Olli Saarela

The Effects of Debonding on the Low-Velocity Impact Response of Steel-CFRP Fibre Metal Laminates

The effect of metal-composite debonding on low-velocity impact response, i.e. on contact force–central deflection response, deformation profiles and strains on the free surfaces was studied. We focused on type 2/1 fibre metal laminate specimens made of stainless steel and carbon fibre epoxy layers, and tested them with drop-weight impact and quasi-static indentation loadings. Local strains were measured with strain gauges and full-field strains with a 3-D digital image correlation method. In addition, finite element simulations were performed and the effects of debonding were studied by exploiting cohesive elements. Our results showed that debonding, either the initial debonding or that formed during the loading, lowers the slope of the contact force–central deflection curve during the force increase. The debonding formation during the rebound phase was shown to amplify the rebound of the impact side, i.e. to lower the ultimate post-impact deflection. The free surface strains were studied on the laminate’s lower surface at the area outside the debond damage. In terms of in-plane strains, debonding formation during impact and indentation, as well as the initial debonding, lowered the peripheral strain and resulted in a positive change in the radial strain.

Tolkku - a toolbox for decision support from condition monitoring data

This paper describes a software toolbox (a software library) designed for condition monitoring and diagnosis of machines. This toolbox implements both new methods and prior art and is aimed for practical down-to-earth data analysis work. The target is to improve knowledge of the operation and behaviour of machines and processes throughout their entire life-cycles. The toolbox supports different phases of condition based maintenance with tools that extract essential information and automate data processing. The paper discusses principles that have guided toolbox design and the implemented toolbox structure. Case examples are used to illustrate how condition monitoring applications can be built using the toolbox. In the first case study the toolbox is applied to fault detection of industrial centrifuges based on measured electrical current. The second case study outlines an application for centralized monitoring of a fleet of machines that supports organizational learning.

Progressive Failure of Composite Laminates; Analysis vs.Experiments

The failure of composite structures divides into three stages; damage initiation, damage progression and final failure. A typical mechanism of damage initiation is matrix cracking in some layer of the laminate. Consequently, damage initiation is often referred to as the first ply failure (FPF). Damage progresses with several mechanisms that often occur interactively. Most important of these are additional matrix cracking and crack propagation, debonding of fibres and the matrix, and delamination of layers. The final failure typically takes place when the ultimate load carrying capability of the fibres is reached in some layer.

Apparent Fracture Toughness Versus Micro-Scale Fracture Toughness of Interfaces—The Challenge of Critical Values

This study aims to shed light on the conflict between macroscopic fracture toughness and small-scale measures of interface strength when the interface is simulated with an increasing resolution. In the design of practical structures, crack initiation is the critical, limiting phase of material behavior and needs to be defined by lucid means. In this study, we model an example structure, namely a fracture test specimen, including a bi-material interface having micro-roughness and thermal residual stresses. The initiation of secondary cracks, which is one of the microscopic mechanisms leading to crack coalescence, is studied. The work focuses on the effects of the bi-materials Youngs modulus, residual stresses, and flaws. The simulation of crack propagation shows that the overall crack tip loading mode mixity distribution is not essentially affected when the Youngs modulus of the bi-materials coating is doubled. The analysis of the secondary crack initiation due to interfacial flaws in turn showed that increasing the Youngs modulus of either the bi-materials substrate or coating resulted in immediate interface collapse via an elongated ‘damage region’ along the interface.

Internal structure and fatigue performance of quasi-unidirectional Non-Crimp Fabric reinforced laminates

The aim of this work was to study the effect of stitch pattern and stitch tension on tension-tension fatigue performance of quasi-unidirectional non-crimp fabric reinforced glass-fibre/epoxy laminates. Internal structure variations of the laminates were characterised using cross-sectional microscopy imaging for the evaluation of fatigue test results. Reinforcements for the study were manufactured with three different stitch patterns and two different stitch tensions. Based on the results, the stitch pattern contributes to in-plane and out-of-plane waviness of fibres, as well as on the size and shape of the axial fibre bundles. The fatigue life and slope parameter of the fitted S-N curve were significantly dependent on the stitch pattern, while they were not dependent on the applied stitch tension.

Intermodulation communication principle: Passive wireless sensor networks for internet-of-things

Intermodulation sensor concept is a solution for intelligent sensing: sensing is wireless and passive. It provides communication distances of several meters, high accuracy, and compliancy with the frequency regulations. The operation of the intermodulation sensor transponder is based on passive mixer diode loaded by a sensor resonator. We introduce the sensor concept, analyze the range of the system, and demonstrate wireless capacitance and voltage measurements.

Data Reconciliation of the Turbine Section: Evaluation of Estimation Uncertainty

Data reconciliation is technique for reducing measurement uncertainty by adjusting measured data to comply with a first-principles process model, most importantly with mass and energy balances. It also provides estimates for modelled unmeasurable process variables and estimates for the uncertainties of the computed values. For computing these estimates the process model has to include estimates of measurement uncertainties defined a priori. A priori consideration of all potential sources of uncertainty is far from trivial. This paper discusses a data-driven approach of uncertainty evaluation, based on identifying and subtracting variability modes affecting multiple measurements. Possible bias in the measurements is not considered. The approach is applied to evaluate the uncertainties of estimates computed with a data reconciliation model of a turbine section of a nuclear power plant.Copyright

Reliability of operating window identified from process data

This paper discusses data-based operating windows as a tool for process management and development. In particular identification of the operating window and its uncertainty are analyzed. The operating window is determined by maximizing either the mutual information (static) or entropy transfer (dynamic). An industrial example shows that the entropy of the indicator variable is reduced to half by an operating window specified with only few variables, selected amongst over 3000 candidates. Test model based simulations suggest that such few-variable operating windows can be reliably identified from datasets having lengths of a few thousand observations.