Auto-calibration principles for two-dimensional residual stress measurements by Barkhausen noise technique

The magnetic Barkhausen Noise (BN) is well suited to evaluate the effects of mechanical stresses of ferromagnetic materials, e.g. the indirect detection of residual stress states. The most common causes for the occurrence of residual stresses are manufacturing processes, such as casting, welding, machining, forming, heat treatment, etc., consecutive repairs and design changes, and installation or assembly and overloads during the operating life of a construction. A significant calibration effort based on a set of reference values and/or test samples is needed for these measurements, which require a great deal of time and material resources. Additionally, it is impossible to determine the stress states of different components (σ xx and σ yy ) at the surface. Therefore, a new auto-calibration method was developed to analyze two-dimensional stresses. A fixed calibration function based on defined parameters (determined experimentally) was applied. To adjust the auto-calibration function to the experimental reference values by varying functional parameters, a large number of measurement points were used. We present a method that can calculate, based on the multi-dimensional stress state at the measuring point, the stress components σ xx and σ yy for two perpendicular magnetization directions using the Barkhausen Noise effect

Verfahren zur Bestimmung von mechanischen Spannungen in Substraten oder Schaltungsträgern, die mit einem keramischen Werkstoff gebildet sind

Zur Bestimmung von mechanischen Spannungen in Substraten oder Schaltungsträgern (3), die mit einem keramischen Werkstoff gebildet sind, wird ein Laserstrahl auf einen Oberflächenbereich eines zu untersuchenden Substrats gerichtet und mit einem zur ortsaufgelösten Erfassung ausgebildeten Detektor(1a) von der bestrahlten Oberfläche reflektierte und gestreute elektromagnetische Strahlung in Form eines Speckle-Musters erfasst. Die Erfassung von Speckle-Mustern erfolgt zu einer ersten und mindestens einem zweiten nachfolgenden Zeit. Eine statische Erfassung einer Abbildung eines erfassten Speckle-Musters zumindest eines Bereiches, der mit elektromagnetischer Strahlung beleuchtet wird, wird auf seine Eigenschaften überprüft und dabei charakterisiert sowie parametrisiert, wenn die Auswertung mit einer Korrelationsfunktion mit einer Grauwertübertragungsmatrix wird. Die mit dem optischen Detektor (1) erfassten Daten werden einer elektronischen Auswerteeinheit zugeführt und in der elektronischen Auswerteeinheit wird ein Vergleich mit in einem elektronischen Speicher gespeicherten durch eine Kalibrierung erhaltenen Speckle-Mustern durchgeführt und mit dem Vergleich die jeweilige mechanische Spannung des keramischen Werkstoffs des Substrats bestimmt

Current Induced Barkhausen Noise Technique: Poster presented at 13th International Conference on Barkhausen Noise and Micromagnetic Testing - ICBM 2019, Prague, September 23-26, 2019

Today Barkhausen Noise (BN) measurement is an efficient nondestructive technique for materials characterization. Usually the noise is detected by a coil while the magnetic hysteresis of the test material is cycled by an electromagnet. Due to magnetostrictive effects BN is highly sensitive to materials stresses and can be used for quantitative determination of residual stresses and external mechanical loads. The interaction of the magnetic structure (magnetic Bloch walls) with the microstructure of the material results in BN signals that are very sensitive to microstructure variations. Quantification of plastic deformation, of hardness, tensile strength, yield strength, and hardness depth are typical applications. Other innovative techniques of creating BN by electric current magnetization (Eddy field technique) (see Figure 1) opens new areas of applications for Electromagnetic Non Destructive Evaluation. When an alternating electric current is used to excite a sample by electric current noise can be detected within the noise related to the Barkhausen Effect and is sensitive to re-magnetization processes. The sensitivity of the effect to residual and applied stresses can be used to develop this technique as a principle for stress sensor technology [1]. The current BN sensors are very large and not optimal application surrounding and must be specially adapted for each kinds of different component geometries. These facts are very cost and time consuming and discourages many potential users. The Eddy field BN sensors are more compact than conventional sensors and can work in the sophisticated small samples geometries. The paper presents an idea of development of Eddy field BN sensor as well as its realization in a first prototype system. The Figure 2 shows the probe for Current Induced Barkhausen Noise Technique. Additionally the paper will shows the results of the application of this novel sensor on samples with different hardness and stress conditions

Laser Speckle Photometry (LSP) - optical sensor system for monitoring of material condition and processing

The Fraunhofer IKTS at Dresden-Klotzsche develops innovative methods and testing systems for characterization of material properties and system components, product quality control and monitoring of manufacturing processes. The IKTS own the extensive know-how of laser-based measurement and testing methods, such as Raman, Fluorescence, Interferometry, Ellipsometry and the Laser Speckle Photometry (LSP). LSP was developed by the Fraunhofer-Gesellschaft completely and it has been successful tested in laboratory. Currently this optical method will be transferred to industrial applications. The Laser Speckle Photometry is a noncontact nondestructive testing method which bases on detection and analysis of thermal, mechanical or manufacturing process activated characteristic speckle dynamics. The determination of the material conditions state bases on understanding of structural changes on different scaling levels during the excitation of the frequency analysis of time resolved speckle fluctuation. The LSP determined rapid relevant material quality characteristics and detect defects. It allowing a complete online monitoring of production processes. In this work, the principle and first results for several materials, objects and applications will be shown. In addition we perform the LSP to obtain the speckle time response of ceramic material for microcrack detection. Even, it will be present the results of the development of a sensing system for determination of gold content of contacts manufactured by overlaying welding with the lateral dimensions <100 microns

Laser Speckle Photometry for Stress Measuring at Industrial Components

Excessive deformations, cracks or failures occur in industrial practice when components are overloaded. It is therefore desirable to continuously monitor the mechanical stress state of industrial components. Due to their high thermal conductivity, direct copper bonded (DCB) substrates are used in power electronics to meet the high reliability requirements. These DCBs usually carry residual stresses, especially at the edges of the copper layers. The stress concentration can increase under operating conditions or even during the manufacturing process, resulting in cracks and fractures. In order to avoid the critical cracking situation, it is necessary to know the residual stresses. The article presents a possible approach to the measurement of stress states. A new optical method, the Laser Speckle Photometry (LSP), was used in the laboratory on ceramics exposed to bending stress. Laser Speckle photometry is a fast and contactless method for the measurement of the spatial-temporal dynamics of speckle fields with high temporal resolution after mechanical or thermal excitation. During the experiment laser light illuminates a defined surface region. Due to the optical rough surface of the sample, a so-called speckle pattern is reflected and recorded by a CMOS camera system. The speckle pattern depends on the sample surface condition and the mechanical strain condition. The stress-induced changes in the materials structure lead to changes in the speckle field, which is formed by a probing laser. The shift of speckle-field is analysed by statistical methods, using co-occurrence matrix and correlation functions. Correlations between stress condition and measurement signal were observed and evaluated. This resulting measurement signal was then calibrated using mechanical stresses determined using finite element (FE) simulations. The authors present the transferability of previous local stress measurements on Al2O3 ceramics to LTCC ceramics. It is shown, that laser speckle photometry is a suitable instrument for non-destructive characterization and monitoring of stress states in ceramics

Stress Characterization of Ceramic Substrates by Laser Speckle Photometry

Direct copper bonding (DCB) substrate consisting of an Al2O3-ceramic layer in between two thick copper layers is one of the most powerful ceramic substrates for electronic applications. During the manufacturing process or under the service loading, high thermal or mechanical stress would be occurred. Extreme stress concentration might increase and in consequence copper structure rip off the ceramic substrates by cracking and conchoidal fractures. To avoid the crack initiation and guarantee a good quality state of ceramic substrates, the residual stress condition should be monitored. In this paper, an optical non-destructive testing method – Laser Speckle Photometry (LSP) will be demonstrated to determine mechanical stresses in ceramic substrates. The static speckle pattern is generated on the sample surface by illumination of a laser source, and sample is loaded by a 3-point-bending device to introduce surface strain, which activates a time resolved speckle signal. By recording and analyzing the quasi static speckle pattern, the speckle signal can be related to applied external mechanical signal. The resulting measurement signal was calibrated by stresses, calculated via a finite-element-model FEM. The potential of LSP for non-destructive characterization and monitoring of stress condition will be shown in details

A laser speckle photometry based non-destructive method for measuring stress conditions in direct-copper-bonded ceramics for power electronic application

1. BACKGROUND Power electronic application come along with very high requirements in terms of reliability. Thermal loadings stress whole electronic packages while powering the semiconductor and by environmental temperature changes. Direct copper bonding (DCB) substrate are widely used in such electronic application due to its high thermal conductivity performance. They consists of copper layers sintered onto alumina ceramic sheet. Because of the very high process temperature thermal inducted stresses arise along the substrate while cooling to ambient temperatures. The DCB substrates carry residual stresses usually. Along the edges of copper structures the residual stresses are concentrated. Under service condition or even during the electronic manufacturing process the stress concentration might increase and in consequence copper structure rip off the substrate by cracking and conchoidal fractures in the ceramic sheet. To avoid the critical cracking situation the knowledge about the residual stress condition is required. The target will be a stress mapping across the DCB substrate. Within the paper on potential approach to measure the stress condition will be presented. / 2. METHODOLOGY The presented work demonstrate a laser-speckle-photometry (LSP) based method to determine mechanical stresses in a non-destructive testing approach. For this purpose, Al2O3-ceramic samples were systematically loaded by a 3-point-bending experiment in a dynamic mechanical analyser to induce certain stress condition. The mechanical test procedure was simultaneously observed by the LSP measurements setup. During the experiment a defined surface on the alumina ceramic region is illuminated by laser light. By reason of the optical rough surface of the sample a so called speckle pattern reflects, which is recorded by a CCD-Camera system. The speckle pattern depends on the sample surface condition and the mechanical strain condition. In the next step the recorded images of the pattern are evaluated by using different mathematical algorithms. Correlations between stress condition and measurement signal was observed and evaluated. This resulting measurement signal was then calibrated bystresses calculated via a finite-element-model of the 3-point-bended Al2O3-ceramic. / 3. RESULTS The authors observe an excellent accordance between the LSP measurement signal and the stresses introduced into the surface of the sample. Furthermore a study of influence was realised, to demonstrate the independence of the measurement signal from external effects for example ambient light, exposure time etc. These results enable to enhance the LSP-method for mapping stresses across processed assemblies in a production line in future