An experimental investigation of multi-functional z-pinned carbon-epoxy composites

Carbon fibre reinforced epoxy composites have high specific strength, stiffness and fatigue resistance whilst being durable and corrosion resistant, and these properties make them suitable for light-weight aerospace structures. Despite the many advantages, there are some factors that limit the use of composites in aircraft structures. Carbon-epoxy laminates are susceptible to delamination cracking due to the low strength and toughness properties of the epoxy matrix and the fibre-matrix interface. Delamination cracks can grow under relatively low interlaminar loads resulting in a potential threat to the structural integrity and safety of composite structures. Other problems with using carbon-epoxy composites in aircraft structures are their low electrical and thermal conductivities. This PhD project aims to experimentally characterise a multi-functional carbon fibre-epoxy laminate that combines high delamination resistance with increased thermal and electrical conductivities. These properties are controllably improved using z-pins, which are thin composite or metal rods inserted in the through-thickness direction of the laminate. The effects of the volume content, diameter, length and material properties of z-pins on the delamination fracture toughness, fatigue resistance, impact damage tolerance, and interlaminar strengthening mechanisms are systematically investigated. In addition, the influence of the material properties and concentration of z-pins on the through-thickness electrical and thermal conductivities of carbon-epoxy laminates is determined. Using this information, it is possible to design multi-functional composites with tailored damage tolerant and electrical/thermal properties

Dynamic mode II delamination in through thickness reinforced composites

Through thickness reinforcement (TTR) technologies have been shown to provide effective delamination resistance for laminated composite materials. The addition of this reinforcement allows for the design of highly damage tolerant composite structures, specifically when subjected to impact events. The aim of this investigation was to understand the delamination resistance of Z-pinned composites when subjected to increasing strain rates. Z-pinned laminated composites were manufactured and tested using three point end notched flexure (3ENF) specimens subjected to increasing loading rates from quasi-static (~0m/s) to high velocity impact (5m/s), using a range of test equipment including drop weight impact tower and a split Hopkinson bar (SHPB). Using a high speed impact camera and frame by frame pixel tracking of the strain rates, delamination velocities as well as the apparent fracture toughness of the Z-pinned laminates were measured and analysed. Experimental results indicate that there is a transition in the failure morphology of the Z-pinned laminates from quasi-static to high strain rates. The fundamental physical mechanisms that generate this transition are discussed

Previsione delle caratteristiche qualitative di prodotti laminati tramite definizione dei parametri termomeccanici di lavorazione: il sistema Promet 4.0

L’obiettivo del progetto di ricerca, del quale viene descritto in questa sede il prodotto finale, era la messa apunto di un metodo di previsione della microstruttura che, tenendo conto delle condizioni di laminazione eraffreddamento, fosse in grado di anticipare le principali caratteristiche meccaniche e microstrutturali dellaminato. Il modello è stato sviluppato partendo dalle conoscenze teoriche sviluppate dai molti ricercatori che sisono occupati di queste problematiche, e dall’esperienza accumulata nell’attività di progettazione di impianti.Al fine di consentire la massima flessibilità operativa all’utilizzatore, il software prevede che siano fornite lecondizioni operative di laminazione (riduzione per passata, temperatura, tempo intergabbia, temperatura dipreriscaldo, eventuale profilo di raffreddamento), e dispone di un database di oltre 100 acciai, contenente lecurve CCT e le proprietà meccaniche in relazione alla velocità di raffreddamento. Lo strumento fornisce lacurva CCT, opportunamente modificata per tener conto della microstruttura del laminando, con sovrimposta latraiettoria di raffreddamento impostata dall’operatore, nonché i dati microstrutturali e meccanici di interesseper quella determinata classe di acciai. Il software è stato validato tramite confronto diretto con le proprietà diprodotti laminati in condizioni controllate, ottenendo un’eccellente capacità previsionale.INTRODUZIONE:L’IMPORTANZA DEL CONTROLLO DELLE CONDIZIONIDI LAMINAZIONE NEI MODERNI IMPIANTILa temperatura, la velocità di deformazione e il raffreddamentosuccessivo alla laminazione possono determinare caratteristichetecnologiche tali da poter eliminare, in alcuni prodotti, successivicostosi trattamenti termici. Alla fine della laminazione ilpezzo lavorato ha una temperatura ancora molto alta, che dipendesia dall'impianto sia dai parametri di laminazione adottati.Le moderne tecnologie prevedono un controllo costante dellatemperatura nelle varie fasi del processo, incluso il raffreddamentofinale dopo l'ultima gabbia in presa, e dell'evoluzione delgrano austenico. Le fasi più interessanti del processo sono: a. laminazionea temperatura controllata nel treno sbozzatore e intermedio;b. raffreddamento ad acqua a monte e a valle dellegabbie di finitura; c. raffreddamento controllato su linee di evacuazione.Per i prodotti in barre vengono usati letti debitamente proporzionatiin lunghezza e larghezza, dove i prodotti, una volta scaricati,vengono traslati tramite appositi longheroni mobili eM. El Mehtedi, S. SpigarelliDipartimento di Ingegneria Industriale e Scienze Matematiche,Università Politecnica delle Marche, 60123 – Ancona (Italy)tel. +390712204746, fax. +390712204801email: [email protected]. Pegorin

Previsione delle caratteristiche qualitative di prodotti laminati tramite definizione dei parametri termomeccanici di lavorazione: il sistema Promet 4.0

The objective of this research project was the setting up of a model able to predict the microstructure which, taking into account the rolling schedule and cooling, will be able to provide the mechanical and microstructural characteristics after rolling. The model was developed starting from the theoretical knowledge proposed by many researchers who have dealt with these issues, and the experience gained in the design of rolling systems. In order to allow the maximum working flexibility to the final user, the PROMET system requires to fill in the thermomechanical conditions for rod rolling (preheating temperature, pass reduction, temperature, interpass time, strain rate and cooling profile), and a database of more than 100 steel types was developed, containing CCT curves and the mechanical properties relative to the cooling rate. The tool provides the CCT curves, suitably modified to take into account the microstructure of the rolled, superimposed with the cooling trajectory set up by the operator, as well as mechanical and microstructural data of interest for that particular class of steels. The PROMET system was validated by direct comparison with the properties of rod rolled products under controlled conditions, obtaining an excellent prediction capability

Prediction Models of the final properties of steel rods obtained by thermomechanical rolling process

The objective of this research project was the setting up of a numerical model able to predict the microstructureof rod rolled products which, taking into account the rolling schedule and cooling, is able to provide themechanical and microstructural final characteristics. The model was developed starting from the theoreticalknowledge proposed by many researchers who have dealt with these issues, and the experience gained in thedesign of rolling systems by Siemens-VAI. In order to allow the maximum working flexibility to the final user, theprediction model requires to fill in the thermomechanical conditions for rod rolling (preheating temperature,reduction pass, rolling temperatures, interpass time, strain rate and cooling profile); a database of more than150 steel types was developed, containing CCT curves and the mechanical properties relative to the coolingrates. The tool provides the CCT curves, suitably modified to take into account the microstructure of the rolled,superimposed with the cooling trajectory set up by the operator, as well as mechanical and microstructural dataof interest for that particular class of steel. The Model was validated by direct comparison with the properties ofrod rolled products under controlled conditions, obtaining an excellent prediction capabilit

Mixed-mode I/II delamination fatigue strengthening of polymer composites using z-pins

An experimental investigation is presented into the improvement to the delamination fatigue resistant properties of z-pinned carbon fibre-epoxy composite under mixed-mode I/II cyclic interlaminar loading. Delamination fatigue tests are performed on unpinned and z-pinned composites under different mixed-mode ratios spanning mode I to mode II interlaminar cyclic conditions. The fatigue resistance and fatigue strengthening mechanisms induced by the z-pins is dependent on the cyclic mixed-mode ratio. The threshold critical strain energy release rate needed to initiate delamination growth in the z-pinned composite increases with the G I -to-G II ratio. The fatigue crack growth rate slows considerably and the critical strain energy release rate for fast fatigue fracture increases with the G I -to-G II ratio. The delamination fatigue strengthening induced by the z-pins increases with the G I -to-G II ratio due to a transition in the crack bridging toughening process from pin pull-out under mode I dominated loads to combined tensile and shear fracture under mixed-mode loads to pin shear rupture under mode II dominated loads. It is also found that the effect of the G I -to-G II ratio on the fatigue properties is greater for the z-pinned composite compared to the unpinned laminate due to the high fatigue sensitivity of z-pins to the mixed-mode ratio

On a coupled mode at sharp notches subjected to anti-plane loading

When a crack or sharp notch is subjected to anti-plane shear loading the boundary conditions negate the transverse shear stress components at the free surfaces and generate a coupled in-plane shear stress field, which can be singular. For sharp V-notches with zero notch opening angle (or cracks) this 3D phenomenon was known for a long period of time. However this mode was largely ignored in stress analysis of notched components. Till now it is still unclear how significant this mode is and whether it has to be taken into consideration in integrity and failure analysis of engineering structures. The present paper is aimed to investigate this singular mode by means of the 3D Finite Element method, which is applied to notched plates with different notch opening angles and plate thicknesses. The stress state associated with the coupled mode is localised and quickly decays with the distance from the notch tip. Therefore, the obtained theoretical results can be applied to a wide range of notched components provided that the boundaries are sufficiently far from the notch tip. Among these results is the effect of the plate thickness on the intensity of the coupled mode; the latter increases with the decrease of the plate thickness. Significance of these new results to failure and integrity assessments of plate and shell components with sharp notches, including through-the-thickness cracks, is discussed in the final section of the paper. © 2012 Elsevier Masson SAS. All rights reserved.Filippo Berto, Andrei Kotousov, Paolo Lazzarin, Fabio Pegori

Comparative study of the mode I and mode II delamination fatigue properties of z-pinned aircraft composites

This paper compares improvements to the mode I and mode II delamination fatigue resistance of an aerospace composite material achieved by z-pin reinforcement. Mode I (cyclic crack opening) and mode II (cyclic crack sliding) interlaminar fatigue tests were performed on a carbon fibre reinforced epoxy composite reinforced in the through-thickness direction with different volume contents and diameters of z-pins. Paris curves obtained from displacement-controlled fatigue tests reveal that z-pins are more effective at resisting the initiation and growth of delamination cracks under mode I than mode II conditions. Both the mode I and II fatigue resistance increase with the z-pin content due to the formation of a large-scale extrinsic crack bridging toughening zone, although fatigue strengthening is greater for mode I. Improvements to the mode I and mode II delamination fatigue resistance are also dependent on the z-pin diameter. Similarities and differences in the mode I and mode II fatigue properties are related to the fatigue strengthening mechanisms induced by z-pins for the two load conditions

Controlling the electrical conductivity of fibre-polymer composites using z-pins

Carbon fibre composites have inherently high through-thickness electrical resistivity, which limits their application when high electrical conductivity is required. The use of z-pins to increase the through-thickness electrical conductivity of composite laminates is investigated in this paper. The through-thickness and in-plane electrical properties of a unidirectional carbon-epoxy laminate reinforced with carbon fibre composite or metal (copper, stainless steel, titanium) z-pins are characterised. Experimental tests and analytical model reveal that z-pins can increase the through-thickness electrical conductivity of the composite material by many orders of magnitude (up to 10 6 ). The through-thickness electrical conductivity can be controllably increased via the judicious choice of the material type and volume content of the z-pins. Large improvements to the through-thickness conductivity can be achieved without the z-pins altering significantly the in-plane conductivity of the composite material

Three dimensional finite element mixed fracture mode under anti-plane loading of a crack

The 3D Finite Element method is applied to mixed fracture under anti-plane loading of a straight through-the-thickness crack in a linear elastic plate. This coupled fracture mode represents one of three-dimensional phenomena, which are currently largely ignored in numerical simulations and failure assessment of structural components weakened by cracks. It arises due to the boundary conditions on the plate free surfaces, which negate the transverse shear stress components corresponding to classical mode III. Instead, a new singular stress state in addition to the well-known 3D corner singularity is generated. This singular stress state (or coupled fracture mode) can affect or contribute significantly to the fracture initiation conditions. The coupled singular mode exists even if the applied anti-plane loading produces no singularities (KIII=0). In this case there is a strong thickness effect on the intensity of the coupled fracture mode. © 2013 Elsevier Ltd