Virgin Specimens Research Articles

Ultimate Capacity of Undercut Fasteners Installed in Heat-Damaged Concrete

The ultimate behavior and bearing capacity of undercut fasteners installed in heat-damaged concrete are investigated in this paper in which the results of a rather comprehensive research project are presented. Sixty-one fasteners (shank diameter ØN=10 mm) are tested under a pull-out force after being installed in as many preheated and postdrilled slabs made of three different concretes: (1) a low-strength concrete (LSC), fc20=20 MPa; (2) a normal-strength concrete (NSC), fc20=52 MPa; and (3) a high-performance concrete (HPC), fc20=63 MPa. Besides room temperature, five reference temperatures (between 200°C and 450°C at a prefixed distance from the heated surface) are considered to represent as many values of the fire duration prior to the instalment of the fastener. Four values of the embedment depth (45%, 60%, 80%, and 100% of the embedment suggested by the producer=10ØN) are investigated. In all cases, except in the virgin unheated specimens, the failure is caused by the thermally-damaged concrete, with the formation of a conical crack. Two finite-element (FE) models are formulated, based on linear-elastic fracture mechanics (LEFM) and on nonlinear fracture mechanics (NLFM), respectively, and their results are shown to envelop the test results. A third simple model based on linear fracture mechanics is also developed to quantify the extra loss of capacity induced by real fires in which the heating rate is one order of magnitude higher than in the electric furnaces generally used in the labs. Last but not least, to have information on the ultimate load associated with the possible formation of a conical crack, even in those cases in which the collapse due to shank yielding precedes concrete-cone failure (typically in high-grade virgin or undamaged specimens), the well-known concrete capacity method (CC-method) is modified to incorporate the maximum aggregate size, which plays a substantial role in crack mechanics.

Thermal shock resistance and bonding strength of tri-layer Yb2SiO5/mullite/Si coating on SiCf/SiC composites

A Yb2SiO5/mullite/Si tri-layer environmental-barrier-coating (EBC) were coated on SiCf/SiC substrates via Air Plasma Spraying (APS). The thermal cycle tests (TCT) were conducted under thermal corrosive condition of vapor-oxygen (50 vol% H2O and 50 vol% O2) with thermal shock from 1200 °C to 200 °C. Microstructures, weight loss and bonding strength of the samples were systemically investigated after 101, 396, 606 and 700 TCT cycles respectively. The results show that the corner of the tri-layer coating peel off from the sample with weight loss of 1.3% after 700 TCT cycles. The bonding strength between substrate and tri-layer coatings gradually decreases to 6.79 MPa (approximately 55.2% of virgin specimens) after 700 cycles due to thermal shock induced cracks distributed horizontally within Si layers and between Si layer and outer layers.

Microstructure optimizations for improving interlaminar shear strength and self-healing efficiency of spread carbon fiber/epoxy laminates containing microcapsules

The purpose of this study is to improve interlaminar shear strength and self-healing efficiency of spread carbon fiber (SCF)/epoxy (EP) laminates containing microcapsules. Microencapsulated healing agents were embedded within the laminates to impart a self-healing functionality. Self-healing was demonstrated on short beam shear specimens, and the healing efficiency was evaluated by strain energies of virgin and healed specimens. The effects of microcapsule concentration and diameter on apparent interlaminar shear strength and healing efficiency were discussed. Moreover, damaged areas after short beam shear tests were examined by an optical microscope to investigate the relation between the microstructure and the healing efficiency of the laminates. The results showed that the stiffness and the apparent interlaminar shear strength of the laminates increased as the microcapsule concentration and diameter decreased. However, the healing efficiency decreased with decreasing the microcapsule concentration and diameter.

Effect of Intra-Ply Hybrid Patches and Hydrothermal Aging on Local Bending Response of Repaired GFRP Composite Laminates

This study investigates the influence of intra-ply hybrid patches and hydrothermal aging on the indentation properties of patch repaired GFRP composites. Fabrics with various proportions of glass and Kevlar fibers were employed as the patches to achieve enhanced mechanical properties by hybridizing Kevlar and glass fibers together. Hydrothermal aging behavior of repaired composite laminates modified with water-resistant glass fibers in the outer layers was further investigated. Specimens were immersed in an environmental chamber containing seawater at temperatures of 30 (ambient), 50, and 70 °C until up to saturation. Damage mechanisms of repaired laminates were monitored using real-time acoustic emission (AE) technique. The experimental results showed that specimens repaired with 50G:50K patches offered superior performance than the virgin specimens. The hydrothermal aging effect on indentation behavior of the modified repaired specimens showed a considerable reduction in indentation properties, with higher strength retention exhibited by the repaired specimens modified with chopped glass fibers compared to the particulate fibers.

Study on Strength and Leaching Behavior of Biogeochemical Cemented Sand

Conventional ground improvement techniques involve densification of soil either by mechanical compaction or chemical grouting while others involve inclusion of reinforcements, etc. Many conventional grout materials were found posing a threat to environment due to their toxic nature and release of greenhouse gases. In this regard, research is initiated in developing more environmentally sustainable additives for soil improvement in which biological based alternatives are gaining momentum. In the present study, a noble technique Microbial Induced Calcite Precipitation (MICP) using Sporosarcina pasteurii was adopted to modify the properties of sand and improve its efficiency by supplementing with Cellulomonas flavigena. The mineralogy of treated specimens was studied using X-Ray Diffraction and Scanning Electron Microscope analyses. The leachability of precipitated calcite was studied under constant flowing conditions and the material was found to be stable. The Unconfined Compressive Strength (UCS) and elastic modulii of the treated specimens were found to be in the range of 266–343 kPa and 14–35 MPa respectively. The angle of internal friction found from Direct Shear Test on treated specimens was observed to be a little lower compared to virgin specimens. The permeability of treated specimen showed a reduction in magnitude by one order approximately.

Experimental and simulation study of the effect of cut-out defect in carbon fibres twill weave composite

The automated fibre placement (AFP) is an effective manufacturing process that produces a large complex structure with high quality at low secondary waste. Although, the AFP is highly accurate and reproducible and very common in the aerospace application, break and cut-out defect in the plies are inevitable. In this study, we have investigated 4 different types of geometrical cut-out defect that have an equal area (38.4 mm2) that cut from identical locations in the specimen plies. The effect of triangular, rectangular, square and circular cut-out defect has been investigated and compared to a virgin material that has no flaw or cut-out defect in the plies. It has been observed and hence reported that the circle induced flaw has caused 73% reduction in the impact strength due its 360° distortion of carrying stress. The failure mode of the circular cut-out defect specimens was proven to be matrix crack due to miscarriage of the stresses in the fibre, while a pull-out fibres failure mode has been observed in the virgin specimen. The experimental work has been validated using FEA analysis that identically simulate the boundary condition of both bending and impact test.

Carbonation Effect on Behavior of Ecological High Ductility Cementitious Composites

Abstract To explore the carbonation effect on the design parameters of a bridge deck link slab made by ecological high ductility cementitious composites (Eco-HDCC), flexural and compressive properties of Eco-HDCC were studied. Besides, the carbonation front and pH profile in the pore solution of Eco-HDCC specimens with different carbonation ages were determined. Results indicate that the ultimate flexural strength of Eco-HDCC has little change as carbonation age increases, and first cracking strength increases for carbonation ages of 3 d∼7 d, then decreases. The ultimate deflection of Eco-HDCC after being carbonated is higher than that of the virgin specimen. Besides, as the carbonation age prolongs, the compressive strength and elastic modulus of Eco-HDCC slightly increase. In addition, with the increase of the carbonation age, carbonation front of Eco-HDCC increases while the pH value of the pore solution decreases. When the depth ranges are 15 mm∼25 mm and 37 mm∼40 mm for carbonation ages of 56 d, the pH values are 10.4 and 11.2, which may destroy the passive film of the steel bar. Moreover, the mechanical property parameters for the design of the link slab made by Eco-HDCC can be selected according to non-carbonated material for the purpose of safe and conservative design. The corrosion-resistant steel bar or fiber-reinforced polymer (FRP) bar could be a promising choice to prevent steel bar corrosion.

Hydrothermally degraded carbon fiber / epoxy plates subjected to underwater explosive loading in a fully submerged environment

An experimental and computational investigation was conducted to evaluate the underwater blast response of fully submerged carbon fiber composite plates after prolonged exposure to saline water. The material was a biaxial carbon fiber/epoxy composite with a [±45°] fiber orientation layup. The plates were placed in a saline water bath with a temperature of 65 °C for 35 and 70 days, which simulates approximately 10 and 20 years of operating conditions in accordance to Fick's law of diffusion coupled with Arrhenius's Equation and a reference ocean temperature of 17 °C. Underwater blast experiments were performed in a 2.1 m diameter pressure vessel. The composite plates were placed in the center of the vessel while fully submerged in water, and an RP-85 explosive was detonated at a standoff distance of 102 mm from the center of the plate. Two cases of fluid hydrostatic gage pressures were investigated: 0 MPa, and 3.45 MPa. Two high speed cameras were utilized for three-dimensional Digital Image Correlation, which provided full-field displacements and velocities of the composite plates during underwater blast loading. A third high speed camera captured the behavior of the explosive gas bubble. Moreover, the pressure fields generated by the explosive detonation and resulting gas bubble were recorded with tourmaline pressure transducers. A water diffusion study was completed which showed that the diffusion of water into the composites reached a point of complete saturation after 35 days of exposure. Quasi-static material characterization tests were performed before and after prolonged exposure to saline water. The properties obtained from quasi-static testing also served as material inputs for the numerical models. The quasi-static test results showed that the tensile modulus E1,2 does not change with exposure to saline water, whereas the in-plane shear modulus G12 decreases with saline water exposure. During blast loading, for the case of 0 MPa hydrostatic gage pressure, the gas bubble interacts with the composite plate substantially. In such an event, the out of plane displacement increased for saline water exposed plates when compared to virgin structures. For the case of 3.45 MPa hydrostatic gage pressure, the gas bubble does not visibly interact with the composite plate. In this case, the out of plane displacement for specimens exposed to saline water was similar to the virgin specimen. A fully coupled Eulerian–Lagrangian fluid structure interaction simulation was performed by using the DYSMAS code. The numerical simulations showed that the displacement of fully submerged composite plates is driven by the displacement of fluid, as well as the size of the gas bubble formed by the explosive rather than the peak pressure generated by the explosive. The numerical simulations were in agreement with the experimental findings in terms of pressure history and plate deformation.

Bayesian analysis of fatigue data with multi-load-level damage accumulation: The benefits of rerun specimens

A hierarchical Bayesian approach to analysing fatigue test data including reinserted specimens is proposed. It is found that the inference model is capable of fitting the fatigue damage model to the observed data well. After the addition of rerun specimen data, the results show a significant change in the predictive SN curves. For the analysed 40CrMo8 fatigue data sets, the deviation of fatigue limit is the primary explanatory mechanism for the observed fatigue life scatter. The change of predictive fatigue limit distribution after the addition of rerun data is compared to the change due to additional (simulated) virgin specimen tests.

Effects of hygrothermal environment on the buckling and postbuckling performances of stiffened composite panels under axial compression

Effects of hygrothermal environment on the buckling and postbuckling performances of stiffened composite panels under axial compression were investigated experimentally and numerically in this paper. Hygrothermal aging experiment on stiffened composite panels was conducted in 70 °C distilled water and the moisture uptake was analyzed every 24 h until to moisture saturation. Moisture uptake process and moisture content distribution along the thickness direction were analyzed by using finite element (FE) method. Thin region and thick region of stiffened composite panels were defined and classified respectively according to moisture uptake process. Then the axial compressive experiments were performed on the hygrothermal aged specimens, with comparison of compression on virgin specimens. Strains, loads, out-of-plane displacements, shortening and failure modes were analyzed comparatively. The results indicate the hygrothermal environment impose much negative effects on the stability and carrying ability of specimens. In the end, a new approach for simulating the dynamic properties of unsaturated specimens was proposed, which considered the gradient of moisture content. Effects of various moisture content on buckling and postbuckling performances were determined.

Investigation on space charge and charge trap characteristics of gamma‐irradiated epoxy micro–nano composites

Epoxy nano–micro composite specimen prepared with micro silica and ion trapping nanoparticle, by shear mixing process, was exposed to gamma radiation and its performance for space charge and charge trap characteristics were analysed. The threshold for space charge accumulation of epoxy nanocomposites reduces and rate of space charge accumulation increases with an increase in dosage of gamma irradiation. The average growth of space charge density during poling and charge decay rate during depoling are relatively higher for gamma-irradiated specimens than the virgin specimen. The initial surface potential has a marginal reduction with increase in the dosage of gamma radiation, but the surface potential decay rate has increased significantly. Trap distribution characteristics indicate more number of shallow traps and increase in charge mobility after irradiation. The relative permittivity and loss tangent of the specimens have high impact due to gamma irradiation. The activation energy calculated from DC conductivity by Arrhenius law reduces with increment in radiation dose. Laser-induced breakdown spectroscopy reflected no change in elemental composition with gamma-irradiated specimen. The variation in plasma temperature and ion line to atomic line intensity ratio with dosage of gamma radiation have direct correlation to the Vickers hardness number of the specimens.

The effect of vascular self-healing pattern on mechanical behaviour and healing performance of epoxy/glass composite

ABSTRACTIn this research, the effect of vascular self-healing configuration on the healing behaviour of epoxy/glass (50/50) composite has been experimentally studied. Hollow glass fibres (HGFs) of 450 ± 10 μm diameter and 30–35% hollowness were produced by using an extruder. The hollow glass fibres were filled with healing components and subsequently employed as vascular healing system in the composite laminate. The orientations of embedded HGFs were selected at three levels of 0, 45 and 90°, and the distances of HGFs were chosen at three levels of 200, 500 and 800 μm in composites. A damage at tensile strain of 1.2% initiated, and following, the self-healing process took place. The virgin and healed specimens were characterised in terms of tensile behaviour, and subsequently, the healing performance was obtained. In addition, the formation of micro-cracks, fracture of filled HGFs and morphology of healed zones in self-healing composites were analysed by employing SEM.

A novel model to simulate the formation and healing of cracks in self-healing cross-ply composites under flexural loading

The formation and healing of cracks in fiber-reinforced composites made of a thermally remendable polymeric matrix were studied. A new thermally remendable polymer, using furan and maleimide moieties based on the Diels–Alder reaction was synthesized and employed for the fabrication of fiber-reinforced composites. To find the healing efficiency of composites, three-point bending specimens with the stacking sequence of [903/0/903] were prepared. The specimens were initially subjected to tensile loading until reaching the characteristic damage state. Then, the flexural modulus and strength after the healing up to different conversion degrees were obtained and compared with the virgin specimens. The results revealed over 95% recovery of the flexural strength and modulus after the complete healing. In addition, the effect of multiple healing on the healing efficiency was investigated and it was observed that the percentage of recovery would remain constant in the absence of fiber breakage. Using the shear-lag analysis and the classical lamination theory, a novel model was developed to simulate the residual flexural modulus of the damaged specimens as a function of the crack density. Moreover, the proposed model was extended to predict the recovered flexural stiffness after the healing of the specimens. A comparison of the results of the model and experiments validated the capability of the proposed model.

Effect of hygrothermal aging on static behavior of quasi-isotropic CFRP composite laminate

Abstract The Carbon fiber reinforced polymer (CFRP) composites used in offshore and marine industries tend to deal with high temperatures and humid conditions throughout its life. This work investigates the effect of long term hygrothermal aging on the physical and mechanical properties of CFRP composites. The IMA/M21 carbon fiber epoxy quasi-isotropic Multidirectional laminate with stacking sequence [±45, 0, 90] 2S was fabricated using vacuum assisted resin transfer moulding . The static specimens were immersed in room temperature (RT) and comparatively high temperature (HT) tap water at 70 °C up to saturation. The kinetics of moisture absorption was monitored up to saturation as a function of time which followed a two-stage model. The static strength of virgin, RT aged and HT aged specimens were determined with a constant cross-head speed of 1 mm/min. The post-saturation static strength showed a major drop however reduction in compressive strength was higher than tensile strength in a matrix dominated failure. Similarly, the hygrothermal aged specimen showed higher moisture absorption and strength reduction as compared to RT aging. The SEM and FESEM micrographs clearly revealed matrix erosion and fiber-matrix interfacial debonding after saturation. The TGA analysis of the moisture absorbed specimen showed a degraded physical property.

Effect of gamma irradiation on space charge and charge trap characteristics of epoxy–MgO nanocomposites

Insulating materials used in power apparatus required to be space charge free. Epoxy–magnesium oxide (MgO) nanocomposites were prepared and gamma rays were irradiated for different dosages to understand the dielectric parameters variation, space charge characteristics, surface potential variations, and for getting charge trap characteristics of the material. It is realised that permittivity increases and then reduces when the irradiation level is increased. Tan (δ) marginally increased with increasing dosage of gamma irradiation. Space charge accumulation reduces with increase in wt% of MgO in epoxy resin. 3 and 5 wt% MgO added samples showed stable space charge accumulation and fast decay during depoling, irrespective of irradiation level. Surface potential decay time reduces with irradiated specimens. After irradiation, epoxy nanocomposites showed both shallow trap formation and deep trap formation and found that shallow trap density is high compared to the deep trap density. All the irradiated specimens showed a decrease in the trap energy level compared to the virgin specimens.

Critical lifetime of HDPE pipes through damage and reliability models

Damage models are not directly applicable on high-density polyethylene (HDPE) pipes. In this paper, static and strain-unified theory damage models are adapted to fit the HDPE case by substituting the dynamic tests’ endurance limits by preloading simulation through notch and stiffness evaluation. Then, tensile and burst tests are following up to evaluate the specimens’ residual life. Compared to virgin specimens, the rupture limit of old HDPE pipes’ specimens had dropped significantly and their elongation decreased from 275 mm to about 26 mm. The degradation of the seven categories of specimens are different. Indeed, the degradation is too noticeable, disappearance of the plastic phase, for the categories 6 and 7, which are in the bottom of the pipe. Then, a reduced plastic phase on the lateral categories 4 and 5 showing an important impact of degradations. Finally, a larger plastic phase for the categories 1 and 2 taken from the top of the pipe, showing a medium impact of degradation. Thus, the use of the stiffness factor, reflecting the variability of degradation of the different categories of specimens, and the thickness reduction as life fractions for both aged and neat HDPE specimens was possible. The developed strains damage model compared to static burst pressures’ one confirmed the damage stages and the critical life fraction of HDPE pipes. By comparing these models, the drastic change of HDPE pipes’ behavior, from a ductile to a brittle one, have been proved. These findings allowed us to find out the critical life fraction of neat and old HDPE pipes, which has been confirmed by comparing the burst pressure curves of a notched and an old pipe. The presented approach is cost effective allowing a deep analysis of HDPE pipes failure and damage quantification through simply made models based on static tensile and burst test instead of tedious and very costly dynamic ones.

Effect of hygrothermal environment on the tension–tension fatigue performance and reliable fatigue life of T700/MTM46 composite laminates

The tension–tension fatigue performance of T700/MTM46 composite laminates after hygrothermal aging was investigated and compared with those of virgin T700/MTM46 laminates. The most significant failure mode of the moisture-saturated fatigue specimens is still severe delamination, and the stiffness degradation of moisture-saturated fatigue specimens can be divided into two distinct stages. However, the hygrothermal conditions will aggravate the stiffness degradation of the composite laminates during fatigue. Damage evolution was studied by the edge view of the specimens. The degree of damage of the saturated specimens is more serious than that of the virgin specimens at the same percentage of fatigue life during the fatigue process, especially in the initial stage. The distribution of fatigue life in each stress level was determined. The p-γ-S-N surfaces were established to predict a reliable fatigue life. The results show that the reliable fatigue life of the moisture-saturated specimens is much lower than that of the virgin specimens under the same conditions. Although the hygrothermal environment does not show a significant effect on the static tensile properties of the T700/MTM46 composite laminates, the fatigue performance is significantly degenerated.

Thermomechanical fatigue behaviour of aged heat resistant austenitic alloys

The increasing demands for efficiency and flexibility result in more severe operating conditions for the materials used in critical components of biomass power plants. These operating conditions involve higher temperature ranges, more pronounced environmental effects and cyclic operations. Austenitic stainless steels have shown to possess promising high temperature properties which makes them suitable as candidates for critical components in biomass power plant. However, their behaviour under such conditions is not yet fully understood. This work investigates three commercial austenitic alloys: Esshete 1250, Sanicro 25 and Sanicro 31HT. The alloys were subjected to in-phase (IP) thermomechanical fatigue (TMF) testing under strain-control in the temperature range of 100–800 °C. Both virgin and pre-aged TMF specimens were tested in order to simulate service degradation resulting from long-term usage. The results show that the pre-aged specimens suffered shorter TMF-life compared to the virgin specimens. The scanning electron microscopy methods electron backscatter diffraction (EBSD) and energy dispersive spectroscopy (EDS) were used to analyse and discuss active failure and deformation mechanisms. The difference in TMF-life produced by the two testing conditions was attributed to an embrittling effect by precipitation, reduced creep properties and oxidation assisted cracking.

Multiple-scale investigations on self-healing induced mechanical property recovery of ECC

The self-healing behavior of Engineered Cementitious Composites (ECC) has attracted much research in the past decade. Nonetheless, very few study attempted to reveal the underlying mechanism of self-healing induced mechanical recovery of pre-damaged ECC. In this paper, the influence of self-healing on the material properties of ECC was investigated on multiple length scales. Plausible underlying mechanisms of mechanical properties recovery of ECC are proposed. The results show obvious regaining of tensile/flexural properties of pre-damaged ECC at macro-scale due to self-healing. At the meso-scale, self-healing leads to fiber bridging strength recovery to about the same level as that of virgin specimens. At the micro-scale, self-healing is found associated with recovery of frictional bond strength between the fiber and matrix. However, chemical bond recovery appears to be absent. The regaining in fiber/matrix interfacial friction results in the recovery of fiber bridging capacity, which forms the root cause for recovery of the mechanical properties of self-healed ECC.

Fatigue analysis of shape memory alloys by self-heating method

In recent years, self-heating method, which is based on temperature measurements, has been proposed as a faster and cheaper alternative to classical fatigue tests. In the present study, one-dimensional thermomechanical constitutive equations of shape memory alloys (SMAs) were implemented into the self-heating method by using a probabilistic two-scale model in order to study fatigue of these alloys. This model was shown to be capable of predicting the S/N curve of these alloys as well as its scatter by using probability relations. To identify parameters of the two-scale model, self-heating experiments were conducted. The presented approach was developed to predict the S/N curve of both raw specimens and heterogeneous ones with surface treatments for any failure probability in a much shorter time compared to the classical methods. The heterogeneous samples were made by hydrogenating the surface of virgin specimens. To investigate validity of the numerical results, the theoretical predictions were compared with experimental data and a good agreement was observed.