Papers by Keyword: Thermography

Abstract: It is well known that the work hardening process of low-carbon steels is highly dependent on the movement and accumulation of dislocations in the crystal grains, which affect the stress and strain magnitudes and their distribution. The aim of this paper is to explain the importance of dislocation movement and density on the temperature, i.e. stress and strain changes during cold plastic deformation of low-carbon steels. Therefore, tests were carried out in this paper using the methods of static tensile testing, thermography, digital image correlation (DIC) and microstructural analysis. The microstructure analysis was carried out using a light and transmission electron microscope (TEM). The transmission electron microscope analysis was performed in two different modes, the TEM and scanning TEM (STEM). The results of static tensile testing, thermography and digital image correlation (DIC) are related to the microstructural changes that occur during the work hardening process of low-carbon steel. At the moment of maximum work hardening (immediately before fracture), significant grain elongation and high dislocation density of low-carbon steel were observed.

Abstract: Nowadays new applications based on the 3D printing technique demand increasingly strict product quality requirements. The in-situ monitoring of variables associated with the manufacturing process through the application of different techniques could help to evaluate the process and ultimately to ensure product quality. In this regard, the acquisition and evaluation of variables and indexes derived from thermographic analysis during the process are key for an early defect detection and can contribute to quality estimation. In this work, a new methodology is proposed for the monitoring and analysis of the additive manufacturing process based on the processing of thermographic images from an LWIR (Long Wave Infrared) camera. The methodology and the suitability of the variables and indexes extracted during the monitoring of the manufacturing process are discussed for the case of a 3D fused filament fabrication of polymers.

Abstract: A Haar cascade classifier is a machine learning (ML) algorithm used for object detection. In this paper, the Haar algorithm is introduced in the context of a non-destructive evaluation of fibrereinforced composite (FRC) structures. The Haar learning model is used for flaw identification from thermal images. Thermal images are created from cross-ply (CP) carbon fibre-reinforced laminates with flat-bottomed holes (6–10 mm) of different depths from the surface (0.5–1.5 mm). After training is complete, the model successfully detects similar artificial flaws in previously unseen thermal images. In doing so, the feasibility of Haar classifiers for automatic evaluation of FRCs is established.

Abstract: For structural health of mechanical structures, non-destructive detection and material defect characterization represent the main useful tools for mechanical decay prediction caused by local composite damage phenomena. In this work, internal delamination due to alternate bending were characterized in flat specimens, performing fatigue and static tests, coupled with thermographic, optical, and ultrasonic analysis for damage detection and evolution purposes. Damage to rupture behavior of CFRP material through mechanical tensile tests is performed on several samples and non-destructive inspection procedures are optimized during successive HCF tests to detect in real time local compliance variations and damage initiation. Thermographic continuous monitoring and occasional ultrasonic analysis are implemented to analyze composite anomalies during fatigue life and to elaborate a procedure for identification of delamination induced damage before failure. IRT and UT results are computed with MATLAB analysis for damage evaluation with strain and compliance data acquired during tests.

Abstract: The availability of reliable fatigue data is of continuous and often urgent need. The paper to be presented therefore intends to show how the potential of non-destructive testing methods, digitisation in metrology as well as signal processing can be combined in order to achieve a significant gain in information concerning the fatigue behaviour combined with a reduction of required experimental effort and cost. The new SteBLife approach is an enhanced short-time calculation method developed at the Chair of Non-Destructive Testing and Quality Assurance at Saarland University, which takes into account that a material’s elastic-plastic reaction and hence relationship is non-linear. With respect to a test strategy, the number of fatigue experiments required to determine a material’s complete S-N-curve can be limited to three to five tests only (SteBLife_mtc, mtc: multiple tests, trend curve and SteBLife_msb, msb: multiple tests, scatter bands) in cases that mean values and/or complete scatter bands of S-N-curves are required. If a trend S-N-curve is sufficient, the effort can be reduced to one single test only (SteBLife_stc, stc: single test, trend curve) with a special step-shaped specimen. This leads to a significant improvement in efficiency when compared to the conventional way an S-N-curve is determined where a minimum of 15 fatigue tests is required. Within the work to be presented the SteBLife method is demonstrated for normalized SAE 1045 (C45E) steel.

Abstract: We developed a wireless communication holder system to monitor the process temperature and vibration of a rotating machining tool. This report presents an estimation of the process temperature in end-milling and the chatter vibration when boring with it. The thermocouple is set in the end-mill tool to measure the process temperature, and the MEMS accelerometer is set in the boring tool to measure the chatter vibration. We also present an investigation of the end-mill temperature observed using infrared thermography and the vibration of the spindle using a traditional accelerometer. The proposed holder system was found to be effective to estimate the process monitoring of the rotating machining tools.

Abstract: Infrared thermographic technology has attracted the attention of various industrial fields. We therefore focus on it as a novel tool temperature monitoring method to improve the end-milling conditions of difficult-to-cut materials. However, it is difficult to measure tool temperature under coolant conditions because coolant prevents the monitoring of end-mill tool surfaces. Thus, we developed a novel temperature monitoring method by applying a pore electric discharge machining process to place a thermocouple sensor in the end-mill tool and by using a wireless telegraphic multifunctional tool holder to directly measure the internal temperature of the end-mill tool under rotation. We evaluated internal temperature distribution under various end-milling conditions on the basis of multipoint simultaneous measurements and discussed the influence of forced convection heat transfer on the internal temperature of the tool under various rotating conditions. In addition, we used a finite element method to analyze unsteady heat conduction on the basis of temperature measurement by high-speed video infrared thermography and compared the results with those of the wireless telegraphic multifunctional tool holder to demonstrate its validity and usefulness.

Abstract: This paper presents the results of experimental and theoretical studies justifying the possibility of using infrared (IR) thermography for detection of deformation debonding of composite material from a reinforced concrete structure that occurs under operating conditions and develops according to the cohesion scenario. The analysis of the results allowed us to determine the optimal inspection parameters of IR thermography to assure best registration of the presence of fiber composite material debonding from the surface of a concrete structure. It has been found that the most accurate and timely information about debonding in a carbon fiber sheet/epoxy/concrete/delamination/concrete system can be obtained during the cooling stage after pulse heating of the structure surface, since at this stage the magnitude of thermal response to debonding reaches its maximum.

Abstract: Friction stir welding (FSW) is an alternative method of joining materials with low melting point, patented in 1991 by the American Welding Institute (UK). This method uses the heat generated by mechanical friction between two moving parts, one is the tool rotates and is fastened on the spindle of a conventional milling machine and the other is the part that is attached and is gagged on the bed of the machine. Among the variables identified as the most important for a successful run of the process are the revolutions per minute (RPM) at which the tool rotates, the speed advance at which the workpiece and the tool design as such moves [1]. In this paper the design of several tools applied to FSW process is studied in specific dissipation of heat generated by mechanical friction between the parts, its relation to tool design and qualities of successful meetings is presented, the methodology to achieve this goal was first identify the possible and applicable materials for the tools, second his respective designs to ensure the right function for operation, and finally define FSW technical parameters (RPM, Head angle, speed advance) for experimental tests. The findings and conclusions attribute a novel analysis in the design of tools for this innovative manufacturing process, in the analysis of the results obtained for each of the assemblies experimentation it was discovered that the use of rings at the parts are not decisive for a good weld even heat dispersion is not good.

Abstract: During technical operation, high performance materials are partially exposed to high frequency cyclic loading conditions. Furthermore, the small strains in the very high cycle fatigue (VHCF)-regime lead to accumulative damage which causes crack initiation related to an appropriate local deformation leading to final fatal fracture. At the same time, quite high requirements with regard to high number of cycles without any damage are demanded for many applications. Fields of application of these light-weight, but expensive materials, are e.g. in the automobile industry (e.g. engine blocks, cylinder heads, brakes).The fatigue behavior of Al-matrix composites (Al-MMCs) reinforced by alumina particles (15 vol.% Al2O3) or short fibers (20 vol.% Saffil), respectively, was already intensively studied in the LCF and HCF range. The present study is focusing on investigations in the very high cycle fatigue regime at stress amplitudes up to 140 MPa to reach fatigue life of about 1010 cycles. All experiments were carried out using an ultrasonic fatigue testing device under symmetric loading conditions (R=-1). Fatigue tests were accompanied by in situ thermography measurements to record the temperature of the whole specimen and to find “hot spots” indicating changes in microstructure and therefore the initiation or growth of cracks. Moreover, the resonant frequency as well as the damage parameter were evaluated to determine the beginning of damage. For a better understanding of the damage mechanism (matrix decohesion, matrix failure or failure of reinforcement) all fractured surfaces were investigated by scanning electron microscopy. The combination of these methods contributes to a better understanding of the underlying mechanism of damage in aluminum-matrix-composites.