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Kouichi Yasuda, Toshiyuki Kawano, [Masanori Kikuchi](https://orcid.org/0000-0002-9451-8147), Mamoru Aizawa, Kanji Tsuru, Sadami Tsutsumi

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Validity check of easy-to-use torsion test method for bioceramicsFull Terms & Conditions of access and use can be found athttp://www.tandfonline.com/action/journalInformation?journalCode=tace20Journal of Asian Ceramic SocietiesISSN: (Print) 2187-0764 (Online) Journal homepage: http://www.tandfonline.com/loi/tace20Validity check of easy-to-use torsion test methodfor bioceramicsKouichi Yasuda, Toshiyuki Kawano, Masanori Kikuchi, Mamoru Aizawa, KanjiTsuru & Sadami TsutsumiTo cite this article: Kouichi Yasuda, Toshiyuki Kawano, Masanori Kikuchi, Mamoru Aizawa, KanjiTsuru & Sadami Tsutsumi (2018) Validity check of easy-to-use torsion test method for bioceramics,Journal of Asian Ceramic Societies, 6:1, 43-53, DOI: 10.1080/21870764.2018.1439613To link to this article:  https://doi.org/10.1080/21870764.2018.1439613© 2018 The Author(s). Published by InformaUK Limited, trading as Taylor & FrancisGroup on behalf of The Korean CeramicSociety and The Ceramic Society of JapanPublished online: 26 Feb 2018.Submit your article to this journal Article views: 78View related articles View Crossmark datahttp://www.tandfonline.com/action/journalInformation?journalCode=tace20http://www.tandfonline.com/loi/tace20http://www.tandfonline.com/action/showCitFormats?doi=10.1080/21870764.2018.1439613https://doi.org/10.1080/21870764.2018.1439613http://www.tandfonline.com/action/authorSubmission?journalCode=tace20&show=instructionshttp://www.tandfonline.com/action/authorSubmission?journalCode=tace20&show=instructionshttp://www.tandfonline.com/doi/mlt/10.1080/21870764.2018.1439613http://www.tandfonline.com/doi/mlt/10.1080/21870764.2018.1439613http://crossmark.crossref.org/dialog/?doi=10.1080/21870764.2018.1439613&domain=pdf&date_stamp=2018-02-26http://crossmark.crossref.org/dialog/?doi=10.1080/21870764.2018.1439613&domain=pdf&date_stamp=2018-02-26FULL LENGTH ARTICLEValidity check of easy-to-use torsion test method for bioceramicsKouichi Yasuda a, Toshiyuki Kawanob, Masanori Kikuchic, Mamoru Aizawad, Kanji Tsurueand Sadami TsutsumifaDepartment of Materials Science and Engineering, Tokyo Institute of Technology, Tokyo, Japan; bAnalytical and Measuring InstrumentsDivision, Shimadzu Corporation, Kyoto, Japan; cBioceramics Group, RCFM, National Institute for Materials Science, Tsukuba, Japan;dDepartment of Applied Chemistry, Meiji University, Tama-ku, Kawasaki-shi, Japan; eSection of Bioengineering, Department of DentalEngineering, Fukuoka Dental College, Fukuoka, Japan; fTokyo Branch, Applied Electronics Laboratory, Kanazawa Institute of Technology,Tokyo, JapanABSTRACTThis article examines the validity of a test method to determine the torsion strength ofbioceramics under in-vivo-mimicking circumstances. The torsion test setup consisted of upperand lower grip jigs, designed to grip dog bone-type bioceramic specimens, and an openingtorque tester for PET bottles. A specimen was set on the torque tester through the lower gripjig at the bottom, and the upper grip jig was then mounted on the top end of the specimen.The upper grip jig was rotated by hand to apply torque until the specimen was fractured bythe torsion. The torsion strength was calculated using the maximum torque at fracture andthe gage diameter. Five calcium phosphate bioceramics were employed for the torsion test.The torsion strength data obtained by this method agreed closely with data measured usinga material testing machine with a convertor from the linear crosshead motion into therotation. Round-robin tests among four different organizations in Japan revealed that thetorsion strength data showed good agreement for each sample immersed for 24 hr underphosphate-buffered solution as in-vivo-mimicking circumstances. These results verified theability of the easy-to-use torsion method to give appropriate strength data with a simpleexperimental setup.0102030405060708090Torsion strength / MPaOrg.1Org.2Org.3Org.4A     B     C     D     ERound-robin test of PBS-immersed samples,tested in air using easy-to-use torsion test.ARTICLE HISTORYReceived 6 June 2017Accepted 22 December 2017KEYWORDSTorsion test method; torsionstrength; hydroxyapatiteceramics; validity check;bioceramics1. IntroductionBioceramics are regarded as key materials amongadvanced technologies because they play a role inmany promising applications designed to improvepeople’s quality of life in our aging society [1]. Associety ages, the elderly will encounter problems suchas bone fracture when they fall accidentally as well asgradual bone collapse resulting from osteoporosis(bone loss due to aging). To turn damaged humanbones to an almost healthy condition, development ofbioceramics is considered to be the task of pressingurgency [2].In the past, dense α-Al2O3 ceramics have beenused for bone implantation and restoration, but itbecame apparent that the patients found it difficultto have α-Al2O3 artificial bones replaced regularly (atabout 10-year intervals). In order to avoid this pro-blem, porous apatite ceramics have been developed,in which cells can penetrate into the pores and pro-liferate. And artificial bones (porous apatite ceramics)directly bind to natural bone at an early date [3–10].This makes it clear that bioceramics must be strongenough not to be fractured or seriously damaged,even when subjected to abrupt loading during a clin-ical course. Until recently bending tests [5–7,10] haveCONTACT Kouichi Yasuda kyasuda@ceram.titech.ac.jpJOURNAL OF ASIAN CERAMIC SOCIETIES, 2018VOL. 6, NO. 1, 43–53https://doi.org/10.1080/21870764.2018.1439613© 2018 The Author(s). Published by Informa UK Limited, trading as Taylor & Francis Group on behalf of The Korean Ceramic Society and The Ceramic Society of JapanThis is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/4.0/), which permitsunrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.http://orcid.org/0000-0002-2681-7516http://www.tandfonline.comhttp://crossmark.crossref.org/dialog/?doi=10.1080/21870764.2018.1439613&domain=pdfbeen used to measure the strength of bioceramics, buttorsion (or shear) tests have not been applied.Torsion tests first appeared in the literature [11–13]in the 1950s in the engineering ceramics field. Theylater came to be used as a part of multiaxial fracturecriterion studies, because the stress state σ1 = −σ2 ispure torsion (σi stresses are principal stresses). Theinclined-surface crack in flexure [14,15], biaxial ten-sion by ball on ring [16], diametrical compression [17],and antisymmetrical bending [18] methods were alsoproposed for such the studies, but the tension/torsiontest was considered the most fundamental test method.In 1981, Petrovic and Stout [19,20] reported tension/torsion tests of α-Al2O3 rod specimens, in which theprincipal stress ratio σ2/σ1 was changed from 0 (puretension) to −1 (pure torsion) and found that the tensilefracture strength σ1f increased with increases in theprincipal compressive stress σ2. In their following arti-cles [21,22], they also conducted axial tension, hooptension, balanced biaxial tension, balanced tension-compression, and pure torsion tests for α-Al2O3 tubespecimens. They reported, however that the balancedbiaxial tensile strength was 10% lower than the uniax-ial tensile strength, although no change was observedin the balanced tensile/compression strength, and alsothat the pure torsion strength was 16% higher than theuniaxial tensile strength. These controversial resultswere picked up again by Kim and Suh [23], who alsoinvestigated the tension/torsion behavior of α-Al2O3tube specimens to consider the effect of minimumprincipal stress. Recently, torsion tests are also appliedin the research on fatigue [24,25], PZT materials [26],and joining [27].Conducting tension/torsion or pure torsion testsof bioceramics in the conventional way requires amaterial testing machine equipped with axial/rota-tional two-way grip chucks [28] (a system with atotal cost as high as $100,000). We must also prepare(1) dog bone-type specimens, (2) metal chuck sleeves,(3) a good setting alignment and strong bondingbetween the specimen and chuck sleeve, and (4) amultiple control system in which load and torque canbe controlled simultaneously. Experts in mechanicaltesting can then conduct the tests, but the manufac-turers and users of bioceramics cannot. As an alter-native, we can purchase a more compact torsiontester [29] based on the lathe turning machine, butthese are designed for torsion tests only. Since thecost will be several tens of thousands of dollars, it isnot easy to prepare these machines, either.With a view to the restricted conditions for torsiontests, the present authors have recently proposed aneasy-to-use torsion test method for bioceramics [30].This method requires only an opening torque testerfor PET bottles [31] (costing approximately $1,000)and simple grip jigs to enable anyone to conducttorsion tests anywhere with no special expensivesetup. In a previous article [30], the present authorsoptimized this torsion test method by changing thespecimen geometry and improving the jig mechanismto prevent bending moments and axial loading dur-ing torsion testing, and showed reasonable torsionstrength data for blackboard chalk and five types ofhydroxyapatite ceramics with different porosities.In this article, we verify that the torsion strengthdata acquired by the easy-to-use torsion test methodis accurate by comparing it with torsion strength datameasured using a material testing machine equippedwith a linear displacement/rotation convertor (usinga rack-and-pinion gear system). For simplicity, thetests were done in air at ambient temperature. In thefollowing, we confirm that good agreement can beseen in easy-to-use torsion strength data from fourdifferent organizations. In this test, the specimensTable 1. Samples to be tested in torsion.Sample MaterialNominalporosity/%A Hydroxyapatite 75B Hydroxyapatite 75C Hydroxyapatite 50D Hydroxyapatite/tricalcium phosphatecomposites35E Hydroxyapatite/tricalcium phosphatecomposites0Figure 1. Specimen geometry.44 K. YASUDA ET AL.were immersed in PBS (phosphate-buffered solution(PBS), which provides in-vivo-mimicking circum-stances) for 24 hr at ambient temperature beforetorsion testing, because the final goal is to propose atorsion strength test method for PBS-immersed speci-mens as an ISO standard.2. Experimental2.1 SampleFive types of bioceramics (denoted A, B, C, D, E,hereafter) were supplied by bioceramics companiesin Japan. Precise information on their processingand characterization was not open to us, except forthe nominal porosity shown in Table 1. Samples A, B,and C were hydroxyapatite ceramics, and samples Dand E were hydroxyapatite/tricalcium phosphatecomposites. These samples had been used in the pre-vious study [30].2.2 Specimen geometryThe specimen geometry was 40 mm in length, 10 mmin gage, and ϕ6 mm in gage diameter, with taperingin a 2/7.5 slope, and prism ends as shown in Figure 1.Optimization of the specimen geometry was dis-cussed in the previous article [30].2.3 Easy-to-use torsion test2.3.1 Opening torque tester for PET bottlesVarious torsion testers were available commercially.Most were developed as specialized machines formetallic automobile drive shafts [32], however, theirtorque capacity was also too large (e.g. maximumtorque = 1000 Nm). We also considered more com-pact torsion testers [29]; their torque capacity wasstill large, however, at about 50 Nm. Thus, neitherwas directly applicable to bioceramics (typically0.1 Nm or less for bioceramics in this study) due tolarge torque range and high cost of the testers.Taking the future proposal for a “Test method fortorsion strength of porous bioactive ceramics under in-vivo-mimicking circumstances” as an ISO standard forthe future into account, the present authors decided toassemble a torsion test system that is as simple aspossible to enable anyone to conduct tests anywherewithout expensive specialized equipment. To satisfythese conditions, we focused on an opening torquetester for PET bottles incorporating simple grip jigs.Figure 2. Opening torque tester for PET bottles.Figure 3. Grip jigs (the left is upper grip jig, and right is lower grip jig attached with a plastic sleeve guide).JOURNAL OF ASIAN CERAMIC SOCIETIES 45Figure 2 shows an opening torque tester for PETbottles (Tohnichi Mfg. Co. Ltd: 2TME500CN2) [31].This tester was used for checking the opening torque ofPET bottles within a designated range using the follow-ing test procedure: to hold PET bottle, four rubber-covered pillars were moved radially toward the centerof the round-based table by rotating a black nob on theright side; the PET bottle cap was rotated by hand; andthe maximum opening torque was displayed in a win-dow at the top. Thus, this tester already had threefunctions: holding the object, measuring the torque (atorque cell was installed below the round-based table),and displaying the maximum torque upon opening.Wetherefore prepared only simple specimen grip jigs toconduct torsion tests for bioceramics. Actually, the pre-sent authors used two types of testers (Tohnichi Mfg.Co. Ltd: 2TME500CN2 (100–500 cNm) and2TME200CN2 (40–200 cNm)) depending on the tor-que range applied. Several companies around the worldmake similar opening torque testers for PET bottles.2.3.2 Grip jigs with sleeve guideAs shown in Figure 3, the grip jigs were two-steppedcircular cylinders with a square hole in the center of thesmaller base planes. The smaller circular cylindrical partof the upper (left) grip jig had a smooth outer surfacethat fit the smooth inner side surface of the plastic sleeveguide to fix the rotation axis when the upper grip jig wasrotated by hand. The smaller circular cylindrical part ofthe lower (right) grip jig had a long threaded outer sidesurface whose tread fit the inner thread of the plasticsleeve guide in order to adjust the height of the guidewhen rotated along the tread. This sleeve guide had twofunctions: to prevent bending moments and to cancelaxial load during torsion tests. The inner structure isshown in Figure 4 (semi-sectioned drawing). To avoidblank test calibrations, the upper grip jig was made ofaluminum alloy (not a heavy metal).2.3.3 Easy-to-use torsion testA lower grip jig was attached to an opening torquetester for PET bottles, and one end of the specimenwas inserted into a square hole in the grip jig(Figure 5(a)). The other end of specimen was alsoinserted into the upper grip jig (Figure 5(b)). Theplastic sleeve guide was moved upward by its rota-tion. When its top end touched the upper grip jig, thesleeve guide was moved up slightly to cancel theweight of the upper grip jig (Figure 5(c)). Theupper grip jig was then rotated by hand to applytorque until the specimen was fractured by the tor-sion (Figure 5(d)). The time to fracture was within1–2 s, and it was thus considered that slow crackgrowth had little effect on the torsion strength values.A blank test should be conducted before the torsiontests by rotating the upper grip jig without a specimen.When the upper grip jig was made of aluminum alloy,the blank test value was almost 0.0 cNm; however, theFigure 4. Semi-sectioned drawing of grip jigs (the left half isoutlook, right half is sectioned).(a) (b) (c) (d)Figure 5. Easy-to-use torsion test using the grip jigs.46 K. YASUDA ET AL.value was around 2–3 cNm when we used stainlesssteel due to the friction between the top end of thesleeve guide and the upper grip jig. In this study, thepresent authors used an aluminum alloy upper grip jig(no need for calibration). If the blank test value is 2–3cNm when using a stainless-steel upper jig, the blanktest value must be subtracted from each torque mea-surement at fracture for calibration.2.4. Torsion test with a material testing machineequipped with a linear displacement/rotationconvertorTo investigate the accuracy of the easy-to-use torsiontest data, a material testing machine (Shimadzu,Autograph AG-X) was used to measure the torsionstrength of the same samples. The material testingmachine was equipped with a linear displacement/rota-tion convertor (Shimadzu, STJ-10). As shown inFigure 6, the convertor had a rack-and-pinion gearsystem to transform the linear displacement of thecrosshead movement into rotation of the output axistoward the backside of the testing machine. One grip jig(right jig in Figure 7) was attached to this output axis,and another grip jig (left jig in Figure 7) was attached tothe torque cell. The torque cell (10 Nm max. capacity,0.01 Nm(=1 cNm) min. accuracy) was fixed to the rigidframe of the testing machine. The rotation test speedwas set at 50 °/min by adjusting the crosshead speed ofthe material testing machine such that the time tofracture was within 3–4 s in this case as well.As shown in Figure 7 (lower figure), a dummyspecimen was set between the two flat metal platesof the grip jig. The centering of the interval betweenthe plates with respect to the output rotation axis canbe adjusted by rotating each hexagonal bolt sepa-rately. Note that the interval of the two plates waslightly fitted to the grip end of the specimen, ratherthan firmly fixed, so that the axial displacement of thespecimen was free during the torsion test. We usedthe dummy specimen to adjust the alignmentbetween the rotation axis and the two grip jigs.In this verification, torsion testing with a materialtesting machine equipped with a convertor was con-ducted in air by the organization ID = 5 and the numberof specimen was 5. The easy-to-use torsion tests, on theother hand, were conducted in air by the organizationID = 1, and the number of specimen was 10 for eachsample. Torsion data from both tests were compared.Convertor   Load cell  Crosshead Output rotation axis    Torque cellGrip jigsFigure 6. A material testing machine (Shimadzu, Autograph AG-X) equipped with a linear displacement/rotation convertor(Shimadzu, STJ-10, using a rack-and-pinion gear system).Grip jigsTorque cellAlignment check by cylindrical metal dummy specimen Figure 7. Expanded view of grip jig and alignment checkusing a cylindrical dummy specimen.JOURNAL OF ASIAN CERAMIC SOCIETIES 472.5 Round-robin testFour organizations (ID = 1–4) in Japan participatedin this round-robin test in order to confirm goodagreement in torsion strength data when differentpeople conducted the easy-to-use torsion test. Priorto this set of torsion tests, all the specimens wereimmersed in PBS (pH = 7.4) for 24 hr at ambienttemperature. During immersion, the specimenswere evacuated once for 30 min or longer to elim-inate air in the pores to the extent. Then, justbefore each torsion test, a specimen was pickedout, and the wet surface was wiped with paper.The easy-to-use torsion tests were conductedaccording to the procedure in Section 2.3. Thenumber of specimen was 10 for each sample, exceptin the case of the organization ID = 3.Organization ID = 3 tested five as-received speci-mens in air and five specimens tested in air afterimmersion in PBS.3. Results and discussion3.1 Accuracy check3.1.1 Easy-to-use torsion test in airEasy-to-use torsion tests of as-received specimensin air were conducted by organization ID = 1, andtheir fracture surfaces are shown in Figure 8.From this figure, it can be seen that samples A,B, and C showed helical fracture surfaces, whichare a typical feature of torsion fracture of brittlematerials. According to the mechanics of materials[33], a pure torsion stress state is equivalent to aSample A Sample BSample C Sample DSample EFigure 8. Fracture surfaces of as-received samples tested in air using the easy-to-use torsion test method (by organization 1).Table 2. Torsion strength in air using easy-to-use torsion test(as-received specimen, measured by organization 1).SampleMaterial(Nominal porosity)Averagetorsionstrength/MPaStandarddeviation oftorsion strength/MPaA Hydroxyapatite(75%)5.21 0.61B Hydroxyapatite(75%)4.85 0.40C Hydroxyapatite(50%)8.21 1.30D Hydroxyapatite/tricalciumphosphate composites(35%)35.1 4.14E Hydroxyapatite/tricalciumphosphate composites(0%)86.9 3.4148 K. YASUDA ET AL.tensile/compression biaxial stress state achieved by45° rotation of the original x-y coordinate system.Thus a crack extended along the normal directionto the tensile stress (viz. the crack always extendedwith an incline of 45° to the longitudinal axis ofthe specimen). On the contrary, samples D and Eshow multiple fractures (fragmentation occurredduring fracture), but if we look at them moreclosely, we find a partial helical fracture surface.Samples D and E show relatively higher torsionstrength due to lower porosity and such multiplefractures can also be seen after bending tests ofhigh-strength dense ceramics.By measuring the maximum torque at fractureTf of each specimen, we calculated the torsionstrength tf using the following equation [34];τf ¼16Tfπd3(1)where d is the gage diameter (ϕ6 mm in thisstudy). The average and standard deviation of torsionstrength are summarized in Table 2. The torsionstrength increased with decreases in the porosity,and the standard deviation was around 1/10 of itsaverage, indicating that the data for the easy-to-usetorsion test method showed good accuracy (no largedata scattering).Time / sec  5Time / sec  Time / sec  Time / sec  0 1 2 3 4 50 1 2 3 4 0 1 2 3 4 50 1 2 3 4 50 2 4 6 8 10Time / sec  0.50.40.30.20.10.00.50.40.30.20.10.00.50.40.30.20.10.02.01.61.20.80.40.0543210Torque / NmTorque / NmTorque / NmTorque / NmTorque / NmSample A Sample BSample C Sample DSample EFigure 9. Torque/time (∝rotation angle) diagrams of as-received samples.Table 3. Torsion strength in air using a material testingmachine (as-received specimen, measured by organization 5).SampleMaterial(Nominal porosity)Averagetorsionstrength/MPaStandarddeviation oftorsion strength/MPaA Hydroxyapatite(75%)4.72 0.26B Hydroxyapatite(75%)4.53 0.29C Hydroxyapatite(50%)7.43 1.20D Hydroxyapatite/tricalciumphosphate composites(35%)29.0 2.45E Hydroxyapatite/tricalciumphosphate composites(0%)77.9 13.5JOURNAL OF ASIAN CERAMIC SOCIETIES 493.1.2 Torsion test in air conducted with a materialtesting machine equipped with a convertorTorsion tests of as-received specimens were conducted inair by organization ID = 5 using a material testingmachine equipped with a convertor, and the resultingfracture surfaces were the same as those in Figure 8.Figure 9 shows their torque/time (∝rotation angle) dia-grams.Most of the samples except B showed linear elasticdeformation and then brittle fracture. In sample B, linearelastic deformation was observed, followed by nonlineardeformation and then brittle fracture. We used the max-imum torque as the torque at fracture in our calculations.As stated earlier, the time to fracture tf was around 3–4 s,so the rotation angle ϕf at fracture was around 5.8*10–2[rad]¼ tf ½sec� � 50½�=min� � π½rad�=180½��ð Þ=60½sec�� �¼ 4� 50� π=180ð Þ=60ð Þ, and the shear strain at frac-ture was therefore γf ¼ r � φf =L ¼ ð3 � 10�3Þ �ð5:8� 10�2Þ=ð10� 10�3Þ ¼ 1:7� 10�2, where L and rwere the gage and its radius.Table 3 shows the average and standard deviation oftorsion strength measured by the material testingmachine with the convertor, and Figure 10 shows acomparison between the data obtained by the easy-to-use torsion method (in Table 2) and the material testingmachine method (Table 3). At a glance, we can see thatalmost the same results were obtained by the two mea-surement methods.In the comparison, all the samples show good agree-ment in average torsion strengthmeasurement results forthe easy-to-use method and the material testing machinemethod, indicating that the easy-to-use torsion testmethod gives data with good accuracy data, comparableto data obtained by the material testing machine.3.2 Round-robin testFour organizations (ID = 1, 2, 3, 4) conducted torsiontests of specimens immersed in PBS (pH = 7.4) for24 hr at ambient temperature. A comparison of theresults obtained by the 4 organizations is shown inFigure 11. Note that organization ID = 3 tested 5specimens and the others tested 10 specimens foreach sample. At first glance, we see good agreementin the average torsion strength for each sample amongthe four organizations, indicating an absence ofexperimenter effect.Easy-to-use method      Material testing machine(10 specimens each)     (5 specimens each)by Org.1               by Org.5Figure 10. Comparative study of easy-to-use method and a material testing machine with a convertor (as-received specimens in air).0102030405060708090Torsion strength / MPaOrg.1Org.2Org.3Org.4A     B     C     D     ERound-robin test of PBS-immersed samples,tested in air using easy-to-use torsion test.Figure 11. Round-robin test of torsion strengths of samplesafter PBS immersion for 24 hr, tested in air using easy-to-usetorsion tests (by organization 1, 2, 3, 4).50 K. YASUDA ET AL.Within the experimental data in this study, theeasy-to-use torsion test method is useful forobtaining reasonable torsion strength data, whichis in good agreement among different organiza-tions and which clearly shows the differences inthe samples.3.3 Comparison between as-received samplesand PBS-immersed samplesOrganization ID = 1 conducted torsion tests for both as-received samples (Table 2) and PBS-immersed samples(Figure 11). Thus we can compare the as-received sam-ples and PBS-immersed samples. Figure 12 shows theSample A Sample BSample C Sample DSample EFigure 12. Fracture surfaces of samples after PBS immersion for 24 hr, tested in air using easy-to-use torsion tests (byorganization 1).0102030405060708090100A B C D ETorsion strength  / MPaPBS immersionAs-received Figure 13. Torsion strength of as-received samples ( ) and samples ( ) after PBS immersion for 24 hr, tested in air using easy-to-use torsion tests (by organization 1).JOURNAL OF ASIAN CERAMIC SOCIETIES 51fracture surfaces of the samples after PBS immersion for24 hr, and testing in air by the easy-to-use torsionmethod. Comparing these with Figure 8, we see thatsamples A, B, C, and D show the same fracture featuresas the as-received samples, but that sample E shows atypical helical fracture surface, not multiple fracture as inFigure 8. This corresponds to the tendency shown inFigure 13, which is a comparison between the torsionstrengths of as-received samples andPBS-immersed sam-ples. Only sample E shows a difference between the twodata, which may have been brought about by slow crackgrowth during PBS immersion.In the torsion tests, the maximum stress wasformed at the cylindrical surface of the gage, and indense sample E, the weakest defect (largest defect) onthe surface was relatively smaller than that in othersamples, and slow crack growth during PBS immer-sion consequently had a significant effect on its crackextension. Since samples A, B, C, and D were porous,the original weakest defect was relatively larger insize; so crack extension by slow crack growth didnot have much effect on the final crack size (weakestdefect size). These data suggest that not only as-received samples but also PBS-immersed samplesare important for estimating the reliability ofbioceramics.4. ConclusionThe easy-to-use torsion test method was verified bytwo experiments. One was an accuracy check compar-ing torsion strength data measured by the easy-to-usemethod with data obtained using a material testingmachine equipped with a linear displacement/rotationconvertor. Good agreement was shown between theresults for the two methods. The other was a round-robin test conducted by four different organizations inJapan, which found that the torsion strength data didnot depend on the organization, and that consistenttest data were obtained. Based on these experimentalresults, it is concluded that the easy-to-use torsionmethod gives reasonable strength data with a simpleexperimental setup, and that it can be proposed as ISOstandard for the future.AcknowledgmentsThis research was supported in part by the Ministry ofEconomy, Trade and Industry, Japan as a project under-taken for International Standardization for test method fortorsion strength of porous bioactive ceramics under in-vivo-mimicking circumstances. The authors wish to expressour gratitude to Mr. Tsutomu ARAI (Tohnichi Mfg. Co.Ltd.) for his technical advice on torque measurements, tobioceramics manufacturers for supplying samples, and tothe Japan Fine Ceramics Association for its invaluableadministrative support.Disclosure statementNo potential conflict of interest was reported by theauthors.ORCIDKouichi Yasuda http://orcid.org/0000-0002-2681-7516References[1] The WHOQOL Group. The World HealthOrganization quality of life assessment (WHOQOL):position paper from the World Health Organization.Soc Sci Med. 1995;41:1403–1409.[2] Tsutsumi S. Trend report on international andJapanese standardization activities for bioceramicsand tissue engineered medical products. Sci TechnolAdv Mater. 2010;11(3):014112.[3] Aoki H, Kato K. Apatites as biomaterials. Ceramics.1975;10: 469–478. in Japanese.[4] Jarcho M, Bolen CH, Thomas MB, et al.“Hydroxylapatite synthesis and characterization indense polycrystalline form“. 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J Am Ceram Soc. 1992;75(4):896–902.[24] Suresh S, Tschegg EK. Combined mode-I-mode IIIfracture of fatigue-precracked alumina. J Am CeramSoc. 1987;70(10):726–733.[25] Schwind T, Kerscher EK, Lang K-H. Fatigue of alu-mina under cyclic torsion loading. Adv Eng Mater.2009;11(7):586–589.[26] Fett T, Munz D, Thun G. Multiaxial deformationbehavior of PZT from torsion tests. J Am CeramSoc. 2003;86(8):1427–1429.[27] Ferraris M, Salvo M, Casalegno V, et al. Torsion testson AV119 epoxy – joined SiC. Int J Appl CeramTechnol. 2012;9[4]:795–807.[28] For example, Simadzu, Axial force and torsion testingmachine [Internet]. [cited 2017 Jun 6]. Available from:http://www.shimadzu.com/an/test/fatigue/axial-torsion.html[29] For example, Instron, MT Series Low CapacityTorsion Testers [Internet]. [cited 2017 Jun 6].Available from: http://www.instron.us/en-us/products/testing-systems/torsion-systems/low-capacity?region=North%20America&lang=en-US[30] Yasuda K, Tsutsumi S. Challenges in development ofeasy-to-use torsion test method for bioceramics - towardISO standard proposal. The Proceedings of the 41stInternational Conference on Advanced Ceramics andComposites (American Ceramic Society, Jan 22-27,2017, Daytona Beach, FL, USA): Ceramic Engineeringand Science Proceedings, Volume 38, Issue 2 (eds J.Salem, J. C. LaSalvia, R. Narayan, D. Zhu), John Wiley& Sons, Inc., Hoboken, NJ, USA. 2018.[31] For Example, Tohnichi, Torque Meter [Internet].[cited 2017 Jun 6]. Available from: https://en.global-tohnichi.com/products/categories/32[32] For Example, Tinius Olsen Testing MachineCompany, Torsion Tester [Internet]. [cited 2017 Jun6]. Available from: https://www.tiniusolsen.com/list-of-products/torsion-testers[33] Gere JM, Timoshenko SP. Mechanics of materials. 3rded. Chapter 3, Torsion. Boston: PWS PublishingCompany; 1990. p. 173.[34] Timoshenko SP, Goodier JN. The theory of elasticity.Torsion: MacGrow-Hill Book Company;1951 264.Chapter 11.JOURNAL OF ASIAN CERAMIC SOCIETIES 53http://www.shimadzu.com/an/test/fatigue/axial-torsion.htmlhttp://www.shimadzu.com/an/test/fatigue/axial-torsion.htmlhttp://www.instron.us/en-us/products/testing-systems/torsion-systems/low-capacity?region=North%20America%26lang=en-UShttp://www.instron.us/en-us/products/testing-systems/torsion-systems/low-capacity?region=North%20America%26lang=en-UShttp://www.instron.us/en-us/products/testing-systems/torsion-systems/low-capacity?region=North%20America%26lang=en-UShttps://en.global-tohnichi.com/products/categories/32https://en.global-tohnichi.com/products/categories/32https://www.tiniusolsen.com/list-of-products/torsion-testershttps://www.tiniusolsen.com/list-of-products/torsion-testers Abstract 1.  Introduction 2.  Experimental 2.1  Sample 2.2  Specimen geometry 2.3  Easy-to-use torsion test 2.3.1  Opening torque tester for PET bottles 2.3.2  Grip jigs with sleeve guide 2.3.3  Easy-to-use torsion test 2.4.  Torsion test with a material testing machine equipped with a linear displacement/rotation convertor 2.5  Round-robin test 3.  Results and discussion 3.1  Accuracy check 3.1.1  Easy-to-use torsion test in air 3.1.2  Torsion test in air conducted with a material testing machine equipped with a convertor 3.2  Round-robin test 3.3  Comparison between as-received samples and PBS-immersed samples 4.  Conclusion Acknowledgments Disclosure statement References