# Study of the Kinetics of Adhesive Bond Formation Using the Ultrasonic Method
## 4 Discussion  
Analyzing the results obtained, it can be said that already preliminary tests indicate the differentiated nature of the failure of the adhesive bond by deadhesion or decohesion (Figure 2) and the time-dependent contributions of these forms of failure to the detachment of the adhesive from the substrate. These regularities are confirmed in the basic, mechanical examinations, normal tear tests of bonded disc specimens (Figure 15), and specimens in the form of steel strips, tangentially torn (Figure 17), on which areas of deadhesion and decohesion of the adhesive layer in different proportions are visible. The results of the non-destructive ultrasonic testing, represented by the courses over time, selected ultrasonic parameters on the screen of the ultrasonic apparatus (Figure 8a), in the form of a decibel decrease in gain between the first and second pulses of the longitudinal ultrasonic wave (Figure 11), which indicate the dependence of these parameters on the time of curing of the adhesive, both in the case of steel disc specimens and bonded steel strips (Figure 12). The nature of these courses is similar, although it promises different values of the ultrasonic parameters that are characteristic for the disc specimens (Figure 11) and strips of the steel sheet specimens (Figure 12). They have a form described by an increasing curve during the crosslinking of the adhesive, with a tendency to stabilize the values after the formation of the adhesive bond. The course at the same measurement time of the ultrasonic surface wave parameter, generated by the probe (Figure 7b), in the form of amplification of the first pulse of this wave on the screen of the ultrasonic apparatus (Figure 8b), deviates from the character of the course of the ultrasonic longitudinal wave parameter, both in terms of the gradual decrease in value, which is understandable, and in terms of the beginning of the stabilization time of the value of this parameter, which significantly differs from this time, in the case of the longitudinal wave for different samples. Mechanical verification of the strength of adhesive bonds by means of tearing tests on a testing machine (Figure 9), both in the case of tearing glued disc specimens (Figure 5) with a normal force (Figure 14) and tearing glued steel strips (Figure 4) with a tangential force (Figure 16), confirms the nature of the kinetics of the constitution of the epoxy adhesive bond, obtained by means of an ultrasonic longitudinal wave, introduced normally (perpendicularly) to the adhesive surface with an adequate ultrasonic probe (Figure 7a,c). Also, the nature of the distribution of the hardness of the adhesive layer (Figure 18), applied to the steel strip (Figure 10) during the crosslinking of this adhesive, coincides with the nature of the kinetics of the formation of the adhesive joint, obtained both by using the non-destructive method--by an ultrasonic longitudinal wave (Figures 11 and 12)--and by using the destructive method--by tearing this bond (Figure 16). A two-component epoxy adhesive was tested, and it was noted that the mechanical strength changed with time. For shear, the greatest changes in the mechanical strength occurred during the first 24 h after the bond was made. The strength increased from 0 MPa to about 10 MPa. Over the next 24 h, the strength increased by only about 0.5 MPa. During the entire period when the values were recorded (216 h), the strength increased by only about 20% with respect to the first 24 h. It was noted that from the 96 th hour, the strength was at a similar level. Figure 18: The distribution of an adhesive hardness over time. For the samples in which the tensile strength was tested, it was noted that the greatest increase in strength occurred during the first 24 h (from 0 MPa to 23 MPa). During the next 72 h, the strength increased by 2 MPa, after which it stabilized at 27 MPa. At the same time, it was noted that the ultrasonic parameters recorded during the tests changed.