PSI - Issue 28

Vinicius Carrillo Beber et al. / Procedia Structural Integrity 28 (2020) 1950–1962 V.C. Beber and M. Brede / Structural Integrity Procedia 00 (2019) 000–000

1953

4

components named principal stresses ( � � � �� � � ��� ) as seen in Eq. 2: 0 0 0 0 0 0 I MP ij II III               

(2)

When the tensor of maximum principal stresses has only one non-zero component the state of stress is named uniaxial stress . This occurs, for instance, in a uniaxial tensile loading of un-notched bulk specimens. However, under a multiaxial state of stress, two or three maximum principal stresses are non-zero (Fatemi and Shamsaei, 2011). A common approach to deal with multiaxial stresses is to define an e quivalent stress ( eq  ) to be used as a comparative parameter between the state of stress of different joints. The equivalent stress is a function of the stress tensor, which is chosen so that it characterises the severity of the stress, e.g. under static loading the material fails at a certain value of the equivalent stress, and under cyclic loading each value of equivalent stress corresponds to a certain fatigue lifetime. From the principal stresses ( , , I II III    ), the hydrostatic pressure ( p ) and the von Mises stress ( q ) can be calculated:

    

(3)

p

 

I

3 II

III

1 (

(4)

2 ) (              2 ) ( 2 )

q







I

II

II

III

I

III

2

Finally, the multiaxiality ratio (  ) is defined as follows:

p

(5)

 

q

3. Experimental Methodology

3.1. Adhesives Two types of commercial adhesives were selected for this investigation: ‐ an elastic adhesive, with a polyurethane-based chemical structure; ‐ a structural adhesive, with an epoxy-based chemical structure.

The elastic adhesive is a single-component boosted system with moisture cure at RT. The structural adhesive is a 2-component system with cure at RT for 24h and then hot cure at 70°C for 2h. As shown in Figure 2 for bulk adhesive specimens, the adhesives have very distinct mechanical behaviours, which are summarised in Table 1. The structural adhesive has a higher static strength and stiffness (i.e. Young’s modulus, E), whereas the elastic adhesive has a greatly larger strain at break. Another important distinction between the adhesives is related to their Poisson’s ratio, which has a direct impact on the compressibility of the adhesives; therefore, altering their hydrostatic