Figure 1. Universal Hydraulic Testing Machine

Transcription

1 Mechanical Teting Mechanical teting i a general ter which refer to a broad range of activity involved with the deterination of echanical propertie and behavior of aterial, tructure and achine. Typically, thi involve applying oe type of tiulu and eauring a repone. The tiulu ay be either tatic or dynaic in nature and ay act in a localized or a ditributed anner at ingle or ultiple location. In a iilar way, the repone eaureent ay be ade at one or ore point or in a ditributed anner. Expanding on thi definition, echanical teting i the proce of applying force, preure, diplaceent, heat, or iilar tiuli to a echanical yte and then eauring it repone. The objective, or coure, i to characterize the behavior of the yte either in order to obtain aurance of the perforance or to verify the one hand, echanical teting ay be carried out priarily to verify the overall perforance of a echanical yte, i.e., the load carrying capability of an aircraft wing, or the thrut production of a rocket otor. On the other hand, echanical teting ay be ued to verify the appropriatene or accuracy of a particular theory. In thi cae, the teting i uually directed at a iplified coponent or odel that ay actually bear no outward reeblance to a practical deign. A third cae could be defined to include thoe tet carried out priarily to characterize the echanical propertie of a new aterial. A with the econd cae, the teting uually involve working with rather abtract pecien which bear no reeblance to a practical product. In all of thee ituation, the tet ethodology ay be either deterinitic or probabilitic in nature. In the firt cae, the approach i to carry out thorough and extenive tet with a ingle or at ot a very few pecien which are aued to be repreentative of a larger et. Thi i often preferred when the pecien i cotly or the tet etup i coplex. The other approach i to ue a large nuber of noinally identical pecien which are aued to contitute a uitable ubet or aple of the larger et of all poible pecien. Statitical ethod are then ued to analyze the data and to draw concluion about the propertie of the whole et. In the Structure Lab coure, the ajor ephai will be directed toward exaining the variou technique, intruent and interpretative ethod for carrying out echanical teting. At the ae tie, thi knowledge will be ued to perfor variou echanical tet for the purpoe of verifying the theorie developed in the copanion coure. Relatively little ephai will be placed on exaining ethod for overall perforance teting. Tet Machine Tet achine or teting achine i the nae ued to refer to any one of a variety of device for applying a controlled load to a pecien. The load ay range fro a few gra to illion of pound uing achine currently available. The load ay either be tatic in nature (changed lowly fro one level to another), or ay vary rapidly at rate up to ten or hundred of cycle per econd (Hertz). In general there are two principal coponent to a tet achine: a loading echani, and a weighing or load-eauring yte. A nuber of different approache have been ued in the deign on each of thee coponent, and conequently, it i logical to decribe tet achine deign by exaining each coponent eparately. The ection will be concluded by decribing the characteritic of everal of the tet achine available in the Aeropace Engineering laboratorie. AE3145 Tet Machine Page 1

2 Loading Mechani Load generation in ot tet achine i done either hydraulically or echanically. Electroagnetic and theral technique have been eployed in a few pecialized cae. Hydraulic: Hydraulic loading echani ue a piton in a cylinder actuated by preurized oil or hydraulic fluid. A hown in Fig. 1, the cylinder i bored in a large aive tructure which for the tationary bae of the tet achine. For iplicity and ruggedne, the piton i deign for unidirectional operation, that i, for applying load in only one direction. It i connected to a oving table on which the tet pecien can be placed. A fixed crohead i upported above on colun attached to the bae o that a copreive load can be applied to a pecien located between the table and crohead. The tet achine daylight pace i the axiu vertical opening between the table and crohead. Typically, the axiu piton troke i about 2, and in order to accoodate a variety of pecien height, the fixed crohead can be adjuted vertically. Thi i uually done by uing large threaded rod for the upporting colun and fitting otorized nut in the crohead. Tenion load can be applied if a driven crohead attached to the table and piton i located above the fixed crohead. Specien are then attached between the fixed and driven crohead. Thi type of deign i frequently referred to a a Univeral Tet Machine becaue of it ability to generate either tenion or copreion load. Hydraulic deign are uually the leat expenive in the larger achine ize and are capable of generating the highet load available. Figure 1. Univeral Hydraulic Teting Machine The load control i obtained by regulating the hydraulic preure in the cylinder beneath the piton. Thi i done by ean of two needle type flow control valve. One valve i connected between pup and the cylinder and i ued to control the buildup of preure. The other i connected between the cylinder and the reervoir and i ued to control the bleedoff of preure. The two valve function are oetie incorporated in a ingle control valve, or in other deign the control unit i iply fitted with eparate loading and unloading valve. Due to the double acting nature of the control and the all leakage alway preent around the piton, it i AE3145 Tet Machine Page 2

3 nearly ipoible to aintain a contant load or crohead poition. However, the load can uually be applied oothly epecially at high level. The newet type of hydraulic tet achine i the electrohydraulic deign. The load i generated in the ae anner a above but the control i accoplihed by ean of an electrically activated 4-way valve. A typical configuration i hown in Fig. 2. Due to the reverible 4-way control valve, it i now poible to ue a double acting hydraulic piton or actuator which i capable of forcefully advancing or retracting. Thi i accoplihed becaue the control valve can continuouly vary the flow fro the pup to either ide of the piton while connecting the oppoite ide to the tank. A a reult, it i not neceary to provide eparate tenion and copreion pace. Both tenile and copreive load can be applied to the pecien without having to relocate or reattach it. In fact, the load can obviouly be revered fro tenion to copreion or vice vera during the ae tet. Figure 2. Typical Electrohydrualic Tet achine Control of an electrohydraulic achine i coniderably ore coplex but the reulting capabilitie are far greater. Since the valve i electrically operated, it can readily be controlled by electronic circuitry or even a coputer. The typical control yte ued a concept called feedback control that i coonly eployed in a large nuber of proce and indutrial control application a well a for any purely electronic circuit deign. In thi approach, an electrical ignal generated by the weighing yte i fed back to the control yte where it i copare with a coand ignal repreenting the deired load. If the difference or error ignal i zero, the valve poition reain unchanged, but if it i nonzero, the valve will be electrically driven in a direction to reduce the error. With proper table adjutent, the load will cloely follow the valve pecified by the coand ignal which itelf ay be generated by a anually operated knob, a function generator, a coputer, or a previouly recorded ignal (on agnetic tape, for exaple). In typical tet achine deign, the valve can be driven at frequencie up to everal ten of Hertz o that dynaically varying load can be generated. Thi i invaluable for fatigue and fracture teting. On a ore general note, the feedback ignal need not be proportional to load but can be derived fro actuator diplaceent, velocity, or to pecien train, for exaple. Thee variable then becoe the controlled or independent variable in the tet. A a final note, thi ae technology wa originally developed to provide boot force for aircraft control AE3145 Tet Machine Page 3

4 urface in the early 1950 and i ued today in fly-by-wire deign in which the only pilot connection to a control urface i by ean of the electrical coand ignal. Mechanical: Mechanical loading echani eploy a crew yte to generate a load in uch the ae way a a crew-jack operate. In the ot coon configuration, a hown in Fig. 3, the load i produced by the oveent of a driven crohead toward or away fro a tationary crohead or the achine table. The driven crohead i activated by threaded upporting colun which are rotated by a otor and gear drive ounted in the achine bae. The figure how a univeral configuration with eparate pace for copreion and tenion. Mot odern achine ue a ingle opening and rive the crohead in either direction. Thi i ipler but require pecial nut in the crohead which do not exhibit any echanical hyterii or lack when the load i revered. Mechanical or crew-jack type achine are generally liited to le than about 500,000 lb. capacity and are ore cotly than hydraulic achine in the larger capacity range. Mechanical deign are often cheaper in the all ize of le than about 2000 lb. capacity. Mechanical achine are oewhat harder to control at very high loading level but they can hold a contant crohead poition eaily and thu can be ued to aintain load for long period of tie. Figure 3. Typical Mechanical (Screw-jack) Tet Machine Control of echanical loading echani can be accoplihed in the iplet cae by uing a reverible variable peed otor with echanically or electrically electable gear ratio. A ore coplex approach involve uing a for of ynchro control called and aplidyne. The haft poition of a ynchro otor i laved to a uch aller ynchro tranitter which provide the control input. A the haft of the ynchro tranitter i rotated anually, the laved ynchro otor follow exactly. Thi otor i connected through gearing to drive the crew colun. Thi ake for iple and convenient anual control. Autoatic control can be provided by rotating the tranitter with a all geared clock otor or ele the ynchro ignal can be electrically created in a coputer controlled ynchro yntheizer. AE3145 Tet Machine Page 4

5 Other: Many other technique have been ued to produce controlled force but few are coonly ued in tet achine deign. Two technique deerve ention, however. Electroagnetic actuator iilar in concept to audio peaker have been eployed to generate force for tet purpoe. The ingle advantage in thi approach i the dynaic perforance obtained, but thi ha in recent deign been nearly equalled by electrohydraulic achine. The diadvantage i that axiu force are typically in the range of a few hundred pound. Another approach involve uing the theral expanion of a confined aterial to produce a force. A uitable aterial i lowly heated or cooled to produce an expanion or contraction. If it along with the pecien i confined between two rigid crohead, the reult will be a copreive or tenile load in both the pecien and the driving aterial. Thi approach reeble the echanical crew type achine, but it i capable of uch lower loading rate with little of the hyterii often encountered in echanical achine becaue of the crew/nut interface. Weighing Mechani Weighing or load eauring echani are eployed in tet achine to indicate the agnitude of the force generated. In contrat to the cae for loading echani, weighing yte are coniderably ore varied in concept although the current deign alot univerally utilize electrical reitance train gage load cell. The different approache are decribed in the following ection. Mechanical: Mechanical weighing yte eploy an arrangeent of bea, lever, and balance to indicate load in uch the ae way that balance bea platfor weighing cale operate. The cheatic in Fig. 3 how a iple balance bea and poie yte ued with a crew type loading echani. Thi require anual poitioning of the poe and i very cuberoe. The approach i ore coonly deigned with an autoated ean for poitioning the poie. One ethod i to attach the poie to a threaded rod or lead crew running the length of the balance bea and driven by a all electric otor. The poition of the bea i ened electrically and if not in balance, current i upplied to the otor to ove the poie in a direction to return to a balanced tate. Thi principle of eaureent in which the yte i alway returned to (or aintained in) the ae balanced tate i often referred to a a null balance eaureent and can be ipleented in either echanical or electrical ter. The balance bea approach often yield high accuracie and by change of poie weight can eaily be extended in range. It i, however, quite low. The concept i often ued in wind tunnel to eaure odel force (the yte are referred to a tunnel balance ). The balance bea can be replaced with a directly calibrated indicating yte. In thi approach the caled down force proportional to load that i tranitted by the bea i applied to a load ening eleent and the reulting deflection i ued to indicate the load. A coon ethod i to ue a pendulu which wing up in repone to a oent generated by the bea force. The pendulu poition i indicated on a large load dial. Elatic eber uch a cantilevered bea and torque tube can alo be ued. In thee cae the load i indicated by the reulting elatic deforation produced by the caled force. AE3145 Tet Machine Page 5

6 Hydraulic: Hydraulic weighing yte are of two ain type. The firt type i uable only with hydraulic loading yte and iply involve eauring the fluid preure in the cylinder beneath a ingle acting piton. The preure i ened by a conventional preure gage whoe dial i calibrated in unit of load. (The ot coon gage, called bourdon tube type, eploy a hood haped thin tube which when preurized tend to uncurl and by ean of gear and linkage to rotate a dial pointer.) Thi direct approach i by far the iplet, but due to friction force between the piton and cylinder, the axiu in accuracy ay not be obtained. Thi concept i illutrated in Fig. 1. Note that the ethod cannot be ued with bi-directional cylinder or actuator in which an oppoing preure ay be acting on the other ide of the piton. Figure 4. Hydraulic Tet Machine with Load Weighing Capule The other type of hydraulic weighing yte i copletely eparate fro the loading echani. A hydraulic capule (a ealed bellow, for exaple) i placed oewhere in the load path and the internal preure i ened by a preure gage or tranducer. A hown in Fig. 4, the capule can be located in a poition to eaure the full load reacted by the tationary crohead. The ue of a capule avoid the proble of piton friction noted above and a iilar iple bourdon tube preure gage can alo be ued to indicate the load. For either type, everal variation in the load indicating yte have been developed. One approach i actually a cobination of hydraulic and echanical technique. The hydraulic preure i ued to activate a all piton which in turn deflect a balance bea. The bea can be autoatically balanced a noted earlier or it can be coupled to a pendulu. Thi approach i often ued becaue variou indicator range can be incorporated iply by varying the balance bea fulcru location. Another approach involve uing a bourdon tube in a null balance configuration. A pneuatic actuator (bellow) i connected to the tip of the bourdon tube and i ued to hold the tip at it undeflected or null poition. Thu, a the load increae, ore force ut be exerted by the bellow to aintain the bourdon tube tip at the null poition. The load indication i derived fro the deflection of an elatic eber in the bellow load path. Thi AE3145 Tet Machine Page 6

7 deign i called a Tate-Eory weighing yte and ha the advantage of allowing or change in fluid volue take place in the bourdon tube. Strain Gage Load Cell: By far the ot popular approach ued in current tet achine weighing yte i to eploy train gage type force tranducer (or load cell) to eaure the load. Thee unit will be decribed in uch greater detail in ubequent ection when train eauring technique are dicued. Baically, however, the unit conit of an elatic load ening eleent whoe reulting deforation under load i ened by electrical reitance train gage. The reitance change i detected by iple electronic circuitry and aplified to either operate a large dial indicator or a digital diplay. The ignal i alo available for recording or proceing by a coputer. Thee load cell are generally bi-directional o they are ideally uited to electrohydraulic tet achine or other achine with a ingle tet pace. Tet Machine Characteritic Hydraulic and echanical tet achine have other ditinguihing characteritic that ay be ore or le deirable for pecific application. Soe of thee have already been noted. Two, however, deerve pecial treatent. Precribed Variable: Hydraulic and echanical tet achine differ in their loading characteritic in one ajor way. Mechanical deign utilize a precribed diplaceent of a crohead to produce a load on a confined pecien. In thi cae the diplaceent i the independent variable and the reulting force depend upon the tiffne of the pecien: F = k x where, k i the pecien tiffne and x i the crohead diplaceent. The force are higher for tiffer pecien and vice vera. Hydraulic deign on the other hand utilize hydraulic preure to produce a precribed force on the confined pecien. In thi cae, force i the independent variable and the reulting deflection depend on the pecien tiffne. x = F/k The deflection are thu lower for tiffer pecien and vice vera. Electrohydraulic tet achine are oewhat unique i thi repect becaue their characteritic can be changed depending on the choice of control variable. If the load a eaured by a load cell i ued a the feedback ignal, then load i the independent variable and the achine behave like a conventional hydraulic achine. On the other hand, if actuator troke i ued for the feedback ignal, then diplaceent i the independent variable and the achine behave like a crew type deign. Thee characteritic are particularly iportant depending on the tet requireent. Hydraulic achine are preferable in application where a relatively contant load ut be aintained even though all increental or continuing deforation occur. Thi i deirable, for exaple, in certain creep tet where it i neceary to eaure the pecien elongation with tie while under a contant load at an elevated teperature. Mechanical crew type achine are preferable in application where load intabilitie exit, that i, in ituation where an increent in load produce deforation that can only be aintained if the load i reduced. Thi occur in colun buckling tet a well a in tenion tet of aterial aple to deterine their echanical propertie. The latter cae i illutrated in Fig. 5 for a typical ductile aterial. If the tenile load i precribed (hydraulic achine) then the vertical axi ark the independent AE3145 Tet Machine Page 7

8 variable. The ituation i table until the loading reache A. Beyond thi point, the pecien can upport no further increae in load o that what happen i that path A-B i never recorded. Intead, the pecien rapidly elongate and failure occur alot intantly. Another way of viualizing thi i to note that for load above P o, there are two equilibriu deflection for a given load. It i thu poible at any tie for the pecien to jup over to a larger diplaceent tate provided the increental force i oentarily available (due for intance to a perturbation fro outide force). The key here i the ultivalued nature of the behavior for precribed loading. If a echanical tet achine i ued, the diplaceent arked on the horizontal axi i the independent variable. In thi cae there i no ultivaluedne and intability i not a proble. Intead, a the crohead i advanced, the load on the pecien rie to A and then fall off to the failure point at B. Figure 5. Typical Load-Deflection Curve for a Ductile Material Machine Stiffne: A ore ubtle factor in tet achine operation i the echanical tiffne of the achine through the load path. Two proble can arie if the achine i not very tiff copared to the pecien. Firt, for a echanical crew type achine, thi finite tiffne act like a pring in erie with the pecien whoe tiffne can alo be repreented by another pring a hown in Fig. 6. If the crohead otion i x and the pecien load i F, then it follow that F = k e = k e kx = And the relative deflection of the pecien and achine are thu given by: Specien: e = F / k Machine: e = F / k It alo follow that: x = F / k = F / k k / ( k + k ) [ ] Thu the pecien deflection (hortening in thi cae) i given a: AE3145 Tet Machine Page 8

9 ( k k ) e = F / k = xk / k = xk / + or by taking tie derivative, the deflection rate i: ( k k ) e = xk / + Thi reult how that for a finite achine tiffne, the actual pecien deflection will be le that the crohead otion which i precribed. For exaple, it the pecien and achine are of equal tiffne, the actual deflection rate will be one half the et crohead rate. Generally, it i deirable to ue a achine whoe tiffne i at leat an order of agnitude or better than the pecien tiffne. Figure 6. Stiffne Analyi of a Typical Tet Machine The econd factor concern the buildup of internal or train energy within the frae of the tet achine. With reference to the above reult, the train energy in the tet achine i the train energy in the equivalent pring: ( e ) 2 = 1 F k 1 2 k U = / 2 2 The tiffer the achine, the le train energy i tored for a given load. The preence and availability of thi energy can be a caue for concern in teting where intabilitie are preent. Given the right trigger, uch a an outide ipule or hock, thi energy i available to quickly upet the equilibriu of the pecien and poible reult in it udden failure. Machine that are very tiff are called hard tet achine, while thoe with a low tiffne are referred to a oft. With a very hard crew type achine it i often poible to watch the ultiate tenile failure of a ductile pecien a a gradual tearing apart of the aterial. The ae proce oberved in a oft achine i uually a rapid and oetie exploive fracture that i ipoible to ee with the unaided eye. AE3145 Tet Machine Page 9