Combined Stress State Research Articles

Experimental and numerical study on progressive damage and failure in composite laminates during open-hole compression tests

Progressive damages in the open-hole compression (OHC) tests of composite laminates were experimentally and numerically studied. In the experiment, the failure mechanisms were investigated via in situ microscopy observation, digital image correlation, X-ray radiography, and X-ray computed tomography. Three layups were tested to examine the dependence of progressive damages on the layups. Additionally, numerical simulation was conducted to comprehensively examine the failure mechanisms. In the numerical studies, the simulation scheme, considering the plasticity, kink-band failure, multiple intra-laminar cracks, and delamination, was developed. From the experiment and simulation, it is clarified that the kink-band is initiated and propagated by the combined stress states consisting of longitudinal compression and in-plane shear around the intra-laminar cracks. Therefore, for the high-fidelity OHC simulation in various layups, it is necessary to capture the interaction between kink-band and intra-laminar cracks by considering the combined stress state in the kink-band failure criteria and modeling the multiple intra-laminar cracks.

A theoretical and experimental assessment of 3D macroscopic failure criteria for predicting pure inter-fiber fracture of transversely isotropic UD composites

A systematic and comprehensive assessment of popular 3D macroscopic failure criteria for predicting pure inter-fiber fracture of transversely isotropic unidirectional (UD) composites is performed in the present study. The maximum stress/strain, Tsai-Hill, Hoffman, Tsai-Wu, as well as the Christensen, Hashin, Puck, and LaRC05 criteria for the inter-fiber fracture (IFF) mode, are briefly reviewed. The theoretical foundations of each criterion are critically examined at first. Afterwards, comparisons between the failure criteria which have passed the theoretical examination and the experimental data available in the literature are conducted. The results indicate that the action-plane based criteria perform best in qualitative description of the failure responses, yet no criterion is able to accurately predict the failure stresses in all loading conditions. Even under the same combined stress state, the performance ranking varies for different material types. The main advantages and shortcomings of the evaluated criteria are identified and summarized using this combined theoretical and experimental analysis approach.

Strength of magnesium at high pressures and strain rates

The strength and deformation mechanisms in magnesium can be significantly affected by anisotropy, high strain rates, and pressure. In this study, pressure shear plate impact (PSPI) experiments are conducted to measure the strength of extruded polycrystalline magnesium at pressures varying from 5 to 10 GPa at a nominal strain rate of 105 s−1. The experimental technique enables to first shock load the material sample to the desired normal stress in one direction, and then shear the material in a perpendicular direction. A recently developed hybrid analysis method for PSPI experiments is used to extract the stress–strain curves of magnesium from the particle velocity records measured at the rear surface of the target. The PSPI experimental results reveal a slower twinning saturation at high pressures. To better understand the material behavior under the combined stress states in the PSPI experiments, the results were compared with that of a specimen deformed by a two-step process of quasi-static compression followed by dynamic shear loading at relatively low pressure. The two-stage loading at low pressure, and the calculation of temperature rise in the PSPI experiment revealed that the combined effect of the reorientation and the temperature rise lower the flow strength of magnesium at high pressures under multiaxial loading.

Investigation of deformation damage and fracture in combined stress state for 5052-Al alloy based on experiment and simulation

In this paper, the deformation damage and fracture behavior of 5052-Al alloy specimens were analyzed in a combined stress state. These were respectively described by relating mathematical models of combination constitutive relationship, cumulative damage criterion, and equal-energy fracture locus. The effective laws of critical damage valueDf, micro-void expansion exponent A, and micro-void coalesce exponent B on damage evolution were also discussed. As well as the influence of the ratio α of critical fracture shear stress τ0 to normal stress σ0, the stress triaxiality T, also represents a relationship between elastic dilatational (dW/dV)v and elastic distortional (dW/dV)d strain energy density. Furthermore, the energy weighting factor β on fracture locus was considered. Simultaneously, the parameters of related mathematical models were quantified by experiments, and some model parameters were calibrated for the first time, such as A, B and β. The deformation damage and fracture process of specimens were simulated and analyzed with the mathematical models of combination constitutive relationship, cumulative damage criterion, and equal-energy fracture locus. Moreover, the fracture mechanism of deformation samples was also discussed through a dominated ductile fracture phenomenon, from macroscopic/microscopic fracture morphology. The results indicated that numerical outcomes are consistent with the experimental ones and the combination constitutive relationship combining damage criterion with fracture locus can describe the deformation behavior of materials.

Effect of contact forms on the wear of hard silicon surfaces by soft polymers

The mechanism of hard surfaces worn by soft polymers is not clearly understood. In this paper, a new hypothesis has been proposed, it holds that the stress acting on the hard surface under certain working conditions is the main reason for wear of the hard surface by a soft polymer. The hypothesis was investigated by changing the contact form between tribo-pairs. For this, friction tests between six polymer spheres and smooth, rough, and inclined monocrystalline silicon surfaces were carried out. The results show that for the same tribo-pair, the silicon surface will not be worn in some contact forms, but in other contact forms it will be worn. We believe the wear of hard surface by a soft polymer is the result of the combined stress state action on the hard surface.

Determining the Parameters of the Hereditary Kernels of Isotropic Nonlinear Viscoelastic Materials in Combined Stress State*

The relations between the heredity kernels of isotropic nonlinear viscoelastic materials in combined and one-dimensional stress states are derived. The constitutive equations are presented in a form corresponding to the proportional deviator hypothesis. The nonlinearity of viscoelastic properties is described by Rabotnov’s type models. The creep strains and stress relaxation in thin-walled tubular elements subject to a combination of tension and torsion are determined and tested experimentally.

The Modeling of Combined Stress State of the Drilling Bit Body during the Selective Press Fitting of Picks in it

A new mathematical model is proposed where breaking down of exploited drilling bits is considered to be a consequence of an unbalanced spreading of charges in drilling bit body, which is caused by the difference in interference values of obstruction in each pair “body-pick”. Fulfilled modeling allow to make a definition of complicated stress-deformed state of the drilling collet’ body with different number of teeth and different combinations of hole sizes and sizes of teeth using different materials for further production.

Microstructure and bonding behavior of fiber-mortar interface in fiber-reinforced concrete

The interfacial properties between fiber and matrix play a critical role in the overall mechanical responses of composite materials. In this paper, the glass fiber-mortar interfacial microstructure in fiber reinforced concrete (FRC) is visualized and characterized using X-ray microscopy. Additionally, three types of fiber-mortar interface (glass fiber, high modulus polyvinyl alcohol (PVA) fiber, and basalt fiber) are analyzed by scanning electron microscopy and energy dispersive X-ray spectroscopy. The results revealed a lot of microcracks along with the glass fiber-mortar interface; moreover, the hydration product of the glass/PVA/basalt fiber-mortar interface was much lower than that of the mortar matrix. Because microcracks or lower hydration product have such a negative effect on the interfacial bonding between fiber and mortar, the objective of this paper was to provide an analysis of this problem through extensive testing of their bonding properties. Specimens made of three types of fiber were tested along with three different mortar types under tensile stress and a combined stress state to investigate the interfacial bond properties between fiber and mortar. Results show that both of the tensile and shear bond strength of the interface were not only improved by stronger mortar matrix, but also significantly affected by fiber type. Furthermore, when the interface failed by slipping along the interfacial area, the interface showed an increasing shear bond strength with the increase of compressive stress. This was not the case when failure was due to the crushing of mortar. Finally, the FRC splitting tensile strength was tested to demonstrate the bonding mechanism effects on the FRC mechanical properties.

Investigation of bond formation behaviour in composite ring rolling

Ring rolling is an incremental bulk metal forming process used to produce seamless rings with a wide variety of sizes as well as processible materials. Combining the principle of roll bonding and ring rolling could enable production of seamless composite rings with different materials on the inside and outside, therefore giving the opportunity to manufacture rings tailored to specific applications. A fundamental condition to achieve this is a stronger growth tendency of the inner ring compared to the outer ring, which is satisfied at respective forming temperature by choosing a material combination of a stainless steel outer ring and a tempering steel inner ring. A closed-loop controlled Finite Element ring rolling model is used in conjunction with a contact interaction subroutine for modelling generation and rupture of local material bonds by implementation of an empiric model for bond generation. Efforts to prevent oxidation of the joining surface for ring rolling trials were successful but did not lead to a lasting material bond after rolling. A lasting material bond between both rings was only achieved after adjusting the process strategy to initially allow free lateral spread of the rings in the radial roll gap without reduction to the original ring height in the axial roll gap. FE simulations provided the crucial hints for this adjustment, as they were able to show repeated bond formation and rupture during the process as well as the impact of the adjustment on the combined stress state between both rings.

Finite element modeling of the puncture-cutting response of soft material by a pointed blade

Finite element analysis of combined puncture and cutting of a soft substrate by pointed blades allows further progress in investigating material failure properties, as well as in determining the stress state during the insertion process of the pointed blade. In the current work, a finite element model of the puncture-cutting test was developed using ABAQUS explicit scheme, assuming a non-linear hyperelastic neoprene rubber-like material. Based on the phenomenological model of Mooney-Rivlin, this study suggests an energy-based method, which was derived from both the curve of the puncture cutting force and the pointed blade displacement. The required energy was computed from the work variation as a function of the occurred fracture surface area. The obtained results revealed that finite element analysis was able to accurately predict the material resistance to a combined puncture and cutting motion. Moreover, the stress field analysis showed that the puncture cutting of soft substrates involved a combined stress state, generating then a mixed fracture mode.

Anisotropic mechanical behavior and corresponding microstructure evolution of extruded AZ31 under combined normal/shear stress states

In order to clarify the anisotropic mechanical behavior and corresponding microstructure evolution of extruded AZ31 Mg alloy under combined stress states, modified shear-tension specimens (STSs) and shear-compression specimens (SCSs) were specifically designed and tested by means of quasi-static uni-axial loading. The specimens with beveled opposed slots of different angles (0°, 15°, 30°, 45°, 60° and 75°) are used to obtain different τ/σn ratio (SN Ratio, τ/σn). With the SN Ratio increasing, different mechanical properties between STSs and SCSs are observed, i.e., equivalent yield strength and strain-hardening rate. These differences in deformation behavior are resulted from the activation stresses for twinning and slips. It is found that the deformation of extruded AZ31 Mg alloy under combined shear-tension stresses is dominated by the typical slips. In contrast, twinning dominates the deformation under combined shear-compression stresses. Noticeably, the ductility of extruded AZ31 could be effectively improved by a small shear (about half normal stress), because the prismatic slip is further activated under τ stress component and promotes the major deformation under σn stress based on microstructure and texture analysis. In addition, it is found that the fracture mechanism transformed from the micro-voids growth and coalescence to the internal voids shearing with the SN Ratio increasing from 0 to 3.732.

Analysis of Cylindrical Masonry Shell in St. Jacob`s Church in Dolenja Trebuša, Slovenia—Case Study

This paper focuses on identifying key reasons for the damage of the cylindrical masonry shell structure in St. Jacob`s church in Dolenja Trebuša, Slovenia. Typical damage patterns which can be formed in shell structures and may affect the load bearing capacity are outlined. Several stress states (membrane, bending and also combined stress state) that can occur in the shell structure are described. Load cases such as the vertical displacement of the support structure, temperature loading, weight of maintenance team and also seismic loading are taken into account in order to identify the actual cause for the registered crack pattern in the shell structure. Analysis of the shell structure is performed using the SAP2000 structural software. Based on the obtained results, which highlighted key reasons for registered damage, the monitoring of cracks is recommended in the first phase, and, in continuation, the most appropriate repair and strengthening measures are proposed.

Validating a Failure Surface Developed for ABS Fused Filament Fabrication Parts through Complex Loading Experiments

Fused Filament Fabrication (FFF) is arguably the most widely available additive manufacturing technology at the moment. Offering the possibility of producing complex geometries in a compressed product development cycle and in a plethora of materials, it has gradually started to become attractive to multiple industrial segments, slowly being implemented in diverse applications. However, the high anisotropy of parts developed through this technique renders failure prediction difficult. The proper performance of the part, or even the safety of the final user, cannot be guaranteed under demanding mechanical requirements. This problem can be tackled through the development of a failure envelope that allows engineers to predict failure by using the knowledge of the stress state of the part. Previous research by the authors developed a failure envelope for acrylonitrile butadiene styrene (ABS) based, Fused Filament Fabrication (FFF) parts by use of a criterion that incorporates stress interactions. This work validates the first quadrant of the envelope by performing uniaxial tensile tests with coupons produced with a variety of raster angles, creating a combined loading stress state in the localized coordinate system. Results show the safe zone encompassed by the failure envelope proved adequate.

Soil compaction and stress propagation after different wheeling intensities on a silt soil in South-East Norway

ABSTRACTThe objective of this study was to evaluate the effect of wheeling with two different wheel loads (1.7 and 2.8 Mg) and contrasting wheeling intensities (1x and 10x) on the bearing capacity of a Stagnosol derived from silty alluvial deposits. Soil strength was assessed by laboratory measurements of the precompression stress in topsoil (20 cm) and subsoil (40 and 60 cm) samples. Stress propagation, as well as elastic and plastic deformation during wheeling were measured in the field with combined stress state (SST) and displacement transducers (DTS). We also present results from soil physical analyses (bulk density, air capacity, saturated hydraulic conductivity) and barley yields from the first two years after the compaction. Although the wheel loads used were comparatively small, typical for the machinery used in Norway, the results show that both increased wheel load and wheeling intensity had negative effects on soil physical parameters especially in the topsoil but with similar tendencies also in the subsoil. Stress propagation was detected down to 60 cm depth (SST). The first wheeling was most harmful, but all wheelings led to accumulative plastic soil deformation (DTS). Under the workable conditions in this trial, increased wheeling with a small machine was more harmful to soil structure than a single wheeling with a heavier machine. However, the yields in the first two years after the compaction did not show any negative effect of the compaction.

Combined Numerical and Analytical Determination of Poisson’s Ratio for Viscoelastic Isotropic Materials

The Laplace–Carson integral transform method and numerical inversion of the solutions are used to establish the relationship between hereditary kernels that define the scalar properties of isotropic linear viscoelastic materials in combined stress state. The hereditary creep kernel characterizing the behavior of the viscoelastic Poisson’s ratio with time is identified. The calculation of shear creep strains and transverse creep under uniaxial loading with allowance for the time-dependent Poisson’s ratio are experimentally validated.

Modelling stress-strain state in oxide ceramics under thermal shock

Properties of ZrO2 -Al2O3 ceramic composites were studied in the framework of this research. The porous medium significantly affects the strength of the composite under thermal shock. In the areas near pores, concentrators emerge in a combined stress state with the maximum normal stresses under tension being equal to the compressing ones in absolute values and reaching 2.6 GPa (clusters of pores, 6% porosity) and 2.7 GPa (irregular distribution of pores, 12% porosity). High levels of stress lead to cracks in bridges between pores. The growth of porosity from 6% to 12% increases the number of dangerous zones

Simplified phenomenological model of the nonlinear behavior of FRPs under combined stress states

Nonlinear material behavior of FRPs under shear loading is widely observed and investigated. In case of combined stress states under tension and shear, an interaction between the macroscopic shear stress–strain curve evolution and the applied tension has been observed and described by several publications in the past. In the present work, the available experimental data with combined stress states are evaluated and a specific threshold shear stress is found, above which nonlinear material behavior occurs for all stress states. Further, a new simplified phenomenological model is derived to model the nonlinear behavior of FRPs when the threshold shear stress is exceeded. This simplified model only needs the threshold shear stress and one evolution parameter, both derived from a pure shear test, to model nonlinear behavior for all combined stress states. A comparison with the available experimental results and with the predictions of the WWFE-III participants for WWFE-III test case 1 shows a very good agreement.

A damage mechanics based failure criterion for fiber reinforced polymers

In this paper an energy-based failure envelope for unidirectional fiber reinforced polymers (FRP) has been developed. It is based on the stress and the damage energy release rate (DERR) components, which makes it suitable for being used as a damage surface in continuum damage mechanics (CDM), when a thermodynamically consistent evolution rule is intended to be used. The proposed failure surface is an interactive quadratic function of the DERR components accompanying with linear stress components in the form of an invariant. The mathematical formulation of the criterion is originated from the fully interactive Tsai-Wu criterion. The failure locus of the two criteria were compared with each other for several unidirectional FRP composites under several combined stress states. The two criteria were found to be in a reasonably good agreement for investigated composites with Eglass and carbon fibers, especially where the experimental data were available.

Identification of the Hereditary Kernels of Isotropic Linear Viscoelastic Materials in Combined Stress State. 2. Proportional deviators

The relationships between the hereditary and creep kernels are established. The hereditary kernels define the scalar properties of isotropic linear viscoelastic materials in a combined stress state. The creep kernels are obtained in uniaxial-tension and pure-torsion tests. The constitutive equations are chosen so as to meet the hypothesis of proportional deviators. The problems of analyzing the creep deformation and stress relaxation of thin-walled tubular specimens under combined tension and torsion are solved and tested experimentally

Identification of the Hereditary Kernels of Isotropic Linear Viscoelastic Materials in Combined Stress state. 1. Superposition of Shear and Bulk creep

Relations between the shear and bulk creep kernels of an isotropic linear viscoelastic material in combined stress state and the longitudinal and shear creep kernels constructed from data of creep tests under uniaxial tension and pure torsion are formulated. The constitutive equations of viscoelasticity for the combined stress state are chosen in the form of a superposition of the equation for shear strains and the equation for bulk strains. The hereditary kernels are described by Rabotnov’s fractional-exponential functions. The creep strains of thin-walled pipes under a combination of tension and torsion or tension and internal pressure are calculated