Gabriel LaPlante

ε-Caprolactam anionic polymerization with two catalyst-activator systems was evaluated. Temperature and activator type/concentration impact polymer properties. PA6 optimization improves thermoplastic hybrid composite reactive processing economically.

This study focuses on how catalyst concentration, activator concentration, and initial polymerization temperature impact in‐situ anionic ring‐opening polymerization (AROP) of ω‐lauryllactam. Catalyst and activator roles are fulfilled by NaH and toluene‐2,4‐diisocyanate (TDI), respectively, with varying catalyst/activator ratios to assess the influence of a bifunctional activator on the polymerization process. The materials produced undergo a thorough analysis, with a specific emphasis on solidification time. The examination extends to scrutinizing how different concentrations of catalyst/activator and polymerization temperatures affect crucial physical and chemical parameters. The study identifies NaH‐6 mol%/TDI‐3 mol% as the optimal formulation for solidification among the three explored temperatures. Notably, at 180 and 200°C, PA12 exhibits enhanced monomer conversion when a catalyst/activator ratio of 1.7 or 2 is applied. These findings underscore the significant impact of the catalyst/activator ratio and their individual concentrations on the polymer's final properties. This influence extends to factors such as crystallinity, polymer chain regularity, and dynamic mechanical properties. Additionally, the experimental conditions utilized for anionic polymerization are observed to shape the characteristics of PA12.

Discontinuous-long-fibre (DLF) composites fabricated from pre-impregnated unidirectional (UD) fibre chips are susceptible to structural deficiency. The in-plane highly anisotropic mechanical properties of the chips combined with the random nature of fibre orientation causes local weaknesses within the material when fibres are perpendicular to the load. Recent experimental results have shown that using woven-fibre chips could improve the performance of DLF composites by increasing their average mechanical properties and reducing their variability. To better understand the underlying phenomenon giving an advantage to the woven chips, a finite element model was developed to predict the mechanical properties obtained from a standard tensile test. DLF chips were modelled based on a voxel method where random chip positions were generated by an algorithm developed in this work. ANSYS® software was utilized to model the non-linear response associated with progressive damage of the composite. The maximum stress and the Puck failure criteria were employed to define damage initiation for the woven and UD fibres, respectively. Tensile modulus predictions for both types of chips showed good results when compared to experimental data. Strength predictions for the UD fibres also showed good correlation with experimental results, but the model overestimated the strength of the woven-fibre DLF composite. It appeared that the failure of the UD-fibre composites was associated with matrix failure (transverse tension and in-plane shear). Woven-fibre composites, however, showed damage modes linked to both fibre failure (longitudinal tension) and matrix failure (transverse tension and in-plane shear).

Discontinuous long-fibre (DLF) composites can be made with randomly oriented unidirectional pre-impregnated composite chips. High fibre volume fraction unidirectional fibre chips provide good mechanical properties to the DLF composite architecture, which enables this material to contribute to bridging the gap between continuous-fibre and randomly-oriented short-fibre composites. However, it is well known that unidirectional fibres have highly anisotropic in-plane behaviour, which causes weak points in the parts when chips are oriented at unfavourable angles. This can be problematic since chips are randomly oriented in DLF composites. To overcome this problem, this research utilizes woven fibre chips instead of unidirectional fibre chips to fabricate DLF composites. Woven fibres diminish the potential for weak points due to their more homogenized in-plane mechanical properties. For comparison purposes, compression moulded carbon/PEI samples were made from both unidirectional chips and 5HS woven chips. Bending and tensile tests following ASTM guidelines were performed to compare both types of fibre arrangement. The results show that woven fibre chips increase the mechanical properties of the DLF composites and reduce their variability.

Elastic properties in critical areas of 3 D shells made from discontinuous long fibre (DLF) composites are difficult to determine via traditional methods, due to the heterogeneity of the material and the geometry of the part. In this paper, a method is proposed to predict the local modulus of DLF composites based on a micrograph of the polished edge of a specimen. The position and orientation of each fibre are extracted from the micrograph and used in conjunction with classical lamination theory to predict the elastic modulus. Fibre discontinuity is accounted for by including a correction factor based on the Cox formula for averaged elastic constants. Model predictions successfully matched the experimental tests results. In a previous study, material flow during compression moulding of recycled DLF panels led to anisotropic behaviour, which was hypothesized to be caused by chip alignment in the flow direction. By using the proposed method, chip alignment due to flow was confirmed and the anisotropy in the elastic modulus was accurately predicted.

An investigation into high flow compression molding for recycling thermoplastic discontinuous long fiber composites is presented. High flow recycled panels and conventional low flow baseline panels were produced with a large rectangular (2:1 aspect ratio) mold. Flow was induced in the recycled panels by stacking cut sections of conventionally produced baseline panels in the center of the mold cavity, representing 25% initial coverage. High flow compression molded panels were found to exhibit significantly higher than baseline tensile strength (+50%) and modulus (+31%) when tested in the direction parallel to flow. When tested in the direction perpendicular to flow, the opposite effect was found, with reductions in tensile strength (−42%) and modulus (−37%). However, when the average results of both directions are compared to baseline, no significant difference was found between the recycled and baseline panels. This severe anisotropic redistribution of mechanical properties suggests chip orientation is affected by flow. Additionally, micrographic analysis revealed that high flow molding induces intra-ply chip shearing and a reduction in resin rich regions within panels. Baseline panels also exhibited in-plane anisotropy, despite initial random distribution of chips and no or near no flow induced during molding. In this case, mechanical properties favored the direction perpendicular to that of the recycled panels.

This article presents the development of an experimental methodology based on acoustic emission wave detection for determining delamination onset and propagation in carbon fibre composite materials under quasi-static and fatigue loading. Delamination was investigated in quasi-static interlaminar fracture testing over a wide range of mixed-mode ratios ( G_II/ G_T = 0, 0.3, 0.5 and 1) for unidirectional and woven samples. An acoustic emission wave detection method was developed to detect delamination onset, and the corresponding fracture toughness was computed. Interlaminar fracture toughness was also calculated by beam theory and from finite element analysis with the virtual crack closure technique. The mechanical testing results, acoustic emission monitoring and numerical model’s interlaminar fracture toughness were used to define delamination initiation criteria by drawing two-dimensional envelopes corresponding to G_C = f( G_II/ G_T). The acoustic emission wave detection method showed damage accumulation before observable crack propagation, and its failure envelope corresponded to lower fracture energies than the standard test and modelling methods. Mode I fatigue testing with acoustic emission monitoring was performed on the woven samples for different energy release rate ratios ( G_IMAX/ G_IC = 0.3–0.8). A first series of samples were tested to construct an onset delamination fatigue curve Δ G = f( N). A second series of samples were used to study the cumulative acoustic emission energy distribution during delamination growth. An unsupervised pattern recognition methodology is presented for crack opening and closing testing, in order to discriminate between fatigue signal noise and acoustic emission signals emitted from crack initiation and crack growth. Correlations were observed between the acoustic emission energy distribution, the load range, the delamination length and the crack growth rate.