The mechanical properties of fiber-reinforced thermoplastics such as short or long glass fiber-reinforced isotactic polypropylene are anisotropic due to the fiber orientation and time- and temperature-dependent due to the viscoelasticity of the matrix material. Consequently, material characterization involves substantial short- and long-term testing campaigns spanning a broad range of testing conditions, that is, rate, temperature, fiber orientation, and so forth. This study presents a framework based on the combination of previously reported experimental observations captured in phenomenological models for both plasticity and slow-crack growth-driven failure. It is based on the observation that for plasticity-driven failure the influence of applied rate and temperature on the yield stress can be multiplicatively decoupled from the fiber orientation. This implies that capturing the rate and temperature dependence at a single fiber orientation combined with the fiber orientation dependence at a single rate and temperature enables the prediction of plasticity-controlled failure over a broad range of rates, temperatures, and fiber orientations. Additionally, the same factorizability concept is found to be applicable to the slow crack growth-controlled failure regime using the same anisotropic scaling function as obtained for the plasticity-controlled failure regime to predict the fatigue time-to-failure as a function of temperature, fiber orientation, load ratio, and frequency.
The identification of relevant adhesion properties is important for the development of microelectronic systems, e.g., microprocessors, light-emitting diodes, etc. This is best done on an actual device, rather than on dedicated specimens, because such properties are highly specimen specific. This paper discusses how the full-field identification method of integrated digital image correlation (IDIC) can be used to identify three mixed-mode cohesive zone parameters of a flexible, organic light-emitting diode (OLED) from a single mechanical test. A tri-axial, micro-mechanical testing setup is employed for the in situ, on-device delamination experiment, employing optical microscopy. The microscopic magnification, required for capturing the relevant kinematics, prevents the experimentally imposed loading conditions to be captured within the field of view. Virtual experiments are firstly conducted to optimize the local boundary conditions therefore required by the finite element simulation within the IDIC-framework, and which constitutes the cohesive zone model. The virtual experiments also allow for selecting an adequate load-case for the actual experiment, in order to maximize the kinematic sensitivity of IDIC with respect to the three mixed-mode cohesive zone parameters, and identify them accurately. The image residual, minimized in IDIC, together with the convergence behavior towards the associated global minimum, provide adequate measures for assessing the trustworthiness of the solution. The optimized load-case is subsequently used to trigger delamination of a 50 [mu m] thick barrier stack from its substrate in a real in situ experiment on a flexible OLED. The optimized boundary conditions from the virtual experiments are used in a corresponding simulation for identifying the three mixed-mode cohesive zone parameters by IDIC. The optimization scheme within IDIC was found to be robust against deviating initial guesses, and consistently converged to a unique solution for the cohesive zone parameters.
Starvation occurs when the lubricated contact uses up the lubricant supply, and there is not enough lubricant in the contact to support the separation between solid surfaces. On the other hand, the use of textures on surfaces in lubricated contacts can result in a higher film thickness. In addition, a modification of the surface’s geometrical parameters can benefit the tribological behaviour of the contacts. In this article, for parallel sliding surfaces in starved lubricated conditions, the effect of surface texturing upon the coefficient of friction is investigated. It is shown that surface texturing may improve film formation under the conditions of starvation, and as a result, the frictional behaviour of the parallel sliding contact. Furthermore, the effect of starved lubrication on textured surfaces with different patterns in the presence of a cavitation effect, and its influence on frictional behaviour, is investigated. It is shown that surface texturing can reduce the coefficient of friction, and that under certain conditions, the texturing parameter could have an influence on the frictional behaviour of parallel sliding contacts in the starved lubrication regime.
This paper proposes a boundary element model for the stick-slip transition at the contact of two bodies of dissimilar materials in presence of adhesion due to van der Waals force. The adhesion is modeled using a Dugdale approximation of adhesive energy. The coupling between the normal pressure and shear stresses is included so that there is no full-stick condition, even in the absence of an external shear stress. Furthermore, the evolution of the slip area over the contact area is different from the well-known Cattaneo-Mindlin solution due to the difference in the mechanical properties of the contacting bodies. It is also shown that the adhesion increases the pre-sliding distance and the static friction. While roughness can only increase the former one. However, the combined effect of roughness and adhesion on the pre-sliding distance is not cumulative as these two parameters are found to be symbiotic.
