BACKGROUND Journal Impact Factor™(JIF)is often used to evaluate the relative reputation and quality of academic journals in their respective fields,and can greatly influence the quality and scope of subsequent manuscr...BACKGROUND Journal Impact Factor™(JIF)is often used to evaluate the relative reputation and quality of academic journals in their respective fields,and can greatly influence the quality and scope of subsequent manuscript submissions.Therefore,many if not all academic journals are interested in increasing their JIF,to improve their academic impact.AIM To determine the importance of the integrity of the editorial and publication process in improving the academic influence of academic journals and the JIF of academic journals.METHODS In this paper,we describe our statistical analysis of bibliometric factors-including the 2021 JIFs released in the Journal Citation Report™2022,discipline rankings,received and published articles in 2019-2021,and webpage visits and downloads-for seven journals published by Baishideng Publishing Group(Baishideng)and indexed in Science Citation Index Expanded™;ultimately,we introduce and discuss the editing and publishing processes of Baishideng’s journals in their entirety,as they form the basis for our objective of safeguarding and bolstering integrity in academic publication.RESULTS For the seven journals assessed,their 2021 JIFs were basically unchanged from 2020,with the current metric ranging from 5.374 for World Journal of Gastroenterology(WJG)to 1.534 for World Journal of Clinical Cases(WJCC).Further assessments of the journals’bibliometrics from 2019 to 2020,showed that World Journal of Stem Cells has the highest self-citation rate(1.43%)and World Journal of Gastrointestinal Surgery has the lowest(0.21%).Additionally,the total 3012 articles published during this period were cited by more than 20000 articles in approximately 8000 academic journals.Of note,the 1102 articles published in WJG were cited by articles in 3059 journals,among which 171 journals have a JIF of>10,including internationally renowned academic journals such as CA-A Cancer Journal for Clinicians(2021 JIF 286.130,record count:1),Lancet(2021 JIF 202.731,record count:4),Nature Reviews Immunology(2021 JIF 108.555,record count:2),Nature Reviews Gastroenterology&Hepatology(2021 JIF 73.082,record count:9),Lancet Gastroenterology&Hepatology(2021 JIF 45.042,record count:8),Gastroenterology(2021 JIF 33.883,record count:19),and Gut(2021 JIF 31.793,record count:21).This suggests that Baishideng’s journals have been widely recognized for their academic quality.In the Reference Citation Analysis(RCA)database,all seven Baishidengpublished journals obtained a 2022 Journal Article Influence Index(JAII).For example,WJG has a 2022 JAII of 22.048,ranking 18th out of 102 journals in the field of gastroenterology&hepatology in the RCA,with 469909 total citations(6/102)and 21313 total articles(5/102).The numbers of manuscripts received and published in 2021 were both higher than those in 2019-2020.For example,WJCC received a total of 3650 manuscripts in 2021,which is 91.1%higher than those in 2019-2020(average:1910 papers/year).In 2021,WJCC published 1296 articles,representing an increase of 105.1%compared to those in 2019-2020(average:632 articles/year).The numbers of webpage visits and downloads received by the seven journals have increased year by year.For example,the number of total visits received by WJG in 2019-2021 was 1974052 in 2019,2317835 in 2020(increased by 17.4%compared with that in 2019),and 2652555 in 2021(increased by 4.4%compared with that in 2020).The visitors were from more than 220 countries and regions worldwide,such as the United States,China,and the United Kingdom.Open access(OA)plays a vital role in improving the quality,efficiency,transparency,and integrity of academic journal publishing.From 2019 to 2021,a total of 5543 OA articles were published in the seven journals,of which 2083(37.6%)were invited and published free-of-charge.During the same period,1683 articles were published in WJG,and the authors were from more than 70 countries and regions.For the total 5543 articles published in the seven journals from 2019 to 2021,3903 article quality tracking reports were received after the online publication of these articles.The quality of the articles was further evaluated through the Baishideng’s article quality and author evaluation tracking system,with 4655 articles(84.0%)having received author evaluation and feedback,which contributes to tracking metrics for authors’satisfaction with the collective publication processes.From March 25,2021 to June 28,2022,the seven journals received a total of 424 reader evaluations and 229 letters from readers;this subsequent reader engagement demonstrates that the popularity of the published articles and the volume of their readership audience were improved through the reader evaluation system.CONCLUSION Ultimately,the findings from our bibliometric assessments indicate that establishing,promoting and actively practicing processes that safeguard and bolster the integrity of the editing and publication process also help to improve the academic influence of academic journals,which itself is the cornerstone for improving JIF.展开更多
Aircraft laminated composite components often suffer a variety of high velocity impacts with large quantity of energy,which usually affects aircraft behavior and would incur component damages,even disastrous consequen...Aircraft laminated composite components often suffer a variety of high velocity impacts with large quantity of energy,which usually affects aircraft behavior and would incur component damages,even disastrous consequences.Therefore,one investigates the impact resistance of a new type of composite material,Ti/CFRP/Ti sandwich structure,and launches impact tests by using an air gun test system.Then one acquires the critical breakthrough rate of the structure and analyzes the damages.The results show that the main failure mode of the front titanium sheet is shear plugging and brittle fracture of adhesive layer with fiber breakage,while the back titanium sheet is severely ripped.The rear damage is worse than the front one.Compared with traditional CFRP laminates,the critical breakthrough rate of Ti/CFRP/Ti sandwich structure is improved by 69.9% when suffered the impact of a bearing ball with 2mm radius.展开更多
A hybrid energy transmission pipeline is proposed with the aim of long-distance cooperative transmission of electricity and chemical fuels, which is composed of an inner high-temperature superconducting (HTS) power ca...A hybrid energy transmission pipeline is proposed with the aim of long-distance cooperative transmission of electricity and chemical fuels, which is composed of an inner high-temperature superconducting (HTS) power cable and outer liquefied natural gas (LNG) pipeline. The flowing LNG could maintain the operating temperature of the inner HTS power cable within the range of 85 K-90 K, thus the Bi-2223 superconductors in the HTS power cable produce little Joule loss with the transmission current below the critical current. Owing to the advantages of high power density, low transmission losses and economical manufacturing costs, the hybrid energy transmission pipeline is expected to be widely utilized in the near future. In order to ensure the safety of the HTS power cable and explosive LNG in case of short-circuit faults, this paper tests and analyzes the characteristics of Bi-2223 HTS tapes of the Type HT-CA, Type HT-SS and Type H models under short-circuit current impacts at the LNG cooling temperature (85 K-90 K). An experimental platform is designed and established for the ampacity tests of HTS tapes above LN2 cooling temperature (77 K). The AC over-current impact tests at 85 K-90 K are carried out on each sample of Bi-2223 tapes respectively, and the experimental results are analyzed and compared to evaluate their performances under different operating conditions. The results indicate that the Type HT-CA tape can withstand 50 Hz short-circuit current impact with the amplitude of 1108 A (10 times of critical current Ic ) for 100 ms at 90 K, and its resistance is the smallest of the three tested samples under similar current impacts. Therefore, the Type HT-CA Bi-2223 tape is the optimal superconductor of the HTS power cable in the hybrid energy transmission pipeline.展开更多
In 2023,Baishideng Publishing Group(Baishideng)routinely published 47 openaccess journals,including 46 English-language journals and 1 Chinese-language journal.Our successes were accomplished through the collective de...In 2023,Baishideng Publishing Group(Baishideng)routinely published 47 openaccess journals,including 46 English-language journals and 1 Chinese-language journal.Our successes were accomplished through the collective dedicated efforts of Baishideng staffs,Editorial Board Members,and Peer Reviewers.Among these 47 Baishideng journals,7 are included in the Science Citation Index Expanded(SCIE)and 6 in the Emerging Sources Citation Index(ESCI).With the support of Baishideng authors,company staffs,Editorial Board Members,and Peer Reviewers,the publication work of 2023 is about to be successfully completed.This editorial summarizes the 2023 activities and accomplishments of the 13 SCIEand ESCI-indexed Baishideng journals,outlines the Baishideng publishing policy changes and additions made this year,and highlights the unique advantages of Baishideng journals.展开更多
以“中国科技期刊卓越行动计划”(以下简称“卓越计划”)为背景,重点研究了入选的22种领军期刊及29种重点期刊在发展现状、办刊模式、发展路径、实践成效4个方面的现状与所取得的成效。结果表明,入选期刊均秉承了良好的开放科学理念,从...以“中国科技期刊卓越行动计划”(以下简称“卓越计划”)为背景,重点研究了入选的22种领军期刊及29种重点期刊在发展现状、办刊模式、发展路径、实践成效4个方面的现状与所取得的成效。结果表明,入选期刊均秉承了良好的开放科学理念,从出版模式的开放获取程度到出版形式的多样性角度考量,都与期刊自身的发展起到了积极的正相关作用。通过Web of Science数据库,获得了2019年和2020年“卓越计划”资助前后期刊影响因子的变化趋势。22种领军期刊的影响因子全部实现了不同程度的提高,提升幅度从3.486%到108.205%不等;29种重点期刊中有26种期刊实现了提升,但也有3种期刊出现了微小的下跌趋势。26种期刊的影响因子的提升幅度从5.663%到78.414%。2022年,影响因子高于5的期刊中,领军期刊占比91%,重点期刊占比66%,且领军期刊中有5种期刊的影响因子高于20,占比23%(集中在细胞与免疫学方面),体现了领军期刊在国际影响力方面的突出成就。展开更多
The stress wave propagation law and dynamic buckling critical velocity are formulated and solved by considering a general axial connecting boundary for a slender elastic straight rod impacted by a rigid body. The infl...The stress wave propagation law and dynamic buckling critical velocity are formulated and solved by considering a general axial connecting boundary for a slender elastic straight rod impacted by a rigid body. The influence of connecting stiffness on the critical velocity is investigated with varied impactor mass and buckling time. The influences of rod length and rod mass on the critical velocity are also discussed. It is found that greater connecting stiffness leads to larger stress amplitude, and further results in lower critical velocity. It is particularly noteworthy that when the connecting stiffness is less than a certain value, dynamic buckling only occurs before stress wave reflects off the connecting end. It is also shown that longer rod with larger slenderness ratio is easier to buckle, and the critical velocity for a larger-mass rod is higher than that for a lighter rod with the same geometry.展开更多
The rebound behaviors of droplets impacting on a self-fabricated superhydrophobic brass surface (WCA=164.5°) were observed and studied by using high-speed-camera. In accordance with energy conversion, theoretical...The rebound behaviors of droplets impacting on a self-fabricated superhydrophobic brass surface (WCA=164.5°) were observed and studied by using high-speed-camera. In accordance with energy conversion, theoretical analysis of different behaviors and rebound mechanism were given. At lower velocities, three behaviors in different velocity ranges were observed: partial rebounding, entire rebounding and ejecting during rebounding. At higher velocities, such two behaviors as rebound after splashing and rebound, ejecting after splashing, occurred alternately and exhibited certain periodicity. A function to predict the critical impact velocity is derived from energy conservation condition, and the prediction values tally with the experimental values, with the maximum relative error about 14%.展开更多
Total protection of fragile goods and equipment during transport has become critically important as fast delivery systems are growing rapidly due to the rise of mass online commerce worldwide.This paper therefore stud...Total protection of fragile goods and equipment during transport has become critically important as fast delivery systems are growing rapidly due to the rise of mass online commerce worldwide.This paper therefore studies the protection of falling eggs—one of the most fragile goods—through packaging.Intensive experiments are first conducted to obtain the critical stress at which the eggshell is damaged by falling impact.Tests are first conducted of unpackaged eggs dropping onto a granite platform from various heights.Finite element method(FEM)models of unpackaged eggs are then built to simulate the behaviour of eggs and compute their stress levels.The experimental data is compared with the results of FEM simulations to determine the critical stress at which the eggshell is damaged.A series of tests and numerical simulations are then carried out for eggs wrapped in expanded polyethylene(EPE)foam.The stress distribution of both a single egg and multiple eggs in packaging are studied.An empirical formula between the drop speed(or height)and the bottom thickness of the EPE foam packaging is obtained,which can be useful in the design of packaging for the total protection of eggs that may fall during usual transport and delivery operations.The experimental and numerical results from this study show that it is viable to protect fragile goods,as long as the EPE form packaging has sufficient thickness.The proposed study and design procedure are helpful for the selection of key parameters of foam packaging to protect fragile industry products from impact.展开更多
Granular matter possesses impact-absorbing property due to its energy dissipation character.To investigate the impact-absorbing capacity of granular matter,the discrete element method(DEM)is adopted to simulate the im...Granular matter possesses impact-absorbing property due to its energy dissipation character.To investigate the impact-absorbing capacity of granular matter,the discrete element method(DEM)is adopted to simulate the impact of a spherical projectile on to a granular bed.The dynamic responses of the projectile are obtained for both thin and thick granular bed.The penetration depth of the projectile and the first impact peak are investigated with different bed thicknesses and impact velocities.Determining a suitable bed thickness is crucial to the buffering effect of granular matter.The first impact peak is independent of bed thickness when the thickness is larger than the critical thickness.展开更多
文摘BACKGROUND Journal Impact Factor™(JIF)is often used to evaluate the relative reputation and quality of academic journals in their respective fields,and can greatly influence the quality and scope of subsequent manuscript submissions.Therefore,many if not all academic journals are interested in increasing their JIF,to improve their academic impact.AIM To determine the importance of the integrity of the editorial and publication process in improving the academic influence of academic journals and the JIF of academic journals.METHODS In this paper,we describe our statistical analysis of bibliometric factors-including the 2021 JIFs released in the Journal Citation Report™2022,discipline rankings,received and published articles in 2019-2021,and webpage visits and downloads-for seven journals published by Baishideng Publishing Group(Baishideng)and indexed in Science Citation Index Expanded™;ultimately,we introduce and discuss the editing and publishing processes of Baishideng’s journals in their entirety,as they form the basis for our objective of safeguarding and bolstering integrity in academic publication.RESULTS For the seven journals assessed,their 2021 JIFs were basically unchanged from 2020,with the current metric ranging from 5.374 for World Journal of Gastroenterology(WJG)to 1.534 for World Journal of Clinical Cases(WJCC).Further assessments of the journals’bibliometrics from 2019 to 2020,showed that World Journal of Stem Cells has the highest self-citation rate(1.43%)and World Journal of Gastrointestinal Surgery has the lowest(0.21%).Additionally,the total 3012 articles published during this period were cited by more than 20000 articles in approximately 8000 academic journals.Of note,the 1102 articles published in WJG were cited by articles in 3059 journals,among which 171 journals have a JIF of>10,including internationally renowned academic journals such as CA-A Cancer Journal for Clinicians(2021 JIF 286.130,record count:1),Lancet(2021 JIF 202.731,record count:4),Nature Reviews Immunology(2021 JIF 108.555,record count:2),Nature Reviews Gastroenterology&Hepatology(2021 JIF 73.082,record count:9),Lancet Gastroenterology&Hepatology(2021 JIF 45.042,record count:8),Gastroenterology(2021 JIF 33.883,record count:19),and Gut(2021 JIF 31.793,record count:21).This suggests that Baishideng’s journals have been widely recognized for their academic quality.In the Reference Citation Analysis(RCA)database,all seven Baishidengpublished journals obtained a 2022 Journal Article Influence Index(JAII).For example,WJG has a 2022 JAII of 22.048,ranking 18th out of 102 journals in the field of gastroenterology&hepatology in the RCA,with 469909 total citations(6/102)and 21313 total articles(5/102).The numbers of manuscripts received and published in 2021 were both higher than those in 2019-2020.For example,WJCC received a total of 3650 manuscripts in 2021,which is 91.1%higher than those in 2019-2020(average:1910 papers/year).In 2021,WJCC published 1296 articles,representing an increase of 105.1%compared to those in 2019-2020(average:632 articles/year).The numbers of webpage visits and downloads received by the seven journals have increased year by year.For example,the number of total visits received by WJG in 2019-2021 was 1974052 in 2019,2317835 in 2020(increased by 17.4%compared with that in 2019),and 2652555 in 2021(increased by 4.4%compared with that in 2020).The visitors were from more than 220 countries and regions worldwide,such as the United States,China,and the United Kingdom.Open access(OA)plays a vital role in improving the quality,efficiency,transparency,and integrity of academic journal publishing.From 2019 to 2021,a total of 5543 OA articles were published in the seven journals,of which 2083(37.6%)were invited and published free-of-charge.During the same period,1683 articles were published in WJG,and the authors were from more than 70 countries and regions.For the total 5543 articles published in the seven journals from 2019 to 2021,3903 article quality tracking reports were received after the online publication of these articles.The quality of the articles was further evaluated through the Baishideng’s article quality and author evaluation tracking system,with 4655 articles(84.0%)having received author evaluation and feedback,which contributes to tracking metrics for authors’satisfaction with the collective publication processes.From March 25,2021 to June 28,2022,the seven journals received a total of 424 reader evaluations and 229 letters from readers;this subsequent reader engagement demonstrates that the popularity of the published articles and the volume of their readership audience were improved through the reader evaluation system.CONCLUSION Ultimately,the findings from our bibliometric assessments indicate that establishing,promoting and actively practicing processes that safeguard and bolster the integrity of the editing and publication process also help to improve the academic influence of academic journals,which itself is the cornerstone for improving JIF.
基金funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions
文摘Aircraft laminated composite components often suffer a variety of high velocity impacts with large quantity of energy,which usually affects aircraft behavior and would incur component damages,even disastrous consequences.Therefore,one investigates the impact resistance of a new type of composite material,Ti/CFRP/Ti sandwich structure,and launches impact tests by using an air gun test system.Then one acquires the critical breakthrough rate of the structure and analyzes the damages.The results show that the main failure mode of the front titanium sheet is shear plugging and brittle fracture of adhesive layer with fiber breakage,while the back titanium sheet is severely ripped.The rear damage is worse than the front one.Compared with traditional CFRP laminates,the critical breakthrough rate of Ti/CFRP/Ti sandwich structure is improved by 69.9% when suffered the impact of a bearing ball with 2mm radius.
基金supported by National Key R&D Project under Grant(2018YFB0904400).
文摘A hybrid energy transmission pipeline is proposed with the aim of long-distance cooperative transmission of electricity and chemical fuels, which is composed of an inner high-temperature superconducting (HTS) power cable and outer liquefied natural gas (LNG) pipeline. The flowing LNG could maintain the operating temperature of the inner HTS power cable within the range of 85 K-90 K, thus the Bi-2223 superconductors in the HTS power cable produce little Joule loss with the transmission current below the critical current. Owing to the advantages of high power density, low transmission losses and economical manufacturing costs, the hybrid energy transmission pipeline is expected to be widely utilized in the near future. In order to ensure the safety of the HTS power cable and explosive LNG in case of short-circuit faults, this paper tests and analyzes the characteristics of Bi-2223 HTS tapes of the Type HT-CA, Type HT-SS and Type H models under short-circuit current impacts at the LNG cooling temperature (85 K-90 K). An experimental platform is designed and established for the ampacity tests of HTS tapes above LN2 cooling temperature (77 K). The AC over-current impact tests at 85 K-90 K are carried out on each sample of Bi-2223 tapes respectively, and the experimental results are analyzed and compared to evaluate their performances under different operating conditions. The results indicate that the Type HT-CA tape can withstand 50 Hz short-circuit current impact with the amplitude of 1108 A (10 times of critical current Ic ) for 100 ms at 90 K, and its resistance is the smallest of the three tested samples under similar current impacts. Therefore, the Type HT-CA Bi-2223 tape is the optimal superconductor of the HTS power cable in the hybrid energy transmission pipeline.
文摘In 2023,Baishideng Publishing Group(Baishideng)routinely published 47 openaccess journals,including 46 English-language journals and 1 Chinese-language journal.Our successes were accomplished through the collective dedicated efforts of Baishideng staffs,Editorial Board Members,and Peer Reviewers.Among these 47 Baishideng journals,7 are included in the Science Citation Index Expanded(SCIE)and 6 in the Emerging Sources Citation Index(ESCI).With the support of Baishideng authors,company staffs,Editorial Board Members,and Peer Reviewers,the publication work of 2023 is about to be successfully completed.This editorial summarizes the 2023 activities and accomplishments of the 13 SCIEand ESCI-indexed Baishideng journals,outlines the Baishideng publishing policy changes and additions made this year,and highlights the unique advantages of Baishideng journals.
文摘以“中国科技期刊卓越行动计划”(以下简称“卓越计划”)为背景,重点研究了入选的22种领军期刊及29种重点期刊在发展现状、办刊模式、发展路径、实践成效4个方面的现状与所取得的成效。结果表明,入选期刊均秉承了良好的开放科学理念,从出版模式的开放获取程度到出版形式的多样性角度考量,都与期刊自身的发展起到了积极的正相关作用。通过Web of Science数据库,获得了2019年和2020年“卓越计划”资助前后期刊影响因子的变化趋势。22种领军期刊的影响因子全部实现了不同程度的提高,提升幅度从3.486%到108.205%不等;29种重点期刊中有26种期刊实现了提升,但也有3种期刊出现了微小的下跌趋势。26种期刊的影响因子的提升幅度从5.663%到78.414%。2022年,影响因子高于5的期刊中,领军期刊占比91%,重点期刊占比66%,且领军期刊中有5种期刊的影响因子高于20,占比23%(集中在细胞与免疫学方面),体现了领军期刊在国际影响力方面的突出成就。
文摘The stress wave propagation law and dynamic buckling critical velocity are formulated and solved by considering a general axial connecting boundary for a slender elastic straight rod impacted by a rigid body. The influence of connecting stiffness on the critical velocity is investigated with varied impactor mass and buckling time. The influences of rod length and rod mass on the critical velocity are also discussed. It is found that greater connecting stiffness leads to larger stress amplitude, and further results in lower critical velocity. It is particularly noteworthy that when the connecting stiffness is less than a certain value, dynamic buckling only occurs before stress wave reflects off the connecting end. It is also shown that longer rod with larger slenderness ratio is easier to buckle, and the critical velocity for a larger-mass rod is higher than that for a lighter rod with the same geometry.
基金supported by The National Natural Science Foundation of China (Grant No.51109178)Science and Technology Innovation Foundation of NWPU (Grant No.JC20120218)
文摘The rebound behaviors of droplets impacting on a self-fabricated superhydrophobic brass surface (WCA=164.5°) were observed and studied by using high-speed-camera. In accordance with energy conversion, theoretical analysis of different behaviors and rebound mechanism were given. At lower velocities, three behaviors in different velocity ranges were observed: partial rebounding, entire rebounding and ejecting during rebounding. At higher velocities, such two behaviors as rebound after splashing and rebound, ejecting after splashing, occurred alternately and exhibited certain periodicity. A function to predict the critical impact velocity is derived from energy conservation condition, and the prediction values tally with the experimental values, with the maximum relative error about 14%.
基金supported by the National Natural Science Foundation of China(Grant No.51805141)the State Key Laboratory of Advanced Design andManufacturing for Vehicle Body(Grant No.31715008)the Hebei Natural Science Foundation(Grant No.E2018202243 and E2019202201).
文摘Total protection of fragile goods and equipment during transport has become critically important as fast delivery systems are growing rapidly due to the rise of mass online commerce worldwide.This paper therefore studies the protection of falling eggs—one of the most fragile goods—through packaging.Intensive experiments are first conducted to obtain the critical stress at which the eggshell is damaged by falling impact.Tests are first conducted of unpackaged eggs dropping onto a granite platform from various heights.Finite element method(FEM)models of unpackaged eggs are then built to simulate the behaviour of eggs and compute their stress levels.The experimental data is compared with the results of FEM simulations to determine the critical stress at which the eggshell is damaged.A series of tests and numerical simulations are then carried out for eggs wrapped in expanded polyethylene(EPE)foam.The stress distribution of both a single egg and multiple eggs in packaging are studied.An empirical formula between the drop speed(or height)and the bottom thickness of the EPE foam packaging is obtained,which can be useful in the design of packaging for the total protection of eggs that may fall during usual transport and delivery operations.The experimental and numerical results from this study show that it is viable to protect fragile goods,as long as the EPE form packaging has sufficient thickness.The proposed study and design procedure are helpful for the selection of key parameters of foam packaging to protect fragile industry products from impact.
基金This study is financially supported by the National Basic Research Program of China(Grant No.2010CB731502)the National Natural Science Foundation of China(Grant Nos.U1234209 and 41176012).
文摘Granular matter possesses impact-absorbing property due to its energy dissipation character.To investigate the impact-absorbing capacity of granular matter,the discrete element method(DEM)is adopted to simulate the impact of a spherical projectile on to a granular bed.The dynamic responses of the projectile are obtained for both thin and thick granular bed.The penetration depth of the projectile and the first impact peak are investigated with different bed thicknesses and impact velocities.Determining a suitable bed thickness is crucial to the buffering effect of granular matter.The first impact peak is independent of bed thickness when the thickness is larger than the critical thickness.