Discontinuous Glass Fiber Research Articles

Assessment of recycling and repair methods for discontinuous glass fiber reinforced vitrimer composites with reclaimed parts

In the context of fiber-reinforced polymer composites, the pursuit of sustainability is of paramount importance to promote environmentally responsible practices. Vitrimer materials have recently gained significant attention for their potential to enable repair and recycling, rendering them an attractive choice for manufacturing eco-friendly composites while maintaining mechanical strength akin to a thermoset resin like epoxy. This study specifically focuses on glass fibers, and delves into the examination of mechanical properties using both chemical and mechanical recycling methods for quasi-isotropic, randomly dispersed discontinuous glass fiber-reinforced vitrimer composites. It places specific emphasis on the optimization of factors to promote sustainable production. The findings indicate that chemical recycling methods are remarkably effective, resulting in the almost complete restoration of mechanical properties when appropriately selected process parameters are applied. Additionally, this study investigates diverse repair techniques, with the overlap repair method exhibiting the highest degree of recovery. These findings underscore the remarkable potential of glass fiber reinforced vitrimer composites, characterized by their high mechanical properties, for applications in various industries, particularly in structural contexts. Overall, this study highlights the significant reduction in composite waste achievable through recycling and repair strategies, aligning with the broader goal of advancing sustainability within the domain of engineering and manufacturing.

Influence of Flow-Induced Polymorphism and Fiber Morphology on Mechanical Behavior in Long Discontinuous Glass Fiber Polyamide Composites

This study investigates the combined flow-induced crystallization in the polymer and fiber reorientation during compression molding of a long discontinuous glass fiber polyamide (PA 6) composite. The composite is molded from an organosheet (a semi-finished pre-impregnated mat); the composite anisotropic tensile properties are evaluated as a function of polymer crystalline morphology and fiber orientation state, both controlled by the extent of material flow in the mold. To study these effects, the full mold coverage and partial center charge of organosheet (OS)-80 %, 60 %, 50 %, and 40 % were compression molded to cause varying anisotropic material flow. Tensile specimens were cut out from the molded plates in the flow and transverse direction and tested to compare their effective tensile properties (modulus and strength). The flow-induced morphological changes in a molded composite at the glass fiber bundle microstructure scale and polymer crystalline phases nano-structure were characterized using optical microscopy and X-ray diffraction, respectively. These morphological changes contributed to the significant change in the tensile strength and modulus; a combined experimental/numerical simulation framework was used to segregate the relative contribution of each factor. Experimentally, the tensile modulus increased in the flow direction from 9.6GPa to 14.9GPa for the specimens produced by full mold coverage and OS-50 % coverage mold, respectively. The tensile strength increased from 162 MPa to 254 MPa for the full and OS-60 % mold coverage. On the contrary, the strength and modulus in the transverse direction to the flow showed a significant drop to 35 MPa and 3GPa, respectively, which was attributed to reduced fiber alignment and anisotropy in the PA6 matrix.

Thermo-visco mechanical behavior of glass fiber reinforced thermoplastic composite

Thermoplastic composite materials are being increasingly used in many domains, such as automotive, marine and aeronautics. This growing interest is due to the relatively good mechanical properties and the recycle-ability of these materials. In various heat assisted forming technologies, thermoplastic composites may be subjected to severe conditions such as high temperatures, complex strain paths and potentially significant strain rates. This is expected to highly influence the overall behavior of these materials and make the characterization of their mechanical behavior more complicated. In this paper, the mechanical behavior of discontinuous glass fiber reinforced polypropylene composite is investigated at temperatures going from room temperature (RT) to 120°C and strain rates ranging from quasi-static conditions to 10 s−1 by means of uniaxial tensile tests. The effect of material orientation is also investigated at RT and quasi-static strain rate of 0.001 s−1 pointing out a clear anisotropic behavior. The tensile properties including Young’s modulus E, ultimate tensile strength σ ult and strain at failure ɛ f are determined based on the Standard ISO-527 reporting an obvious sensitivity of the material to temperature and strain rate. Based on literature review, the phenomenological isotropic constitutive model of G’Sell and Jonas, originally designed for unreinforced semi-crystalline polymers, is selected to describe the mechanical behavior of the studied material. A modification of the original constitutive equation is proposed for a better representation of the overall behavior of the material at different temperature and strain rate conditions. Finally, the validity of this phenomenological law calibrated by means of the uniaxial tests, is evaluated for an equi-biaxial loading state close to that encountered by the material during heat assisted sheet forming process. The significant disagreement highlighted between numerical and experimental results proved that the uniaxial tensile tests are not sufficient to characterize the behavior of the material when a complex loading condition is imposed.

Notched tensile strength of long discontinuous glass fiber reinforced nylon composite

In this study, the hole size effect in the open hole tension (OHT) of discontinuous glass fiber-polyamide 6 organosheet composite material was studied. Organosheet used in this study is a novel long discontinuous fiber-reinforced polymer composite with long glass fiber and in-situ polymerized polyamide 6 (nylon) matrix. A progressive failure analysis (PFA) using finite-element model was proposed to predict OHT strength using measured fiber orientation distribution. The generated virtual ogranosheet samples were used to conduct OHT Monte Carlo simulations to predict variability in strength. The notch sensitivity was observed experimentally and analyzed using PFA model, which captured interactions between various failure modes. The experimental and PFA results of OHT strength were used for calibrating analytical models, Point Stress Criterion (PSC) and Modified Point Stress Criterion (MPSC). Both, experimental and modeling approaches, yielded nearly identical analytical curves, indicating that virtual testing of organosheet can be used for OHT characterization of organosheet. The results show that the notched strength of organosheet with the diameter of 2 mm was at the same level as strength of unnotched organosheet. The notch insensitivity was attributed to the increased plasticity in the matrix due to ductility in nylon.

Fatigue behavior of hybrid continuous-discontinuous fiber-reinforced sheet molding compound composites under application-related loading conditions

Hybrid continuous-discontinuous sheet molding compound (SMC) composites are considered suitable candidates for structural automotive applications, due to their high mass-specific mechanical properties combined with high geometrical flexibility and low costs. Since structural automotive parts are subject to repeated loading, profound knowledge of their fatigue behavior is required. This paper presents an experimental study on the bending fatigue behavior of hybrid SMC with discontinuous glass fibers in the core and unidirectional continuous carbon fibers in the face layers. Effects of hybridization on the S-N behavior and stiffness degradation have been analyzed in constant amplitude fatigue tests under 3-point bending load at different temperatures and frequencies. Microscopic investigations on polished specimen edges were used to study the damage behavior. The ultimate flexural strength at quasi-static (UFSS) and fatigue strain rate (UFSF) of the hybrid composite was 54 % and 59 % higher than that of discontinuous SMC, respectively. In contrast, the flexural fatigue strength at 2.6⋅106 cycles increased by 258 %. The relative stiffness degradation of the hybrid composites was smaller during most of their fatigue lives due to the continuous carbon fiber reinforcement. The carbon fiber ply on the compression loaded side was the first ply to fail. Fatigue stress significantly decreased at 80 °C due to early kinking of the continuous carbon fiber-reinforced ply on the compression loaded side. Variation of frequency had no significant effect on the fatigue behavior of both discontinuous and continuous-discontinuous SMC.

Mechanical behavior of long discontinuous glass fiber nylon composite produced by in-situ polymerization

The present work studied the mechanical behavior of organosheets produced using 1″ long glass fiber, which are incorporated within an in-situ polymerized polyamide 6 (nylon) matrix. The meso-scale morphology of the glass fiber bundles is preserved during the impregnation process, resulting in the collimated fibers. A finite-element computational model was developed to predict the effective tensile and flexural properties (modulus as well as strength) and was used to understand the failure behavior of the fiber reinforced PA6 organosheet. The continuum damage modeling was employed to describe various failure modes in bundles and PA6 pockets. The proposed computational framework was used to analyze the role of bundle degree of alignment in organosheet on the effective tensile and flexural response. Furthermore, the effect of specimen size on the flexural behavior was studied. The material system revealed a strong dependence of the tensile and flexural response with the degree of alignment in bundles.

Dynamic-mechanical-thermal analysis of hybrid continuous–discontinuous sheet molding compounds

Sheet molding compounds (SMC) are very promising for the production of lightweight structural components, due to the specific mechanical properties combined with the suitability for a large-scale manufacturing process. Automotive industry already implements SMC materials for structural components in their vehicle concepts. Polymeric materials, hence also fiber reinforced polymers, show a viscoelastic behavior and dynamic-mechanical-thermal analysis (DMTA) is an important method of determining the influence of temperature and loading speed of this material class. In this work, SMC which based on a novel hybrid resin system were examined under bending loads using a electric-dynamic test system to realize high-force dynamic-mechanical-thermal analysis. The examined SMC materials were either discontinuously (Dico) or continuously (Co) reinforced. In addition a hybrid continuous–discontinuous reinforcement was realized by stacking different SMC materials. The mechanical characterization aimed to investigate the influence of the reinforcement architecture and the effect of hybridization on the temperature- and frequency-dependent material properties. Glass transition temperature of the hybrid SMC was comparable to glass transition temperature of the discontinuous glass fiber reinforced component. Compared to the continuous carbon fiber SMC, the decrease of storage modulus of the hybrid SMC could be shifted to higher temperatures and damping was also significantly increased due to hybridization.

Fiber reinforcement of a resin modified glass ionomer cement

ObjectivesUnderstand how discontinuous short glass fibers and braided long fibers can be effectively used to reinforce a resin modified glass ionomer cement (RMGIC) for carious lesion restorations. MethodsTwo control groups (powder/liquid kit and capsule) were prepared from a light cured RMGIC. Either discontinuous short glass fibers or braided polyethylene fiber ribbons were used as a reinforcement both with and without pre-impregnation with resin. For the former case, the matrix was the powder/liquid kit RMGIC, and for the latter case the matrix was the capsule form. Flexural strength was evaluated by three-point beam bending and fracture toughness was evaluated by the single-edge V-notch beam method. Compressive strength tests were performed on cylindrical samples. Results were compared by analysis of variances and Tukey’s post-hoc test. Flexural strength data were analyzed using Weibull statistical analysis. ResultsThe short fiber reinforced RMGIC both with and without pre-impregnation showed a significant increase of ∼50% in the mean flexural strength and 160–220% higher fracture toughness compared with the powder/liquid RMGIC control. Reinforcement with continuous braided fibers gave more than a 150% increase in flexural strength, and pre-impregnation of the braided fibers with resin resulted in a significant flexural strength increase of more than 300% relative to the capsule control. However, for the short fiber reinforced RMGIC there was no significant benefit of resin pre-impregnation of the fibers. The Weibull modulus for the flexural strength approximately doubled for the fiber reinforced groups compared to the control groups. Finally, compressive strength was similar for all the groups tested. SignificanceBy using a RMGIC as a matrix, higher flexural strength was achieved compared to reported values for short fiber reinforced GICs. Additionally, the short fibers provided effective toughening of the RMGIC matrix by a fiber bridging mechanism. Finally, continuous braided polyethylene fibers gave much higher flexural strength than discontinuous glass fibers, and their effectiveness was enhanced by pre-impregnation of the fibers with resin.

Predicting Microstructural Void Nucleation in Discontinuous Fiber Composites through Coupled in-situ X-ray Tomography Experiments and Simulations

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.

FEM analysis of the elastic behavior of composites and nanocomposites with arbitrarily oriented reinforcements

This work is aimed at studying the effect of different morphological features on the stiffness of discontinuous fiber reinforced composites and nanocomposites by means of FEM analysis. The morphological features include not only volume fraction and aspect ratio of reinforcement, but also its orientation. The FEM approach, based on continuum mechanics, required the definition of a proper representative volume element, in which reinforcement is randomly dispersed in the simulation domain. The developed model has been used in order to calculate the stiffness matrix of unidirectional discontinuous glass fiber reinforced composite (DGFRC) as well as carbon nanotubes reinforced nanocomposites (CNTRNC). Comparison with closed form analytical models showed that the Halpin-Tsai (HT) model is able to predict the evolution of the elastic constants over a wide range of volume fractions and aspect ratio. The Tandon-Weng (TW) model, in contrast, shows a poor agreement with the simulation results.On the other hand, the developed FEM model also allowed accounting for reinforcement orientation. The calculated elastic properties are in excellent good agreement with the corresponding values calculated according to the classical lamination theory, indicating the accuracy of the developed FEM model for estimation of stiffness of composites and nanocomposites, also accounting for reinforcement orientation.

Effect of Glass Fibers on Self Compacting Concrete

Self Compacting Concrete (SCC) is a specially developed concrete for concreting under extreme condition of inaccessibility from heights. It is capable to flow under influence of its own weight. It could be used when encountered with dense reinforcement and complex structural design. Problem of segregation as well as bleeding is eliminated and vibration is not required for compaction. As concrete is strong in compression and weak in tension. Hence to make it strong in tension, discontinuous Anti-Crack high dispersion glass fibers are added. SCC mix prepared with addition of discontinuous glass fibers is called as Glass Fiber reinforced Self Compacting Concrete (GFRSCC). In this paper an experimental study has been carried out to check the effect of Anti-Crack high dispersion glass fibers on the compressive strength, split tensile strength and flexural strength of SCC. The result show that, as compared to the Normal SCC, the compressive strength of GFRSCC increases by 2.80% and 12.42%, the split tensile strength of GFRSCC increases by 4.47% and 25.12% and the flexural strength of SCC increases by 6.57% and 14.34% when the Cem-FIL Anti-Crack HD glass fibers were added as 0.25% and 0.50% respectively by the weight of total cementitious material contents. The addition of 0.25% Cem-FIL Anti-Crack HD glass fibers to SCC has not much affect on the workability of Normal SCC. Whereas, addition of 0.50% Cem-FIL Anti-Crack HD glass fibers reduces the workability of SCC.

Prediction of elastic anisotropic thermo-dependent properties of discontinuous fiber-reinforced composites

This study focuses on the micromechanical prediction of temperature-dependent elastic properties of a composite made of a polypropylene matrix reinforced with discontinuous glass fibers. Firstly, an experimental investigation of the mechanical behavior is presented. Specimen are cut from injection-molded rectangular plates using a pattern based on fiber orientation. The microstructure is investigated by X-ray tomography at the specimen center and an average orientation tensor is calculated. Tensile tests are performed over a temperature range from ambient temperature to 85℃ and dispersion of mechanical properties is rather low; moreover, they are representative of the ones measured out of an industrial injected part. Then, the evolution of elastic properties with orientation and temperature is analyzed and compared with numerical predictions calculated with Mori–Tanaka homogenization scheme.

Rising R-curves in particulate/fiber-reinforced resin composite layered systems

BackgroundAn anisotropic fracture behavior has already been successfully induced in a condensable resin composite by reinforcement with discontinuous glass fibers, developing a steep rise in the fracture resistance curve. There is still the need for developing such concepts further, by evaluating the behavior of layered structures that attempt to replicate the natural gradient structure of enamel and dentin. MethodsMonolithic and bilayer specimens of a flowable short-fiber particulate-reinforced composite (SFPRC) material (GC Corp.) and a commercial flowable particulate-reinforced composite (PRC) (Essentia HiFlo, GC Corp.) were produced with the fibers aligned perpendicular or randomly oriented to the plane of crack propagation. Pre-cracked specimens were tested in 3-point bending using the single-edge-V-notch-beam method with accurate displacement tracking recordrecord. Controlled crack growth experiments were conducted at progressive crack extension intervals to build R-curves using linear-elastic and elastic-plastic fracture mechanics. ResultsWe demonstrate firsthand the development of R-curves in resin composite bilayers, with crack arrest at the interface and a steep rise in crack resistance with subsequent drop within the SFPRC layer. Crack deflection and fiber bridging were found to contribute to a higher rise in the R-curves for specimens with aligned fibers; still, randomly oriented fibers showed significant R-curve development, especially at short range intervals. An increase in the vol% of fibers and a decrease in the Young's modulus contributed to a higher overall performance of the flowable vs. the condensable SFPRF materials. ConclusionsSFPRC materials show an up to 5-fold improvement in the fracture resistance over conventional PRCs, and show the potential to be used to build gradient structures by varying the fiber alignment and fibers parameters (length and packing).

Effects of defects in hybrid sheet moulding compound – Evaluation of defects and the impact on mechanical properties

AbstractSheet moulding compound is a widely used fibre‐reinforced material. Generally, it consists of discontinuous glass fibres in a thermoset matrix system. Due to the finite fibre length, mechanical properties of structural components are limited. To overcome this drawback, sheet moulding compound is locally reinforced with a unidirectional carbon fibre sheet moulding compound material in the approach presented in this contribution. The manufacturing of this hybrid material consisting of discontinuous glass fibre sheet moulding compound and continuous carbon fibre sheet moulding compound can result in different defects, such as folds or fibre misalignments. These defects may affect mechanical properties of the hybrid material. Consequently, this article deals with the investigation and analysis of defective hybrid sheet moulding compound components, which were examined by means of tensile tests. Results point out that investigated defects have different effects on mechanical properties. However, independent from the type of defect, mechanical properties were reduced. With a reduction of 68.86 %, folds have one of the greatest influences on tensile strength. In addition, depending on the angle deviation, even greater reductions can occur. Furthermore, the reduction of the mechanical properties can be identified clearly with increasing angle deviation.

Comparing non-destructive 3D X-ray computed tomography with destructive optical microscopy for microstructural characterization of fiber reinforced composites

Due to their lightweight, relatively high stiffness properties, and formability into complex shapes, discontinuous fiber composites are advantageous for producing small and medium size components. Improved characterization techniques and post-processing methodologies are required for more reliable quantification of the microstructure and defect distributions in these materials, in order to employ model-based approaches to assess their structural integrity. This work compares a non-destructive X-ray approach with a destructive optical microscopy approach for characterizing the microstructural attributes, specifically the fiber volume fraction, porosity volume fraction, fiber orientation distribution, and fiber length distribution of discontinuous glass fibers in a polypropylene matrix. Additionally, a method for destructively determining the ambiguous components of the orientation tensor (related to the sign ambiguity of the out-of-plane angle in a destructive cross-sectional cut of a fiber) over a large surface area is included. It was found that fiber volume fraction and average fiber aspect ratio matched well, while fiber orientation and porosity detection had small but notable differences. The differences in the detection capabilities of each technique are quantified and discussed shedding light on the specific advantages and disadvantages of each approach, and enabling engineers to quantify uncertainty in their microstructural characterization measurements especially as they relate to model based structural integrity activities.

Combined <i>In Situ</i> X-Ray Computed Tomography and Acoustic Emission Analysis for Composite Characterization - A Feasibility Study

Fiber reinforced plastics show a wide range of different damage mechanisms such as matrix cracking, fiber breakage and interface failure. These can be observed in damaged specimens by means of volumetric images acquired by computed tomography (CT). As each failure mechanism causes a characteristic acoustic emission (AE) signal, AE analysis is a promising tool to identify damage mechanisms and offers the advantage that a real-time observation of the damage evolution during the testing period is possible. For a correlation of damage mechanisms and AE events, AE analysis was combined with in- situ CT measurements. This combined approach was validated by means of a 3-point-bending test on a discontinuous glass fiber reinforced sheet molding compound (GF-SMC) in which AE signals were acquired during loading using two high frequency piezoelectric sensors. At times of increasing AE activity, the test was interrupted in order to carry out a CT-scan of the specimen under load. AE events could subsequently be linked with the damage mechanisms observed in the CT-scans at different stages of damage to identify signal features that are characteristic for a certain mechanism. The sources of the signals could be localized and were in line with the actual location of damage.

Dynamic mechanical analysis of pure and fiber‐reinforced thermoset‐ and thermoplastic‐based polymers and free volume‐based viscoelastic modeling

Fiber‐reinforced polymer composites have gained significant importance in engineering applications and are widely used as structural components. The optimal choice of combinations of the type of polymer matrix and the fiber content depend on the required applications. In view of this, polypropylene (PP), a thermoplastic, reinforced with 30 wt.% of short glass fibers (PP‐GF30) is considered for this study. Additionally, a rather new composite system based on unsaturated polyester‐polyurethane hybrid resin (UPPH) is introduced. This thermoset‐based material is reinforced with 41 wt.% of long discontinuous glass fibers (UPPH‐GF41). Dynamic mechanical analysis (DMA) is performed on the pure polymer and the composite samples to experimentally characterize the temperature‐ and frequency‐dependent material behavior. It is observed that the stiffness of the materials is highly temperature‐dependent. PP exhibits a nonlinear viscoelastic behavior in the temperature range of possible applications whereas the UPPH material shows a less pronounced behavior. The resulting experimental data not only give general information on the material behavior of the composite subjected to a temperature and frequency load but also provide input as well as validation data for the developed material modeling methods. In an industrial environment, homogenized elastic material properties are desirable for analysis of the structural components at room temperature conditions. A mean field homogenization method is introduced to estimate the effective elastic material properties on a macroscopic scale. This method is formulated explicitly in terms of orientation tensors of second‐ and fourth‐order, describing quantitatively the microstructure of the corresponding composite. Additionally, the thermoviscoelastic material behavior for the considered temperature range is modeled by the free volume concept. In this context, the material parameters for the pure PP and UPPH material are identified. The calculated simulation results are compared with the experimental data entailing good agreements.

Cruciform Specimen Design for Biaxial Tensile Testing of SMC

This paper presents an investigation of different cruciform specimen designs for the characterization of sheet molding compound (SMC) under biaxial loading. The considered material is a discontinuous glass fiber reinforced thermoset. We define various (material-specific) requirements for an optimal specimen design. One key challenge represents the achievement of a high strain level in the center region of the cruciform specimen in order to observe damage, at the same time prevention of premature failure in the clamped specimen arms. Starting from the ISO norm for sheet metals, we introduce design variations, including two concepts to reinforce the specimens’ arms. An experimental evaluation includes two different loading scenarios, uniaxial tension and equi-biaxial tension. The best fit in terms of the defined optimality criteria, is a specimen manufactured in a layup with unidirectional reinforcing outer layers where a gentle milling process exposed the pure SMC in the center region of the specimen. This specimen performed superior for all considered loading conditions, for instance, in the uniaxial loading scenario, the average strain in the center region reached 87 % of the failure strain in a uniaxial tensile bone specimen.

Effect of discontinuous glass fibers on mechanical properties of glass ionomer cement

Aim: This study investigated the reinforcing effect of discontinuous glass microfibers with various loading fractions on selected mechanical properties of self-cure glass ionomer cement (GIC).Method: Experimental fiber reinforced GIC (Exp-GIC) was prepared by adding discontinuous glass microfiber (silane/non-silane treated) of 200–300 µm in length to the powder of self-cure GIC (GC Fuji IX) with various mass ratios (15, 20, 25, 35, and 45 mass%) using a high speed mixing device. Flexural strength, flexural modulus, work of fracture, compressive strength and diametral tensile strength were determined for each experimental and control materials. The specimens (n = 8) were wet stored (37 °C for one day) before testing. Scanning electron microscopy equipped with energy dispersive spectrometer was used to analysis the surface of silanized or non-silanized fibers after treated with cement liquid. The results were analyzed with using multivariate analysis of variance MANOVA.Results: Fiber-reinforced GIC (25 mass%) had significantly higher mechanical performance of flexural modulus (3.8 GPa), flexural strength (48 MPa), and diametral tensile strength (18 MPa) (p < .05) compared to unreinforced material (0.9 GPa, 26 MPa and 8 MPa). No statistical significant difference in tested mechanical properties was recorded between silanized and non-silanized Exp-GIC groups. Compressive strength did not show any significant differences (p > .05) between the fiber-reinforced and unreinforced GIC.Conclusion: The use of discontinuous glass microfibers with self-cure GIC matrix considerably increased the all of the studied properties except compressive strength.

DMA based characterization of stiffness reduction in long fiber reinforced polypropylene

This paper describes an experimental technique based on dynamic mechanical analysis (DMA) that is shown to be useful in separating nonlinear viscoelastic effects from the characterization of irreversible stress-induced damage responsible for stiffness reduction in fiber-reinforced polymer composites. In this work, we characterize the damage evolution of polypropylene (PP) reinforced with long, discontinuous glass fibers (GF) produced by the Direct Long-Fiber Thermoplastic/Compression Molding (D-LFT/CM) process. The experimental technique is comprised of three phases: 1) dynamic stabilization at low load to measure the time-dependent storage and loss moduli, followed by 2) a frequency sweep to provide rate-dependent viscoelastic properties and 3) a quasi-static application of a peak load. These three phases are repeated with the peak load of Phase 3 increased in each iteration. Experimental results for D-LFT/CM PP/GF30 presented here show a direct correlation between peak load and the irreversible stiffness reduction. Furthermore, the stabilization phase following peak load application is shown to lead to a stiffness recovery of up to 40% due to nonlinear viscoelastic recovery.