P. Priolo

Static and fatigue behaviour of stitched graphite/epoxy composite laminates

Abstract The paper studies the effect of edge stitching on tensile static and fatigue properties of graphite fiber reinforced laminates. The influence of Kevlar stitchings on edge delaminations, ultimate tensile strength and tension–tension fatigue life were examined on laminates of two stacking sequences ([±45/0/90]s and [±30/90]s), as representative of two classes of fiber-dominated and matrix-dominated laminates. Through-thickness stitching offered a significant improvement in the static delamination resistance of laminated composites, but had varying effects on the ultimate tensile stress of the laminates under study, with an increase in strength of [±30/90]s laminates and a decrease in strength of [±45/0/90]s laminates. Similarly the fatigue life of matrix dominated [±30/90]s specimens was considerably extended by stitching, while the fatigue resistance of fiber-dominated [±45/0/90]s specimens was reduced, particularly at high fatigue stresses. Stitching appeared very efficient in arresting delamination in [±30/90]s specimens for both static and fatigue loads, but did not prove to be as efficient in stopping fatigue delaminations in [±45/0/90]s specimens, since Kevlar threads were found to be prone to breakage under cyclic loading. As a conclusion, while the static delamination resistance of laminated composites is improved by stitching, this does not automatically result in better fatigue performance for all lamination sequences.

Viscoplastic Analysis of the Off-axis Rate-dependent Inelastic Behavior of Unidirectional AS4/PEEK:

Using the finite element program generator (FEPG), the three-dimensional viscoplasticity model is introduced into this finite element automatic generating system. The corresponding element subroutine is developed, which is verified by comparing the finite element computing results with the experimental ones. With the generated procedure the monotonic tensile stress–strain curves are obtained under the strain rate of 0.1 s 1for the off-axis angles 5, 10, 15, 30, 45, 60, 75, and 90, respectively. The above stress versus strain curves show marked nonlinearity for the off-axis angles in the range 10 < <60. Furthermore, in order to observe the rate-dependent nonlinear stress–strain behavior for unidirectional thermoplastic composites of different off-axis angles, we have acquired the stress–strain curves under the strain rate of 1 10 5, 1 10 3, and 1from which we can conclude that the unidirectional AS4/PEEK composites have distant rate-dependent behavior when the stress extends some values which are different especially for the off-axis angles.

Numerical Analysis of Interlaminar Stresses of Angle-ply AS4/PEEK Laminate with a Central Hole

This article addresses the distributions of interlaminar stresses of centrally notched angle-ply AS4/PEEK laminates by finite element method (FEM). The calculation results indicate: interlaminar stress concentration always occurs in the vicinity of free edges (including two free sides and hole edge), which is the so-called free edge effect; when two adjacent plies have the same layup angles, the free edge interlaminar normal stress dominates; when two adjacent plies have the opposite layup angles, interlaminar shear stress dominates. For both conditions, the highest interlaminar stresses generally occur at the hole edge. The results of digital speckle correlation measurement test indicate that the result predicted by FEM is reliable.

Interlaminar Stress Analysis of Thick Thermoplastic AS4-PEEK Composite Laminates

The three-dimensional viscoplasticity model, which can characterize the nonlinear behavior and the strain-rate dependence behavior of the thermoplastic composite AS4-PEEK, is realized through the procedure developed. The interlaminar stresses at different interfaces for two kinds of strain rates are computed. Depending on the plots of the three-dimensional distribution of the interlaminar stresses, the distribution characters of the interlaminar stresses at the different interfaces are analyzed and the major factors that may result in delamination are demonstrated. According to the computing results of the interlaminar stresses for the different strain rates, the effects of the variation of the strain rates on the interlaminar stresses are studied.

Signal processing for passive impact damage detection in composite structures

The problem of impact damage detection in composite structures using piezoceramic sensors is addressed. Piezoceramic sensors are bonded on the composite specimen and used in a passive mode to acquire the strain data from dropweight impact tests. The paper is focused on the comparative study of various signal processing techniques which can extract features related to different impact energy levels associated with different types of damage. The analysis includes time, frequency and wavelet domain features. The results show the potential of the methods fo impact damage identification.

Deformation Measurement of Composite Laminate with Impact Damage under Compressive Loads

The out-of-plane displacement fields for a cross-ply laminate [03/903]s with impact damage under axial compression are investigated using carrier electronic speckle pattern interferometry (carrier-ESPI) in this paper. The compression after impact (CAI) test is conducted on rectangular specimen 65mm × 87.5mm in size using an anti-buckling fixture. In order to apply a uniform load to the top of composite laminate, a self-designed device insures that an accurate loading is given. The experimental results reveal the deformation characteristics of laminate under CAI test.

Static and Fatigue Damage Progression in Graphite Fibre Reinforced Peek

The relatively low amount of structural damage present in some graphite-peek laminates and the related need to increase the sensitivity of damage indicators such as stiffness and crack density have led to an investigation of various experimental techniques suitable for following damage progression during static and fatigue tests. Stiffness measurement is completed with real time detection of the hysteresis cycle area during the test. Traditional optical microscopy on the lateral surface is integrated by the observation and measurement of the pitch of a photographic grating bonded to the same surface. In this way it is possible to follow the complete strain field of the lateral surface and assess any different behaviour of the layers. Some examples of application of the above techniques to graphite-peek specimens under static and fatigue loading are reported and discussed.

Multiscale Finite Element Modelling of Gallery Failure in Epoxy-Clay Nanocomposites

A multiscale finite element (FE) methodology is applied to study failure behaviour of an intercalated epoxy-clay nanocomposite. A 2D FE model of the nanocomposite is built to capture nanocomposite morphology and gallery failure mechanism. Intercalated morphology is reconstructed using a random dispersion of clay tactoids within the epoxy matrix, while the galleries are modeled using cohesive zone elements. The nanocomposite response is predicted by numerical homogenization technique. The effects of cohesive law parameters (particularly the fracture energy) and clay volume fraction on the macroscopic behavior of the nanocomposite are investigated. The analysis shows that gallery failure is the main cause of strength reduction of the nanocomposite. Moreover, the strength reduction is found to increase with the clay content, which is in a qualitative agreement with available experimental results.

Deformation in Impacted Stitched Composite Plates Subjected to Compressive Load

The study on deformation in impact-damaged graphite-fibre/epoxy stitched composite plates subjected to compressive load is presented. A delaminated cross-ply laminate [03/903]S obtained in low-velocity impact test has been examined using a self-designed anti-buckling device in compressive experiment. The out-of-plane displacement field of the specimen has been measured with an optical whole-filed measurement technique, which is carrier electronic speckle pattern interferometry (carrier-ESPI). Finite element (FE) simulation is also carried out to predict the deformation. The effect of the stitching line on compressive deformation is discussed for various stitched laminates. Finally, the numerical results are compared with experimental measurement deformations under different compressive loads.

A Numerical and Experimental Study of Composite Plates with Delamination under Compression Load

The results on a numerical and experimental study of graphite/epoxy composite plates [03/903]S are presented. Each specimen was firstly impacted by the low velocity with different energy level. Compression experiment based on carrier electronic speckle pattern interferometry (carrier-ESPI) is made for the post-impact composite plate. The finite element method (FEM) is used to gain further understanding of the deformation behavior of impacted specimens. The compressive deformation of various delaminated composite plates are analyzed with software ANSYS to see that the impact parameters and delamination damage data, such as impact energy, maximum impact force and delamination area, affect the deformation fields. In addition, the comparisons between the finite element results and experimental measurements are considered under different compression loads.