## 5 Repair Qualification Study
### Second Stage--Strength Test of the Bonding Systems
#### 5.2.1 Shear Strength Test  
It was assumed that, if the strength is greater than the value of the allowable stress in the bonding or if the fracture of the rubber is decohesive in nature, the adhesive system will be considered correct. The strength criterion results from the fact that the tests carried out during the development of this technology concern the connection of a fragment of the rotor blade covered with a layer of rubber and a sheet composed of Ti-6 Al-4V alloy [49], the properties of which are similar to those of the alloy of the shield (Figure 12). The allowable stress value of the bonding layer is \(\tau^{\rm{ult}}\) = 3.92 MPa, which was taken from the repair instructions developed by the rotor blade manufacturer and published by the Polish Air Force [12]. The description of the tests contained in the instructions pointed out that the value of the allowable stress in the bonding layer \(\tau^{\rm{ult}}\) = 3.92 MPa obtained during laboratory tests was equivalent to that in single-lap joint tests [50], and it concerned only the strength of the bonding layer. Therefore, the tests presented in this article and the  
Figure 11: Exposure conditions of samples in a climatic chamber during the aging program. test by which the limit value was obtained differ from one another. Focusing only on one criterion, consisting of comparing the obtained results and the limit values, was considered insufficient. A fragment of the sample with rubber was taken from the lower part of the leading edge of the rotor blade (Figure 13). The surfaces for the bonding process were treated according to the proposed repair technology. The limited possibilities of obtaining spar fragments resulted in only eight samples being produced for each variant:  
1. Bonding 2216B/A + BR-127 primer;
2. Bonding DP490. Four samples after aging and four samples without aging were strength-tested. The shear strength test of bonding was carried out on an Instron Fatigue system. A constant speed of actuator movement equal to 2 mm/min was applied. Force was recorded as a function of displacement, on the basis of which the shear stresses were determined. The damage caused by the shearing of bonding 2216B/A + BR 127 primer after aging, was characterized by adhesive fracture surfaces between the bonding layer and rubber (Figure 14a). The rest of the samples were characterized by the cohesive fracture surface of the rubber (Figure 14b-d). Figure 12: Scheme of the sample for shear strength tests of the leading edge shield and rubber bonding. Figure 13: Place from where the spar fragments were extracted: (**a**) scheme of spar; (**b**) Mi-8 rotor blades after collecting samples. The results of the bonding 2216B/A + BR primer samples without aging and after aging obtained during the shear strength test are given in Figure 15. In the case of the samples after aging, the spread of the results was considerable, and, for this reason, the average value and standard deviation were not calculated. The reason for the large discrepancy in the strength test results of the joint (Figure 15b) may be the degradation of the polymeric materials in the joint. Degradation is referred to as the partial disintegration of a polymer--not into low-molecular-weight products, but into fragments of high but lower molecular weight than the initial polymer. Factors initiating degradation may be physical or chemical interactions, such as radiation, temperature, or exposure to chemicals, including water [51; 52; 53; 54]. The shear stresses of the joints of samples without aging were calculated. Despite the fact that the results were less than \(\tau^{\text{ult}}\) = 3.92 MPa, the rubber was damaged instead of the bonding. Therefore, it should be assumed that the strength condition was met. A very large dispersion of the results after the aging of the samples and adhesive fracture surfaces between the rubber and bonding layers testify to the high influence of the environmental conditions on bonding.