactive compatibilization on structure and properties of melt-drawn HDPE/PA6 microfibrillar composites

Microfibrillar composites (MFCs) of HDPE matrix with PA6 reinforcing fibrils formed in situ by melt drawing were modified by ethylene/glycidyl methacrylate copolymer (PEGMA) and addition of layered silicates using different mixing protocols. The goal was enhancement of adhesion between fibrils and matrix, their reinforcement by clay, and evaluation of the effect of clay on the MFC morphology, especially the fibril dimensions. Improved mechanical properties, including toughness, were found in the case of pre-blending Cloisite 15A (C15) with both polymeric components. Pre-blending of Cloisite C30B (C30) in PA6 is effective at low draw ratios only, whereas simultaneous addition of both clays leads to significant worsening of properties. In many cases, low stiffness increase indicates existence of clay-induced controversial effects which may eliminate reinforcement induced by fibrils and clay. The results indicate that these effects are significantly affected by the extent and course of clay migration between polymer phases during extrusion-mixing and melt drawing. Possible explanation of affecting the properties due to low-modulus interface supported by finite element analysis (FEA) is presented.

Modelling Tissue Behaviour Based on Hyperelasticity Theory

The tissues are during their physiological function, e.g., in the course of growth, adolescence, and aging, subjected to a cyclic mechanical loading and to large displacements and rotations as well. A tissue free of all external tractions is in a state that minimizes its internal power. In the course of aging of the tissues, for instance in the wall of the aorta, the vein, and also in the myocardium or heart valves, the decrease of the water content and increase of the collagen content occurs; while in compact and trabecular bone the contents of both mineral substances and collagen, undergo reduction. In accordance with it, the strain energy function and the constitutive equations of living tissue based on the hyperelasticity theory using rotationless strain were studied. On the base of the proposed eigenvalue decomposition of the rotationless strain tensor and hyperelasticity the strain energy function was formulated as depending on biological time of tissue. The quantity of strain energy function per unit of the biological time, which essentially characterizes the velocity of change of mechanical response of tissue in the course of its aging, was also defined. The coefficient of tissue aging is the further diagnostic parameter, which is independent of the rotationless strain tensor and expresses the relative change of mechanical response of tissue during the biological time. The corresponding constitutive equation of tissue depending on the biological time is also determined. On the base of the regression analysis the theoretical stress-strain curves for myocardium and blood vessels were determined. The numerical results reveal that the coefficient of aging progressively increases in hardening tissues (coronary artery, vena cava inferior) whereas at the softening tissues it has a relatively slow increase at the dependence on tissue aging.

Experimental Testing of the Composite Energy Absorber

This paper deals with the development of a deformation element designed to be used in transportation industry. The aim is to develop a design of thermoplastic composite that begins to warp under a considerable impact loading and thus ensures a high energy absorption. This is accomplished using several modes of deformation. Fiber fracture and delamination of the composite layers can be counted among the most important ones. The shape of the deformation element was designed to utilize the advantages of its thermoplastic matrix when creating the shape of the composite plate as well as when joining the plates together (welding, bonding).

Multiscale Modeling of Hybrid Composite Structures

A novel type of hybrid composite structure has been developed, experimentally investigated and used for many practical applications. The main supporting elements of composite structures are formed by the stamping process of partially cured and axially-oriented carbon fibre rods. This system can fill relatively thick parts of cross sections of beams without risk of delamination. Typical macroscopic sub-cells are formed in the transversal cross section of the part due to this technology. An advantage of this final 3D composite structure is its high shear strength and stiffness in comparison with thick unidirectional composite parts. To absorb the dynamic energy and increase the damping, a rubber-cork layer can be inserted during production, before the final pressing and curing of the whole part. The final stiffness property of the whole 3D composite is obtained from multiscale modeling. It is based on an averaging process and a homogenization technique in FEA. A parametric study was carried out to determine the influence of the size, orientation and thickness of the cell border winding layer on the components of the global elastic material matrix. A comparison of a numerical analysis prediction with experimental results shows acceptable agreement of the elastic modules. A mezzo scale model can be applied for designing a real part on a macro scale.

Fatigue threshold and crack growth in an electron beam weld made of steel and bronze

In this research topic some experimental tests with single-edge notched beams were performed to determine the threshold load value for fatigue crack growth and to characterize fatigue crack behaviour of an electron beam weld made of steel and bronze. Subsequently, numerical analyses were done to estimate the threshold value ΔKth and to simulate the fatigue crack growth. The calculated crack path was compared to those determined experimentally. The objective was to find out the necessary fracture properties for an analysis of an electron beam welded worm wheel and to asses the capability of usual fracture analysis software to simulate fatigue crack growth in welds.

Analysis of Mechanical Properties of the “Bearing” Part Based on Polished Samples

Abstract Presented work describes the use of algorithm for the computation of thermo-elastic properties of randomly reinforced composite which is based on histogram from image analysis done on ITAM, CAS. Three polished samples from “Bearing” part were analyzed. Results are used for verification of the algorithm functionality and primarily for computation of thermo-elastic properties which were compared with each other and used in modified FE analysis.

Comparison of the Thermoelastic Properties of a Randomly Reinforced Composite Computed by the Classical Lamination Theory and by the Monte Carlo Simulation

The thermoelastic properties of randomly reinforced composites can be computed with the idealization that a randomly reinforced layer is composed of a large number of thin layers oriented from 0 to 180°. This approach is a modification of the classical lamination theory. The calculation algorithm is based on an image processing method which identifies the fiber orientation histogram. This histogram is used as an input for the Monte Carlo simulation and for computing reduced stiffness matrices. The method is verified on flat plates and is found to be useful for computing the thermoelastic properties of real parts, because it takes into account the actual orientational distribution of fibers.

Health and usage monitoring system for the small aircraft composite structure

This paper is focused on the design of the health and usage monitoring system (HUMS) of the composite ultra-light aircrafts. A multichannel measuring system was developed and installed for recording of the long-term operational measurements of the UL airplane. Many fiber Bragg grating sensors were implemented into the composite aircraft structure, mainly in the glue joints. More than ten other analog functions and signals of the aircraft is monitored and can be correlated together. Changing of the FBG sensors responses in monitored places and their correlations, comparing with the calibration and recalibration procedures during a monitored life may indicate damage (eg. in bonded joints) and complements the HUMS system.

Simulation of Damage in Hybrid Composite Cell Structure

A novel type of hybrid cell composite structure has been developed and used for many practical applications. Typical macroscopic sub-cells in the cross section structure are formed by the stamping process of partially cured and axially-oriented high modulus carbon fibre bundles. Each bundle is wrapped around by a thin layer of high strength fibres. Main goal of this paper is the simulation of damage of this new composite structure. Modified hierarchy homogenization method (Non-uniform Transformation Field Analysis) was proposed for simulation of damage progress. Modification is based on introducing damage modes. The method is based on assumption that field of in-elastic strain describing damage in each constituents can be decomposed on finite set of fields, called damage modes. Modified NTFA method was incorporated into FEM code and verified in several four-point bending tests.