Key Engineering Materials Vol. 627

Abstract: Damage detection in anisotropic composite plates based on Lamb wave technique has been investigated. A network of transducers is used to detect barely visible damage caused by impact. A CFRP composite plate has been impacted and tested to verify the proposed damage detection algorithms. The difference in the propagational properties of Lamb waves in the pristine state and the damage state is used through data fusion and imaging algorithms to detect, locate and characterise the damage. The influence of directionality of the velocity on the validity of the detection algorithm is examined and some results are presented.

Abstract: Derivation of the integral equation for general 3D crack problems was examined based on the theory of body force method. In the present analysis, stress intensity factors (SIFs) along a front of arbitrary shaped 3D planar crack are obtained directly only by solving simultaneous equations expressing a boundary condition. The crack surface is discretized using number of triangular elements and the variation of the force doublet embedded in each triangle is assumed at constant. The derived boundary integral equation was transformed into a set of simultaneous equations and was solved computationally. In order to improve the accuracy of the numerically examined boundary integral, a polar transformation scheme combined with Tayler expansion of the fundamental solutions is introduced. Not only a single crack problem but also an interference among coplanar cracks can be calculated using the unique program developed in this research. It was verified that as the number of triangular elements increases, the evaluated SIF converges to the reference solution.

Abstract: The fully reversed long life fatigue cycle behaviour of shot peened steels has been investigated. In the case of air cooled forged 0.4%C and 0.7%C steels, shot peening resulted in a relatively small effect on fatigue life (+2.2% and-2.0% respectively) owing to cyclic softening. Fatigue cracks in the shot peened specimens have been observed to initiate in sub-surface layers, reducing the detrimental effect of surface roughness. Neither cyclic softening nor hardening occurred in the smooth non shot peened samples cycled under the same conditions. Shot peening quench and tempered 0.5%C steel samples resulted in a reduced fatigue limit of 12.0% due to cyclic softening. Relaxation of the residual stresses occurred quickly in these steels due to adjustment and rebalancing of the residual stresses caused by the plastic strain. The effect of cyclic softening and shift in crack initiation site rather than the residual peening stresses was significant in determining the fatigue life of these shot peened steels.

Abstract: A new model based on Finite Fracture Mechanics (FFMs) has been proposed to predict the open-hole tensile strength of composite laminates [1]. Failure is predicted when bothstress-based and energy-based criteria are satisfied. This model is based on an analytical solution, and no empirical adjusting parameters are required, but only two material properties: the unnotched strength and the fracture toughness. In the present work, an extension of the proposed FFMs model to predict the notched response of composite laminates with notch geometries other than a circular opening [2] is presented and applied to the prediction of size effects on the tensile and compressive notched strength of composite laminates. The present model is also used to assess the notch sensitivity and brittleness of composite laminates by means of versatile design charts and by the identification of a dimensionless parameter designated as notch sensitivity factor. A further extension of the FFMs model is proposed, which takes into account the crack resistance curve of the laminate in the model's formulation, and it is used to predict the large damage capability of a non-crimp fabric thin-ply laminate [3].

Abstract: The development of composite materials has been following a growing trend, with its use ranging from sporting goods to construction materials and defense. The aerospace industry in particular has observed a minute increase in demand for these materials, since the associated weight reductions promise significant savings in fuel related costs. Composite materials, however, are prone to important and very specific failure modes which are invisible to the naked eye (e.g., delaminations, fiber ruptures, debondings) and occur in response to everyday events such as impacts. Thus, usage of these materials in critical areas of airplanes (e.g. wings and fuselage) is subject to the development of technologies able to continuously and precisely monitor existence, type and extent of failures. Impact localization plays a key role within this paradigm allowing identification of hot spots, that is, areas subjected to potentially damaging phenomena, for further analysis. Several such techniques have already been developed, but anisotropy, precision, robustness and cost are common drawbacks amongst their implementation. The present study was directed to the use of conditioned piezoelectric sensors along with low cost commercial off-the shelf data acquisition system to implement a reliable impact localization method.

Abstract: One of the most critical aspects of composite structures is indeed associated to delamination phenomenon, especially with reference to their fatigue behavior. As a matter of facts, delaminations are strongly influenced by the fatigue induced degradation phenomena which can lead to a significant increase of delaminated area with the number of cycles, reducing the structural load carrying capability. In the present paper, an advanced numerical approach, very similar to the Paris Law formulation and based on the Energy Release Rate, is presented. The proposed formulation, in the frame of a geometrical non-linear analysis, is able to take into account the local damage accumulation along the delamination front in order to evaluate the delamination growth under fatigue loading conditions. In order to test the effectiveness of the proposed numerical approach, the fatigue behavior of a delaminated panel with a central hole has been simulated and the obtained numerical results have been compared with literature experimental results.

Abstract: The aim of this work is to introduce a novel constitutive material model for the progressive failure analysis of composite laminates able to predict the Gradual degradation of laminates up to their complete failure. As a relevant added value with respect to state of the art constitutive models, the presented model uses energy considerations to avoid mesh and time-step dependences. In order to prove its effectiveness and accuracy, the proposed material constitutive model has been implemented in the FE software ANSYS© as a User defined material subroutine (USERMAT) and used to simulate the mechanical behavior of an Open Hole Tension specimen. The obtained numerical results have been compared to experimental literature data and to the numerical outputs of Instantaneous and Standard Gradual Degradation Models.

Abstract: Aim of this paper is in presentation of results of static and dynamic tests of round timber bolted connections with slotted – in steel plates. Round timber joints static tests in tension were made on pressure machine. Round timber joints multicyclic (fatigue) tests in tension were made on pulsator. Results of laboratory tests have been statistically evaluated and completed by graphical records of deformation response on loading. Samples of round timber bolt connections with slotted - in steel plates were tested for carrying capacity and deformation of a single tension – up to the failure of connection.

Abstract: A two-scale three-dimensional approach for degradation and failure in polycrystalline materials is presented. The method involves the component level and the grain scale. The damage-induced softening at the macroscale is modelled employing an initial stress boundary element approach. The microscopic degradation is explicitly modelled associating Representative Volume Elements (RVEs) to relevant points of the macro continuum and employing a cohesive-frictional 3D grain-boundary formulation to simulate intergranular degradation and failure in the Voronoi morphology. Macro-strains are downscaled as RVEs' periodic boundary conditions, while overall macro-stresses are obtained upscaling the micro-stress field via volume averages. The comparison between effective macro-stresses for the damaged and undamaged RVEs allows to define a macroscopic measure of local material degradation. Some attention is devoted to avoiding pathological damage localization at the macro-scale. The multiscale processing algorithm is described and some preliminary results are illustrated.