长期未解决的问题 meaning in Chinese

issue of long standing

As the variable stiffness of testing specimen affects load stiffness , the theoretical analysis and simulation study of the effect of variable load stiffness on the system are done . the degree and tendency of the effect of variable load stiffness on the control quality and accuracy are given , furthermore , a new extension control algorithm for material testing machine is presented in this thesis . by these algorithm , the problem existed and confused in testing machine engineering in a long term can be solved completely
针对试样刚度变化对负载刚度的影响问题，进行了负载刚度变化对控制系统影响的理论分析和仿真研究，从理论上分析了材料拉伸试样进入屈服阶段时试验过程产生震荡的原因，并研究开发了相应的可拓智能控制策略的解决办法，解决了国内外材料试验机工程界长期未解决的问题。