Undamaged Specimens Research Articles

Laminated Glass Plates Subjected to High-Velocity Projectile Impact and Their Residual Post-Impact Performance

This study aims to analyze the performance of laminated glass against ballistic loading and investigates its residual load-bearing capacity. Two groups of specimens were used in quasi-static four-point bending experiments, first without prior ballistic damage and then with it. The main objective was to compare the load-bearing capacity of these two groups to see the effect of ballistic damage. Three different layer compositions were used. The ballistic loading was conducted using an in-service 9 mm bullet fired from a semiautomatic carbine with the glass specimens hanging on steel ropes in a free boundary setup. Numerical simulation and analytical methods were used and validated against the measured response of the undamaged specimens. The simulations were in good agreement with the experimental results. All of the glass specimens were able to withstand the ballistic loading, and the subsequent performance during the quasi-static bending loading was similar to that of the undamaged specimens. The quality of the glass edges seemed to be more important than ballistic damage. The front-plate damage played a negligible role, and the back-plate damage needed to be extensive to influence subsequent performance. Provided that ballistic damage is mainly localized only to the centers of the plates, it did not affect the post-impact loading capacity.

Application of SBF Test to Fatigue Damage Assessment of Type 316 Steel

Abstract A new fatigue testing technique, the small bulge fatigue (SBF) test, was recently developed by some of the present authors. In this technique, a cyclic oil pressure is alternatively applied to both surfaces of a small disk-type specimen (8 mm in diameter) with flat and concave surfaces. This SBF test is a kind of biaxial fatigue test as well as the disk bending fatigue (DBF) test. In this study, damaged specimens were subjected to the SBF test to examine the adaptability of this test to the fatigue damage assessment of actual components. To prepare these damaged specimens, Type 316 austenitic stainless steel was subjected to the DBF test using large disk specimens (87 mm in diameter) by considering that most of components were generally operating under multiaxial loading condition. The test was stopped at several stages of the fatigue life. Then, the SBF test specimens were taken from the surfaces of the interrupted DBF test specimens. It was expected that fatigue life of the SBF tests corresponded to the degrees of fatigue damage induced by the DBF test. The SBF test results showed that the fatigue life of the damaged SBF specimen, which contained microcracks formed by the DBF test, was only one tenth as long as that of the undamaged specimen. It was noteworthy that some microcracks introduced by the DBF test were opened and became clearly visible after the SBF test, although they were closed and not visible just after the DBF tests.

Performance of Hair Testing for Cocaine Use-Comparison of Five Laboratories Using Blind Reference Specimens.

The purpose of this study was to compare results from five commercial hair testing laboratories conducting workplace drug testing with regard to bias, precision, selectivity and decontamination efficiency. Nine blind hair specimens, including cocaine-positive drug user specimens (some contaminated with methamphetamine) and negative specimens contaminated with cocaine, were submitted in up to five replicates to five different laboratories. All laboratories correctly identified cocaine in all specimens from drug users. For an undamaged hair specimen from a cocaine user, within-laboratory Coefficients of Variation (CVs) of 5-22% (median 8%) were reported, showing that it is possible to produce a homogenous proficiency testing sample from drug user hair. Larger CVs were reported for specimens composed of blended hair (up to 29%) and curly/damaged hair (19-67%). Quantitative results appeared to be method-dependent, and the reported cocaine concentrations varied up to 5-fold between the laboratories, making interlaboratory comparisons difficult. All laboratories reported at least one positive result in specimens contaminated with cocaine powder, followed by sweat and shampoo treatments. Benzoylecgonine, norcocaine, cocaethylene and hydroxylated cocaine metabolites were all detected in cocaine powder-contaminated specimens. This indicates that current industry standards for analyzing and reporting positive cocaine results are not completely effective at identifying external contamination. Metabolite ratios between meta- or para-hydroxy-cocaine and cocaine were 6- and 10-fold lower in contaminated specimens compared to those observed in cocaine user specimens, supporting their potential use in distinguishing samples positive due to contamination and drug use.

Foreign object damage and post-impact tensile behavior of plain-woven SiC/SiC composites

Plain-woven SiC/SiC composite plates, fabricated using the chemical vapor infiltration (CVI) technology, were impact-tested at different velocities using 3-mm-diameter steel spheres. The states of the impacted targets were divided into three categories: surface damage, critical penetration, and full penetration. The microstructures of the foreign object damage (FOD) in the targets were carefully observed using microscope and X-ray computerized tomography (CT) scans. Based on the results of the observations, the forms of impact damage of ceramic matrix composites (CMCs), including cone cracking, yarn fracture and bending, and delamination, are revealed and summarized. The mechanisms of the penetration process were further analyzed by combining these damage modes of the CMCs. Subsequently, tensile tests of the impacted specimens were performed to investigate the effect of FOD on the tensile behavior of the CMCs. The digital image correlation (DIC) method was used to obtain the strain distribution of the impacted targets during tension. The tensile curves of the undamaged and impacted specimens were obtained and compared. The evolution of the tensile failure of the damaged beams was then analyzed. Finally, the residual strengths of CMC plates impacted at different velocities were compared.

Stress assessment and fracture surface analysis in a foreign object damaged 7075-T6 specimen under rotating bending

The damage caused by impact of hard particles on aircraft components is called Foreign object damage (FOD) and it may cause premature failure. 7075-T6 is one of the alloys frequently used for manufacturing aircraft parts and components. In this paper, the failure mechanism of a damaged 7075-T6 specimen subject to rotating bending was explained from a phenomenological point of view using the Finite Element (FE) method and the fracture surface observation. The results showed that the crack initiation occurred in the locations of the highest residual tensile axial stresses after the impact, where also the tensile axial stresses in the simulated fatigue cycle are the highest, and at the surface of the FOD dent. A discontinuity was observed under the dent, in the area subject to compression after the impact and in the fatigue cycle. The damage induces a 30 ± 10% reduction in fatigue strength compared to undamaged specimens.

Post-failure behavior of 2-ply laminated glass plates with different interlayers

Modern design codes, such as the new Eurocode on the design of structural glass components, consider the prediction of the post-failure behavior of these elements of paramount importance for a proper and safe design. In this view, an experimental investigation is presented in the paper, on the mechanical response of undamaged and damaged 2-ply Laminated Glass (LG) under quasi-static loads, with different interlayers, namely polyvinyl butyral (PVB), SentryGlas (SG) and a plasticized version of PVB, Saflex DG41 (DG41). Firstly, undamaged specimens (UDLG) were tested in simply supported configuration (configuration 0) with a vertical load in the middle increased up to the failure of the bottom glass ply. damaged LG specimens (PDLG) were then tested into two different configurations: configuration I, with broken ply below (bottom ply) and configuration II, with broken ply above (top ply). All the presented tests were performed under displacement control. For both configurations, the interlayers influence on the post-breakage behavior was discussed. It is underlined that the global response of the two configurations was completely different: configuration II, which was characterized by a deformed shape opposite to the load direction, showed a good residual stiffness and reached high failure loads. On the contrary, in configuration I, the response was characterized by a low stiffness and load carrying capacity. In addition, also the influence of different glass types was discussed, focusing attention on tempered and toughened glass. The results showed that the contribution of the fractured layer cannot be disregarded in the evaluation of the global stiffness of a PDLG and its contribution is strictly related to the interlayer and glass typologies.

Experimental Study on Mechanical Properties of Q690 High-Strength Steel after High Cycle Fatigue Damage

Through the static tensile test of Q690 high-strength steel, the relevant mechanical parameters are obtained and the maximum fatigue load is determined. The fatigue life is measured by the fatigue test under the load. According to the fatigue cumulative damage method, the number of fatigue pre-damage vibration is designed in proportion. Then the fatigue pre-damage test is carried out on the high-strength steel, the stress-strain curve and the variation of residual mechanical property reduction coefficient with fatigue damage were drawn. The results show that: compared with the undamaged specimens, the yield strength and tensile strength of Q690 steel are less affected by fatigue damage, but the elongation changes more significantly, and the elastic modulus is not significantly affected. Finally, through the change of mechanical properties of Q690 high-strength steel with different fatigue damage, it provides a scientific basis for the performance evaluation of existing Q690 high-strength steel structure after fatigue damage.

Mechanical evaluation of different repair concepts in thermoplastic and thermoset materials

The repair process of thermoplastic and thermoset polymeric matrix composite materials is being increasingly demanded in the aeronautic industry for the need to know, in relation to the performed approximation, the recovery of residual strenght. To ensure the security and the viability of the repair processes, as well as its durability throughout its life cycle, it is necessary to perform the validation of the mechanical properties of structural elements after the repair process to eliminate defects and damages. The aim of this work is to validate, after aging, different damaged composite repaired coupons that have undergone different repair processes. The analyzed repair concepts are named as “Prepreg”, “Dry” and “Hybrid” for the thermoset material, and “Reconsolidation” and “Pre-consolidated patch” for different integration techniques of thermoplastic materials. Test coupons have reached H/W conditions according to applicable specifications. Additionally, different protections have been performed on the specimens to faithfully simulate ageing conditions. Furthermore, for each type of material and configuration, a reference test series meaning undamaged and unrepaired specimens has been tested to monitore in each case the level of recovered structural capacity. This activity is part of the CERES (Advanced Rear End Structural test program – low level tests) project, which is framed under Clean Sky 2 JU.

An experimental investigation on the net cross-section failure of damaged plates containing holes

In Eurocode 3, a design rule is given for predicting the ultimate resistance of the net cross-section at bolt holes. This is based on the average stress evaluated in the net cross-section, and contains both a reduction factor of 0.9 and a partial factor. According to previous research, this reduction factor makes the design rule over-conservative for uncracked components. There are indications that this conservatism accounts for the possible presence of small cracks at the bolt holes. Very limited experimental research has been conducted on the influence of potential cracks at the bolt hole, particularly at the notch root. This paper presents a few experimental results on the ultimate load of pre-cracked notched plate specimens made of S275JR grade steel. Four pre-cracked specimens, with crack depths ranging from 1 to 7mm, have been tested and their ultimate load has been compared with the one of non-pre-cracked specimens, exhibiting lower ultimate load and ductility as compared to the undamaged specimens. However, despite the significant pre-cracking, the ultimate strength is slightly lower than calculated with the design rule of Eurocode 3, considering mean material properties and excluding the partial factor. Moreover, the pre-cracked specimens have been assessed using a constraint-corrected version of the failure assessment diagram validating its prediction with the experimental results.

Constitutive models and statistical analysis of the short-term tensile response of geosynthetics after damage

The objective of this research was to analyse the tensile response of three geosynthetics, to apply constitutive equations, and propose values for model parameters to represent the nonlinear behaviour of these materials in the short-term, supported by statistical analysis. Data for specimens of a nonwoven geotextile, a woven geogrid and a reinforcement geocomposite previously submitted to mechanical damage, abrasion damage, and mechanical damage followed by abrasion damage were analysed. Nonlinear regressions of the experimental results were performed to fit the load vs. strain curves to constitutive equations. For each geosynthetic, the results of damaged specimens were statistically compared to those of the undamaged ones to observe the influence of the induced damage on the tensile behaviour of the material. Experimental results were statistically compared with those obtained by the constitutive models to verify if the tensile properties and the model parameters were properly estimated. For the geotextile, significant changes in tensile properties were noticed only after sequential mechanical damage and abrasion damage. For the geogrid and the reinforcement geocomposite, abrasion damage was predominant due to considerable changes in the tensile properties and the shape of the load vs. strain curves. In general, the polynomial models fitted the ultimate tensile strength slightly better, while the hyperbolic-based models presented better approximation of the secant stiffness. For hyperbolic-based models, estimating curves for damaged materials from model parameters of undamaged specimens by applying adjustment coefficients and reduction factors allowing for damage was considered promising, with slight differences for average and median curves. Contrary to the literature, model parameter α was not a material constant, as it varied according to the material condition and the shape of the load vs. strain curve.

Structural Damage Detection Using Supervised Nonlinear Support Vector Machine

Damage detection, using vibrational properties, such as eigenfrequencies, is an efficient and straightforward method for detecting damage in structures, components, and machines. The method, however, is very inefficient when the values of the natural frequencies of damaged and undamaged specimens exhibit slight differences. This is particularly the case with lightweight structures, such as fiber-reinforced composites. The nonlinear support vector machine (SVM) provides enhanced results under such conditions by transforming the original features into a new space or applying a kernel trick. In this work, the natural frequencies of damaged and undamaged components are used for classification, employing the nonlinear SVM. The proposed methodology assumes that the frequencies are identified sequentially from an experimental modal analysis; for the study propose, however, the training data are generated from the FEM simulations for damaged and undamaged samples. It is shown that nonlinear SVM using kernel function yields in a clear classification boundary between damaged and undamaged specimens, even for minor variations in natural frequencies.

Residual Strength of Reinforced Concrete Beams under Sequential Small Impact Loads

Sequential small impact loads may not collapse structures directly but could weaken the strength of structures. This study aimed to investigate the impact of these sequential small impact loads on the strength of the reinforced concrete beams. First, six sequential impact loads were applied to the test specimens. Then, the residual static capacity of the impacted specimens was determined by the ultimate static load test, compared with those of undamaged specimens. The experiment was composed of 12 specimens having identical dimensions. The variable parameters were the magnitude of the axial load and shear reinforcement. Under the sequential small impacts, the axial load improves the impact performance. It reduces the tensile strain of the longitudinal reinforcement. Hence, the flexural tensile crack propagation is limited. In addition, the local damage at the impact location is minimized and the shear plug induced diagonal shear crack is prevented. The axial force is also able to diminish the adverse effect of the large spacing stirrups. Large impact load could alter the failure of a designed flexural critical reinforced concrete beam without axial load to the shear failure. Although the axial load improves the impact response, the Residual Resistance Index (RRI) decreases with axial load. For the damaged specimens with axial load, the ultimate static load is lower than the calculated concrete shear capacity and more severe diagonal shear cracks were found. It can be obviously said that the prior impact damage decreases the concrete shear capacity.

Determination of Fatigue Damage Initiation in Short Fiber-Reinforced Thermoplastic through Acoustic Emission Analysis

This study investigates the damage initiation in short glass fiber-reinforced polyamide 6.6 under fatigue loading using acoustic emission analysis. An optimized specimen geometry was developed to meet the specific requirements of this testing method, at the same time allowing further micromechanical studies. Specimens were preloaded with tensile–tensile fatigue loading, varying the maximum stress and the number of load cycles. Subsequently, the acoustic emission signals in residual strength tests were compared to those of undamaged specimens. The idea behind this approach is that only the damage that has not already occurred under fatigue load can be recorded in the residual strength tests. Using the analysis of acoustic energy, a stress threshold for damage initiation was identified. Furthermore, with tension–tension fatigue tests, the SN curve of the material was determined to estimate the lifetime for the identified stress threshold. The presented approach allows us to estimate a so-called endurance limit of short glass fiber-reinforced polyamide 6.6.

Development and Validation of a Three-Dimensional Printed Multifunctional Trap for Surveillance of Mosquitoes.

An essential component of vector-borne disease monitoring programs is mosquito surveillance. Surveillance efforts employ various collection traps depending on mosquito species and targeted life-history stage, i.e., eggs, larvae, host-seeking, resting, or gravid adults. Surveillance activities often use commercial traps, sometimes modified to accept specific mosquito species attractants. The advent of widely available and affordable 3D printing technology allows the construction of novel trap designs and components. The study goal was to develop and assess a cost-effective, multipurpose, 6-volt mosquito trap integrating features of both host-seeking and gravid mosquito traps to collect undamaged live specimens: a multifunctional mosquito trap (MMT). We tested the MMT in comparison to commercial traps, targeting gravid Aedes albopictus, host-seeking Ae. albopictus, and total number of host-seeking mosquitos regardless of species. Field evaluations found the MMT performed as well as or better than comparable commercial traps. This project demonstrates an easy to construct, inexpensive, and versatile mosquito trap, potentially useful for surveying multiple mosquito species and other hematophagous insects by varying attractants into the MMT.

Study on prediction method of crack propagation in absorber weld by experiment and simulation

As an important equipment for purifying crude oil and natural gas in energy industry, absorber is subjected to harsh working environment of high stress and severe corrosion. Especially, weld is the weak region of the absorber, which faces significant risk of crack propagation due to residual stress caused by the welding process, working pressure and temperature. Meanwhile, crack propagation in absorbers with high pressure, high temperature and toxic media can often cause disastrous consequences. Therefore, to solve this problem, a systematic method considering welding, heat treatment and working process is presented in this paper to predict crack propagation completely in the weld. In the case, based on actual conditions, a finite element model (FEM) of welding process considering transient heat transfer and heat radiation for residual stress field analysis is established to obtain the far-field stress for cracks. Then, the detailed analysis of stress field and stress intensity factor (KI) under the obtained far-field stress at the crack tip is carried out through the method of integrating the strain energy density (SED) in the crack tip region. Finally, experiments of fracture toughness (KIC) of undamaged weld specimens and post-corrosion weld samples are carried out to compare with KI obtained by FEM and evaluate the safety of existing cracks. The method presented in this paper is a complete system that can provide a universal technological basis for crack propagation prediction and safety assessment of pressure vessel.

Textile reinforced mortars (TRM) for repairing and retrofitting masonry walls subjected to in-plane cyclic loads. An experimental approach

Masonry walls exhibit low tensile strength and high material heterogeneity, which makes them especially vulnerable against cyclic loading conditions, such as those typical in earthquakes. This paper presents the experimental results obtained from tests on three masonry walls reinforced with textile reinforced mortar (TRM) materials subjected to in-plane cyclic loading. These full-scale masonry walls were tested in the LARGE laboratory at the University of Alicante (Alicante, Spain). The walls had been built using a traditional construction technique, with solid clay bricks layered with lime mortar. One specimen was tested and damaged by in-plane cyclic loading and was subsequently strengthened by a vertical layer of TRM with an overlapping of 200 mm. It was then tested again until failure in a second test. In addition, another undamaged specimen had been previously reinforced with the same TRM technique and tested until failure, thus providing a third test. A network of sensors and digital image correlation systems were used to monitor displacements and crack patterns. The comparison between these experimental results made it possible to assess the effectiveness of TRM in restoring the structural integrity of damaged masonry walls and almost doubling their load-bearing capacity under cyclic loads. Conclusions obtained here provide valuable information to the scientific community, architects and structural engineers about the strengthening and repair of severely damaged masonry walls.

Behavior of Auxetic Steel Wire RC Columns Exposed to Elevated Temperature

Twenty-four rectangular RC column specimens, constructed at 1/3 scale, were tested under axial loading to investigate the use of ASWM (one, two, three, four, and five layers) exposed to elevated temperatures. Six of the column specimens were subjected to laboratory temperature (control or un-damaged specimens) of 23oC. Eighteen of the column specimens were subjected to elevated temperatures (six columns at 250oC, six columns at 500oC, and six columns at 750oC) for 2 hr. The obtained results clearly showed that each axial load resistance, toughness, and stiffness were all negatively affected by escalated temperature degrees. Also, the confinement effectiveness in terms of the ultimate load was decreased with the increase in concrete compressive strength (un-damaged). The number of ASWM layers significantly influenced the ductility, energy absorption, and ultimate load improvement percentage. The number of the ASWM layers considerably enhanced the ductility up to a certain number of erected layers; however, adding more layers had no apparent effect.

Bending residual strength of damaged Al foam sandwiches through UT and IRT analyses

In the present work, the effect of low-speed impact on the static residual strength of aluminum foam sandwiches having GFRP skins has been evaluated. Two different configurations have been considered, which differs both for thickness and layup used for the manufacturing of GFRP skins. The detection and the quantification of the damage induced by impact have exploited the potentiality of pulsed thermography and phased array ultrasonic techniques, which gives the possibility to provide synergistic and complementary information. Mechanical strength has been evaluated through three-point bending tests carried out on undamaged and damaged specimens. Static tests allowed also evaluating the failure mode of panels and the changes induced by the presence of a low-speed impact zone.

Repair of severely damaged RC columns through FRCM composites

This study presents the results of an experimental campaign aimed at testing the effectiveness of carbon fiber reinforced cementitious matrix (CFRCM) confinement as a repair technique for severely damaged reinforced concrete (RC) columns. To evaluate shape effect and transverse steel reinforcement (TSR) – fiber interaction, two different cross-section shapes, and two TSR rates were investigated. Nine full-scale specimens were analyzed considering three control ones, three undamaged specimens confined with two CFRCM layers, and three severely damaged specimens repaired through confinement by two CFRCM layers. Axial stress-strain curves were analyzed to derive strength and ductility enhancement. Finally, TSR and fiber strains were monitored to better comprehend the contributions of internal and external reinforcement to the overall confinement and possible TSR-FRCM interaction.

Evaluating Applicability of ASTM C 928 Approach in Assessing Adequacy of Patch Repair of Bridge Piers

Severe environmental conditions in many parts of Iran could adversely affect infrastructures, especially bridge piers. This research evaluates the efficiency of fiber-reinforced polymer-modified self-compacting concrete in patch repair of bridge piers. The efficiency of this material for repair has been verified using ASTM C 928 and a novel testing method (patch test) that uses a cylindrical compression test simulating indirect force transfer from old (existing) concrete to new (patch repaired) concrete. To investigate the efficiency of self-compacting concrete in patch repair and derive the correlation between bond strength and patch strength, two sets of specimens have been included in the experimental program. These include 24 and 27 specimens which are prepared for patch and slant shear tests, respectively. Test results show significant improvement in strength due to use of polymer modified and fiber reinforced self-compacting concrete in slant shear tests, where strength enhancement as much as 50% (compared to undamaged specimens) was observed. Meanwhile, in the patch tests, repaired specimens are only able to barely exceed the original strength of undamaged specimens. In the slant shear tests, the use of polymer is very effective in increasing bond strength and using fibers reduces observed variation in the strength of repaired specimens. Considering force path in the patch repair and regardless of materials used for repair, the results show that judging the efficiency of the repair method based on the slant shear test, as proposed by ASTM C 928, in the case of patch repaired elements could be misleading.