composite materials have become widely used in engineering applications, in order to reduce the overall weight of structures while retaining their required strength. in this work, a composite material consisting of discontinuous glass fibers in a polypropylene matrix is studied at the microstructural level through coupled experiments and simulations, in order to uncover the mechanisms that cause damage to initiate in the microstructure under macroscopic tension. Specifically, we show how hydrostatic stresses in the matrix can be used as a metric to explain and predict the exact location of microvoid nucleation that occurs during damage initiation within the composite's microstructure. furthermore, this work provides evidence that hydrostatic stresses in the matrix can lead to coupled microvoid nucleation and early fiber breakage, and that small fragments of fibers can play an important role in the process of microvoid nucleation. These results significantly improve our understanding of the mechanics that drive the initiation of damage in the complex microstructures of discontinuous fiber reinforced thermoplastics, while also allowing scientists and engineers to predict the microstructural damage behavior of these composites at sub-fiber resolution and with high accuracy. Composite materials have gained attention in many engineering applications, especially in the aerospace and automotive industries due to their low weight and high strength. Specifically, polymer matrix composites have allowed for major weight savings and higher performance aircraft and vehicles. Despite their high rate of implementation, scientists and engineers have faced challenges in predicting their mechanical behavior and performance, especially past the small strain regime and into the damage initiation and damage propagation regimes, because there exist a number of damage mechanisms which are often coupled and are active throughout the life of a component. Compared to continuous fiber composites, discontinuous fiber reinforced polymers exhibit vastly heterogeneous microstructures that can vary significantly depending on the component geometry, making mechanical behavior predictions even more complicated 1-3. Until recently, most efforts in predicting the mechanical performance of discontinuous fiber composites have been focused on the elastic loading regime during which damage has not yet initiated and progressed 4,5. Efforts in understanding and predicting the behavior of fiber composites past the elastic regime have typically focused on attempting to replicate the macroscopic bulk stress-strain behavior of a specimen through phenomenological damage parameters 6. In recent years, some microstructural approaches have been used to explore the damage mechanisms in certain composites using homogenization approaches 7 as well as unit cell methods 8. Experimentally, one in-situ study of a thermoset polymer, reinforced with discontinuous carbon fibers, showed that fiber tips play an important role in microvoid nucleation due to high shear stress...

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Through the piezospectroscopic effect, certain photo-luminescent materials, once excited with a laser, produce spectral emissions which are sensitive to the stress or strain that the material experiences. A system that utilizes the piezospectroscopic effect for non-contact stress detection over a material's surface can capture important information on the evolution of mechanical response under various conditions. Therefore, the components necessary for piezospectroscopic mapping and analysis have now been integrated into a versatile and transportable system that can be used with photo-luminescent materials in any load frame or on a variety of structures. This system combines compact hardware components such as a portable laser source, fiber optics, spectrograph, charge-coupled device (CCD), and an X-Y-Z stage (with focusing capabilities) with a series of data analysis algorithms capable of analyzing and outputting high resolution photo-luminescent (PL) maps on-site. Through a proof of concept experiment using a compressed polycrystalline alumina sample with sharp machined corners, this system successfully captured high resolution PL maps with a step size of 28.86μm/pixel and located high stress concentrations in critical areas, which correlated closely with the results of a finite element model. This work represents an important step in advancing the portability of piezospectroscopy for in-situ and non-contact stress detection. The instrumentation developed here has strong implications for the future of non-destructive evaluation and non-invasive structural health monitoring.

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