Load-point Displacement Research Articles

Predicting base resistance of super-long piles using a random forest model: A case study from Ho Chi Minh city

The prediction of the base resistance for long piles is usually challenging because of the impact of material characteristics and the influence of the nature of the surrounding soil. Artificial intelligence models have been applied in various geotechnical engineering fields, and significant results have been achieved. Based on a well-instrumented static load test dataset (1131 data points) from various projects in the soft soil area of Ho Chi Minh City, this study established a random forest (RF) model considering five input parameters, including the applied load, load point displacement, axial stiffness, standard penetration test value of the soil beneath the pile toe, and the distance from the load point to the pile toe. Twenty percent of the data was designated as the test set, which was used to make predictions using the established model. The results show that the RF model has good predictive performance in terms of prediction accuracy and reliability. A sensitivity analysis of the input factors provided a deeper understanding of the base resistance mechanism, which is important in pile foundation design practice.

J-integral testing on micro-scale cantilever beam specimens

The current paper presents a modified procedure for the J-integral testing on micro-scale cantilever beam specimens. For such tests, a non-dimensional factor is required that relates the plastic part of the area below the load vs. load-point displacement curve to J. This parameter is called the “plastic eta-factor”, ηpl. Up to now, a constant value of ηpl≈2 has been used, which is also valid for deeply notched single-edge bend specimens. A comprehensive numerical case study is performed to determine ηpl for cantilever beam specimens as a function of the relative crack length a/W, the relative cantilever length L/W, the relative base distance d/W, and the load, i.e. the degree of plasticity in the specimen. It is shown that ηpl≈2 is applicable only, if the crack length is large, a/W⩾0.5. For smaller a/W-ratios, which are commonly used in the fracture mechanics testing of nano- and micrometer sized materials, ηpl can become much smaller and also dependent on the d/W-ratio. A fitting procedure is presented that enables the determination of the correct value of ηpl, and modifications of the J-integral testing procedure for cantilever beam specimens are proposed. Finally, a numerical validation of the procedure is presented.

Determination of Toughness Variation and Cohesive Zone Parameters for De-bonding of Adhesively Bonded PMMA and Steel Using the Shaft-loaded Blister Test

ABSTRACT In this work, the shaft-loaded blister test (SLBT) is employed to investigate the de-bonding of a poly-metha-methyl-acrylate (PMMA) layer adhesively bonded to a steel layer. During de-bonding, the mode-mixity does not remain constant in this test; hence, the variation of mode-mixity with the change in de-bond size (blister radius) and the out-of-plane displacement of the PMMA layer is established through Finite Element Analysis (FEA). Experiments are performed in which the load, load-point displacement, and de-bond size is recorded synchronously. Three different thicknesses of PMMA layer are considered in experiments. The variation in toughness as a function of mode-mixity is evaluated. FE simulations with embedded cohesive zone model (CZM) are carried out, and the mixed-mode cohesive zone modelling parameters are identified by comparing the numerical results with experimental observations. By using this approach, the toughness variation as a function of mode-mixity and the CZM parameters is determined using the SLBT.

Soft computing for determining base resistance of super-long piles in soft soil: A coupled SPBO-XGBoost approach

Prediction of base resistance for long piles is usually challenging because of the complex mobilization of load over the depth. This study hence proposes a novel machine learning (ML) method where the eXtreme Gradient Boosting (XGBoost) algorithm is coupled with the state-of-the-art Student Psychology-Based Optimization method (SPBO) to predict base resistance of piles. A dataset (796 datapoints) from well-instrumented static load tests among various projects in the soft soil region of Ho Chi Minh City is used to develop the model. Five input parameters including the applied load, loading point displacement, axial stiffness, SPT value of the soil beneath the pile toe, and the distance from the loading point to the pile toe are considered. Performance of the coupled SPBO-XGBoost model is comprehensively assessed by comparing it with other optimized XGBoost models including the Teaching-Learning Based Optimization, Bayesian Optimization, and other well-known ML techniques. The established model is used to predict data obtained from independent studies and compared with practice design codes of pile foundation. The results show that the proposed SPBO-XGBoost model outperforms all other models considering the prediction accuracy and reliability. Sensitivity analysis and Partial Dependence Plots are conducted to gain insights into the base resistance mechanism, giving considerable implications to design practice of pile foundation.

Evaluation of modelling parameters for AA2014-T6 in simulating dynamic fracture event

The material behavior of the aluminum alloy AA2014-T6 under different strain rates and temperatures must be examined in order to simulate their responses under extreme loading conditions like dynamic fracture and impact. In the present study, several uniaxial tensile experiments, including quasi-static, dynamic, and triaxiality, were carried out at different temperatures to evaluate the material constants for the Johnson cook constitutive and failure models of AA2014-T6. It was observed that material flow stress exhibited positive strain rate sensitivity at high strain rates and negative sensitivity when temperature increased from 25 to 250 °C. From the triaxiality experiments, it was also observed that the plastic flow increased while the ductility of the material decreased with increasing stress triaxiality. The dynamic fracture behavior of a pre-cracked 3-point bend specimen of AA2014-T6 was studied at different loading rates. Applied load and load point displacement (LPD) were measured using strain gauges. In addition, crack mouth opening displacement (CMOD) and crack initiation time were calculated using the 3D digital image correlation technique. It was found that the loading rates highly influence the dynamic fracture initiation and propagation toughness of the material. A 3D finite element simulation of dynamic 3-point bend experiments was performed in ABAQUS using the J-C damage evaluation criteria, and incident stress obtained from the dynamic experiments was used as an input load to minimize the error. In accordance, with the actual experiments conducted, the material geometry, boundary conditions, and contact circumstances were considered. The authors found good agreement between both the experimental and numerical results.

The effective crack extension and its implications on the single cantilever beam test for sandwich composites

The response of Single Cantilever Beam sandwich specimens may be analyzed effectively through one-dimensional theories accounting for the actual deformation of the core and its effects on fracture parameters and compliance. The model of a beam on an elastic Winkler foundation describes the detaching facesheet as an Euler–Bernoulli beam resting, in the bonded region, on an elastic foundation which generates oscillating deflections with characteristic length [Formula: see text]. Modified beam theory (MBT) describes the debonded part of the face sheet as a cantilever beam, built-in at the debond tip, with an effective length which is longer than the actual detached length, by a quantity [Formula: see text]. If [Formula: see text] the two models predict the same energy release rate and similar load-point displacements. Effective crack extension and characteristic length depend on the thicknesses and elastic constants of facesheets and core and may be derived through experimental measurements of the specimen compliance at various increments of crack growth, as prescribed by experimental protocols. In the theoretical analyses, [Formula: see text] is typically assumed, either by directly using formulas proposed for other specimens or by calibrating their parameters to case specific finite element results. In this paper, we investigate the applicability of such formulas to a wide selection of sandwich composites for naval and aeronautical applications having thicknesses or materials other than those used for the original fitting; we then highlight their limitations and effects on the design of experimental protocols for material characterization. The applicability of a recent isotropic-elasticity based formula for [Formula: see text] is then verified using extensive data from the literature and novel finite element (FE) results for typical sandwich composites. Finally, a novel elasticity solution for the load-point displacement of a layer on a half plane is derived which supports the test data reduction procedure for [Formula: see text].

An improved fixture for better measuring the displacements and mode‐I fracture toughness of chevron bend specimens

The load versus load point displacement (LPD) curve is significant for the nonlinearity corrections for the mode-I fracture toughness of chevron bend (CB) specimens. The testing fixture plays a crucial role in measuring load versus LPD curves. To better measure the LPDs of CB specimens, we developed an improved fixture system consisting of an understructure and a yoke based on the traditional saddle arrangement fixture suggested by ISRM. With the aid of the improved fixture, continuous synchronized LPDs and CMODs are measured, and nonlinearity corrections are conducted. After nonlinearity corrections, the data of mode-I fracture toughness become more comparable and present a better regularity among different specimen diameters and variant rock types. Repeated tests verify that the improved fixture has the advantages of higher stability and more convenient installation, removal, and adjustment, which can enhance the testing robustness and efficiency.

Investigations into the static and dynamic fracture initiation and propagation toughness of AA2014-T6 incorporating temperatures effects

The fracture toughness of a material is an important parameter to quantify the reliability and safety of an engineering structure. In this article, the effect of loading rate and temperature on static and dynamic initiation and propagation toughness fracture behavior of AA2014-T6 is studied. Pre-crack three-point bend specimens are used in determining the static and dynamic fracture initiation and propagation toughness. Static three-point bend experiments are performed using a conventional UTM combined with the heating chamber at different displacement rates (1 mm/min, 10 mm/min, and 100 mm/min) and a wide temperature ranging from −150 °C to 200 °C. Similarly, the dynamic experiments are carried out on modified Hopkinson pressure bar (MHPB) under different loading rate and temperature. ASTM E399 was utilized in evaluating the static fracture initiation toughness, while the dynamic fracture initiation toughness is measured with the help of load point displacement (LPD) and crack initiation time. The 3D digital image correlation (DIC) in combination with ultra-high-speed imaging is used to calculate the crack initiation time and crack mouth opening displacement (CMOD). The energy conservation-based method was utilized in calculating the propagation fracture toughness in both static and dynamic loading conditions. Results show that the propagation toughness increases with temperature while the static fracture initiation toughness declines linearly. In addition, both initiation and propagation fracture toughness increases with the loading rates. Also the dynamic fracture investigation reveals that the positive rate sensitivity for dynamic propagation toughness increases with the increase in temperature. Results also show that the value of dynamic fracture initiation toughness is maximum at room temperature as the yield strength and plastic zone size are optimum in room temperature environments.

Dynamic fracture behaviour of AA7475-T7351 alloy at different strain rates and temperatures

The fracture behaviour of the aluminium alloy AA7475-T7351 at different loading rates and temperatures (-150 °C to 200 °C) is investigated here. The three-point bend experiments are conducted at Zwick/Roell-50 UTM (crosshead speed up to 100 mm/min), MTS-250 machine (crosshead speed upto 1000 mm/min) and modified Hopkinson pressure bar (striker velocity 11–19 m/s) to understand the initiation and propagation fracture toughness of AA7475-T7351 aluminium alloy along the rolling (L-T) and transverse (T-L) directions. The digital image correlation (DIC) technique is used to obtain the load point displacement (LPD) and crack mouth opening displacement (CMOD) and the same is used to calculate the stress intensity factor (SIF). The fracture surface is studied using a digital stereo microscope (DVM6) and scanning electron microscope (SEM). The fracture toughness of AA7475-T7351 is found to depend on the loading rates and temperatures and found to vary between 50 and 66 MPm1/2 along the L-T direction, whereas the corresponding value along the T-L direction is comparatively less.

Experimental study of dynamic fracture behavior of Al7075-T651 under different loading rates

The dynamic fracture initiation toughness for an aluminium alloy, Al7075-T651, has been estimated experimentally at different loading rates of 6.20, 7.87, and 10.1 M P a m / μ s . In this study, dynamic 3-point bend experiments were performed with a modified Hopkinson Pressure Bar (MHPB) associated with two high-speed cameras. A 3D digital image correlation (DIC) technique was used to get the full-field deformation of the specimen, such as crack initiation time and crack mouth opening displacements. Load point displacement as well as crack mouth opening displacement methods were used to determine the stress intensity factor (SIF). It was discovered that the dynamic fracture toughness significantly increased with increasing loading rates. The fractography carried out on the surface of fractured samples revealed transgranular facets and large dimples, which illustrated an increase in the fracture toughness with loading rates. 2D and 3D profiles of fractured surface were drawn, which relate undulation with fracture toughness.

Fracture Behavior and Digital Image Analysis of GFRP Reinforced Concrete Notched Beams.

This study presents three-point bending fracture tests on glass fiber-reinforced polymer (GFRP) reinforced concrete notched beams. Few studies have been conducted to date to understand the fracture behavior of this type of specimens. The specimens have nominal depth, width, and length equal to 150 mm, 150 mm, and 550 mm. Plain concrete notched beams with the same dimensions are cast from the same batch of concrete to compare the responses with GFRP reinforced concrete notched beams. The notch of the plain concrete specimens is either saw cut or cast. These two notch fabrication methods are compared based on the load responses. The peak load, crack mouth opening displacement (CMOD), GFRP bar slip at two ends, and load point displacement are used to discuss the results of the fracture tests. In addition, digital image analysis is performed to identify the fracture process zone (FPZ) and the location of the neutral axis, which are used to determine the force in the GFRP bar via cross-sectional analysis. Finally, the GFRP bar force versus slip responses are compared with those from the pull-out tests performed on the same bar to show that the bond of the bar in the pull-out tests represents an upper bound limit compared to the behavior in bending.

Experimental Study on Damage Characteristics of Corroded Steel pipe Based on Acoustic Emission Technology

The corrosion damage of steel pipe is mainly characterized by crack initiation and local wrinkling. One of the effective methods to evaluate the damage degree of steel pipe is to find out the law of crack initiation. The research object of this test was Q235 steel pipe specimen with diameter of 700mm, wall thickness of 5mm and height of 260mm. Acoustic emission technology was used to conduct real-time axial compression detection on corroded steel pipe specimens. The relationship between acoustic parameters (cumulative ringing number and amplitude) and the damage degree of steel pipe specimens was established, and the crack propagation law and crack initiation load of steel pipe after corrosion damage were obtained. By analyzing the different characteristics of the relationship between the displacement of loading point and acoustic parameters, the general position of steel pipe damage was detected and the exact value P of crack initiation load was determined. The experimental results provide a basis for further research on the damage mechanism of steel tube specimens. The test results provide a certain basis for further research on the damage mechanism of steel pipe specimens.

Size effects and failure regimes in notched micro-cantilever beam fracture

Fracture tests using notched micro-cantilever (MC) specimens are increasingly being used to measure the fracture toughness of materials at the micro-scale. Detailed finite element analyses (FEAs) of loading of self-similar micro- and bulk cantilever beam fracture specimens using isotropic, elastoplastic constitutive models, are conducted to critically examine the validity of the toughness data obtained using MC specimens. From the simulated normalized load versus load point displacement and the corresponding normalized energy release rate versus load curves, the transition of the failure regimes from crack propagation to plastic collapse of the uncracked notch ligament are identified. While the crack propagation regime allows for the estimation of valid fracture toughness of the probed material, the plastic collapse regime does not. The effects of specimen aspect ratios, material hardening, and yield criteria on the master curves and transition point are examined. A method to interpret the failure regimes, assess the validity and size effects in micro-cantilever fracture tests is proposed. An expression is derived for the minimum size requirements of MC fracture specimens in order to get a valid fracture test. The data reported in the literature from fracture tests on bulk metallic glasses, nano-crystalline materials and ultra-fine grained materials are assessed using the proposed methodology. The present work has important implications for specimen design, interpretation of failure regimes, and potential size effects in MC fracture tests.

A beam model for the fracture of adhesive joints with dissimilar metal adherends

The natural ability to join different materials is one of the main advantages of adhesive joints. It is thus important to characterize the fracture resistance of adhesive joints with dissimilar materials, as they are increasingly used in relevant industrial applications. There are, however, well-known problems in data analysis of such tests, especially where mode partitioning is concerned. A beam model is here developed for mixed-mode I-II fracture testing on adhesive joints with dissimilar metal adherends. The model predicts quite accurately the strain-energy release rate, the mode-mix and the load-point displacements with fully closed-form equations. Therefore, it can be the basis of future data reduction schemes for fracture tests. Furthermore, it is shown that a wide range of mode-mix combinations can be achieved with simple well-known tests on dissimilar metal adherend joints. They could thus be an interesting alternative to developed mixed-mode I-II tests that require complex test fixtures.

Rate dependent fracture along a silicon/epoxy interface under mixed-mode loading conditions

This paper describes the development of a dual-actuator loading device that was then used to apply asymmetric, transverse end-displacements to laminated beam specimens (silicon/epoxy/silicon) over a range of separation rates. Measurements of the reaction forces, as well as load-point displacements and rotations, were used to determine the normal and tangential components of the crack tip displacements and the corresponding components of the J-integral. This was made possible because the specimens identically satisfied a balance condition. The resulting data set obtained from experiments conducted at five separation rates at each of five mode-mix phase angles is a testimony to the efficiency of the approach. A mixed-mode beam on elastic foundation analysis established that the stiffness of the normal and shear interactions of the silicon/epoxy interface was independent of the separation rate and mode-mix. Furthermore, the stiffness values thus determined were considerably lower than those based on the bulk behavior of the epoxy in tension and shear. The analysis also allowed the crack growth to be tracked in order to establish its onset and the corresponding critical values of the normal and shear components of the J-integral, along with the corresponding strengths and critical crack tip displacements. For each mode-mix, these critical values increased with the separation rate. This increase in properties is in spite of the glassy nature of the bulk epoxy and further suggests the presence of an interphase region in the epoxy adjacent to the silicon. However, the change of mode-mix was accompanied by a change in local separation rates, leading to non-monotonic behavior in the critical J-integral. Following the onset of crack growth, the application of the transverse end-displacements along radial loading paths resulted in simultaneous changes in the local separation rates and mode-mix, implying a fracture criterion that depends on both mode-mix and rate-dependent damage evolution processes.

Numerical study for dynamic fracture toughness of AA7075-T651 under dynamic loading

Finite element simulations for three-point bend specimens of aluminium AA7075-T651 were performed by using ABAQUS software. The simulation parameters, load point displacement and crack mouth opening displacement obtained, were used to evaluate the dynamic stress intensity factor. Three-point bend specimens impacted with an incident bar at three different striker velocities on modified Hopkinson pressure bar (MHPB) used to evaluate load point displacements and crack mouth opening displacements during the experiment were simulated. Load point displacements and crack mouth opening displacements were obtained using finite element simulation with maintaining same experimental conditions and it was found a good agreement between experimental and numerical results. These results were used to evaluate dynamic initiation fracture toughness at different loading rates using dynamic stress intensity factor and the time of initiation of fracture obtained with an experimental method using MHPB conjugation with digital image correlation (DIC) method.

Study of quasi-static crack growth in aluminum/alumina functionally graded material using crack gauge

Quasi static crack growth of Al/Al2O3 functionally graded materials prepared through powder metallurgy up to 30% weight fraction of Al2O3 is studied with crack on stiff (FGM I) and compliant (FGM II) sides using crack gauge in three point bend test. The variation of weight fraction of Al2O3 with distance from 100% aluminum face is measured through image analyzers to identify the composition corresponding to crack tip location. The measurement of crack tip location, load point displacement and load with time are carried out simultaneously. It is observed that crack grows stably when the crack traverses from stiff to compliant side and unstably from Compliant to stiff side. For stable quasi static crack growth (crack on stiff side), variation of the fracture toughness and crack tip opening displacement (CTOD) are evaluated and increase with distance from stiff side. Further, the fracture toughness is also evaluated using Raveendran’s micromechanics prediction techniques and found that the experimental values are closer to Raveendran’s lower bound for greater weight fraction of alumina and closer to Raveendran’s upper bound for lower ceramic weight fraction. From fractography, mixed mode failure with greater deviation of crack path for FGM I is observed as compared to mode-I failure for FGM II.

Propagation Speed of Dynamic Mode-I Cracks in Self-Compacting Steel Fiber-Reinforced Concrete.

The objective of this study is to measure the crack propagation speed in three types of self-compacting concrete reinforced with steel fibers loaded under four different loading rates. Central-notched prismatic beams with two types of fibers (13 mm and 30 mm in length), three fiber volume ratios, 0.51%, 0.77% and 1.23%, were fabricated. Four strain gages were glued on one side of the specimen notch to measure the crack propagation velocity, a fifth one at the notch tip to estimate the strain rates upon the initiation of a cohesive crack and the stress-free crack. A servo-hydraulic testing machine and a drop-weight impact device were employed to conduct three-point bending tests at four loading-point displacement rates, the former to perform tests at 2.2 m/s, 22 mm/s and the latter for those at 1.77 m/s, 2.66 m/s, respectively. With lower fiber contents, smooth mode-I cracks were formed, the crack speed reached the order of 1 mm/s and 20 m/s. However, crack velocities up to 1417 m/s were obtained for the concrete with high content of fibers under impact loading. This value is fairly close to the theoretically predicted terminal crack velocity of 1600–1700 m/s. Numerical simulations based on cohesive theories of fracture and preliminary results based on the technique of Digital Image Correlation are also presented to complement those obtained from the strain gages. In addition, the toughness indices are calculated under all four loading rates. Strain hardening (softening) behavior accounting from the initiation of the first crack is observed for all three types of concrete at low (high) loading rates. Significant enhancement in the energy absorption capacity is observed with increased fiber content.

A comparative study of cohesive zone models for predicting delamination fracture behaviors of arterial wall

AbstractArterial tissue delamination, manifested as the fracture failure between arterial layers, is an important process of the atherosclerotic plaque rupture, leading to potential life-threatening clinical consequences. Numerous models have been used to characterize the arterial tissue delamination fracture failure. However, only a few have investigated the effect of cohesive zone model (CZM) shapes on predicting the delamination behavior of the arterial wall. In this study, four types of CZMs (triangular, trapezoidal, linear–exponential, and exponential–linear) were investigated to compare their prediction of the arterial wall fracture failure. The Holzapfel–Gasser–Ogden (HGO) model was adopted for modeling the mechanical behavior of the aortic bulk material. The CZMs optimized during the comparison of the aortic media delamination simulations were also used to perform the comparative study of the mouse plaque delamination and human fibrous cap delamination. The results show that: (1) the numerical predicted the relationships of force–displacement in the delamination behaviors based on the triangular, trapezoidal, linear–exponential, and exponential–linear CZMs match well with the experimental measurements. (2) The traction–separation relationship results simulated by the four types of CZMs could react well as the corresponding CZM shapes. (3) The predicted load–load point displacement curves using the triangular and exponential–linear CZMs are in good agreement with the experimental data, relative to the other two shapes of CZMs. All these provide a new method combined with the factor of shape in the cohesive models to simulate the crack propagation behaviors and can capture the arterial tissue failure response well.

Loading-Rate Effect on the Fracture Response of Natural Hydraulic and Aerial-Lime Mortars

This paper presents the influence of the loading velocity on the fracture response of mortars with both natural hydraulic and aerial limes under three different rates (loading-point displacement rates, 5.0×10−4, 5.0×10−1, and 1.6×101 mm/s). According to the results, the fracture energy and the peak load are sensitive to the loading rate. The maximum dynamic increase factors (DIF), of the peak load are 1.4 and 1.6 for the natural hydraulic and aerial lime mortars, respectively, while it is 1.9 for the fracture energy for both mortars. Moreover, the increase in the peak load and fracture energy of the natural hydraulic lime mortar is basically the result of the viscous effect of free water in the mortar. However, for the aerial lime mortar, the rate effect is chiefly related to the crack path and propagation velocity.