Experimental Load Displacement Research Articles

Bearing performance and progressive failure analysis of bolted joint in 3D printed pseudo-woven CFRP composite with fibre steering

This study investigates the bearing failure process of 3D printed pseudo-woven carbon fibre reinforced polymer (CFRP) composite joints, with a particular focus on the damage mechanisms influenced by steered fibres. A multiscale finite element model employing LaRC05 failure criteria is developed and validated against the experimental load–displacement curves and micro-computed microtomography (CT) images of four distinct cases. The model clearly demonstrates the critical importance of maintaining fibre continuity around the bolt hole, as this significantly influences the ability to reduce stress concentrations caused by the direct bearing loads from the bolt. Moreover, the model reveals that fibre steering can substantially improve the composite joint’s performance. This enhancement is achieved by adjusting the level of shear-induced damage propagation in individual filaments. The results demonstrate the potential and capability of the model to capture individual filament behaviour for the failure analysis of 3D printed composites, achieving good correlations with experimental measurements and observations, in terms of failure modes and load-bearing capacities.

Estimation of CZM Parameters for Investigating the Interface Fracture of Adhesively Bonded Joints Under Mode I, Mode II, and Mixed‐Mode (I/II) Loading

ABSTRACTThe mechanical performance of joints for any engineering applications requires accurate characterization of joint interfaces. This paper presents a simplified experimental procedure to estimate the cohesive zone model (CZM) parameters for predicting the interface failure of adhesively bonded joints subjected to mode I (opening) and mode II (shear) loading. The novelty of this procedure lies in predicting all CZM parameters—cohesive stiffness, strength, and cohesive energy (fracture toughness), of the bonded interface using one common experimental setup. It is observed that the cohesive strength and cohesive energy in shear loading are approximately 3.8 and 11.6 times, respectively, compared to the opening loading condition. Employing a triangular cohesive law, the CZM parameters are combined with appropriate damage initiation (quadratic stress) and evolution (power law) criteria. The proposed methodology is verified against experimental load–displacement responses of the bonded interface under mixed‐mode I/II (opening/shear) loading across a wide range of mode mixity (α = 0°, 15°, 30°, 45°, 60°, 75°, and 90°). The maximum error between the experimental and the numerical peak load is 5.11%. The acceptable agreement between the numerical and experimental results confirms the effectiveness of this method in investigating interface fracture in adhesively bonded joints under different loading conditions.

A Machine Learning Boosted Data Reduction Methodology for Translaminar Fracture of Structural Composites

AbstractThis work explored a machine learning (ML) algorithm as a fast data reduction method for translaminar fracture energy in composite laminates. The method was validated with translaminar fracture tests on compact tension (CT) specimens on AS4/8552 and IM7/8552 cross-ply lay-ups. Experimental fracture energy and R-curves for both materials were determined using the most common data reduction methods, such as the compliance calibration (CC), the area (AM) and the Irwin relationship (IM). Our new data reduction method uses a surrogate model based on an artificial neural network (ANN) trained with synthetic data generated with the cohesive crack finite element model. Such a surrogate model maps the cohesive properties with the corresponding load–displacement, crack-displacement and energy-displacement curves with interrogation times in the order of 20 ms and relative errors in the load–displacement and crack growth less than 2%. Such performance enabled its encapsulation to approximate the inverse problem to infer the cohesive parameters with the maximum likelihood estimator (MLE) directly from the experimental load–displacement and crack-displacement curves. The results demonstrated the ability of the model to deliver cohesive parameter inference directly from the macroscopic tests carried out at the laboratory level.

A simple continuum approach to predict the drained pull-out response of piles for offshore wind turbines

The article presents a continuum approach to predict the response of pile foundations for jacket-supported offshore wind turbines. Tensile loading conditions are examined, which may be critical for piles used in combination with this structure type, generally adopted to exploit wind energy in intermediate water depths. The approach is developed to guarantee a simple implementation with a limited number of input data easily attainable from cone penetration tests and laboratory tests, and to ensure computational cost-effectiveness. Data from technical-scale tests on open-ended steel piles driven in dense sand and subjected to drained pull-out are used to assess the performance of the approach. The results are shown to be accurate, approximating rather closely the experimental load–displacement curves. The accuracy of the approach is also compared to that obtained with a recently proposed design method, to investigate the predictive capacity of the approach and its potential to support preliminary design activities.

Effects of different running intensities on the micro-level failure strain of rat femoral cortical bone structures: a finite element investigation

BackgroundRunning with the appropriate intensity may produce a positive influence on the mechanical properties of cortical bone structure. However, few studies have discussed the effects of different running intensities on the mechanical properties at different levels, especially at the micro-level, because the micromechanical parameters are difficult to measure experimentally.MethodsAn approach that combines finite element analysis and experimental data was proposed to predict a micromechanical parameter in the rat femoral cortical bone structure, namely, the micro-level failure strain. Based on the previous three-point bending experimental information, fracture simulations were performed on the femur finite element models to predict their failure process under the same bending load, and the micro-level failure strains in tension and compression of these models were back-calculated by fitting the experimental load–displacement curves. Then, the effects of different running intensities on the micro-level failure strain of rat femoral cortical bone structure were investigated.ResultsThe micro-level failure strains of the cortical bone structures expressed statistical variations under different running intensities, which indicated that different mechanical stimuli of running had significant influences on the micromechanical properties. The greatest failure strain occurred in the cortical bone structure under low-intensity running, and the lowest failure strain occurred in the structure under high-intensity running.ConclusionsModerate and low-intensity running were effective in enhancing the micromechanical properties, whereas high-intensity running led to the weakening of the micromechanical properties of cortical bone. Based on these, the changing trends in the micromechanical properties were exhibited, and the effects of different running intensities on the fracture performance of rat cortical bone structures could be discussed in combination with the known mechanical parameters at the macro- and nano-levels, which provided the theoretical basis for reducing fracture incidence through running exercise.

On the failure behavior of a GFRP composite cylinder under Brazier crushing force – An experimental and computational study

The induced ovalization or out-of-plane deformation in the circular and transition airfoils of modern MegaWatt (MW) size wind turbine rotor blades is caused by the combined effect of the applied far-field bending loads and the local structural curvature. In general, the phenomenon of the ovalization or out-of-plane deformation of the circular elastic cross sections under the applied bending load or the internal/external pressure is termed the Brazier effect [1 2]. Considering the poor mechanical performance in the out-of-plane direction of the typically used laminated glass composites for wind turbine rotor blade construction, the current research work aims to develop a thoroughly validated computational methodology to understand the ovalization (out-of-plane deformation) induced failure behavior in circular cross-sections located between the root and transition region of a rotor blade, using a substructure level testing and modeling under representative load conditions. To this end, detailed manufacturing, testing, and a computational modeling campaign is devised and executed at different length scales starting from coupon to cylinder (substructure) level. The required elastic, strength, and fracture properties for the cylinder progressive damage analysis are evaluated using coupon-level experiments following various ASTM standards.The developed numerical methodology is thoroughly validated using the substructure level experimental load–displacement curves, strain, and damage profiles. Detailed experimental and numerical studies reveal that the induced ovalization leads to a non-linear relation between the applied crushing force and the local radius of curvature until the unstable collapse of the cylinder. Based on the thorough local stress and damage analysis, it is evident that along with the dominant interlaminar shear stress (ILSS), the presence of the interlaminar tensile (ILT) stress component is the key to the delamination induced collapse of the cylinder.

Fracture mechanical stability of DCB and 4ENF tests complemented with linear springs

In this work the stability of the DCB and 4ENF fracture specimens is analyzed using some beam theories and the compliance calibration method. The main goal is that the systems are complemented with linear springs and it is investigated how the springs influence the stability of crack propagation from the theoretical standpoint. Large number of experiments were carried out including crack initiation tests providing the experimental load displacement responses, the compliance datasets and the qualitative observation of crack initiation. Apart from the analytical method the compliance calibration method was employed to evaluate the experimental data and to determine the fit curves of the compliance datasets, the R-curves and finally the bifurcation curves or in other words the critical displacement and crack length relationships including several additional spring stiffnesses for both test configurations. The results indicate that - as expected - the smaller the spring stiffness is the worst the stability of the DCB specimen will be, leading to the possibility of destabilization. On the contrary the analysis and experiments confirm that the 4ENF test cannot be destabilized independently of the stiffness of additional springs.

Bolted steel to laminated timber and glubam connections: Axial behavior and finite-element modeling

This study aims to characterize the axial monotonic and cyclic tensile behavior of bolted steel connections to laminated timber and bamboo, using slotted-in steel plates, and to provide a high-fidelity simulation using finite-element modeling. Forty-eight experimental tests were conducted, including eight different configurations, varying in material (Laminated Veneer Lumber (LVL) and Glued Laminated Bamboo (glubam)), number of bolts, and diameter of bolts, under tension, both monotonic and cyclic (three repetitions). The force and displacement were recorded, and used to define the initial stiffness, yielding strength, ultimate strength, ductility, viscous damping, and energy dissipation. The failure modes of the connections were monitored using Digital Image Correlation (DIC) techniques. A novel high-fidelity finite-element model integrating Hill’s yielding and element removal criteria was proposed to predict the mechanical performance and fracture behavior of the examined connections. The numerical simulation was validated by comparing the experimental load–displacement curves and the strain field measurements obtained with DIC. Finally, theoretical formulas for predicting the stiffness of bolted connections were proposed based on the FEM simulations.

Improved simpler R/C hysteresis model considering biaxial interaction

From practical point of view, it is evident that the seismic wave acted on structures through two mutually orthogonal components which have significant combined effect on inelastic demands of the structure. Usually a smaller amount of ground motion generally applied in only one direction than the other and in most of the cases the studies considered only limited inelasticity. Therefore, further research is needed that considers ground motion in both horizontal directions with corresponding bi-directional intensities. Study shows, the combined effects cause significant amount of degradation of strength and stiffness which in turn cause early yielding of R/C(Reinforced Concrete) structural member. Conventional uni-axial model does not consider this type of bi-directional interaction. Some existing simpler bi-axial hysteresis model which can capture biaxial interaction shows satisfactory results, but still the deviation from experimental results are very prominent. On the other hand, some existing sophisticated bi-axial hysteresis model shows better accuracy in predicting response of R/C structural elements but these types of models require large number of calibrated test results which makes them case-specific. In this backdrop, by using principle of yield surface, this study aims to incorporate bi-directional interaction over a simpler hysteresis model which primarily able to capture uni-directional loading effects. Detailed computational scheme of the enhanced simpler uni-axial hysteresis model and extension of the model to capture bi-directional interaction with its performance in prediction of inelastic cyclic behaviour of R/C structural members are presented in this paper. From the performance point of view, it is clearly visible that the proposed simple bi-axial hysteresis model can predict experimental load–displacement curves of R/C structural members under bi-directional reversible loading with acceptable accuracy. The requirement of limited number of input parameters for the present proposed bi-axial hysteresis model make it more convenient to assess the progressive seismic damage under bi-directional ground motion.

Progressive failure analysis of U-notched thin composite plates repaired with a double-sided composite patch

In this study, carbon fiber-reinforced U-notched composite plates were repaired with composite patches made of the same material using adhesive bonding and their mechanical performance was investigated experimentally and numerically. Repairs were made by applying a double-sided composite patch to the notched thin composite plates. The repaired composite plates were tested under tensile load. The effect of the variation in notch width and depth on the repaired plate failure load and type was investigated. At the end of the experiments, load–displacement graphs of each specimen were obtained. The effect of variation in notch depth and width in the repaired specimens was not as dominant as in the non-repaired specimens. While the variation in notch dimensions changed the failure load between 34% and 37% in the non-repaired specimens, this rate was between 18% and 21% in the repaired specimens. In the numerical analysis, progressive failure analysis was performed using the Hashin Failure Criteria. Subroutine codes were written in the program Ansys, which uses the finite element method for numerical analysis. In the results, numerical and experimental load–displacement graphs and failure types of the specimens are presented comparatively. The convergence rate of the experimental and numerical data was between 92% and 99.8%.

Experimental and numerical characterization of pultruded GFRP structural elements with stiffened Web-Flange junctions

The research presented herein pertains to the experimental characterization and numerical simulation of the behavior of pultruded Glass Fiber-Reinforced Polymer (GFRP) structural elements, with specific attention devoted to enhancing ultimate capacity of the Web-Flange Junctions (WFJs). An experimental campaign, performed on as-manufactured I-beam specimens is conducted to serve as a baseline for comparing behavior of pultruded GFRP members with the proposed stiffening strategy. The specimens were produced through the addition of external elements, using bonded L-shaped profiles with variable lengths that were installed in the proximity of the WFJ zone of the I-beams. The experimental tests aimed at highlighting the inherent premature failure that potentially occurs at the WFJ as a result of lack of reinforcement continuity between the flanges and the web that are required to withstand localized radial stress concentration at these locations. To this end, the top flanges of the thin-walled GFRP I-beams were clamped while a localized concentrated load was applied on the bottom flange by means of a steel jaw. Two sets of tests were performed, namely Mid-Point (MP) and End-Point (EP), according to the location of the applied load that caused delamination, and eventually separation of the bottom flange from the web. Failure modes observed in the tests, together with experimental load–displacement curves, are used to investigate the influence of external stiffeners length on the ultimate strength and stiffness of the I-beams. In executing the tests, Digital Image Correlation (DIC) techniques were used to track the spatial distribution of the deformation in the elements. Based on the results of the experimental campaign, a numerical investigation was conducted, to characterize the influence of different stiffeners’ arrangements on the mechanical response of the GFRP elements. The computational model was validated by comparing the observed response, both in terms of measured force–displacement curves, and bi-dimensional displacement maps obtained by means of the DIC technique. Numerical results highlighted the importance and influence of an explicit representation of the mechanical response of the bonding agent (i.e., the epoxy resin used to attach the stiffeners to the I-beams) to correctly characterize the deformation behavior of the strengthened WFJs.

Experimental study on out-of-plane flexural behavior of steel-concrete-steel composite walls

In this study, six steel-concrete-steel composite wall (SCSCW) specimens have been designed and manufactured, and their out-of-plane flexural behaviors under combined axial and horizontal loads are experimentally studied. The study parameters include the connector arrangement scheme and applied axial load. The effects of these parameters on the failure modes, experimental load–displacement curves, bearing capacities, ductility coefficients, stiffness, energy dissipation capacities, and strain distributions of the specimens are analyzed. All specimens exhibit sectional flexural failure. The failure phenomena include the out-of-plane local buckling, concrete crushing, and welding fracture of the steel plates. These specimens show excellent ductility with an average ductility coefficient 2.27. The strength degradation coefficients of the specimens are >0.85. Increasing the number of connectors can restrict the local buckling of external steel plates, and the bearing capacities of the specimens are also improved. The connector arrangement scheme and axial load significantly influence the secant stiffness of the specimens during the initial stage of the experiments. When the connector arrangement schemes are identical, the higher the axial load, the better the energy dissipation capacity of the specimens. A simplified calculation model for the flexural capacity of SCSCWs is proposed considering the effects of local buckling and side plates, which has high accuracy and efficiency, and average ratio between the calculated and experimental peak loads is 0.96. A brief design example for the SCSCWs is provided to illustrate the design process considering the axial and out-of-plane loads.

Effect of Fiber Wrapping on Bending Behavior of Reinforced Concrete Filled Pultruded GFRP Composite Hybrid Beams.

The application of pultruded fiber reinforced polymer (FRP) composites in civil engineering is increasing as a high-performance structural element or reinforcing material for rehabilitation purposes. The advantageous aspects of the pultrusion production technique and the weaknesses arising from the 0° fiber orientation in the drawing direction should be considered. In this direction, it is thought that the structural performance of the profiles produced by the pultrusion technique can be increased with 90° windings by using different fiber types. This paper presents experimental studies on the effect of FRP composite wrapping on the flexure performance of reinforced concrete (RC) filled pultruded glass-FRP (GFRP) profile hybrid beams with damage analysis. The hybrid beams are wrapped fully and partially with Glass fiber reinforced polymer (GFRP) and carbon fiber reinforced polymer (CFRP) composites. Hybrid beam specimens with 0° to 90° fiber orientations were tested under three- and four-point bending loads. Based on the experimental load–displacement relationship results, initial stiffness, ductility, and energy dissipation capacity were compared. The experimental findings revealed that the maximum load-carrying capacities of beams produced with pultrude profiles increased by 24% with glass wrapping and 64.4% with carbon wrapping due to the change in the damages. A detailed damage analysis is provided. Similarly, significant increases were observed in structural performance ratios such as initial stiffness and ductility ratio.

FEA Modelling of Externally-Strengthened Concrete Beam with CFRP Plates Under Flexural Test

This study concentrates on FEA modelling of concrete beam strengthened with externally bondedCFRP lates under bending by using Traction Separation Law (TSL) as constitutive law to require maximum cohesive stress and fracture energy values. The FEA models were developed following experimental work reported by Al-Rousan et al. [23] and Ding et al. [22]. Combination of two numerical techniques were adopted, i.e., Extended Finite Element Method (XFEM) and Cohesive Zone Method (CZM) assigned within cracked beam region and adhesive layer respectively. The consistence of FEA beam deformations to capture debonding failure as seen during experimental observations and load-displacement was evaluated accordingly. Additionally, combination of XFEM-CZM techniques provides good strength predictions with experimental dataset. It is clearly shown that the failure mode exhibited are determined by testing method, CFRP width and CFRP length. CFRP sheets provides a significant contribution to concrete ductility, which is noticeable in longest CFRP sheet. All testing series were examined, the discrepancies of less than 25% were found. Note that current approach used calibrated fracture energy values from similar concrete grade and CFRP plates, however better prediction can be produced if fracture energy values were independently determined from experimental set-up.

Experiments on the mode II fracture toughness in ENF tests of CFRP curved beams

The purpose of this study is to establish end-notched flexure (ENF) tests of curved beam specimens cut from tanks or cylinders. To provide data reduction methods for experiments, a compliance-based beam method (CBBM) for curved beams in ENF tests is proposed based on the previously established Euler–Bernoulli curved beam theory formulations of ENF tests. ENF tests were conducted using curved beam specimens with three different curvature radii, and the results were in good agreement with the curved beam theory. In addition, the fracture toughness values obtained from the proposed CBBM for curved beams were validated using three-dimensional finite element (FE) analysis. The results revealed that the FE analysis with the measured fracture toughness agreed well with the experimental load–displacement curves, verifying the validity of the obtained fracture toughness values for curved beams.

Effects of Fe atoms on hardening of a nickel matrix: Nanoindentation experiments and atom-scale numerical modeling

Significant hardening effect due to Fe concentrations in Ni-based alloys with face-centered-cubic structure has been studied by using a combined experimental and atomistic-based computational approach via nanoindentation tests. The obtained experimental load–displacement data for the [001] crystal orientation reached a qualitative good agreement with molecular dynamics simulations results, leading to strong evidence that the main strengthening factors are associated to sluggish dislocation diffusion, reduced defect sizes and the nucleation of tetrahedral stacking faults. Here, interstitial type prismatic dislocation loops mainly formed by ⅙〈112〉 Shockley dislocations are nucleated during the loading process, where their interaction leads to the formation of pyramidal shaped stacking fault which are mainly created by ⅓〈100〉 Hirth dislocations lines. Observing both types of defects coexisting in the same plastic deformation zone by both approaches. Reported mechanical data, measured experimentally and interpreted numerically, are also in accordance with microstructural SEM and TEM investigations.

Numerical analysis of the mixed-mode fracture of bonded joints depending on the adhesive thickness

The mixed-mode strength of bonded joints can be predicted by techniques such as cohesive zone models (CZM), which requires the estimation of the adhesive strength and fracture toughness ( GC). Under the scope of fracture properties, the tensile and shear fracture toughness ( GIC and GIIC, respectively) and the corresponding mixed-mode behaviour are particularly relevant. Moreover, these parameters highly depend on the adhesive thickness ( tA), making it relevant to validate and propose CZM laws for bonded joint design. This work numerically addressed the tA influence on the mixed-mode fracture process of adhesive joints. For this purpose, single-leg bending (SLB) test experimental data was used, considering joints with composite adherends and a ductile adhesive, and tA from 0.1 to 2.0 mm. A numerical CZM analysis was performed, including the experimental and numerical load-displacement ( P- δ) curves’ comparison for validation, followed by the CZM law estimation and fracture envelope validation, for all tA. The effect of the different CZM parameters on the results was finally evaluated. Overall, it was possible to numerically ascertain the tA effect on the fracture behaviour of adhesive joints and to propose a numerical technique for mixed-mode bonded joint analysis.

A semi-analytical method for determining mode-II fracture toughness and bridging law of composite laminates

Accurate determinations of fracture toughness and the bridging law are important for the design and analysis of composite laminates. In this work, based on the Castigliano theorem and the point friction theory, a semi-analytical method for mode-II fracture toughness and bridging law is established. ENF tests on T800 carbon/epoxy laminates are carried out and fracture toughness are obtained. Public data from laminates made by other different materials are also used for verification. It is found the fracture toughness calculated by the semi-analytical method agree well with experimental results, verifying the accuracy of the proposed method for determining fracture toughness. Furthermore, the relative shear displacement at the pre-crack tip is also calculated and its relation with fracture toughness is established, from which bridging law can be determined by J-integral theory. The determined bridging law is integrated into cohesive zone model for delamination growth modelling. Good agreements between numerical and experimental load–displacement curves exhibit, which further verifies the proposed method. Using the proposed method, only experimental load and displacement data are required when determining fracture toughness and bridging law. Problems of unstable crack growth and difficult observation due to the compression of delamination are avoided in a simple and efficient way.

Fracture toughness and crack growth resistance of sandwich panels with grooved cores

The delamination phenomenon in sandwich panels occurs in the form of face/core debonding. The grooved foams are usually employed in curved sandwich structures for easier formability and better adhesion. In this research, the debonding toughness of facesheet to the core in a sandwich beam with grooved core is investigated using numerical and experimental methods. The sandwich beam consisted of two Kevlar 49/polyester facesheets and a polyurethane foam core. To obtain the values of strain energy release rate, the Cracked Sandwich Beam (CSB) specimen along with the Modified Beam Theory (MBT) method was used for the data reduction of the test results. In the numerical part, a model based on the cohesive zone theory was employed to simulate the crack growth in the specimens. Comparison of the numerical and experimental load–displacement curves indicated that the proposed model can reasonably predict the behavior of the structure under similar loading conditions. The effect of foam-related parameters such as the number and width of core grooves on crack growth resistance was examined numerically. The results showed that increasing the number and width of the grooves enhanced the resistance of the sandwich beams against the growth of interfacial cracks.

Efficiently determining the R-curve and bridging traction-separation relation of mode I delamination in a simple way

R-curve and bridging traction-separation relation play important roles for characterizing the large scale fibre bridging behavior during the delamination growth process whereas their accurate determinations are laborious. Existing methods depend on complex devices or visual measurements of the delamination length and the crack opening displacement (COD) during tests. To deal with this problem, a simple semi-analytical method is proposed in this study. Using this method, only experimentally recorded load and displacement are required. Difficult measurements of the delamination length and the COD are avoided. The relation between the strain energy release rate (SERR) and the delamination length, and the relation between the SERR and the COD can be easily and efficiently determined, from which the bridging traction-separation relation can be calculated. Validation on this method is performed by comparing the obtained R-curve and bridging traction-separation relation with the experimental results. Furthermore, the determined bridging traction-separation relation is integrated into a tri-linear cohesive zone model for delamination simulation. Experimental load–displacement responses can be accurately predicted, which again illustrates the applicability of the proposed method. The proposed method provides a simple and efficient way for determining the R-curve and the bridging traction-separation relation of mode I delamination.