Loss Of Load-carrying Capacity Research Articles

Enhancing the bending strength, load-carrying capacity and material efficiency of aspen and black alder plywood through thermo-mechanical densification of face veneers

Many research papers highlight the positive impact of wood densification on mechanical properties of solid wood as well as wood veneers. Previous experimental studies comparing densified to non-densified wood materials of the same thickness consistently demonstrated higher strength properties in bending for densified materials. However, these studies often overlooked an important comparison: the strength and load-carrying capacity of non-densified wood material to its densified state, in which the thickness is reduced and thus the material’s moment of inertia. The current study, aims to address this gap for plywood made from low-quality, underutilized hardwood species, encompassing both non-densified and densified veneers. Additionally, these plywood panels are compared with high-quality birch plywood. Plywood samples made from common aspen (Populus tremula L.) and black alder (Alnus glutinosa L.) species, incorporating densified veneers, are compared to conventional silver birch (Betula pendula Roth.) plywood. The study also considers material utilization efficiency. Results showed that plywood made from non-densified aspen and black alder veneers exhibited comparable strength to standard birch plywood, positioning them as competitive alternatives to high-quality wood species. While densified black alder veneers increased strength in average from 83.6 MPa to 126.3 MPa, a loss in load-carrying capacity was observed from 1930 N to 1637 N due to reduced thickness and moment of inertia.

Free vibration and buckling behavior of porous orthotropic doubly-curved shallow shells subjected to non-uniform edge compression using higher-order shear deformation theory

Due to the continuous enhancement of manufacturing technology for porous materials, it is expected to be increasingly used in aerospace, aviation, and other engineering applications, where the necessity for lightweight structures is paramount. Also, the non-uniform edge loads caused by service conditions, complex adjacent structures, and external loads lead to localized stress concentrations within the shells. Localized stress concentrations result in a loss of load-carrying capacity in specific regions, causing step-by-step failure of the shell. This study investigates the buckling and free vibration behavior of porous orthotropic doubly-curved shallow shells with four different porosity distribution patterns subjected to non-uniform edge compressive loads. The equations of motion of doubly-curved shallow shells are established by using higher-order shear deformation theory and Hamilton’s principle. Then, the pre-buckling in-plane stress distributions of doubly-curved shallow shells are determined and appended to the equations of motion. These fundamental partial differential equations are solved by Galerkin’s method, and the critical buckling load and natural frequency formulations are achieved. By comparing with the results in published literature, the feasibility and accuracy of present formulations are validated. Finally, systematic parametric studies are carried out to discuss the effects of different non-uniform edge compression patterns, porosity distribution patterns, porosity coefficients, aspect ratios, arc length-to-thickness ratios, radius-to-arc length ratios, orthotropy ratios, and shell types on the buckling and free vibration responses of porous orthotropic doubly-curved shallow shells. Parametric studies indicate that the reduction or increment in critical buckling loads (CBLs) and fundamental natural frequencies (FNFs) of spherical shells (SSs) are more sensitive than those of hyperbolic paraboloidal shells (HPSs). The CBLs and FNFs of SSs are larger than those of HPSs. The maximum and minimum CBLs are achieved for the triangular and uniform edge compression, respectively. The porosity effect is independent of edge compression pattern types.

Characterizing properties of fungal-decayed cross laminated timber (CLT) connection assemblies

The effects of brown rot decay fungi on mechanical properties of CLT connection assemblies were investigated. CLT connections evaluated were assembled with code approved angle bracket connectors and modelled as floor-to-wall systems in mass timber buildings. Physical changes, mass loss and quasi-static cyclic tests were used to assess the performance of connection assemblies up to 40 weeks after fungal inoculation. Peak load, stiffness, energy dissipation and ductility of connections were characterized based on force–displacement data generated from the destructive test of connections. Assemblies experienced up to 57 % loss in load carrying capacity and 90 % loss in energy dissipating capacity of the connections after 40 weeks of fungal exposure. Connection stiffness was only slightly impacted over this period but wetting and redrying caused significant degradation of connection ductility.

A Displacement-Based Fiber Element to Simulate Interactive Lateral Torsional and Local Buckling in Steel Members

Collapse in steel structures is often controlled by loss of load carrying capacity of steel columns due to interactive buckling, which involves interactions between local and global (i.e., lateral and lateral torsional) buckling. Commonly used concentrated plastic hinge or fiber-based elements do not simulate the physics of this response, potentially leading to inaccuracy in performance assessment. A nonlinear fiber-beam-column element [termed the Torsion Fiber Element (TFE)] to simulate monotonic interactive buckling in steel beam-columns is presented. The element, implemented in the OpenSees platform, incorporates St. Venant as well as warping torsion through enrichment of strain interpolation functions, in addition to axial and flexural deformation modes. Local buckling is represented through a softening multiaxial constitutive relationship. The efficacy of this approach is examined by comparing its results against those obtained from continuum finite element simulations as well as experimental data on beam-columns subjected to monotonic loading. The comparisons indicate that the element can functionally represent the physics underlying interactive buckling, resulting in effective prediction of the overall monotonic load-deformation response, as well as internal deformation and stress fields. Limitations of the element in its current form are summarized, along with prospective improvements.

Damage Propagation by Cyclic Loading in Drilled Carbon/Epoxy Plates

Fiber reinforced composites are widely used in the production of parts for load bearing structures. It is generally recognized that composites can be affected both by monotonic and cyclic loading. For assembly purposes, drilling is needed, but holes can act as stress concentration notches, leading to damage propagation and failure. In this work, a batch of carbon/epoxy plates is drilled by different drill geometries, while thrust force is monitored and the hole's surrounding region is inspected. Based on radiographic images, the area and other features of the damaged region are computed for damage assessment. Finally, the specimens are subjected to Bearing Fatigue tests. Cyclic loading causes ovality of the holes and the loss of nearly 10% of the bearing net strength. These results can help to establish an association between the damaged region and the material's fatigue resistance, as larger damage extension and deformation by cyclic stress contribute to the loss of load carrying capacity of parts.

Foreword to the special issue on new developments in structural stability.

Foreword to the special issue on new developments in structural stability.

Fatigue in a class of viscoelastic solids

We study fatigue (weakness induced by cyclic loading) in a viscoelastic body described by a generalization of the Kelvin-Voigt constitutive relation, employing a novel damage initiation criterion developed by Alagappan et al. [13-15]. The main premise is that damage is a consequence of the inhomogeneity of the material which leads to some locations in the body being naturally weaker, say for instance due to the density being lower and the material moduli depending on the density and decreasing with density, leading ultimately to failure at that location. This approach has been used successfully for polymers, elastomers and concrete subject to monotonic loading. In this study, we consider the initiation of damage due to cyclic loading, which is referred to as fatigue. Since the body under consideration is viscoelastic, it dissipates energy in each cycle which leads to an increase in temperature. We shall not take the effect of the temperature of the material moduli, instead we assume that the material moduli depend on the density and the rate of dissipation. In the case of our specific study the shear modulus of the material depends on the density and dissipation (in the case of the constitutive relation considered the shear rate), and the structure of the shear modulus is such that it decreases with decrease in density and decreases with increase in dissipation (tantamount to the assumption that it decreases with increasing shear rate for the constitutive relation under consideration) leading to damage of the material. We find that after sufficient number of cycles, the body under consideration undergoes significant loss in load carrying capacity due to fatigue.

Shear band formation in porous thin-walled tubes subjected to dynamic torsion

In this paper, we have performed 3D finite element calculations of thin-walled tubes subjected to dynamic twisting to investigate the effect of porous microstructure on the formation of shear localization bands under simple shear conditions. For that purpose, we have incorporated into the finite element model the porous microstructures of four different additive manufactured metals – aluminium alloy AlSi10Mg, stainless steel 316L, titanium alloy Ti6Al4V and Inconel 718 – for which the void volume fraction varies from ≈0.001% to ≈2%, and the voids size between ≈6μm and ≈110μm (Marvi-Mashhadi et al., 2021). For each microstructure, we have created up to 10 realizations varying the spatial location of the voids and the distribution of voids size. The matrix material is elastic/plastic, with yielding defined by the von Mises yield criterion and associated flow rule. The yield stress evolution is considered to be dependent on strain, strain rate and temperature, with parameters corresponding to Titanium and HY-100 Steel, taken from Molinari (1997) and Batra and Kim (1990), respectively. Moreover, we have assumed the deformation process to be adiabatic. The calculations have been performed for shear strain rates ranging from 100s−1 to 10000s−1. To the authors’ knowledge, this is the first study ever that simulates dynamic torsion testing of porous materials with actual representation of voids, providing new results which bring to light the influence of porosity on dynamic shear banding under simple shearing. Namely, the numerical calculations have shown that both the location of the shear band and the critical strain leading to the shear band formation depend on the spatial and size distribution of the voids in the specimen, evidencing the influence of material defects on the localization pattern. Notably, the shear band nucleation strain decreases with both the void volume fraction in the specimen and the size of the voids, the size of the largest pore being the main microstructural feature controlling the loss of load carrying capacity of the specimen. In addition, we have carried out a parametric analysis varying the temperature and strain rate sensitivities of the material, and the loading rate. For the strain rates investigated, increasing the loading speed leads to a mild decrease of the shear strain leading to shear band formation, while the strain rate sensitivity is shown to stabilize material behavior and delay localization. Moreover, the numerical results have made apparent that for the hardening materials considered, thermal softening is essential to trigger the shear band formation, so that the porous microstructure alone does not lead to shear localization.

Comparison of Failure for Thin-Walled Composite Columns.

The novelty of this paper, in relation to other thematically similar research papers, is the comparison of the failure phenomenon on two composite profiles with different cross-sections, using known experimental techniques and advanced numerical models of composite material failure. This paper presents an analysis of the failure of thin-walled structures made of composite materials with top-hat and channel cross-sections. Both experimental investigations and numerical simulations using the finite element method (FEM) are applied in this paper. Tests were conducted on thin-walled short columns manufactured of carbon fiber reinforced polymer (CFRP) material. The experimental specimens were made using the autoclave technique and thus showed very good strength properties, low porosity and high surface smoothness. Tests were carried out in axial compression of composite profiles over the full range of loading—up to total failure. During the experimental study, the post-buckling equilibrium paths were registered, with the simultaneous use of a Zwick Z100 universal testing machine (UTM) and equipment for measuring acoustic emission signals. Numerical simulations used composite material damage models such as progressive failure analysis (PFA) and cohesive zone model (CZM). The analysis of the behavior of thin-walled structures subjected to axial compression allowed the evaluation of stability with an in-depth assessment of the failure of the composite material. A significant effect of the research was, among others, determination of the phenomenon of damage initiation, delamination and loss of load-carrying capacity. The obtained results show the high qualitative and quantitative agreement of the failure phenomenon. The dominant form of failure occurred at the end sections of the composite columns. The delamination phenomenon was observed mainly on the outer flanges of the structure.

Progressive fatigue damage model for FRP wires under longitudinal cyclic tensile loading

This paper develops a progressive fatigue damage model to study the initiation and progression of fiber damage in the unidirectional FRP considering the boundary effect. A characteristic representative volume element is proposed and a damage indicator describing the loss of load carrying capacity of fiber elements is introduced and combined with the principle of superposition to realize the stress redistribution caused by fiber breakage; the maximum fatigue stresses of the neighboring fiber elements affected by fiber breakage are updated. Monte Carlo simulation is conducted to study the fatigue behavior of FRP wires considering the effect of variabilities in the static strength and the fatigue life of single fibers. Results indicate that the proposed model can predict the fatigue life and stiffness degradation of FRP wires well compared with the experimental data. Specifically, the cluster sizes of critical damage for FRP wires during the fatigue process can be determined, which provides quantitative information for the fatigue mechanism of the FRP wires.

Experimental investigation of damaged RC joints retrofitted by stiffened angles and bars

Experimental studies reveal that weakness in joint core leads to the formation of a shear hinge in this area, loss of load carrying capacity in the both regions of cyclic curve and decrease of ductility. Generally, the recent mentioned results are related to the previous studies, in which the effect of slabs and lateral beams (2D joints) have been neglected. However, these effects can lead to different responses in real joints compared to ideal 2D joints, in the term of changes in capacity and demand. Consequently, in this study the effect of slab and lateral beam has been considered (3D joints). Four 3D external joints with a scale of 1/2 are made in the laboratory. Specimens include two control joints and two damaged joints retrofitted by stiffened angles and post-tensioned bars. The results show that the use of retrofit method for damaged joints results in the formation of a flexural plastic hinge in the beam and shift of the shear hinge out of the joint core. Furthermore, the effect of slab and lateral beam in the bare non-seismic joint may lead to the increase of the ductility, compared to the similar 2D joints in previous studies.

Impact of concrete strength on seismic behavior of T-shaped double-skin composite walls

This study investigates the seismic performance of T-shaped double-skin composite walls (DSCWs), which are comprised of double-skin composite flange and web walls and concrete filled steel tubular (CFST) boundary elements, through experimental tests. Six T-shaped DSCW specimens, which are categorized into three groups, according to flange width and type of mechanical connectors (shear studs/tie bars), are tested under combined axial compressive loads and lateral cyclic loads. The DSCWs fail in a ductile flexural mode with similar damage patterns, characterized by buckling of steel tubes and faceplates, concrete crushing, and fracture of steel tubes at the wall base, and exhibit asymmetric hysteresis behaviors, with higher stiffness and strength but lower deformation capacities when the flange walls are under tension. Special focus should be placed on the design of CFST boundary element connected to the web wall, whose compressive failure triggers the loss of load carrying capacity of the specimens. The comparisons between the behaviors of two specimens in each group, which are infilled with high strength (C70) and normal strength concrete (C30) respectively, suggest that employment of high strength concrete improves the seismic behavior of T-shaped DSCW significantly regardless of the flange wall width when shear studs are used. Tie bars may impair the seismic behaviors of DSCWs by inducing premature local buckling of perforated steel faceplates. Numerical studies are conducted using detailed finite element models to investigate the buckling behaviors of steel faceplates which are connected to infilled concrete by tie bars and shear studs. The simulation results reveal that the former buckles earlier than the latter due to smaller critical stress. The slenderness ratio limits to prevent local buckling prior to yielding of steel faceplates are also suggested when these two types of mechanical connectors are used.

Three-dimensional thermohydrodynamic investigation on the micro-groove textures in the main bearing of internal combustion engine for tribological performances

A three-dimensional thermohydrodynamic numerical simulation study was used to investigate the impact of the micro-groove surface texturing on the tribological performances in the main bearing of the internal combustion engine. For this purpose, various number of grooves and groove height to the bearing surface were applied to determine the optimal texture surface parameters by comparing the load-carrying capacity and friction force in the engine main bearing. In the multiphysics numerical model, the three-dimensional Navier–Stokes equation was employed considering the cavitation mechanism based on the Elrod method in the solution. Using the transverse grooves on the bearing surface altered the cavitation response and film reformation. To validate the use of the current numerical model for analyzing the bearings, the obtained results were compared with those of the published theoretical papers, where a good agreement was obtained. The bearing performance was studied in thermal interface conditions to find the optimal set textures parameters that gave minimum fiction force with minimum loss in load-carrying capacity. The bearing with the optimal micro-groove texture parameter showed a reduction in friction (around 16%) with the minimum reduction in load-carrying capacity (around 6%) and the maximum reduction of the flow work (around 15%) compared with the untextured bearing surface. This paper focuses on the thermohydrodynamic investigation with a combination of thermal effects of the fluid film in the textured bearing. Meanwhile, the heat transfer characteristic, temperature distribution of solid bodies, and convection heat transfer coefficient in the contact surfaces of the textured bearing were investigated. The proposed multiphysics numerical model can be widely used for predicting the optimal texture surface parameters in different engineering systems modeling. Moreover, using the three-dimensional-based numerical model is more cost-effective compared with the experimental evaluation of the textured surface.

Life-Extending Benefit of Crack Sealing for Pavement Preservation

Crack sealing and filling is used as a pavement preservation treatment to prevent water infiltration and loss of load-carrying capacity, hence extending the life of the pavement. This study used field performance data of test sections from the Pavement Preservation Group (PG) Study being conducted in part by the National Center for Asphalt Technology (NCAT). Data from test sections located in a low traffic volume road with a hot, wet, no-freeze climate collected over a period of more than 6 years were used to evaluate the effect of crack sealing and filling, either as a stand-alone treatment or in combination with other surface treatments. A semi-parametric survival analysis was performed to determine the difference in median time to failure (MTTF) for sections with and without crack sealing. The results showed that when used as a stand-alone treatment on a pavement in “good” condition, the life-extending benefit from crack sealing/filling could not be calculated for the selected analysis period because the MTTF would not be reached within 10 years. The estimated benefit for pavements in “fair” and “poor” condition ranged from 1.1 to 7.3 years, depending on pretreatment condition and travel lane. If used in combination with a chip seal, the additional life-extending benefit for a pavement in “good” condition is approximately 2 years, and is slightly decreased to approximately 1.5 years if the pavement is in “fair” to “poor” category. Test sections continue to be monitored as part of a broader long-term study.

Full scale experiments on splitting behaviour of concrete slabs in steel concrete composite beams with shear stud connection

This paper presents the results of 15 push-off tests to determine, with a high degree of precision, the longitudinal splitting characteristics of a concrete slab in a novel steel-concrete composite beam with headed shear stud connectors. A modified test rig was developed, instead of the standard push-off test rigs. Both monotonic and cyclic loadings were considered. The experiments allowed the exploration of several currently unknown effects of various loads on components of steel-concrete composites, such as transverse loading and low-level cyclic loading on shear stud capacity. The results allowed the splitting failure mode to be characterised in unprecedented detail. It was found that for the case of cyclic loading, when many cycles were applied, it did not lead to premature failure of the composite beam, but instead increased the capacity of the beam against longitudinal splitting. This is significant for composite members under variable loading, such as floor supporting beams in car parks. It was also shown that the transverse compression forces across the base of the studs would increase the capacity of the beam against longitudinal splitting. Also studied were the differences and limitations of EuroCode 4 and New Zealand (NZS) standards and their effects on composite beam failure characteristics. It was shown that the presence of anti-splitting reinforcement would result in a gradual loss of load carrying capacity, compared to a sudden loss when not present. Even with sufficient side cover from the stud to the edge of the concrete, following the NZS standard, longitudinal splitting in composite beams continues to be observed.

Reactive Powder Concrete with Steel, Glass and Polypropylene Fibers as a Repair Material

Repairing of reinforced concrete structures is currently a major challenge in the construction industry and is being put back into operation with a slight loss in load carrying capacity. Damage occurs due to many factors that reduce the strength of concrete structures and their durability. The aim of this paper is study the compatibility between three types of reactive powder concrete with (steel fibre, glass fibre and polypropylene fibre) as a repair materials and normal strength concrete as a substrate concrete. Compatibility was investigated in three steps. First: individual properties for substrate concrete were studied, these are (slump test, compressive strength, splitting strength, and flexural strength) also, for repair material these are (compressive strength and flexural strength) were determined by using standard ASTM test methods. Second: bond strength of composite cylinder for substrate concrete with different repair materials were evaluated by using slant shear test. Third: compatibility was investigated by using composite prisms of substrate concrete with different repair materials under two-point loading (flexural strength test). From the experimental results concluded, bond strength between reactive powder concrete with glass fibre as a repair material and normal strength concrete as a substrate layer is higher (17.38Mpa) compared with RPC with steel fibre (13.13Mpa) and polypropylene fibre (14.31MPa). Also, it is more compatible due to flexural strength for composite prisms (having higher flexural strength (8.13MPa). Compared with steel fibre (7.44MPa) and polypropylene fibre (6.47MPa). These results due to RPC with glass fibre have good workability with suitable flowability and glass fibre have higher tensile strength compare with other fibre.

Weakened Zones in Wood – Based Composite Beams and Their Strengthening by CFRP: Experimental, Theoretical, and Numerical Analysis

This paper looks at the mechanical behaviour of wood-based beams. Laboratory tests of an I-beam show a considerable decrease of capacity in the weakened zones, e.g., at the connection between two pieces of pine wood used as a flange. We propose strengthening of the zones using fibre composite tapes based on CFRP. This article demonstrates the way for protection of the I-beam with defects in the bottom flange against the loss of load-carrying capacity. The required anchor length is determined by an analytical method formulated in the paper. The numerical results are presented. The full adhesion between CRFP and wood is assumed in the numerical simulations.

Reliability analysis of shallow foundation in the vicinity of the existing buried conduit

ABSTRACTThe present study proposes reliability-based approach for assessing the performance of shallow foundation placed in the vicinity of an existing buried flexible pipe or utility tunnel. Performance function for the reliability analysis is defined in terms of % bearing capacity loss in the load carrying capacity of the shallow foundation due to the presence of buried flexible pipe or utility tunnel, and, allowable bearing capacity loss in load carrying capacity that can be tolerated. For the reliability analysis, an explicit functional relationship between input variables, such as geotechnical parameters of in situ soil as well as material properties of pipe, and, output response, i.e. % bearing capacity loss in load carrying capacity of foundation soil is needed. Using concept of response surface methodology (RSM) combined with the results of the numerical analysis; such an explicit functional relationship is easily established. Thereafter, reliability analysis can be performed, conveniently, using standard First Order Second Moment (FOSM) approach and performance of the foundation soil system with buried flexible pipe, present in the vicinity, can be assessed in terms of an index, popularly known as ‘reliability index (β)’.

On the micro- and macroscopic elastoplastic deformation behaviour of cast iron when subjected to cyclic loading

The complicated constitutive behaviour of cast iron, involving a non-linear elastic regime, tension-compression stress asymmetry, varying elastic modulus and an inflection in the tension-to-compression hardening curve, is investigated using a micromechanical modelling approach. In this way, it is demonstrated that the abnormalities observed in the constitutive behaviour are qualitatively and quantitatively explained by the interaction behaviour between the matrix and graphite constituents. In initial tension, the absence of linearity is rationalised by the successive loss in load-carrying capacity of the graphite phase due to debonding, which in subsequent cycling, results in the opening and re-contact of the matrix-graphite interface. This effect is demonstrated to result in tension-compression asymmetry in stress and elastic modulus, as well as the inflection in tension-to-compression loading. The given model of explanation is validated by comparison to the experimentally acquired microscopic strain field in EN-GJV-400 at locations where stress concentrations are generated due to the matrix-graphite debonding, using high-resolution digital image correlation of scanning electron images.

Monotonic and cyclic testing of clay brick and lime mortar masonry in compression

This research presents an experimental programme on the mechanical characterization of masonry under monotonic and cyclic uniaxial compression. Two different types of standard specimens, running bond walls and stack bond prisms, were built using handmade clay bricks and hydraulic lime mortar. The experimental results are compared and discussed in terms of strength, stiffness and deformability. It was observed that the two specimen types provided very similar results on both strength and stiffness. Cyclic loading tests carried out on a set of samples provided new experimental evidence on the stiffness degradation, loss of load carrying capacity for increasing irreversible compressive strains and energy dissipation. The paper presents eventually a thorough discussion about the comparison between the obtained experimental results with available predictive models for strength, stiffness and fracture energy of masonry under monotonic and cyclic compression loading.