scholarly journals The causes of holes and loss of physical integrity in long‐lasting insecticidal nets

2021 ◽  
Vol 20 (1) ◽  
Author(s):  
Amy Wheldrake ◽  
Estelle Guillemois ◽  
Hamidreza Arouni ◽  
Vera Chetty ◽  
Stephen J. Russell
Keyword(s):  
Mechanical Damage ◽  
Damage Mechanism ◽  
Fracture Morphology ◽  
Physical Integrity ◽  
Rapid Accumulation ◽  
Underlying Causes ◽  
Textile Testing ◽  
Underlying Mechanisms ◽  
Long Lasting Insecticidal Nets ◽  
Cutting Burn

Abstract Background Long-lasting insecticidal nets (LLINs) are expected to last for at least 3 years, but whilst this may be achieved from an insecticidal perspective, physical protection is frequently compromised much earlier because of the rapid accumulation of holes during use. To understand why LLINs are so susceptible to loss of physical integrity, thousands of hole damage sites in LLINs retrieved from the field in Africa and Asia were forensically studied to identify the persistent underlying causes. Methods A total of 525 LLINs consisting of six different brands from five different countries across Africa and Asia were collected from the field after 1 to 3 years in use. More than 42,000 individual sites of hole damage were analysed based on the morphology and size of each individual hole, aided by optical microscopy (OM) and scanning electron microscopy (SEM). The fracture morphology enabled positive identification of the underlying mechanisms of the damage. Results Across all LLINs and geographical settings, mechanical damage is the primary cause of holes and loss of physical integrity in LLINs (63.14% by frequency and 81.52% by area). Snagging is the single most frequent mechanical damage mechanism, whilst the largest sized holes in LLINs result from seam failure and tearing. Abrasion and hole enlargement are also responsible for a progressive loss in the physical integrity of nets. Collectively, these five modes of mechanical damage can be expected to result from normal use of LLINs by households. Evidence of deliberate cutting, burn holes and rodent damage was observed to a lesser degree, which LLINs are not designed to withstand. Conclusions Loss of physical integrity in LLINs is an inevitable consequence of using a vector control product that has an inherently low resistance to mechanical damage during normal use. To improve performance, new specifications based on laboratory textile testing is needed, to assess the resistance of LLIN products to the primary causes of mechanical damage when in use, which are snagging, tearing, abrasion and hole enlargement. Seam construction also needs to meet a revised minimum standard to reduce the risk of a rapid loss of physical integrity during use.

scholarly journals Development of a single resistance to damage metric for mosquito nets related to physical integrity in the field

2021 ◽  
Vol 20 (1) ◽  
Author(s):  
Amy Wheldrake ◽  
Estelle Guillemois ◽  
Vera Chetty ◽  
Albert Kilian ◽  
Stephen J. Russell
Keyword(s):  
Mechanical Damage ◽  
End User ◽  
Physical Integrity ◽  
Good Physical Condition ◽  
Mosquito Nets ◽  
Performance Metric ◽  
Physical Deterioration ◽  
Testing Data ◽  
Key Factor ◽  
Textile Testing

Abstract Background In common with the majority of personal protective equipment and healthcare products, the ability for long-lasting insecticidal nets (LLINs) to remain in good physical condition during use is a key factor governing fitness for purpose and serviceability. The inherent ability of a product to resist physical deterioration should be known in advance of it being used to ensure it has maximum value to both the end-user and procurer. The objective of this study was to develop a single performance metric of resistance to damage (RD) that can be applied to any LLIN product prior to distribution. Methods Algorithms to calculate RD values were developed based on consideration of both human factors and laboratory testing data. Quantitative reference forces applied to LLINs by users during normal use were determined so that aspirational performance levels could be established. The ability of LLINs to resist mechanical damage was assessed based on a new suite of textile tests, reflecting actual mechanisms of physical deterioration during normal household use. These tests quantified the snag strength, bursting strength, abrasion resistance and resistance to hole enlargement. Sixteen different unused LLINs were included in the analysis. The calculated RD values for all LLINs and the corresponding physical integrity data for the same nets retrieved from the field (up to 3 years of use) were then compared. Results On a RD scale of 0 (lowest resistance) – 100 (highest resistance), only six of the sixteen LLINs achieved an RD value above 50. No current LLIN achieved the aspirational level of resistance to damage (RD = 100), suggesting that product innovation is urgently required to increase the RD of LLINs. LLINs with higher RD values were associated with lower hole damage (PHI) in the field when adjusted for normal use conditions. Conclusions The RD value of any LLIN product can be determined prior to distribution based on the developed algorithms and laboratory textile testing data. Generally, LLINs need to achieve higher RD values to improve their ability to resist hole formation during normal use. Innovation in LLIN product design focused on the textile material should be actively encouraged and is urgently needed to close the performance gap.

A Rational Methodology for Determination of Service Life for a Delayed Coker Drum

2015 ◽  
Author(s):  
John J. Aumuller ◽  
Jie Chen ◽  
Vincent A. Carucci
Keyword(s):  
Service Life ◽  
Low Cycle Fatigue ◽  
Damage Model ◽  
Mechanical Damage ◽  
Damage Mechanism ◽  
Modern Trend ◽  
Cold Spot ◽  
Chromium Alloy ◽  
Cycle Fatigue ◽  
Service Environment

Delayed unit coker drums operate in a severe service environment that precludes long term reliability due to excessive shell bulging and cracking of shell joint and shell to skirt welds. Thermal fatigue is recognized as the leading damage mechanism and past work has provided an idealized description of the thermo-mechanical mechanism via local hot and cold spot formation to quantify a lower bound life estimate for shell weld failure. The present work extends this idealized thermo-mechanical damage model by evaluating actual field data to determine a potential upper bound life estimate. This assessment also provides insight into practical techniques for equipment operators to identify design and operational opportunities to extend the service life of coke drums for their specific service environments. A modern trend of specifying higher chromium and molybdenum alloy content for drum shell material in order to improve low cycle fatigue strength is seen to be problematic; rather, the use of lower alloy materials that are generally described as fatigue tough materials are better suited for the high strain-low cycle fatigue service environment of coke drums. Materials such as SA 204 C (C – ½ Mo) and SA 302 B (C – Mn – ½ Mo) or SA 302 C (C – Mn – ½ Mo – ½ Ni) are shown to be better candidates for construction in lieu of low chromium alloy steel materials such as SA 387 grades P11 (1¼ Cr – ½ Mo), P12 (1 Cr – ½ Mo), P22 (2¼ Cr – 1 Mo) and P21 (3 Cr – 1 Mo).

Near-Wellbore Hydraulic Fracture Non-Planar Propagation and Torturous Morphology in Tight Sandstone Formation

2021 ◽  
Author(s):  
Ruxin Zhang ◽  
Qinglin Shan ◽  
Wan Cheng
Keyword(s):  
Stress Distribution ◽  
Fracture Initiation ◽  
Damage Mechanism ◽  
Fracture Morphology ◽  
Propagation Model ◽  
Hydraulic Fractures ◽  
Tight Sandstone ◽  
Propagation Behavior ◽  
Stress Shadow ◽  
Sandstone Formation

Abstract In this paper, a 3D near-wellbore fracture propagation model is established, integrating five parts: formation stress balance, drilling, casing and cementing, perforating, and fracturing, in order to investigate fracture initiation characteristics, near-wellbore fracture non-planar propagation behavior, and torturous hydraulic fracture morphology for cased and perforated horizontal wellbores in tight sandstone formation. The method is based on the combination of finite element method and post-failure damage mechanism. Finite element method is used to determine the coupling behavior between the pore fluid seepage and rock stress distribution. Post-failure damage mechanism is adopted to test the evolution of hydraulic fractures through simulating rock damage process. Moreover, a user subroutine is introduced to establish the relation between rock strength, permeability, and damage, in order to solve the model. This model could simulate the interaction between fractures during their propagation process because of the stress shadow. The simulation results indicate that each operation could cause redistribution and reorientation of near-wellbore stress. Therefore, it is important to know the real near-wellbore stress distribution that affects near-wellbore fracture initiation and propagation. Initially, hydraulic fractures initiate independently from each perforation and propagate along the direction of maximum horizontal stress. However, hydraulic fractures divert from original direction gradually to interconnect and overlap with each other, because of stress shadow, resulting in non-planar propagation behavior. Individual fractures coalesce into a spiral-shaped fracture morphology. In addition, a longitudinal fracture could be observed because of wellbore effect, which is a result of weak cementing strength or near-wellbore weak plane. Finally, the complex and torturous fracture morphologies are created near the wellbore, incorporating Multi-spiral shaped fracture and horizontal-vertical crossing shaped fracture. However, the propagation behavior of fracture far away from wellbore is controlled by in-situ stress, forming a planar fracture. The highlights of this 3D near-wellbore fracture propagation model are following: 1) it considers near-wellbore stress change caused by each construction to ensure the accuracy of near-wellbore stress distribution; 2) it achieves 3D simulation of fracture initiation and near-wellbore propagation from perforations; 3) the interaction between fractures is involved, resulting in complex and torturous morphology. This model provides the theoretical basis for fracture initiation and propagation, which also could be applied into heterogenous formations considering the effect of discontinuities.

Unique microstructure of kapok fibers in longitudinal microscopic images

2016 ◽  
Vol 87 (18) ◽  
pp. 2255-2262 ◽  
Author(s):  
Lixia Hu ◽  
Fumei Wang ◽  
Guangbiao Xu ◽  
Bugao Xu
Keyword(s):  
Cell Wall ◽  
Residual Deformation ◽  
Damage Mechanism ◽  
Fracture Morphology ◽  
Kapok Fiber ◽  
Microscopic Images ◽  
Deformation And Fracture ◽  
Assembly Structure ◽  
Before And After ◽  
Kapok Fibers

This paper is focused on investigating the microstructure of kapok fiber through the observation of residual deformation and fracture morphology of kapok fibers in longitudinal microscopic images. By comparing the morphology of kapok fibers before and after different treatments, unique weak spiral lines inside the cell wall were found. The spiral lines that divide the cell wall into a wound ribbon appear to be the weak locations (e.g., the fracture or bending points) of the fiber. The damage mechanism of kapok fibers in assemblies was summarized. Details of structures inside the ribbon were also examined to reveal that macro-fibrils of 0.2 µm in diameter and >1.0 µm in length were packed neatly along the fiber axial on the surface. A framework of the multi-assembly structure of kapok fiber was summarized.

A mathematical model for the freezing process in biological tissue

1988 ◽  
Vol 234 (1276) ◽  
pp. 343-358 ◽  
Keyword(s):  
Experimental Data ◽  
Mathematical Model ◽  
Blood Vessel ◽  
Vascular System ◽  
Mechanical Damage ◽  
Thermal Conditions ◽  
Damage Mechanism ◽  
Freezing Process ◽  
Unit Structure ◽  
Krogh Cylinder

A mathematical model has been developed to study the process of freezing in biological organs. The model consists of a repetitive unit structure comprising a cylinder of tissue with an axial blood vessel (Krogh cylinder) and it is analysed by the methods of irreversible thermo­dynamics. The mathematical simulation of the freezing process in liver tissue compares remarkably well with experimental data on the structure of tissue frozen under controlled thermal conditions and the response of liver cells to changes in cooling rate. The study also supports the proposal that the damage mechanism responsible for the lack of success in attempts to preserve tissue in a frozen state, under conditions in which cells in suspension survive freezing, is direct mechanical damage caused by the formation of ice in the vascular system.

scholarly journals Creep Modelling of Rootstock during Holding in Watermelon Grafting

Agriculture ◽  
2021 ◽  
Vol 11 (12) ◽  
pp. 1266
Author(s):  
Kang Wu ◽  
Jianzhong Lou ◽  
Chen Li ◽  
Wei Luo ◽  
Congcong Li ◽  
...  
Keyword(s):  
Creep Deformation ◽  
Viscoelastic Model ◽  
Mechanical Damage ◽  
Viscoelastic Behavior ◽  
Damage Mechanism ◽  
Deformation Characteristics ◽  
Mechanical Compression ◽  
Creep Tests ◽  
Viscoelastic Parameters ◽  
Test Velocity

The fragile structure of a rootstock predisposes the stem to mechanical damage during grafting. Thus, it is necessary to take into account the rootstock’s rheological properties under mechanical compression when designing a clamping mechanism. This study focused on cucurbit, a typical rootstock for watermelon grafting. Firstly, we adopted a four-element Burgers model to analyze viscoelastic behavior and deformation characteristics of the rootstock, then conducted creep tests to obtain the parameters of the viscoelastic model. Next, we developed a model for the rootstock during holding based on viscoelastic parameters, loading force and contact time. Moreover, we evaluated the effect of various loading forces and test velocities on creep deformation to reveal the least damage on the rootstock. Results showed that the influence of loading force on the creep deformation was greater than test velocity. Finally, the holding test indicated that the clamping mechanism with silicone rubber can effectively prevent the damage to the stem. Specifically, the loading force should be controlled below 4 N to reduce the associated damage. Taken together, our findings provide a theoretical basis for analyzing the holding damage mechanism during watermelon grafting.

scholarly journals Understanding urbanicity: how interdisciplinary methods help to unravel the effects of the city on mental health

2020 ◽  
pp. 1-12 ◽  
Author(s):  
Lydia Krabbendam ◽  
Mark van Vugt ◽  
Philippe Conus ◽  
Ola Söderström ◽  
Lilith Abrahamyan Empson ◽  
...  
Keyword(s):  
Mental Health ◽  
Rural Areas ◽  
Urban Areas ◽  
Mental Health Problems ◽  
Epidemiological Studies ◽  
Population Based ◽  
Urban Environments ◽  
Country Level ◽  
Underlying Causes ◽  
Underlying Mechanisms

Abstract Twenty-first century urbanization poses increasing challenges for mental health. Epidemiological studies have shown that mental health problems often accumulate in urban areas, compared to rural areas, and suggested possible underlying causes associated with the social and physical urban environments. Emerging work indicates complex urban effects that depend on many individual and contextual factors at the neighbourhood and country level and novel experimental work is starting to dissect potential underlying mechanisms. This review summarizes findings from epidemiology and population-based studies, neuroscience, experimental and experience-based research and illustrates how a combined approach can move the field towards an increased understanding of the urbanicity-mental health nexus.

Mechanical damage mechanism of frozen coal subjected to liquid nitrogen freezing

Fuel ◽  
2022 ◽  
Vol 309 ◽  
pp. 122124
Author(s):  
Lei Qin ◽  
Chao Ma ◽  
Shugang Li ◽  
Haifei Lin ◽  
Ping Wang ◽  
...  
Keyword(s):  
Liquid Nitrogen ◽  
Mechanical Damage ◽  
Damage Mechanism
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