Delamination growth mechanisms in woven glass fiber reinforced polymer composites under Mode II fatigue loading at cryogenic temperatures

The purpose of this research is to characterize the cryogenic delamination growth behavior in woven glass fiber reinforced polymer (GFRP) composite laminates subjected to Mode II fatigue loading. Mode II fatigue delamination tests were performed at room temperature, liquid nitrogen temperature (77 K) and liquid helium temperature (4 K) using the four-point bend end-notched flexure (4ENF) test method, and the delamination growth rate data for the woven GFRP laminates were obtained. The energy release rate range was determined by the finite element method. Microscopic examinations of the specimen sections and fracture surfaces were also carried out. The present results are discussed to obtain an understanding of the fatigue delamination growth mechanisms in the woven GFRP laminates under Mode II loading at cryogenic temperatures.     

Temperature effects on mixed mode I/II delamination under quasi-static and fatigue loading of a carbon/epoxy composite

This paper addresses the effect of temperature on the mixed-mode interlaminar fracture toughness and fatigue delamination growth rate of a carbon-fibre/epoxy material, namely IM7/8552. Quasi-static and fatigue characterisation tests were carried out at −50 °C, 20 °C, 50 °C and 80 °C, using asymmetric cut-ply coupons. The experimental results show that temperature may have an accelerating or delaying effect on delamination growth, depending on the loading regime, i.e. either quasi-static or fatigue. Fractographic examinations were also carried out in order to assist the interpretation of the experimental data. A semi-empirical equation is introduced to describe the experimentally observed fatigue delamination growth rates at elevated temperatures.     

Cryogenic delamination growth in woven glass/epoxy composite laminates under mixed-mode I/II fatigue loading

This paper investigates the fatigue delamination growth behavior in woven glass fiber reinforced polymer (GFRP) composite laminates under mixed-mode I/II conditions at cryogenic temperatures. Fatigue delamination tests were performed with the mixed-mode bending (MMB) test apparatus at room temperature, liquid nitrogen temperature (77 K) and liquid helium temperature (4 K), in order to obtain the delamination growth rate as a function of the range of the energy release rate, and the dependence of the delamination growth behavior on the temperature and the mixed-mode ratio of mode I and mode II was examined. The energy release rate was evaluated using three-dimensional finite element analysis. The fractographic examinations by scanning electron microscopy (SEM) were also carried out to assess the mixed-mode fatigue delamination growth mechanisms in the woven GFRP laminates at cryogenic temperatures.     

Experimental evidence of a common local mode II growth mechanism of fatigue cracks loaded in modes II,III and II + III in niobium and titanium

The paper is focused on an identification of the local mode II mechanism of fatigue cracks loaded under the remote mode III and the mixed mode II + III and presents a convincing experimental evidence of such a mechanism in materials with a nearly coplanar crack growth. Closure-free data were obtained by applying fatigue experiments in modes II, III and II + III in commercially pure titanium and niobium. The results revealed that the micromechanism of propagation of all kinds of shear-mode cracks can be described by a common model of advances of local mode II crack segments nearly in the direction of applied shear stress. These segments nucleated at spatial geometrical irregularities of the precrack front generating fibrous patterns at fracture surfaces.     

Effect of stitch density on fatigue characteristics and damage mechanisms of stitched carbon/epoxy composites

The effect of stitch density (SD) on fatigue life, stiffness degradation and fatigue damage mechanisms in carbon/epoxy (T800SC/XNRH6813) stitched using Vectran thread is presented in this paper. Moderately stitched composite (SD = 0.028/mm²; ‘stitched 6 × 6’) and densely stitched composite (SD = 0.111/mm²; ‘stitched 3 × 3’) are tested and compared with composite without stitch thread (SD = 0.0; ‘unstitched’). The experiments show that the fatigue life of stitched 3 × 3 is moderately better than that of unstitched and stitched 6 × 6. Stitched 3 × 3 pattern is also able to postpone the stiffness degradation onset. The improvement of fatigue properties and postponement of stiffness degradation onset in stitched 3 × 3 is primarily due to an effective impediment of edge-delamination. Quantification of damage at various cycles and stress levels shows that stitch density primarily affects the growth rate of delamination.     

Carbon/epoxy composite delamination analysis by acoustic emission method under various environmental conditions

New multifunctional materials for aerospace industry with exceptional properties must be tested under various environmental conditions to find out possible scatter factors for evaluated properties. Delamination is a typical damage mode observed for laminated composites. Therefore, reliable information regarding the delamination growth behaviour is needed for all operational environments of an aircraft operated at cryogenic and elevated temperatures. In this paper, delamination crack growth monitoring in a climatic chamber on double-cantilever beam (DCB) specimens using optical devices and acoustic emission (AE) techniques is described. A relationship between cumulative AE energy, events localization, clusters, and crack growth in a plain-weave carbon fibre–reinforced epoxy is investigated under constant displacement rate loading at + 80 °C, and − 55 °C. Test results are evaluated for specimens with multi-walled carbon nanotubes (MWCNT) in the microstructure and for a reference material. The mechanical properties during delamination are represented by fracture toughness G_IC, and they are also correlated with the AE data. The elevated test temperature caused a decreased rate of released AE energy. The crack growth in material with more significant fibre breakage caused increase of the AE release rate.     

Liquid helium free mechanical property test system with G-M cryocoolers

In the present work, a cryogenic mechanical property testing system conduction-cooled by two G-M cryocoolers was developed. The testing sample can be cooled from room temperature to 2.7 K within 7.5 h. The sample was first cooled down to 11.1 K directly by the two G-M cryocoolers and then cooled down to 2.7 K by decompressing the chamber. Instead of liquid helium, the cooling process is characterized by cooling with recycled helium gas as heat transfer medium. The heat load of the system was analyzed and optimizations were adopted in terms of material selections and design. The static load capacity of the system reaches 200 kN and the fatigue load capacity can reach 50 kN. This system can be installed onto an electronic universal testing machine or a fatigue testing machine to characterize static tension, fracture mechanics or fatigue properties at tunable low temperatures. Tensile properties of 316L austenitic stainless steels at 4.2 K were tested with the system and the results were compared with those obtained by cooled using liquid helium, which demonstrates high reliability.     

Influence of mode ratio and hygrothermal condition on the delamination toughness of a thermoplastic particulate interlayered carbon/epoxy composite

Double cantilever beam, end-notched flexure and single leg bending tests were used to determine the effects of temperature and moisture on the toughness of a thermoplastic particulate-toughened carbon/epoxy composite. Tests were performed on both dry and moisture-saturated specimens at temperatures of ?43 °C, 21 °C and 98 °C, and on dry specimens only at 125 °C. In-situ observations and post-test scanning electron microscopy showed increasing matrix ductility with increasing temperature and moisture content. This correlated to an increase in the mode I and a decrease in the mode II toughness. The mixed-mode toughness data and fracture surface morphologies displayed a blend of the mode I and mode II behaviors.     

Studies on the electrochemical preparation of MgCeCo alloy thin films on Cu substrates in urea–DMSO system

Cyclic voltammetry was used to investigate the electrochemical behaviors of Mg(II), Ce(III) and Co(II) in 3.00 mol L^− 1 urea–DMSO (dimethylsulfoxide). The electrode processes of Mg(II), Ce(III) and Co(II) reducing on Pt electrodes were irreversible steps. The transfer coefficient of Mg(II), Ce(III) and Co(II) in 3.00 mol L^− 1 urea–DMSO system was calculated as 0.07, 0.05 and 0.05 at 298.15 K, respectively. The diffusion coefficient of Mg(II), Ce(III) and Co(II) in 3.00 mol L^− 1 urea–DMSO system was calculated as 2.27 × 10^− 10, 1.77 × 10^− 10 and 3.16 × 10^− 10 m² s^− 1 at 298.15 K, respectively. The MgCeCo alloy thin films with smooth, uniform and metallic luster were obtained on Cu substrates by cyclic electrodeposition in 0.01 mol L^− 1 Mg(ClO₄)₂–0.01 mol·L^− 1 Ce(CH₃SO₃)₃₋0.01 mol L^− 1 CoCl₂–3.00 mol L^− 1 urea–DMSO system. The potential sweep rate was found to be important with respect to the adhesion of the thin films.     

High-temperature low cycle fatigue behavior of a gray cast iron

The strain controlled low cycle fatigue properties of the studied gray cast iron for engine cylinder blocks were investigated. At the same total strain amplitude, the low cycle fatigue life of the studied material at 523 K was higher than that at 423 K. The fatigue behavior of the studied material was characterized as cyclic softening at any given total strain amplitude (0.12%–0.24%), which was attributed to fatigue crack initiation and propagation. Moreover, this material exhibited asymmetric hysteresis loops due to the presence of the graphite lamellas. Transmission electron microscopy analysis suggested that cyclic softening was also caused by the interactions of dislocations at 423 K, such as cell structure in ferrite, whereas cyclic softening was related to subgrain boundaries and dislocation climbing at 523 K. Micro-analysis of specimen fracture appearance was conducted in order to obtain the fracture characteristics and crack paths for different strain amplitudes. It showed that the higher the temperature, the rougher the crack face of the examined gray cast iron at the same total strain amplitude. Additionally, the microcracks were readily blunted during growth inside the pearlite matrix at 423 K, whereas the microcracks could easily pass through pearlite matrix along with deflection at 523 K. The results of fatigue experiments consistently showed that fatigue damage for the studied material at 423 K was lower than that at 523 K under any given total strain amplitude.     

Measurements of the microhardness,electrical and thermal properties of the Al–Ni eutectic alloy

Al–5.7 wt% Ni eutectic alloy was directionally solidified upward with different temperature gradients (0.83–4.02 K/mm) at a constant growth rate (0.008 mm/s) and with different growth rates (0.008–0.483 mm/s) at a constant temperature gradient (4.02 K/mm). Microhardness (HV) and electrical resistivity (ρ) of directionally solidified samples were measured using microhardness test device and a standard d.c. four-point probe technique, respectively. Dependency of the microhardness and electrical resistivity on the solidification processing parameters (temperature gradient, G and growth rate, (V) were analyzed. According to these results, it has been found that the values of HV and ρ increase with the increasing values of G and V. Variations of electrical resistivity and electrical conductivity (σ) for casting Al–Ni eutectic alloy were also measured at the temperature in range 300–720 K. The enthalpy of fusion (ΔH) for the Al–Ni eutectic alloy was determined by differential scanning calorimeter (DSC) from heating trace during the transformation from solid to liquid.     

Mode III fatigue delamination growth of glass fiber reinforced polymer woven laminates at cryogenic temperatures

This paper investigates the cryogenic fatigue delamination behavior of glass fiber reinforced polymer woven laminates under Mode III loading. Fatigue delamination tests were conducted using split cantilever beam specimens at room temperature, liquid nitrogen temperature (77 K) and liquid helium temperature (4 K). A finite element analysis was also employed to calculate the energy release rate. The temperature dependence of the fatigue delamination growth rate vs. energy release rate range is discussed. Fracture surfaces were examined by scanning electron microscopy to identify the delamination mechanisms under fatigue loading. The important conclusion we reach is that the Mode III fatigue delamination growth rates of woven laminates at cryogenic temperatures are lower than that at room temperature.     

Fatigue crack growth and damage characteristics of high-speed rail at low ambient temperature

Low temperature can be a significant problem affecting safety and maintenance of railway. In this study, the fatigue crack growth rate and rolling contact fatigue damage behaviors of high-speed rail material under different temperature conditions were investigated by a series of experiments. The results indicate that the stress and strength of rail material increase with the decrease of ambient temperature. The crack growth rate at 0 °C and − 20 °C is similar with that at 20 °C. While, when the temperature decreases to − 60 °C, the growth rate of crack increases sharply. The promotion of rail embrittlement at low temperature accompanied with the action of high stress causes the rapid failure and increase of surface crack length and subsurface crack damage. Meanwhile, three crack growth mechanism models at different temperatures can be inferred. The brittle fracture mode is increasingly apparent with the temperature decreasing.     

Crystallization kinetics and structure of poly(trimethylene terepthalate)/monolayer nano-mica nanocomposites

In this work, the structure of poly(trimethylene terepthalate) (PTT)/monolayer nano-mica (MNM) nanocomposites are investigated by wide angle X-ray diffraction (WAXD), small angle X-ray scattering (SAXS) and scanning electron microscope (SEM). In the PTT nanocomposites, the crystallization induces the segregation of MNM in which three morphologies, including interlamellar, interfibrillar, and interspherulitic segregations, are observed with changing the MNM content. Avrami analysis of isothermal crystallization demonstrates that MNM enhances the bulk crystallization rate in the nanocomposites. Moreover, the non-integral values of Avrami exponent n between 2 and 4 with increasing crystallization temperature indicate the mixed growth and nucleation mechanisms. The analysis of secondary nucleation theory for neat PTT and the PTT nanocomposites exhibit the same regime transition of crystallization behaviour in which the classical transition temperatures of regime I to II and regime II to III take place at 488 K and 468 K, respectively. The growth rate of spherulites of the PTT nanocomposites is twofold larger than that of neat PTT in regime III, implying that MNM plays an effective role as a nucleating agent, since the addition of MNM enormously reduces the activation energy of nucleation, folding surface free energy and average work of chain folding for PTT nucleation. However, experimental results show that the MNM content below 1 wt% is the most effective for nucleation of PTT crystallization.     

Crack growth behavior of 7075-T6 under biaxial tension–tension fatigue

Crack growth behavior of aluminum alloy 7075-T6 was investigated under in-plane biaxial tension–tension fatigue with stress ratio of 0.5. Two biaxiality ratios, λ (=1 and 1.5) were used. Cruciform specimens with a center hole, having a notch at 45° to the specimen’s arms, were tested in a biaxial fatigue test machine. Crack initiated and propagated coplanar with the notch for λ = 1 in L–T orientation, while it was non-coplanar for λ = 1.5 between L–T and T–L orientations. Uniaxial fatigue crack growth tests in L–T and T–L orientations were also conducted. Crack growth rate in region II was practically the same for biaxial fatigue with λ = 1 in L–T orientation and for the uniaxial fatigue in L–T or T–L orientations, while it was faster for biaxial fatigue with λ = 1.5 at a given crack driving force. However, fatigue damage mechanisms were quite different in each case. In region I, crack driving force at a given crack growth rate was smallest for biaxial fatigue with λ = 1.5 and for uniaxial fatigue in T–L orientation, followed by biaxial fatigue with λ = 1 and uniaxial fatigue in L–T orientation in ascending order at a given crack growth rate.     

Exploring mechanical property balance in tufted carbon fabric/epoxy composites

The paper details the manufacturing processes involved in the preparation of through-the-thickness reinforced composites via the ‘dry preform–tufting–liquid resin injection’ route. Samples for mechanical testing were prepared by tufting a 5 harness satin weave carbon fabric in a 3 mm × 3 mm square pitch configuration with a commercial glass or carbon tufting thread, infusing the reinforced preforms with liquid epoxy resin and curing them under moderate pressure. The glass thread reinforcement increases the compression-after-impact strength of a 3.3 mm thick carbon fabric laminate by 25%. The accompanying drop-downs in static tensile modulus and strength of the same tufted laminate are below 10%. The presence of tufts is also shown to result in a significant increase in the delamination crack growth resistance of tufted double-cantilever beam specimens and has been quantified for the case of a 6 mm thick tufted carbon non-crimped fabric (NCF)/epoxy composite.     

Investigation on influence of dynamic strain ageing on fatigue crack growth behaviour of modified 9Cr–1Mo steel

Fatigue crack growth behaviour of modified 9Cr–1Mo steel is examined in the temperature range 300–823 K. An improvement in fatigue crack growth resistance is observed in the dynamic strain ageing regime. The activation energy for the process leading to this is estimated from the temperature-dependence of crack tip strain rate as 55–80 kJ/mole. This indicates that dynamic strain ageing due to interaction of dislocations with interstitial solute elements is responsible for the improved fatigue resistance in this range.     

Ultrasonic fatigue of a single crystal Ni-base superalloy at 1000 °C

An ultrasonic fatigue testing system capable of operating at temperatures up to 1000 °C has been developed and utilized to study the fatigue behavior of a single crystal superalloy (PWA 1484) at a temperature of 1000 °C and loading frequency of approximately 20 kHz. The stress-life data generated from the ultrasonic testing system were comparable to those from conventional servo-hydraulic fatigue tests for similar single crystal alloys. Interior Ta-rich carbides were the major microstructural feature responsible for crack initiation in the alloy. Crack growth under ultrasonic loading frequency at 1000 °C for PWA 1484 occurred in a crystallographic manner on {1 1 1} octahedral slip planes, in contrast to the normal Mode-I growth mode typically observed for single crystal superalloys at high temperature (>850 °C) with conventional servo-hydraulic loading frequencies (<100 Hz).     

Selective detection of toxic Pb(II) ions based on wet-chemically prepared nanosheets integrated CuO–ZnO nanocomposites

The present study describes a selective detection methodology for hazardous metal ions based on low-dimensional nanosheets (NSs) integrated CuO–ZnO composite materials. A large-scale synthesis of NSs by wet-chemical process is performed using alkaline reducing agents at higher pH medium. The prepared NSs are characterized in terms of their morphological, structural and optical properties, and efficiently applied for the toxic metal ions detection. The detailed structural, compositional, and optical characterization of NSs are evaluated by XRD, FT-IR, XPS, EDS, and UV–vis spectroscopy, which confirmed that the obtained NSs are well-crystalline CuO–ZnO and possessed good optical properties. The CuO–ZnO NS morphology is investigated by FE-SEM, which confirmed that the NS possesses microstructure shape and growth in large-quantity. The analytical application of CuO–ZnO NSs is studied for a selective extraction of toxic lead-divalent [Pb(II)] ions prior to its determination by inductively coupled plasma-optical emission spectrometry (ICP-OES). The selectivity of doped NSs phase is investigated for eight different metal ions, including Cd(II), Cu(II), Hg(II), La(III), Mn(II), Pb(II), Pd(II), and Y(III) under similar experimental conditions. From the selectivity study, it is confirmed that the composite CuO–ZnO NS phase is the most toward Pb(II) ions according to the magnitude of distribution coefficient (K_d) values, such as Pb(II) > Y(III) > Cd(II) > La(III) > Hg(II) > Cu(II) > Mn(II) > Pd(II). The uptake capacity for Pb(II) is experimentally calculated to be ∼82.66 mg g⁻¹.