The body of a floor model for modal analysis 1 ForewordIn the three major auto assembly, auto body representing the level of vehicle development, development of the car in the dominant position of possession. As the traffic process, the body structure will be in a variety of incentives vibration source under vibration, if these incentives frequency vibration source close to the body as a whole or in part, the natural frequency, it will happen resonance phenomenon, a violent vibration and noise, And even caused structural damage. Therefore, in order to improve vehicle safety, stability and comfort, it is necessary to the body structure of the natural frequency analysis, and its structure can be designed to avoid the various vibration source of encouragement. The paper was useded to the finite element analysis methods, the body of a floor model for modal analysis, analysis of its natural frequency and vibration mode, to provide reference for actual production.2 The establishment of the floor body finite element modelBody floor is a typical convex slot board structure, and its simulation modeling There are two ways, first by the true trough convex shape modeling and the other is in accordance with the method mentioned in the literature, that is used in the slot and convex Strengthen the beam to simulate the plate structure, strengthen cross-section of the beam parameters and the actual structure in line with the text of the original model using the first method.2.1Modeling2.1.1 Plane issue and thin bendingBody floor of the CAD model is the creation of the Catia software completed.Body parts are the most pieces of sheet metal, sheet metal thickness h plane is much smaller than its size. The load plate deformation and the role of the way, when the load in parallel to the surface (the equally flat plate thickness) and the thickness along the same direction, that is the plane stress problems if the load perpendicular to the surface, it will cause the curved sheet Deformation.To the plate in the face for the x-y plane, perpendicular to the surface in the shaft for the z axis. In the plane stress problem, only parallel to the x-y plane of the three stress components as:These three components along the same thickness h, which is a function of x and y, and z coordinates nothing to do, and the remaining amount is zero. Plane stress of the physical equation as:Plate bending deformation, in the face by a flat surface, known as flexible surfaces. In the face of all the points in the perpendicular to the surface in the direction of the displacement w known as the deflection. When w much smaller than the thickness t, that is met, you can think of in wireless strain has no contingency Kok, now known as the small deflection plate bending problems. If the deflection w t close to the thickness of the order of magnitude, you can not believe that flexible fiber surface, the length of the same, the problem will become non-linear, this situation known as the large deflection plate bending problems. Most of the works in question will be regarded as small plate of bending deflection problems, such problems can greatly simplify. Plate bending small deflection of the stress and physical deformation of the equation can be expressed as: To sum up, because the floor is a body of the more complicated pieces of sheet metal, in the role of the load by the time their plane stress there is the problem, there plate bending problems. So in practical engineering analysis, taking into account both the role of the two.2.1.2 Unit type In the establishment of Body Parts finite element model, taking into account both of these pieces of sheet metal to resist the direction parallel to its plane of Rafah pressure, but also to resist bending and reverse the load, should be used with both types of units. The unit has a shell thickness, the full description of these characteristics, the unit used to shell modeling.2.1.3 Definition of material Body flooring materials commonly used sheet steel, its elastic modulus E = 2.1x105 Mpa, Poissons ratio for = 0.3, the density = 7.9 x103 kg/m3. In a shell unit to modeling, comes plate thickness t = 1 mm.2.2 MeshingThe left side of the floor model of the mesh used finite element software HyperWorks in HyperMesh divided into modules, including the number of nodes for 7730, the number of quadrilateral element for 7306, the number of units to 370 triangles, triangular element of the entire unit to the percentage of the total 4.8%. In order to meet these technical requirements, the need for strict control of their cell size and the warping of the unit and the quadrilateral triangle and the interior angle of perspective, the left side of the floor plans for a grid model.Figure 1 left the floor grid model3 The calculation of results and analysis3.1 Modal analysisModal analysis is to study the structure of a dynamic modern way is t