Experimental characterization of the crack-tip-opening angle in fibre metal laminates

doi:10.1016/j.engfracmech.2010.02.014

Engineering Fracture Mechanics

Volume 77, Issue 6, April 2010, Pages 1012-1024

https://doi.org/10.1016/j.engfracmech.2010.02.014 Get rights and content

Abstract

Although the crack-tip-opening angle (CTOA) has been shown to be well suited for modelling stable crack growth in monolithic sheet aluminium alloys, its applicability for fibre metal laminates has not been fully analyzed yet. Fracture test were performed on M(T) panels made of Glare 2-3/2-0.4, Glare 3-3/2-0.4 and laminated 2024-T3 3/2-0.4. Different fatigue pre-crack lengths were created to study the effect of bridging fibres on the CTOA measured on the external layer. The effect of bridging fibres resulted in small deviations of the CTOA vs. crack extension curve with respect to the reference panel made of metal laminate. The CTOA criterion could be successfully used for predicting the residual strength in fibre metal laminates.

Introduction

The concept of damage tolerance is a key aspect in ensuring and maintaining safety of an airframe over its design life. From a design point of view, implementing the damage tolerance concept requires an understanding of the growth and evolution of damage. An example of damage tolerant materials are fibre metal laminates (FMLs) [1], which have been developed in the past to increase the fatigue characteristics of laminated aluminium structures by adding fibres in the bond line.

One of the most important aspects concerning damage tolerance is the residual strength of materials and structures, which is defined as the maximum load a structure or material can withstand in presence of damage before catastrophic failure occurs.

The fracture process in ductile metals, such as Al alloys which are mainly used in the aeronautical industry, is normally characterized by the development of certain amount of plastic deformation which precedes the crack extension. From a macro-scale point of view, the fracture process can be schematically divided in three phases: crack-tip blunting, stable crack extension and instable crack extension. It has been reported in [2], [3] that the same fracturing process occurs also in the metal constituent of FMLs. Understanding the failure process and the related mechanisms is of vital importance in assessing failure criteria, which can be applied to predict the crack growth in real structures. The well-known crack-tip-opening angle (CTOA) has been proven to be a material parameter which can be used as fracture criterion for ductile monolithic metals. Many studies [4], [5], [6], [7], [8], [9], [10], [11] have demonstrated that the CTOA is nearly constant after a small amount of crack extension. The non-constant CTOA region has been shown to be related to the change of the fractured surface from flat-to-slant and to the tunnelling of the crack during the initiation of the stable tearing [7]. In addition, the effects of the metal thickness and metal orientation [11] on the CTOA have been investigated for monolithic Al alloy, especially for 2024-T3 [4] and for 2024-T351 [8]. Different techniques available for CTOA measurements have been reported in [4], [10], [12]. The CTOA has been also numerically calculated by means of cohesive zone modelling [13], [14] and the good agreement observed between the predicted and experimental data demonstrated that the CTOA approach is attractive for investigating the structural integrity of thin-walled structures.

Based on the knowledge present in literature, this paper presents a preliminary study of the CTOA in FMLs. The driving idea is to evaluate the applicability of CTOA as fracture parameter for FMLs. The effect of bridging fibres on the CTOA vs. crack extension curve has been investigated considering different lay-ups and fatigue pre-crack lengths. Since the CTOA is a material parameter independent of the specimen geometry [4], [5], [6], [7], [8], [9], [10], [11], it is plausible to assume that the presence of fibres, both ahead and behind the crack tip, should not strongly affect the cracking behaviour of the metal layers. Despite the presence of fibres, the thin metal sheets should present a CTOA vs. crack extension behaviour similar to an equivalent metal laminate, or monolithic metal with comparable thickness.

This investigation is part of a more extensive research with the aim to develop an analytical prediction model for residual strength in FMLs, where the CTOA represents the fracture criterion for the metal constituent.

Section snippets

Fibre metal laminates

Fibre metal laminates (FMLs) such as Glare [1], ARALL [15], and CentrAl [16], consist of alternating layers of metals and fibre-reinforced polymer composites, as illustrated in Fig. 1. FMLs are known for exceptionally slow fatigue crack growth compared to monolithic metallic materials, while still exhibiting many desired properties of metals, such as formability, impact resistance, machinability, and reparability.

The slow crack growth of FMLs is a result of crack bridging by the fibres. While

Experimental work

In the following sections the experimental work is described in details, providing information regarding the characteristics of the selected materials, the description of the test matrix and test execution.

CTOA vs. crack extension

The measurements of the CTOA are plotted versus the crack extension Δa, which is the distance measured from the tip of the fatigue pre-crack to the tip of the quasi-statically extended crack. The “critical” crack-tip-opening angle, CTOA_c, is calculated by averaging the CTOA values starting from Δa_t, which represents the value of the crack extension at the end of the transition phase. The calculation of Δa_t based on the definition provided in [22], which applies only for specimens with a

Conclusions and recommendations

Several fracture tests were performed on M(T) specimens made of metal laminate 2024-T3 3/2-0.4, Glare 2-3/2-0.4 and Glare 3-3/2-0.4. For all the specimens CTOA measurements were performed using the optical measurement technique. The analysis of CTOA vs. Δa, Force vs. displacement and Force vs. crack extension curves led to the following conclusions:

•
For ML configurations the ratio 2a_f/W does not influence the CTOA vs. Δa curve. The CTOA_c reached the same value independently of the ratio 2a_f/W.

Acknowledgements

This research was carried out under the Project No. MC 1.06268 in the framework of the Research Program of the Materials Innovation Institute–M2i–(www.m2i.nl), the former Netherlands Institute for Metal Research.

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Experimental characterization of the crack-tip-opening angle in fibre metal laminates

Abstract

Introduction

Section snippets

Fibre metal laminates

Experimental work

CTOA vs. crack extension

Conclusions and recommendations

Acknowledgements

Eng Fract Mech

Eng Fract Mech

Eng Fract Mech

Eng Fract Mech

Eng Fract Mech

Eng Fract Mech

Eng Fract Mech

Eng Fract Mech

Eng Fract Mech

Eng Fract Mech

CTOA and crack-tunneling measurements in thin sheet 2024-T3 aluminum alloy

Exp Mech