Jacob Waldbjørn’s research while affiliated with Technical University of Denmark and other places

The design of a thin-walled composite beam is presented whose purpose is to facilitate the study of intra-laminar tunneling cracks at the sub-structural length-scale. The beam utilizes a variable thickness reinforcement in the longitudinal direction to develop a gauge zone of near-uniform strains when loaded in a cantilever beam configuration. The beam is manufactured from E-glass fibers and epoxy resin and is tested under static and cyclic multi-axial loading. The results presented include experimental strain measurements at the gauge zone, which agree with numerical strain predictions, and observations of the developed tunneling cracks.

Background Efficient biaxial strain control to study fatigue damage growth in composite materials at the coupon scale level under realistic multi-axial stress states observed in structures is needed. Objective Two biaxial cyclic strain control algorithms, referred to here as the active and the passive control method, are presented as a fatigue testing strategy for composite materials. Method The strain is measured at a sampling frequency of 116 Hz through a real-time digital image point tracking system. The active control method utilizes a cascade control algorithm to accommodate the low sampling rate of the digital image point tracking system relative to the servo-hydraulic PID controller. The passive control method is a conditional control algorithm where the force command is adjusted when the measured peak-valley strain violates the predefined strain tolerance. The performance of these control techniques are initially evaluated on uniaxial coupon specimens to investigate the effect of testing frequency, strain magnitude, accuracy of the stiffness parameter and the tolerance band. Subsequently, the two control methods are verified on cruciform specimens under different biaxial strain states. ResultsUnder biaxial loading, the peak feedback strains had a maximum error of 6.4% and 9.0% for the active and the passive control method respectively, depending on the required biaxial strain state. Conclusions The active control method was found to offer a better accuracy relative to the passive control method. However, it suffered from a lower fidelity as the test stopped whenever the real-time digital image point tracking system lost track of the point markers.

Transverse cracks in the double curved trailing edge panels within the transition zone are among one of the increasingly encountered in-field damages found on wind turbine blades today. Believed to be root cause of these transverse cracks, are the out-of-plane deformation of the double curved trailing edge pressure side panels. These deformations are evaluated on the inner 15 m section of a 34 m wind turbine blade – referred to here as the root section. Through a parametrical study the free end of the root section is loaded in the quasi-static regime comprising edgewise loading (Fy) and torsional moment (Mz) around the longitudinal axis of the blade. The root section is through a multi-scale numerical analysis found to exhibit representative structural behavior in terms of out-of-plane deformations within the area of interest. A combination between Fy and Mz are found to generate the highest peak-to-peak out-of-plane deformation of 15.9 mm.

Real-time hybrid simulation (RTHS) is a powerful cyber-physical technique that is a relatively cost-effective method to perform global/local system evaluation of structural systems. A major factor that determines the ability of an RTHS to represent true system-level behavior is the fidelity of the numerical substructure. While the use of higher-order models increases fidelity of the simulation, it also increases the demand for computational resources. Because RTHS is executed at real-time, in a conventional RTHS configuration, this increase in computational resources may limit the achievable sampling frequencies and/or introduce delays that can degrade its stability and performance. In this study, the Adaptive Multi-rate Interface rate-transitioning and compensation technique is developed to enable the use of more complex numerical models. Such a multi-rate RTHS is strictly executed at real-time, although it employs different time steps in the numerical and the physical substructures while including rate-transitioning to link the components appropriately. Typically, a higher-order numerical substructure model is solved at larger time intervals, and is coupled with a physical substructure that is driven at smaller time intervals for actuator control purposes. Through a series of simulations, the performance of the AMRI and several existing approaches for multi-rate RTHS is compared. It is noted that compared with existing methods, AMRI leads to a smaller error, especially at higher ratios of sampling frequency between the numerical and physical substructures and for input signals with high-frequency content. Further, it does not induce signal chattering at the coupling frequency. The effectiveness of AMRI is also verified experimentally.

This paper presents a quasi-static hybrid simulation performed on a single component structure. Hybrid simulation is a substructural technique, where a structure is divided into two sections: a numerical section of the main structure and a physical experiment of the remainder. In previous cases, hybrid simulation has typically been applied to structures with a simple connection between the numerical model and physical test, e.g. civil engineering structures. In this paper, the method is applied to a composite structure, where the boundary is more complex i.e. 3 degrees of freedom. In order to evaluate the validity of the method, the results are compared to a test of the emulated structure – referred to here as the reference test. It was found that the error introduced by compliance in the load train was significant. Digital image correlation was for this reason implemented in the hybrid simulation communication loop to compensate for this source of error. Furthermore, the accuracy of the hybrid simulation was improved by compensating for communication delay. The test showed high correspondence between the hybrid simulation and the reference test in terms of overall deflection as well as displacements and rotation in the shared boundary.

Test control is traditionally performed by a feedback signal from a displacement transducer or force gauge positioned inside the actuator of a test machine. For highly compliant test rigs, this is a problem since the response of the rig influences the results. It is therefore beneficial to control the test based on measurements performed directly on the test specimen. In this paper, fibre Bragg grating (FBG) and Digital Image Correlation (DIC) are used to control a test. The FBG sensors offer the possibility of measuring strains inside the specimen, while the DIC system measures strains and displacement on the surface of the specimen. In this paper, a three-point bending test is used to demonstrate the functionality of a control loop, where the FBG and DIC signals are used as control channels. The FBG strain control was capable of controlling the test within an error tolerance of 20 µm m−1. However, the measurement uncertainty offered by the FBG system allowed a tolerance of 8.3 µm m−1. The DIC displacement control proved capable of controlling the displacement within an accuracy of 0.01 mm.

A holistic approach to strain monitoring in fibre-reinforced polymer composites is presented using embedded fibre Bragg grating sensors. Internal strains are monitored in unidirectional E-glass/epoxy laminate beams during vacuum infusion, curing, post-curing and subsequent loading in flexure until failure. The internal process-induced strain development is investigated through use of different cure schedules and tool/part interactions. The fibre Bragg grating sensors successfully monitor resin flow front progression during infusion, and strain development during curing, representative of the different cure temperatures and tool/part interfaces used. Substantial internal process-induced strains develop in the transverse fibre direction, which should be taken into consideration when designing fibre-reinforced polymer laminates. Flexure tests indicate no significant difference in the mechanical properties of the differently cured specimens, despite the large differences in measured residual strains. This indicates that conventional flexure testing may not reveal residual strain or stress effects at small specimen scale levels. The internal stresses are seen to influence the accuracy of the fibre Bragg gratings within the loading regime. This study confirms the effectiveness of composite life cycle strain monitoring for developing consistent manufacturing processes.