Malte Frövel

Consumed Fatigue Life Assessment of Composite Material Structures by Optical Surface Roughness Inspection

A strong knowledge of the fatigue state of highly advanced carbon fiber reinforced polymer composite (CFRP) structures is essential to predict the residual life and optimize intervals of structural inspection, repairs, and/or replacements. Current techniques are based mostly in measurement of structural loads throughout the service life by electric strain gauge sensors. These sensors are affected by extreme environmental conditions and by fatigue loads in such a way that the sensors and their systems require exhaustive maintenance throughout system life.This work is focused on providing a new technique to evaluate the fatigue state of CFRP structures by means of evaluating the surface roughness variation due to fatigue damage. The surface roughness is a property that can be measured in the field by optical techniques such as speckle and could be a useful tool for structural health monitoring. The relation between surface roughness and fatigue life has been assessed on CFRP test specimens. A tensile fatigue load with an R=0.1 (T-T) and a maximum load of 60% of the material ultimate strength has been applied. The surface roughness of the specimens has been determined from the surface topography measured by a high precision confocal microscope. Results show that the surface roughness of the specimens increases with the accumulation of fatigue cycles in such a way that the roughness could be taken into account as a fatigue damage metrics for CFRP.

Optical Sensing of the Fatigue Damage State of CFRP under Realistic Aeronautical Load Sequences

We present an optical sensing methodology to estimate the fatigue damage state of structures made of carbon fiber reinforced polymer (CFRP), by measuring variations on the surface roughness. Variable amplitude loads (VAL), which represent realistic loads during aeronautical missions of fighter aircraft (FALSTAFF) have been applied to coupons until failure. Stiffness degradation and surface roughness variations have been measured during the life of the coupons obtaining a Pearson correlation of 0.75 between both variables. The data were compared with a previous study for Constant Amplitude Load (CAL) obtaining similar results. Conclusions suggest that the surface roughness measured in strategic zones is a useful technique for structural health monitoring of CFRP structures, and that it is independent of the type of load applied. Surface roughness can be measured in the field by optical techniques such as speckle, confocal perfilometers and interferometry, among others.

Surface irregularity factor as a parameter to evaluate the fatigue damage state of CFRP

This work presents an optical non-contact technique to evaluate the fatigue damage state of CFRP structures measuring the irregularity factor of the surface. This factor includes information about surface topology and can be measured easily on field, by techniques such as optical perfilometers. The surface irregularity factor has been correlated with stiffness degradation, which is a well-accepted parameter for the evaluation of the fatigue damage state of composite materials. Constant amplitude fatigue loads (CAL) and realistic variable amplitude loads (VAL), representative of real in- flight conditions, have been applied to “dog bone” shaped tensile specimens. It has been shown that the measurement of the surface irregularity parameters can be applied to evaluate the damage state of a structure, and that it is independent of the type of fatigue load that has caused the damage. As a result, this measurement technique is applicable for a wide range of inspections of composite material structures, from pressurized tanks with constant amplitude loads, to variable amplitude loaded aeronautical structures such as wings and empennages, up to automotive and other industrial applications.

Fiber Bragg gratings for optical sensing (FIBOS) for an aerospace application

FIBOS, as one of the payloads of a picosatellite called OPTOS, will be used to measure temperature during the mission with Fiber Bragg Gratings. Description and calibration of FIBOS are presented.

Calibration of FIBer Bragg gratings for Optical Sensing (FIBOS) for an aerospace application

Fiber Bragg grating Sensors, FBGs, have been widely used as optical sensors for structural health monitoring of different materials. They can be embedded in composite structures or attached on their surface to monitor the entire life cycle of the material or to measure different physical parameters. FIBOS contains two FBGs and will be used to measure temperature and strain during the aerospace mission OPTOS. OPTOS is a picosatellite, designed and manufactured by the Spanish Institute for Aerospace Technology, INTA that will be launched during the summer 2009. The main goal of the mission is to demonstrate the possibility of using some novel technologies for space applications inside a miniaturized space and with big restrictions in terms of mass and power consumption. The paper describes the different units that constitute the FIBOS payload: one tunable laser, two FBGs mounted onto one steel mechanical structure to monitor independently temperature and strain and the processing unit that include all the electronics to control and connect the payload with the DOT of the satellite. Calibration measurements at different temperatures inside a thermalvacuum chamber as well as FIBOS operation during the mission are also presented.

Electronic speckle pattern interferometry technique for the measurement of complex mechanical structures for aero-spatial applications

Using the electronic speckle pattern interferometry (ESPI) technique in the in-plane arrangement, the coefficient of thermal expansion (CTE) of a composite material that will be used in a passive focusing mechanism of an aerospace mission was measured. This measurement with ESPI was compared with another interferometric method (Differential Interferometer), whose principal characteristic is its high accuracy, but the measurement is only local. As a final step, the results have been used to provide feedback with the finite element analysis (FEA). Before the composite material measurements, a quality assessment of the technique was carried out measuring the CTE of Aluminum 6061-T6. Both techniques were compared with the datasheet delivered by the supplier. A review of the basic concepts was done, especially with regards to ESPI, and the considerations to predict the quality in the fringes formation were explained. Also, a review of the basic concepts for the mechanical calculation in composite materials was done. The CTE of the composite material found was 4.69X10-6 ± 3X10-6K-1. The most important advantage between ESPI and differential interferometry is that ESPI provides more information due to its intrinsic extended area, surface deformation reconstruction, in comparison with the strictly local measurement of differential interferometry

Temperature and humidity dependent performance of FBG-strain sensors embedded in carbon/epoxy composites

Fiber Bragg Grating Sensors, FBGSs, are very promising for Structural Health Monitoring, SHM, of aerospace vehicles due to their capacity to measure strain and temperature, their lightweight harnesses, their multiplexing capacities and their immunity to electromagnetic interferences, within others. They can be embedded in composite materials that are increasingly forming an important part of aerospace structures. The use of embedded FBGSs for SHM purposes is advantageous, but their response under all operative environmental conditions of an aerospace structure must be well understood for the necessary flight certification of these sensors. This paper describes the first steps ahead for a possible in future flight certification of FBGSs embedded in carbon fiber reinforced plastics, CFRP. The investigation work was focused on the validation of the dependence of the FBGSs strain sensitivity in tensile and compression load, in dry and humid condition and in a temperature range from -150°C to 120°C. The test conditions try to simulate the in service temperature and humidity range and static load condition of military aircraft. FBGSs with acrylic and with polyimide coating have been tested. The FBGSs are embedded in both, unidirectional and quasi isotropic carbon/epoxy composite material namely M21/T800 and also MTM-45-1/IM7. Conventional extensometers and strain gages have been used as reference strain sensors. The performed tests show an influence of the testing temperatures, the dry or wet specimen condition, the load direction and the coating material on the sensor strain sensitivity that should be taken into account when using these sensors.