In Situ Deformation Characterization of Density-Graded Foams in Quasi-Static and Impact Loading Conditions

Digital image correlation is utilized to characterize deformation and strain fields developed within the layers of density-graded multilayered foam structures subjected to uniaxial quasi-static and dynamic compression. Three-layered graded structures fabricated from rigid polyurethane foams with nominal densities of 160, 240, and 320 kg/m³ are subjected to both quasi-static and dynamic loading. The quasi-static measurements show that, irrespective of the loading direction, the densification initiates in the lowest density layer and propagates into other layers later once the first layer is fully densified. The deformation mechanisms are seen to be different in the case of dynamic loading conditions compared to the quasi-static loading. The deformation mechanism, in the case of dynamic loading, depends on the sample orientation relative to the direction of the applied load. In cases where the higher density layers are impacted, the propagation of the elastic and compaction waves leads to partial deformation of the lowest density layer. Sample deformation continues in all layers upon the reflection of the stress waves from the distal end of the sample. In cases where the lowest density layer is oriented towards the impact face, a completely different deformation response is observed. A detailed full-field analysis of strain and stress is performed. The mechanisms associated with the formation and propagation of stress waves from the impact ends to the distal ends of the samples are discussed

High Strain Rate Response of Adhesively Bonded Fiber-Reinforced Composite Joints A Computational Study to Guide Experimental Design

Adhesively bonded carbon fiber-reinforced epoxy composite laminates are widely used in aerospace applications. During a high energy impact event, these laminates are often subjected to high strain rate loading. However, the influence of high strain rate loading on the response of these composite joints is not well understood. Computational finite element (FE) modeling and simulations are conducted to guide the design of high strain rate experiments. Two different experimental designs based on split Hopkinson bar were numerically modeled to simulate Mode I and Mode II types loading in the composite. In addition, the computational approach adopted in this study helps in understanding the high strain rate response of adhesively bonded composite joints subjected to nominally Mode I and Mode II loading. The modeling approach consists of a ply-level 3D FE model, a progressive damage constitutive model for the composite material behavior and a cohesive tie-break contact element for interlaminar delamination

The COVID-19 pandemic and healthcare systems in Africa:A scoping review of preparedness, impact and response

BACKGROUND: The COVID-19 pandemic has overwhelmed health systems in both developed and developing nations alike. Africa has one of the weakest health systems globally, but there is limited evidence on how the region is prepared for, impacted by and responded to the pandemic. METHODS: We conducted a scoping review of PubMed, Scopus, CINAHL to search peer-reviewed articles and Google, Google Scholar and preprint sites for grey literature. The scoping review captured studies on either preparedness or impacts or responses associated with COVID-19 or covering one or more of the three topics and guided by Arksey and O’Malley’s methodological framework. The extracted information was documented following the Preferred Reporting Items for Systematic Reviews and Meta-Analyses extension checklist for scoping reviews. Finally, the resulting data were thematically analysed. RESULTS: Twenty-two eligible studies, of which 6 reported on health system preparedness, 19 described the impacts of COVID-19 on access to general and essential health services and 7 focused on responses taken by the healthcare systems were included. The main setbacks in health system preparation included lack of available health services needed for the pandemic, inadequate resources and equipment, and limited testing ability and surge capacity for COVID-19. Reduced flow of patients and missing scheduled appointments were among the most common impacts of the COVID-19 pandemic. Health system responses identified in this review included the availability of telephone consultations, re-purposing of available services and establishment of isolation centres, and provisions of COVID-19 guidelines in some settings. CONCLUSIONS: The health systems in Africa were inadequately prepared for the pandemic, and its impact was substantial. Responses were slow and did not match the magnitude of the problem. Interventions that will improve and strengthen health system resilience and financing through local, national and global engagement should be prioritised

The COVID-19 pandemic and healthcare systems in Africa : a scoping review of preparedness, impact and response

Funding Information: Funding GAT was supported with funding from the Australia National Health and Medical Research Council (NHMRC) Investigator Grant #1195716.Peer reviewedPublisher PD

Static and dynamic constitutive behavior and fracture of titanium based FGM under thermo-mechanical loading

An experimental study has been conducted to investigate the static and dynamic constitutive behaviors and fracture of titanium based functionally graded material (FGM) under different temperature. The material used for the present study is a Ti/TiB FGM plate consisting of seven layers with100 % Ti at one end and 85% TiB at the other. The specimens are machined using a wire electrical discharge machining (EDM) technique. The dynamic constitutive behavior in compression is investigated using the split Hopkinson pressure bar (SHPB) technique in conjunction with Infra red spot heater. At elevated temperature the material showed high thermal softening with reduction in failure stress and an increase in failure strain. Quasi-static and dynamic fracture-initiation toughness under thermo-mechanical loading is also investigated using three-point bending experiment and the effect of temperature and loading rate on the fracture-initiation toughness of this material is discussed. ©2009 Society for Experimental Mechanics Inc

Failure and Fracture Behavior of Brittle Polymer Foam

Experimental study was performed to investigate the fracture behavior of relatively brittle polymer foam. A single notch bending specimen made of a PVC core cell foam A-series, A 800 and A 1200, are used for the investigation. To measure the strain around the defect section, a 2D digital image correction (DIC) technique was used. The fracture initiation toughness was calculated from the load displacement curve and a strain fields obtained from DIC technique. Furthermore a study was performed to investigate the failure behavior of foam core with sharp cracks, notch and circular hole. To reduce the size effect, the net cross-sectional areas of the specimen for all the geometries considered are kept constant. An Instron tensile loading machine was used and the tensile load was measured directly through the load cell. The full strain field around the section was measured using DIC and the data points at the interest location were extracted. The result was compared with the dog-bone tensile experiment of intact specimen. It was observed that, the net section strength for specimen with cracks, notch and circular hole is higher than that of the intact foam core

Quasi-static and dynamic fracture initiation toughness of Ti/TiB layered functionally graded material under thermo-mechanical loading

Quasi-static and dynamic fracture initiation toughness of Ti/TiB layered functionally graded material (FGM) is investigated using a three point bend specimen. The modified split Hopkinson pressure bar (SHPB) apparatus in conjunction with induction coil heating system is used during elevated temperature dynamic loading experiments. A simple and accurate technique has been developed to identify the time corresponding to the load at which the fracture initiates. A series of experiments are conducted at different temperatures ranging from room temperature to 800 °C, and the effect of temperature and loading rate on the fracture initiation toughness is investigated. The material fracture toughness is found to be sensitive to temperature and the fracture initiation toughness increases as the temperature increases. Furthermore, the fracture initiation toughness is strain rate sensitive and is higher for dynamic loading as compared to quasi-static loading

Effect of temperature on the dynamic constitutive behavior of TiB/Ti FGM

An experimental investigation is conducted to evaluate the thermo-mechanical constitutive behavior of a TiB/Ti functionally graded material (FGM) under dynamic loading. A Split Hopkinson Pressure Bar (SHPB) apparatus with infrared (IR) spot heaters is used to investigate the effect of temperature on the mechanical behavior of FGM materials. A series of experiments are conducted at different temperatures and the stress strain relation for different strain rates are obtained. The material showed high thermal softening at elevated temperature resulting in a large reduction in compressive strength and an increase in failure strain. © 2008 Society for Experimental Mechanics Inc