New research findings on non-proportional low cycle fatigue

One of the challenges regarding multiaxial fatigue damage predictions is non-proportional loading. Relevant studies have shown that these multiaxial loadings cause significant additional hardening and reduction in durability due to non-proportionality. Fatigue life predictions due to non-proportional loadings are based on an equivalent non-proportional strain range that considers a material constant related to additional hardening and a non-proportionality factor. In this paper an analysis of the non-proportional factor for three multiaxial loadings forming a square in γ/√3 – ε coordinates is carried out. One of the observations revealed by this analysis is the sensitivity of the non-proportional factor to variable shear strain rate

Introduction

The aerospace industry is aiming for a cleaner means of transport. One way to achieve this is by making transportation lighter, thus directly improving fuel efficiency and reducing environmental impact. A further aim, of the industry, is to reduce maintenance time to lessen operating costs, which can result in a reduction of air transport costs, benefitting both passenger and freight services. Current developments to support these aims include using advanced materials, with the current generation of aerospace structures being 50% composite materials. These materials offer a weight reduction whilst maintaining adequate stiffness; however, their damage mechanics are very complex and less deterministic than those of metals. This results in an overall reduced benefit. Structures are manufactured thicker using additional material to accommodate unknown or unpredictable failure modes, which cannot be easily detected during maintenance. A way to overcome these issues is the adoption of a structural health monitoring (SHM) inspection system. Structural health monitoring (SHM) is understood to be the continuous or periodic and automated method for determining and monitoring the condition of a monitored object within condition monitoring (according to DIN ISO 17359). This is conducted through measurements with permanently installed or integrated transducers and the analysis of the measurement data. Its purpose is to detect damage, for example, cracks or deformations, at an early stage to initiate countermeasures. [...

Damage detection in a composite wind turbine blade using 3D scanning laser vibrometry

As worldwide wind energy generation capacity grows, there is an increasing demand to ensure structural integrity of the turbine blades to maintain efficient and safe energy generation. Currently, traditional non-destructive testing methods and visual inspections are employed which require the turbine to be out-of-operation during the inspection periods, resulting in costly and lengthy downtime. This study experimentally investigates the potential for using Lamb waves to monitor the structural integrity of a composite wind turbine blade that has been subject to an impact representative of damage which occurs in service. 3D scanning laser vibrometry was used to measure Lamb waves excited at three different frequencies both prior to, and after, impact to identify settings for an optimal system. Signal processing techniques were applied to the datasets to successfully locate the damage and highlight regions on the structure where the Lamb wave was significantly influenced by the presence of the impact damage. Damage size resulting from the impact was found to correlate well with the laser vibrometry results. The study concluded that acousto-ultrasonic-based structural health monitoring systems have great potential for monitoring the structural integrity of wind turbine blades

Optimized placement of parasitic vibration energy harvesters for autonomous structural health monitoring

Energy harvesting, based on sources including vibration and thermal gradients, has been exploited in recent years to power telemetry, small devices, or to charge batteries or capacitors. Generating the higher levels of power which have thus far been required to run sensor systems such as those needed for structural health monitoring has been more challenging. In addition, harvesters such as those required to capture vibration often require additional elements (e.g. cantilevers) to be added to the structure and harvest over a relatively narrow band of frequencies. In aerospace applications, where weight is at a premium and vibrations occur over a broader range of frequencies, this is non-ideal. With the advent of new, lower power monitoring systems, the potential for energy harvesting to be utilized is significantly increased. This article optimizes the placement of a set of parasitic piezoelectric patches to harvest over the broad band of frequencies found in an aircraft wing and validates the results experimentally. Results are compared with the requirements of a low-power structural health monitoring system, with a closing of the gap between the energy generated and that required being demonstrated

Improved acoustic emission source location during fatigue and impact events in metallic and composite structures

In order to overcome the difficulties in applying traditional Time-Of Arrival (TOA) techniques for locating Acoustic Emission (AE) events in complex structures and materials, a technique termed “delta-t mapping” was developed. This paper presents a significant improvement on this, in which the difficulties in identifying the precise arrival time of an AE signal are addressed by incorporating the Akaike Information Criteria (AIC). The performance of the TOA, the delta-t mapping and the AIC delta-t mapping techniques is assessed by locating artificial AE sources, fatigue damage and impact events in aluminium and composite materials respectively. For all investigations conducted the improved AIC delta-t technique shows a reduction in average Euclidean source location error irrespective of material or source type. For locating H-N sources on a complex aluminium specimen the average source location error (Euclidean) is 32.6, (TOA), 5.8 (delta-t) and 3mm (AIC delta-t). For locating fatigue damage on the same specimen the average error is 20.2, (TOA), 4.2 (delta-t) and 3.4mm (AIC delta-t). For locating H-N sources on a composite panel the average error is 19.3, (TOA), 18.9 (delta-t) and 4.2mm (AIC delta-t). Finally the AIC delta-t mapping technique had the lowest average error (3.3mm) when locating impact events when compared with the delta-t (18.9mm) and TOA (124.7mm) techniques. Overall the AIC delta-t mapping technique is the only technique which demonstrates consistently the lowest average source location error (greatest average error 4.2mm) when compared with the delta-t (greatest average error 18.9mm) and TOA (greatest average error 124.7mm) techniques. These results demonstrate that the AIC delta-t mapping technique is a viable option for AE source location, increasing the accuracy and likelihood of damage detection, irrespective of material, geometry and source type

Audible acoustics for detecting and locating damage in composite structures

Carbon fibre composites are increasingly being used for large scale load bearing structures in a variety of industries, which include aerospace and renewable energy sectors. The main benefit of composite structures over traditional metallic equivalents is the inherent strength to weight ratio. This allows for the development of more optimised and efficient structures. However, the development of these large-scale composite structures can be very costly and time-consuming exercise. As a part of the development process extensive verification testing is undertaken which can range from small-scale coupon and full scale structural tests. Non-Destructive Testing (NDT) techniques are often employed to detect the early onset of damage and avoid catastrophic failures. This requires periodic test down time to gain access to the structure to undertake these inspections. Structural Health Monitoring (SHM) techniques can be employed but these often require sensors coupled to the structure and vast amounts of cabling which can be time consuming to install. Audible Acoustics (AA) uses an array of microphones to audible detect the sound signatures from damage initiation and growth in composite materials and could potential be used as a non-contact NDT technique, creating a quick and easy to setup damage detection system. Therefore, offering a reduction in testing downtime, avoiding unexpected failures and enabling the re-use of specimens, which will ultimately reduce time and costs. This paper explores the use of an Acoustic Camera to detect and located damage in carbon fibre coupon specimens with different lay-ups. An acoustic camera is an array of microphones which can be used to visualise the location of a sound source. Results showed using delay and sum beam forming it was possible locate damage in the structures. Further frequency analysis and traditional NDT translated signal processing techniques were used to enhance the detection of damage