Effects of singular and non‐singular stress fields on very high cycle fatigue life of adhesive joints

Abstract

Singular stress fields represent a concern during the design of mechanical connections as they could trigger cracks within the joint, under low‐amplitude loads, generating failures and reducing its service life. Appropriate design plans should therefore be implemented to produce more reliable apparatus. This paper explores the effects induced by both the presence and the absence of stress singularities on the very high cycle fatigue (VHCF) performance of adhesively bonded butt joints through analytical, numerical, and experimental methods. Analytical models defined the conditions to design a stress singularity‐free joint, global–local finite element descriptions studied the stress distributions in detail, and then VHCF experiments extracted S–N outcomes. The suitability of this approach is confirmed numerically while a substantial increase of the joint life is experimentally observed if the singularity is removed. Nonetheless, a non‐singular configuration leads to higher scatter data compared to the original specimen configuration.

Effect of Loading Frequency on the Fatigue Response of Adhesive Joints up to the VHCF Range

“…The capability of sustaining and transmitting loads between Aluminum 7075 T6 and SikaPower ® -1277 epoxy resin was assessed in [43], where dedicated tensile tests were performed at 2.5 mm/min. The load-displacement curves were comparable, and processed stress data were in line with those extracted from the bulk adhesive in Table 1.…”

“…To perform fatigue tests, the UFTM adopts the following devices: (i) an electric generator (Branson DCX 4 kW, not visible in Figure 1) which generates an electrical sinusoidal wave at 20 kHz; (ii) a piezoelectric transducer, responsible for converting the electrical wave into a mechanical vibration; (iii) a booster and a horn made of Ti-6Al-4V that amplify and convey the vibration to the specimen under testing; (iv) the specimen under investigation, which was designed for the UFTM resonance conditions; (v) a laser displacement sensor that measures the displacement at the specimen free end; (vi) vortex tubes to keep the temperature controlled in the range (20-24) • C; and (vii) a temperature sensor to observe the temperature evolution during the test execution. The capability of sustaining and transmitting loads between Aluminum 7075 T6 and SikaPower ® -1277 epoxy resin was assessed in [43], where dedicated tensile tests were performed at 2.5 mm/min. The load-displacement curves were comparable, and processed stress data were in line with those extracted from the bulk adhesive in Table 1.…”

“…In a previous study [47], it was observed that a nominal cylindrical bi-material configuration is affected by stress singularity issues. A specific design methodology [43] should be adopted to collect fatigue properties without the presence of this detrimental effect, which reduces the joint mechanical performance.…”

“…This goal is achievable through the analysis of mechanical and geometrical conditions that prevent this issue [39,40]; additionally, innovative testing methods for better material characterization have become accessible [41,42]. Furthermore, a significant increase in fatigue performance has been exhibited when a stress singularity is removed from a specimen under investigation [43].…”

“…For example, in recent years, VHCF tests have been carried out to investigate a wide set of materials such as stainless steel [46], steels produced with different processes [47], steels with large risk volume [48], and adhesively bonded joints [49,50], confirming the industrial and academic interest towards this research field. Particular attention has also been paid to the quantification of the influence of stress singularities on the joint life [43], size effects [51], and, in general, the fatigue responses of adhesives under the VHCF regime.…”