PSI - Issue 64

Arij Fawaz et al. / Procedia Structural Integrity 64 (2024) 89–96 Arij FAWAZ/ Structural Integrity Procedia 00 (2019) 000 – 000

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The second sample configuration allows for the combination of both the creep setup and the geometry of the ELS test sample, which is the mode II mechanical characterization test used in this study. The creep configuration adopted in this study is four-point bending. It was chosen to ensure a constant stress in the steel adherends along the bonded interfaces between both applied loads. In order to obtain a non-zero stress xx value (stress along the x-axis) in the adhesive layer, it was decided to use a non-symmetric sample. Specifically, the thickness of the upper adherend (t upper adherend ) and lower adherend (t 1 lower adherend ) is not the same, t upper adherend > t 1 lower adherend (Fig. 2).

Fig. 2. A 4-point non-symmetrical bending creep test.

Once the non-symmetric sample is prepared, an ELS test will be conducted on this sample. However, in order to ensure mainly shear in the adhesive joint during the ELS test, a symmetric sample is required. This implies that the upper and lower adherends, made from the same material, must have the same thickness to obtain a neutral axis located in the middle of the adhesive layer. To address this issue, a new methodology is proposed. It involves bonding an additional plate onto the thinner adherend, which is the lower adherend in this case, until it reaches an equivalent stiffness equal to that of the upper adherend (Fig. 3). As a result, the newly modified lower adherend of t lower adherend thickness is composed of three layers: the thin lower adherend having a thickness of t 1 lower adherend , a small adhesive layer with a thickness of a 0.4 mm, and the second bonded adherend of t 2 lower adherend thickness.

Fig. 3. Configuration 2 specimen. All the samples from configuration 1 and 2 have the same length =300 mm and width b=15 mm. In configuration 1, the thickness of both adherends is equal to 5 mm, with an adhesive thickness of approximately 1 mm. For configuration 2, the following dimensions were chosen: t upper adherend = 5 mm, t 1 lower adherend =0.6 mm, t 2 lower adherend =4 mm, t adhesive 1 ≈ 1 mm and t adhesive 2 ≈ 0.4 mm (i.e., total lower adherend thickness t lower adherend ≈ 5 mm). The initial artificial crack consists of a Teflon insert with a length of a 0 =75 mm and an initial free length of L =100 mm. 2.2. Material properties For configurations 1 and 2, the adherends used in this study are high-strength steel plates of type STRENX 700, having a yield strength of 700 MPa. The adhesive 1 in both configurations is a commercial epoxy-based adhesive, the resin Biresin® CR92 mixed with the hardener Biresin® CH92-4. This adhesive has a glass transition temperature (Tg)