Stress Pattern Analysis By Thermal Emission Research Articles

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

Stress Pattern Analysis by Thermal Emission of Plain Weave Carbon Fiber/Epoxy Resin Composite Iosipescu Specimens with Three Different Notch Angles

Abstract Iosipescu specimens were developed to examine the shear properties of isotropic materials. In the past decade, there has been much effort made to analyze this specimen configuration for measuring anisotropic materials, particularly laminated and woven composites. Some authors have used notch angles ranging from 90 to 110° in these materials and other authors have performed finite element analysis (FEA) to determine the notch angle and depth with the most uniform shear stress zone based on the elastic moduli. The object of this study was to verify the preferred angle of woven fiber composites through the use of stress pattern analysis by thermal emission (SPATE), which is a technique that can image the hydrostatic stress state in a material under load. SPATE images were collected for various specimens at different notch angles (90, 100, and 110°) and different weave lay ups. Mean load was held constant while the SPATE image was collected but later increased to examine the stress state of the Iosipescu specimens at high loads. The results indicate that for a plain woven carbon fiber/epoxy resin composite, the 100° notched specimen is the best specimen of the three conditions examined due to the delay in occurrence of damage and the large size of the pure shear zone.

Experimental exploration to thermal infrared imaging for detecting the transient process of solid impact

Based on the analysis and the comparison of stress pattern analysis by thermal emission (SPATE) and remote sensing rock mechanics (RSRM), the idea to detect the transient process of solid impact with thermal infrared (TIR) imaging technology is introduced. By means of TVS-8100MKII TIR imaging system, which has high recording speed, high space distinguishability and high temperature sensibility, TIR imaging experiments on free falling steel ball impacting on marble, granite, concrete, steel, organic-glass and wood plate are conducted. It was discovered that: (i) the target’s TIR temperature increases remarkably after impact; when (ii) if ball’s size is not changed, the variation amplitude of target’s TIR temperature proportionally increases with the ball’s potential energy or falling height; (iii) the variation amplitude of target’s TIR temperature is involved with the material type and the surface glabrous condition of the target, and the amplitudes are in order as concrete, unpolished marble, steel plate, wood plate, polished granite, polished marble and organic-glass plate; and (iv) the TIR radiation of fragile targets decreases gradually after impact, while there is delayed TIR radiation strengthening for plastic target. It is deduced that once the relational functions and technical parameters, which are involved with certain impact body and target material, are set up through experimental study, the remote detection and back analysis based on TIR imaging for the transient process of solid impact will be no problem. Besides, there is also important scientific meaning for the omen mechanics study and satellite TIR detection and prediction for structural earthquake.

ON DETERMINING STRESS INTENSITY FACTORS FOR MIXED MODE CRACKS FROM THERMOELASTIC DATA

Abstract— An alternative methodology is presented for determining stress intensity factors for cracks subject to mixed‐mode displacements. The methodology involves thermoelastic data generated from a SPATE (Stress Pattern Analysis by Thermal Emission) system and has been adapted from one used successfully in photoelasticity. The thermoelastic data is collected throughout the elastic stress field dominated by the crack tip singularity. The stress field is described using a Fourier series within Muskhelishvili's approach. This method allows different applied stress fields to be described which may include transient or non‐uniform stress fields. The results obtained using the new methodology are at least as good as those obtained previously for pure mode I cases, and generally better for mixed mode displacement conditions.

Measurement of Stress Intensity Factor of Orthotropic Material Using SPATE

SPATE(Stress Pattern Analysis by Thermal Emission) can be effectively used to analyze the stress distribution of the orthotropic structure under the repeated load by non-contact. In this research, the measuring conception and method of stress intensity factor of orthotropic material using SPATE are suggested. The relationships between the maximum values of SPATE signal and (or ) are theoretically established in the vicinity of crack tip of the orthotropic material. It is certified through SPATE experiment that their linear quality is very excellent.

The role of strain measurement technology in structural evaluation

AbstractThis paper describes the importance of structural evaluation to the process of design assessment and outlines the significant role played by strain measurement technology in extending the scope of strain history data collection and facilitating strain monitoring of full‐scale structures in the laboratory. After some general discussion of the various models of structural behaviour, it then proceeds to give a brief historical account and outlines the principles of thermoelastic stress analysis (TSA). A general desciption is provided of a specific TSA technique namely Stress Pattern Analysis by Thermal Emission (SPATE).Despite the level of innovation and development in strain measurement technology, the use of bonded strain gauge elements is still by far the most commonly used strain measurement and monitoring technique in structural evaluation work at NEL. The paper highlights several recent test programmes carried out at NEL in the form of case studies which demonstrate different aspects of measuring, monitoring and validation of this and other strain measurement techniques; and verification of analytical models.

Calibration of the thermoelastic stress analysis technique under sinusoidal and random loading conditions

For the past five years the National Engineering Laboratory (NEL) has been engaged in a research programme investigating the applications of thermoelastic stress analysis to engineering structures. This paper discusses the use of a uniformly stressed component for the calibration of SPATE (Stress Pattern Analysis by Thermal Emission). Experimental results for sinusoidal loading on a carbon steel are compared with a theoretical prediction based upon material properties and instrument characteristics. A calibration technique which is suitable for the measurement of quantitative stresses under random loading, including modal behaviour, is also described.

Appraisal Of A New Infrared-Based Stress Analysis Technique

A new and versatile experimental stress analysis technique has recently been developed, based on the measurement of the infrared radiation emitted from the surface of a body as a result of temperature changes caused by cyclic loading within the elastic range ( i.e., the thermoelastic effect). The theory is outlined and the apparatus [referred to as the SPATE (stress pattern analysis by thermal emission) equipment] is described. A number of applications are described to illustrate the principal characteristics of the technique, which is now regarded as a valuable new approach in the stress analysis and design assessment of a wide range of engineering components and structures. Possible future developments in this area are outlined.