Pearl Lee-Sullivan

Determination of Activation Energy for Glass Transition of an Epoxy Adhesive Using Dynamic Mechanical Analysis

The activation energy associated with the glass transition relaxation of an epoxy system has been determined by using the three-point bending clamp provided in the recently introduced TA Instruments DMA 2980 dynamic mechanical analyzer. A mathematical expression showing the dependency of modulus measurements on the sample properties and test conditions has also been derived. The experimental results showed that the evaluation of activation energy is affected by the heating rate and test frequency, as well as the criterion by which the glass transition temperature (Tg) is established. It has been found that the activation energy based on the loss tangent (tanδ) peak is more reliable than on the loss modulus (E2) peak, as long as the dynamic test conditions do not cause excessive thermal lags.

Nonlinear finite element analysis of stress and strain distributions across the adhesive thickness in composite single-lap joints

Abstract A geometrically nonlinear, two-dimensional (2D) finite element analysis has been performed to determine the stress and strain distributions across the adhesive bond thickness of composite single-lap joints. The results of simulations for 0.13 and 0.26 mm bond thickness are presented. Using 2-element and 6-element mesh schemes to analyze the thinner bond layer, good agreement is found with the experimental results of Tsai and Morton. Further mesh refinement using a 10-element analysis for the thicker bond has shown that both the tensile peel and shear stresses at the bond free edges change significantly across the adhesive thickness. Both stresses became increasingly higher with distance from the centerline and peak near but not along the adherend–adhesive interface. Moreover, the maximum shear and peel stresses occur near the overlap joint corner ends, suggesting that cohesive crack initiation is most likely to occur at the corners. The dependence of stress and corresponding strain distributions on bond thickness and adhesive elastic modulus are also presented. It is observed that the peak shear and peel stresses increase with the bond thickness and elastic modulus.

Finite element and experimental studies on single-lap balanced joints in tension

Adhesively bonded balanced single-lap joints have been studied using finite element analysis and verified either experimentally or theoretically. In the finite element method, a geometrically nonlinear two-dimensional method was applied using ANSYS code Version 5.3. The effects of (i) plane strain and plane stress conditions; (ii) simply supported and fully fixed boundary conditions at the adherend ends; (iii) filleted and unfilleted overlap end geometries; and (iv) two different adhesive materials on the bending moment factor, k, and the adhesive stresses have been evaluated. The finite element simulations were compared with experimental results only for the filleted joint bonded with a relatively flexible adhesive. The results for unfilleted joints bonded with flexible and rigid adhesives were compared with the analytical solution proposed by Oplinger.

Stiffness behaviour due to fracture in adhesively bonded composite-to-aluminum joints II. Experimental ☆

Abstract Experimental testing of composite-to-aluminum single-lap joints has been performed to verify the theoretical model proposed in Part I. Using specimens bonded with a flexible and a rigid adhesive, tests were conducted at room temperature and −40°C. Results show that the joint stiffness is more affected by the response of the adherends to the test temperature than by the modulus of the thin adhesive layer. It is found that the theoretical model is able to predict the joint stiffness and the rate of stiffness loss with crack growth. The best correlation is found for the rigid adhesive joint tested at −40°C in which the fracture mode is pure adhesive cracking. Since the model does not account for other failure modes such as delamination and interfacial tearing, the stiffness loss due to such failures is underestimated.

Guidelines for performing storage modulus measurements using the TA Instruments DMA 2980 three-point bend mode: I. Amplitude effects

An experimental study of the effects of oscillation amplitude on the elastic storage modulus, E′, for a rigid thermoplastic measured using three point-bending is presented. The results are part of a wider investigation on the operating conditions needed to improve repeatability and reproducibility of E′ measurements for the recently commercialized TA Instruments 2980 Dynamic Mechanical Analyzer. It has been found that sample alignment is required to achieve reliable E′ values. Based on experiments performed on polycarbonate at 1 Hz using a span-to-thickness ratio of 11, 17 and 33, respectively, the optimum amplitude for obtaining the lowest standard deviation under ambient conditions is found to be about 100 μm. It is also found that a span-to-thickness ratio of <17 is very sensitive to changes in amplitude.

Guidelines for performing storage modulus measurements using the TA instruments DMA 2980 three-point bend mode. II. Contact stresses and machine compliance

Abstract The effects of contact stresses and instrument compliance on the storage modulus, E′, measurements for a rectangular sample with span-to-thickness ratio of about 17 is presented. Using finite element analysis, it has been found that contact stress effects are negligible even when the applied force is near the maximum limit. Machine compliance, however, appears to affect the stiffness, and hence E′ values, particularly at static loads less than 4 N. An analytical model describing machine compliance effects has been developed and can be used for correcting the stiffness measurements of the geometry studied. The model has been verified for three different rigid thermoplastics.

Detection of debonding in composite-aluminum joints using gamma-ray Compton scattering

Abstract Gamma-ray Compton scattering is used for the detection of debonding in adhesively bonded composite-aluminum joints. A collimated narrow beam of monochromatic photons, generated by a 137 CS source, is directed towards the joint and scattered photons are recorded, using a detector located on the same side as the source. The energy of the scattered radiation is measured and related to the angle of scattering. The occurrence of debonding is indicated by a change in the count rate at an energy corresponding to its location. The performance of the technique is successfully demonstrated experimentally for joints of different adhesive-bonding thicknesses and for artificially induced debonds.

Delamination Initiation Around Cut Holes in Symmetric Laminates: Effects of PET Film Interleaving

Abstract The effects of PET film interleaving on delamination initiation in centrally notched (±θ/0)s composite laminates with fiber directions θ = 15°, 30°, 45° and 60° respectively have been investigated. A pair of interleaves were selectively placed at delamination prone interfaces in each laminate. The circular holes were cut in glass-epoxy laminates using high-speed drilling (HSD) and abrasive waterjet cutting (AWC) in order to compare the influence of hole surface finish with interleaving. Surface profilometry was used as a nondestructive technique for detecting delamination onset during tensile testing. It was found that although HSD interleaved holes were the roughest, the delamination onset stresses were significantly increased particularly for 30° < θ < 60°. For ±θ = 60°, however, delamination onset was unaffected by either hole surface finish or interleaving.

Simultaneous temperature and strain monitoring of composite cure using a Brillouin-scattering-based distributed fiber optic sensor

A fiber optic distributed sensing system has been used for in-situ measurement of in-plane strain and temperature variation during the curing of AS4-3501 composite. The distributed sensing system based on Brillouin scattering has a spatial resolution of 15 cm. In this paper, we present preliminary experimental results on the Brillouin frequency shift measured by an optical fiber embedded within a 16-ply composite panel during the heat-up, isothermal hold and cool-down stages of the cure process. By deducting the temperature effects, the average strain profile along the mid-plane of the composite panel at various stages of the cure process can be seen. The distributed sensor can detect the reaction advancement by measuring the cure shrinkage at the gelation and vitrification stages. Shrinkage is then correlated with the degree of cure data from Differential Scanning Calorimetry (DSC) obtained for the same cure cycle. The thermal response of the solidified composite during cooling is also profiled. Details of the data processing the Brillouin frequency shift data to obtain strain as a function of cure temperature, time and location have been explained. The measurement accuracy is discussed. The ultimate goal of this research is to detect in real time the evolution of process-induced strains within these materials.

Stress Relaxation Profiles of Molded and Extruded Thermoplastics Using Dynamic Mechanical Analysis

The stress relaxation profiles across the skin-core-skin layers of injection molded polycarbonate and extruded polyethylene sheet are presented. The profiles were obtained by progressively removing the layers followed by stress relaxation tests using a Dynamic Mechanical Analyzer (DMA). The machined layers were characterized using the elastic modulus, Eo , and the relaxation time, o. It was found that the DMA was able to distinguish the differences in viscoelastic response across the thickness of the polymer samples. The variation in behaviour was consistent with the expected morphology and molecular orientation developed due to the processing method.