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A PECVD dielectric layer of Si3N4 on Si substrate; (a) without and (b) with platinum dots; and a single crystal nickel alloy sample without (c) and with (d) platinum dots. In both cases, the dot pattern serves as a marker for the DIC procedure.
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This focused ion beam-digital image correlation (FIB-DIC) Good Practice Guide (GPG) is one of the key outputs from the iSTRESS project, aimed at providing users with practical advice for making reliable residual stress measurements on their own systems and materials using this technique. It brings together the expertise and experience of the projec...
Citations
... The residual stress measurements were performed at the cut edges using a method called Focused Ion Beam-Digital Image Correlation (FIB-DIC), see Lord et al. (2018). Punched specimens of CP980 with cutting clearances of both 8.5% and 24.1% were studied. ...
... Additionally, the largely consistent strain relief throughout the middle of the core makes it easier to average effectively and improves interpretational robustness in accordance with the suggestions by Lord et al. (2018). Multiple milling stages and pictures were used to improve the strain estimation accuracy, in contrast to the direct comparison between only two sets of SEM images collected before and after milling, even if the DIC approach could have limits regarding the measured area size. ...
This paper presents a validated finite element modeling approach for simulating shear cutting, needing a minimal amount of experimental characterization. Only one uniaxial tensile test and one force–displacement relationship from a punching experiment are needed for calibration, with maintained prediction accuracy compared to more experimentally demanding approaches. A key ingredient is the observation that the Lode angle parameter is close to zero in the fracture region, postulating that the fracture strain only depends on stress triaxiality, with one free calibration parameter. The true stress–strain behavior is provided from inverse modeling of the tensile test, whereas the fracture model is calibrated using the punching test. The model is verified for different materials by comparing force–displacement curves for punching experiments not used in the calibration. The prediction error for the intrusion is below 4%. A validation is made for two setups. The local residual stresses are measured using Focused Ion-Beam Digital Image Correlation (FIB-DIC). The simulated values are within the experimental bounds. Cut edge morphology and plastic strains obtained by nano-indentation mappings are compared to simulation results, showing a decent agreement. For trimming, the cut edge morphology prediction performance decreases at 17% cutting clearance while it is maintained over the whole range for punching. The predicted hardness values have a mean absolute percentage error below 7.5%. Finally, the effect of element size and remeshing is discussed and quantified. The minimal experimental characterization and simulation effort needed, enables an efficient optimization of the cutting process in the industry.
... Double slot geometry was used to measure residual stress [36]. Since the thickness of the coating was around 2 μm, the distance between the two slots (d) was chosen to be between 1 and 2 μm. ...
... Eq. (4) [34,36]. ...
... Whilst the sectioning performed prior to RS measurement may have caused a relief of the long-range global scale stresses, the local RS is fully captured by the investigation. The most suitable ion milling regimes were found and optimized empirically for the specific FIB-SEM equipment in accordance with the general good practice recommendations [24]. The optimized regime for FIB-DIC micro-ring-core milling was as follows: 30 kV, 4 nA, incremental step ion milling time 30 s, total ion beam milling time per location 15 min. ...
Additive manufacturing technologies attract significant interest as a route to fabricate structural elements with complex net shapes. 3D printing may offer the option of combining dissimilar materials within one build, like dissimilar metals in the case of laser powder bed fusion. The strength of bimetallic interfaces formed by immiscible metals such as stainless steel and copper alloys depends on the mechanism of bonding, the presence and nature of defects, and residual stresses, placing the focus of interest on the structure–mechanical performance relationship in the vicinity of the bimetallic interface. In this article, we report an investigation of laser powder bed fusion fabrication of thin plate bimetallic bronze-stainless steel samples. Sample characterization was performed using several techniques including surface topography and fractography, energy dispersive X-ray spectroscopy, electron backscatter diffraction, mechanical testing with digital image correlation mapping of strains during in situ tension inside scanning electron microscope. Spatially resolved evaluation of the local residual stress was carried out using the Korsunsky Focused Ion Beam micro-ring-core drilling method (FIB-DIC). Pure bronze and stainless steel samples fabricated separately possessed good strengths (ultimate tensile strengths of 150 MPa and 850 MPa, respectively) and significant amounts of ductility (local strains to failure of 0.8% and 1.2%, respectively), in spite of the presence of a relatively coarse and porous structure. Although the values for the bimetallic sample were somewhat reduced, they also demonstrated good mechanical properties, with the ultimate tensile strength of 105 MPa and 0.7% ductility. The fracture of bimetal takes place within the bronze at a distance of about 150 μm from the interface. The present report discusses the insights obtained, and charts the pathways towards further improvement of bimetal joint strengths through simple modifications of the laser powder bed fusion printing procedures.
... This included a hole diameter of 2 mm with a depth of 1 mm at 10,000 RPM, with a feed rate of 0.1 m per minute and increments of 0.05 mm of the drilling depth. Assessment of the drilling depth was performed, and it was determined that this could Furthermore, the images were pre-processed in accordance with good practice DIC corrections [61]. This included rigid-body compensation, which aligned the image sequence and compensated for minor displacement errors due to actuation. ...
Residual stress often has a significant impact on part performance and lifetime. Existing measurement techniques using strain gauges or non-destructive methods are often expensive and time-consuming. This paper presents a low-cost, novel measurement device that uses digital image correlation with the hole-drilling method to quantify the magnitude and preferred orientation of these locked-in forces. A two-axis measurement device that rapidly drills and images the surface around the hole was developed to measure residual stresses as a function of depth with sub-millimetre resolution. Validation of the device and DIC methodology was performed using a four-point bending specimen and comparison with conventional strain gauge methods. The results showed strong correlations between the two measurement techniques, as well as the theoretical estimates. The total cost of production was estimated to be approximately £380, which is significantly cheaper than competitors. The device also substantially reduced the cost per measurement point (less than £1 vs. £50+) and shortened the experiment duration from 2 h per point to 45 min per measurement. A functional, rapid, economical device has been designed and produced, which is currently being used for residual stress analysis of industrial samples. The presented design is completely open-source, and the relevant links are provided.
... As an alternative to the traditional material-removal methods, the FIB-DIC method can be used to measure RS on a micrometer or nanometer scale. In this case, a focused ion beam (FIB) is used to remove material and digital image correlation (DIC) measures the strain relief [44]. ...
This study investigates the effects of welding parameters on residual stresses (RS) in the weld bead of low carbon hot-rolled steel plates welded by gas metal arc welding (GMAW). The ultrasonic nondestructive technique using Lcr waves is employed. The wave speed is estimated through the measured time-of-flight (TOF) in the material subsurface and the current internal stresses were calculated using acoustoelastic relations. Aiming to find the best set of welding parameters, arc voltage (WAV), wire feed speed (WFS), and welding speed (WS) were tested using a design of experiments (DOE) approach and analyzed with the response surface methodology (RSM); other parameters were kept fixed. The methodology enabled the construction of the RS profiles, even facing surface welding defects that led to the disposal of some samples. Results indicated that RS increases with WAV and WFS. As for WS, a stress peak was found at 30 cm/min. The results also highlight the importance of setting the right parameters to get low resultant stresses and the effectiveness of the acoustoelastic technique for quality control in welding processes.
... HM 111/6 The evaluation of strains from the displacements of the markers was performed using the Matlab based tool iSTRESS [14]. iSTRESS is an open-source software which uses the DIC method from the Matlab Image Processing Toolbox. ...
It is well known that WC-Co hard metals exhibit a pronounced residual stress state at room temperature. Determining the exact residual stress distribution in the constituent phases experimentally is still a challenging task. The averaged values of microscopic residual stresses are considered as tensile stresses in the cobalt binder and compressive stresses in the WC phase. Earlier studies based on analytical models suggest significant residual stresses in the binder phase. In the present study, digital image correlation (DIC) is used in combination with focused ion beam (FIB) to estimate residual stresses in the binder phase. This method, referred to as micro-ring-core drilling, involves measurements of displacement fields based on SEM images. Experiments were performed on samples of industrial grades WC6wt.%Co and WC20wt.%Co, adapting the methodology to hard metal specific microstructure. In contrast to the literature, high tensile residual stresses were found only in the WC6wt.%Co sample while no clear results were obtained for WC20wt.%Co. The results are discussed and the effect of superposition with residual stresses of the WC phase is suggested as a possible reason for the deviation from analytical model.
... EBSD characterization of grain structure becomes a necessary preliminary step for correct selection of geometrical parameters of FIB ring-core drilling. The particular voltage and current of ion milling were empirically found and optimized in accordance with general recommendations given in [16] for this method to be the following: high voltage 30 kV, current 10 nA, tilt stage 55 • , 10 µm inner diameter and 15 µm outer diameter of a ring, milling depth 1 µm/step, and dataset of 12 images. ...
Ga-ion micro-ring-core FIB-DIC evaluation of residual stresses in shot peened VT6 (Ti-6Al-4V) alloy was carried out and cross-validated against other non-destructive and semi-destructive residual stresses evaluation techniques, namely, the conventional sin2 ψ X-ray diffraction and mechanical hole drilling. The Korsunsky FIB-DIC method of Ga-ion beam micro-ring-core milling within FIB-SEM with Digital Image Correlation (DIC) deformation analysis delivered spatial resolution down to a few micrometers, while the mechanical drilling of circular holes of ~2 mm diameter with laser speckle interferometry monitoring of strains gave a rough spatial resolution of a few millimeters. Good agreement was also found with the X-ray diffraction estimates of residual stress variation profiles as a function of depth. These results demonstrate that FIB-DIC provides rich information down to the micron scale, it also allows reliable estimation of macro-scale residual stresses.
... The ring-core method was applied as it allows simultaneous evaluation of three components of in-plane normal and shear strain relaxation ( x , y ,and xy ) (27). Figure 1C schematically shows the ring-core incremental milling steps and scanning electron microscopy (SEM) acquisition processes. The incremental milling was stopped when the depth was equal to the ring-core diameter to ensure that the residual stress is fully released in the ring-core region (27)(28)(29). The ring-core displacement that results from the ring-core expansion, or shrinkage, during incremental milling is recorded by the markers (the displacement between red and green markers in Fig. 1D by DIC). ...
The plasma-facing components of future fusion reactors, where the Eurofer97 is the primary structural material, will be assembled by laser-welding techniques. The heterogeneous residual stress induced by welding can interact with the microstructure, resulting in a degradation of mechanical properties and a reduction in joint lifetime. Here, a Xe+ plasma focused ion beam with digital image correlation (PFIB-DIC) and nanoindentation is used to reveal the mechanistic connection between residual stress, microstructure, and microhardness. This study is the first to use the PFIB-DIC to evaluate the time-resolved multiscale residual stress at a length scale of tens of micrometers for laser-welded Eurofer97. A nonequilibrium microscale residual stress is observed, which contributes to the macroscale residual stress. The microhardness is similar for the fusion zone and heat-affected zone (HAZ), although the HAZ exhibits around ~30% tensile residual stress softening. The results provide insight into maintaining structural integrity for this critical engineering challenge.
... Double slot geometry was used to measure residual stress [36]. Since the thickness of the coating was around 2 μm, the distance between the two slots (d) was chosen to be between 1 and 2 μm. ...
... An average strain among the area between the slots was calculated, and regions that deviated significantly from the mean were considered as ion beam affected areas and were discarded. This average strain in x-direction was defined as and residual stress (σ res ) was measured using Equation 4 [34], [36]. ...
... At present, this method gives unprecedented opportunities to correlate structural peculiarities and residual stresses and is clearly relevant for 3D-printed materials. The FIB-DIC micro-ring-core drilling method requires the use of relatively complex dual FIB-SEM microscopes and skilled operators, but the robustness of the measurements has already reached the pre-standardization stage [27]. There are very few reports of using the FIB ring-core drilling method to measure residual stresses in SLM 3D-printed materials [22] and no reports on SLM 3D-printed Al-alloys were found in the literature to date. ...
SLM additive manufacturing has demonstrated great potential for aerospace applications when structural elements of individual design and/or complex shape need to be promptly supplied. 3D-printable AlSi10Mg (RS-300) alloy is widely used for the fabrication of different structures in the aerospace industry. The importance of the evaluation of residual stresses that arise as a result of the 3D-printing process’ complex thermal history is widely discussed in literature, but systematic assessment remains lacking for their magnitude, spatial distribution, and comparative analysis of different evaluation techniques. In this study, we report the results of a systematic study of residual stresses in 3D-printed double tower shaped samples using several approaches: the contour method, blind hole drilling laser speckle interferometry, X-ray diffraction, and Xe pFIB-DIC micro-ring-core milling analysis. We show that a high level of tensile and compressive residual stresses is inherited from SLM 3D-printing and retained for longer than 6 months. The stresses vary (from −80 to +180 MPa) over a significant proportion of the material yield stress (from −⅓ to ¾). All residual stress evaluation techniques considered returned comparable values of residual stresses, regardless of dramatically different dimensional scales, which ranged from millimeters for the contour method, laser speckle interferometry, and XRD down to small fractions of a mm (70 μm) for Xe pFIB-DIC ring-core drilling. The use of residual stress evaluation is discussed in the context of optimizing printing strategies to enhance mechanical performance and long-term durability.
