Uniaxial compression tests are the most common tests for characterizing the strength of concrete-like materials. The dynamic compression strength of concrete-like material is typically obtained by ...Split Hopkinson Pressure Bar (SHPB) tests. The increase in material strength under dynamic loading is usually attributed to the strain rate effect and modelled with a dynamic increase factor (DIF). However, it was observed by some researchers that the radial inertial confinement caused apparent increase of dynamic strength of concrete-like specimen in SHPB tests. They attributed the material strength increase to this inertial effect, instead of the strain rate effect. In the present study, numerical analyses are performed to investigate the compressive behaviour of concrete-like material at high strain rates. A homogeneous macroscale model and a heterogeneous mesoscale model are developed in the study. In the macroscale model, the material is assumed to be homogeneous and isotropic. In the mesoscale model, the test sample is modelled as a three-phase composite consisting of aggregate, mortar matrix and interfacial transaction zone (ITZ) between the aggregate and the mortar matrix. The aggregate is assumed to be circular and the ITZ is modelled as a thin boundary around the aggregate. In the both models, the materials are assumed to be insensitive to the strain rate first. Therefore, the obtained strength enhancement is only due to the inertial confinement. Strain rate sensitive material properties are then used in the two models in the calculations. Numerical simulations of the concrete samples under compression at different strain rates are carried out. The relative contribution of the inertial effect and the strain rate effect on the compressive strength DIF is examined based on the numerical results. The failure process of concrete specimen is also studied.
Abstract Studies have suggested that maternal infection/inflammation maybe a major risk factor for neurodevelopmental brain damage. In the present study, we evaluated the effects of prenatal exposure ...to a low level of inflammatory stimulation lipopolysaccharide (LPS) repeatedly on spatial learning and memory performances in rat offspring's lifetime. Sixteen pregnant Sprague–Dawley rats were randomly divided into two groups. The rats in the LPS group were treated i.p. with LPS (0.79 mg/kg) at gestation day 8, 10 and 12; meanwhile the rats in the control group were treated with saline. After delivery, the rat offspring at 3- (young), 10- (adult) and 20-mon-old (aged) were allocated. Spatial learning and memory abilities were tested by Morris water maze. The structure of hippocampal CA1 region was observed by light microscopy. The expression of synaptophysin (SYP) and glial fibrillary acidic protein (GFAP) in hippocampal CA1 region were measured by immunohistochemistry. Results showed that the rat offspring of LPS group needed longer escape latency and path-length in the Morris water maze and presented a significant neuron loss, decreased expression of SYP, increased expression of GFAP in CA1 region in histological studies. All these changes were more significant with the age increasing. These findings support the hypothesis that maternal systemic inflammation may alter the state of astrocytes in rat offspring for a long time, the alteration may affect neurons and synapse development in neural system, increase the neurons' vulnerability to environment especially as the age increasing, at last result in distinct learning and memory impairment.
At mesoscale, concrete may be regarded as a three-phase composite consisting of coarse aggregate, mortar matrix and interfacial transition zone (ITZ) between the aggregate and the mortar matrix. In ...the present paper, mesoscale model is adopted to analyze the dynamic tensile behaviour of concrete at high strain rates; especially, the effects of the ITZ on the failure properties are analyzed. In the mesoscale model, to simplify the problem, the shape of the coarse aggregate is assumed to be circular and the ITZ zone is modelled as a thin boundary layer around the aggregate. Dynamic material properties and continuum damage mechanics theory are employed to simulate the material behaviour of the three phases. Numerical simulation of the concrete samples under tension at different strain rates are carried out. Different aggregate size, different aggregate distribution and different material properties are considered. Strain rate effect is also analyzed. From the numerical results, it is found that the dynamic failure (crack) pattern is highly affected by the aggregate distribution. It is also found that the properties of the interfacial transition zone significantly influence the failure mechanism and the tensile strength of concrete.
Summary Objective Chitosan has been widely used as an injectable scaffold in cartilage tissue engineering due to its characteristic biocompatibility and biodegradability. In this study, chitosan was ...used in its hydrogel form as a scaffold for chondrocytes that act to reconstruct tissue-engineered cartilage and repair articular cartilage defects in the sheep model. This study aims to find a novel way to apply chitosan in cartilage tissue engineering. Methods Temperature-responsive chitosan hydrogels were prepared by combining chitosan, β-sodium glycerophosphate (GP) and hydroxyethyl cellulose (HEC). Tissue-engineered cartilage reconstructions were made in vitro by mixing sheep chondrocytes with a chitosan hydrogel. Cell survival and matrix accumulation were analyzed after 3 weeks in culture. To collect data for in vivo repair, reconstructions cultured for 1 day were transplanted to the freshly prepared defects of the articular cartilage of sheep. Then at both 12 and 24 weeks after transplantation, the grafts were extracted and analyzed histologically and immunohistochemically. Results The results showed that the chondrocytes in the reconstructed cartilage survived and retained their ability to secrete matrix when cultured in vitro . Transplanted in vivo , the reconstructions repaired cartilage defects completely within 24 weeks. The implantation of chitosan hydrogels without chondrocytes also helps to repair cartilage defects. Conclusions The chitosan-based hydrogel could support matrix accumulation of chondrocytes and could repair sheep cartilage defects in 24 weeks. This study showcased the success of a new technique in its ability to repair articular cartilage defects.
Highlights • Prenatal LPS (0.4 mg/kg) results in pathological impairment in the hippocampus in offspring. • Prenatal exposure to high-fat diet results in systemic inflammation in offspring rats. • ...Prenatal LPS and high-fat diet result in protective effect on the hippocampus in offspring. • Predictive adaptive response may underlie the protective effect on the hippocampus.
Stem cells were characterized by their stemness: self-renewal and pluripotency. Mesenchymal stem cells (MSCs) are a unique type of adult stem cells that have been proven to be involved in tissue ...repair, immunoloregulation and tumorigenesis. Irradiation is a well-known factor that leads to functional obstacle in stem cells. However, the mechanism of stemness maintenance in human MSCs exposed to irradiation remains unknown. We demonstrated that irradiation could induce reactive oxygen species (ROS) accumulation that resulted in DNA damage and stemness injury in MSCs. Autophagy induced by starvation or rapamycin can reduce ROS accumulation-associated DNA damage and maintain stemness in MSCs. Further, inhibition of autophagy leads to augment of ROS accumulation and DNA damage, which results in the loss of stemness in MSCs. Our results indicate that autophagy may have an important role in protecting stemness of MSCs from irradiation injury.
Experimental results and numerical simulations have demonstrated that a protective barrier can effectively reduce blast load and, therefore, protect structures from an external explosion. However, ...there are no formulae in the open literature that can be used to estimate the blast loads on a structure behind a barrier. In this paper, pseudo-analytical formulae based on numerical results are derived to estimate the reflected pressure-time history on a rigid wall behind a protective barrier. Numerical simulations of blast wave propagation are carried out to estimate the peak reflected pressure and the impulse on a rigid wall behind a blast barrier. The shock wave front arrival time and positive phase duration are extracted from the numerical results. Pseudo-analytical formulae, which are derived from the best-fitted curves of the numerical results, are suggested. These formulae can be used with those given in TM5-1300 or other methods for blast pressure estimation in the no-barrier case, to estimate pressure-time histories at various building locations behind a protective barrier.
This study presents an effective procedure for extracting the first few bridge frequencies using the data collected by a moving test vehicle. Previously, the effectiveness of the vehicle scanning ...method for bridge frequencies was hampered by factors such as vehicle frequency and road surface roughness. To this end, the
contact-point response
of the vehicle with the bridge that is
free of the vehicle frequency
is adopted in the analysis. To enhance the visibility of the first few bridge frequencies for extraction, the
variational mode decomposition with band-pass filter
(VMD-BPF) is proposed herein. The VMD is neater and more elegant than the empirical mode decomposition (EMD) in that less decompositions are needed, while there exists no mode-coupling problem, and the BPF serves to remove the undesired roughness frequencies. To verify the feasibility of the proposed procedure, both the vehicle and contact-point responses generated either numerically or by the field test are analyzed. It is demonstrated that the VMD-BPF is an effective method for extracting the bridge frequencies using the contact-point response for the scenarios considered.
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