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New Design Concept for a Lifting Platform Made of Composite Material L Solazzi these structures have to reach great heights and obviously they have to satisfy the constraints induced by the highway standards like the maximum axle load and the maxi mum overall dimensions To satisfy these requests the material of the structures changed from the classic structural steel S235 JR S275 JR or S355JR to high strength steel S700 to S1100 or more characterized by a significantly higher specific resistance The idea of this paper is to use a composite material for the construction of the arms of an elevating platform in order to reduce the global weight of the machine The analyses on the new kind of platform show the technical possibility to change the material of the arms with composite materials and this produces a significant reduction of the weight of the machine components about 50 Being a feasibility study still remain open some problems such as the mechanical behavior of the used composite materials fatigue environment effects etc Keywords Compositestructuresdesign Carbonfiber Liftingplatform 1 Introduction Great highs platforms had a great diffusion in recent years Fig 1 its development was made possible thanks to the usage of high strength steels able to optimize the mechanical characteristics and allowing in this way the weight reduction in fact is fundamental in a movable structure to keep under control this peculiarity in order to guarantee the road transport Appl Compos Mater 2013 20 615 626 DOI 10 1007 s10443 012 9287 2 L Solazzi R Scalmana Department of Mechanical and Industrial Engineering University of Brescia via Branze 38 Brescia Italy e mail luigi solazzi unibs it However the height of these structures is increasing constantly bringing to their limit the transport possibilities The aim of this work is the design of a work platform with arms made both by high strength steel and by composite materials and its comparison it with a platform made only by high strength steel The platform is designed to work at a height of 85 m which represents a very high size for the Italian market There are prototypes constructed using for the different components arms cylinders joints metallic materials mostly high strength steel alloys that are increasingly used in work machines like equipment for the handling of materials 1 2 and truck components 3 5 which show great problems due to the great weight and also to the transport of the platform structure actually a small portion of the jib the last part of the telescopic arm is made of plastic material especially in the platforms used for the maintenance of electrical lines in order to introduce an electrical insulator between the operator in the basket and the rest of the machine The ultimate target is to reduce considerably the weight of the system thus allowing to increase the load capacity and reducing the mass which has to be transported by the handling truck allowing also the transport of the entire machine in accordance with the road stand ards A first application of composite materials for the realization of arms of trucks has occurred for cement mixers a company has recently been developed a concrete pump provided with an arm composed of 5 sections with the last two made of carbon fiber However the design specifications for that machine are very different both for the working area and for the load spectrum there is for example no limit for the displacement as in a lifting platform Starting from a preliminary geometry and design specifications the paper deals with all the major issues related to the implementation of composite structures 6 7 the definition of the required dimensions of the arm moving concept and the consequent devices the sizing of the arm sections the dynamical analyses and the failure verification with a finite element method Fig 1 A steel platform working on a wind turbine 616Appl Compos Mater 2013 20 615 626 2 Materials and Methods 2 1 Design Specifications The platform has to be connected with a truck and has to reach the maximum height of about 85 m at the extremity of the arm there is a device called JIB used for the fine movement of the platform the operating range of the platform is shown in Fig 2 The platforms with these characteristics that are commercially available are composed by 7 telescopic arms at which end is connected the JIB structure 2 2 Load Conditions The first development of carbon fiber instead of steel to realize working arms of heavy vehicle is findable in cement mixers where are used in order to replace the traditional steel booms this application presents substantial differences with the elevating platform in what concerns the load conditions and the allowed behaviour the principal differences are the supporting structure crane is divided in two functional groups the first four telescopic arms are com posed by high strength steel the second consist of four closable arms this choice is based on several considerations the use of long fibers instead of short ones is also justified because the work technique allows to realize the arms using specific molds the short fibers also have a lower fatigue resistance and are more difficult to joint with steel components 2 4 Properties of the Materials The used material for the closable arms chosen as the best compromise between the performance of the material and the related costs is a low modulus carbon fiber with an epoxide matrix their characteristics of interest are reported in Table 1 The fiber percentage of each layer is 60 with these data is possible to compute the mechanical characteristics of the lamina 7 reported in Table 2 The directions 1 and 2 depend on the fiber orientation and are defined in this case following the schema reported in Fig 3 Table 1 Mechanical characteristics of the materials composing the used lamina MaterialDensity kg m3 Young modulus E MPa Shear modulus G MPa Poisson ratio Epoxidic resin1 2004 5001 6000 4 Carbon fiber1 750230 00050 0000 3 618Appl Compos Mater 2013 20 615 626 2 5 Dimensioning of the Arms The size of the arms depends on several factors some of these are once computed the maximum reactions axial forces and bending moments the next step regards the definition of the arm sections and also the stresses using the FEM software ABAQUS The stacking sequence of the layers composing the laminate has to comply the following rules to drive the arm is used an hydraulic cylinder the computation of the reaction forces is made following the schema of Fig 4 All the kinematic system of the structure Fig 5 is schematized using the software MECAD developed by the Department of Mechanical and Industrial Engineering of the University of Brescia used for the computation of the reaction forces in each arm and Fig 4 Schema for the computation of the reaction forces in the structure of the first arm starting from the concentrated force of the platform F and the distributed load P dimensions in mm Fig 5 Schema for the computation of the reaction forces in the composite structure using the software MECAD 620Appl Compos Mater 2013 20 615 626 device as well as displacements velocities and accelerations of each part during the opening of the arms The computation of the stresses is made following the classical composite theory for two dimensional laminates 10 the considered stacking sequence is 902 45 45 04 s Introducing a reference system with the first two axis on the lamina and neglecting the deformation 3perpendicular to the lamina plane is possible to compute the deformations with the following formula 1 2 g12 2 4 3 5 S11S120 S12S220 00S66 2 4 3 5 1 2 t12 2 4 3 5 1 The Sijcomponents form the compliance matrix S are computed starting from the elastic constants of the lamina S11 1 E1 S12 n12 E1 n21 E2 S22 1 E2 S66 1 G12 2 To compute the stresses starting from the deformations is necessary to use the stiffness matrix Q the inverse of the compliance one 1 2 t12 2 4 3 5 Q11Q120 Q12Q220 00Q66 2 4 3 5 1 2 g12 2 4 3 5 3 These formulas are usable with lamina having the fiber reinforcement aligned with the principal axis direction if there is a relative angle is necessary to use the rotational matrix T T c2s22sc s2c2 2sc scscc2 s2 2 4 3 5 4 Fig 6 Schema for the computation of the stiffness characteristics of the laminate Appl Compos Mater 2013 20 615 626621 where c20cos2 s20sen2 moreover S T T S T Q T 1 Q T T 5 The expression usable for the computation of the stresses in a generic lamina becomes also x y txy 2 4 3 5 Q11Q12Q16 Q12Q22Q26 Q16Q26Q66 2 4 3 5 x y gxy 2 4 3 5 6 In case of a laminate composed of a certain number of lamina is possible to compute the forces and the moments Nx Ny Nxy Mx My Mxy 2 6 6 6 6 6 6 4 3 7 7 7 7 7 7 5 A11A12A16B11B12B16 A12A11A26B12B22B26 A16A26A66B16B26B66 B11B12B16D11D12D16 B12B22B26D12D22D26 B16B26B66D16D26D66 2 6 6 6 6 6 6 4 3 7 7 7 7 7 7 5 0 x 0 y g0 xy kx ky kxy 2 6 6 6 6 6 6 4 3 7 7 7 7 7 7 5 7 Were 0 i e kijare respectively the deformations and the bendings of the laminate in the principal directions in compact form N M AB BD 0 k 8 Fig 8 Values of the static safety coefficient in the first arm Fig 7 Values of the displacements for the arm in the specific load condition measures in mm 622Appl Compos Mater 2013 20 615 626 The components of the ABBD matrix are computable in the following way Aij X N k 1 Qij k zk zk 1 Bij 1 2 X N k 1 Qij k z 2 k z 2 k 1 Dij 1 3 X N k 1 Qij k z 3 k z 3 k 1 9 With Ziposition of the layer as shown in Fig 6 The components of the B matrix as the components A16and A26 are quite equal to 0 because the laminate is balanced and symmetric Is now possibly to compute the deformations and the bendings using the inverse ABBD matrix 0 k A0B0 C0D0 N M 10 Fig 10 First four vibration modes of the JIB arm without the basket a first mode 4 5 Hz b second mode 22 3 Hz c third mode 5 9 Hz d fourth mode 30 5 Hz Fig 9 Stress trend for each lamina in one of the most solicited regions of the laminate Appl Compos Mater 2013 20 615 626623 Due to the balanced and symmetrical disposition of the layers are obtained equal deformations in the lamina and bendings equal to 0 the greatest stresses are located in the 0 lamina and reach the value of about 100 MPa significantly below the breaking limit this allows to ensure in a first approximation a satisfactory fatigue resistance also seen the excellent characteristics of the carbon fiber 11 12 Following this procedure are determined all the sections of the arms which are realized with a variable section in order to optimize the flexional resistance by increasing the section dimensions where the bending moment is higher this characteristic very difficult to obtain with metal arms is practicable with composite materials realizing a suitable mold In order to evaluate the global mechanical behavior of the structure concerning aspects like global stresses displacements buckling coefficients is necessary to execute several analyzes using a finite element program 2 6 FEM Analysis The FEM analysis is performed using the ABAQUS STANDARD code the composite arms are modeled as shell elements while the metallic parts are modeled as 3D parts to each arm is given the defined stacking sequence of the laminate with the correspondence thickness of each layer to improve a correct behavior is given a local orientation for each arm as reported in Fig 7 Once computed the stresses is possibly to apply a failure criterion for the structure like the Tsai Hill one as shown in Fig 8 Is also possible to inspect the stresses in the laminate varying the position along the thickness the founded values are significantly different for each lamina due to the variation of the orientation the greatest stress value is findable in the 0 lamina as reported in Fig 9 Fig 12 Views of the entire structure in the reclosed position with the principal dimensions in evidence the sections of the arms measures in mm Fig 11 View of the entire structure in the reclosed position 624Appl Compos Mater 2013 20 615 626 The last performed analysis regards the calculation of the natural vibration modes of the arms in Fig 10 are shown the first four vibration modes for the JIB arm 2 7 Final Aspect of the Structure The final structure is composed by four closable composite arms and four telescopic steel arms the aspect is reported in Fig 11 In order to minimize the height the steel and the composite arms are connected in a staggered arrangement The principal dimensions of the structure including the sections of the composite arms are reported in Fig 12 The performed analyses show a great weight reduction due to the material change the density of the carbon fiber is 5 times lower than the density of steel the weight of the structural devices will conversely increase but globally there is a significant weight reduc tion as reported for the JIB in Table 3 Reprocessing the same numerical methodology for the other three arms in composite material the weight reduction is quite the same percentage as in the JIB up to 50 Regarding the costs is possible to make a comparison of the material costs between a composite arm and its equivalent made of steel The cost of a lamina made with low modulus carbon fiber STS fiber and epoxy matrix is about 35 m2and the cost for the high strength steel yield stress about 1 300 MPa is 4 kg considering the thickness 0 63 mm and the density 1 530 kg m3 is possible to compute the material cost per kg equal to 34 and so the total material cost of the composite arm reported in Table 4 The total cost of the machine comprehends even many other factors like the mold cost and the quantity of manufactured products which are not currently computable 3 Conclusions This paper considers the design of a new concept of platform able to optimize the most important characteristics for this purpose was studied the possibility to replace the steel arms of a platform using composite materials this required a new type of design analyzing the possibilities given by various materials the choice fell on a long fiber composite Table 4Material costs of the JIB of the steel platform and of the composite one Arm cost Devices cost Total cost Steel platform2 5306403 170 Composite platform5 7807206 500 Table 3Weights of the steel and the composite JIB MachineArm weight kg Devices weight kg Total weight kg Steel platform632160792 Composite platform170180350 Percentage variation 73 12 56 Appl Compos Mater 2013 20 615 626625 The first analyses show that this substitution combined with new handling sys tems allows a significant weight reduction up to 50 this permits its application for extremely high platforms mainly used in wind power systems where the dimensions are increasingly growing that otherwise would not be employable and drivable on the road network There remain some open issues related to the use of composite materials such as fatigue and local buckling and the interaction with the environment The final aspect regards the costs which increase significantly in comparison with steel but globally will be profitable if compared to a machine that could not travel on the road due for example to the excessive weight References 1 Solazzi L Design a scrap loader using different alloy steels II International Conference on Super High Strength Ste