Elsevier

Composites Part B: Engineering

Volume 163, 15 April 2019, Pages 536-547
Composites Part B: Engineering

Fatigue life reduction of GFRP composites due to delamination associated with the introduction of functional discontinuities

https://doi.org/10.1016/j.compositesb.2019.01.005Get rights and content

Abstract

This paper reports on an experimental investigation into the fatigue life of Glass Fibre Reinforced Polymer (GFRP) when essential design discontinuities are introduced to a GFRP part to enable correct function. Specifically, the impact of high speed drilled holes on fatigue life is investigated. Fatigue life is a critical mechanical property, in particular, for industrial applications where both minimal weight and high reliability are sought.

The paper investigates the impact of high speed drilling parameters on the delamination created around the hole, and subsequently on the static strength and fatigue life of GFRP composite laminates. Delamination damage in GFRP specimens is monitored using novel Acoustic Emission (AE) and image processing techniques. The progress of delamination under fatigue testing is used to predict GFRP mechanical performance and associated GFRP mechanical properties are proposed.

What follows is an outline of the experimental method used. First, the extent of delamination after high speed drilling was measured in both unidirectional and woven GFRP specimens under different feed rate and cutting speed parameters. Quasi-static three point bending tests were then performed to investigate the effect of delamination on strength and to assist the determination of appropriate fatigue load magnitudes. Then, three point fatigue bending tests were completed. In this step, Acoustic Emission and image processing techniques were applied simultaneously. Experimental results indicated that drilling parameters have negligible effects on the static strength of GFRP specimens, however, the fatigue life of GFRP specimens varied significantly with the changes of drilling parameters. The experimental results also showed that AE and image processing techniques produced consistent data, offering a validation to the data itself and to indicating that both techniques have merit when completing experiments of this type more generally to determine the mechanical behavior (and associated mechanical properties) of specimens.

Introduction

Fiber reinforced polymer-matrix composite materials are widely used due to favorable engineering properties such as corrosion resistance, high strength and stiffness with low specific weight. As such, composites offer advantages for the design of aerospace components. Safety critical aerospace components must also satisfy stringent fatigue fracture performance requirements to be considered. To be a candidate material within a systematic design program, composites must be able to achieve adequate safety margins. This issue makes composites interesting field for researchers [[1], [2], [3], [4], [5]]. Of critical importance in fatigue performance is the existence of stress-raising geometric discontinuities that are, of course, essential if a component is to be integrated into an assembly.

The introduction of a hole into a composite laminate can result in subsequent fatigue loading to introduce multiaxial stresses into the laminate [6]. Given the prevalence of holes in aerospace parts, investigating the impact of holes on fatigue performance is of critical importance when considering safe function.

Section snippets

Prior investigations

Prior published investigations into fatigue behavior of drilled composites are outlined. Belmonte and colleagues [7] investigated the influence of fiber volume fraction on the fatigue behavior of short glass fiber reinforced polyamide (PA66) where specimens with fiber content of between 0% and 50% were drilled and tested. Field Emission Scanning Electron Microscopy (FESEM) was used to investigate the influence of fiber content specimen damage due to fatigue loading. Their results concluded that

Materials

The studied materials were unidirectional (GF 12/200 DLN) and woven (VV 770), with IMP503 resin, prepreg glass/epoxy supplied by G. Angeloni Srl. Fiber volume fraction was 20.16% for unidirectional and 28.23% for woven specimens. They were vacuumed and cured under 125 °C for 1 h after manufacturing, composite plates' quality were checked by the ultrasonic C-scan method. To manufacture specimens, the plates were cut by water jet machine with dimensions of 170 × 20 mm2 and thickness of 5 mm

Experimental results and analysis

This study shows when feed rate increases and cutting speed reduces, Fda increases. In addition, the woven specimens were found to be more resistant to delamination than the unidirectional specimens with the same drilling parameters. Table 2 shows the adjusted delamination factors by diverse drilling parameters in the woven and unidirectional specimens. Last column in this table shows difference in amount of Fda for unidirectional and woven specimens. It is obvious that by improving drilling

Concluding remarks

This paper has identified the importance of drilling parameters (feed rate and cutting speed) selection and their effect on the extent of delamination, static strength and fatigue life of composite materials. It was found that the extent of cyclic-load induced delamination decreases as the hole cutting speed increases or as the feed rate decreases.

Quasi-static three point bending tests showed that the static strength of the high speed drilled specimens using different cutting speeds and feed

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) received no financial support for the research, authorship, and/or publication of this article.

Acknowledgments

The authors wish to thank the Department of Mechanical Engineering at Amirkabir University of Technology, Tehran, Islamic Republic of Iran; the Department of Industrial Engineering at Bologna University, Italy; and, the Department of Mechanical Engineering at the University of Melbourne, Australia, for providing facilities and resources for this study.

References (32)

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