Forced assembly is a common countermeasure for eliminating the interface gap during composite airframe assembly, which modifies the bolt-hole bearing behavior and brings failure risks. In this study, an experimental investigation on the effect of forced assembly on the fatigue behavior of single-lap, countersunk composite bolted joints is presented. The hole elongation and dynamic stiffness are obtained based on ASTM D6873. The S-N curves are fitted according to ASTM E739. The fatigue damage is observed through microscopy, and the residual strength is acquired by static tensile tests. The results indicate that forced assembly significantly shortens the fatigue life of joints. After forced assembly, the hole elongation evolves faster and reaches the fatigue failure threshold earlier. In addition, the initial dynamic stiffness is reduced, and the loss of stiffness is accelerated. Apart from the designed hole bearing failure, another failure mode, i.e., bolt head cracking, is observed under low fatigue load level (70% of 2% offset bearing static strength), leading to faster degradation of fatigue property. Under equal hole elongation (4% of the hole diameter), the joints with forced assembly display severe hole bearing damage, and visible shearing cracks are observed, lowering the residual strength.

中文翻译：

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强制装配对单搭接沉头复合材料螺栓接头疲劳性能影响的实验研究


强制装配是复合材料机身装配过程中消除界面间隙的常见对策，它会改变螺栓孔轴承性能并带来失效风险。在本研究中，对强制装配对单搭接、沉头复合材料螺栓接头疲劳行为的影响进行了实验研究。孔伸长率和动态刚度是根据ASTM D6873获得的。 SN 曲线根据 ASTM E739 进行拟合。通过显微镜观察疲劳损伤，通过静态拉伸试验获得残余强度。结果表明，强制装配显着缩短了接头的疲劳寿命。强制装配后，孔伸长率演变更快，更早达到疲劳失效阈值。另外，初始动刚度降低，刚度损失加速。除了设计的孔轴承失效之外，在低疲劳载荷水平（2%偏置轴承静强度的70%）下观察到另一种失效模式，即螺栓头开裂，导致疲劳性能更快退化。在等孔伸长率（孔径的4%）下，强装接头表现出严重的孔承压损伤，并观察到明显的剪切裂纹，残余强度降低。