383 research outputs found
Creep/fatigue/relaxation of angle-ply GFRP composite laminates
No full textFiber-reinforced polymer (FRP) composites are used in engineering structures of several domains, such as wind energy, bridges, and automotive industry. Usually such material systems include polymeric matrices and exhibit behavior that is sensitive to the loading pattern due to their cyclic- and time-dependent mechanical properties. Although the majority of the aforementioned engineering applications undergoes a significant number of fatigue cycles, of random loading profiles, throughout their lifetime, the research efforts assigned to the investigation of the loading effects on their fatigue behavior are still very limited. Fatigue design allowables are today derived from standardized experimental investigations, mainly subjecting the examined materials under constant amplitude continuous fatigue loading; not corresponding to actual loading profiles seen by the structures in open air applications. This chapter aims to present the effect of different loading patterns on the fatigue life and damage development of angle-ply thermoset composites, discussing the influence of load interruptions including periods of zero load, when the material recovers, or periods of load hold times, during which the material creeps.CCLA
Fatigue life modeling and prediction methods for composite materials and structures—Past, present, and future prospects
No full textThis chapter aims to provide an overview of the fatigue life modeling and prediction methods for composite materials and structures, recalling methods used in the past, discovering the present status, and attempting to foresee future trends.CCLA
On the fatigue behavior of thin and thick adhesively bonded composite joints
No full textInvestigations of the fatigue performance of adhesively bonded joints were initiated since the 1950 s. An abundance of publications has emerged dealing with the experimental investigation of the fatigue performance of adhesively bonded joints of various geometries and material combinations to serve the needs of various structures operating under various loading and environmental conditions. This work reviews the fatigue of composite adhesively bonded joints by analyzing relevant literature and reviewing current testing standards for primary applications. It classifies joints based on the materials used and the bondline thickness moving from thin film adhesive joints to thick paste adhesive joints to cover a wide range of contemporary engineering applications. The findings lead to a comprehensive discussion that lays the groundwork for further research in this field
Computational intelligence methods for the fatigue life modeling of composite materials
No full textNovel computational methods such as artificial neural networks, adaptive neuro-fuzzy inference systems and genetic programming are used in this chapter for the modeling of the nonlinear behavior of composite laminates subjected to constant amplitude loading. The examined computational methods are stochastic nonlinear regression tools, and can therefore be used to model the fatigue behavior of any material, provided that sufficient data are available for training. They are material independent methods that simply follow the trend of the available data, in each case giving the best estimate of their behavior. Application on a wide range of experimental data gathered after fatigue testing glass/epoxy and glass/polyester laminates proved that their modeling ability compares favorably with, and is to some extent superior to, other modeling techniques.CCLA
Fatigue life prediction under realistic loading conditions
No full textThe accurate modeling of the fatigue behavior of composite materials under constant amplitude (CA) cyclic stress is in itself a difficult task but when loading is of variable amplitude (VA), the situation is even more complicated since an appropriate damage metric and rules for its summation must be defined. One of the widely used approaches is based on the theoretical formulation and use of a damage summation rule to predict life under VA loading without recourse to experimental observation of the damage accumulation process. The most popular and best-known example of this category, which does not always lead to accurate results however, is the linear Palmgren-Miner rule. Other summation rules were also proposed as alternatives to the use of Palmgren-Miner rule to accurately predict the fatigue lifetime of glass-fiber reinforced plastic (GFRP) and CFRP composites loaded under block or VA loading patterns. An alternative to this classic fatigue life prediction methodology is the residual strength fatigue theories, where residual strength is used as the damage metric. Comparison of the remaining strength of the material to the static strength allows the estimation of the fatigue cycles until failure. The basic fatigue modeling introduced in previous chapters of this book for the interpretation of the fatigue data (Chapter 2), residual strength theories (Chapter 3), and constant life diagrams (Chapter 6) is combined here to establish fatigue life prediction methodologies.CCLA
DIC-based monitoring on debonding crack propagation in wrapped composite joints
No full textWrapped composite joint is an innovative technique which connects steel hollow sections through bonding such that the fatigue performance is improved compared to welded joint. In this paper, a DIC-based method of monitoring surface strains is proposed to quantify the debonding crack propagation within the composite wrap layers during high cycle fatigue loading. A constant strain threshold was used to obtain crack length based on strain distribution curves extracted from DIC. Sensitivity analysis of such threshold showed that within the ‘steady strain slope’ region, the influence of threshold choice on calculated crack length is insignificant, but a good choice of threshold can help obtain more stable results. During cyclic loading, it was found that stiffness degradation and crack development of the joint is arrested due to friction effect at the cracked interface. Static tests after cyclic loading showed that the joint can still sustain its original static resistance.Steel & Composite Structure
Measurement of damage growth in ultrasonic spot welded joint
No full textUltrasonic spot welding is a joining technique for thermoplastic composites with great potential regarding processing speed and cost. To investigate the damage tolerance and possible inherent damage arresting behavior of multi-spot welded joints, a technique is necessary to measure damage growth in the joints under cyclic loading. Visual inspection is not possible because the damage is not located on the outside surface and conventional techniques such as C-scan are not practical during a fatigue test because the specimen would have to be removed from the setup. This paper details a methodology for quantifying damage growth rates in singlespot welded joints using surface strain measurements made by Digital Image Correlation. This represents the first step towards developing a methodology for quantifying damage progression behavior in complex multi-spot welded joints.Structural Integrity & CompositesAerospace Structures & Computational Mechanic
Enhancement of mode I fracture toughness of adhesively bonded secondary joints using different layup patterning of CFRP
No full textDelamination growth in fibre reinforced polymer composites is generally evaluated with experiments that have been standardized for quasi-static load conditions. These tests characterize unidirectional delamination growth in mode I (DCB), mode II (ELS or ENF) of mixed mode conditions (MMB). However, little attention is paid in literature to the applicability of these tests to in-service delamination problems that are generally characterized by planar delamination growth. In this study, the relation between planar delamination growth, induced by transverse quasi-static indentation loading, and these unidirectional delamination tests was investigated. To that aim, prior planar delamination growth tests reported in literature, performed at EPFL, were analysed to identify up to what extent this planar growth could be correlated to the concepts of strain energy release and strain energy density. Once this appeared to successful, an experimental setup was designed to measure the delamination boundary during the transverse indentation loading of planar delamination specimens made of nontransparent carbon fibre reinforced polymer composites. With that set-up, quasi-static and fatigue planar delamination growth experiments were performed, and delamination contours could be successfully captured. While the quasi-static tests revealed limited growth, evaluation with numerical simulations revealed that the indentation force required to extend the delamination quasi-statically would cause damage to the specimen. This is attributed to the increasing length of the delamination contour when delaminations expand, which is not the case with standard unidirectional specimen. With the fatigue tests, however, delamination growth was achieved, but interestingly enough two phases were observed; first the delamination propagated in a planar fashion, while at some point in time work did not exceed an apparent threshold. Instead of no growth, however, the delamination still increased but then in a transverse manner. What makes this study of particular interest, is that the strain energy density as criterion could capture the strain energy offered (work) along the entire delamination contour, while the strain energy release rate described the resistance to delamination growth. This latter observation is in agreement with the original concept employed by Griffith when he formulated the basis of linear elastic fracture mechanics. This presentation present the experiments performed, the analysis of results, and will conclude with a proposal how to relate standard unidirectional tests to planar growth, considering that these standard tests contain little to no information on transverse phenomena with respect to strain energy density (work) and strain energy release (dissipation).Structural Integrity & Composite
Multilayer Leading Edge Protection systems of Wind Turbine Blades: A review of material technology and damage modelling
No full textIn the immediate future, wind power will provide more electricity than any other technology based on renewable and low-emission energy sources. As a result, the size of offshore wind turbines has increased to harvest more wind energy in order to achieve the 2050 EU carbon neutral targets. The use of composites opens great prospects in the design and manufacture of the wind turbine blades due to their optimization versatility but composites perform poorly under impact and are sensitive to environmental factors. To combat this, blade manufacturers employ polymer-based surface coatings, caps or tapes to protect the composite structure. However, it is the repeated impact of rain droplets combined with the high blade tip speed, which are mostly contributing to the erosion of wind turbine blades [1]. The hindering of leading-edge erosion could be obtained through its multilayer material optimization i.e. Leading Edge Protection LEP [2]. Both the surface erosion and the intra-layer adhesion are affected by the shock wave propagation through the thickness of the LEP system produced from the collapsing water droplet after impact [3]. It is necessary to increase the interfacial fracture toughness resistance of the multy-layered system from the surface to the interface boundaries to damp the surface damage and avoid subsurface delamination [4]. Therefore, validated models considering the developed multicomplex stress states and the material degradation due to environmental loads are required for design purposes toward anti-erosion protection performance. This investigation summarizes the review of the current literature conducted in the framework of the IEA Wind TCP (International Energy Agency Wind Technology Collaboration Programme) - Task 46 Erosion of wind turbine blades [5]. It focuses on two main issues: firstly, the LEP material configuration used in industry considering the blade integration technology and, secondly, the modelling techniques and numerical procedures currently used to predict both wear surface damage and interface delamination failure. This work will allow for the identification of gaps within the research that can be explored during IEA Wind Task 46.Aerospace Manufacturing Technologie
Effect of dwell stage in the cure cycle on toughening of epoxy using thermoplastic multilayers
No full textEpoxies with high cross-linking densities are brittle and hence have a low fracture toughness. However, different methods are known to increase fracture toughness. Numerous approaches are known to incorporate a second phase into the epoxy matrix, such as rubber, inorganic nanoparticles or thermoplastics, referred to as bulk resin modification. These tougheners usually form specific morphologies during the curing phase of epoxy, resulting in improved fracture toughness of the system. Unfortunately, for some tougheners, the addition of second phase into the epoxy system also results in a reduction in overall modulus and limitation in end-use temperature of the system. In the case of thermoplastic tougheners, the second phase is created by diffusion and dissolution, followed by reaction induced phase separation, leading to a morphology in the micrometer range. However, the influence of the curing history beyond phase separation, using two dwell cure cycles with varying dwell time/degree of cure, on the interphase dimension and final morphology for PEI having a contrasting phase behaviour (UCST), is not well understood. The research presented in this work aims to understand the interphase formation, to later attain the desired droplet size and interphase morphology for improved material toughness. This aim is achieved by analyzing the influence of dwell time by considering two main cases for each selected 1st dwell temperature (120-180˚C): (i) wait until the onset of phase separation (OPS) before increasing the temperature to 200°C (second dwell), (ii) wait until 80% degree of cure (80% DOC) before the second dwell. At all processing temperatures, a distinct gradient morphology (Fig. 1a ) was clearly observed for both cases (OPS and 80% DOC). The SEM micrographs revealed the formation of a larger interphase region (71 μm) for the OPS case as compared to the 80% DOC case (56 μm). Figure 1b shows the interphase thickness as a function of 1st dwell temperature for both OPS and 80% DOC cases. It can be seen that the interphase thickness increased with 1st dwell temperature for both cases, until 160˚C after which it slightly decreased for a 1st dwell temperature of 180˚C. This work highlights, i) the importance of the curing process beyond phase separation to control interphase dimension and final morphology and, ii) the influence of both these parameters on the toughness enhancement.Aerospace Manufacturing Technologie
- …
