Toan Vu-Khanh

Forming of Woven Fabric Composites

Forming woven fabric composites into complex parts consisting of double curvature shells leads to complex redistribution and reorientation of fibers in the composite. The thermo-mechanical performance of the molded part is highly dependent on this rearrangement. Consequently, prediction of the properties of double curvature parts requires good understanding of the structural rearrangement in each layer of the composite laminate. This paper presents the results of a study on the prediction of the complex rearrangement that occurs during molding of complex components made of woven fabric prepregs. The deformation of the composite laminate to conform to a given geometry has been modeled using the pin-jointed net idealization and the volume conservation assumption. Experimental measurements of the fiber orientation and the ply thickness variation have also been carried out on molded parts with double curvature shape made of different fabric structures. The numerical prediction has been found to be in good agreement with experimental measurements.

The effects of injection molding on the mechanical behavior of long-fiber reinforced PBT/PET blends

Abstract Long-fiber reinforced thermoplastics have received much attention because of their processability by conventional technologies for thermoplastics. However, fiber degradation represents a major problem. This work has been undertaken with the objective of investigating the effects of injection molding parameters on fiber degradation and fracture performance in PBT-PET blend/glass fiber composites. The influences of six parameters and their interactions are analyzed, i.e. peak cavity pressure, holding pressure, back pressure, screw speed, melt temperature and barrel profile. The results show that most of the molding variables, as well as their interactions, affect the properties of the composite. Fiber-length retention is not the sole parameter to be optimized; the matrix system is also affected by molding conditions and has significant effects on composite properties. The influence of the composite microstructure, which is controlled by the molding process, on the rigidity, strength and toughness of the composite is also discussed.

Fracture Mechanics and Mechanisms of Impact-Induced Delamination in Laminated Composites

The fracture mechanism of impact-induced delamination is studied in carbon fiber/PEEK (polyetheretherketone) cross-ply laminates under drop weight impact. The study is based on the energy theory of fracture mechanics and the concept of crack arrest toughness. The damaged laminate is modeled by a finite element method which simulates delaminations and transverse cracks. The numerical results are combined with test data to study the delamination behavior. It is found that the delamination occurs in a deflection-controlled condition and is a process of Mode II dominated unstable crack growth and subsequent arrest. The fracture behavior can be described by strain energy release rate and the delamination size is governed by the delamination arrest toughness of the composite.

Impact fracture behavior of nylon‐6/ABS blends

Tensile and impact properties of uncompatibilized nylon-6/ABS blends have been studied over the entire range of compositions. The blends were prepared by extrusion and, subsequently, injection molded into tensile specimens and rectangular plaques. The impact fracture performance was characterized using recently proposed models based on fracture mechanics, for various fracture behaviors. The results showed that nylon-6 breaks in a brittle manner. With the addition of ABS, the blend exhibits the same behavior with a slightly lower impact resistance up to about 60 wt %. A sudden jump in the value of impact fracture energy is observed around 70 wt % ABS with a brittle—ductile transition in the mechanism of fracture. The transition in fracture mechanisms is confirmed through observation of the fracture surfaces by scanning electron microscopy (SEM). Tensile tests showed that the elongation at break increases only slightly between 0 and 50% ABS content, but a significant jump occurs around 70% ABS, reaching a 6-fold increase in comparison to that of the pure components. SEM observation of etched samples shows that a cocontinuous morphology occurs around 70 wt % ABS. The peak observed for the elongation at break and the jump in impact performance, as well as the onset of brittle–ductile transition, are attributed to this morphological effect.

Damage Extension in Carbon Fiber/PEEK Crossply Laminates under Low Velocity Impact

Low-velocity impact in carbon fiber/PEEK crossply laminates has been studied by test and analysis. Emphases of the study were focused on the material properties which may control the damage extension of transverse crack and delamination. It was found that, considering the thermal residual stress and the crack constraining effect, extension of transverse cracks could not be predicted by the Strength of Materials approach. The impact-induced delamination could be characterized by the crack arrest concept of fracture mechanics. The delamination resulted from a Mode II-dominated unstable fracture, which occurred under displacement-controlled conditions and seemed to be arrested at a constant interlaminar fracture energy. It was found that the thermoplastic APC-2 composite exhibits the same damage modes as epoxy composites under low velocity impact. Both the matrix-controlled damage and the fiber-controlled penetration may become the dominant failure mode, depending on the stacking sequence of the laminate. The residual stress in the thermoplastic laminates is as high as half of the transverse strength of the unidirectional material. The crack constraining effect tends to increase the in situ transverse strength of the lamina as the lamina thickness decreases. Considering the residual stress and crack constraining effect, the transverse crack extension cannot be predicted by the Strength of Materials approach. The crack arrest concept of fracture mechanics seems to be a useful approach to predict the extension of impact-induced delamination. The delamination resulted from a Mode II-dominated unstable fracture, which occurred under displacement-controlled conditions and seemed to be arrested at a constant interlaminar fracture energy. By assuming the delamination arrest at about the time of maximum impact load, the delamination arrest toughness could be evaluated from the test data of [05/905/05] laminates. The delamination arrest toughness is also found to be close to the Mode II-propagation toughness of the material.

Effects of physical aging on yielding kinetics of polycarbonate

Abstract The purpose of this work was to investigate the effects of physical aging on the kinetics of yielding in polycarbonate. PC samples were annealed over a wide range of aging times and temperatures. Both tensile and compressive tests were performed over various loading rates and temperatures to analyze the effects of aging time and aging temperature on yielding kinetics. Two grades of polycarbonate, Makrolon, of different molecular weights, PC-2608 (low Mw), and PC-3208 (high Mw), supplied by Bayer were analyzed. In unaged condition, PC is hard and tough, but after aging, it becomes more brittle. In terms of molecular movement, the yielding process is a thermally activated process involving inter- and intra-molecular motions. The time–temperature dependence of yielding behavior can be separated into two regions. Aging does not affect localized molecular motions of the β process during yielding. Physical aging in PC results in a slower jump rate of the main segments of macromolecules between two equilibrium positions. It reduces the flexibility of the macromolecules and thus, makes the polymer more brittle. Heat aging also causes a decrease of the entropy (ΔS) in polycarbonate, and this decrease is more important when the molecular weight is reduced. Increasing the annealing time and temperature results in a continuous reduction of ΔS. The rate of aging decreases with decreasing annealing temperature and below about 30 °C, no aging takes place. Annealing also strongly affects the excess of enthalpy in PC. However the effect of physical aging on yielding differs to that on enthalpy excess. The kinetics of yielding and aging processes in polycarbonate are also different. An increase in the strain rate does not have the same effect on the yield stress as an increase in the aging time by a same factor.

Prediction of fibre rearrangement and thermal expansion behaviour of deformed woven-fabric laminates

Abstract Forming woven-fabric composites into parts consisting of double-curvature shells leads to complex redistribution and reorientation of fibres in the composite. The thermo-mechanical behaviour of woven-fabric composites in the moulded part is highly dependent on this rearrangement. The paper presents the results of a study on the prediction of the forming-induced fibre rearrangement and the complex thermal expansion behaviour of deformed woven-fabric laminates. It is proposed that the thermo-mechanical properties of deformed fabric composites (non-orthogonal structure) can be characterized by a sub-plies laminate composed of four fictional unidirectional plies. Based on the sub-plies model, a workable approach is developed to analyse the thermal expansion properties of general woven-fabric laminates with any arbitrary in-plane shearing deformation. The equivalent on-axis thermal expansion coefficients of the constituent plies in the sub-plies model, which include the fibre undulation effect, are also determined. The orthogonal and various specimens made of 8-harness satin fabric composite, subjected to different in-plane shearing deformations before moulding, were tested from 21 °C to 177 °C to verify the proposed model. The predicted results are in good agreement with experimental data.

Recent developments and needs in materials used for personal protective equipment and their testing.

The field of personal protective equipment (PPE) has led to several high technology innovations. Indeed, improved protection against the various possible encountered risks is looked for, in particular at the workplace. This has generated the development of new materials and new manufacturing technologies, as well as the introduction of new applications for existing ones. However, the remaining challenges are numerous. This paper presents some of the new technologies introduced in the field of protective clothing against heat and flames, mechanical risks and chemical aggressors. It also describes new challenges that are currently worked on, in particular the effect of service aging and the need for testing methods that reproduce real-use conditions. Finally, it discusses various existing and potential applications of nanomaterials and smart textiles for PPE.

Effects of time and temperature on physical aging of polycarbonate

Abstract The effects of time and temperature on physical aging of polycarbonate (PC) have been investigated by differential scanning calorimetry (DSC). Aging of PC is a progressive process involving several mechanisms. The results agree with the concept of lateral entanglement. An increase in the density of entanglement with aging stage leads to a more stronger morphology that requires a higher temperature and a higher energy to disentangle. The method used to calculate the excess enthalpy from the DSC scan affects the measurement results and the interpretation of the aging process. The subtraction between the DSC scans of aged and de-aged samples would include the change in both the upper and the lower endothermic peaks. The method of extrapolating the base line above Tg back in temperature until it touches the peak would indicate only the change in the upper endothermic peak at the early stage of annealing, when the lower peak is small. During annealing, rejuvenation and aging take place simultaneously. At the early stage of annealing, rejuvenation is more important and can be easily detected by DSC analysis. At a later annealing stage, aging becomes predominant and outweighs the rejuvenation process.

Evaluation of the Flexibility of Protective Gloves

Two mechanical methods have been developed for the characterization of the flexibility of protective gloves, a key factor affecting their degree of usefulness for workers. The principle of the first method is similar to the ASTM D 4032 standard relative to fabric stiffness and simulates the deformations encountered by gloves that are not tight fitted to the hand. The second method characterizes the flexibility of gloves that are worn tight fitted. Its validity was theoretically verified for elastomer materials. Both methods should prove themselves as valuable tools for protective glove manufacturers, allowing their existing products to be characterized in terms of flexibility and the development of new ones better fitting workers’ needs.