轮轨黏着系数对钢轨直裂纹瞬态扩展行为的影响

采用ANSYS/LS-DYNA建立了含有钢轨直裂纹的三维显式有限元模型，考虑了高速条件下轮轨间的瞬态滚动接触和裂纹面之间的滑动接触，于时域内分析了轮轨滚滑作用下裂纹面之间的瞬态法、切向接触和裂尖动态应力场强度因子。轮轨接触和裂纹面间的接触，均采用“面-面”接触算法定义，切向接触满足库伦摩擦定律。裂尖应力场强度因子采用虚拟闭合法计算，不考虑裂纹面之间的间隙，即两个裂纹面上的离散节点完全重合。牵引工况的计算结果表明，滚滑状态下的轮轨法、切向接触力不随轮轨黏着系数的增加而变化，但裂纹面间的最大法、切向接触力却随黏着系数的增加而不断减小。车轮滚到裂纹时，钢轨的弯曲变形使得两个裂纹面相互挤压闭合，即K_I值为0，而K_Ⅱ最大值约为K_Ⅲ最大值的8.1倍左右。这意味着钢轨表面的直裂纹，如果可以扩展，其在无横移牵引工况下的扩展应该以Ⅱ型撕裂为主导。随着粘着系数由0.1增至0.5时，无横移牵引工况下K_Ⅱ最大值仅增大3.98%，而K_Ⅲ最大值反而降低20.8%。

INFLUENCE OF WHEEL-RAIL ADHESION COEFFICIENT ON TRANSIENT PROPAGATION OF A VERTICAL RAIL CRACK

A 3D explicit finite element model has been established with ANSYS/LS-DYNA to simulate the high-speed transient rolling contact between a wheelset and a cracked rail, for which a vertical rail crack is considered. By defining another contact pair, the normal as well as tangential contact forces between the crack faces and the resulted dynamic stress intensity factors (SIFs) at the crack tip are analyzed in the time domain. The virtual crack closure technique is employed to calculate the SIFs, so that the discrete nodes on the two crack surfaces are kept coincident. No gap between the crack faces has been considered. The wheel-rail contact and the contact between crack faces are both defined by a ‘surface-to-surface’ contact algorithm, and the Coulomb's law of friction is implemented to treat tangential contact problems for both contact pairs. The results under traction have shown that the normal and tangential contact forces between the wheel and rail do not change with the adhesion coefficient, while those between crack faces decrease correspondingly. As the wheel reaches the crack, the two faces are pressed against each other due to rail bending under wheel loading, i. e., the crack is closed and K_I is zero value, while the maximum of K_Ⅱ is 8.1 times that of K_Ⅲ. This means that mode II shear probably dominates the propagation of the vertical crack, if it can, under traction conditions and without lateral shift. With the increase of the adhesion coefficient from 0.1 to 0.5, the maximum of K_Ⅱ increases by only 3.98% under traction conditions and without lateral shift, while the maximum of K_Ⅲ decreases by 20.8%.