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以有限元方法(FEM)为基础的计算机程序是被公认的,而且还广泛地用于内燃机零部件的高精度强度分析。为了研究简化的三维物体的效果和使用两维有限元方法,在下列的内燃机零部件上和负荷情况下使用三维和两维有限元模型进行应力分析:1.一个半组合的柴油机曲轴由于轴颈的过盈配合和弯曲力矩而引起的曲柄销圆角上的应力。2.一个带有分散的内部加强筋的大缸径柴油机活塞顶由于燃烧压力而在活塞顶和加强筋之间的过渡圆角上引起的应力。计算的应力值和在实物尺寸的模型上用应变片测得的应力值作比较。如果实物尺寸的模型和柴油机零部件的实际形状相同,而且在要进行应力分析的区域内元素网格足够细密,则用三维模型计算的应力值在应变片测量值的公差范围内。用两维模型的一种组合可以精确地计算曲柄销圆角处的过盈配合的应力。将三维分析的位移作为边界条件可以令人满意地计算出设计细节,如圆角的不同形状。采用这样的方法,例如使用圆筒形壳体而不用单独的加强筋来模拟整个三维物体或其某些部份将会使得重要的设计细节部分的应力分布情况面目全非。看来用于三维物体应力计算的两维模型的主要用途是将三维模型计算所得的位移作为边界条件来分析各种设计细节。
Computer programs based on the finite element method (FEM) are well-established and are also widely used for high-precision intensity analysis of internal combustion engine components. In order to study the effects of simplified three-dimensional objects and the use of two-dimensional finite element methods, stress analyzes were performed using three-dimensional and two-dimensional finite element models on the following internal combustion engine components and under load: 1. A semi-combined diesel engine crankshaft due to journal The interference fit and bending moment caused by the crankpin fillet stress. 2. Stress in a transitional fillet between a piston crown and a tendon due to combustion pressure on a large bore diesel engine piston with dispersed internal stiffeners. The calculated stress values are compared with the stress values measured with strain gauges on a physical dimension model. If the physical dimension model is identical to the actual shape of the diesel engine component and the element mesh is fine enough in the area where stress analysis is to be performed, the stress values calculated with the three-dimensional model are within the tolerances of the gage measurements. Using a combination of two-dimensional models, the stress of the interference fit at the crankpin fillet can be accurately calculated. Using the displacement of the 3D analysis as the boundary condition can satisfactorily calculate the design details such as the different shapes of the fillets. Using such a method, for example, the use of a cylindrical shell instead of a separate stiffener to simulate an entire three-dimensional object or some portion thereof will make the stress distribution of critical design detail unrecognizable. It appears that the main purpose of a two-dimensional model for stress calculations of three-dimensional objects is to analyze various design details using the displacements computed by the three-dimensional model as boundary conditions.