Effect of polybutylene terephthalate (PBT) on impact property improvement of hybrid kenaf/glass epoxy composite

Environmental regulations, costs and lightweight encourage car manufacturers to develop new reliable products. Epoxy provides a reliable fibre impregnation and creates substantial three-dimensional (3D) cross-linking for proper load transmission and impact strength improvement, but their low toughness decreases their energy absorption. Thermoplastic toughening improves the epoxy impact property with a low thermo-mechanical defect. This study, focused on improving the impact property of hybrid kenaf/glass fibre epoxy composite by use of a modified sheet moulding compound (GMT). The results indicated that most of the mechanical properties of developed material were almost the same as those of the GMT, except impact. This result highlights the potential for utilisation of the toughened hybrid bio-composite in some automotive structural components. Moreover, geometric parameters, e.g., cross-section, thickness, and reinforcement ribs suggest an improvement of structural impact resistance to comply with the bumper beam product design specification (PDS).     

Dynamic crack resistance of steel pipe mains during crack initiation, growth, and arrest

Methods for evaluating the strength and crack resistance of steel pipe mains over a wide range of temperatures and loading rates are given. A large volume of data taken from complex studies on the dynamic strength and crack resistance of two types of pipe steels during crack initiation and arrest is examined. A quantitative analysis of the microstructure of these steels in three mutually perpendicular planes is given.Translated from Problemy Prochnosti, No. 7, pp. 3–16, July, 1993.     

Dynamic crack resistance of steel pipe mains during crack initiation,growth, and arrest. Report 2

On the basis of results obtained earlier, the effect of temperature and loading rate on the crack resistance of the steels under study has been analyzed. It has been shown that they are in good agreement with those predicted by the K model. Measurements on the length of the extension zone using stereoscopic indicate that there is quantitative agreement with crack resistance measurements obtained by conventional fractography methods. Translated from Problemy Prochnosti, No. 9, pp. 3–14, September, 1993.     

聚对苯二甲酸丁二醇酯(PBT)抗水解性能的研究

聚对苯二甲酸丁二醇酯（PBT）处于玻璃化温度以上的潮湿环境中时,由于其自有端羧基催化的酸性水解过程具有自加速的特点,造成材料力学性能急剧下降,通过封闭PBT的端羧基,使端羧基浓度下降到10meq/kg以下,并且添加抗水解稳定剂,消耗水解过程新生的端羧基,以冲缓PBT的酸性水解速度,提高PBT树脂耐水解性。     

Local stress and strain during crack growth by steady state creep

Experimental methods are used to measure the distribution of plastic strain ahead of a crack propagating under steady state creep conditions. Using these strains the local strain-rates are known, and from these the steady state stress distribution is deduced assuming power law behaviour. The resulting information indicates that the stress distribution is closer to _ss X ^–1/(m+1) than to _ss X ^–1/(2m). It is shown that for low values of the exponent,m, in the power law, that creep crack growth should correlate with the elastic stress intensity factor, whereas at largem values a better correlation is expected with the net stress.     

The initiation of crack growth in linear polyethylene

The initiation of crack growth was observed in single edge notched tensile specimens under plane strain conditions. The shape of the deformation zone varied from being balloon-like above 12 MPa and nearly planar below but neither shape conformed to that predicted by the conventional shear type yield criteria. The initial damaged zone grew at a constant velocity with a stress dependence ofV=V ₀ ⁿ . The spectrum of damage morphologies that preceed crack growth consists of a leading edge of crack-like pores followed by a region of increasing fibrillation which is terminated by a region of increasing fibril fracture.     

Effect of nanosilica and polyphosphazene elastomer on the in situ fibrillation of liquid crystalline polymer (LCP) and thermo-mechanical properties of polybutylene terephthalate (PBT)/LCP blend system

Nanocomposites of polybutylene terephthalate (PBT) and liquid crystalline polymer (LCP) with either polyphosphazene or nanosilica, or in combination of both were prepared by melt blending. The compatibility between the polymeric phases (PBT&LCP) was observed to be increased by the addition of polyphosphazene while the nanosilica promoted the LCP domain deformation from spherical to ellipsoidal shape. LCP fibres were produced in presence of both polyphosphazene and nanosilica due to the compatibilization of polyphosphazene and bridging effect of nanosilica through hydrogen bonding. All these above structural changes were confirmed by scanning electron microscope (SEM). Transmission electron microscope (TEM) images showed better dispersion of nanosilica in presence of polyphosphazene than nanosilica alone. There is remarkable increase in storage modulus with the addition of nanosilica, individually and in combination with polyphosphazene. Percentages of crystallinity for the concerned nanocomposites were calculated through X-ray diffraction study (XRD). Tensile strength and Young modulus were increased with addition of nanosilica and polyphosphazene but percentage of elongation at break was higher for polyphosphazene added nanocomposite. This is due to flexible compatibilizing effect of polyphosphazene, which delays the detachment of liquid crystalline polymer (LCP) domain from the polybutylene terephthalate (PBT) matrix and thus detains the fracture.     

Low-temperature cyclic cracking resistance of high-strength aluminum alloys in crack initiation and growth stages

Translated from Fiziko-Khimicheskaya Mekhanika Materialov, No. 3, pp. 40–49, May–June, 1990.     

Evaluation of copolymers of polyethylene oxide and polybutylene terephthalate (polyactive): mechanical behaviour

Polyether–polyester segmented block copolymers (Polyactive®) on the basis of polybutylene terephthalate (PBT) and polyethylene oxide (PEO) were mechanically tested. Tensile strength and modulus of elasticity in compressive and tensile deformation were recorded according to ASTM standards. These tests were done in vitro under dry and wet conditions, and after 3, 9 and 25 wk subcutaneous implantation of these materials in goats. Strength and modulus of elasticity were higher with increased contents of PBT in the copolymers. After water uptake, the polymer displayed a lower strength and stiffness. Disintegration of the materials with 70% PEO content and dumb-bell shape was noted at 3 wk. Disintegration of the cylinders of the same material was seen after 25 wk implantation. Of the materials with 60% PEO content, only four of the five dumb-bells had disintegrated after 25 wk implantation. The in vivo test results of all other implants did not show a clinically relevant decrease of strength and stiffness with time after implantation of the copolymers in the goats. Mechanical behavior of the various copolymers seemed mainly determined by the amount and integrity of the PBT phase.     

The DFZ model of fracture at crack tip and crack initiation criterion

Direct observation by transmission electron microscopy (TEM) has been made on the distribution of dislocations in front of the crack tip during tensile deformation of aluminum. A microfracture model has been established to describe the equilibrium configuration of the dislocations in the presence of a dislocation-free zone (DFZ). The site of void nucleation observed from TEM experiments was found to be at about the place of maximum dislocation density predicted from the model. The relationship between the size of crack, DFZ and crack opening displacement (COD) was obtained as a function for a crack initiation criterion.     

Fatigue crack initiation and growth in solder alloys

In modern electronic packaging, especially surface mount technology (SMT), thermal strain is usually induced between components during processing, and in service, by a mismatch in the thermal expansion coefficients. Since solder has a low melting temperature and is softer than other components in electronic packaging, most of the cyclic stresses and strains take place in the solder. Fatigue crack initiation and fatigue crack propagation are likely to occur in the solder even when the cyclic stress is below the yield stress. It is an objective of this research to study the behaviour of fatigue crack initiation and propagation in both lead‐containing solder (63Sn‐37Pb), and lead‐free solders (Sn‐3.5Ag). The effect of alloying (Cu and Bi addition), frequency, tensile hold time and temperature on low cycle fatigue (LCF) behaviour of the solders is discussed. Mechanisms of LCF crack initiation and propagation are proposed and LCF life prediction, based on the various models, is carried out.     

The effect of steady torsion on fatigue crack growth in shafts

Most of catastrophic mechanical failures in power rotor shafts occur under cyclic bending combined with steady torsion: Mode I (ΔK_I) combined with Mode III (K_III). An analysis of the influence of steady torsion loading on fatigue crack growth rates in shafts is presented for short as well as long cracks. Long cracks growth tests have been carried out on cylindrical specimens in DIN Ck45k steel for two types of testing: rotary or alternating bending combined with steady torsion in order to simulate real conditions on power rotor shafts. The growth and shape evolution of semi-elliptical surface cracks, starting from the cylindrical specimen surface, has been measured for several loading conditions and both testing types. Short crack growth tests have been carried out on specimens of the same material DIN Ck45k, under alternating bending combined with steady torsion. The short crack growth rates obtained are compared with long crack growth rates. Results have shown a significant reduction of the crack growth rates when a steady torsion Mode III is superimposed to cyclic Mode I. A 3D Finite Element analysis has also shown that Stress Intensity Factor values at the corner crack surface depend on the steady torsion value and the direction of the applied torque.     

Fatigue crack initiation from notch root (local-strain damage accumulation process on crack initiation)

Local deformation (i.e. local-strain behavior) at the notch root in a crack initiation process in annealed 0.48% carbon steel was investigated by the real-time fine-grid method. The fatigue crack initiation cycle was controlled by local-strain damage accumulation. For a quantitative expression of cumulative fatigue damage, we propose a new parameter, the “average local-strain accumulation value,” , which is defined by the integration of local-strain histories until crack initiation. The relationship between average local-strain accumulation range, , and crack initiation cycles, N_c, showed a line whose slope was nearly −0.5 on a log-log coordinate graph. This line we term the “local-strain damage accumulation curve.” The mean stress effect in cases of R = −1, 0 on this line ( vs N_c) was very small or negligible. From the results of variable-loading tests, the linear cumulative damage law based on the local-strain value was also confirmed.     

Fatigue crack initiation and crystallographic crack growth in an austenitic stainless steel

The lifetime of a specimen under cyclic loading is usually limited by the initiation and growth of microcracks. Experimental results for the austenitic stainless steel X6 CrNiNb 18-10 are given and a model for crack initiation and crack growth in the first grains of a polycrystalline aggregate is proposed.     

The effect of repeating load on the crack growth initiation and crack propagation in delayed failure

The delayed failure test under repeating load was carried out with pre-cracked specimen. The incubation time and the crack propagation rate were correlated with the stress intensity factor K.

The incubation time is decreased by the superposition of repeating load, as the range of stress intensity factor ΔK or the repeating frequency f increase. The reason can be explained by the promotion of corrosion reaction due to, e.g. the destruction of oxide film on the crack tip, which facilitates the invasion of hydrogen atoms into the material.

The crack propagation rate da/dt is decreased by the superposition of repeating load, and there exist two valleys of crack propagation rate minima on the da/dt vs f and da/dt vs ΔK curves. One valley corresponds to the interaction between the cyclic movement of the region with tri-axial tensile stress and the hydrogen atoms diffused from crack tip, which disturbs the concentration of hydrogen atoms. Another seems te correspond to the generation of retained compressive stress which reduces the effective stress intensity at crack tip and supresses the invasion and diffusion of hydrogen atoms.     

The crack growth resistance of polyimide film

The crack growth resistance of Dupont Kapton film is measured using the constrained short centre-notched tension specimen that is clamped along its loaded edge. Poisson's effect induces a lateral tension on the specimen which eliminates the buckling problems associated with standard fracture tests. Polyimide film is shown to possess considerable crack growth resistance that decreases slightly with film thickness. SEM examination reveals the likely cause of the crack growth resistance to be cold drawing behind the crack tip.