Point Bending Experiments Research Articles

Strain distribution prediction in UHPC beams using deep learning model

AbstractOver the last three decades, ultra‐high‐performance concrete (UHPC) has emerged as a highly innovative cementitious engineering material. This paper utilizes a distributed fiber optic sensor based on optical frequency domain reflectometry (OFDR) combined with the deep learning model to monitor and predict the strain distribution in UHPC T‐beams subjected to two point bending experiment. These sensors are deployed on the side surfaces of the UHPC T‐beams to capture the strain distribution when subjected to vertical loads, facilitating the determination of crack locations based on strain distribution peaks. While time series modeling is widely used in civil engineering to monitor potential damage using sensor data, its application in tracking and predicting known cracks is less explored. To address this gap, a Long Short‐Term Memory (LSTM) neural network model is developed to forecast the increase in the peak value of the strain distribution prior to structural damage, thus predicting crack initiation. The accuracy of the proposed LSTM model in predicting the UHPC strain distribution was thoroughly investigated. The results demonstrate excellent agreement between the predicted strain distributions and those detected by the ODISI 6000 monitoring system. The root‐mean‐square errors of the model‐predicted strains are generally below 10 με, with an average coefficient of determination (R2) reaching up to 98%, indicating a high degree of accuracy.

Implementation of the adaptive phase-field method with variable-node elements for cohesive fracture

In this paper, an adaptive phase-field model based on variable-node elements and error-indicator is presented to predict cohesive fracture evolution. The phase-field cohesive-zone model is characterized by the parameterized energetic degradation and crack geometric functions. A staggered iteration scheme is used to solve the coupled non-linear system of displacement field and phase field. The adaptive local refinement algorithm facilitated with error-indicator and variable-node elements is developed to improve computational efficiency. To determine the adaptive local refinement regions, we apply the phase-field threshold to characterize the crack regions and a novel history field threshold to denote crack tips. The variable-node elements are acted as transition elements to flexibly yet simply link the fine and coarse meshes. In addition, a fast method to generate the variable-node elements is given. Several representative numerical examples are studied to demonstrate the performance and accuracy of the proposed adaptive phase-field model, consisting of a mixed-model fracture of L-shape panel, the wedge-splitting test, the classic three point bending experiment and a plate with a hole and multi-cracks under tensile/shear loadings. The computer codes can be accessed at: https://github.com/hhuztc/Adaptive-PFM.git.

Analysis of the stress state in QFN package during four point bending and temperature experiments utilizing piezoresistive stress sensor

This article reports a comprehensive study on four point bending and temperature experiments and their correlation with numerical simulation. The test vehicle is a printed circuit board (PCB) with 16 Quad Flat No Leads Packages (QFNs) components mounted on top and bottom side. Each QFN has 120 stress sensing cells distributed over the surface of the silicon die. In our experiments we have investigated the stress state in the QFN utilizing these piezoresisitve stress sensors for thermal, four point bending and combined loading conditions. In our study we compare these measurements with the numerical model. Stress analysis for the isothermal bending case showed that components very close to the edge are exposed to slightly higher stresses (20 %) compared to the components placed in the center of the PCB.

Failure analysis of delaminated carbon/epoxy composite under pure bending: validation with numerical analysis

PurposeDelamination is a common and crucial damage mode which occurs during manufacturing of layered composites or their service life. Its existence leads to degradation in mechanical properties or even structural failure of composites. Hence, the purpose of this article is to study the effect of induced delamination on flexural performance of CFRP composites.Design/methodology/approachIn this article, the flexural behaviors of intact and delaminated carbon/epoxy laminates were investigated under pure bending. A circular PTFE film was introduced during fabrication to create artificial delamination. Moreover, finite element models were developed for intact and delaminated composites using ANSYS. The created models were discretized using 3D structural eight node solid elements.FindingsThe delamination influenced considerably flexural properties of composite. The composite exhibited a linear elastic nature prior to the damage of top ply on the compression side. The flexural strength and stiffness of the composite reduced to 44.5% and 18.2% respectively due to the existence of artificial delamination. The results of four point bending experiments and finite element analysis agreed for both intact and delaminated composites within acceptable error. Finally for same composites, first ply failure analysis was carried out using Tsai-Hill, Tsai-Wu and Hashin failure criteria.Originality/valueIn pure bending, beam section of the middle portion is free from shear. It is not so in case of three-point bending. Hence, the effect of embedded artificial defect on bending performance of CFRP composite due to pure bending has been investigated.

Experimental simulation of bending damage of silicon nitride yarn during 3D orthogonal fabric forming process

The aim of this study executed on Silicon Nitride (Si3N4) yarn is to examine some bending damage behaviors and fracture mechanisms that occur during the 3D orthogonal fabric forming process. A three point bending experiment device has been developed in order to simulate the Z-binder yarn bending condition. The effects of weft density, fabric thickness, and yarn tension have been studied. The Weibull analysis of the tensile strength show that the bending damage increases with the increase of weft density, fabric thickness, and yarn tension. The resulting bending damage causes a reduction in yarn strength of between 2.5 and 17.2% depending on the bending parameters of yarn. The growth of the fibrillation area also reflects similar trends with tensile strength loss rate. The fibrillation length produced by the yarns is mostly distributed within the range of 0.3 to 0.9 mm. A comparison of the calculation result to experimental data shows the bending fracture probability of filaments inside yarn are less than that of monofilament. The tensile and bending fracture of Si3N4 filaments exhibit typical brittle fracture characteristics.

Prestressing low clinker structural concrete elements by ultra-high modulus carbon fibre reinforced polymer tendons

The combination of low clinker high-performance concrete (LCHPC) and ultra-high modulus (UHM) carbon fibre reinforced polymer (CFRP) tendons was recently proposed for prestressed structural elements. The 70% reduction in cement content resulting in limited creep and shrinkage of the LCHPC in comparison to a conventional high-performance concrete (HPC) and the very high UHM-CFRP tendon stiffness (> 509 GPa) were expected to impact the mechanical behaviour of such structures. This study focuses on the behaviour of 3 m-long beam specimens during prestressing, concrete hardening and in 4 point-bending experiments. Fibre optic sensors were implemented inside the CFRP tendons to measure strain during those stages and a digital image correlation system was employed to monitor the 4-point-bending tests. After 28 days, the LCHPC recipe, despite a 70% cement reduction and much smaller environmental footprint, did not show measurable differences in the prestress loss behaviour in comparison to a conventional HPC. The UHM-CFRP prestressing tendons, because of their stiffness, showed both higher prestress losses of around 40% and on average a nearly doubled prestress transfer length. However, they increased the beam`s maximum load-bearing capacity by 21% and showed 47% less deflection at failure in comparison to beams prestressed with the standard modulus (UTS)-CFRP tendons.

Study on the Long Bone Failure Behaviors Under the Indenter Rigid-Contact by Experiment Analysis and Subject-Specific Simulation.

The bending fracture behaviors of long bone have gained great attention due to the high bending fracture risk during sports events, traffic accidents, and falling incidents, etc. For evaluating bone bending behaviors, most of the previous studies used an indenter in three point bending experiments while the effect of its rigidity was never considered. In this work, using the porcine long bones, the three point bending tests were conducted to explore the bone fracture behaviors under a rigid indenter. In addition to collecting the force applied, the bone fracture dynamic process was recorded by high-speed photography, and the fracture surface profile in mesoscale was observed by the scanning electron microscope (SEM). Based on CT scanning of long bones, the cross section properties of test specimens were calculated by a homemade matlab script for correlating with their failure strengths. Also, a subject-specific finite element (FE) model was developed to identify the outcomes induced by a rigid indenter on simulation. Findings led to conclusions as follows: (1) The tension fracture came with fracture path deflection, which was caused by the bone indentation induced mesoscale crack-opening. Due to this damage before the whole bone fracture, a bone fracture moment correction was established to compensate experimental data. (2) The plastic indentation caused the force fluctuation as suggested by correlation analysis. (3) The bone failure moment correlated with the inertial moment of the bone cross section at the fracture location higher than the traditional cross section area. (4) In the subject-specific simulation, the indentation caused compression fracture under a much lower failure force. Removing the element erosion on the indenter-contacted area only during the validation was verified as a good option to solve this issue.

Experimental study of the effect of submersion time on the strength development of freeze bonds

Freeze bonds have been found to influence the mechanical properties of ice rubble and ridges, and it is therefore important to study the fundamental physical properties affecting these features. The shear strength of freeze bonded ice blocks has been investigated through a series of asymmetric four point bending (AFPB) experiments for different submersion times and initial ice temperatures. Ice blocks were subject to a confinement of 25 kPa and were sheared at an actuator rate of 5 mm/s. Effects of submersion time on shear strength of freeze bonds was investigated by varying the submersion time from 1 min to 14 days, for two initial ice temperatures of −18 °C and −10 °C. Shear strength values measured are believed to be dominated by two concurrent mechanisms. Thermal bond growth is the first mechanism, where an initial increase in strength with submersion time is observed, reaching a peak after four (4) minutes of submersion time and gradually decreasing to a constant value. Sintering-creep bond development is the second mechanism that significantly influences the bond strength in submersion times longer than 24 h, where the strength increases and eventually asymptotes to the strength of solid ice. An empirical equation which estimates the bond strength as a function of submersion time and initial ice temperature is developed.

Improving the Adhesion of BFRP Strips to the Concrete Surface

Fiber Reinforced Polymer (FRP) composites are widely used in repair and strengthening of reinforced concrete structural elements. The FRP composite adhered to the concrete surface may be separated from the concrete surface in the form of debonding before reaching the ultimate strength. Epoxy resin, concrete strength, fiber properties and application method have an important role in bonding of FRP composites to concrete surfaces. In this study, concrete beam specimens were produced in order to investigate the adhesion of Basalt Fiber Reinforced Polymer (BFRP) composites to the concrete surface using conventional concretes. Stress distribution between concrete and BFRP was investigated by opening a gap in the bottom center of the samples. Unidirectional basalt fiber fabric was used in the production of the test specimens. The effects of concrete surface properties and U winding method on the end of fiber adhesion ability were investigated by bonding BFRP composite to the lower surfaces of the Specimens. Specimens were tested by four point bending experiment. According to the results obtained, the grinding of the concrete surface and the U-winding method significantly improve the adhesion.”

Fracture Properties and Failure Analysis of Zirconia Toughened Hydroxyapatite Bioceramic Coating

In order to improve the weak mechanical properties of bioceramic HA, composite coatings were prepared by adding the toughening zirconia to HA powder so as to improve the biological and the physical properties but not to reduce the biocompatibility of HA bioceramic coating. The pure HA coating and HA/ZrO2 composite coating containing 30% stabilized zirconia on titanium alloy by employing plasma spraying technology were prepared. The relation curves of load-displacement for the two coatings were obtained with three point bending experiment, and obtained the slope about the straight fitting line. Then the elastic modulus and fracture toughness of the both coatings were calculated. The fracture morphology of the two coatings after their failure was observed using electron microscope.

Fatigue testing of GFRP materials for the application in automotive leaf springs

Due to their high specific properties, fiber reinforced plastics are ideal candidates for the substitution of steel parts in the automotive sector enabling high weight savings and increasing the efficiency of automobiles. One possible application are leaf springs made of glass fiber reinforced plastics. Due to the simple bending load case, the fiber orientation can be aligned with the main stress direction. In real life such components as leaf springs are loaded under stochastic loads originated from different driving maneuvers. To replicate those fatigue load case, a high number of component tests are required. Aim of the present work (as a result of a joint research project between the Institute for Plastics Processing and Ford Motor Company) is to develop a simplified testing program that is able to reproduce the damage and the fatigue behavior which occurs in real composite components. For this purpose, three point bending experiments on unidirectional specimens made of glass fiber reinforced epoxy pre-preg material are performed. However, since the loading of the real component introduces additional transverse stresses in the region of the clamping group, the results on specimen level are not transferable to component level. For this purpose, a special clamping group is developed on specimen level to enable a better prediction of fatigue life and the transferability of the results of the fatigue tests to component level. This results in a more realistic representation of the leaf spring stress state in the fatigue experiments. As a result of this research, a higher safety margin in the design phase of such components and less testing time is achieved.

High-temperature DIC based on aluminium dihydrogen phosphate speckle

Digital image correlation (DIC) method has been widely used for measurement in high temperature environment, where speckle pattern that can withstand high temperature and maintain good stability is required. In this work we develop a simple method to fabricate speckle pattern for high temperature application. The curing temperature of the speckle is controlled under 350 °C to avoid potential damage to the substrate material. The speckle can resist temperature up to 1050 °C for minimum 0.5 h. Three point bending experiment was carried out at 700 °C to test the speckle. Results show that this high temperature speckle exhibits excellent adhesion to the substrate. This non-invasive speckle shows great potential for improving the measurement reliability at high temperature in engineering application.

Camera array-based digital image correlation for high-resolution strain measurement.

Digital image correlation (DIC) is a well-known technique for non-contact, non-destructive, full-field deformation measurement in experimental solid mechanics. Although DIC has been widely used in science and engineering, the resolution of strain measurement with DIC is limited by imaging resolution and is much lower than that obtained with a strain gauge. To achieve a breakthrough in strain measurement using DIC, a camera array-based DIC method is proposed herein for high-resolution strain measurement. Twenty-five industrial cameras were assembled into a plane array, with each camera capturing a part of the specimen. A novel calibration-based image stitching method is proposed and was applied to these images and their corresponding displacement fields. The strain field was then calculated based on the stitched displacement fields. The use of the camera array greatly improved the measurement spatial resolution of DIC and made high-resolution strain measurement possible. Both static error analysis and four point-bending experiments were performed to demonstrate the feasibility and effectiveness of the proposed method, and a full-field strain resolution of 10 was achieved.

Whole bone testing in small animals: systematic characterization of the mechanical properties of different rodent bones available for rat fracture models

ObjectivesRat fracture models are extensively used to characterize normal and pathological bone healing. Despite, systematic research on inter- and intra-individual differences of common rat bones examined is surprisingly not available. Thus, we studied the biomechanical behaviour and radiological characteristics of the humerus, the tibia and the femur of the male Wistar rat—all of which are potentially available in the experimental situation—to identify useful or detrimental biomechanical properties of each bone and to facilitate sample size calculations.Methods40 paired femura, tibiae and humeri of male Wistar rats (10–38 weeks, weight between 240 and 720 g) were analysed by DXA, pQCT scan and three-point-bending. Bearing and loading bars of the biomechanical setup were adapted percentually to the bone’s length. Subgroups of light (skeletal immature) rats under 400 g (N = 11, 22 specimens of each bone) and heavy (mature) rats over 400 g (N = 9, 18 specimens of each bone) were formed and evaluated separately.ResultsRadiologically, neither significant differences between left and right bones, nor a specific side preference was evident. Mean side differences of the BMC were relatively small (1–3% measured by DXA and 2.5–5% by pQCT). Over all, bone mineral content (BMC) assessed by DXA and pQCT (TOT CNT, CORT CNT) showed high correlations between each other (BMC vs. TOT and CORT CNT: R2 = 0.94–0.99). The load–displacement diagram showed a typical, reproducible curve for each type of bone. Tibiae were the longest bones (mean 41.8 ± 4.12 mm) followed by femurs (mean 38.9 ± 4.12 mm) and humeri (mean 29.88 ± 3.33 mm). Failure loads and stiffness ranged from 175.4 ± 45.23 N / 315.6 ± 63.00 N/mm for the femurs, 124.6 ± 41.13 N / 260.5 ± 59.97 N/mm for the humeri to 117.1 ± 33.94 N / 143.8 ± 36.99 N/mm for the tibiae. Smallest interindividual differences were observed in failure loads of the femurs (CV% 8.6) and tibiae (CV% 10.7) of heavy animals, light animals showed good consistency in failure loads of the humeri (CV% 7.7). Most consistent results of both sides (left vs. right) in failure loads were provided by the femurs of light animals (mean difference 4.0 ± 2.8%); concerning stiffness, humeri of heavy animals were most consistent (mean difference of 6.2 ± 5%). In general, the failure loads showed strong correlations to the BMC (R2 = 0.85–0.88) whereas stiffness correlated only moderate, except for the humerus (BMC vs. stiffness: R2 = 0.79).DiscussionAltogether, the rat’s femur of mature specimens showed the most accurate and consistent radiological and biomechanical results. In synopsis with the common experimental use enabling comparison among different studies, this bone offers ideal biomechanical conditions for three point bending experiments. This can be explained by the combination of a superior aspect ratio and a round and long, straight morphology, which satisfies the beam criteria more than other bones tested.

Target-based calibration method for multifields of view measurement using multiple stereo digital image correlation systems

Multiple digital image correlation (DIC) systems can enlarge the measurement field without losing effective resolution in the area of interest (AOI). However, the results calculated in substereo DIC systems are located in its local coordinate system in most cases. To stitch the data obtained by each individual system, a data merging algorithm is presented in this paper for global measurement of multiple stereo DIC systems. A set of encoded targets is employed to assist the extrinsic calibration, of which the three-dimensional (3-D) coordinates are reconstructed via digital close range photogrammetry. Combining the 3-D targets with precalibrated intrinsic parameters of all cameras, the extrinsic calibration is significantly simplified. After calculating in substereo DIC systems, all data can be merged into a universal coordinate system based on the extrinsic calibration. Four stereo DIC systems are applied to a four point bending experiment of a steel reinforced concrete beam structure. Results demonstrate high accuracy for the displacement data merging in the overlapping field of views (FOVs) and show feasibility for the distributed FOVs measurement.

Simulation of inextensible elasto-plastic beams based on an implicit rate type model

The aim of this paper is to show that it is possible to model the elasto-plastic behavior of an inextensible beam undergoing finite bending by using a novel implicit rate type response relation directly between the bending moment, curvature, and their rates. This is in contrast to conventional approaches that require the integration of elasto-plastic stress–strain relations across the thickness of the beam at each time step. The model proposed here (a) is rate independent and exhibits hysteresis, (b) requires no notion of plastic strain and no integration across the thickness, and (c) does not have a sharp yield point and instead transits smoothly between nominally elastic and inelastic response. The governing equations are solved numerically as a set of first order differential equations with very low order interpolation functions for quasi-static response and are capable of modeling both hardening and softening behavior as well as the formation of plastic hinges, etc. Since there is no yield function, there is no necessity to use a return mapping algorithm that was developed specifically for use in elasto-plasticity with sharp yield functions. This simplifies the numerical algorithm also. The simulations for an elastic perfectly plastic beam are compared with elasto-plastic models in ABAQUSTM, and they show good agreement at a fraction of the computational time. Moreover we have compared simulations of large deformations of an aluminum alloy bar subject to three point bending experiments. The simulations also match the experimental results very well. We demonstrate that it is possible to incorporate frictionless sliding contact (which is needed for the three point bending comparison) in a relatively straightforward manner without the complexity that arises when it is handled in a full three-dimensional (3D) model.

Fresh and mechanical properties, and strain sensing of nanomodified cement mortars: The effects of MWCNT aspect ratio, density and functionalization

A comprehensive analysis on the effect of aspect ratio, bulk density and functionalization of multi walled carbon nanotubes (MWCNTs) in the development of nanomodified mortars, reinforced with different types of MWCNTs is presented herein. A structural characterization of the pristine and functionalized carbon nanotubes was carried out with scanning electron microscopy (SEM), transmission electron microscopy (TEM), and thermogravimetric analysis (TGA). A simple one step dispersion method, involving the application of ultrasonic energy and the use of a superplasticizer (SP) was utilized for the preparation of uniformly dispersed MWCNT suspensions. The experimental determination of the fresh and 28d mechanical properties of mortars with w/c = 0.5 and s/c = 3.0, using four different types of well dispersed pristine and functionalized MWCNTs at an amount of 0.1 wt% of cement took place through: (i) flow and time of setting tests; (ii) three point bending experiments on 4 × 4 × 16 cm specimens; and (iii) uniaxial compression on the half prisms of the flexural test specimens (4 × 4 × 8 cm). The piezoresistive behavior of the mortars reinforced with the pristine MWCNTs was experimentally determined using the 4-pole method, and compared with the strain sensing ability of the mortars reinforced with the functionalized MWCNTs. All MWCNT reinforced mortars exhibit a remarkable enhancement in the mechanical properties. However, the 28d flexural strength, young's modulus and energy absorption capability of the mortars reinforced with the mechanically functionalized MWCNTs at an amount of 0.1 wt% increased by 120%, 124%, and 103% respectively. Finally, depending on the procedure of the functionalization, chemical or mechanical, a different effect on the intrinsic properties of MWCNTs was observed. The carboxylic groups attached to the surface of the chemically functionalized MWCNTs indeed provided them with the ability of a uniform and effective dispersion, without the need of a sonication procedure. On the other hand, it was found that functionalized MWCNTs do not always retain the electrical properties of pristine MWCNTs.

Effect of CNT and CNF loading and count on the corrosion resistance, conductivity and mechanical properties of nanomodified OPC mortars

In this study, the influence of the addition of carbon nanotubes (CNTs) and nanofibers (CNFs) and their dispersion on the corrosion behavior, bulk electrical resistivity and mechanical properties of nanomodified Portland cement mortars was investigated. The experimental determination of the mechanical properties of nanocomposites reinforced with CNFs at an amount of 0.1wt% of cement, and CNTs at amounts of 0.1 and 0.5wt% took place, through three point bending and uniaxial compression experiments. The electrical properties were measured using alternating current (AC) and following the two pole method. For the evaluation of corrosion resistance the corrosion potential (Ecorr) and corrosion current density (Icorr) were measured in specimens partially immersed in a 3.5% NaCl solution, using the Linear Polarization Technique (LPR). The electrochemical mass loss was then calculated using Faraday’s Law. The addition of 0.1wt% CNTs and CNFs remarkably improves the flexural strength and the Young’s modulus, while enhancing the bulk electrical conductivity of the mortar nanocomposites. The corrosion study shows the addition of 0.1wt% CNTs and CNFs decreases the corrosion rate and significantly increases the resistance to corrosion by delaying the onset of the corrosion reaction.

A method to control delaminations in composites for adjusted energy dissipation characteristics

Concepts to adjust the delamination behaviour of textile reinforced composites are investigated. The composite interfaces are modified by adjusting the interlaminar contact area using perforated PTFE-foils. According mode I and mode II energy release rates are determined and a progressive correlation between the interlaminar contact area and energy release rates is identified. The results are exploited within three point bending experiments to adapt the structural delamination and subsequent energy dissipation behaviour with the proposed interface modification concept. Two structural designs concepts are evaluated numerically: adjusting structural energy dissipation capacity and adjusting the peak levels as well as the characteristic trends of the structural reactive forces. It is demonstrated, that the mechanical response of composite structures can be tailored by controlling their delamination behaviour.

Bending collapse behaviour of hat-shaped tubular structure strengthened with CFRP sheet

Three point bending experiment of aluminium tubular structure with hat cross-section was carried out under quasi-static and impact conditions. The structure was strengthened with carbon fibre reinforced plastic (CFRP) sheet. The objective is to exhibit the effective strengthening method for such tubular structure. The anvil speed was 10 m/s in impact test. The anvil force commonly showed a rapid increase in the early deformation stage and it changed to the gradual increase after yielding. The force reached peak followed by the decrease with saturating manner. The absorbed energy was approximately 10% greater under impact condition comparing with that under quasi-static one. The thickness of CFRP sheet was less effective. The energy absorption performance was slightly improved when the CFRP sheet was attached to the hat bottom. When the sheet was attached to both the hat top and the hat bottom, the performance was improved about 40% in maximum. It was improved about 10% when the hat sides were strengthened. It should be noted that the force did not exhibit the sharp drop, though other structures exhibited force decreasing stage.