Towards a Damage Tolerance Philosophy for Composite Materials and Structures

O’Brien, TK

doi:10.1520/stp24105s

Cited by 83 publications

(45 citation statements)

References 27 publications

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“…The interlaminar mixed-mode fracture toughness is usually expressed via semi-empirical laws, such as the linear interaction or the Benzeggagh-Kenane criteria [2]. Similarly, fatigue delamination growth is described by semi-empirical power laws that relate the interlaminar crack propagation rate to the energy release rate (ERR) [3,4].…”

Section: Literature Reviewmentioning

confidence: 99%

A cut-ply specimen for the mixed-mode fracture toughness and fatigue characterisation of FRPs

Charalambous

Allegri

Lander

et al. 2015

Composites Part A: Applied Science and Manufacturing

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The characterization of mixed-mode fracture toughness and fatigue delamination growth in fibrereinforced composites is crucial for assessing the integrity of structural elements in service. An asymmetric cut-ply coupon (ACP) loaded in four-point bending is here proposed to carry out the aforementioned characterisations. Analytical expressions of the energy release rate and mode-mixity for the ACP are derived and validated by means of finite element analysis. A fracture toughness and fatigue characterisation of the carbon/epoxy material IM7/8552 is carried out via ACP specimens. It is proved that the material data obtained from ACP specimens match those generated using ASTM standard mixed-mode bending (MMB) coupons. The main reason for the introduction of the ACP test resides in its applicability to characterisation scenarios where measuring the delamination length with optical means, as required for MMB coupons, is difficult. Such scenarios include the investigation of static and fatigue delamination growth at low and high temperatures, which requires the usage of environmental chambers. This poses significant constraints in terms of volume available for the test rigs, and, most importantly, limitations on visual access to observe delamination propagation.However, the manufacturing of ACP coupons is more complex than for MMB specimens and the testing requires several additional precautions that are here discussed in detail.

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Section: Literature Reviewmentioning

confidence: 99%

A cut-ply specimen for the mixed-mode fracture toughness and fatigue characterisation of FRPs

Charalambous

Allegri

Lander

et al. 2015

Composites Part A: Applied Science and Manufacturing

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“…Typically, a power curve is fit through this data set, to give a threshold below which delamination should not occur [4,5]. A curve was fit first through the complete data set.…”

Section: Delamination Onsetmentioning

confidence: 99%

Effect of data reduction and fiber-bridging on Mode I delamination characterization of unidirectional composites

Murri

2013

Journal of Composite Materials

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Reliable delamination characterization data for laminated composites are needed for input in analytical models of structures to predict delamination onset and growth. The double-cantilevered beam (DCB) specimen is used to measure fracture toughness, G Ic , and strain energy release rate, G Imax , for delamination onset and growth in laminated composites under mode I loading. The current study was conducted as part of an ASTM Round Robin activity to evaluate a proposed testing standard for Mode I fatigue delamination propagation. Static and fatigue tests were conducted on specimens of IM7/977-3 and G40-800/5276-1 graphite/epoxies, and S2/5216 glass/epoxy DCB specimens to evaluate the draft standard "Standard Test Method for Mode I Fatigue Delamination Propagation of Unidirectional FiberReinforced Polymer Matrix Composites." Static results were used to generate a delamination resistance curve, G IR , for each material, which was used to determine the effects of fiber-bridging on the delamination growth data. All three materials were tested in fatigue at a cyclic G Imax level equal to 90% of the fracture toughness, G Ic , to determine the delamination growth rate. Two different data reduction methods, a 2-point and a 7-point fit, were used and the resulting Paris Law equations were compared. Growth rate results were normalized by the delamination resistance curve for each material and compared to the nonnormalized results. Paris Law exponents were found to decrease by 5.4% to 46.2% due to normalizing the growth data. Additional specimens of the IM7/977-3 material were tested at 3 lower cyclic G Imax levels to compare the effect of loading level on delamination growth rates. The IM7/977-3 tests were also used to determine the delamination threshold curve for that material. The results show that tests at a range of loading levels are necessary to describe the complete delamination behavior of this material.

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“…Quaresimin [7] used 0.3 mm as the value of the crack to be considered detectable by a microscope, justifying that small changes in the initial crack length have a small effect on the fatigue life, within the scatter of the experimental data in any case. On the other hand, O'Brian proposed a new damage tolerance philosophy for composite materials [8], assuming the existence of matrix cracks throughout the off-axis plies, and the onset of delamination being predicted by a strain energy release rate threshold. This threshold can be obtained by running several tests at several high loads, whereby the onset of delamination is always unstable and easily detectable because of catastrophic change in stiffness.…”

Section: Introductionmentioning

confidence: 99%

Damage initiation and structural degradation through resonance vibration: Application to composite laminates in fatigue

Magi

Maio

Sever

2016

Composites Science and Technology

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General rightsThis document is made available in accordance with publisher policies. Please cite only the published version using the reference above. AbstractThe definition of failure is fundamental to the characterisation of fatigue strength of components and structures and is often expressed as a percentage of stiffness degradation. This article proposes a new method to capture damage initiation and structural degradation during a fatigue test, by exploiting resonance vibrations. The method entails monitoring dynamic parameters, including the phase angle between excitation and response, which suddenly changes as soon as the overall stiffness changes as a consequence of damage. In this paper, a theoretical approach based on equations of motion is presented in order to describe the possible scenarios in which a component undergoes structural degradation. In the proposed approach, an abrupt change in the time history of the stiffness of the component is assumed and analysed in both the quasi-static case at low excitation frequencies and the dynamic case, when the excitation frequency is close to the resonance frequency.The resulting analytical solution shows that any small stiffness variation is amplified by the dynamic response of the component as a large phase change, even when it is too small to be captured by the quasi-static response. The idea is successfully applied to capture the initiation of delamination in CFRP laminates, and the relative S-N curve to initiation is drawn. The experimental evidence of a critical event in the fatigue life of a component is found both in the dynamic phase response and in the base acceleration. The critical event is captured from a sudden change in the dynamic parameters of the structure as soon as a delamination is initiated. Thermography, C-scan and micrograph confirmed that the onset of delamination occurs at the moment of the critical event,after an initial stage of microcracking.

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Towards a Damage Tolerance Philosophy for Composite Materials and Structures

Cited by 83 publications

References 27 publications

A cut-ply specimen for the mixed-mode fracture toughness and fatigue characterisation of FRPs

A cut-ply specimen for the mixed-mode fracture toughness and fatigue characterisation of FRPs

Effect of data reduction and fiber-bridging on Mode I delamination characterization of unidirectional composites

Damage initiation and structural degradation through resonance vibration: Application to composite laminates in fatigue

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