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双轴拉伸试验机的设计 S.A.Kurkin, V.F.Lukyanov, M.N.KrumbolDT当加载内部压力时，焊接薄板结构及其外壳将受到双向拉伸的张力。在这种情况下，往往有一个灵敏度增加的金属片，由于局部应力集中的存在或者由于与制造工艺相关的金属的机械性能的局部改变，从而导致结构的强度明显下降。因此，在至关重要的薄板结构的材料和制造工艺的选择时，平时所进行的单轴试验所获得的数据是不够的。这就需要测试的大量的组装件和最大限度地反映实际工作条件下的金属基材和焊接接头的模型。这样的测试对检查其生产的最后阶段的结构强度是很重要的，但他们是相当昂贵的，并且作为一项惯例，由于过早失败，传达的信息也不足。在这方面，我们应该进行测试，测试应该充分地反映机构在实际工作条件下，并且在实验室调查的方法将适用。参考薄板结构我们应该考虑到这样的测试：a）应力状态（主应力分量下的双向平等或不平等）；b）载荷特性（静态或反复加载不同周期的静态）；c）环境的影响；d）工作温度的影响这些试验设备应该容易设计，并提供一份高输出的调查。在本文中我们将描述在双向应力的状态下，测试金属和焊接接头机器的设计实验。在 1，2 的分析中表明，通过静压弯曲的方法，金属和薄板结构的焊接接头的工作条件在测试中被体现得最充分。在这种情况下，标本是安全的，并且负载了液压。 图1 产生双向拉伸的加载方案应力状态的金属所产生的力取决于试样的形状和模具(图1)。因此，支持在一个平面标本静态荷载压力情况下对管芯圆轮廓进行，双向弯曲产生的张力和标本的外表面的凸出面的相当多的部分受到的张力与平等的组成部分张力均为1 =2（图1a）。如果平面标本被可靠的固定在管芯圆轮廓上，双向拉伸将被取代为双向弯曲。在加载的情况下，不是一个平面标本，而是一个模具的孔的直径和标本的厚度的比率足够大的球形标本，弯曲的部分是小的，并且我们可以认为试样的中心部分受到的双向拉伸为2/1=1。比率在2/1 = 1.0-0.75之内的不等组件的双向拉伸张力，可通过使用了具有椭圆孔的模型的标本的屈曲来根据给出的图表1b得出。进一步的减小比例在2/1（0.7-0.3）的双向拉伸张力是通过在图1c所示的计划方案得到的，在此比例中，由法兰部份抑制圆柱之间的模具和冲压圆柱形式的圆柱面板形状的标本加载静水压力3。工作压力下薄板结构的两种特征载荷的类型：单（静态）和低循环（重复静态）。第一，使用计划A和B是有用的（见图1）。根据试验计划，静态加载可以做到在平面形式的标本，以及在预成球形部分的标本上4，5。薄板是可取的，因为他们的制造耗时少。在球形段形成固定标本许可减少对轮廓试样的边缘效应的影响；然而，这些标本的制备要求塑性变形，从而会导致改变材料的组织和性能，并不总是可求的力学性能的变。所有这三个方案如图1可用于在低周反复荷载条件下进行测试。然而，根据方案a应优先考虑双向弯曲，因为后者允许更大的测试厚度。加强与应力均匀分布的区域，标本的轮廓可以被铰接到死点。根据方案b和c(参阅图1)，低周载荷测试在一个球形或圆柱形面板形式的预成标本上进行。必须考虑到,在相当大的弯曲应力的标本的附件的地方，超过了应力在样板中心的弯曲应力。周围介质在一个长期和反复的静态测试下，具有特别是强烈的效果。引起我们极大的兴趣的是，在腐蚀性环境中反复加载静载荷情况下的材料寿命。从环境影响角度来看，后者可以被用来作为压力下的标本的工作流体。在这种情况下,现在的任何试验方案都被使用了。为了防止主系统和机械零件腐蚀，腐蚀性液体放置于标本之下并且被从主要工作流体与活塞密封装备分开。位于下腔试样的弹性膜起到着同样的作用。这个方案的主要缺点是不能观察断裂的过程。因此，在这些情况下的没有压力的腐蚀性介质是没有益处的，强腐蚀性的介质应放置在标本上面，其更换应该容易，因为它是污染腐蚀产物，并通过视觉和静止画面摄影手段观察断裂的过程。为防止上板腐蚀可以涂上一层环氧树脂漆,或圆形橡胶防止腐蚀介质超出了被测试样品的极限，可以粘在标本的局部，限制扩散层。在与上述标本中的腐蚀试验情况下，加载的方案建议如图1a所示。温度是确定测试结果的一个重要因素。调查的数据6表明负温度降到-196o，在静载荷（通过用液氮或蒸汽局部冷却的试样）的条件下可以得到。在长期试验的情况下，把它放置在一个冷却室来冷却整个机器的方法是合适的。加热的温度约200-250o，标本可以通过上述试样放在电热器上。为了更好的热交换应该有应该有一层以上的标本，在测试过程中试样油层是激烈运动的。本机设计的初步数据是板材的机械性能，是在厚度范围内进行测试，并是在加载的条件下得到的。这种机器的设计和操作的10年经验表明，在一个单一的静态负荷情况下，在液压系统的最大压力应不超过600-700测量atm，并且在反复加载的情况下，它不应该超过150-200测量atm。主要参数是模型2r的孔的尺寸，这确定着机器的尺寸和结构,而这又取决于标本厚度t。我们将考虑不同测试方案的r/t的比值的选择。在方案a（见图1），r/t的比值的增加是伴随着薄膜应力和标本弯曲应力的对比的增加而增加的，这是不可取的，因为薄膜应力影响较大的裂缝发生率，因此可以阻碍了测试结果的分析。此外,压力测试所需的标本的增加是随着r/t比值的减小的。考虑到上述表示的考虑，我们建议选择符合的比率是在模具半径R和试样厚度t的不等式：其中y是材料的屈服点; E为弹性模量; P是标本下的最大压力。在标本的静态试验中，对轮廓的克制（图1，方案B）的比值R / T是按以下方式确定的。实验数据表明，标本的中心部分的拉伸张力产生的弯曲由表达式描述，其中是对数缩颈单轴拉伸变形断裂。 其中的最好的断裂时间的值不超过0.03，为此满足其中的不等式:是非常必要的。此外，有相当多的压力P对试样在R /T为小比值的情况下的断裂是有必要的。利用Tomlenov7，Sandier和Khodulin 8的研究，我们可以推荐一对强度，材料可塑性，压力P为试样断裂比率关系:其中的sk是真正的抗断裂数。从(3)和(4)的关系可得出，我们必须采取更大的r/t的比值。考虑到在测试试样的曲率半径，不仅可以减少由于金属变形的结果，而且可以减少由于紧固法兰的延误，我们必须增加10-15的比例获得。为了防止法兰被拖进模型,法兰宽度部分标本应不少于模型直径的0.25-0.3。在一个预成球台形式标本的测试中，半径段R应选为符合不平等式：从而减少了边缘效应的影响。为测试方案c(见图1),我们用一种形式为圆柱面板的标本。圆筒状弧形板应该在断面上,并且有一个中央的角度不少于120-160o。该面板长度(没有法兰部分)不应小于2.5-3其截面的半径。为了增加其数值，模具应在一个椭圆形，其主要轴线与面板的母线重合。图2 双向拉伸试验机的设计对检测机的模型的孔的尺寸设计，参考了莫斯科东北鲍曼高等工业学校（MVTU）和农业机械建设研究所（RISKHM）的罗斯托夫设计试验机，上述表示的考虑皆按照表1。各种各样设备的主要不同之处在于装配设计上面，比如锁定装配1,模型2和液压钳3在检测中是用于固定标本4的。液压钳被用于确保标本的轮廓。当根据方案b和c(参阅图1)测试，标本的一个可靠约束的法兰部分必须予以提供。该模具对试样的压力N，模型的孔2R的直径与标本之下的压力P有以下关系：锁紧装置防止模具和液压钳的相互位移。标本在测试时，锁紧装置吸收了相当大的负载（由200至5000吨）,因而对组装设计必须给予特别注意。 最简单的锁定设备的设计如图2c所示。液压钳和模具是固定在均匀布置有螺栓的圆周上。然而,这种设计不提供标本的迅速附件。只有在其长期试验的情况下才使用,例如,在反复的静态测试。图2a显示锁定元素的设计是在液压下进行的。设计很简单,并允许使用标准的设备,但它是麻烦的,特别重要的是,在检测中液压钳的上部位置限制标本的接近。锁定装置如图2b所示，它是根据插销栓的操作原则来制作的。模型和液压钳的外壳被固定住，这是当锁止环弯曲到与其设计的一致的时候。 带螺纹千斤顶的设备9是用于解除上盘（当紧固和拆卸标本时）。这种方式使得结构紧凑和高效率的工作。这个方案，建议适用紧固力在1000吨以下的标本。在图2e所示的设计，锁定装置具有块的结构。该框架的计算，使他们的垂直元素与一个很小的组成部分承受弯曲张力。设计紧凑，并且块体结构系统大大方便了机器的组装。在机器中，标本是悬挂在模具上的，可移动的块框架在一个特殊的运输装置上起到阻挡的作用10。使用从1000至5000吨的锁紧力的锁定装置是便利的。锁定元素在一个环的形式（见图2d）11允许与该锁定元素重量大大减少，其重量比按前面的方案作出规划。该标准密封在这种情况下使用不提供所需的松紧度。可接受的密封件的设计应该考虑以下内容。为活塞直径小于500毫米和活塞直径大于500毫米的液压夹具的设计分别如图3a和图3b所示。该钳的铰链的组装也如图3c所示。在第一种情况下，活塞6是在磁盘的一个中心孔处，其中缸瓦1的导杆和密封圈也放置在该处。该杆的作用是消除活塞的初始失调，并用来传递流体进入标本下面的腔体。活塞与缸壁之间是通过一个圆截面的橡胶圈和一个T形截面2的钢垫圈来实现密封的。活塞组装后，橡胶圈的预紧是通过固定栓5的方法实现的。在测试过程中，活塞下的流体压力的增加使得垫圈变紧，这保证了活塞的密封是可靠的。橡胶圈4防止流体通过固定栓的孔流出。这种密封设计提供了方便的密封液压钳的密封圈，甚至允许活塞下腔有0.3-0.5ram的间隔。橡胶密封圈的截面应不小于100平方毫米。当活塞的直径超过500mm的时候，它应该设计为环形（见图3b），这种情况下密封组件的设计是比照上述。橡胶密封圈的截面不应超过150-200平方毫米。 图3 标本液压钳的设计参考文献1Ya.B.Fridman等人.双向拉伸下板材的习性.合金和有色金属的研究俄罗斯，4号（1963）2W.E.Copper.压力容器设计的拉伸试验的意义.焊接J.36 ，2号（1956）3S.A.Kurkin和N.S.Meshaikin.圆柱面静压屈曲下的板材和焊接接头的测试.Svarochnoe Proizvodstvo，7号（1970）4S.A.Kurkin 和 V.F.Lukyanov.基于双向拉伸条件下的测试结果的焊接薄壁容器设计的评价.Svarochnoe Proizvodstvo,9号(1965).5B.A.Drozdovskii等人.碳含量对张力状态下钢板结构强度的影响.Obrabotka Metallov,5号(1964).6S.A.Kurkin和D.I.Umarov.双向拉伸试验机板材和在高低温度条件下的焊接接头.IZV，VUZ,Mashinostroenie,2号(1968).7A.D.Tomlenov.塑料的受压状态和扩展过程的稳定性.金属的压力成形问题俄罗斯，Izd.AN SSSR,莫斯科(1958). 8N.I.Sandler和A.K.Khodulin.薄板金属双向拉伸的测验仪器.Zavod.Lab,12号(1951). 9S.A.Kurkin等人.焊接薄壁容器的模拟式双向拉伸试验机.Svarochnoe Proizvodstvo,5号(1965). 10V.F.Lukyanov等人.作者的证书号：254177，Byul.Otkr,Izobr, Prom.Obr,Tov.Zn,31号(1969). 11S.A.Kurkin等人.作者的证书号：261745，板材和焊接接头的双向拉伸试验机.应用于1968年11月27日，Byul.Otkr,Izobr,Prom.Obr,Tov.Zn,5号(1970).DESIGN OF BIAXIAL TENSILE TESTING MACHINESS.A.Kurkin, V.F.LukWyanov, and M.N.KrumbolDTWelded 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 tensionThe 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 components1=2(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 with2/1=1.Biaxial tension with unequal components within the ratio 2/1 = 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 2/1(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 investigations6 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 inequalitywhere y 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 expressionwhere b 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 inequalityis 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 inequalitywhich 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 testingThe 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) 11permits 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 specimenThe 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
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