Failure Mode Changes Research Articles

Low-velocity impact response of wood-strand sandwich panels and their components

AbstractProfiled hollow core sandwich panels (SPs) and their components (outer layers and core) were manufactured with ponderosa and lodgepole pine wood strands to determine the effects of low-velocity impact forces and to observe their energy absorption (EA) capacities and failure modes. An instrumented drop weight impact system was applied and the tests were performed by releasing the impact head from 500 mm for all the specimens while the impactors (IMPs) were equipped with hemispherical and flat head cylindrical heads. SPs with cavities filled with a rigid foam insulation material (SPfoam) were also tested to understand the change in EA behavior and failure mode. Failure modes induced by both IMPs to SPs were found to be splitting, perforating, penetrating, core crushing and debonding between the core and the outer layers. SPfoams absorbed 26% more energy than unfilled SPs. SPfoams with urethane foam suffer less severe failure modes than SPs. SPs in a ridge-loading configuration absorbed more impact energy than those in a valley-loading configuration, especially when impacted by a hemispherical IMP. Based on the results, it is evident that sandwich structure is more efficient than a solid panel concerning impact energy absorption, primarily due to a larger elastic section modulus of the core’s corrugated geometry.

A molecular dynamics study of the effects of hydrogen bonds on mechanical properties of Kevlar® crystal

Effects of loading direction and hydrogen bonds on the anisotropic elastic and strength properties of p-phenylene terephthalamide (PPTA) crystal (i.e. Kevlar®) are studied using molecular dynamics simulations with reactive force field ReaxFF. Simulations of monotonic tension, compression and shear loadings of the crystal indicate that the elastic properties exhibit orthotropic symmetry but strengths in the various crystal directions are anisotropic and highly dependent on the direction of loading. Tensile strength in the chain axis of the crystal fails by chain scission at a stress level that is twenty times higher than the compression strength governed by chain buckling. To study the effects of hydrogen bonds, simulations are repeated by removing the hydrogen bonding interactions from the ReaxFF force field parameter file. Simulation results indicate that hydrogen bonds do not affect the axial properties that are dominated by covalent bonds in the chains. However, in the absence of hydrogen bonds, transverse tensile, compressive and shear moduli (40–44%) and strengths (47–69%) are reduced significantly. The absence of hydrogen bonding on changes in failure modes for the various loading directions are identified. The results presented in this study provide insight into moisture induced property degradation mechanisms of Kevlar® reported in the literature attributed to reduction in hydrogen bonding.

Durability of aluminium alloy adhesive joints in cyclic hydrothermal condition for high-speed EMU applications

The representative adhesive structure of high-speed electric multiple units (EMU) is the side-window, the durability of which is concerned due to the degradation caused by changeable temperature and moisture in hostile service environment. It is of great significance to evaluate the adhesive structure's durability of side-window to prevent bonding failure. In this study, aluminium alloy single lap joints (SLJ), scarf joints (SJ) and butt joints (BJ) were manufactured to conduct accelerated aging tests under cyclic hydrothermal (CH)condition, in which the temperature and moisture vary from 80 °C/95% relative humidity (RH) to −40 °C/30%RH periodically. The adhesive Sikaflex®-265 were studied by Fourier transform infrared (FTIR), thermogravimetric analysis (TGA) and Differential scanning calorimetry (DSC). The failure strength of adhesive joints was tested after different aging cycles and the scanning electron microscopy (SEM) was also employed to investigate the microscopic fracture mechanisms. It has been found that the composition, glass transition temperature (Tg), melting temperature(Tm) and thermal stability of adhesive changes after CH aging. The failure strengths of SLJ, SJ and BJ show cubic polynomial descending with aging cycles, and loss about 30% of initial strength after 60 aging cycles. Moreover, the failure mode changes from cohesive failure to mixed failure after CH exposure, which is verified by SEM analysis. The failure criteria before and after CH aging were consistent with quadratic stress criteria, and an equation for a surface describing the variations of quadratic stress criterion with different aging cycles was built, providing reference for the durability evaluation of high-speed EMU's adhesive structures.

Finite Element Analysis of the Void Growth and Interface Failure of Ductile Adhesive Joints

Progressive failure of ductile porous adhesive joint generally includes two competitive failure modes: the void growth of ductile adhesive layer and the interface debonding between the adhesive layer and bonding plates. The damage evolution behavior and ultimate strength of ductile adhesive joint are largely dominated by their evolving interactions. However, most of the existing research failed to predict the damage evolution of these two failure modes simultaneously. After the variational weak form of dynamic equilibrium for two adhesive solids with a finite-thickness adhesive layer and two discontinuous cohesive interfaces is given, this paper studies theoretically the competition between these two failure modes using explicit finite element analysis (FEA). The finite-deformation Gurson–Tvergaard–Needleman (GTN) model is used to predict the void growth of adhesive layer, and the bilinear cohesive model as a ABAQUS module is used to simulate the interface debonding. For single-lap joint under tensile loads, effects of the cohesive strengths, the initial void volume fraction, and the thickness of adhesive layer on their interactions are explored. Besides, the ultimate strengths by FEA are also compared with analytical solutions. Numerical results show that dominating failure mode changes from the interface debonding to the failure of adhesive layer at about the cohesive strength 40 MPa and the thickness of adhesive layer 0.5 mm for FM-73 ductile adhesive joint.

The Carbon Nanotubes Effect into Single-lap Joint Failure Modes and Load Capacity: a Macromechanical Analysis

This study investigates the influence of the adhesive (epoxy resin) thickness and the dispersions of non-functionalized carbon based nanostructures (carbon nanotubes - CNT) on mechanical properties of single-lap bonded joints. To achieve this goal, three CNT concentrations (0.5%, 1.0% and 2.0% m/m); and three different bondline thicknesses (0.05 mm, 0.15 mm and 0.40 mm) were evaluated. The mechanical properties were measured using the apparent shear tensile test, based on ASTM D5868. The results showed that the addition of 1.0 wt% CNTs improved the interface strength, leading to an increase on delaminated areas in failure region up to 55.36% and also improving the peak force up to 13.85%. Decrease in adhesive thickness from 0.40 mm to 0.05 mm promoted a stress redistribution inside adhesive layer improving the peak force up to 13.91%. CNT seems to promote changes in failure modes, light fiber tear (LFT) failure significantly increases up to 45.96%, indicating that interface strength between adhesive and adherent was improved.

Thermal creep effect on the behavior of shear tab connections due to fire temperatures

This study investigates the effect of thermal creep of steel on the behavior of shear tab beam-column connections due to fire temperatures. FE simulations are first developed under fast steady-state temperature conditions to study the behavior of the shear tab connections at different temperatures and validated against experimental data available in the literature. Parametric studies are performed to investigate the impact of major geometric parameters on the behavior of the shear tab connections during fire temperatures with and without creep effect. Then, creep effect is explicitly included in the FE models under steady-state temperature conditions to study the time-dependent deformations of shear tab connections when applied forces and temperatures are kept constant. Also, isochronous force-rotation curves are developed to better understand the time effect on the strength and rotational capacity. Results from the simulations and parametric studies show that the connection experiences additional rotation/displacement and a change in failure mode is observed when the effect of thermal creep is included in predicting the response of the shear tab connections. These results clearly indicate the importance of including creep in predicting the response of shear tab connections subjected to fire temperatures and its effect on the behavior.

Failure characteristics of jointed rock-like material containing multi-joints under a compressive-shear test: Experimental and numerical analyses

Extensive efforts have been made to gain a better understanding of the failure behaviour of rocks and rock-like materials, but crack propagation and failure processes under compressive-shear loading have not yet been comprehensively investigated. To address this area of research, the peak shear strengths (τ) and failure processes of specimens with multiple joints are studied by lab testing and particle flow code (PFC2D). Four types of failure modes are observed: (a) shear failure through a plane (Mode-I), (b) intact shear failure (Mode-II), (c) oblique shear crack connection failure (Mode-III), and (d) stepped path failure (Mode-IV). The failure mode gradually transformed to Mode-III as α (joint inclination angle) increases from 0° to 90° in the specimens. In addition, with increasing joint distance (d) in the specimens, the failure mode changes to Mode-II. As the non-overlapping length between joints (c) in the specimens increases, the failure mode changes to Mode-IV. The joint geometry has a major influence on the shear strength of the jointed specimens. The peak shear strength of specimens with different joint inclination angles is obtained when α=45°. Additionally, the peak shear strength increases as the joint distance (d) and non-overlapping length (c) increase.

A mechanism for energy absorption: Sequential micro-kinking in ceramic reinforced aluminum alloy lattices during out-of-plane compression

The current study examines the out-of-plane compressive response of ceramic/aluminum hybrid lattice materials with an anodic alumina outer shell and an aluminum alloy core. The combination of increasing truss angle and oxide coating thickness results in a six-fold increase in compressive strength, two-fold increase in densification strain, twelve-fold increase in energy absorption per volume, and twenty-fold increase in energy absorption per mass. Such improvements are caused by the change in failure mode from mid-strut buckling to a hinge kinking mode as the oxide coating thickness increases. Microscopy and analytical modelling reveal that kink formation is most likely initiated by aluminum shear band formation followed by oxide rotation and fracture (analogous to micro-kinking in fiber composites). In terms of energy absorption, the best performing lattice materials in the current study were on par with the best available cellular materials in existing literature.

Dependence of ultimate bearing capacity and failure behavior of T-shaped deep cement mixing piles on enlarged cap shape and pile strength

A numerical investigation of the ultimate pile capacity (Qult) and failure behavior of T-shape deep cement mixing (TDM) piles is conducted under equivalent volume. Physical model tests are conducted to verify the numerical findings. The results reveal that both the shape of the enlarged pile cap and pile strength play key roles. Sensitivity analyses indicate that the effects of cap shape and pile strength on the complex load carrying behavior of TDM piles are attributable to changes in failure mode, which depend on the mobilized skin friction and bearing of both the cap and pile body.

Acoustic Emission and Damage Characteristics of Granite Subjected to High Temperature

Acoustic emission (AE) signals can be detected from rocks under the effect of temperature and loading, which can be used to reflect rock damage evolution process and predict rock fracture. In this paper, uniaxial compression tests of granite at high temperatures from 25°C to 1000°C were carried out, and AE signals were monitored simultaneously. The results indicated that AE ring count rate shows the law of “interval burst” and “relatively calm,” which can be explained from the energy point of view. From 25°C to 1000°C, the rock failure mode changes from single splitting failure to multisplitting failure, and then to incomplete shear failure, ideal shear failure, and double shear failure, until complete integral failure. Thermal damage (DT) defined by the elastic modulus shows logistic increase with the rise of temperature. Mechanical damage (DM) derived by the AE ring count rate can be divided into initial stage, stable stage, accelerated stage, and destructive stage. Total damage (D) increases with the rise of strain, which is corresponding to the stress‐strain curve at various temperatures. Using AE data, we can further analyze the mechanism of deformation and fracture of rock, which helps to gather useful data for predicting rock stability at high temperatures.

Direct shear testing of brittle material samples with non-persistent cracks

The mechanical behavior of the brittle material samples containing the internal and edge cracks are studied under direct shear tests. It is tried to investigate the effects of stress interactions and stress intensity factors at the tips of the pre-existing cracks on the failure mechanism of the bridge areas within these cracks. The direct shear tests are carried out on more than 30 various modeled samples each containing the internal cracks (S models) and edge cracks (E models). The visual inspection and a low power microscope are used to monitor the failure mechanisms of the tested samples. The cracks initiation, propagation and coalescences are being visualized in each test and the detected failure surfaces are used to study and measure the characteristics of each surface. These investigations show that as the ratio of the crack area to the total shear surface increases the shear failure mode changes to that of the tensile. When the bridge areas are fixed, the bridge areas in between the edge cracks have less strength than those of internal cracks. However, the results of this study show that for the case of internal cracks as the bridge area is increased, the strength of the material within the bridge area is decreased. It has been shown that the failure mechanism and fracture pattern of the samples depend on the bridge areas because as the bridge area decreases the interactions between the crack tip stress fields increases.

Effect of low-temperature annealing on tensile behavior of electrodeposited bulk nanocrystalline Ni–W alloys

Relatively low-temperature thermal treatment of nanocrystalline metals results in grain boundary relaxation and the absence of clear grain growth. However, the manner in which the relaxation affects the tensile properties of bulk nanocrystalline metals is unclear. In this study, the effect of low-temperature annealing on the tensile behavior of nanocrystalline metals was studied using electrodeposited Ni–W alloys as a model system. Tensile tests were conducted using the electrodeposited bulk nanocrystalline Ni–W alloys in the as-deposited state and after annealing at 200–300°C for 24h. After annealing, the yield strength and tensile strength increased by 0.25–0.34 and 0.13–0.18GPa, respectively, while uniform elongation decreased from 4.8% to 2.5%. Good local elongation of 6.0–7.0%, which was comparable with the value in the as-deposited state, was observed after annealing at 200 and 250°C. However, annealing at 300°C induced a degradation of local elongation to 2.6%. The results of this study indicate that reduction of excess boundary defects affects dislocation emission from grain boundaries, leading to increasing yield strength and decreasing uniform elongation, whereas the relaxation has no effect on the local elongation. In addition, discussion of the microstructures of Ni–W alloys also suggest the possibility that the failure mode changes at annealing temperatures near 250–300°C by changing the shape of grain boundaries more linearly, in addition to a reduction of excess boundary defects.

Relationship between unusual high-temperature fatigue crack growth threshold behavior in superalloys and sudden failure mode transitions

An investigation of high temperature cyclic fatigue crack growth (FCG) threshold behavior of two advanced nickel disk alloys was conducted. The focus of the study was the unusual crossover effect in the near-threshold region of these type of alloys where conditions which produce higher crack growth rates in the Paris regime, produce higher resistance to crack growth in the near threshold regime. It was shown that this crossover effect is associated with a sudden change in the fatigue failure mode from a predominant transgranular mode in the Paris regime to fully intergranular mode in the threshold fatigue crack growth region. This type of a sudden change in the fracture mechanisms has not been previously reported and is surprising considering that intergranular failure is typically associated with faster crack growth rates and not the slow FCG rates of the near-threshold regime. By characterizing this behavior as a function of test temperature, environment and cyclic frequency, it was determined that both the crossover effect and the onset of intergranular failure are caused by environmentally driven mechanisms which have not as yet been fully identified. A plausible explanation for the observed behavior is proposed.

Analytical and finite element studies on hybrid FRP strengthened RC column elements under axial and eccentric compression

Analytical and finite element studies on the behavior of Reinforced Concrete (RC) column elements strengthened using a hybrid Carbon Fiber Reinforced Polymer (CFRP) laminates and externally bonded fabric is explored in this study. The main objective of this study is to evaluate the effect of hybrid strengthening on the initial and post-cracking stiffness, peak and post-peak behavior, and change in failure modes of RC column elements. Both axial and eccentric loads are considered in this study. Experimental program is carried out by testing the column elements with and without different FRP strengthening. Analytical predictions are obtained using the strain compatibility approach based on the existing constitutive models for unconfined and confined concrete, steel and FRP composites. A numerical model of column elements is developed using a commercial software ABAQUS. The predictions obtained from the analytical and FE analysis were validated with the experimental results. Analytical and FE predictions exhibited a fair correlation with discrepancy of less than 5% when compared to test results. A parametric study is carried out using the validated FE analysis by varying the CFRP fabric ratio, CFRP laminate ratio and their hybrid combinations. Hybrid strengthening technique is found to be more efficient in improving the initial and post-cracking stiffness, strength and ultimate displacement ductility of RC column elements under compression.

On the in-plane shear response of the high bond strength concrete masonry walls

Conventional unreinforced masonry walls subject to in-plane shear loading fail due to exceedance of shear and tensile bond strengths. This paper examines whether or not the in-plane shear capacity of masonry walls would increase with the increase in the bond strengths through experimental and numerical investigations. For these investigations, shear walls were built with high bond strength polymer cement mortar; they were applied in thin layers of 2 mm thickness each. Material tests were carried out to characterise the bond and the compressive strengths of the high bond strength thin layer mortared masonry; the bond strengths were found approximately double that of the conventional 10 mm thick cement mortars. The shear walls, however, exhibited significantly lower capacity (contrasting the expectation) and displayed base course sliding mode of failure. To ascertain the validity of the experimental results, a combined surface contact—interface element micro finite element (FE) modelling technique was formulated; the results adequately reproduced the experimental datasets. The validated FE model was then applied to examine the effect of the aspect ratios and pre-compression levels to the failure modes, deformation and strength of the high bond strength shear walls and is shown that once the pre-compression exceeds 15% of the masonry compressive strength, the base sliding failure mode changes to the diagonal cracking mode with corresponding increase in in-plane shear capacity. Therefore, it is concluded that the increase the bond strength without regard to pre-compression could adversely affect the safety of the high bond strength unreinforced masonry shear walls.

Resistance assessment of beam-to-column joints with different blind bolt systems

Blind bolted connections enable simple joining of various beams to tubular columns. In the parametric analysis several different blind bolts systems were compared through the example of I-beam-to-hollow section column joint with extended endplate. 1155 combinations of parameters defining behaviour of considered joint type were analysed. The size of a bolthole or more precisely blind bolt geometry has major effect on joint resistance. The most common failure mode is bending of the column flange. In the cases where the thickness of the endplate is lower than or equal to the thickness of the column wall, endplate most frequently fails by bending. Obviously the use of an elliptical yield line mechanism and appreciation of the membrane action effect increase the joint resistance. It is suggested to investigate further the membrane action effect with the use of elliptical yield line mechanism especially in the region where the failure mode changes.

On the assessment of the load-bearing capacity of Z-pinned composite T-joint under out-of-plane tension

Z-pin technology, as a promising through-thickness reinforcement, offers an efficient solution for the weak composite parts joints, especially for large integrated composite structures. This paper presents a two-dimensional finite element model to analyze the enhancing efficiency of Z-pins for composite T-joints. The modeling used a new parameter definition method for delamination growth simulation in Z-pinned joining interface via a regional cohesive zone model. The finite element model was validated by experiments for unpinned and Z-pinned T-joints under web pullout tension. The parametric analysis on the structural configuration shows that shear stress ratio in Z-pins declines with the span ratio increasing and the skin thickness ratio decreasing, which leads to the failure mode change of a single pin from pull-out to shear fracture so that the improvement on the out-of-plane tension declines gradually. Within 1.7–2.1 span ratio (the constant 4 mm skin thickness) or 1.8–2.5 thickness ratio (the 200 mm ...

Preliminary Numerical Analysis of a Masonry Panel Reinforced with Pultruded GFRP Profiles

The peculiarities of pultruded FRP profiles, i.e. low mass, durability and ease of construction, make them suitable for retrofitting traditional masonry structures, particularly in seismic areas. This could represent an effective solution, not yet sufficiently explored, that allows for non-invasive and reversible interventions, which improve the structural performance with a very small structural mass addition. The paper presents a FEM study on a hypothesis of retrofit of a traditional masonry building with pultruded FRP frame, adjacent to the masonry structure and connected to it with mechanical fasteners. The results appear promising and enlighten much increased in-plane strength and stiffness, as well as the change of the masonry failure mode into a more dissipative one.

Compression load-carrying capacity of 3D-integrated weft-knitted spacer composites

Recent developments in composite manufacturing have been resulted in formation of newly-known 3D integrated weft-knitted fabrics which can be used as the composites’ reinforcing materials. In this paper, the compression-resistivity of 3D composite panels reinforced with these newly designed 3D textile-preforms from E-glass fibers has been studied. Following this research, the composites mechanical functionality under flatwise and edgewise compression loadings was evaluated. Using VIP method, three groups of glass/epoxy composite with different core thicknesses and structural geometries were prepared. It was concluded that the compressive strength of the flat-wisely loaded samples would significantly decrease by increasing the thickness. Moreover, changing the composites’ geometrical shape leads to some changes in failure mode; in this regard, the produced single-decker U-shaped panels only suffer from the pure buckling failure, while the double-decker U-shaped panel failed due to a combination of facing bending stress, core shear stress, and buckling failure. Thickness changes are not as effective as structural geometry changes on the panels’ compress-resistivity under edgewise compression. As compared with the conventional 3D woven sandwich composites, it was approved that mechanical functionality of the produced 3D integrated weft-knitted spacer panels is completely improved so that they can be considered as good alternatives especially in building constructions.

Influence of Radial Stress Gradient on Strainbursts: An Experimental Study

Strainbursts, which are violent disasters that are accompanied by the ejection failure of rocks, usually occur in hard brittle rocks around highly stressed underground openings. The release of the radial stress at excavation boundaries is one of the major inducing factors for strainbursts in tunnels. After excavation, the radial stress usually exhibits different but apparent gradient variations along the radial direction near the boundary within a certain depth under different in situ stress conditions. In this study, the influence of the radial stress gradient on strainbursts of granite was investigated using an improved true-triaxial rockburst testing system, which was equipped with an acoustic emission monitoring system. The stress state and boundary conditions (i.e., one face free, other faces loaded and increasing tangential stress) of the representative rock element in the vicinity of the excavation boundary were simulated. High-speed cameras were used to capture the ejection failure processes during strainbursts, and the kinetic energy of ejected fragments was quantitatively estimated by analyzing the recorded videos. The experimental results indicate that with an increasing radial stress gradient, the strength increases, the apparent yield platform prior to the peak stress on the stress–strain curves decreases, the failure mode changes from strainburst characterized by tensile splitting to strainburst characterized by shear rupture, and the kinetic energy of ejected fragments during strainbursts significantly increases.