Mechanical property variability in FRP laminates and its effect on failure prediction

Results are presented from an experimental study for the modeling of stochastic behavior of a unidirectional Glass/Polyester composite. An analytical approach is developed for the prediction of failure under general in-plane loading including the variability of strength, stiffness and the thermal expansion coefficients. Monte Carlo simulation and the first-order reliability method are used for comparison and the new method is proved to be in good agreement. Following international design codes, a direct comparison is also presented for failure locii at a specific reliability level as derived by the various probabilistic approaches. Results reveal that a serious overestimation of the reliability of the composite structure is being made when the stochastic nature of the material elastic properties is not taken into account.     

Acoustic structural health monitoring of composite materials : Damage identification and evaluation in cross ply laminates using acoustic emission and ultrasonics

The characterisation of the damage state of composite structures is often performed using the acoustic behaviour of the composite system. This behaviour is expected to change significantly as the damage is accumulating in the composite. It is indisputable that different damage mechanisms are activated within the composite laminate during loading scenario. These “damage entities” are acting in different space and time scales within the service life of the structure and may be interdependent. It has been argued that different damage mechanisms attribute distinct acoustic behaviour to the composite system. Loading of cross-ply laminates in particular leads to the accumulation of distinct damage mechanisms, such as matrix cracking, delamination between successive plies and fibre rupture at the final stage of loading. As highlighted in this work, the acoustic emission activity is directly linked to the structural health state of the laminate. At the same time, significant changes on the wave propagation characteristics are reported and correlated to damage accumulation in the composite laminate. In the case of cross ply laminates, experimental tests and numerical simulations indicate that, typical to the presence of transverse cracking and/or delamination, is the increase of the pulse velocity and the transmission efficiency of a propagated ultrasonic wave, an indication that the intact longitudinal plies act as wave guides, as the transverse ply deteriorates. Further to transverse cracking and delamination, the accumulation of longitudinal fibre breaks becomes dominant causing the catastrophic failure of the composite and is expected to be directly linked to the acoustic behaviour of the composite, as the stiffness loss results to the velocity decrease of the propagated wave. In view of the above, the scope of the current work is to assess the efficiency of acoustic emission and ultrasonic transmission as a combined methodology for the assessment of the introduced damage and furthermore as a structural health monitoring tool.     

A non-local criterion for modelling unbalanced woven ply laminates with stress concentrations

A non-local ply scale criterion [Hochard C, Lahellec N, Bordreuil C. A ply scale non-local fibre rupture criterion for CFRP woven ply laminated structures. Compos Struct 2007;80:321–26] was previously developed for predicting the failure of balanced woven ply structures with stress concentrations. This non-local criterion was based on the mean values determined over a Fracture Characteristic Volume (FCV) corresponding to a cylinder with a circular area and the same thickness as the ply. This non-local approach along with a ply scale continuum damage behavioural model was implemented in the ABAQUS Finite Element Code. The behavioural model was developed from a classical Continuum Damage Mechanics (CDM) model [Ladevèze P. A damage computational method for composite structures. Comput Struct 1992;44:79–87]. In the present study, this approach was extended to the case of unbalanced woven ply. The FCV approach and the CDM behavioural model are presented and comparisons are made between the experimental data and the modelling predictions obtained on plates with open holes, notches and saw cuts.     

Compression fatigue failure of CFRP laminates with impact damage

The objective of this study is to investigate failure mechanisms of impact-damaged CFRP laminates subjected to compression fatigue. Two kinds of composite materials, UT500/Epoxy and AS4/PEEK, were used to examine the dependence of failure behavior on the material properties such as interlaminar toughness. Impact-induced delaminations in the UT500/Epoxy specimen were considerably larger than those in the AS4/PEEK specimen. The S–N curves for the UT500/Epoxy specimens with impact damage exhibited a similar tendency to those without impact. The impact-induced delamination in the UT500/Epoxy specimen grew widthwise to the free edge on the rear side of the specimen during the fatigue test. On the other hand, the AS4/PEEK specimens without impact exhibited a more steeply declining S–N curve than those with impact damage. The delaminations in the impacted AS4/PEEK specimen did not reach the free edge before the fatigue fracture.     

Hybrid effects in thin ply carbon/glass unidirectional laminates: Accurate experimental determination and prediction

Experimental results are presented which allow the hybrid effect to be evaluated accurately for thin ply carbon/epoxy–glass/epoxy interlayer hybrid composites. It is shown that there is an enhancement in strain at failure of up to 20% for very thin plies, but no significant effect for thicker plies. Hybrid specimens with thick carbon plies can therefore be used to measure the reference carbon/epoxy failure strain. The latter is significantly higher than the strain from all-carbon specimens in which there is an effect due to stress concentrations at the load introduction. Models are presented which illustrate the mechanisms responsible for the hybrid effect due to the constraint on failure at both the fibre and ply level. These results give a good understanding of how variability in the carbon fibre strengths can translate into hybrid effects in composite laminates.     

Normal and oblique penetration of woven CFRP laminates by a high velocity steel sphere

In this research, two thicknesses of a woven CFRP laminate have been subjected to impact by a steel sphere in a velocity regime ranging from 170 to 374 m/s. Impact and penetration of targets at normal and oblique incidence were studied using high speed video. For the normal incidence targets at the higher velocities of impact, a conical mass of laminate was ejected ahead of the projectile. Furthermore, despite the energy transferred to the plate increasing with impact energy, the degree of delamination in the thicker targets decreased indicating a change in projectile penetration mechanism. Eventually, the degree of delamination in the thicker targets appeared to approach an asymptotic level whereas for the thinner targets the degree of delamination appeared constant regardless of impact energy. For oblique targets, more of the kinetic energy was transferred from the projectile when compared to the same thickness of target that had been subjected to a normal incidence impact. However, this was merely due to a geometrical effect. Further, thicker panels appeared to behave more efficiently by absorbing more kinetic energy per effective linear thickness at the lower impact energies where petalling is a dominant factor in the penetration. This advantage appeared to disappear as the impact energy was increased.     

Non-destructive testing of low-energy impact in CFRP laminates and interior defects in honeycomb sandwich using scanning pulsed eddy current

With the growing interest to use composite materials and honeycomb sandwich panels in industrial fields, much attention is devoted to the development of non-destructive testing (NDT) techniques for the detection and evaluation of defects. In this work, scanning pulsed eddy current (PEC) was investigated and two features, representing the magnetic field intensity and conductivity, were used to characterise the different types of defects in carbon fibre reinforced plastics (CFRP) laminates and honeycomb sandwich panels. The experimental results show that the low energy impact from 4 J to 12 J, conductive and non-conductive insert defects can be effectively detected and evaluated using the proposed methods. The effectiveness was verified and the advantages of scanning PEC were addressed through comparative studies with flash thermography and shearography.     

Reinforcement effects of MWCNT and VGCF in bulk composites and interlayer of CFRP laminates

The reinforcement effects of two nanofillers, i.e., multi-walled carbon nanotube (MWCNT) and vapor grown carbon fiber (VGCF), which are used at the interface of conventional CFRP laminates, and in epoxy bulk composites, have been investigated. When using the two nanofillers at the interface between two conventional CFRP sublaminates, the Mode-I interlaminar tensile strength and fracture toughness of CFRP laminates are improved significantly. The performance of VGCF is better than that of MWCNT in this case. For epoxy bulk composites, the two nanofillers play a similar role of good reinforcement in Young’s modulus and tensile strength. However, the Mode-I fracture toughness of epoxy/MWCNT is much better than that of epoxy/VGCF.     

Detection of fibre fracture and ply fragmentation in thin-ply UD carbon/glass hybrid laminates using acoustic emission

This paper investigates the link between acoustic emission (AE) events and the corresponding damage modes in thin-ply UD carbon/glass hybrid laminates under tensile loading. A novel configuration was investigated which has not previously been studied by AE, where the laminates were fabricated by embedding thin carbon plies between standard thickness translucent glass plies to produce progressive fragmentation of the carbon layer and delamination of the carbon/glass interface. A criterion based on amplitude and energy of the AE event values was established to identify the fragmentation failure mode. Since the glass layer was translucent, it was possible to quantitatively correlate the observed fragmentation during the tests and the AE events with high amplitude and energy values. This new method can be used as a simple and advanced tool to identify fibre fracture as well as estimate the number and sequence of damage events that are not visible e.g. in hybrid laminates with thick or non-transparent layers as well as when the damage is too small to be visually detected.     

Characterization and evolution of matrix and interface related damage in [0/90]S laminates under tension. Part I: Numerical predictions

This paper considers damage development mechanisms in cross-ply laminates using an accurate numerical method that assumes a Generalized Plane Strain (GPS) state. A 2D Boundary Element Method (BEM) model is generated to investigate the two types of damage progression in a [0/90]_S laminate: transverse cracks in the 90° lamina and delamination between both laminae. The model permits the contact between the surfaces of the cracks. The study is carried out in terms of the dependence of the Energy Release Rates (ERR) of the two types of crack on their respective lengths. A special emphasis is put on the mechanisms of the joining of the two aforementioned types of crack, including the study of the distribution of the stresses along the interface between the two plies when the transverse crack is approaching this interface.     

Experiments and analytical comparison of RC beams strengthened with CFRP composites

This paper deals with strengthening, upgrading, and rehabilitation of existing reinforced concrete structures using externally bonded composite materials. Five strengthened, retrofitted, or rehabilitated reinforced concrete beams are experimentally and analytically investigated. Emphasis in placed on the stress concentration that arises near the edge of the fiber reinforced plastic strip, the failure modes triggered by these edge effects, and the means for the prevention of such modes of failure. Three beams are tested with various edge configurations that include wrapping the edge region with vertical composite straps and special forms of the adhesive layer at its edge. The last two beams are preloaded up to failure before strengthening and the ability to rehabilitate members that endured progressive or even total damage is examined. The results reveal a significant improvement in the serviceability and strength of the tested beams and demonstrate that the method is suitable for the rehabilitation of severely damaged structural members. They also reveal the efficiency of the various edge designs and their ability to control the characteristic brittle failure modes. The analytical results are obtained through the Closed-Form High-Order model and are in good agreement with the experiment ones. The analytical and experimental results are also used for a preliminary quantitative evaluation of a fracture mechanics based failure criterion for the strengthened beam.     

Two-dimensional strain-based interactive failure theory for multidirectional composite laminates

A 2-D strain-based interactive failure theory is developed to predict the final failure of composite laminates subjected to multi-axial in-plane loading. The stiffness degradation of a laminate during loading is examined based on the individual failure modes of the maximum strain failure theory, and a piecewise linear incremental approach is employed to describe the nonlinear mechanical behavior of the laminate. In addition, an out-of-plane failure mode normal to the laminate is also investigated to more accurately predict the failure of multidirectional laminates. The theoretical results of the failure model presented are compared with the experimental data provided by the World-Wide Failure Exercise, and the accuracy of the model’s predictive capabilities is investigated.     

An experimental study on the long-term behavior of CFRP pultruded laminates suitable to concrete structures rehabilitation

The rheological behavior of structural materials has a significant role indeed in Civil Engineering, where concrete and FRP (Fiber Reinforced Polymer) materials undergo creep in normal environmental conditions, while steel exhibits a sizable creep only at high temperature (above 400 °C). With reference to RC structures strengthened by means of FRP laminates, FRP creep generally coexists with concrete cracking. The interaction between these phenomena should be taken into account in order to evaluate the structural durability. Here, the first results of a research program on creep in composite pultruded laminates used in Civil Engineering are presented, under various stress levels and in constant environmental conditions (many theoretical and experimental studies on creep have been performed so far in the aerospace and naval fields, but not as many in Civil Engineering). The specimens tested in this project are made of high modulus carbon fiber reinforced polymer – CFRP, whose mechanical properties are tailored for Civil Engineering applications. The tests are still in progress in the Material and Structures Testing Laboratory of the Civil Engineering Department of the University of Salerno (Italy).     

Experimental study on delamination migration in composite laminates

The transition of delamination growth between different ply interfaces in composite tape laminates, known as migration, was investigated experimentally. The test method used promotes delamination growth initially along a 0/θ ply interface, which eventually migrates to a neighbouring θ/0 ply interface. Specimens with θ = 60° and 75° were tested. Migration occurs in two main stages: (1) the initial 0/θ interface delamination turns, transforming into intraply cracks that grow through the θ plies; this process occurs at multiple locations across the width of a specimen, (2) one or more of these cracks growing through the θ plies reaches and turns into the θ/0 ply interface, where it continues to grow as a delamination. A correlation was established between these experimental observations and the shear stress sign at the delamination front, obtained by finite element analyses.Overall, the experiments provide insight into the key mechanisms that govern delamination growth and migration.