In this study, we demonstrate that the strength of a short‐fiber reinforced polymer as measured in uniaxial tension is not a good indicator for its strength under multiaxial loading conditions. To illustrate this fact, the influence of physical aging on the strength of a 20 wt% short‐fiber reinforced polycarbonate is studied for a uniaxial and biaxial loading condition. Results demonstrate that aging strongly reduces the strength in multiaxial loading, whereas, it increases in uniaxial loading. To rationalize these observations, a micromechanical analysis of the local stress state is performed using three‐dimensional (3D) representative volume elements (RVE) in combination with a constitutive model that adequately describes the intrinsic deformation response of the matrix. RVE simulations demonstrate that biaxial loading results in higher local hydrostatic stresses and triaxiality than the uniaxial loadcase for the composite system considered, and aging results in a shift towards higher values. The high triaxiality, the magnitude of the hydrostatic stress, and the shift in hydrostatic stress upon aging, presents a clear rationalization why physical aging induces a strength reduction in biaxial loading and not in uniaxial loading. These results highlight that care should be taken when predicting strength under complex loading conditions with only uniaxial data available.

Investigating and understanding the intrinsic material properties of biogenic materials, which have evolved over millions of years into admirable structures with difficult to mimic hierarchical levels, holds the potential of replacing trial-and-error-based materials optimization in our efforts to make synthetic materials of similarly advanced complexity and properties. An excellent example is biogenic silica which is found in the exoskeleton of unicellular photosynthetic algae termed diatoms. Because of the complex micro- and nanostructures found in their exoskeleton, determining the intrinsic mechanical properties of biosilica in diatoms has only partly been accomplished. Here, a general method is presented in which a combination of in situ deformation tests inside an SEM with a realistic 3D model of the frustule of diatom Craspedostauros sp. (C. sp.) obtained by electron tomography, alongside finite element method (FEM) simulations, enables quantification of the Young’s modulus (E = 2.3 ± 0.1 GPa) of this biogenic hierarchical silica. The workflow presented can be readily extended to other diatom species, biominerals, or even synthetic hierarchical materials.

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.