自扣緊固件接合品質之非破壞性檢測方法開發與性能探討

Abstract

自扣緊固技術是將板材以塑性變形的方式擠壓入緊固件的齒形結構和流動槽中，產生形鎖與緊配的雙重效果，以期在薄板上提供螺紋接合的功能。目前針對使用時可能發生的破壞，已有破壞性的檢測方法，但對於鉚壓緊固的過程，尚未開發非破壞性檢測系統，作為全面性檢測的方法。因此本論文針對自扣緊固件的接合品質進行非破壞性檢測方法的開發，以鉚壓過程中會變異的薄板孔徑大小作為檢測的依據。 首先利用理論計算、有限元素分析以及鉚壓成品的切片探討其應力應變的型態，配合鉚壓力、曲線特徵以及材料變形的驗證，明確地了解鉚壓接合面的機理。當孔徑越大時，板材擠壓入流動槽的體積就越少，進而可以得知緊固件與板材的接觸面積與接觸壓力越小。理論上當面積與壓力變小時，接觸電阻則反向變大，所以利用有限元素分析的結果，建立一套預估緊固件與薄板之間接觸電阻的模型，以探討影響接觸電阻之因素。其次設計開發量測接觸電阻的裝置，進行接合品質的檢測，驗證薄板孔徑大小與接觸電阻呈現正相關的結果，進而確定接觸電阻方法的可行性。 延續緊固件的鉚壓分析，將已鉚壓好的緊固件承受衝擊，以分析其動態響應，並且設計開發衝擊設備與量測位移響應的裝置進行實際的量測，證實緊固件的動態響應的半周期與孔徑大小呈現正相關，說明振動方法可以檢測孔徑大小的變化。 本論文之貢獻在於完整的設計開發流程，利用量測接觸電阻與衝擊振動的方法檢測孔徑大小的變化，包括理論模型的推導、有限元素的分析和檢測機構的設計製作。未來的方向在於探討更多鉚壓過程的變異因子，包括偏心、鉚壓不足、過度鉚壓和傾斜等，使得檢測緊固接合的技術得以提升。

Self-clinching technique utilizes squeezing force to fasten the sheet metal into the special geometric features of tooth structure and undercut of the fastener, enabling thread engagement on the sheet metal as a result of shaped lock and interference fit. Destructive testing systems have been used to detect the destruction on the joints between fasteners and sheet metal during product usage; however, no methods have been developed to examine the defects caused by the fastening process. Therefore, the objective of this research is to develop two different non-destructive testing systems for evaluating the fastening quality of self-clinching fasteners base on aperture size. Theoretical calculation, finite element analysis and experimental slices were first performed to analyze the stress-strain pattern. To clearly understand the mechanism of self-clinching fastener joints, the riveting force and features of experimental curves and material deformation were verified. It was found that the larger the aperture in the sheet, the less the metal extrusion into the undercut. This has led to a decreased contact area and pressure between the fastener and the sheet metal. On the other hand, the contact resistance increases as the contact area and pressure decreases. Therefore, finite element analysis was used to establish a set of model to predict the contact resistance whose measurements were conducted using the designed device for fastening quality evaluation. The results showed a positive correlation between the sizes of the aperture and the contact resistance, which indicated the feasibility of the designed method. Besides evaluation of static riveting fastening quality, features of fasteners under dynamic loads should also be identified. Both finite element analysis and the designed displacement measuring device were used to examine the dynamic response of the riveted fasteners after controlled mechanical impact. It was confirmed that the sizes of the holes in the sheet metal were positively correlated with half cycles of the dynamic response of the fasteners after the impact, which illustrated the aperture sizes resulted from the fastening process could be detected by the designed method. In sum, this research has demonstrated the feasibility to apply the developed non-destructive testing systems to the evaluation of the fastening quality of self-clinching fasteners. However, others factors affecting the fastening quality should be further examined to strengthen the application of this method, which will contribute to the full inspection of self-clinching qualities in the future.