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雙軸拉伸試驗(yàn)機(jī)的設(shè)計(jì) S.A.Kurkin, V.F.Luk’yanov, M.N.Krumbol’DT 當(dāng)加載內(nèi)部壓力時(shí)，焊接薄板結(jié)構(gòu)及其外殼將受到雙向拉伸的張力。在這種情況下，往往有一個(gè)靈敏度增加的金屬片，由于局部應(yīng)力集中的存在或者由于與制造工藝相關(guān)的金屬的機(jī)械性能的局部改變，從而導(dǎo)致結(jié)構(gòu)的強(qiáng)度明顯下降。 因此，在至關(guān)重要的薄板結(jié)構(gòu)的材料和制造工藝的選擇時(shí)，平時(shí)所進(jìn)行的單軸試驗(yàn)所獲得的數(shù)據(jù)是不夠的。這就需要測(cè)試的大量的組裝件和最大限度地反映實(shí)際工作條件下的金屬基材和焊接接頭的模型。這樣的測(cè)試對(duì)檢查其生產(chǎn)的最后階段的結(jié)構(gòu)強(qiáng)度是很重要的，但他們是相當(dāng)昂貴的，并且作為一項(xiàng)慣例，由于過(guò)早失敗，傳達(dá)的信息也不足。 在這方面，我們應(yīng)該進(jìn)行測(cè)試，測(cè)試應(yīng)該充分地反映機(jī)構(gòu)在實(shí)際工作條件下，并且在實(shí)驗(yàn)室調(diào)查的方法將適用。 參考薄板結(jié)構(gòu)我們應(yīng)該考慮到這樣的測(cè)試： a）應(yīng)力狀態(tài)（主應(yīng)力分量下的雙向平等或不平等）； b）載荷特性（靜態(tài)或反復(fù)加載不同周期的靜態(tài)）； c）環(huán)境的影響； d）工作溫度的影響 這些試驗(yàn)設(shè)備應(yīng)該容易設(shè)計(jì)，并提供一份高輸出的調(diào)查。 在本文中我們將描述在雙向應(yīng)力的狀態(tài)下，測(cè)試金屬和焊接接頭機(jī)器的設(shè)計(jì)實(shí)驗(yàn)。 在[ 1，2 ]的分析中表明，通過(guò)靜壓彎曲的方法，金屬和薄板結(jié)構(gòu)的焊接接頭的工作條件在測(cè)試中被體現(xiàn)得最充分。在這種情況下，標(biāo)本是安全的，并且負(fù)載了液壓。 圖1 產(chǎn)生雙向拉伸的加載方案 應(yīng)力狀態(tài)的金屬所產(chǎn)生的力取決于試樣的形狀和模具(圖1)。因此，支持在一個(gè)平面標(biāo)本靜態(tài)荷載壓力情況下對(duì)管芯圓輪廓進(jìn)行，雙向彎曲產(chǎn)生的張力和標(biāo)本的外表面的凸出面的相當(dāng)多的部分受到的張力與平等的組成部分張力均為σ1 =σ2（圖1a）。如果平面標(biāo)本被可靠的固定在管芯圓輪廓上，雙向拉伸將被取代為雙向彎曲。在加載的情況下，不是一個(gè)平面標(biāo)本，而是一個(gè)模具的孔的直徑和標(biāo)本的厚度的比率足夠大的球形標(biāo)本，彎曲的部分是小的，并且我們可以認(rèn)為試樣的中心部分受到的雙向拉伸為σ2/σ1=1。 比率在σ2/σ1 = 1.0-0.75之內(nèi)的不等組件的雙向拉伸張力，可通過(guò)使用了具有橢圓孔的模型的標(biāo)本的屈曲來(lái)根據(jù)給出的圖表1b得出。進(jìn)一步的減小比例在σ2/σ1（0.7-0.3）的雙向拉伸張力是通過(guò)在圖1c所示的計(jì)劃方案得到的，在此比例中，由法蘭部份抑制圓柱之間的模具和沖壓圓柱形式的圓柱面板形狀的標(biāo)本加載靜水壓力[3]。工作壓力下薄板結(jié)構(gòu)的兩種特征載荷的類型：?jiǎn)危o態(tài)）和低循環(huán)（重復(fù)靜態(tài)）。第一，使用計(jì)劃A和B是有用的（見(jiàn)圖1）。根據(jù)試驗(yàn)計(jì)劃，靜態(tài)加載可以做到在平面形式的標(biāo)本，以及在預(yù)成球形部分的標(biāo)本上[4，5]。薄板是可取的，因?yàn)樗麄兊闹圃旌臅r(shí)少。在球形段形成固定標(biāo)本許可減少對(duì)輪廓試樣的邊緣效應(yīng)的影響；然而，這些標(biāo)本的制備要求塑性變形，從而會(huì)導(dǎo)致改變材料的組織和性能，并不總是可求的力學(xué)性能的變。 所有這三個(gè)方案如圖1可用于在低周反復(fù)荷載條件下進(jìn)行測(cè)試。然而，根據(jù)方案a應(yīng)優(yōu)先考慮雙向彎曲，因?yàn)楹笳咴试S更大的測(cè)試厚度。加強(qiáng)與應(yīng)力均勻分布的區(qū)域，標(biāo)本的輪廓可以被鉸接到死點(diǎn)。根據(jù)方案b和c(參閱圖1)，低周載荷測(cè)試在一個(gè)球形或圓柱形面板形式的預(yù)成標(biāo)本上進(jìn)行。必須考慮到,在相當(dāng)大的彎曲應(yīng)力的標(biāo)本的附件的地方，超過(guò)了應(yīng)力在樣板中心的彎曲應(yīng)力。 周圍介質(zhì)在一個(gè)長(zhǎng)期和反復(fù)的靜態(tài)測(cè)試下，具有特別是強(qiáng)烈的效果。引起我們極大的興趣的是，在腐蝕性環(huán)境中反復(fù)加載靜載荷情況下的材料壽命。 從環(huán)境影響角度來(lái)看，后者可以被用來(lái)作為壓力下的標(biāo)本的工作流體。在這種情況下,現(xiàn)在的任何試驗(yàn)方案都被使用了。 為了防止主系統(tǒng)和機(jī)械零件腐蝕，腐蝕性液體放置于標(biāo)本之下并且被從主要工作流體與活塞密封裝備分開(kāi)。位于下腔試樣的彈性膜起到著同樣的作用。 這個(gè)方案的主要缺點(diǎn)是不能觀察斷裂的過(guò)程。因此，在這些情況下的沒(méi)有壓力的腐蝕性介質(zhì)是沒(méi)有益處的，強(qiáng)腐蝕性的介質(zhì)應(yīng)放置在標(biāo)本上面，其更換應(yīng)該容易，因?yàn)樗俏廴靖g產(chǎn)物，并通過(guò)視覺(jué)和靜止畫(huà)面攝影手段觀察斷裂的過(guò)程。 為防止上板腐蝕可以涂上一層環(huán)氧樹(shù)脂漆,或圓形橡膠防止腐蝕介質(zhì)超出了被測(cè)試樣品的極限，可以粘在標(biāo)本的局部，限制擴(kuò)散層。在與上述標(biāo)本中的腐蝕試驗(yàn)情況下，加載的方案建議如圖1a所示。 溫度是確定測(cè)試結(jié)果的一個(gè)重要因素。調(diào)查的數(shù)據(jù)[6]表明負(fù)溫度降到-196o，在靜載荷（通過(guò)用液氮或蒸汽局部冷卻的試樣）的條件下可以得到。在長(zhǎng)期試驗(yàn)的情況下，把它放置在一個(gè)冷卻室來(lái)冷卻整個(gè)機(jī)器的方法是合適的。 加熱的溫度約200-250o，標(biāo)本可以通過(guò)上述試樣放在電熱器上。為了更好的熱交換應(yīng)該有應(yīng)該有一層以上的標(biāo)本，在測(cè)試過(guò)程中試樣油層是激烈運(yùn)動(dòng)的。 本機(jī)設(shè)計(jì)的初步數(shù)據(jù)是板材的機(jī)械性能，是在厚度范圍內(nèi)進(jìn)行測(cè)試，并是在加載的條件下得到的。 這種機(jī)器的設(shè)計(jì)和操作的10年經(jīng)驗(yàn)表明，在一個(gè)單一的靜態(tài)負(fù)荷情況下，在液壓系統(tǒng)的最大壓力應(yīng)不超過(guò)600-700測(cè)量atm，并且在反復(fù)加載的情況下，它不應(yīng)該超過(guò)150-200測(cè)量atm。 主要參數(shù)是模型2r的孔的尺寸，這確定著機(jī)器的尺寸和結(jié)構(gòu),而這又取決于標(biāo)本厚度t。 我們將考慮不同測(cè)試方案的r/t的比值的選擇。 在方案a（見(jiàn)圖1），r/t的比值的增加是伴隨著薄膜應(yīng)力和標(biāo)本彎曲應(yīng)力的對(duì)比的增加而增加的，這是不可取的，因?yàn)楸∧?yīng)力影響較大的裂縫發(fā)生率，因此可以阻礙了測(cè)試結(jié)果的分析。此外,壓力測(cè)試所需的標(biāo)本的增加是隨著r/t比值的減小的。考慮到上述表示的考慮，我們建議選擇符合的比率是在模具半徑R和試樣厚度t的不等式： 其中y是材料的屈服點(diǎn); E為彈性模量; P是標(biāo)本下的最大壓力。 在標(biāo)本的靜態(tài)試驗(yàn)中，對(duì)輪廓的克制（圖1，方案B）的比值R / T是按以下方式確定的。 實(shí)驗(yàn)數(shù)據(jù)表明，標(biāo)本的中心部分的拉伸張力產(chǎn)生的彎曲由表達(dá)式 描述，其中Ψ是對(duì)數(shù)縮頸單軸拉伸變形斷裂。 其中的最好的斷裂時(shí)間的值不超過(guò)0.03，為此滿足其中的不等式: 是非常必要的。 此外，有相當(dāng)多的壓力P對(duì)試樣在R /T為小比值的情況下的斷裂是有必要的。利用Tomlenov[7]，Sandier和Khodulin [8]的研究，我們可以推薦一對(duì)強(qiáng)度，材料可塑性，壓力P為試樣斷裂比率關(guān)系: 其中的sk是真正的抗斷裂數(shù)。 從(3)和(4)的關(guān)系可得出，我們必須采取更大的r/t的比值?？紤]到在測(cè)試試樣的曲率半徑，不僅可以減少由于金屬變形的結(jié)果，而且可以減少由于緊固法蘭的延誤，我們必須增加10-15％的比例獲得。 為了防止法蘭被拖進(jìn)模型,法蘭寬度部分標(biāo)本應(yīng)不少于模型直徑的0.25-0.3。 在一個(gè)預(yù)成球臺(tái)形式標(biāo)本的測(cè)試中，半徑段R應(yīng)選為符合不平等式： 從而減少了邊緣效應(yīng)的影響。 為測(cè)試方案c(見(jiàn)圖1),我們用一種形式為圓柱面板的標(biāo)本。圓筒狀弧形板應(yīng)該在斷面上,并且有一個(gè)中央的角度不少于120-160o。該面板長(zhǎng)度(沒(méi)有法蘭部分)不應(yīng)小于2.5-3其截面的半徑。為了增加其數(shù)值，模具應(yīng)在一個(gè)橢圓形，其主要軸線與面板的母線重合。 圖2 雙向拉伸試驗(yàn)機(jī)的設(shè)計(jì) 對(duì)檢測(cè)機(jī)的模型的孔的尺寸設(shè)計(jì)，參考了莫斯科東北鮑曼高等工業(yè)學(xué)校（MVTU）和農(nóng)業(yè)機(jī)械建設(shè)研究所（RISKHM）的羅斯托夫設(shè)計(jì)試驗(yàn)機(jī)，上述表示的考慮皆按照表1。 各種各樣設(shè)備的主要不同之處在于裝配設(shè)計(jì)上面，比如鎖定裝配1,模型2和液壓鉗3在檢測(cè)中是用于固定標(biāo)本4的。液壓鉗被用于確保標(biāo)本的輪廓。當(dāng)根據(jù)方案b和c(參閱圖1)測(cè)試，標(biāo)本的一個(gè)可靠約束的法蘭部分必須予以提供。 該模具對(duì)試樣的壓力N，模型的孔2R的直徑與標(biāo)本之下的壓力P有以下關(guān)系： 鎖緊裝置防止模具和液壓鉗的相互位移。標(biāo)本在測(cè)試時(shí)，鎖緊裝置吸收了相當(dāng)大的負(fù)載（由200至5000噸）,因而對(duì)組裝設(shè)計(jì)必須給予特別注意。 最簡(jiǎn)單的鎖定設(shè)備的設(shè)計(jì)如圖2c所示。液壓鉗和模具是固定在均勻布置有螺栓的圓周上。然而,這種設(shè)計(jì)不提供標(biāo)本的迅速附件。只有在其長(zhǎng)期試驗(yàn)的情況下才使用,例如,在反復(fù)的靜態(tài)測(cè)試。圖2a顯示鎖定元素的設(shè)計(jì)是在液壓下進(jìn)行的。設(shè)計(jì)很簡(jiǎn)單,并允許使用標(biāo)準(zhǔn)的設(shè)備,但它是麻煩的,特別重要的是,在檢測(cè)中液壓鉗的上部位置限制標(biāo)本的接近。 鎖定裝置如圖2b所示，它是根據(jù)插銷栓的操作原則來(lái)制作的。模型和液壓鉗的外殼被固定住，這是當(dāng)鎖止環(huán)彎曲到與其設(shè)計(jì)的一致的時(shí)候。 帶螺紋千斤頂?shù)脑O(shè)備[9]是用于解除上盤(pán)（當(dāng)緊固和拆卸標(biāo)本時(shí)）。這種方式使得結(jié)構(gòu)緊湊和高效率的工作。這個(gè)方案，建議適用緊固力在1000噸以下的標(biāo)本。 在圖2e所示的設(shè)計(jì)，鎖定裝置具有塊的結(jié)構(gòu)。該框架的計(jì)算，使他們的垂直元素與一個(gè)很小的組成部分承受彎曲張力。設(shè)計(jì)緊湊，并且塊體結(jié)構(gòu)系統(tǒng)大大方便了機(jī)器的組裝。在機(jī)器中，標(biāo)本是懸掛在模具上的，可移動(dòng)的塊框架在一個(gè)特殊的運(yùn)輸裝置上起到阻擋的作用[10]。使用從1000至5000噸的鎖緊力的鎖定裝置是便利的。鎖定元素在一個(gè)環(huán)的形式（見(jiàn)圖2d）[11]允許與該鎖定元素重量大大減少，其重量比按前面的方案作出規(guī)劃。該標(biāo)準(zhǔn)密封在這種情況下使用不提供所需的松緊度?？山邮艿拿芊饧脑O(shè)計(jì)應(yīng)該考慮以下內(nèi)容。 為活塞直徑小于500毫米和活塞直徑大于500毫米的液壓夾具的設(shè)計(jì)分別如圖3a和圖3b所示。該鉗的鉸鏈的組裝也如圖3c所示。在第一種情況下，活塞6是在磁盤(pán)的一個(gè)中心孔處，其中缸瓦1的導(dǎo)桿和密封圈也放置在該處。該桿的作用是消除活塞的初始失調(diào)，并用來(lái)傳遞流體進(jìn)入標(biāo)本下面的腔體?；钊c缸壁之間是通過(guò)一個(gè)圓截面的橡膠圈和一個(gè)T形截面2的鋼墊圈來(lái)實(shí)現(xiàn)密封的?；钊M裝后，橡膠圈的預(yù)緊是通過(guò)固定栓5的方法實(shí)現(xiàn)的。在測(cè)試過(guò)程中，活塞下的流體壓力的增加使得墊圈變緊，這保證了活塞的密封是可靠的。橡膠圈4防止流體通過(guò)固定栓的孔流出。這種密封設(shè)計(jì)提供了方便的密封液壓鉗的密封圈，甚至允許活塞下腔有0.3-0.5ram的間隔。橡膠密封圈的截面應(yīng)不小于100平方毫米。 當(dāng)活塞的直徑超過(guò)500mm的時(shí)候，它應(yīng)該設(shè)計(jì)為環(huán)形（見(jiàn)圖3b），這種情況下密封組件的設(shè)計(jì)是比照上述。橡膠密封圈的截面不應(yīng)超過(guò)150-200平方毫米。 圖3 標(biāo)本液壓鉗的設(shè)計(jì) 參考文獻(xiàn) 1]Ya.B.Fridman等人.雙向拉伸下板材的習(xí)性.合金和有色金屬的研究[俄羅斯]，4號(hào)（1963） [2]W.E.Copper.壓力容器設(shè)計(jì)的拉伸試驗(yàn)的意義.焊接J.36 ，2號(hào)（1956） [3]S.A.Kurkin和N.S.Meshaikin.圓柱面靜壓屈曲下的板材和焊接接頭的測(cè)試.Svarochnoe Proizvodstvo，7號(hào)（1970） [4]S.A.Kurkin 和 V.F.Lukyanov.基于雙向拉伸條件下的測(cè)試結(jié)果的焊接薄壁容器設(shè)計(jì)的評(píng)價(jià).Svarochnoe Proizvodstvo,9號(hào)(1965). [5]B.A.Drozdovskii等人.碳含量對(duì)張力狀態(tài)下鋼板結(jié)構(gòu)強(qiáng)度的影響.Obrabotka Metallov,5號(hào)(1964). [6]S.A.Kurkin和D.I.Umarov.雙向拉伸試驗(yàn)機(jī)板材和在高低溫度條件下的焊接接頭.IZV，VUZ,Mashinostroenie,2號(hào)(1968). [7]A.D.Tomlenov.塑料的受壓狀態(tài)和擴(kuò)展過(guò)程的穩(wěn)定性.金屬的壓力成形問(wèn)題[俄羅斯]，Izd.AN SSSR,莫斯科(1958). [8]N.I.Sandler和A.K.Khodulin.薄板金屬雙向拉伸的測(cè)驗(yàn)儀器.Zavod.Lab,12號(hào)(1951). [9]S.A.Kurkin等人.焊接薄壁容器的模擬式雙向拉伸試驗(yàn)機(jī).Svarochnoe Proizvodstvo,5號(hào)(1965). [10]V.F.Lukyanov等人.作者的證書(shū)號(hào)：254177，Byul.Otkr,Izobr, Prom.Obr,Tov.Zn,31號(hào)(1969). [11]S.A.Kurkin等人.作者的證書(shū)號(hào)：261745，板材和焊接接頭的雙向拉伸試驗(yàn)機(jī).應(yīng)用于1968年11月27日，Byul.Otkr,Izobr,Prom.Obr,Tov.Zn,5號(hào)(1970). DESIGN OF BIAXIAL TENSILE TESTING MACHINES S.A.Kurkin, V.F.LukWyanov, and M.N.Krumbol’DT Welded sheet structures and shells experience biaxial tension when loaded with an internal pressure.Under these conditions there is often an increased sensitivity of the sheet metal to the presence of stress raisers or to a local change of he mechanical properties of the metal related with the manufacturing process, which can lead to a marked decrease of the strength of the structure. Therefore, when selecting the material and manufacturing process of crucial sheet structures the data obtained in the usual uneasily tests of specimens are insufficient. This necessitates testing large full-scale assemblies and mock-ups of articles maximally reflecting the real operating conditions of the base metal and welded joints. Such tests are of interest for checking the strength of structure at the final stage of its manufacture, but they are quite expensive and as a rule convey insufficient information on the causes of premature failure. In this connection we should conduct tests which would most fully reflect the real working conditions of structure and in which laboratory methods of investigation would be applicable. With reference to sheet structures we should take into account in such tests: a) The state of stress (biaxial with equal or unequal components of the principal stresses); b) Character of loading (static or repeated static with different cycles); c) Effect of the ambient medium; d) Effect of operating temperature. The equipment for the tests should be simple in design and provide a high output of investigations. In this article we will present the experience of designing machines for testing metal and welded joints in a state of biaxial stress. An analysis conducted in [1, 2] showed that the working conditions of metal and welded joints in sheet structure are reproduced most fully in testing by the hydrostatic buckling method. In this case the specimen is secured about the contour and loaded by a hydraulic pressure. Fig.1 loading schemes for producing biaxial tension The stressed state arising in the metal depends on the shape of the specimen and die(Fig.1).Thus, in the case of hydrostatic pressure loading of a plane specimen supported about the contour of the round hold of the die, biaxial bending occurs and a considerable part of the outer convex surface of the specimen experiences uniform tension with equal componentsσ1=σ2(Fig.la).If the plane specimen is reliably fixed about the contour of the hole of the die, biaxial tension is superposed on biaxial bending. In the case of loading, not a plane specimen, but a spherical segment(Fig.lb) with a sufficiently large ratio of the diameter of the hole of the die to the thickness of the specimen, the bending component is small, and we can consider that the central part of the specimen experiences biaxial tension withσ2/σ1=1. Biaxial tension with unequal components within the ratio σ2/σ1 = 1.0-0.75 can be produced by buckling the specimens according to the scheme given in Fig.lb with the use of dies having elliptic holes. A further decrease of the ratio σ2/σ1(0.7-0.3)is achieved by means of the scheme shown in Fig.1c, where a specimen in the form of a cylindrical panel restrained by a flange part between the cylindrical die and cylindrical punch is loaded by hydrostatic pressure [3]. For sheet structures working under pressure two types of loading are characteristic: single (static) and low cycle (repeated static).For the first it is expedient to use schemes a and b (see Fig.1). Testing according to scheme a under a static load can be done both on specimens in the form of plane sheets and in the form of reshaped spherical segments [4, 5]. Sheets are preferable, since their manufacture is less timeconsuming.Specimens in the form of spherical segments permit reducing the influence of the edge effect from securing the specimen about the contour; however, the preparation of such specimens requires plastic deformation, and this can lead to a change of the mechanical properties of the material which is not always rectifiable even by subsequent heat treatment. All three schemes shown in Fig.1 can be used in tests under low-cyclic loading conditions.However, preference should be given to biaxial bending according to scheme a, since the latter permits testing greater thickness. To increase the zone with uniform distribution of stresses, the contour of the specimen can be hinged to the die. Tests by low-cyclic loading according to schemes b and c(see Fig. 1) are conducted only on reshaped specimens in the form of a spherical segment or cylindrical panel. It is necessary to take into account that at the place of attachment of such specimens considerable bending stresses, exceeding the stresses in the center of the specimen, occur. The ambient medium has an especially strong effect on the results of long-term and repeated static tests. Of great interest is the life of materials in the case of repeated static loading in corrosive environments. From the standpoint of the effect of the environment, the latter can be used as the working fluid acting on the specimen under pressure. In this case any of the present test schemes is used. To prevent corrosion of the main systems and parts of the machine, the corrosive fluid is placed under the specimen and is separated from the main working fluid by a partitioning piston equipped with seals. The same role can be played by an elastic membrane located in the cavity under the specimen. The main shortcoming of this scheme is the impossibility of observing the process of fracture.Therefore,in those cases where the action of the corrosive medium without pressure is of interest, the corrosive medium should be placed over the specimen, which permits its easy replacement as it is contaminated by the corrosion products and also observation of the course of fracture visually and by means of still and motion-picture photography. For protection against corrosion the upper plate can be coated with a layer of epoxy resin or lacquer, or a circular rubber molding preventing the spread of the corrosive medium beyond the limits of the part of the specimen being tested can be glued on the specimen. In the case of corrosion tests with the medium above the specimen the loading scheme shown in Fig.la is recommended. The temperature is an important facto determining the test results. The data of investigations[6] showed that negative temperatures down to -196can be obtained in static loading( by local cooling of the specimen with liquid nitrogen or its vapors).In the case of long-time tests it is expedient to cool the entire machine by placing it in a cooling chamber. Heating the specimen to temperatures 200-250can be done by electric heaters placed above the specimen. For better heat exchange there should be a layer of mineral oil above the specimen which is intensely agitated during testing. The initial data for designing the machine are the mechanical properties of the sheet metal, range of thicknesses to be tested, and the loading conditions. The 10-year experience of the design and operation of such machines indicates that in the case of a single static load the maximum pressure in the hydraulic system should not be above 600-700 gauge atm and in the case of repeated static loading it should not exceed 150-200 gauge atm. The main parameter determining the dimensions and construction of the machine is the size of the hole of the die 2r, which depends on the thickness t of the specimen. We will consider the selection of the value of the ratio r/t for the different test schemes. For scheme a (see Fig.1) an increase of the ratio r/t is accompanied by an increase of membrane tresses in comparison with the stresses in the specimen from bending, which is undesirable, since membrane stresses affect considerably the rate of development of fracture and can hamper an analysis of the test results. In addition, the pressure under the specimen required for testing increases with a decrease of the ratio r/t. Taking into account the considerations expressed above, we recommend selecting the ratio between the radius r of the die and the thickness t of the specimen in conformity with the inequality where σy is the yield point of the material; E is the modulus of elasticity; P is the maximum pressure under the specimen. In static tests of specimens restrained about the contour (Fig.1, scheme b) the ratio r/t is determined in the following way. The experimental data showed that the bending component of strain in the central part of the specimen is characterized by the expression where Ψb is the logarithmic necking deformation in fracture by uneasily tension. It is desirable that by the time of fracture the value of ebend does not exceed 0.03, for which fulfillment of the inequality is required. Moreover, a considerable pressure P is necessary for fracture of the specimen in the case of small values of the ratio r/t. Using the studies of Tomlenov [7], Sandier and Khodulin [8], we can recommend a dependence of the ratio r/t on strength, plasticity of the material, and pressure P for fracture of the specimen: where Sk is the true resistance to breaking. From relationships (3) and (4) we must take the larger r/t.Taking into account that during the test the radius of curvature of the specimen can decrease not only as a consequence of deformation of the metal but also due to slippage of the flange part in the fastening, we must increase the ratio obtained by 10-15%. To prevent the flange from being pulled into the die, the width of the flange part of the specimen should be not less than 0.25-0.3 of the diameter of the die. In testing specimens in the form of a reshaped spherical segment the radius of the segment R should be selected from the inequality which lessens the influence of the edge effect [8]. For testing by scheme c (see Fig.1) we use a specimen in the form of a cylindrical panel. The cylindrical panel should form an arc in the cross section with a central angle not less than 120-160.The length of the panel (without the flange part) should be not less than 2.5-3 radii of its cross section. To increase, the working zone of the specimen, the die should be made in the form of an oval whose major axis coincides with the generatrix of the panel. Fig.2 Designs of machines for biaxial tension testing The dimensions of the hole of the dies of the testing machines designed at the N.E.Bauman Moscow Higher Technical School (MVTU) and Rostov-on-Don Institute of Agricultural Machine Construction (RISKHM) in accordance with the considerations expressed above are presented in Table 1. Various devices(Fig.2) differing mainly in the design of such assemblies as the locking assembly 1, die 2,and hydraulic clamp 3 are used for fastening the specimen 4 during the test. The hydraulic clamp is used to secure the specimen about the contour. When testing according to schemes b and c(see Fig. 1) a reliable restraint of the flange part of the specimen must be provided. The force N of pressing the specimen against the die is assigned in relation to the diameter of the hole 2r of the die and pressure P under the specimen: The locking device prevents mutual displacement of the die and hydraulic clamp. During testing of the specimens the locking device absorbs considerable loads(from 200 to 5000 tons),and therefore special attention must be given to the design of this assembly. The simplest design of the locking device is shown in Fig.2c. The hydraulic clamp and die are fastened by bolts uniformly arranged about the circumference. However, this design does not provide quick attachment of the specimen. Its uses expedient only in the case of long term tests, for example, in repeated static tests. Figure 2a shows the design of a locking element made in the manner of a hydraulic press. The design is simple and allows using standard equipment, but it is cumbersome and, what is especially important, the upper location of the hydraulic clamp limits access to the specimen during the test. The locking device shown in Fig.2b is made according to the operating principle of a bayonet lock. The die and the housing of the hydraulic clamp have pin projections which after twisting the lock ring rest against the corresponding projections of this ring. A device with screw jacks [9] is used for lifting the upper plate (when fastening and removing the specimen).Machines made in this manner are compact and highly productive. The use of this scheme can be recommended in the case of a fastening force of the specimen less than 1000 tons. In the design shown in Fig. 2e the locking device has the form of a block of frames. The frames are calculated so that their vertical elements experience tension with an insignificant bending component. The design is compact and the block system greatly facilitates assembly of the machine. In the machine the specimen is suspended from the die, which can be moved out of the block of frames on a special carriage [10]. It is expedient to use this locking device for fastening forces from 1000 to 5000 tons. The locking element made in the form of a ring (see Fig.2d) [11]permits a considerable reduction of its weight in comparison with the weight of the locking element made according to the preceding scheme. In addition, this scheme simplifies considerably the testing of specimens in the form of a cylindrical panel. Fig.3 Designs of hydraulic clamp of the specimen The hydraulic clamp has some special design features. To provide compactness of the machines the working stroke of the hydraulic cylinder is limited to the minimum value (40-60ram) sufficient for convenience of loading and unloading the specimen. Therefore, the ratio of the height of the piston to its diameter is so small that the danger of misalignment and jamming of the piston during operation. Jamming can be eliminated by increasing the gap between the piston and cylinder. The use of standard seals in this case does not provide the required tightness. Acceptable designs of seals are considered below. The designs of hydraulic clamps for a piston diameter less than 500 mm are shown in Fig.3a and for a piston diameter greater than 500 mm in Fig.3b.The clamp of the hinge assemblies also shown there (Fig.3c).In the first case the piston 6 is made in the form of a disk with a central hole in which the guide rod of tile cylinder 1 and the seal are located. The rod serves for eliminating initial misalignments of the piston and is used for delivering the fluid into the cavity under the specimen. The seal between the piston and cylinder wall is accomplished by means of a rubber ring