Thermoelastic stress analysis (TSA) is an established active thermographic approach which uses the thermoelastic effect to correlate the temperature change that occurs as a material is subjected to elastic cyclic loading to the sum of the principal stresses on the surface of the component. Digital image correlation (DIC) tracks features on the surface of a material to establish a displacement field of a component subjected to load, which can then be used to calculate the strain field. The application of both DIC and TSA on a composite plate representative of aircraft secondary structure subject to resonant frequency loading using a portable loading device, i.e. ‘remote loading’ is described. Laboratory based loading for TSA and DIC is typically imparted using a test machine, however in the current work a vibration loading system is used which is able to excite the component of interest at resonant frequency which enables TSA and DIC to be carried out. The accuracy of the measurements made under remote loading of both of the optical techniques applied is discussed. The data are compared to extract complimentary information from the two techniques. This work forms a step towards a combined strain based non-destructive evaluation procedure able to identify and quantify the effect of defects more fully, particularly when examining component performance in service applications.
Thermoelastic Stress Analysis (TSA) has been proposed as a method of obtaining residual stresses. The results of a preliminary study demonstrated that when Al-2024 plate containing holes that were plastically deformed by cold expansion process to 2% and 4% strain the thermoelastic response in the material around the hole was different to that obtained from a plate that had not experienced any plastic cold expansion (i.e. a reference specimen). This observation provides an opportunity for obtaining residual stresses based on TSA data. In many applications a reference specimen (i.e. residual stress free specimen) may not be available for comparison, so a synthetic, digital bitmap has been proposed as an alternative. An elastic finite element model is created using commercially available software Abaqus/Standard and the resultant stress field is extracted. The simulated stress field from the model is mapped onto a grid that matches the TSA pixel data from a physical reference specimen. This stress field is then converted to a ΔT/T field that can be compared to the full-field TSA data. When the reference experimental data is subtracted from the, bitmap dataset the resultant ΔT/T field is approximately zero. Further work proposes replacing the experimental reference data with that from specimens that have undergone cold expansion with the aim of revealing the regions affected by residual stress through a departure from zero in the resultant stress field. The paper demonstrates the first steps necessary for deriving the residual stresses from a general specimen using TSA.
We have systematically measured the differential stress-optic coefficient, ΔC, and Young's modulus, E, in a number of
PMMA fibers drawn with different stress, ranging from 2 up to 27 MPa. Effect of temperature annealing on those
parameters was also investigated. ΔC was determined in transverse illumination by measuring the dependence of
birefringence on additional axial stress applied to the fiber. Our results show that ΔC in PMMA fibers has a negative sign
and ranges from -4.5 to -1.5×10-12 Pa-1 depending on the drawing stress. Increase of the drawing stress results in greater
initial fiber birefringence and lower ΔC. The dependence of ΔC and initial birefringence upon drawing stress is nonlinear
and gradually saturates for higher drawing stress. Moreover, we find that ΔC is linearly proportional to initial fiber
birefringence and that annealing the fiber has no impact on the slope of this dependence. On the other hand, no clear
dependence was observed between the fiber drawing stress and the Young's modulus of the fibers as measured using
microscopic digital image correlation with the fibers tensioned using an Instron tension tester.
Fibre Bragg gratings (FBGs) in polymer optical fibres (POFs) have been used to measure the strain in a woven textile.
FBGs in both POFs and silica optical fibres were attached to a woven textile specimen, and their performance
characterised. It was demonstrated that the POF FBGs provide improved strain transfer coefficients and reduce local
structural reinforcement compared to silica FBGs and therefore make a more suitable proposition for textile monitoring.
Thermoelastic Stress Analysis is used to investigation two damage types in composite materials, namely delamination and fibre breakage. The 'damage' is introduced into the material at the manufacturing stage using PTFE patches to model delamination and by cutting fibers to model breakage. Both Glass Fibre-Reinforced Plastic (GRP) and Carbon Fibre-Reinforced Plastic (CFRP) specimens were tested using an Instron 8800 servohydraulic test machine, and the Deltatherm TSA equipment was used to obtain full-field images of the temperature change in the cyclically loaded specimens. Results are presented to show how the damage introduced produces a change in the measured thermoelastic signal. Issues such as non-adiabatic behavior are discussed in the context of quantitative damage assessment.
Thermoelastic stress analysis is used to obtain the stress concentration factors from a variety of circular holes in cylinders. The cylinders are loaded in uniaxial tension, uniaxial compression and a combination of bending and compression. Firstly, radial holes are investigated and the results from the thermoelastic data obtained by the SPATE equipment are compared to previous experimental and numerical work. SCFs are then obtained form offset, oblique and offset-oblique holes using the Deltatherm system. The effect of hole obliquity and direction of applied load relative to the hole geometry is discussed.