Improving the interlaminar fracture toughness of carbon fiber/epoxy composites using clustered microcapsules

The composite laminates are susceptible to delamination between reinforcing plies during their long-term service. In this paper, we propose a modified carbon fiber/epoxy composite laminate with embedded clustered dual-component microcapsules in order to increase the interlaminar fracture toughness of the lamina. The details of microcapsules were illustrated using scanning electron microscope (SEM). The modified CF/EP composite laminates were fabricated using hot-compaction technique. Mode I interlaminar fracture tests were conducted using double cantilever beam specimens, then the values of opening fracture toughness G_IC were calculated to evaluate the toughening effect of modified laminates. The toughening mechanism was revealed and discussed through micrographs of the fracture surfaces obtained by ultra-depth microscope and SEM. The results show that clustered microcapsules after polymerization are equal to special Z-pinning, significantly enhancing the ability of crack arrest, and largely and roundly improved the G_IC values of resultant composite laminates. Meanwhile, the clustered microcapsules and matrix resin formed a second-phase material layer, which also absorbed the fracture energy and suppressed the expansion of cracks.     

Numerical Assessment of PE 80 and PE 100 Pipe Lifetime Based on Paris-Erdogan Equation

Summary: A novel accelerated fracture mechanics extrapolation procedure based on cyclic test with cracked round bar (CRB) specimens was verified by a correlation of real pipe failure time to simulated failure times at a temperature of 60 °C. The procedure was applied to predict the long-term failure of modern PE 80 and PE 100 pipes 23 °C. Moreover, the used stress intensity factor concept also allows to consider the impact of arbitrary additional loading situations like soil loads or point loads and to assess pipe lifetime under complex loading situations.     

Dynamic 4 ENF test for a strain rate dependent mode II interlaminar fracture toughness characterization of unidirectional carbon fibre epoxy composites

A dynamic 4ENF testing procedure has been proposed to characterize the influence of loading rate on mode II fracture interlaminar toughness of a unidirectional composite material. The stable crack growth accomplished by the proposed dynamic 4ENF procedure allows achievement of the R-curves and the in-situ compliance calibration of each specimen. This enables performance of a monotonic dynamic test and presents a great advantage for the dynamic interlaminar characterization of the composite material. The dynamic G_IIC-s values obtained are similar to those determined by quasi-static loading conditions, which suggests that the proposed testing procedure is able to determine accurately the dynamic G_IIC of a composite material. However, for the tested material and within the analysed loading rate range, both the determined R-curves and G_IIC values do not show any clear sign of loading rate dependence.     

Lifetime prediction of a blue PE100 water pipe

The traditional method to assess the lifetime of plastic pipes is based on hydrostatic pressure testing. A complementary approach has been conducted to monitor the depletion of antioxidants and initiation of thermo-oxidative degradation on a PE100 blue water pipe that had been exposed to hydrostatic pressure in water at low test temperatures (maximum 80 °C).Depletion of antioxidants was monitored using OIT testing and initiation of thermo-oxidative degradation was assessed by iodometric detection of hydroperoxides. An empirical model based on the Arrhenius fit of the data was developed to extrapolate the lifetime of the PE100 pipe material at various service temperatures (10-25 °C). Associated activation energies, E_a, were determined and appeared to be in line with the values obtained from experiments carried out at low test temperatures. The combination of pressure testing and chemical analyses proved to be a very powerful tool to extrapolate the lifetime of plastic pipes.     

On the use of edge cracked short bend beam specimen for PMMA fracture toughness testing under mixed-mode I/II

In this paper an inclined edge cracked short beam specimen subjected to symmetric three-point bend loading was designed and examined for conducting mixed-mode I/II fracture toughness experiments. The aspect ratio (i.e. length to width ratio) and the loading span distance are considered much lower than the other conventional cracked bend beam samples. Crack tip parameters such as stress intensity factors and T-stress were computed numerically for this specimen by several finite element analyses and it was demonstrated that the specimen is able to produce full combinations of mode I and II including pure mode II. The practical capability of the short bend beam specimen was studied experimentally by conducting a set of mixed-mode fracture tests on PolymethylMethacrylate (PMMA) as a well-known model brittle material. The critical stress intensity factors, the direction of fracture kinking and the path of fracture trajectory were investigated both experimentally and theoretically using two stress and strain-based fracture criteria. The fracture toughness of tested PMMA was decreased by moving towards mode II case due to the effect of T-stress on the fracture mechanism of the short bend beam specimen.     

Interlaminar fracture toughness of unidirectional DCB specimens: A novel theoretical approach

A novel theoretical approach is presented to calculate the mode I interlaminar fracture toughness (G_Ic) of double cantilever beam (DCB) specimens with low ratio of initial crack length-to-thickness (a₀/2h). This method is based on a sixth-order beam theory, namely Reddy-Bickford beam (RB), on Winkler elastic foundation (WEF) to account for both transverse shear deformation of the beam and local effects at the delamination front (root rotation). RB with only two generalized displacements w and ?; and three boundary conditions at ends and loading points of a shear deformable beam gives more accurate results than the fourth-order Timoshenko beam theory. The accuracy of the proposed method in prediction of initiation G_Ic values is evaluated together with other available models considering the experimental fracture toughness for moderately thick unidirectional E-glass/epoxy DCB specimens with small initial delamination lengths.     

The effect of heat ageing on fracture toughness in uPVC pressure pipe

This paper reports a study of the effect of accelerated heat ageing in air on the fracture toughness of two uPVC pipes. The pipes were extruded at 179°C and 195°C respectively and curved fracture toughness specimens were aged at temperatures between 100 and 140°C. After testing at a crosshead rate of 1 mm/min, a similar activation energy of between 104 and 115 kJ/mol was calculated, for both pipes, from the variation of fracture toughness with ageing time and temperature. Extrapolation of the elevated-temperature data down to 20°C showed thermo-oxidative degradation to have a small effect on the fracture toughness during the expected service lifetime of 50–100 years.     

A new method for the detection of thermal oxidation in polyethylene pipes

Dumb-bell shaped specimens containing the inner wall surface of the pipe cut from 12 polyethylene pipes of different origin were subjected to constant uniaxial tensile loads at 313 K in air. The brittleness of the inner wall surface layer of the thermally oxidized pipes manifested itself in a shift of the unstable/stable necking transition to lower stresses and longer times. With the aid of optical and scanning electron microscopy, the brittleness of the inner wall surface layer of the oxidized pipes was demonstrated in distinctive surface cracks, which were the dominant feature of the drawn samples of the oxidized pipes. No such surface cracks were observed in the non-oxidized samples. The surface crack patterns were characterized in terms of crack frequency (longitudinal and transverse), width of fragments of cracked top layer (l_B), plastic strain of the fragments of the cracked top layer (ε_pl) and thickness of the cracked top layer (H). Correlations were found that can be interpreted according to basic principles of fracture mechanics. Through knowledge of, for example, ε_pl and l_B the values of the other variables can be predicted. The thickness of the cracked top layer corrected for the reduction in thickness due to plastic deformation is approximately equal to the thickness of the oxidized layer as determined by polarized microscopy. Drawing in an Instron Tensile Testing Machine at an elongation rate of 10 mm/min at 298 K also revealed the distinctive surface cracks in the oxidized samples. On the basis of these results, a new method for the detection of thermal oxidation in polyethylene pipes is proposed.     

Fracture toughness of polymers in the ductile‐to‐brittle transition region: Statistical approach and lower bound determination

A method available in literature was adapted and proposed for treating scatter and nonlinearity effects in fracture toughness of polymers in the ductile‐to‐brittle transition regime. The materials used were polypropylene homopolymer (PPH) and a polypropylene‐elastomeric polyolefin blend (PPH/POes 20 wt %), at room temperature and at 20‐mm/min test rate. Under such conditions, the fracture toughness presents a large scatter and a mean value can not be used as a design parameter because it leads to toughness overestimation. Then, there is a need to find a threshold of toughness, as a safe characteristic value for design. The toughness was evaluated by using the J‐integral approach. Large sets of specimens, 53 samples per each material, were tested with the purpose to reveal a reliable tendency in fracture behavior. As the toughness was considered nonuniform throughout the material, a weakest link model was assumed, and then results were analyzed statistically by means of a three‐parameter Weibull model (3P‐W). The PPH responded well to this 3P‐W model, whereas some deviations from the original model were observed in the PPH/POes blend. However, lower‐bound toughness values could be determined for both materials by censoring nonvalid data (Δa > 0.1b₀). From an engineering point of view, the results are very encouraging, since this methodology allows to obtain a threshold of fracture toughness from a given population, that is suitable to characterize the material fracture toughness at a given temperature and strain rate. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 3674–3684, 2005     

Effect of molecular relaxations on the fracture behavior of poly(vinyl chloride)

The fracture toughness of PVC has been measured by using three-point bend specimens tested over a wide range of strain rate and temperature. A method has been described of deriving fracture load indirectly by measurement of stiffness from a preliminary “low blow” test in instrumented impact testing. Some limitations of the method, when used with semi-ductile material of low stiffness, have been discussed. Fracture toughness results have also been evaluated by an alternative energy method, which is however more suitably applied to the lower speed impact test. The curves of K_1c versus temperature for PVC contain a weak maximum at a temperature below T_g, the location of which varies with testing speed. The position of the maximum in the time–temperature field has been compared with the locus of the β damping peak derived from mechanical (flexural vibrations, torsion pendulum) and dielectric loss measurements, with reasonable agreement. Static toughness was higher than dynamic, and this correlated with fracture surface appearance, thereby indicating a real difference in toughness probably associated with crack tip craze development.     

Dynamic fracture testing of polymethyl-methacrylate (PMMA) single-edge notched beam

Dynamic fracture in single-edge notched polymethyl-methacrylate (PMMA) beams have been investigated by three-point-bending impact testing with a drop-weight machine. A high-speed camera combined with the digital image correlation (DIC) method is used to capture the impact-induced crack initiation and propagation, as well as the beam deformation fields and the open mode strain at the original notch tip. The crack propagation length is recorded and the instantaneous crack velocity is calculated. Furthermore, the dynamic fracture toughness K_Id is quantified from the loading-displacement relations at different impact velocities. The effects of the impact velocity and impact energy on dynamic fracture toughness, fracture initiation strain, as well as the corresponding influences on the fracture propagation velocity, are discussed.     

The time and temperature dependence of fracture toughness for thermoplastics

A method for obtaining comparative and intrinsic toughness of thermoplastics involves deriving two apparent material properties, yield stress and a critical value of stress field intensity factor (fracture toughness). These are obtained at various times under load and temperatures. A ductility factor can then be derived and is the ratio of the square of fracture toughness to yield stress. Ductility factors can create a league table of toughness for thermoplastics which correlates reasonably with impact toughness energies.Individual values of fracture toughness and yield stress provide an intrinsic measure of toughness. Consequently, the relative toughness of a range of thermoplastics are explained in these terms. Materials discussed include homopolymers and copolymers of propylene, uPVC, PMMA (and rubber toughened versions) and PES.There is an importance in obtaining geometry independent fracture toughness measurements, otherwise likely artefacts are introduced. Such considerations are discussed for uPVC and polypropylene materials.     

Fracture toughness of nylon‐6 blends with maleated rubbers

The fracture toughness of blends of nylon‐6 with maleated ethylene–propylene rubber and maleated styrene/hydrogenated butadiene/styrene triblock copolymer was investigated with a single‐edge‐notched three‐point‐bending instrumented Dynatup test. The blends for which the rubber particle size was less than 0.7 μm fractured in a ductile manner over the whole range of ligament lengths, whereas the blends with particles larger than 0.7 μm showed a ductile‐to‐brittle transition with the ligament length. In this regime, ductile fracture was observed for specimens with short ligaments, whereas brittle fracture was seen for those with long ligaments. The ductile fracture behavior was analyzed with the essential‐work‐of‐fracture model, whereas linear elastic fracture mechanics techniques were used to analyze the brittle fracture behavior. The fact that the ductile fracture energy was larger for the blends with the styrene/hydrogenated butadiene/styrene triblock copolymer than for those with ethylene–propylene rubber was due to the larger dissipative energy density of the blends based on the styrene/hydrogenated butadiene/styrene triblock copolymer. Both the critical strain energy release rate (G_IC) and the plane‐strain critical stress intensity factor (K_IC) increased as the rubber particle size decreased for both blend systems. The G_IC and K_IC parameters had similar values, regardless of the rubber type, when the rubber particle size was fixed. The transition ligament length was near the size criterion for plane‐strain conditions for both blend systems. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 1739–1758, 2004     

Using developed creep constitutive model for optimum design of HDPE pipes

Unlike metal pipes, high density polyethylene (HDPE) pipes are not susceptible to erosion and corrosion. However, the most important mechanical feature of the HDPE pipes is that this material creeps even at room temperature. Therefore, it is essential to study the creep behavior of this material in order to develop a model. In this paper, creep behavior of HDPE at different temperature and stress levels has been experimentally studied to obtain the creep constitutive parameters of the material. These parameters are used to predict the creep behavior of different structures such as HDPE pipes. For this purpose, a number of specimens have been machined from industrial manufactured pipe walls. Uniaxial creep tests have been carried out and creep strain curves with time for each test were recorded. Then, a constitutive model is proposed for HDPE based on the experimental data and optimization methods. The results of this model have been compared with the test data and good agreement is observed. The developed constitutive model and reference stress method (RSM) were used to produce graphs which provide optimum creep lifetime and design conditions for HDPE pipes that are subjected to combined internal pressure and rotation. These graphs can facilitate the design process of HDPE pipes.     

Visualisation de l'écoulement discontinu gaz-solide dans un changement de direction

With the use of 25 images per s camera, the process of solid plugs of a gas-solid plug flow in a pipe bend with constant curvature (R_b/D=0) can be highlighted. This process can be seen on frame sequences 1, 2, 3 and 4. This study is realised in horizontal-vertical pipes, for solid-gas mass flow rates (q=q_s/q_g) equal to 12.7 and 17.9. The visualisations show that the flow deceleration and propulsion stages of the solid plug are not correlated because at the end of the deceleration stage, the plug velocity is zero. This work has allowed us to validate the different hypotheses necessary for the stress model between solid plug and the outer face of a pipe bend.     

Fracture process and molecular kinetics of PMMA

The critical value of the stress intensity factor, K_1c, (fracture toughness) has been measured for poly(methyl methacrylate) (PMMA) over a wide range of testing speed (K ? 4 to K? ? 50 × 10⁶ lb/in^3/2-sec) and temperature (from ?197°C to +21°C) in air and inert gas, by use of single-edge notch, double cantilever beam (normal and compact types) and instrumented impact tests. Some features of the toughness curves were found to be subject to time–temperature shifts and were explained in terms of relaxation motions of parts of the polymer molecule, (second-order thermodynamic transitions). Correlation with published data on mechanical energy losses in vibration experiments, dielectric losses and NMR results provided an identification of the operative relaxations. The coincidence of the fracture mode transition in the glassy state with the peak of the β relaxation was observed and the trend of K_1c values in a transition region was attributed to the close relationship of K_1c to the complex modulus for the hard glassy state. Impact and propagating crack values of K_1c have been discussed in general terms, and the limitations of fracture mechanics in studying a time-dependent material property have been considered in the light of recent rheological studies on the fracture of polymers.     

Imidazolium ionic liquids as fracture toughening agents in DGEBA-TETA epoxy resin

Although epoxy resins are used in a broad variety of applications due to their good mechanical and thermal properties, their low fracture toughness is a limitation, exhibiting brittle behavior. This study explored the potential use of imidazolium ionic liquids (IL) as toughening agents for epoxy resin based on diglycidyl ether of bisphenol A (DGEBA) with triethylenetetramine (TETA) as curing agent. Fracture toughness was evaluated for DGEBA-TETA epoxy resins with eleven imidazolium IL and the best results were found for the IL with the chloride anion and the shortest N-alkyl side chain, C₄MImCl. The use of 1.0 phr of C₄MImCl lead to the reduction of the crosslink density of the post-cured resin, resulting in the increase of 25.5% in stress intensity factor and 8.2% in tensile strength with no significant loss in other mechanical properties.     

Determination and comparison of the plane stress essentialwork of fracture of three polyesters PET, PPT and PBT

The essential work of fracture (EWF) method has been used to study the relationship between molecular structure and thin film fracture toughness for three ductile polyesters at ambient temperature. The fracture toughness of PPT is of particular interest. Successful fracture characterisation of thin film polyesters has been achieved by the EWF method using double edge notched tension (DENT) specimens. The specific essential work of fracture, w _e, for polyethylene terephthalate (PET), polypropylene terephthalate (PPT) and polybutylene terephthalate (PBT) films is found to be 35.54±2.56, 41.03±3.23 and 31.34±8.60 kJ m^–2, respectively. Differential scanning calorimetry (DSC) has been employed to investigate the crystallinity of the polymers concerned and the effect of this on their EWF values.     

Determination and comparison of the essentialwork of fracture (EWF) of two polyester blends

The fracture toughness of blends of polypropylene terephthalate (PPT) with polyethylene terephthalate (PET) and polybutylene terephthalate (PBT) were investigated. Binary blends were prepared comprising 10:90, 30:70, 50:50, 70:30 and 90:10 mass/mass%. The fracture toughness was determined for each blend using the essential work of fracture (EWF) method and thin film double edge notched tension (DENT) specimens. The specific essential work of fracture, w _e, values obtained for blends of PET/PPT ranged from 27.33 to 37.38 kJ m^–2 whilst PBT/PPT blends yielded values ranging from 41.78 to 64.23 kJ m^–2. Differential scanning calorimetry (DSC) was employed to assess whether or not crystallinity levels influence the mechanical properties evaluated. The fracture toughness of PPT deteriorated with PET incorporation. However, high we values exceeding that of pure PPT were obtained for PBT/PPT blends across the composition range studied.