Elsevier

Renewable Energy

Volume 126, October 2018, Pages 1102-1112
Renewable Energy

Impact fatigue damage of coated glass fibre reinforced polymer laminate

https://doi.org/10.1016/j.renene.2018.04.043Get rights and content

Highlights

  • A novel assessment for repeated high speed impact damage relevant to leading edge erosion of wind turbine blades is presented.

  • A single point impact fatigue test is developed.

  • Incubation time can be evaluated by in-situ video observation, correlating to in-situ acoustic emission data.

  • The ex-situ characterizations by the ultrasonic and CT scans show internal fracture before visible damage started.

  • An FEM model indicates intense shear-stress orientations, showing a good agreement with observed cracks in the coating by the CT-scan.

Abstract

Impact fatigue caused by rain droplets, also called rain erosion, is a severe problem for wind turbine blades and aircraft. In this work, an assessment of impact fatigue on a glass fibre reinforced polymer laminate with a gelcoat is presented and the damage mechanisms are investigated. A single point impact fatigue tester is developed to generate impact fatigue damage and SN data. Rubber balls are repeatedly impacted on a single location of the coated laminate. Each impact induces transient stresses in the coated laminate. After repeated impacts, these stresses generate cracks, leading to the removal of the coating and damage to the laminate. High-resolution digital imaging is used to determine the incubation time until the onset of coating damage, and generate an SN curve. An acoustic emission sensor placed at the back of the laminate monitors changes in acoustic response as damage develops in the coated laminate. The subsurface cracks are studied and mapped by 3D X-ray computed tomography. A finite element method model of the impact shows the impact stresses in the coating and the laminate. The stresses seen in the model are compared to cracks found by 3D tomography. The damage is also evaluated by ultrasonic scanning.

Introduction

Wind energy is recognized as a key renewable energy source, reducing dependency on fossil fuels [[1], [2], [3], [4]]. There are a variety of designs for energy generation by wind, but in all cases, the kinetic energy of wind is converted to electrical energy. The three bladed horizontal axis wind turbine is a common design, comprising rotor blades, a tower and a power converting part including a generator and a gear box. Since the power generation capacity of a wind turbine highly depends on the swept area of the blades, lighter and larger blades are demanded [5,6]. Fibre reinforced polymer composite materials can meet the demand for lighter and larger wind turbine blades due to their high strength-to-mass ratio, a high stiffness-to-mass ratio, good fatigue resistance, corrosion resistance, flexible formability and low thermal expansion.

Surfaces of wind turbine blades in both onshore- and offshore-installations are exposed environmental and tribological effects over their operational lifetimes [7], including extreme wind/gusts, rain showers, hailstone showers, airborne particles of sand, snow, icing, extreme temperatures and ultraviolet light exposure. Among them rain erosion is often thought to be a major damage source [8]. In particular, the leading edge of the blade tips, whose speed is commonly greater than 80 m s−1 [9], can experience significant damage, and thus a protective coating is usually applied. Such damages are collectively called “leading edge erosion”. However, erosion is only one of the damage phenomena, and in fact, very little is known about the different damage modes and mechanisms. All wind farms require frequent visual inspection of the blades and subsequent repair or replacement due to unpredictable damages [18].

Attempts to reproduce the rain erosion damage phenomena in laboratory scale can be found in literature eg Ref. [10]. Rain erosion testing (RET) is used to experimentally assess rain erosion performances. The most common test for rain erosion in wind energy applications is the rotating arm rig, where a rotor is rotating in a rain field of generated droplets [11]. It was originally developed for aerospace materials. Despite the prevalence of this test method there is a problem in reproducibility when different test setups are compared. This is mainly due to turbulence in the test rigs, different droplet size distributions, and other parameters which are difficult to control. In addition, the evaluation mainly relies on visual inspection of the protective coatings to document damage and delamination, and a measurement of the material mass loss, without taking account of damage modes. The testing requires these operation to be stopped and the blade sample unmounted, making observation limited to few discrete intervals, and an observation of exact failure times and mechanisms difficult. Considering these circumstances, development of a highly controlled impact test method, in-situ monitoring and damage characterisation techniques are needed for studying the impact damage mechanisms in detail. Impact damage and impact fatigue are also studied by other means like drop weight tests and shooting with projectiles [12].

The effect of repeated mechanical loading in a material's lifetime has been studied based on a concept of cumulative fatigue damages [13,14]. This concept can be applied to the impact fatigue damage of coated systems. There is usually an initial incubation period during which damage accumulates in the material while no visible damage and no functional loss is detected, as shown in Fig. 1. After the incubation period, a steady damage evolution may be measured. For the leading edge erosion of wind turbine blades, a delta mass method is commonly used, in which mass loss of a material is measured at discrete time intervals. With this method, the impact damage process may be simplified to two discrete stages. In the incubation period, small cracks begin to form inside the material and each crack further extends as the impacts continue. At some point, the cumulative damage results in cracks merging and an initial removal of a portion of the material at the surface, followed by a period of steady mass loss. Drawbacks of the delta mass methods are that in-situ measurements are difficult to perform due to severe mechanical impacts and the fact that initial damages including crack creation are not detected. Therefore, it is desirable to use enhanced inspection techniques suitable for in-situ observation and monitoring, such as visual imaging and acoustic detection.

In the present work, a newly developed single point impact fatigue test (SPIFT) is presented. Ex-situ measurements of ultrasonic scanning and X-ray tomography were performed to identify initial defects in specimens. After that, the impact fatigue test was carried out. Rubber balls are used to impact specimen surfaces with a defined impact-speed and interval. During the impact test, the specimen surface was observed in-situ by digital imaging, and acoustic signals were measured. After the impacts, the specimens were again characterised by the ex-situ methods for damage analysis. The results were compared with stress wave propagation in the specimen simulated by a finite element method (FEM) model.

Section snippets

Materials

Flat glass fibre reinforced polymer (GFRP) laminates coated with an epoxy based gelcoat were manufactured by vacuum infusion as shown in Fig. 2. The uncured gelcoat layer (Huntman, RenGel, SW 5200/Ren NY 5212, density = 1.5 g cm−3) was first placed on a flat mould surface, on which fibre fabric layers were placed in addition to process aid foils. The symmetric fibre layup was established using twelve fabric layers. The fabric layers in the lay-up were biaxial (±45°) 444 g m−2 layers (Saertex,

Failure criterion by visual damage

One of the advantages of SPIFT is that it allows simple in-situ damage assessment. In conventional RET, the rotating target, in a rain and water mist environment, makes the capture of high-resolution images difficult. Therefore, high-resolution imaging is usually performed outside the RET setup.

Since samples used in SPIFT are stationary, high-quality image or video capture is a relatively simple task and is achieved using the camera described in section 2.4.

From the captured visual data the

Conclusion

Experimental and theoretical assessments for damages caused by repeated high speed impacts were presented. SPIFT demonstrated repeated impacts on a coated laminate with rubber balls, damaging the coating and the laminate. In-situ video observation and AE monitoring and ex-situ characterisations by ultrasonic scanning and X-ray CT-scan were carried out.

In-situ video data enabled determination of the incubation time and plotting of SN curves in terms of coating removal at an impact speed ranging

Acknowledgements

The authors are grateful to Lars Lorentzen for developing SPIFT, Erik Vogeley for operation of the X-ray CT-scan, and Christian H. Madsen and Jonas K. Heininge for fabricating laminate specimens. This research was partly supported by EUDP project ‘Rain erosion tester for accelerated test of wind turbine blades’, Case no.: 64015-0045 (EUDP) and the Innovation Fund Denmark being part of the Fast-Track consortium (5152-00002B). This research was conducted using mechanical testing equipment from

References (20)

There are more references available in the full text version of this article.

Cited by (0)

View full text