The fatigue of carbon fibre reinforced plastics - A review

Engineering structures are often subjected to the conditions of cyclic-loading, which onsets material fatigue, detrimentally affecting the service-life and damage tolerance of components and joints. Carbon fibre reinforced plastics (CFRP) are high-strength, low-weight composites that are gaining ubiquity in place of metals and glass fibre reinforced plastics (GFRP) not only due to their outstanding strength-to-weight properties, but also because carbon fibres are relatively inert to environmental degradation and thus show potential as corrosion resistant materials. The effects of cyclic loading on the fatigue of CFRP are detailed in several papers. As such, collating research on CFRP fatigue into a single document is a worthwhile exercise, as it will benefit the engineering-readership interested in designing fatigue resistant structures and components using CFRP. This review article aims to provide the most relevant and up-to-date information on the fatigue of CFRP. The review focuses in particular on defining fatigue and the mechanics of cyclically-loaded composites, elucidating the fatigue response and fatigue properties of CFRP in different forms, discussing the importance of environmental factors on the fatigue performance and service-life, and summarising the different approaches taken to modelling fatigue in CFRP

Erosion Mapping of Through-Thickness Toughened Powder Epoxy Gradient Glass-Fiber-Reinforced Polymer (GFRP) Plates for Tidal Turbine Blades

Erosion of tidal turbine blades in the marine environment is a major material challenge due to the high thrust and torsional loading at the rotating surfaces, which limits the ability to harness energy from tidal sources. Polymer-matrix composites can exhibit leading-blade edge erosion due to marine flows containing salt and solid particles of sand. Anti-erosion coatings can be used for more ductility at the blade surface, but the discontinuity between the coating and the stiffer composite can be a site of failure. Therefore, it is desirable to have a polymer matrix with a gradient of toughness, with a tougher, more ductile polymer matrix at the blade surface, transitioning gradually to the high stiffness matrix needed to provide high composite mechanical properties. In this study, multiple powder epoxy systems were investigated, and two were selected to manufacture unidirectional glass-fiber-reinforced polymer (UD-GFRP) plates with different epoxy ratios at the surface and interior plies, leading to a toughening gradient within the plate. The gradient plates were then mechanically compared to their standard counterparts. Solid particle erosion testing was carried out at various test conditions and parameters on UD-GFRP specimens in a slurry environment. The experiments performed were based on a model of the UK marine environment for a typical tidal energy farm with respect to the concentration of saltwater and the size of solid particle erodent. The morphologies of the surfaces were examined by SEM. Erosion maps were generated based on the result showing significant differences for materials of different stiffness in such conditions

Powder Epoxy for One-Shot Cure, Out-of-Autoclave Applications: Lap Shear Strength and Z-Pinning Study

Large composite structures manufactured out-of-autoclave require the assembly and bonding of multiple parts. A one-shot cure manufacturing method is demonstrated using powder epoxy. Lap shear plates were manufactured from powder epoxy and glass fiber-reinforced plastic with four different bonding cases were assessed: secondary bonding using standard adhesive film, secondary bonding using powder epoxy, co-curing, and co-curing plus a novel Z-pinning method. This work investigates the lap shear strength of the four cases in accordance with ISO 4587:2003. Damage mechanisms and fracture behavior were explored using digital image correlation (DIC) and scanning electron microscopy (SEM), respectively. VTFA400 adhesive had a load at break 24.8% lower than secondary bonding using powder epoxy. Co-curing increased the load at break by 7.8% compared to powder epoxy secondary bonding, with the co-cured and pinned joint resulting in a 45.4% increase. In the co-cured and co-cured plus pinned cases, DIC indicated premature failure due to resin spew. SEM indicated shear failure of resin areas and a large amount of fiber pullout in both these cases, with pinning delaying fracture phenomena resulting in increased lap joint strength. This highlights the potential of powder epoxy for the co-curing of large composite structures out-of-autoclave

Number needed to treat with ursodeoxycholic acid therapy to prevent liver transplantation or death in primary biliary cholangitis

Objective: The clinical benefit of ursodeoxycholic acid (UDCA) in primary biliary cholangitis (PBC) has never been reported in absolute measures. The aim of this study was to assess the number needed to treat (NNT) with UDCA to prevent liver transplantation (LT) or death among patients with PBC. Methods: The NNT was calculated based on the untreated LT-free survival and HR of UDCA with respect to LT or death as derived from inverse probability of treatment weighting-adjusted Cox proportional hazard analyses within the Global PBC Study Group database. Results: We included 3902 patients with a median follow-up of 7.8 (4.1-12.1) years. The overall HR of UDCA was 0.46 (95% CI 0.40 to 0.52) and the 5-year LT-free survival without UDCA was 81% (95% CI 79 to 82). The NNT to prevent one LT or death within 5 years (NNT5y) was 11 (95% CI 9 to 13). Although the HR of UDCA was similar for patients with and without cirrhosis (0.33 vs 0.31), the NNT5y was 4 (95% CI 3 to 5) and 20 (95% CI 14 to 34), respectively. Among patients with low alkaline phosphatase (ALP) (≤2× the upper limit of normal (ULN)), intermediate ALP (2-4× ULN) and high ALP (>4× ULN), the NNT5y to prevent one LT or death was 26 (95% CI 15 to 70), 11 (95% CI 8 to 17) and 5 (95% CI 4 to 8), respectively. Conclusion: The absolute clinical efficacy of UDCA with respect to LT or death varied with baseline prognostic characteristics, but was high throughout. These findings strongly emphasise the incentive to promptly initiate UDCA treatment in all patients with PBC and may improve patient compliance

Obeticholic acid for the treatment of non-alcoholic steatohepatitis: interim analysis from a multicentre, randomised, placebo-controlled phase 3 trial

Background Non-alcoholic steatohepatitis (NASH) is a common type of chronic liver disease that can lead to cirrhosis. Obeticholic acid, a farnesoid X receptor agonist, has been shown to improve the histological features of NASH. Here we report results from a planned interim analysis of an ongoing, phase 3 study of obeticholic acid for NASH. Methods In this multicentre, randomised, double-blind, placebo-controlled study, adult patients with definite NASH,non-alcoholic fatty liver disease (NAFLD) activity score of at least 4, and fibrosis stages F2–F3, or F1 with at least oneaccompanying comorbidity, were randomly assigned using an interactive web response system in a 1:1:1 ratio to receive oral placebo, obeticholic acid 10 mg, or obeticholic acid 25 mg daily. Patients were excluded if cirrhosis, other chronic liver disease, elevated alcohol consumption, or confounding conditions were present. The primary endpointsfor the month-18 interim analysis were fibrosis improvement (≥1 stage) with no worsening of NASH, or NASH resolution with no worsening of fibrosis, with the study considered successful if either primary endpoint was met. Primary analyses were done by intention to treat, in patients with fibrosis stage F2–F3 who received at least one dose of treatment and reached, or would have reached, the month 18 visit by the prespecified interim analysis cutoff date. The study also evaluated other histological and biochemical markers of NASH and fibrosis, and safety. This study is ongoing, and registered with ClinicalTrials.gov, NCT02548351, and EudraCT, 20150-025601-6. Findings Between Dec 9, 2015, and Oct 26, 2018, 1968 patients with stage F1–F3 fibrosis were enrolled and received at least one dose of study treatment; 931 patients with stage F2–F3 fibrosis were included in the primary analysis (311 in the placebo group, 312 in the obeticholic acid 10 mg group, and 308 in the obeticholic acid 25 mg group). The fibrosis improvement endpoint was achieved by 37 (12%) patients in the placebo group, 55 (18%) in the obeticholic acid 10 mg group (p=0·045), and 71 (23%) in the obeticholic acid 25 mg group (p=0·0002). The NASH resolution endpoint was not met (25 [8%] patients in the placebo group, 35 [11%] in the obeticholic acid 10 mg group [p=0·18], and 36 [12%] in the obeticholic acid 25 mg group [p=0·13]). In the safety population (1968 patients with fibrosis stages F1–F3), the most common adverse event was pruritus (123 [19%] in the placebo group, 183 [28%] in the obeticholic acid 10 mg group, and 336 [51%] in the obeticholic acid 25 mg group); incidence was generally mild to moderate in severity. The overall safety profile was similar to that in previous studies, and incidence of serious adverse events was similar across treatment groups (75 [11%] patients in the placebo group, 72 [11%] in the obeticholic acid 10 mg group, and 93 [14%] in the obeticholic acid 25 mg group). Interpretation Obeticholic acid 25 mg significantly improved fibrosis and key components of NASH disease activity among patients with NASH. The results from this planned interim analysis show clinically significant histological improvement that is reasonably likely to predict clinical benefit. This study is ongoing to assess clinical outcomes

Interlaminar fracture behaviour of fibre reinforced powder epoxy composites

The aim of this work is to study delamination in fibre reinforced composites with applications in wind and tidal turbine blades. Powder epoxy composites are a very promising option for the manufacturing of large composite structures due to their low exotherm and viscosity, a suitability for out-of-autoclave manufacturing and their ability to perform a separate melting and curing phase allowing complicated parts such as wind turbine blades to be formed separately and cured in a one-shot process. It was shown that quasi-unidirectional carbon fibre and glass fibre reinforced powder epoxy composites could be manufactured out-of-autoclave with a low void content. However, a relatively high variation in the thickness between samples and fibre distortion, especially in the CFRP samples were measured. This shows that, although powder epoxy is a promising resin for the manufacturing of composites, an automated process such as the fabrication of powder epoxy prepregs is preferable to the hand spraying process for the fabrication of consistent composites. The powder epoxy resin was show to be very ductile with a strain to failure of nearly 10%. However, it lies within the mid-range of reported strength values for epoxy with an ultimate strength of 61 MPa and in the low range of reported elastic modulus values with a value of 1.64GPa. The tensile properties of powder epoxy composites were shown to be comparable to those of commonly used epoxy resins but the compressive strength was shown to be quite low with a 48% and 59% reduction for the GFRP and CFRP composites compared to their tensile strengths. The interlaminar fracture toughness of carbon and glass fibre composites was studied in pure mode I by performing Double Cantilever Beam (DCB) tests and at 25% mode II, 50% mode II and 75% mode II by performing Mixed Mode Bending (MMB) testing and in pure mode II by performing an End-Loaded Specimen (ELS) test. The strain energy release rate (SERR) at both crack initiation and propagation were shown to be significantly higher than both conventional and toughened epoxy composites for which published data is available. While the powder epoxy is not the only factor explaining the high fracture toughness, with the off-axis fibres, evidenced by fibre bridging, played a role, the powder epoxy composites used in this study still had a higher toughness than published data for epoxy quasi-unidirectional fabrics which also contain off-axis fibers. To understand the influence of moisture on the mechanical properties of the studied powder epoxy composites, they were hygrothermally aged in 60°C seawater. The CFRP and GFRP samples were saturated after around 4 months. Water diffusion in the composites induced plasticisation of the resin, leading to a 25°C reduction in the resin Tg. The longitudinal elastic modulus of the CFRP and GFRP were unaffected by the presence of water but reductions in the longitudinal, transverse and shear strengths of 45-50% and 15-30% were measured for the GFRP and CFRP respectively. SEM micrographs showed a degradation in the fibre/matrix interfacial strength, especially in the GFRP samples. In contrast, water absorption reduced the compressive strength properties of CFRP to a greater degree than GFRP. Hygrothermal ageing led to reductions in the interlaminar fracture toughness but the reductions were dependent on the mode ratio. The propagation GC decreased between 25% for mode I and 40% for mode II for the GFRP samples, while a 25% decrease in the mode I was observed for CFRP while the mode II Gc remained unaffected. The degradation of the fibre/matrix interface was a likely explanation for the reduction in mode I toughness for the CFRP as fibre pull-out likely occurred at a lower energy while the 90° fibres of the UD glass fabric were not able to arrest mode II crack growth as efficiently. A finite element delamination analysis was carried out using the virtual crack closing technique (VCCT). A parametric model was developed to allow for automated modelling of a variety of ply drop configurations. This was followed by a sensitivity analysis to understand the influence of the various model parameters on the predicted delamination failure load. Finally, a method was developed for the optimization of chamfered ply drop specimens with regards to the chamfered slope ratio and the vertical height at the end of the ply drop, called the toe height. It was shown that the delamination failure load increased with the chamfered slope ratio but decreased as the toe height increased. As a result of this study, the combination of chamfer slope and vertical height leading to failure in the plank occurring prior to delamination initiation were obtained, allowing for the manufacturing of chamfered ply drops where the risk of delamination is suppressed. The methodology for the characterisation of the fatigue delamination properties needed for an accurate prediction of delamination growth under fatigue loading was defined. Then a finite element predicting fatigue crack growth in chamfered ply drop specimens, based on the Paris law and VCCT models to extract SERR as a function of crack length and applied strain, was developed for a variety of geometric configurations. The comparison of predicted fatigue life with experimental results shows that the FE model captured the same trends observed during testing, but the predicted strain values leading to a given number of cycles to failure were up to 30\% different from the experimental ones. In the absence of fatigue delamination properties for the materials used in this study, the fatigue life predictions were carried out using Paris law parameters obtained from literature, giving a likely explanation for this relatively large difference

Influence of Hygrothermal Ageing on the Mechanical Properties of Unidirectional Carbon Fibre Reinforced Powder Epoxy Composites

A systematic investigation was carried out in this work to evaluate the effect of seawater ageing on the mechanical properties of different composites manufactured with three types of unidirectional carbon fibres, based on a novel powder epoxy system. The flexural behaviour of the samples with and without accelerated seawater ageing was evaluated. A significant strength reduction due to seawater ageing was observed in the composites. Changes within the materials are also evaluated by interlaminar fracture toughness testing and dynamic mechanical thermal analysis. Interlaminar fracture toughness mode-I testing using the Double Cantilever Beam (DCB) test, revealed that the interfacial adhesion differences in combination with the moisture uptake could alter the fracture resistance of the composites, hence emphasising the importance of the interfacial bonding strength. A Finite Element analysis was carried out to gain insight on the important modelling parameters, for mode I dominated failure, and compared with the experimental results. Fracture surface examination by scanning electron microscopy revealed delamination, fibre debonding/bridging and plasticising of resin due to seawater effect