Resistance of a polyetherimide to environmental stress crazing and cracking

The environmental stress crazing and cracking (ESC) behavior of an aromatic polyetherimide (PEI) has been characterized in a wide spectrum of organic liquids and compared to the behavior of several other glassy thermoplastics. PEI's response is qualitatively similar to that of the other resins for each of which ESC resistance reaches a minimum in solvents having solubility parameters close to that of the resin. Taken as a whole, the ESC resistance of PEI is found to be quantitatively superior to that of any other glassy resin for which similar data are available for comparison.     

Fatigue failure mechanisms in polymer composites

Axial fatigue of two, commercial, talc-filled polypropylenes was studied. A significant result of our investigation was the identification and characterization of the failure mechanisms and the effects of frequency and hysteretic heating. Frequency was found to be important to the fatigue response of the polymers. At low frequency, fatigue failure appeared to be a process of crack initiation and growth with very little dissipative heating. At high frequency, the fatigue process was dominated by dissipative heating resulting in significant creep and modulus loss, and failure was much accelerated and due mainly to material softening and melting. Correlation of temperature rise with on-line energy loss in selected experiments enabled us to extract information from the fatigue process. The results provided some quantitative understanding of the different fatigue failure mechanisms at low and high frequencies.     

Miscibility in molecular composites of polyamide-imide/polyetherimide

Summary Compositional dependence of miscibility in molecular composites of polyamide-imide/polyetherimide prepared by coagulation of the blend of the two polymers from dimethyl acetamide solutions has been established by calorimetric and dynamic mechanical thermal studies. This study reports on the thermodynamics of miscibility in the molecular composites.     

Solvent-induced crystallization in polyetherimide thermoplastics and their carbon fiber composites

Solvent-induced crystallization (SINC) was observed in a polyetherimide (PEI), a thermoplastic used as a matrix in carbon fiber composites. This observation was made using wide angle X-ray scattering (WAXS), differential scanning calorimetry (DSC), and optical microscopy. It was discovered that methylene chloride induces crystallization in the PEI by penetrating the surface and swelling the bulk polymer. Prepreg processed using N-methyl pyrrolidone (NMP) was also crystalline. One processed above the crystalline melting point (T_m), no crystallinity in the sample was found, as the PEI did not crystallize from the melt. The observed crystallization of both the neat polymer and its carbon fiber prepreg was exclusively through a solvent-induced process, although it is likely that the mechanism through which crystallization occurs during solvent prepreg processing is different than the diffusion-controlled mechanism demonstrated with methylene chloride. A solvent prepregging process may involve a low molecular weight or monomer solution as well as other polymerization by products. Measurements using WAXS showed a maximum degree of crystallinity of 30%, as induced by methylene chloride. A value of 85 J/g for the heat of solvent-induced crystallization in the PEI was calculated from the DSC measurements.     

Strength and bonding mechanisms in vibration-welded polycarbonate to polyetherimide joints

Under the right conditions, high strengths are shown to be achievable in vibration welded polycarbonate to polyetherimide Joints. While welding of thermoplastic interfaces of the same material can be understood in terms of interchain diffusion at elevated temperatures, this mechanism is severely limited in the case of dissimilar materials. Scanning electron microscopy is used to show that part of the bond strength in such dissimilar materials results from mechanical interlocking of the two polymers, which is caused by viscous mixing. The effects of the weld parameters on the weld morphology are considered in detail.     

Cryogenic failure mechanisms of fiber-epoxy composites for energy applications

Fiber-reinforced composites used in the production, storage, and transport of energy are often exposed to extreme environments such as cryogenic temperatures and/or nuclear radiation. The fraction resistance of composites subjected to these conditions are of fundamental interest. A study was undertaken to examine the effects of Gammaradiation on the structure and properties of fibers, matrix and their interfacial bonding in terms of their influence on composite failure mechanisms. Also thermomechanical analysis was conducted to estimate the residual thermal stresses due to differential thermal expansion coefficients both between fiber and matrix and between two laminae. Results indicate that the impact energy of a composite laminate could be increased by 100 percent by immersing the specimen in liquid nitrogen for only five min. Samples impact-tested at cryogenic temperatures were found to possess a great degree of delamination and crack bifurcation. Thermomechanical analysis and SEM investigation both reveal that microcracking and small-scale delamination are promoted by differential thermal contraction. Such effects could be responsible for the observed crack branching and delamination phenomena during impact loading. Although under certain circumstances the Gammaradiation may yield a small increase in fracture resistance it generally degrades the cohesive strength of the matrix and reduces the interfacial bonding between fiber and matrix. Results of a mechanical and microscopic analysis are presented and discussed.     

On temperature-dependent interfacial fracture energy/stress intensity factor and matrix cracking in ceramic composites

The first matrix cracking stress is a crucial indicator to appraise the mechanical properties of ceramic-matrix composites, which is the starting point of permanent damage. Based on the classic energy balance method and stress intensity method, two temperature-dependent first matrix cracking stress models of fiber-reinforced ceramic matrix composites are established, respectively. The model established by the energy balance method considers the evolution of interfacial fracture energy with temperature, and the model established by the stress intensity method takes into account the evolution of stress intensity factor with temperature. The predictions of the above models in a wide temperature range are verified using experimental data available in the literature, which shows the rationality and accuracy of the above models. Moreover, on the basis of the energy balance method model, the main factors controlling the first matrix cracking stress at different temperatures are analyzed by the numbers. Finally, in view of the analysis results, some suggestions on how to optimize and enhance the first matrix cracking stress at different temperatures are put forward.     

Controlled energy dissipation in fibrous composites. II: Microscopic failure mechanisms

Mechanical properties and microscopic fracture mechanisms of continuous fiber reinforced polymer composites were investigated. Perforated polyimide films (e.g. Kapton®) were added between composite prepreg layers to modify the interlaminar bonding strength. Addition of highly perforated films can increase the toughness of unidirectional glass/epoxy composites without an appreciable reduction in strength. The fibrous composites studied exhibit two fracture modes (compressive and tensile) when failed by three-point bending. In general, the compressive failure mode preceded the tensile failure mode. Real-time acoustic emission (AE) analysis was found to provide more fracture information which is otherwise not discernible from mechanical testing alone. The crack initiation stress level and the subsequent crack propagation mode were identified by real-time AE during deformation and by post-failure scanning electron microscopy fracture surface analysis.     

Effect of chemical etching of resistance wire surface on the strength and failure mechanism of the resistance-welded joint of polyetherimide composites

The surface modification of stainless-steel (SS) mesh was carried out by chemical etching for different time, and then, the etched SS meshes were used as the interlayer heating element for resistance welding of glass fabric-reinforced polyetherimide (GF/PEI) composite laminates. The hydrophobicity and tensile strength of the etched and untreated SS wires and interfacial shear strength (IFSS) between SS wire and PEI resin were investigated by dynamic contact angle (DCA) analyzer and single fiber pull-out test to evaluate the influence of the degree of chemical etching on the bonding strength of PEI resin and SS surface. Tensile lap-shear strength (LSS) and scanning electron microscope (SEM) analysis were performed to investigate the welding interface strength and fracture morphologies of the resistance-welded joint of the GF/PEI laminates. The hydrophobicity and tensile strength of SS wires were increased and decreased respectively with etching time (t_e). The IFSS of SS wire/PEI resin showed an improvement with t_e. The LSS was increased first and then decreased with t_e, and the optimal t_e was 30 min when LSS reached the maximum. Fracture analysis showed the main failure mode of the welded joints was the interlayer debonding when welding time (t_w) was insufficient. With the extension of t_w, the failure mode gradually switched into the intralayer tearing of the SS implant. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 47879.     

Effectual dispersion of carbon nanofibers in polyetherimide composites and their mechanical and tribological properties

The use of proliferation of nanotechnology in commercial applications is driving requirements for minimal chemical processing and simple processes in industry. Carbon nanofiber (CNF) products possess very high purity levels without the need of purification processing before use and are in growing demand for this quality. Polyetherimide (PEI) has excellent mechanical and thermal performance, but its high viscosity makes its nanocomposites processing very challenging. In this study, a facile melt‐mixing method was used to fabricate PEI nanocomposites with as received and physically treated CNFs. The dispersion of CNFs was characterized by scanning electron microscopy, transmitted optical microscopy, and electrometer with large‐area electrodes. The results showed that the facile and powerful melt‐mixing method is effective in homogeneously dispersing CNFs in the PEI matrix. The flexural and tribological characteristics were investigated and the formation of spatial networks of CNFs and weak interfacial bonding were considered as competitive factors to enhanced flexural properties. The composites with 1.0 wt% CNFs showed flexural strength and toughness increased by more than 50 and 550%, respectively, but showed very high wear rate comparable with that of pure PEI. The length of the CNFs also exerted great influences on both mechanical and tribological behaviors. POLYM. ENG. SCI., 50:1914–1922, 2010. © 2010 Society of Plastics Engineers     

Analysis of failure mechanisms of Oxide - Oxide ceramic matrix composites

The failure mechanisms of Oxide-Oxide ceramic matrix composites AS-N610 were studied at both room temperature and high temperature using tensile and fatigue tests with and without lateral and laminar notches. The unnotched coupons had an average tensile strength of 423 MPa with elastic modulus of 97 GPa at room temperature showing a perfect elastic behaviour whereas the laminar notched samples shown similar strength of 425 MPa with elastic modulus (98 GPa) revealing pseudo-ductile behaviour. A reduction in tensile strength of the oxide ceramic matrix composites was observed at high temperatures. Thermal shock experiments revealed that the retained strength of the samples quenched from 1100 °C deteriorated by ～10 % (395 ± 15 MPa). In all samples, fracture origin was observed on the mid-plane showing a higher degree of fiber pull-out, delamination and pseudo ductile behaviour. Finite element analysis confirmed higher stress concentration on the areas of failures.     

Temperature dependent first matrix cracking stress model for the unidirectional fiber reinforced ceramic composites

Based on a kind of equivalence between heat energy and fracture energy, assuming that there is a constant maximum storage of energy that includes both heat energy and fracture energy, a new temperature dependent fracture surface energy model is developed. Using the new model and the classical ACK theory, a temperature dependent first matrix cracking stress model is obtained for the fiber reinforced ceramic composites. According to the model, the temperature dependent first matrix cracking stress of materials can be easily predicted using some basic material parameters such as matrix fracture surface energy and Young’s modulus. The model is verified by comparison with experimental data of SiC fiber reinforced reaction-bonded Si₃N₄ composites at different temperatures. Good agreement is obtained between predicted and experimental data of first matrix cracking stress. The dependency of first matrix cracking stress on fracture surface energy and interfacial shear strength is systematically analyzed.     

Preparation of polyetherimide/carbon fibre composites by a cataphoresis process

Preliminary results in this study show that it is possible to achieve a good Ultem 1000/carbon fibres covering by means of electrophoretical deposition. The expected amount of deposited polymer is well controlled and the deposited films have a good appearance. In this way a variety of carbon fibre composite materials can be manufactured. © 1999 Society of Chemical Industry     

Graphene nanoplatelet‐polyetherimide composites: Revealed morphology and relation to properties

In this research, a novel sample preparation technique was applied to reveal the morphology exfoliated graphene nanoplatelets [GNP, ～10 nm thick and ～5 µm in diameter)] and Polyetherimide (PEId) nanocomposite and study the relationship between processing and properties. The morphology of nanoscale fillers used to be hard to capture through conventional sample preparation. The polish‐plasma etching approach presented in this article successfully created contrast between filler and matrix. As a result, distribution and orientation of the fillers were obtained to study the effect of injection molding, compression molding and annealing. It was found that the orientation was significantly different depending on processing routes. The information obtained from morphology study also led to the modification of Tandon–Weng model, resulting in improved prediction of elastic modulus of the composite. The SEM images also clearly revealed change of filler orientation after annealing. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 130: 4081–4089, 2013     

An assessment of crazing criteria for polyetherimide in three-dimensional stress space

Three point bend tests of notched, polyetherimide (PEI) beams are performed for varying notch radii and beam thicknesses. Finite element stress results including pressure sensitive yielding effects are correlated with experimental load-to-craze observations to determine the three-dimensional stress state at craze initiation. Three crazing criteria; maximum principal stress, hydrostatic stress, and the critical strain criterion of Oxborough and Bowden are examined for consistency. Over the range of geometries studied, the critical strain criterion provides the best fit to the experimental data. This criterion could be used as a brittle failure criterion to help understand and prevent brittle failures.     

Environmental stress cracking in impact polystyrene

Environmental stress cracking (ESC) measurements for various impact polystyrenes were performed using a constant load technique with the specimens in contact with a 50/50 solution of cotton seed oil and oleic acid. It was shown that ESC in impact polystyrene is controlled by the transport of the aggressive liquid through a pre-established dry craze structure where capillary pressure is the driving force. At moderate stress levels just above the critical stress for environmental cracking, there is an apparent incubation time for the dry craze formation. The craze incubation time is strongly influenced by thermal stresses induced by the gel particles. As a consequence, ESC is two-stage process involving both an incubation time and actual crack growth. Control of the craze structure to maximize fibril content is essential for good ESC resistance. The craze fibril content can be altered by variables such as gel particle size, matrix molecular weight, plasticizer content, and rubber content.     

Friction and wear studies of polyetherimide composites under oscillating sliding condition against steel cylinder

Tribological properties of neat polyetherimide (PEI), glass, carbon fiber, and solid lubricants filled PEI composites are presented in this article. The aim of this study was to investigate the friction and wear properties of these composites under dry oscillating sliding condition at room temperature (RT) as well as at elevated temperature (120°C). The polymer specimens were made to oscillate against steel cylinder as a counterpart. The friction and wear properties of PEI and composites were strongly influenced by the temperature. Incorporation of carbon fiber in the PEI matrix has increased the wear rate at RT, while at elevated temperature this trend was opposite. Abrasive action of carbon fibers has severely damaged the counterpart and resulted in accelerated wear of the composite at RT. Solid lubricants filled (PTFE, MoS₂, graphite) along with glass fiber is beneficial in improving the friction and wear performance of the PEI composite at RT, whereas at elevated temperature wear performance was deteriorated. Tribological performance of neat PEI and glass fiber composite was similar with each other at RT. Scanning electron micrographs and optical micrographs of the worn polymer specimens and the steel cylinders was used to study the possible wear mechanisms. The present test results were also compared with data available on the reciprocating wear of PEI and composites in the literature and trends have been reported. POLYM. COMPOS., 38:48–60, 2017. © 2015 Society of Plastics Engineers     

Environmental stress cracking of polyethylene

Deformation of polyethylene in environmental stress cracking (ESC) agents results in changes in both the mechanism of deformation and structure of the resulting drawn material. Stress-cracked failure surfaces are highly fibrillar, the fibrils having less elastic recovery than those in samples drawn in air. In thin films drawn in ESC agents, small blocks of the lamellae remain undrawn and attached to the fibrils drawn across micronecks. The ESC agents are suggested to weaken the cohesion between the fibrils in samples drawn beyond yield as well as the cohesion between mosaic blocks or similar structural elements in the original lamellae as they are being reoriented to form the fibrils. The stress is thus supported by a number of independent, nonuniform fibrils rather than a coherent structure; the weakest of these fibrils fail in turn as the crack propagates through the sample.     

Prediction of failure and fracture mechanisms of polymeric composites using finite element analysis. Part 2: Fiber reinforced composites

A robust finite element scheme for the micro‐mechanical modeling of the behavior of fiber reinforced polymeric composites under external loads is developed. The developed model is used to simulate stress distribution throughout the composite domain and to identify the locations where maximum stress concentrations occur. This information is used as a guide to predict dominant failure and crack growth mechanisms in fiber reinforced composites. The differences between continuous fibers, which are susceptible to unidirectional transverse fracture, and short fibers have been demonstrated. To assess the validity and range of applicability of the developed scheme, numerical results obtained by the model are compared with the available experimental data and also with the values found using other methods reported in the literature. These comparisons show that the present finite element scheme can generate meaningful results in the analysis of fiber reinforced composites.