Probabilistic Assessment of Fracture Toughness of Epoxy Resin EPOLAM 2025 Including the Notch Radii Effect

Many design scenarios of components made of polymer materials are concerned with notches as representative constructive details. The failure hazard assessment of these components using models based on the assumption of cracked components leads to over-conservative failure estimations. Among the different alternative approaches proposed that are based on the apparent fracture toughness, KcN is considered. In so doing, the current deterministic underlying concept must be replaced by a probabilistic one to take into account the variability observed in the failure results in order to ensure a reliable design. In this paper, an approach based on the critical distance principle is proposed for the failure assessment of notched EPOLAM 2025 CT samples with each different notch radii (ρ) including a probabilistic assessment of the failure prediction. First, each apparent fracture toughness is transformed into the equivalent fracture toughness for ρ=0 based on the critical distances theory. Then, once all results are normalized to the same basic conditions, a Weibull cumulative distribution function is fitted, allowing the probability of failure to be predicted for different notch radii. In this way, the total number of the specimens tested in the experimental campaign is reduced, whereas the reliability of the material characterization improves. Finally, the applicability of the proposed methodology is illustrated by an example using the own experimental campaign performed on EPOLAM 2025 CT specimens with different notch radii (ρ).

Effect of the different debonding strength of metal and ceramic brackets on the degree of enamel microcrack healing

ABSTRACT Objective: This study aims to determine shear debonding strength of metal and ceramic brackets, and the degree of enamel crack healing. Material and Methods: Extracted human maxillary premolars were flattened on the buccal surface, and randomly separated into five groups (n = 15). In control groups (groups 1 and 2), metal and ceramic brackets were bonded on flat polished enamel, while in experimental groups (groups 3 and 4), metal and ceramic brackets were bonded on the surface with boundary where corner cracks were created. Additionally, fifteen specimens (group 5) were also prepared for an indentation procedure with no bracket installation. The degree of crack healing was measured. All brackets were detached with a universal testing machine, and the adhesive remnant index (ARI) was also identified. Healing degree and apparent fracture toughness were then calculated. Results: Between groups with similar bracket types, there was no statistically significant difference in debonding strength. Regarding bracket types, ceramic brackets provided significantly higher debonding strength than metal brackets. There was a significant difference in ARI scores between metal and ceramic brackets. The corner cracks showed signs of healing in both horizontal and vertical directions. No statistically significant difference in the healing rates among the groups was found and the apparent fracture toughness increased from the initial to the final measurement. Conclusions: Within the limitations of this study, even though ceramic brackets required significantly higher debonding force compared to metal brackets, debonding stress was limited to the bonding site and did not affect the surrounding cracks on enamel.

Elastic-viscoplastic characterization of S2 columnar freshwater ice

This work addresses the time-dependent response of 3 m × 6 m floating edge-cracked rectangular plates of columnar freshwater S2 ice by conducting load control (LC) mode I fracture tests at −2 °C in the Ice Tank of Aalto University. The loading profile consisted of creep/cyclic-recovery sequences followed by a monotonic ramp to fracture. The LC test results were compared with previous monotonically loaded displacement control (DC) experiments of the same ice, and the effect of creep and cyclic sequences on the fracture properties were discussed. To characterize the nonlinear displacement-load relation, Schapery's constitutive model of nonlinear thermodynamics was applied to analyze the experimental data. A numerical optimization procedure using Nelder-Mead's (N-M) method was implemented to evaluate the model functions by matching the displacement record generated by the model and measured by the experiment. The accuracy of the constitutive model is checked and validated against the experimental response at the crack mouth. Under the testing conditions, the creep phases were dominated by a steady phase, and the ice response was elastic-viscoplastic; no viscoelasticity or major recovery were detected. In addition, there was no clear effect of the creep loading on the fracture properties: the apparent fracture toughness, failure load, and crack opening displacements.

Modeling of electromechanical response and fracture resistance of multilayer piezoelectric energy harvester with residual stresses

The article focuses on a modeling and subsequent optimization of a novel layered architecture of the vibration piezoceramic energy harvester composed of ZrO2/Al2O3/BaTiO3 layers and containing thermal residual stresses. The developed analytical/numerical model allows to determine the complete electromechanical response and the apparent fracture toughness of the multilayer vibration energy harvester, upon consideration of thermal residual stresses and time-harmonic kinematic excitation. The derived model uses the Euler–Bernoulli beam theory, Hamilton’s variational principle, and a classical laminate theory to determine the first natural frequency, steady-state electromechanical response of the beam upon harmonic vibrations, and also the mechanical stresses within particular layers of the harvester. The laminate apparent fracture toughness is computed by means of the weight function approach. A crucial point is the further optimization of the layered architecture from both the electromechanical response and the fracture resistance point of view. Maximal allowable excitation acceleration of the harvester upon which the piezoelectric layer will not fail is determined. It makes possible to better use the harvester’s capabilities in a given application and simultaneously guarantee its safe operation. Outputs of the derived analytical model were validated with finite element method simulations and available experimental results, and a good agreement between all approaches was obtained.

Microstructure and Materials Properties of Understoichiometric TiBx Thin Films Grown by HiPIMS

<p>In the present research article we report synthesis of TiB_x, 1.43<i>n-situ</i> mass- and energy-spectroscopy is used to explain the obtained compositional range. Excess B in overstoichiometric TiB<i>_x</i>thin films from DCMS results in a hardness up to 37.7±0.8 GPa, attributed to the formation of an amorphous B-rich tissue phase separating stoichiometric TiB₂ columnar structures. With a particular focus on characterization of the understoichiometric samples, we show that understoichiometric TiB_1.43 thin films synthesized by HiPIMS exhibit a superior hardness of 43.9±0.9 GPa, where the deficiency of B is found to be accommodated by Ti planar defects. The apparent fracture toughness, electrical resistivity and thermal conductivity of the same sample is 4.2±0.1 MPa√m, 367±7 μΩ·cm and 5.1 W/(m.K), respectively, as compared to corresponding values for overstoichiometric TiB_2.20 DCMS thin film samples of 3.2±0.1 MPa√m, 309±4 μΩ·cm and 3.0 W/(m.K). </p>

Microstructure and Materials Properties of Understoichiometric TiBx Thin Films Grown by HiPIMS

<p>In the present research article we report synthesis of TiB_x, 1.43<i>n-situ</i> mass- and energy-spectroscopy is used to explain the obtained compositional range. Excess B in overstoichiometric TiB<i>_x</i>thin films from DCMS results in a hardness up to 37.7±0.8 GPa, attributed to the formation of an amorphous B-rich tissue phase separating stoichiometric TiB₂ columnar structures. With a particular focus on characterization of the understoichiometric samples, we show that understoichiometric TiB_1.43 thin films synthesized by HiPIMS exhibit a superior hardness of 43.9±0.9 GPa, where the deficiency of B is found to be accommodated by Ti planar defects. The apparent fracture toughness, electrical resistivity and thermal conductivity of the same sample is 4.2±0.1 MPa√m, 367±7 μΩ·cm and 5.1 W/(m.K), respectively, as compared to corresponding values for overstoichiometric TiB_2.20 DCMS thin film samples of 3.2±0.1 MPa√m, 309±4 μΩ·cm and 3.0 W/(m.K). </p>

Does the Biaxial Loading Affect the Apparent Fracture Toughness (KC)?

From linear elastic fracture mechanics (LEFM), it is well accepted that only the singular stress near the crack tip contributes to the fracture event through the crack tip stress intensity factor K. In the biaxial loading, the stress component that adds to the T-stress at the crack tip, affects only the second term in the Williams' series solution around the crack tip. Therefore, it is generally believed that biaxial load does not change the apparent fracture toughness or the critical stress intensity factor (Kc). This paper revisited several specimen geometries under biaxial loading with finite element method. The sources of discrepancy between the theory and the test data were identified. It was found that the ideal biaxial loading would not be achieved for typical fracture specimens with finite geometry. Comparison to available test data shows that, while the biaxial load could affect the apparent fracture toughness, the contribution is relatively small.