Final Failure Load Research Articles

Experimental and numerical investigation on failure behavior of hybrid bonded/bolted GFRP single-lap joints under static shear loading

The mechanical performance of the GFRP (Glass Fiber Reinforced Polymer) hybrid bonded/bolted single-lap joints under static shear loading is investigated. Static loading tests considering the hybrid joints with one bolt, four bolts, and nine bolts were first conducted to study the failure mechanism of hybrid joints, where failure process, rigidity, failure load, and failure modes were focused. Finite element (FE) analysis involving the material degradation and failure laws was also carried out for exploring the internal mechanism of joints’ failure. Based on the verified FE model, the effects of plate thickness and bolt edge distance on the failure capacity and strength of the hybrid joints were further investigated by performing a numerical parametric analysis. Results show that, the failure process of the hybrid joints can be divided into two stages which correspond to the failures of adhesive layer and GFRP plates, respectively. The joint rigidity of joints increases by 98.9 %∼133.1 % comparing the second loading stage to the first stage, and it is found that the rigidity in the first stage is hardly affected by the number of fasten bolts, while it decreases with more bolts in the second stage due to the severe incompleteness of GFRP plates. The failure patterns of the hybrid joints mainly include the slippage between connected plates, the compressive failure and shear failure of GFRP material, and the inclination of bolts. A proper arrangement of bolts can lead to shear failure of GFRP which makes full use of the material property. The final failure loads of four-bolt and nine-bolt joints are respectively 14.5 % and 22.9 % higher than the single-bolt specimen. As the plate thickness getting greater, the shear bearing capacity of hybrid joint increases almost linearly, while it would increase first and then descend with increasing bolt edge distance. In general, the achievement of this work can provide references for the design and further study of the GFRP hybrid joints.

SHEAR BEHAVIOUR OF REINFORCED-CONCRETE BEAMS INCORPORATING IRON FILINGS AS SAND REPLACEMENT

This study examined the influence of partial sand replacement with iron filings on the mechanical and physical characteristics of concrete, as well as conducted experimental tests of the shear behaviour of reinforced-concrete beams. Four replacement rates were used in this study, i.e., 5 %, 10 %, 20 %, and 30 % with a reference mixture containing no iron filings. At ages 7, 14, and 28 days, mechanical property tests (slump, density, ultrasonic pulse velocity, compressive strength, and splitting strength tests) were conducted. In addition, the shear behaviour of five reinforced-concrete beams with the same replacement rates (0 %, 5 %, 10 %, 20 %, and 30 %) were experimentally tested. Tests were conducted to determine the ultimate load failure, final deflection, energy absorption, stiffness, ductility index, compressive stress, and crack formations. According to the results, the correlation between the slump test and iron filings is positive; however, that for absorption is negative. With a higher percentage of iron-filing replacement, the density and ultrasonic pulse velocity increased. For specimens with 30,00 % iron filings, the densities, pulse velocities, and slumps were raised by 6,27 %, 2,44 %, and 58,33 %, respectively, compared to the reference specimens, whereas the absorption rate decreased by 20,00 %. Having 20,00 % iron filings produced the maximum compressive and splitting strengths of 28,00 %, which was 4,60 % higher than the reference mixture, whereas 30,00 % iron filings produced the highest flexural strength, which was 9,50 % higher than the reference mix. The findings of beam testing revealed that increasing the iron-filing content in concrete beams increased the final failure load, final deflection, ductility index, and energy absorption by 6,70 %, 10,29 %, 11,30 %, and 35,00 %, respectively. The initial and secant stiffnesses decreased at rates of 12,60 % and 3,10 %, respectively.

Unified machine-learning-assisted design of stainless steel bolted connections

For the design of stainless steel bolted connections, current design codes firstly calculate the design resistance of each potential failure mode and then take the minimum of the design resistances calculated from all the potential failure modes as the final design failure load. However, this design framework has been found to be tedious and also leads to inaccurate failure load predictions. In this paper, a more accurate and unified predictive framework for stainless steel bolted connections made of different materials with various configurations and bolt hole patterns at both room and elevated temperatures is proposed based on machine learning. This paper starts with a collection of 301 experimental data of stainless steel bolted connections to establish a database. Then, regression models for failure load predictions were developed and trained by six machine learning algorithms, including Decision Tree, Random Forest, Support Vector Machine, Adaptive Boosting, Extreme Gradient Boosting and Cat Boosting, with the key hyperparameters for each machine learning algorithm tuned. This is followed by the evaluation of the model performance by means of a series of statistical indices, with the results indicating that the regression model trained by Support Vector Machine has the best model performance. On the basis of the collected experimental data, the regression model trained by Support Vector Machine and current design codes were assessed and compared, showing that the current design codes are rather inaccurate, while the machine-learning-assisted design method is a unified method and can provide accurate and consistent failure load predictions for stainless steel bolted connections made of different materials with various configurations and bolt hole patterns at both ambient and elevated temperatures.

Investigation of the medium-term effect of osteoprotegerin/bone morphogenetic protein 2 combining with collagen sponges on tendon-bone healing in a rabbit.

Osteoprotegerin (OPG) and bone morphogenetic protein-2 (BMP-2) could be administered sequentially to promote tendon-bone healing. There remain several unresolved issues in our previously published study: a) the release kinetics of OPG/BMP-2 from the OPG/BMP-2/collagen sponge (CS) combination in vitro remained unclear; b) the medium-term effect of the OPG/BMP-2/CS combination was not analyzed. Hence, we design this study to address the issues mentioned above. 30 rabbits undergoing anterior cruciate ligament reconstruction (ACLR) with an Achilles tendon autograft randomly received one of the 3 delivery at the femoral and tibial tunnels: OPG/BMP-2, OPG/BMP-2/CS combination, and nothing (blank control). At 8 and 24 weeks post-surgery, the biomechanical tests and histologic analysis were used to evaluate the tendon-bone healing. In mechanical tests, the OPG/BMP-2/CS group showed a higher final failure load and stiffness than the other groups at 8 and 24 weeks. Additionally, the maximum stretching distance showed a decreasing trend. The mechanical failure pattern of samples shifted from a tunnel pull-away to a graft midsubstance rupture after OPG/BMP-2/CS-treated. From histological analysis, the OPG/BMP-2/CS treatment increased the amount of collagen fibers (collagen I and II) and promoted fibrocartilage attachment. CS as a carrier promotes the medium-term effect of OPG and BMP-2 on tendon-bone healing at the tendon-bone interface in a rabbit ACLR model. OPG, BMP-2 and CS were already applied in several clinical practice, but a further study of clinic use of OPG/BMP-2/CS is still needed.

Experimental Study on External Loading Performance of Large Diameter Prestressed Concrete Cylinder Pipe

A prestressed concrete cylinder pipe (PCCP) is created with a complex composition of concrete core, welded steel cylinder, prestressed steel wire, protective mortar and anti-corrosion coating. Due to the economy and complexity of structural prototype tests, the ultimate loading test on PCCP is rarely conducted. In this paper, the three-edge bearing test was carried out on a 3.2 m diameter PCCP with embedded steel cylinder. The strains of concrete core, prestressed steel wire and protective mortar were monitored, and the distribution and width of the concrete cracks were recorded. The test results show that the cracking on the inside concrete at the crown zone occurred before those on the invert zone of the pipe. The prestressed steel wire postponed the tensile stress of out-side concrete at the springing line until subjected to the calculated cracking load (Pc). Due to the moment redistribution caused by the cracking on the inside of the concrete at the crown/invert, the protective mortar at the springing line was cracked at 1.2 Pc and exhibited visible cracking after 1.4 Pc. The prestressed steel wire reached the elastic and strength limit states of 1.4 Pc and 1.6 Pc, respectively. The PCCP designed by the limit state design method can resist the increased external loads after reaching the serviceability limit state. The final failure load of the test pipe is greater than 1.6 Pc, and there is sufficient safety in actual operation. Due to the fact that the damage to the concrete at the crown/invert zone may become a fuse of PCCP failure, the tensile stress of the inside concrete at the crown/invert zone of the PCCP should be accurately verified in the design process.

Experimental and numerical study of a novel cold-formed steel T-Stub connection subjected to tension force

Cold-formed steel (CFS) is a popular form of construction in New Zealand and the developed world because it can be cost-effective, durable, sustainable, resilient to extreme environmental and seismic loads and easy to install. When used in domestic and medium-rise buildings, T-Stub connection between CFS channel sections is formed by using self-drilling screws which connect the flanges. In the literature, no prior investigation on the tension capacity of such connection has been reported. Therefore, this paper describes a novel test procedure for determining the tension capacity of such CFS T-Stub connection and the associated mechanism of failure. An experimental test program that comprises thirty experimental tests is reported in this paper. Three channel section thicknesses were examined: 0.55 mm, 0.75 mm, and 0.95 mm. All specimens failed as a result of tilting of the screws and bearing failure. Non-linear elasto-plastic finite element (FE) models were developed, and the predictions were validated against the experimental test results. It was adjudged that there was reasonable agreement in terms of connection strength and failure mode prediction. Finally, the design strengths derived from current design standards were compared with the experimental failure loads. It was found that the American Iron and Steel Institute (AISI 2016) and Australian and New Zealand Standard (AS/NZS 2018) are conservative by 6% and unconservative by 10% for the initial and final failure loads of T-Stub CFS connections, respectively.

Effects of fillers on the impact damage and compressive residual properties of single hat-stiffened composite panels

Hat-stiffened composite panels have been widely used in the primary structures of airplanes, such as fuselage sections and wing boxes. Impact damage in the structures can significantly reduce the residual load capacities and inserting fillers into the deltoid regions between the stiffener and skin could be a method of enhancing the damage tolerance of the hat-stiffened composite skin. In this study, impact and compression after impact (CAI) tests have been conducted on the single hat-stiffened composite skin specimens, and the effects of the fillers on the buckle load, failure load and impact damage tolerance are analyzed through comparing the compressive test results of the specimens with/without fillers and with/without impact damage. The usage of the fillers increases not only the buckle load and final failure load of the single hat-stiffened composite skins, but also the impact damage tolerance largely. Furthermore, finite element models are developed for the simulations of the post-buckling and failure process, and the influential mechanisms of the impact damage and fillers are revealed through analyzing the post-buckling failure process. This work provides a study about the influences of the usage of fillers on the mechanical properties of the single hat-stiffened composite skin and it can be a method for initial design of large multi-stringer composite panels.

In Vitro Evaluation of Bond Strength to Dentin of Two Post Systems: Computer-aided Design and Computer-aided Manufacturing Fiber Posts vs Splendor Single Adjustable Post

To assess dentin-post bond strength and mode of failure through tensile strength testing of two endodontic post systems: CAD/CAM custom-milled fiber posts vs Splendor SAP. Thirty extracted single-rooted mandibular premolars were sectioned 2 mm coronal to the cementoenamel junction. Root length was standardized at 15 mm, and the root canals were instrumented with #20 K-files followed by 30/0.03 and 30/0.05 ProDesign Logic rotary files, under irrigation with 2.5% NaOCl, and then submitted to final irrigation with 17% EDTA. Canals were filled with gutta-percha and AH PLUS sealer. After 24 hours, the teeth were prepared for post placement to a depth of 10 mm and randomly allocated into two experimental groups (n = 15): CAD/CAM (CC) and Splendor SAP (SS). All posts were cemented with RelyX U200 dual-cure self-adhesive resin cement. The roots were embedded in acrylic resin, and the specimens were stored for 7 days in moist heat (37°C). Tensile strength testing until failure was then performed in a universal testing machine using a crosshead speed of 0.5 mm/minute. The final failure load was tabulated for statistical analysis, and the G test was used to compare the failure modes observed under light microscopy (5× magnification). There was no significant difference between groups regarding tensile bond strength to root dentin (p = 0.325). Conversely, failure mode differed significantly between groups (p = 0.037). The tensile bond strength observed for the CAD/CAM and Splendor SAP post systems was similar. Adhesive failure was predominant in both groups; however, the CAD/CAM custom-milled fiber posts failed predominantly at the dentin-resin cement interface, whereas Splendor SAP posts failed mostly at the post-resin cement interface. A strong post-dentin bond is a key to the success of dental restorations and prosthetic rehabilitation. In teeth with severe coronal decay and wide canals, both of the tested systems would be able to achieve good cervical fit.

Machine-learning-based design of high strength steel bolted connections

For the design of high strength steel bolted connections, all existing standards adopt the same framework – (i) calculating the design resistance for each potential failure mode and (ii) defining the final design failure load as the minimum of the design resistances calculated from all the potential failure modes. However, this framework has been found to be tedious and also incapable of considering the complex nature of connection behaviour, in particular the transition of different failure modes (e.g., net section fracture, bearing and block tearing), and thus leads to inaccurate failure load and mode predictions. To address the aforementioned shortcomings, this paper presents a more accurate and reliable predictive framework based on machine learning. Firstly, a database including 543 experimental and numerical data was collected. Then, regression models for failure load predictions and classification models for failure mode predictions were developed based on eight machine learning algorithms, including Decision Tree, Random Forest, Support Vector Machine, K-Nearest Neighbour, Adaptive Boosting, Light Gradient Boosting Machine, Extreme Gradient Boosting and Cat Boosting. The performance of the developed models was assessed based on the collected data, indicating that the Support Vector Machine models led to the best predictions of failure loads and modes. The Support Vector Machine models were then compared with existing design standards and shown to yield substantially improved failure load and mode predictions for high strength steel bolted connections. Specifically, the mean ratio of experimental and numerical to predicted failure loads from the machine-learning-based approach is equal to 1.00, while the corresponding mean ratios from the design standards range from 1.10 to 1.39. The machine-learning-based approach is capable of accurately predicting 97.2% of the total failure modes, while the design standards can only accurately predict 67.9%–85.3% of the total failure modes.

Failure analysis of composite F-joint structures under tensile loading: Experimental and numerical study

This paper presents an investigation on the failure process of two different types of composite frame joints (F-joints), one is connected by high-lock bolts (FH) and the other reinforced by high-lock bolts and adhesive (FHA). The mechanical behavior of F-joints subjected to out-of-plane tension load is analyzed by experiment and numerical simulation. Test results show significant differences in the damage process, failure modes and strain distribution between FH and FHA. The strength of FHA increases nearly 13% compared to FH. Three-dimensional finite element models considering failure criterion and damage evolution are developed and implemented in the commercial code Abaqus/Standard through a user-defined subroutine UMAT. Cohesive zone model is used to simulate the adhesive layer damage of FHA. Simulating failure load and the damage process match the experimental data. The information on the failure mechanism and final failure load of composite F-joints is useful for the optimal design of such joints.

Evaluation of the geometrical discontinuity effect on mixed-mode I/II fracture load of FDM 3D-printed parts

The main objective of this manuscript is to evaluate the capability of the fracture criteria and numerical methods to predict fracture properties of the specimens fabricated using the Fused Deposition Modelling (FDM) 3D-Printing method. Different thermoplastic materials are 3D printed in Semi-Circular Bending (SCB) specimens configuration and subjected to three-point bending loading to investigate their failure behaviour, including final load-bearing and crack path trajectory. The Acrylonitrile Butadiene Styrene (ABS) due to its brittle behavior, was selected for the mixed mode fracture study. The SCB samples with various pre-crack angles indicating different modes of loading conditions are manufactured. Cracked SCB specimens are produced with and without geometrical discontinuities as holes to verify the effect of stress concentration on their fracture behavior. The results of the three-point bending tests demonstrated that the Extended Finite Element-Cohesive Zone Model (XFEM-CZM) and Average Strain Energy Density (ASED) criterion are capable of predicting the fracture behavior of the 3D-Printed ABS material under mixed-mode I/II loading. The appropriate XFEM-CZM damage criterion was used to estimate the crack growth path. The final failure load was obtained using the ASED and XFEM criteria. The maximum error rate between the experimental results and the presented theories was 10%, which confirms the employed criteria' accuracy. The results showed that a stress raiser like a hole in the 3D-Printed specimens might reduce the loading capacity of the material by 17%.

Effect of Stringer Design on the Stability of Stiffened Composite Panel Under In-plane Shear: An Experimental Study

The safe exploitation of the buckling and post-bulking region of stiffened composite panels is of prime interest in aerospace engineering. Experiments were carried out to investigate the effects of stringer design on the buckling and post-buckling behavior of stiffened composite panels subjected to in-plane shear loading in this study. Three configurations of I-shape stringers were designed with the same skin in the stiffened composite panels. The shear loading was applied through a customized picture-frame rig which is loaded by a mechanical test machine. Ten specimens in total were manufactured and tested to obtain the buckling loads and the final failure loads, respectively. The experimental results show that the stringer design has much more influence on the buckling load than the final failure load of the stiffened composite panel.

Measurement of Fracture Strength of Zirconia Dental Implant Abutments with Internal and External Connections Using Acoustic Emission.

The aim of the study was to investigate the fracture behaviour of four different groups of zirconia abutments with internal and external connections: (A) Astra Tech ZirDesign™ abutment on Astra Tech Implants, (B) Procera® Esthetic abutment on Nobel Biocare MK III Groovy Implants, (C) IPS e.max® on Straumann Implants, and (D) ZiReal® Posts on Biomet 3I implants. The load was applied on the assemblies using a Zwick universal testing machine: the initial and final failure loads and amplitude were recorded using acoustic emission technique. Mean initial and final failure force was found to be significantly different in each group (P < 0.001). IPS e.max® Straumann abutments exhibited the highest resistance to final fracture force compared to other abutment types. Acoustic emission can be used as one of the methods to detect fracture behaviour of implant abutments. There were no significant differences in fracture loads between the internal and externally connected zirconia abutments studied. However, externally connected abutments demonstrated screw loosening and some deformations.

Stress analysis and strength prediction of composite laminates with two interacting holes

In this study, stress analysis and strength prediction were performed for carbon/epoxy laminates with holes in specific orientations using experimental and numerical techniques. The interaction between the holes was studied with reference to their positions and load direction. The maximum laminate stress at the edge of the holes was identified for different hole orientations. The critical region was determined by considering the maximum stress location at the hole edges. Linear and non-linear behavior around the hole edge was investigated by strain gauge measurements for different hole orientations. Final failure load was predicted for specimens with specific hole orientations by using the point stress criteria (PSC) method. Furthermore, the extended PSC (EPSC) method was proposed to predict the failure load for the specimens with specific hole orientations. The EPSC method was verified by experimental study.

Progressive damage behaviour of RTM-made composite T-joint under tensile loading

The paper deals with mechanical behaviour of the RTM (resin transfer moulding)-made composite T-joints under tensile loading. Initial failure and damage evolution behaviors as well as the load-displacement curves were determined by experiments. Progressive damage models (PDMs) of composite T-joint were presented based on the Hashin, Chang-Chang, Hou criteria and the mixed criterion, and the results predicted from the aforementioned PDMs were compared with experimental data, demonstrating the better correlation with the experiments for the predictions from the mixed criterion than from other criteria. The reason for this is that the interlaminar delamination failure can be identified and isolated by the mixed criterion. Using the PDM based on mixed criterion, the predicted initial and final failure loads have a good agreement with experiments, and the effects of triangular fillers and radius of boundary angle were investigated and discussed on initial failure loads of composite T-joint. The findings in this work are helpful to improve mechanical behaviour of composite T-joint.

Effects of the initial adhesive disbonding on patch repaired composites

The adhesive layer plays an important role on the stress and deformation transfer between composite patch and parent plate in a repaired structure. In this paper, based on the nonlinear material properties and continuum damage mechanics, the three-dimensional progressive damage model of the repaired specimen has been built to investigate the effect of initial adhesive disbonding on the ultimate strength and the damage state. The results indicate that the initial adhesive disbonding does not change the final failure mode, but results in the decrease of the final failure load. The decreasing amount of final failure load for the repaired specimen with four-layer patch is larger than that for the repaired specimen with two-layer patch.

Experimental and numerical study on residual strength of aircraft carbon/epoxy composite after lightning strike

Mechanical properties of aircraft composite laminate after lightning strike are predicted and verified based on the combination of experiment and numerical simulation. Axial compression experiment of composite laminate containing lightning strike damage is carried out and failure modes of experimental samples under three kinds of electrical current waveforms are evaluated. The main failure modes include fiber fracture, matrix cracking and delamination. The final failure load decreases with the increase of actual integral action of lightning strike. Compression failure process of composite laminate is simulated using progressive damage analysis methods such as Hashin criterion, Maximum stress criterion and TSERPES criterion, respectively. Numerical simulation and experiment results are compared and coincided well. Percentage error of Hashin criterion is the minimum compared with those of TSERPES criterion and Maximum stress criterion. Stress concentration mainly appears in both angle sides on the fixed end and lightning damage regions of composite laminate.

Suppression of initial failure at tapered end-closure sandwich panel joint by taper angle change

To examine the configuration of CFRP face plates/foamed plastic core sandwich panel joints, a tapered end-closure-type joint is selected and studied. In this type of joint, the sandwich panel is tapered to form a solid laminate comprising two face plates near the joint, and the two panels to be joined are mechanically fastened at the solid laminate with a splice plate. This type of joint may be suitable for aircraft panels because a flat surface can be obtained at the joint, which is advantageous from an aerodynamic viewpoint. However, in a previous study on such a joint, it was found that a delamination crack initiated from the tapered core end and propagated through the interface between the two face plates as an initial failure mode at a much lower tensile load than the final failure load in a tensile strength test. In this study, the angle of the tapered panel was focused on and the effect of changing the taper angle on suppressing the initial failure was investigated through experiments and numerical analysis. It was found that a smaller taper angle is more effective for suppressing the initial failure.

Experimental Investigation on Flexural Behaviour of Reinforced Geopolymer Concrete with and without Glass Fibres

Experimental investigations were carried out on flexural behaviour of geopolymer reinforced concrete beams with and without fibres. The mix is composed of ternary blend of mineral admixtures, ground granulated blast furnace slag (GGBS), silica fume, metakaolin and fly ash in varying proportions. Two different molarities, namely, 6M and 10M of NaOH and Na2SiO3solution were considered to study the behaviour of geopolymer concrete beams with and without fibres. Beams were cast for various mixes and the behaviour was studied under static standard two point loading. From the flexural experiments, it was found that the first crack load varied from 24 kN to 46 kN and the final failure load varied from 71 kN to 139 kN. It was found that the ultimate load and the corresponding deflection is increased by about 5–10% for the case of beams with fibres. Many multiple cracks were observed in the case fibre incorporated beams which shows the higher energy absorption and improved ductility. It was further noted that for the case of 10M mix beams, there is no significant difference in terms of load carrying capacity, occurrence of first crack load and deflection compared to 6M mixes.

Physical and mechanical properties of a novel hydrogen transport membrane

A novel cermet (ceramic/metal composite) hydrogen transport membrane (HTM) was characterized for its physical and mechanical properties at both room temperature and at elevated temperature of 850 °C. The density of the HTM cermet was measured as 8.9 g/cm3 at ambient temperature with dynamic Young’s modulus of 145.5 GPa and the dynamic shear modulus (G-value) of 54.29 GPa, whereas the Poisson’s ratio (υ) is calculated to be 0.34. Vickers hardness numbers for the as-received HTM cermet are constant and in the range of 2.0–2.2 GPa, with the change of loading force from 100 to 1000 g. The flexural strength (σ fs) of the HTM cermet is about 356 MPa at room temperature and decreases to 284 MPa at the elevated temperature of 850 °C in both air and nitrogen. A difference in testing atmosphere (air or N2) had insignificant effect on the load–displacement curves during the flexural strength test at the elevated temperature of 850 °C. From the load–displacement curves, HTM cermet behaves much more like elastic material at room temperature; with the temperature increased to 850 °C, its behaviour is much more like a metallic material with fractures preceded by plastic deformation. The initial yielding strength of HTM could be mainly controlled by the yield strength of the soft Pd metal material, and the zirconia fractures before yielding in Pd at room temperature, whereas at 850 °C, the yield stress of the Pd drops dramatically and allows it to work and harden in the constraint of a rigid ceramic. The final failure load is mainly dictated by ceramic fracture, while the initial yielding stress is determined on the yield strength of the soft Pd metal at both room temperature and the elevated temperature of 850 °C.