Thermoelastic Measurements Research Articles

Thermo-elasticity of multi walled carbon nanotube- natural rubber/acrylonitrile butadiene rubber blends

Natural rubber nano-composites are emerging as one of the key players in the industry and research due to its novel material properties and nano functionalities. In order to create high performance natural rubber nano-composites, new multifunctional nano-fillers can be created using a various approaches due to the advances of nanotechnology. Blends of natural rubber (NR) and acrylonitrile butadiene rubber (NBR) were used to evaluate the effects of various constitutional ratios between the two substances of fixed weight of multi walled carbon nanotubes (MWCNTs). The increase in the frequency of alternating extension (fatigue deformation) for samples (B2, B3) causes an increase in the thermo-elastic temperature change. The breakdown of the structure is thought to be prevailing at high strain amplitude and consequently may account for the observed effect. The thermo-elastic measurements also involve the determination of the thermo-elastic coefficient, change in entropy, internal energy per unit length, and Gruneisen constant. All these parameters were found to be highly dependent on both fatigue parameters and NR/NBR polymer ratios in the composite. The prepared sample (50/50 phr) can be used as thermo-elastic sensors.

Study of Effective Stress Intensity Factor through the CJP Model Using Full-Field Experimental Data.

In this work, the Christopher-James-Patterson crack tip field model is used to infer and assess the effective stress intensity factor ranges measured from thermoelastic and digital image correlation data. The effective stress intensity factor range obtained via the Christopher-James-Patterson model, which provides an effective rationalization of fatigue crack growth rates, is separated into two components representing the elastic and retardation components to assess shielding phenomena on growing fatigue cracks. For this analysis, fatigue crack growth tests were performed on Compact-Tension specimens manufactured in pure grade 2 titanium for different stress ratio levels, and digital image correlation and thermoelastic measurements were made for different crack lengths. A good agreement (~2% average deviation) was found between the results obtained via thermoelastic stress analysis and digital image correlation indicating the validity of the Christopher-James-Patterson model to investigate phenomena in fracture mechanics where plasticity plays an important role. The results show the importance of considering crack-shielding effects using the Christopher-James-Patterson model beyond considering an exclusive crack closure influence.

The Study on Stress Analysis of Compound Steel-Foam-Glass Fiber Reinforced Polymer (GFRP) Structure by Lock-in Thermography

The application of thermoelastic stress analysis in compound structure is particularly complicated because of the different material components, which determines the different thermoelastic effect to be depended on the different material property and mechanical performance. This paper describes a theoretical and experimental analysis on full-filed stress distribution from thermoelastic measurements and its application to determination of stress concentration for compound Steel-Foam-GFRP structure. A finite element modeling is proposed to calculate the sum of the principal stress under the condition of dynamic cyclic load. The sum of the principal stress can be measured by means of thermal stress analysis (TSA). Lock-in thermography has been applied to measure the sum of principal stress distribution of component by its high thermal resolution. In this study, Experiments were carried out with Steel-Foam-GFRP compound structure under dynamic periodic load. The thermoelastic constant is calibrated for different component of compound structure, respectively. An artificial neural network (ANN) is proposed to identify the different component stress distribution on whole compound structure. The experimental result shows that the stress distribution of compound structure can be measured and analyzed using lock-in thermography. It is found that the stress distribution of compound structure can be evaluated with good accuracies by lock-in thermography.

Analysis of Stress in Aircraft Components Using Lock-In Thermography

This paper describes a theoretical and experimental analysis on full-filed stress distribution from thermoelastic measurements and its application to determination of stress concentration. The sum of the principle stress can be measured by Thermal Stress Analysis (TSA). Lock-in Thermography is very effective tool to measure the structure stress distribution by its high thermal resolving. In this study, the thermoelastic effect theory is described and the relationship between the temperature and the applied stress is developed in an elastic material. Experiments were carried out with 2A12 aluminium alloys plate and ones with hole structure under cyclic load. The thermoelastic effect coefficient is obtained for 2A12 aluminium alloys materials, and the effect law is analyzed that the stress value measured was affected by load frequencies. The optional load frequency is obtained, and that is, the load frequency is selected greater than 3.5Hz for 2Al12 materilas, and it was found that the structure stress can be evaluated with good accuracies by the lock in thermography. The experiment was carried out for aircraft components stress distribution measurement and structure stress analysis. The experimental results show the stress concentration position is easy found from stress distribution by lock-in thermography.

Research on Structure Stress Analysis on Composite by Lock-In Thermography

The application of thermoelastic stress analysis in composite materials is particularly complicated because of the anisotropy of the material, which determines the thermoelastic effect to be depended on the material property and mechanical performance. This paper describes a theoretical and experimental analysis on full-filed stress distribution from thermoelastic measurements and its application to determination of stress concentration. The sum of the principal stress can be measured by Thermal Stress Analysis (TSA). Lock-in Thermography has been applied to measure the sum of principal stress distribution of component structure by its high thermal resolving. In this study, Experiments were carried out with GFRP composite ply and foam materials under cyclic load. The thermoelastic constant is obtained for GFRP and foam composite materials. The stress concentration is analyzed for a specimen with a hole. The experimental results show the stress distribution can be measured and analyzed using Lock-in thermography. It is found that the composite material structure stress can be evaluated with good accuracies by lock in thermography.

Research on High Strength Aluminum Alloys Components Stress Analysis by Lock-In Thermography

This paper describes a theoretical and experimental analysis on full-filed stress distribution from thermoelastic measurements and its application to determination of stress concentration. The sum of the principal stress can be measured by Thermal Stress Analysis (TSA). Lock-in Thermography has been applied to measure the sum of principal stress distribution of component structure by its high thermal resolving. In this study, Finite element method is used to calculate the sum of principal stress distribution, and the thermoelastic effect model is developed to study the relationship between the temperature deviation and the applied stress in an elastic material. Experiments were carried out with ANSI 7071 high strength aluminum alloys ply and ones with a crack under cyclic load. The thermoelastic constant is obtained for ANSI 7071 high strength aluminum alloys materials. The stress concentration factor is calculated for a ply with modeling crack under the condition of different loads. The experiment was carried out with high strength aluminum alloys component structure with rivet joints. The experimental results show the stress distribution can be measured and analyzed the contact stress distribution between ply and rivet by using Lock-in thermography. It was found that the structure stress can be evaluated with good accuracies by the lock in thermography.

Bent-core liquid crystal elastomers

Liquid crystal (LC) elastomers with bent-core side-groups incorporate the properties of bent-core liquid crystals in a flexible and self-supporting polymer network. Bent-core liquid crystal elastomers (BCEs) with uniform alignment were prepared by attaching a reactive bent-core LC to poly(hydrogenmethylsiloxane) and crosslinking with a divinyl crosslinker. Phase behavior studies indicate a nematic phase over a wide temperature range that approaches room temperature, and thermoelastic measurements show that these BCEs can reversibly change their length by more than a factor of two upon heating and cooling. Small-angle X-ray scattering studies reveal multiple, broad low-angle peaks consistent with short-range smectic C order of the bent-core side groups. A comparison of these patterns with predictions of a Landau model for short-range smectic C order shows that the length scale for smectic ordering in BCEs is similar to that seen in pure bent-core LCs. The combination of rubber elasticity and smectic ordering of the bent-core side groups suggests that BCEs may be promising materials for sensing, actuating, and other advanced applications.

The role of proline in the elastic mechanism of hydrated spider silks

This study used thermoelastic measurements to investigate the role of proline in the elastic mechanism of hydrated, spider major ampullate (MA) and flagelliform (FL) silks. Experiments on hydrated MA silk from Araneus diadematus (proline content 16%) reveal that conformational entropy elasticity accounts for about 90% of the elastic force at small extensions, but entropy elasticity drops to about half by 50% extension. The decrease in the entropic component with extension is due to the presence of relatively short and conformationally restricted network chains in Araneus MA silk. Experiments on hydrated Araneus FL silk (proline content 16%) indicate that entropy elasticity dominates the elastic mechanism up to extensions of 100% and beyond, which likely reflects the fact that the glycine-rich network chains in FL silk are longer and less conformationally restricted than those in the MA silk. Thus, the rubber-like, entropic elasticity of these two proline-rich silks is consistent with networks of amorphous chains that become mobile when hydrated. By contrast, the elastic mechanism of hydrated Nephila clavipes MA silk (proline content 3.5%) shows a small contribution from entropic elasticity for extensions of 5% or less, and by 10% extension the elastic force is due entirely to bond-energy elasticity, probably associated with the deformation of stable secondary structures. These results indicate that there are major differences in the structural organization of the glycine-rich network chains and the mechanism of elasticity in proline-rich and proline-deficient fibroins.

Detection and monitoring of fatigue damage in SMCR26 specimens using spate, for a constant mean stress – a qualitative approach

The primary aim of the work presented here was to establish the use of thermoelastic measurements in detecting the presence of damage and its subsequent accumulation in sheet moulding compound (SMCR26). To this end, several dog bone shaped specimens of SMCR26 were cycled at a constant mean stress of 38.87 MPa and at varying stress amplitudes, the latter in order to investigate any dependencies on stress amplitude. Surface scans were taken from each specimen at various stages in their load history using the Stress Pattern Analysis by Thermal Emission (SPATE) measurement equipment. The first scan obtained from each specimen was used to determine its initial state of damage, while subsequent scans were used to monitor the accumulation of damage. The thermoelastic signal in the initial and subsequent scans exhibited different magnitudes and distribution for the different specimens, which denoted heterogeneity of the SMCR26 specimens and of the occurrence and accumulation of damage. This was in sharp contrast to the uniform thermoelastic signal that is emitted from the surfaces of cycled, undamaged homogeneous materials. The absence of any apparent relationship between the stress amplitude and the accumulation of damage in the tested specimens was thought to be a result of the in-homogeneity inherent in SMCR26, and of the heterogeneous nature of the accumulation of damage, rather than a limitation of the thermoelastic equipment used. The results obtained in this work demonstrated SPATE to be a fast and useful means of detecting the presence of damage and of monitoring its accumulate Keywords : Sheet Moulding Compound (SMC), fatigue, damage Journal of Agriculture, Science and Technology Vol. 7(1) 2005: 91-121

Liquid Crystalline Dendritic Networks Derived from Poly(propylene imine) (PPI) Dendrimers

AbstractSummary: A nematic liquid crystalline network based on a poly(propylene imine) dendrimer [PPI‐(NH2)32 (G = 4)] was prepared in the form of films. In the network the PPI dendritic molecules are partially functionalised with a chiral aldehyde and are connected to each other by an achiral dialdehyde. The phase behaviour of the network was investigated by polarising‐light optical microscopy, differential scanning calorimetry, and X‐ray diffractometry. Thermoelastic measurements ruled out the possible elastomeric nature of the material.Schematic of the nematic dendritic network prepared here.imageSchematic of the nematic dendritic network prepared here.

Interfacial interactions-controlled thermoelastic behaviour of synthetic rubber/organoclay nanocomposites

Abstract Thermoelastic measurements of synthetic rubber/organoclay nanocomposites provided the first experimental evidence for the dramatic dependence on interfacial interactions of the scale of nanoparticle displacements within filler aggregates of synthetic rubber/organoclay nanocomposites at high uniaxial extensions.

Thermoelasticity of Polymer Networks

The thermoelastic properties of polymer networks are of interest owing to the information they provide on κ = d ln〈R2〉0/dT, the temperature coefficient of chain dimensions of the network species. Stress−strain measurements at constant volume provide that information directly, through a determination of the energetic fraction of the restoring force fe/f. Almost always, however, the measurements are conducted under constant-pressure conditions, and a correction is applied to account for the effect of volume change. The correction formula commonly used is known to be inaccurate, and here we present a new one that predicts the volume change and κ quite satisfactorily. Some comparisons of κ obtained from thermoelastic measurements, from small-angle neutron scattering, and from intrinsic viscosity data in athermal solvents are given.

FINITE ELEMENT DYNAMIC MODEL UPDATING USING MODAL THERMOELASTIC FIELDS

In this paper, the use of thermoelastic measurements to improve a finite element model is investigated. The originality of the procedure lies in the use of this stress sum field measurement and in the new solution method of the modelling error location stage. Measurement inaccuracies and expansion errors are taken into account through an inequality constraint. Finally, the correction stage is done owing to a variable metric Gauss–Newton method. This updating process has been applied to modal thermoelastic measurements carried out on a thin plate bending with different kinds of defects

Polymer coating as a strain witness in thermoelasticity

The work described in this paper offers the possibility of using a polymer coating as a strain witness in thermoelasticity. In particular, the efficacy of a polymer coating for making thermoelastic measurements is investigated by experiment and the supporting theory is presented. It was found that the thermoelastic response is greatest with thick coatings at high frequencies. However, thicknesses of more than 0.5 mm and frequencies greater than 5 Hz provide adequate results.

Orientation behavior of smectic polymer networks by uniaxial mechanical fields

AbstractPolymer networks showing different smectic polymorphism with either an interdigitated smectic A to undulated smectic A phase transformation or an interdigitated smectic A to double layer smectic C phase transformation are synthesized. Their orientation behavior in uniaxial mechanical fields is investigated by means of X‐ray scattering techniques and thermal expansion measurements. By applying uniaxial extension to the smectic elastomers in the isotropic state ordered smectic networks result with the director uniformly aligned parallel to the strain axis. These networks are optically transparent. Uniaxial deformations of the ordered smectic networks in the direction parallel and perpendicular to the director are performed that also help to elucidate the structure of the smectic phases. We find a reorientation process of layers of the ordered smectic C network. The spontaneous elongation in the direction of the director at the isotropic to smectic A phase transformation of the smectic elastomers is observed by thermoelastic measurements. This indicates that the polymer backbone conformation in the smectic elastomers is prolate in contrast to previous observations of oblate chain conformation in linear smectic polymers.

A SANS-Based Evaluation of the Chain Dimension Temperature Dependence of Poly(ethylethylene) under .THETA.-Conditions

The unperturbed chain dimensions of atactic near-monodisperse poly(ethylethylene) were evaluated in different Θ-solvents over the temperature range 10-80 °C. Radii of gyration, measured by small angle neutron scattering (SANS), were found to increase with increasing temperature and led to a value of 0.4 x 10 -3 deg -1 for d In (R 2 ) 0 /dT ; behavior which replicates that seen in the melt state via SANS and thermoelastic measurements. Those findings are contrary to those from intrinsic viscosity measurements.

Determining reliable edge isopachic data from interior thermoelastic measurements

The maximum stresses in a plane-stressed component typically occur on the boundary. However, it is generally difficult to obtain reliable experimental data at an edge and thermoelastic stress analysis is no exception. The inability to measure reliable edge isopachic stresses has caused many previous thermoelastic stress analyses to be more qualitative than quantitative. This paper develops and implements an effective iterative least-squares method for calculating reliable edge isopachic stresses from measured interior values. The method is based upon the plane-stress isotropic compatibility equation. A regularization scheme is employed to minimize the sensitivity to measurement error and to improve the stability of the algorithm by controlling the rate of convergence. An illustrative example with actual measured thermoelastic data is included. The processes thermoelastically determined results compare well with those obtained using strain gages.

Temperature coefficients for the chain dimensions of polystyrene and polymethylmethacrylate

We have made precise measurements of the unperturbed radius of gyration 〈S2〉1/20 of polystyrene and polymethylmethacrylate molecules in the melt over the temperature range 120 °C–240 °C by small-angle neutron scattering. Contrary to most previous reports, based on dilute solution measurements, we find that neither polymer exhibits a significant change in dimensions with temperature, and we deduce temperature coefficients, κ=d ln〈S2〉0/dT, of (0.0±0.1)×10−3 K−1 for polystyrene and (0.1±0.1)×10−3 K−1 for polymethylmethacrylate. These κ values are close to those given by thermoelastic measurements, but are difficult to reconcile with predictions obtained from the rotational isomeric state model.

Adiabatic thermoelastic measurements : Bakis, C.E.; Reifsnider, K.L. Manual on Experimental Methods for Mechanical Testing of Composites. Edited by R.L. Pendleton and M.E. Tuttle. pp. 139–145. Elsevier Applied Science, 169pp. (1990) ISBN 1 85166 3754

Adiabatic thermoelastic measurements : Bakis, C.E.; Reifsnider, K.L. Manual on Experimental Methods for Mechanical Testing of Composites. Edited by R.L. Pendleton and M.E. Tuttle. pp. 139–145. Elsevier Applied Science, 169pp. (1990) ISBN 1 85166 3754

Analysis of Thermal Conduction Effects on Thermoelastic Temperature Measurements for Composite Materials

Measurement of the temperature changes which occur as a body undergoes a change in stress is becoming a widely used technique for the analysis of surface stress fields. In this paper, an investigation into the effects of thermal conduction on surface thermoelastic temperature changes for composite materials is reported. A mathematical model which shows the effects of thermal conduction is developed, and the results from this model are compared with experimental data. The mathematical model is then extended to solve for heat transfer between two thermally dissimilar materials. It is shown how this model can be used to account for the effects of a surface epoxy layer on the observed thermoelastic temperature changes.