Thomas Gmür

Through-the-thickness distribution of strains in laminated composite plates subjected to bending

In this paper, the through-the-thickness deformation of laminated composite plates subjected to out-of-plane line and concentrated loads is studied experimentally and numerically using different span to depth ratios. Experimental inspection of the specimens is carried out by combining two different techniques: embedded fibre Bragg grating sensors for internal strain measurements and surface-mounted resistive strain gauges for surface strain measurements at selected locations. To eliminate the contribution due to the strain concentration in the vicinity of the loading point and highlight that due to shear effects, measurements are carried out at various distances from the load application by displacing the specimens in the loading frame. A departure from linearity in the through-the-thickness strain distribution is highlighted for small span to depth values. Results are compared to numerically calculated values from finite-element simulations using both laminated-shell and solid elements.

A set of three-dimensional solid to shell transition elements for structural dynamics

Abstract The development of efficient and reliable transition elements which can connect solid and shell finite elements is of prime importance for an accurate modelling of the transition area between three-dimensional solid continuum and thin shell-like regions of complex structures. This paper presents a large set of C0 compatible transition elements that are based upon standard isoparametric solid and superparametric shell elements. The proposed elements, which incorporate the properties of both solids and shells, are adapted to structural dynamics and satisfy the classical finite element convergence requirements. Several numerical examples which compare the current formulation to previously published results or to analytical and experimental solutions are included.

Deformation characteristics of composite laminates—part II: an experimental/numerical study on equivalent single-layer theories

Abstract The problem of defining a range of validity for the most commonly used equivalent single-layer theories for laminated composite plates is addressed in this study. The experimental method described in part I (Bosia F, Botsis J, Facchini M, Giaccari P. Deformation characteristics of composite laminates—part I: speckle interferometry and embedded Bragg grating sensor measurements. Composites Science and Technology [in press]) to determine surface and internal strains in laminates is used to reconstruct strain distributions through the thickness of laminated plates in various cases, from the thin- to the thick-plate range. Three- and four-point bending of simply supported specimens are considered. Additionally, electronic speckle pattern interferometry is employed to visualize the through-the-thickness displacement distribution on the lateral side of the specimens, and highlight the onset of a nonlinear behaviour. Experimental results are compared to finite element simulations using both laminated shell and solid elements. The effect of non-uniform contact between specimens and both loading and support members in the experimental configuration is discussed, and finite-element simulations are modified accordingly. A discrepancy between the predictions of the two considered equivalent-single-layer theories and experimental results is found in the thick-plate range.

Monitoring of composite structures using a network of integrated PVDF film transducers

Aiming to reduce costs, polyvinylidene difluoride (PVDF) film patches are an emerging alternative to more classic piezoelectric technologies, like ceramic patches, as transducers to measure local deformation in many structural applications. This choice is supported by advantages such as the low weight and mechanical flexibility of PVDF, making this polymer suitable for embedding inside full scale polymer based composite structures. Piezoelectric transducer patches can be used as actuators to dynamically excite full-scale composite structures, and as sensors to measure the strain. The main objective of this paper is to verify that the PVDF transducers can provide exploitable signals in the context of structural health monitoring. In order to do so, two aspects of the design of transducer network are investigated: the optimization of the sensor network, for which the effective independence method is proposed, and the use of operational modal analysis (OMA), since it is a simple method to extract the natural frequencies of a structure from a time series. The results of the analysis are compared to a reference set issued from experimental modal analysis (EMA), a simple, well-known, classic method, which is carried out using accelerometers and an impact hammer. By statistical means, it is shown that there is no significant difference between the two methods, and an optimized PVDF transducer network combined with OMA can perform the dynamic analysis of a structure as well as a classic EMA setup would do. This leads the way to the use of low-cost PVDF embedded transducer networks for robust composite material characterization.

Long FBG sensor characterization of residual strains in AS4/PPS thermoplastic laminates

The consolidation of thermoplastic composites produces internal residual strains due to the differences between the coefficients of thermal expansion of the component materials. In the case of AS4/PPS (carbon fibre-polyphenylene sulphide), where the melting/solidification temperature is 280°C, there exists a 255°C range wherein the various constituents will contract/expand to different degrees. A fibre Bragg grating (FBG) sensor may be embedded into this laminate with the goal of characterizing the residual strains; however, these strains may be non-uniform in the longitudinal and transverse directions, and may also vary depending on the laminate architecture. Non-uniform axial strains typically broaden and split the FBG sensors spectral response, making it difficult to measure the strain distribution. Also, load-induced birefringence caused by the consolidation process will complicate the interpretation of the spectral response. This research is directed at understanding the residual strain state in FBG sensors due to the fabrication process. It is the aim of this study to experimentally investigate the residual strains in long and short gauge length FBG sensors embedded in the 0° plies of AS4/PPS unidirectional and cross-ply laminates (200 x 50 x 3.6 mm). Long gauge length sensors are monitored throughout the fabrication process, to provide insight into the development of the residual strains.

A subspace iteration algorithm for the eigensolution of large structures subject to non-classical viscous damping

Reference LMAF-ARTICLE-2000-002View record in Web of Science Record created on 2005-09-14, modified on 2017-05-10

Finite element analysis of microscopic biological structures

Keywords: nanomechanics ; Finite Element Analysis ; Protein elasticity ; DNA elasticity ; Cell Elasticity ; Atomic Force Microscopy Reference EPFL-CHAPTER-149878 Record created on 2010-07-06, modified on 2017-05-12

Experimental Analysis of Composite Laminates Subjected to Bending

Composite materials are becoming increasingly important in many engineering domains. Often the estimation of their mechanical properties is not trivial and simulations need to be accompanied by experimental verification. In this paper, the use of optical methods to characterize the mechanical behaviour of composite laminated plates subjected to bending is described. Speckle Interferometry and fibre optic sensors are used in a combined manner to reach a full understanding of the specimen behaviour. In- and out-of-plane Speckle Interferometry is employed to measure full-field displacements on the free surfaces of the plates, while the strain distribution through the thickness is derived using embedded fibre Bragg grating sensors. Measurements are carried out in the “thin” and “thick” plate ranges and results are compared to analytical and numerical calculations.

Finite-Element Analysis of Microbiological Structures

The recent development of nanomanipulation tools such as optical tweezers or atomic force microscopes has paved the way to the exploration of the mechanical properties of biological material at the micro- and nanometric scale. The use of these instruments is nowadays generalized, however, the interpretation of the results remains a challenge. It is well known that the deformation of an object under a load depends not only on the mechanical properties of the material but also on its geometry. This factor seriously hinders the interpretation of these measurements. To master this complexity, it is nowadays possible to numerically simulate the measuring experimental procedure with various parameters until the simulation fits the measurement results. In this chapter we briefly explain this analysis technique, review its application domains, and mention some recent works that employed this approach.

The solution of large undamped gyroscopic eigensystems by a subspace iteration method

Abstract This paper presents the development of an efficient and numerically stable algorithm for the accurate solution of the free vibration problem arising in the analysis of undamped spinning systems. Closely related to the standard subspace iteration method, the procedure fully exploits the banded nature of the associated structural matrices and avoids complex algebra in all computations. A practical numerical example is presented to demonstrate the effectiveness and accuracy of the proposed algorithm.