D. A. Crump

Design and commission of an experimental test rig to apply a full-scale pressure load on composite sandwich panels representative of an aircraft secondary structure

This paper describes the design of a test rig, which is used to apply a representative pressure load to a full-scale composite sandwich secondary aircraft structure. A generic panel was designed with features to represent those in the composite sandwich secondary aircraft structure. To provide full-field strain data from the panels, the test rig was designed for use with optical measurement techniques such as thermoelastic stress analysis (TSA) and digital image correlation (DIC). TSA requires a cyclic load to be applied to a structure for the measurement of the strain state; therefore, the test rig has been designed to be mounted on a standard servo-hydraulic test machine. As both TSA and DIC require an uninterrupted view of the surface of the test panel, an important consideration in the design is facilitating the optical access for the two techniques. To aid the test rig design a finite element (FE) model was produced. The model provides information on the deflections that must be accommodated by the test rig, and ensures that the stress and strain levels developed in the panel when loaded in the test rig would be sufficient for measurement using TSA and DIC. Finally, initial tests using the test rig have shown it to be capable of achieving the required pressure and maintaining a cyclic load. It was also demonstrated that both TSA and DIC data can be collected from the panels under load, which are used to validate the stress and deflection derived from the FE model.

The Manufacturing Procedure for Aerospace Secondary Sandwich Structure Panels

This study provides a detailed consideration of five manufacturing options that are used to produce aerospace sandwich panels used in secondary structure. The structural performance of each of the manufacturing options is considered along with a cost analysis. By considering the traditional preimpregnated (prepreg), autoclave-cured process, the sources of cost have been investigated, and it has been shown that by removing a portion of the large labor content and the autoclave cure, in favor of an oven-only cure, it would be possible to make significant savings. Monitoring the time to manufacture representative full-scale sandwich panels using the five manufacturing options has shown that by using a resin film infusion (RFI) oven cure, a 30% reduction in time to production is possible. To make an initial assessment of the comparative structural performance of laminates produced using the five manufacturing options, this article also presents results of material quality, in-plane and out-of-plane loading tests. The results of these tests show that the laminates produced using RFI are comparable in quality and performance to laminates produced using the current aerospace industry standard prepreg/autoclave process.

Characterization of an infrared detector for high frame rate thermography

The use of a commercially available photodetector based infrared thermography system, operating in the 2–5 µm range, for high frame rate imaging of temperature evolutions in solid materials is investigated. Infrared photodetectors provide a very fast and precise means of obtaining temperature evolutions over a wide range of science and engineering applications. A typical indium antimonide detector will have a thermal resolution of around 4 mK for room temperature measurements, with a noise threshold around 15 to 20 mK. However the precision of the measurement is dependent on the integration time (akin to exposure time in conventional photography). For temperature evolutions that occur at a moderate rate the integration time can be relatively long, enabling a large signal to noise ratio. A matter of increasing importance in engineering is the behaviour of materials at high strain rates, such as those experienced in impact, shock and ballistic loading. The rapid strain evolution in the material is usually accompanied by a temperature change. The temperature change will affect the material constitutive properties and hence it is important to capture both the temperature and the strain evolutions to provide a proper constitutive law for the material behaviour. The present paper concentrates on the capture of the temperature evolutions, which occur at such rates that rule out the use of contact sensors such as thermocouples and electrical resistance thermometers, as their response times are too slow. Furthermore it is desirable to have an indication of the temperature distribution over a test specimen, hence the full-field approach of IRT is investigated. The paper explores the many hitherto unaddressed challenges of IRT when employed at high speed. Firstly the images must be captured at high speeds, which means reduced integration times and hence a reduction in the signal to noise ratio. Furthermore, to achieve the high image capture rates the detector array must be windowed down, therefore there is a compromise made between the extent of the full-field imaging and the temporal resolution of the image capture. In the present work a maximum image capture speed of 15 kHz was achieved with a detector array of 64 × 12 elements and an integration time was 60 µs. Results from initial work on woven E-glass/epoxy tensile specimens are presented.

Challenges in synchronising high speed full-field temperature and strain measurement

The overall motivation for the research described in the paper is an enhanced understanding of the behaviour of fibre reinforced polymer composites subjected to high velocity loading. In particular, the work described here considers a method that allows the collection of synchronised high speed full-field temperature and strain data to investigate the complex viscoelastic behaviour of fibre reinforced polymer composites material that occurs at high strain rates. The experimental approach uses infra-red thermography (IRT) and digital image correlation (DIC). Because high strain rate events occur rapidly it is necessary to capture the images at high speeds. The paper concentrates on the challenges of the use of IRT and DIC at high speeds to obtain temperature and strain fields from composite materials, and in particular using them in a synchronised manner. In the future such data-rich techniques provide the opportunity for detailed investigation into the viscoelastic behaviour and allow in-depth material characterisation for input to future finite element or numerical models.

The use of infrared thermography at high frame rates

Composite materials are finding increased use in applications where impact and high strain rate loading form a significant part of a component’s service loads. It is therefore imperative to fully characterise the thermomechanical response of composite materials at high strain rates. The work described in the paper forms part of a project investigating the thermomechanical response of composite materials at high strain rates. To obtain the temperature evolutions during the high strain rate event (thermoelastic, viscoelastic and fracture energy), full-field infrared thermography is used. In contrast to visible light photography, the measurand in thermography is the intensity of the emitted radiation from the specimen surface, as opposed to reflected radiation. At increasing recording rates, the emittance available for measurement reduces proportional to the exposure time; the faster the data capture the less the exposure time. Hence, signal noise and detector calibration present a major challenge. This is accompanied by challenges arising from controlling an infrared detector that has not been optimised for the purpose of high speed data acquisition. The present paper investigates the possibility of applying infra-red thermography to high strain rate events and discusses the challenges in obtaining reliable values of the temperature changes that occur over very short time scales during high strain rate events.

Performance assessment of aerospace sandwich secondary structure panels using thermoelastic stress analysis

Abstract Full scale tests carried out on polymer composite sandwich panels, which model the loads experienced by aircraft secondary wing structure, are described. The results from the tests form the basis of a performance assessment of the sandwich construction. The sandwich panel face sheets were manufactured from five different carbon fibre material architectures and were consolidated with epoxy resin using a variety of approaches. In previous work, it was shown that it was possible to significantly reduce the manufacturing costs of producing the sandwich structure by replacing the standard certificated process of unidirectional prepreg cured in an autoclave with a method that used non-crimp fabric infused in a conventional oven. In the present paper, thermoelastic stress analysis (TSA) is used during the full scale tests to obtain stress data from the five different panel types. The TSA data and measured deflection data are used to validate finite element (FE) models of each panel. The experimental validation highlighted some interesting features in the modelling approach. The face sheet materials were treated as homogeneous orthotropic blocks, which resulted in a conservative prediction of deflections. The models did not consider the woven and stitched material face sheet material configurations and therefore, omitted some of the features apparent in the experimental work. However, the validation showed that this did not affect the performance evaluation and most importantly, the validated FE showed that using different face sheet materials had little effect on the stresses in the panels.

Analysis of large scale composite components using TSA at low cyclic frequencies

Thermoelastic stress analysis (TSA) has been applied to large scale honeycomb core sandwich structure with carbon fibre face sheets. The sandwich panel was subjected to a pressure load using a custom designed test rig that could only achieve low cycle frequencies of 1 Hz. Two calibration approaches have been discussed and investigated to allow the use of the thermoelastic response as a validation tool for the stress distribution predicted by an FE model. The TSA data was calibrated using thermoelastic constants derived experimentally using tensile strips of the face sheet material. It has been shown that by using constants obtained from the tensile strips manufactured with the same lay-up as the face sheet of the sandwich panel it was possible to achieve a good correspondence between the predicted stress distribution and the measured TSA response.

Approaches to Synchronise Conventional Measurements with Optical Techniques at High Strain Rates

Polymer composites are increasingly being used in high-end and military applications, mainly due to their excellent tailorability to specific loading scenarios and strength/stiffness to weight ratios. The overall purpose of the research project is to develop an enhanced understanding of the behaviour of fibre reinforced polymer composites when subjected to high velocity loading. This is particularly important in military applications, where composite structures are at a high risk of receiving high strain rate loading, such as those resulting from collisions or blasts. The work described here considers an approach that allows the collection of full-field temperature and strain data to investigate the complex viscoelastic behaviour of composite material at high strain rates. To develop such a data-rich approach digital image correlation (DIC) is used to collect the displacement data and infra-red thermography (IRT) is used to collect temperature data. The use of optical techniques at the sampling rates necessary to capture the behaviour of composites subjected to high loading rates is novel and requires using imaging systems at the far extent of their design specification. One of the major advantages of optical techniques is that they are non-contact; however this also forms one of the challenges to their application to high speed testing. The separate camera systems and the test machine/loading system must be synchronised to ensure that the correct strain/temperature measurement is correlated with the correct temporal value of the loading regime. The loading rate exacerbates the situation where even at high sampling rates the data is discrete and therefore it is difficult to match values. The work described in the paper concentrates on investigating the possibility of the high speed DIC and synchronisation. The limitations of bringing together the techniques are discussed in detail, and a discussion of the relative merits of each synchronisation approach is included, which takes into consideration ease of use, accuracy, repeatability etc.

Investigating the Flexural Behaviour of Foams at High Strain Rate Using Optical Measurement Techniques

Polymer closed cell foam beam specimens manufactured from H100 Divinycell (Diab) are tested in four point bend at three loading speeds using a specially designed rig and an Instron VHS test machine. Synchronised high speed images are captured using white light and infra-red thermography (IRT) to obtain the mid-point full-field deflection and strains using digital image correlation (DIC) along with the temperature evolutions. There is a marked increase in the maximum load to failure with loading rate and the optical techniques provide an opportunity to analyse the strain and temperature evolution within the specimens.