The ASTM UNS S43940 ferritic stainless steel was provided by Outokumpu Stainless Oyj. Research is supported by Academy of Finland project &#8470; 298762 and Aalto University School of Engineering and by post-doctoral funding No 9155273 Aalto University School of Engineering. Video publication was performed with support of Mikko Raskinen from Aalto Media Factory.

1. Specimen preparation and annealing
<ol>
	<li>Mill the original ferritic stainless-steel plates with a thickness of 3 mm (see Table of Materials) to form the plate with characteristic size of about 200 mm x 15 mm x 1 mm.</li>
	<li>Place the produced steel plate into the quartz tube and pump (see Table of Materials) it until the pressure of about 10-6 mbar.</li>
	<li>Provide argon gas (see Table of Materials) into the quartz tube until the pressure reaches about 0.2 mbar.</li>
	<li>Seal the quartz tube with the specimen inside by heating the quartz tube up to the melting temperature19. 
	CAUTION: The sealing procedure is hazardous. Use appropriate precautions such as proper eye protection, etc.20.</li>
	<li>Anneal the steel plate sealed inside of the quartz tube using the chamber furnace (see table of materials) at temperature of 1200 &#176;C for 1 h and quench in water. 
	NOTE: The annealing procedure increases the average grain size of the studied steel up to 350 &#181;m without extensive formation of chromium carbide particles21. 
	CAUTION: The annealing procedure is hazardous. Use appropriate precautions and follow the instructions of the chamber furnace manual.</li>
	<li>Cut notched specimens (with thickness of 1 mm) from the annealed plate of the studied ferritic steel using electrical discharge machining (EDM, see Table of Materials). The scheme of the specimen is shown in Figure 1. 
	CAUTION: The EDM cutting procedure is hazardous. Use appropriate precautions and follow the instructions of the EDM manual.</li>
	<li>Grind and polish the specimen surface.
	<ol>
		<li>Grind the specimen surfaces using grinding machine with emery paper (Table of Materials) until the surface of the specimen is uniform.</li>
		<li>Polish the specimen surfaces using the polishing machine with 3 &#181;m and 1 &#181;m diamond paste (see Table of Materials) for 10 min each.</li>
		<li>Polish the specimen surface using 0.02 &#181;m colloidal silica vibratory polishing (see table of materials) for about 4 h; this is required for EBSD analysis.</li>
	</ol>
	</li>
</ol>
2. Fatigue pre-cracking
<ol>
	<li>Experimentally define the displacement controlled fatigue test parameters.
	<ol>
		<li>Adjust the displacement limits &#949;min and &#949;max of the servo hydraulic machine (see Table of Materials) so that the &#963;min and &#963;max are in range of about -50 MPa and 300 MPa, respectively. 
		CAUTION: The servo hydraulic machine is hazardous. Use appropriate precautions and follow the instructions of the servo hydraulic machine manual.</li>
		<li>Examine the initial crack formation after 2,000, 5,000 and 10,000 cycles using optical microscopy (see Table of Materials) to define the optimal number of fatigue cycles and avoid extensive crack growth.</li>
	</ol>
	</li>
	<li>Subject the specimen to displacement controlled uniaxial cyclic loading for defined amount of cycles.</li>
	<li>Examine the initial crack formation after defined amount of cycles using optical microscopy. Initial cracks with lengths up to about 20 &#181;m are produced at the notch tip.</li>
	<li>Increase the number of the fatigue loading cycles if the initial crack was not produced.</li>
	<li>Replace the specimen if the initial crack length exceeds 50 &#181;m.</li>
</ol>
3. Microstructural characterization
<ol>
	<li>Clean the pre-cracked specimen.
	<ol>
		<li>Clean the pre-cracked specimen with acetone for 20 min using the ultrasonic bath (see Table of Materials).</li>
		<li>Clean the pre-cracked specimen with ethanol for 20 min using the ultrasonic bath (see Table of Materials).</li>
	</ol>
	</li>
	<li>Mark the studied area using Vickers microindentations as shown in Figure 2a.
	<ol>
		<li>Follow the instructions of the Vickers microindentor (see Table of Materials) to perform the microindentation marks.</li>
		<li>Insert the specimen into the micro Vickers hardness tester (see Table of Materials).</li>
		<li>Set the indentation force at 500 N.</li>
		<li>Adjust the position for the first Vickers indentation mark at about 500 &#181;m sideways from the notch tip. Prepare the second indentation at another side.</li>
		<li>Adjust the position for the third indentation mark at about 500 &#181;m sideways and about 400 &#181;m away from the notch tip.</li>
	</ol>
	</li>
	<li>Analyze the microstructure of the steel from the side surface of the specimen in the vicinity of the notch using electron backscatter diffraction (EBSD) analysis (see Table of Materials).
	<ol>
		<li>Follow the instruction manual of scanning electron microscope to perform EBSD analysis.</li>
		<li>Set the magnification at 200x.</li>
		<li>Adjust the position of the specimen under EBSD detector. Ensure that the notch tip and three Vickers microindentation marks are within the framework of the EBSD scanning (see Figure 2b).</li>
		<li>Set the step size of the EBSD scanning at 2 &#181;m. Scanning duration is about 1 h.</li>
	</ol>
	</li>
</ol>
4. Decoration with a pattern
<ol>
	<li>Clean the specimen surface with ethanol (see Table of Materials) for 10 min using the ultrasonic bath.</li>
	<li>Dry the specimen using a fan.</li>
	<li>Clean a microscope slide using a paper napkin soaked with ethanol (see Table of Materials).</li>
	<li>Deposit a thin layer of ink on the glass surface of the microscope slide. A permanent marker provides uniform layer of ink on the glass surface by hand.</li>
	<li>Press down on the silicone stamp with the pattern on the glass surface to transfer a layer of ink to the stamp surface.</li>
	<li>Press down on the silicone stamp covered with the ink on the specimen surface.</li>
	<li>Check the speckle pattern quality using optical microscopy. An example of the speckle pattern is shown in Figure 3. See references22,23 for details of pattern and microcontact printing.</li>
	<li>Ensure that the speckle pattern size is at least 10 times smaller than the grain size of the studied material. 
	&#8203;NOTE: Perform the steps 2, 3 and 4 in sufficient time to avoid the ink drying. Define the drying time experimentally.</li>
</ol>
5. Fatigue testing with DIC
<ol>
	<li>Set the specimen into the servo hydraulic machine (see table of materials). 
	CAUTION: The servo hydraulic machine is hazardous. Use appropriate precautions and follow the instructions of the servo hydraulic machine manual.</li>
	<li>Adjust the load-controlled fatigue test parameters using R = 0.1 (&#963;min = 35 MPa, &#963;max = 350 MPa) and test frequency of 10 Hz using the control software of the fatigue machine.</li>
	<li>Set up an optical microscope with 16x precision zoom lens (see Table of Materials) for optical observation of the specimen notched area.</li>
	<li>Equip the optical microscope with a digital camera with resolution of 2,048 pixels x 1,536 pixels.</li>
	<li>Adjust the magnification of the optical microscope manually.
	<ol>
		<li>Ensure that the whole notched area of the specimen fits into the image area of the digital camera.</li>
		<li>Ensure that the pixel size is at least 5 times smaller than the pattern size.</li>
	</ol>
	</li>
	<li>Run the fatigue test and synchronize with the image recording system.
	<ol>
		<li>Capture the images during temporary (10 s) stops of the fatigue test in intervals of 500 cycles.</li>
		<li>Ensure that the load is held constant with an average stress of about 210 MPa during image acquisition.</li>
	</ol>
	</li>
	<li>Continue the fatigue testing until the crack length approaches a critical value or net-section plasticity starts to dominate.</li>
</ol>
6. Results analysis
<ol>
	<li>Use the obtained raw images to perform the crack growth rate (CGR) and DIC analysis using a commercial software (see Table of Materials).
	<ol>
		<li>Use the operation manual to perform CGR analysis. Note that the crack growth rate analysis is possible to perform using the commercial software automatically or manually.</li>
		<li>Perform the CGR analysis manually using the raw image dataset by measurement of the crack length increment after each 500 cycles.</li>
	</ol>
	</li>
	<li>Analyze of the shear strain deformation for the studied area using commercial software.
	<ol>
		<li>Use the operation manual to perform shear strain deformation analysis.</li>
		<li>Ensure that correlation mode in time series settings of the software is chosen to be &#34;relative to the first&#34;.</li>
	</ol>
	</li>
	<li>Perform Schmid factor and grains misorientation analysis of EBSD data using the open source MTEX toolbox (see Table of Materials). 
	NOTE: Details about Schmid factor and grains misorientation analysis are available in user guide of MTEX toolbox24.</li>
	<li>Perform cumulative analysis of the obtained results. 
	NOTE: The cumulative analysis is discussed in Ref.18.
	<ol>
		<li>Use Vickers microindentation marks to match the grain boundary map, misorientation map and Schmid factor map on top of the shear strain deformation field18.</li>
		<li>Define the correlation between CGR, strain field and microstructure (misorientation and Schmid factor maps)18.</li>
	</ol>
	</li>
</ol>

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The authors have no competing financial interests to disclose.

A novel in situ measurement approach is introduced to measure the cumulative deformation field at a grain micro-scale level. In order to demonstrate the approach capability, the microstructurally small fatigue crack propagation behavior is studied in ferritic stainless steel with 18% chromium. The studied steel was provided in the shape of hot rolled plate with a thickness of 3 mm (see Table of Materials) and average grain size of about 17 &#181;m21.
A successful measurement requires that an initial fatigue crack is produced at the notch tip of the specimens for further propagation behavior analysis. In order to study a microstructurally small crack, the length of the initial crack should be significantly smaller than the grain size of the studied steel. Fatigue testing is displacement controlled to prevent crack growth after fatigue crack initiation. It was found that fatigue crack initiation time decreases significantly with the decrease of stress ratio (R). Thus, only 10,000 cycles were required for fatigue crack initiation in the specimens tested with R-ratio -0.16, while with Rratio 0.1, the fatigue crack did not initiate even after 100,000 cycles. The use of the load ratio R=-0.16 allows to increase the stress range from 315 MPa to 350 MPa, having still smaller maximum stress for pre-cracking than that of actual fatigue testing.
The intermittent small fatigue crack growth is usually associated with the microstructure. In particular, grain boundaries are widely considered as microstructural features responsible for small crack growth retardation4,5,6,7,8,9,10,11,12. The dislocation formulation in the boundary element by Hansson et al.13 shows that the low-angle grain boundaries lying in the way of the crack path can result in both an increase and decrease of the crack growth rate; however, the high-angle grain boundaries do not affect the crack growth rate. The physical reasons causing the anomalous crack growth behavior are not well known. In order to reveal the microstructural features causing the small crack retardation, a microstructural characterization was performed before fatigue testing of the specimen. The polishing procedure described in step 1 is crucial for reliable microstructural analysis using EBSD. In step 3, just before EBSD analysis, the cleaning of the specimen in ethanol is only allowed, since acetone vapor is hazardous for EBSD detector.
In order to reveal deformation processes within individual grains, the size of the speckle pattern must be significantly smaller than the grain size of the studied steel. Since the average grain size of the steel after annealing is about 350 &#181;m, the characteristic size of the speckle pattern required for DIC calculation was chosen to be approximately 10 &#181;m22,12. The speckle pattern size must be at least 10 times smaller than the grain size of the studied steel for proper implementation of step 5. The surface of the specimen is decorated with a speckle pattern using a silicone stamp. We use a custom-made pneumatic tool (see Figure 6) for fast and precise operation of the stamp.
Small fatigue crack propagation behavior is studied during load-controlled fatigue testing of the pre-cracked specimens using the R-ratio of 0.1 (&#963;min = 35 MPa, &#963;max = 350 MPa) and the frequency of 10 Hz. Fatigue testing follows together with digital image correlation (DIC) measurement. The area of interest is monitored using an optical microscope, 16x Precision Zoom Lens, with a resolution of 2 &#181;m/pixel. Images are captured during temporary (10 s) stops of the fatigue test in intervals of 500 cycles. During image acquisition, the loading is held constant, with an average stress of approximately 210 MPa, in order to have equal loading conditions for all images, stabilize plastic deformation, and avoid fatigue crack closure and extensive creep accompanied with min and max of loading force, respectively. The novelty of the method is based on high-resolution in situ DIC image recording that allows to reveal tiny deformation zones forming during small fatigue crack growth. The success of the experiment depends on the proper implementation of the pre-cracking procedure, selection of image capture interval and magnification to prevent the blurring of small features such as the observed shear strain localization zones. Thus, proper selection of camera resolution, optical magnification and speckle pattern size as described in step 5 of the protocol can be crucial for investigation of the strain localization phenomena. However, morphology of the shear strain localization zones is still unclear and needs further improvements of the speckle pattern and resolution of the image recording equipment.
The methodological approach described in this paper is suitable for crack growth analysis of small fatigue cracks in coarse-grained materials. A combination of crack growth rate measurement and strain-field analysis at the sub grain level helps to reveal the mechanism that are responsible for anomalous growth of the small fatigue cracks18, in addition to the widely observed grain boundary effects on SFCs. Deeper understanding of the fatigue fracture mechanisms makes development of new theoretical approaches possible and thus, enables design of lighter and more energy efficient structures in the future.

New lightweight solutions are required to improve the energy efficiency of vehicles such as ships. Weight reduction of large steel structures is possible using advanced steel materials. The efficient utilization of new material and the lightweight solution requires high manufacturing quality and robust design methods1,2. A robust design method means structural analysis under realistic loading conditions, such as wave-induced loading in the case of a cruise ship, as well as response calculations to define deformation and stresses. The allowed stress level is defined based on the strength of critical structural details. In the case of large structures, these are typically welded joints with an inhomogeneous microstructure. One of the key design challenges for new lightweight solutions is fatigue due to its cumulative and localized nature, often taking place at weld notches. For high manufacturing quality, the fatigue behavior is dominated by small fatigue crack (SFC) growth since manufacturing induced defects are very small1,3. Thus, the fundamental understanding of small fatigue crack growth in metallic materials is crucial for sustainable use of new steels in high performance structures.
The effective modeling of such a complicated process as fatigue crack propagation in polycrystalline metallic materials is impossible without a clear understanding of the physical processes accompanying the fatigue fracture mechanism. A significant effort from the research community has been focused on investigating fatigue crack propagation using visual observation and statistical analysis. So far, small fatigue crack growth behavior has mainly been investigated by theoretical methods due to the limitations of experimental techniques. The anomalous fatigue crack growth rate retardation for SFCs is usually associated with the grain boundaries (GB)4,5,6,7,8,9. However, the reasons for anomalous SFC growth are still under discussion. The results obtained by theoretical modelling using a discrete dislocation method shows formation of a dislocation wall, or a short low-angle grain boundary caused by dislocations emitted from the fatigue crack tip affecting the fatigue crack growth rate10,11,12,13. Until recently, there has been a challenge in accurate experimental analysis of the small fatigue crack growth behavior. Experimental observations are required for the development of physical principles based computational models.
For analysis of cyclic material deformation behavior at micro-scale it is desirable to have full-field deformation measurements that can be carried out in situ during cyclic loading using standard mechanical testing equipment, with spatial resolution at least an order of magnitude below the characteristic length scale of the microstructure. In order to understand the variations in fatigue crack growth rate, measured strain fields are often linked to electron backscatter diffraction (EBSD) measurements of material microstructure. Carrol et al.14 provide a quantitative, full-field ex situ measurement of plastic strain near a growing long fatigue crack in a nickel-based super alloy, showing the formation of asymmetric lobes in the plastic wake of the propagating fatigue crack. At higher magnification, electron microscopy digital image correlation (DIC) revealed strain inhomogeneities associated with strain localization on the slip bands, with twin and grain boundaries affecting the fatigue crack growth behavior. However, the used ex situ measurement approach is not able to capture the strain field during fatigue crack propagation. An experimental study of plastic blunting during long fatigue crack propagation was performed by Peralta15 using in situ DIC for commercial purity Ni (99.6%). Results revealed that the accumulation of plastic deformation was dominated by shear along the slip bands that extended ahead of the crack and were inclined with respect to the crack growth direction. The observed strain localization at the slip bands is probably caused by overloading, since the low stress intensity factor values result in a mixed nature of the deformation (shear and normal strain)14,15. A heterogeneous strain field distribution at the sub-grain level has been observed for coarse grained aluminum alloy16 and duplex steel17, where the activation of the dislocation slip systems was associated with Schmid`s law16,17.
A recent study performed by Malitckii18 manifests that anomalous SFC growth behavior is controlled by strain inhomogeneities related to the grain structure or, in particular, by accumulation of shear strain localization zones ahead of the crack. With high-quality micro-scale patterns and grains larger than 100 &#181;m, optical microscopy DIC enabled in situ sub-grain deformation measurements for the first time. However, in Malitckii18, the novel methodology applied to measure plastic strain field in situ over hundreds of thousands of load cycles was not presented or discussed in detail. Therefore, the objective of this paper is to introduce this new experimental approach for studying small fatigue crack growth behavior in polycrystalline materials in the high cycle regime. The novelty of the approach consists of in situ full-field strain measurement using a unique pattern technique, in addition to crack growth rate measurement. Because this method uses optical image sensors it enables capturing thousands of frames during the fatigue test. Electron backscatter diffraction (EBSD) is used for microstructural characterization and combined with DIC measurements to reveal the impact of grain boundaries on small fatigue crack growth retardation18. The approach is applied for the measurement of small fatigue crack propagation in bcc 18%Cr ferritic stainless steel18 simulating the behavior of the structural steel for large structural applications. In this paper, we explain the main steps of the measurement procedure and provide a summary discussion of the main finding.

<table><tbody><tr><td>Acetone</td><td>Sigma-Aldrich</td><td>STBH7695</td><td>Acetone pyrity &amp;ge; 99.5 %</td></tr><tr><td>Argon gas</td><td>Oy AGA Ab, Industrial Gases (Finland)</td><td>UN 1006</td><td>Gas purity &amp;ge; 99.9999 %</td></tr><tr><td>Chamber furnace</td><td>Lenton</td><td>4934</td><td>heat range 20-1200 &lt;sup&gt;o&lt;/sup&gt;C</td></tr><tr><td>Commercial software DaVis 8</td><td>LaVision Inc.</td><td></td><td>Commercial software used for crack growth rate and strain field analysis</td></tr><tr><td>Custom-made pneumatic stamping tool</td><td>Aalto University</td><td></td><td>Made in Aalto University</td></tr><tr><td>Diamond paste</td><td>Struers Inc.</td><td>DP-Mol. 3 &amp;micro;m, DP-Nap. 1 &amp;micro;m,</td><td>Paste for polishing</td></tr><tr><td>Emery paper</td><td>Struers Inc.</td><td>FEPA P #800, FEPA P #1200, FEPA P #2500</td><td>Paper for grinding</td></tr><tr><td>Ethanol</td><td>Altia Industrial</td><td>ETAX Ba</td><td>Ethanol pyrity &amp;ge; 99.5 %</td></tr><tr><td>FEG-SEM scanning electron microscope</td><td>ZEISS</td><td>ULTRA 55</td><td>EBSD analysis</td></tr><tr><td>Ferritic stainless steel</td><td>Outokumpu Stainless Oyj (Finland)</td><td>Core 441/4509 (ASTM UNS S43940)</td><td>3 mm rolled plate</td></tr><tr><td>For Vacuum pump</td><td>Leybold-Heraeus</td><td>D4B/WS</td><td></td></tr><tr><td>Grinding machine</td><td>Struers Inc.</td><td>LaboPol-21</td><td>Hand grinding</td></tr><tr><td>MasterMet 2 Non-Crystallizing Colloidal Silica Polishing Suspension</td><td>Buehler Inc.</td><td>40-6380-064</td><td>0.02 &amp;micro;m colloidal silica&amp;nbsp;</td></tr><tr><td>MatLab software</td><td>MathWorks Inc.</td><td></td><td>MatLab software used as a platform for MTEX toolbox</td></tr><tr><td>Micro Vickers hardness tester</td><td>Buehler Inc.</td><td>1600-6400</td><td></td></tr><tr><td>MTEX software</td><td>Open source</td><td></td><td>Open source toolbox based on MatLab for analysis of the EBSD data (http://mtex-toolbox.github.io/)</td></tr><tr><td>Optical microscope</td><td>Nikon Corporation</td><td>EPIPHOT 200</td><td></td></tr><tr><td>Polishing machine</td><td>Struers Inc.</td><td>LaboPol-5</td><td>Hand polishing</td></tr><tr><td>Servo hydraulic machine</td><td>MTS system corporation</td><td>858 Table Top System</td><td></td></tr><tr><td>Turbomolecular pump</td><td>Leybold-Heraeus</td><td>Turbovac 50</td><td></td></tr><tr><td>Vibratory polisher</td><td>Buehler Inc.</td><td>VibroMet 2</td><td>Automatic polishing</td></tr><tr><td>Wire-cut EDM</td><td>TamSpark Oy</td><td>Charmilles robofil 400</td><td>wire diameter 0.15 mm</td></tr></tbody></table>

full-field strain measurements for microstructurally small fatigue crack propagation using digital image correlation method

A novel measurement approach is used to reveal the cumulative deformation field at a sub-grain level and to study the influence of microstructure on the growth of microstructurally small fatigue cracks. The proposed strain field analysis methodology is based on the use of a unique pattering technique with a characteristic speckle size of approximately 10 &#181;m. The developed methodology is applied to study the small fatigue crack behavior in body centered cubic (bcc) Fe-Cr ferritic stainless steel with a relatively large grain size allowing a high spatial measurement accuracy at the sub-grain level. This methodology allows the measurement of small fatigue crack growth retardation events and associated intermittent shear strain localization zones ahead of the crack tip. In addition, this can be correlated with the grain orientation and size. Thus, the developed methodology can provide a deeper fundamental understanding of the small fatigue crack growth behavior, required for the development of robust theoretical models for the small fatigue crack&#160;propagation in polycrystalline materials.

Using the proposed methodology, we can analyze the sub-grain deformation field accumulating during small fatigue crack propagation under cyclic loading. The characterization is performed at sub grain level showing tiny features of the material behavior under fatigue loading even within a single grain. In particular, formation of shear strain localization fields was observed as shown in Figure 4. A number of tests were performed to verify the observed phenomena.
The deformation field is easily combined with the grain boundary image for a comprehensive characterization of the features responsible for the anomalous growth behavior of the small fatigue cracks (see Figure 5). Cumulative analysis of the deformation field, microstructure, crack growth rate and crack path reveal a dependence between the small crack growth rate retardation and accumulation of the shear strain localization zone18, as shown in the video.
<img alt="Figure 1" class="xfigimg" src="/files/ftp_upload/59134/59134fig1.jpg" /> 
Figure 1: Schematic view of the fatigue test specimen of the studied ferritic stainless steel (dimensions are in mm). <a href="https://jove.unicartagenaproxy.elogim.com/files/ftp_upload/59134/59134fig1large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 2" class="xfigimg" src="/files/ftp_upload/59134/59134fig2.jpg" /> 
Figure 2: SEM image of the side surface of the ferritic stainless steel specimen in the vicinity of the notched area (a) and its inverse pole figure (IPF) map with IPF key in the inset (b). The alignment of the DIC strain field and EBSD image was performed with help of Vickers microindentations shown by dashed circles (a). <a href="https://jove.unicartagenaproxy.elogim.com/files/ftp_upload/59134/59134fig2large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 3" class="xfigimg" src="/files/ftp_upload/59134/59134fig3.jpg" /> 
Figure 3. Optical microscopy of the specimen side surface decorated with a pattern.
<img alt="Figure 4" class="xfigimg" src="/files/ftp_upload/59134/59134fig4.jpg" /> 
Figure 4. Intermittent accumulation of the shear strain localization zones during small fatigue crack growth. <a href="https://jove.unicartagenaproxy.elogim.com/files/ftp_upload/59134/59134fig4large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 5" class="xfigimg" src="/files/ftp_upload/59134/59134fig5.jpg" /> 
Figure 5. Two examples (a and b) of the combined view of the shear strain field and microstructure of the studied steel tested in fatigue. <a href="https://jove.unicartagenaproxy.elogim.com/files/ftp_upload/59134/59134fig5large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 6" class="xfigimg" src="/files/ftp_upload/59134/59134fig6.jpg" /> 
Figure 6. Custom-made pneumatic machine for pattern decoration of the specimens.&#160;<a href="https://jove.unicartagenaproxy.elogim.com/files/ftp_upload/59134/59134fig6large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>

Watch this Scientific Journal Video about Full-field Strain Measurements for Microstructurally Small Fatigue Crack Propagation Using Digital Image Correlation Method at JoVE.com

Full-field Strain Measurements for Microstructurally Small Fatigue Crack Propagation Using Digital Image Correlation Method

Microstructurally small fatigue crack growth behavior is investigated using a novel methodological approach combining crack growth rate measurement and strain-field analysis to reveal the cumulative deformation field at sub-grain level.

A novel measurement approach is used to reveal the cumulative deformation field at a sub-grain level and to study the influence of microstructure on the ...

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9.6K Views.  Aalto University School of Engineering. Microstructurally small fatigue crack growth behavior is investigated using a novel methodological approach combining crack growth rate measurement and strain-field analysis to reveal the cumulative deformation field at sub-grain level.

Video: Full-field Strain Measurements for Microstructurally Small Fatigue Crack Propagation Using Digital Image Correlation Method

Concurrent Quantitative Conductivity and Mechanical Properties Measurements of Organic Photovoltaic Materials using AFM

Organic photovoltaic (OPV) materials are inherently inhomogeneous at the nanometer scale. Nanoscale inhomogeneity of OPV materials affects performance of photovoltaic devices. Thus, understanding of spatial variations in composition as well as electrical properties of OPV materials is of paramount importance for moving PV technology forward.1,2 In this paper, we describe a protocol for quantitative measurements of electrical and mechanical properties of OPV materials with sub-100 nm resolution. Currently, materials properties measurements performed using commercially available AFM-based techniques (PeakForce, conductive AFM) generally provide only qualitative information. The values for resistance as well as Young's modulus measured using our method on the prototypical ITO/PEDOT:PSS/P3HT:PC61BM system correspond well with literature data. The P3HT:PC61BM blend separates onto PC61BM-rich and P3HT-rich domains. Mechanical properties of PC61BM-rich and P3HT-rich domains are different, which allows for domain attribution on the surface of the film. Importantly, combining mechanical and electrical data allows for correlation of the domain structure on the surface of the film with electrical properties variation measured through the thickness of the film.

Organic photovoltaic (OPV) materials are inherently inhomogeneous at the nanometer scale. Nanoscale inhomogeneity of OPV materials affects performance of photovoltaic devices. In this paper, we describe a protocol for quantitative measurements of electrical and mechanical properties of OPV materials with sub-100 nm resolution.

Organic photovoltaic  (OPV) materials are inherently inhomogeneous at the nanometer scale. Nanoscale  inhomogeneity of OPV materials affects performance ...

Real-Time DC-dynamic Biasing Method for Switching Time Improvement in Severely Underdamped Fringing-field Electrostatic MEMS Actuators

Mechanically underdamped electrostatic fringing-field MEMS actuators are well known for their fast switching operation in response to a unit step input bias voltage. However, the tradeoff for the improved switching performance is a relatively long settling time to reach each gap height in response to various applied voltages. Transient applied bias waveforms are employed to facilitate reduced switching times for electrostatic fringing-field MEMS actuators with high mechanical quality factors. Removing the underlying substrate of the fringing-field actuator creates the low mechanical damping environment necessary to effectively test the concept. The removal of the underlying substrate also a has substantial improvement on the reliability performance of the device in regards to failure due to stiction. Although DC-dynamic biasing is useful in improving settling time, the required slew rates for typical MEMS devices may place aggressive requirements on the charge pumps for fully-integrated on-chip designs. Additionally, there may be challenges integrating the substrate removal step into the back-end-of-line commercial CMOS processing steps. Experimental validation of fabricated actuators demonstrates an improvement of 50x in switching time when compared to conventional step biasing results. Compared to theoretical calculations, the experimental results are in good agreement.

The robust device design of fringing-field electrostatic MEMS actuators results in inherently low squeeze-film damping conditions and long settling times when performing switching operations using conventional step biasing. Real-time switching time improvement with DC-dynamic waveforms reduces the settling time of fringing-field MEMS actuators when transitioning between up-to-down and down-to-up states.

Mechanically underdamped electrostatic fringing-field MEMS actuators are well known for their fast switching operation in response to a unit step input ...

Micro/Nano-scale Strain Distribution Measurement from Sampling Moir&#233; Fringes

This work describes the measurement procedure and principles of a sampling moir&#233; technique for full-field micro/nano-scale deformation measurements. The developed technique can be performed in two ways: using the reconstructed multiplication moir&#233; method or the spatial phase-shifting sampling moir&#233; method. When the specimen grid pitch is around 2 pixels, 2-pixel sampling moir&#233; fringes are generated to reconstruct a multiplication moir&#233; pattern for a deformation measurement. Both the displacement and strain sensitivities are twice as high as in the traditional scanning moir&#233; method in the same wide field of view. When the specimen grid pitch is around or greater than 3 pixels, multi-pixel sampling moir&#233; fringes are generated, and a spatial phase-shifting technique is combined for a full-field deformation measurement. The strain measurement accuracy is significantly improved, and automatic batch measurement is easily achievable. Both methods can measure the two-dimensional (2D) strain distributions from a single-shot grid image without rotating the specimen or scanning lines, as in traditional moir&#233; techniques. As examples, the 2D displacement and strain distributions, including the shear strains of two carbon fiber-reinforced plastic specimens, were measured in three-point bending tests. The proposed technique is expected to play an important role in the non-destructive quantitative evaluations of mechanical properties, crack occurrences, and residual stresses of a variety of materials.

A sampling moir&#233; technique featuring 2-pixel and multi-pixel sampling methods for high-accuracy strain distribution measurements at the micro/nano-scale is presented here.

This work describes the measurement procedure and principles of a sampling moir&#233; technique for full-field micro/nano-scale deformation measurements. ...

Ultrasonic Fatigue Testing in the Tension-Compression Mode

Ultrasonic fatigue testing is one of a few methods which allow investigating fatigue properties in the ultra-high cycle region. The method is based on exposing the specimen to longitudinal vibrations on its resonance frequency close to 20 kHz. With use of this method, it is possible to significantly decrease the time required for the test, when compared to conventional testing devices usually working at frequencies under 200 Hz. It is also used to simulate loading of material during operation in high speed conditions, such as those experienced by components of jet engines or car turbo pumps. It is necessary to operate only in the high and ultra-high cycle region, due to the possibility of extremely high deformation rates, which can have a significant influence on the test results. Specimen shape and dimensions have to be carefully selected and calculated to fulfill the resonance condition of the ultrasonic system; thus, it is not possible to test the full components or specimens of arbitrary shape. Before each test, it is necessary to harmonize the specimen with the frequency of the ultrasonic system to compensate for deviations of the real shape from the ideal one. It is not possible to run a test until a total fracture of the specimen, since the test is automatically terminated after initiation and propagation of the crack to a certain length, when the stiffness of the system changes enough to shift the system out of the resonance frequency. This manuscript describes the process of evaluation of materials' fatigue properties at high-frequency ultrasonic fatigue loading with use of mechanical resonance at a frequency close to 20 kHz. The protocol includes a detailed description of all steps required for a correct test, including specimen design, stress calculation, harmonizing with the resonance frequency, performing the test, and final static fracture.

A protocol for ultrasonic fatigue testing in the high and ultra-high cycle region in axial tension-compression loading mode.

Ultrasonic fatigue testing is one of a few methods which allow investigating fatigue properties in the ultra-high cycle region. The method is based on ...

Uncoupling Coriolis Force and Rotating Buoyancy Effects on Full-Field Heat Transfer Properties of a Rotating Channel

An experimental method for exploring the heat transfer characteristics of an axially rotating channel is proposed. The governing flow parameters that characterize the transport phenomena in a rotating channel are identified via the parametric analysis of the momentum and energy equations referring to a rotating frame of reference. Based on these dimensionless flow equations, an experimental strategy that links the design of the test module, the experimental program and the data analysis is formulated with the attempt to reveal the isolated Coriolis-force and buoyancy effects on heat transfer performances. The effects of Coriolis force and rotating buoyancy are illustrated using the selective results measured from rotating channels with various geometries. While the Coriolis-force and rotating-buoyancy impacts share several common features among the various rotating channels, the unique heat transfer signatures are found in association with the flow direction, the channel shape and the arrangement of heat transfer enhancement devices. Regardless of the flow configurations of the rotating channels, the presented experimental method enables the development of physically consistent heat transfer correlations that permit the evaluation of isolated and interdependent Coriolis-force and rotating-buoyancy effects on the heat transfer properties of rotating channels.

Here, we present an experimental method for decoupling the interdependent Coriolis-force and rotating-buoyancy effects on full-field heat transfer distributions of a rotating channel.

An experimental method for exploring the heat transfer characteristics of an axially rotating channel is proposed. The governing flow parameters that ...

Intermediate Strain Rate Material Characterization with Digital Image Correlation

The mechanical response of a material under dynamic load is typically different than its behavior under static conditions; therefore, the common quasistatic equipment and procedures used for material characterization are not applicable for materials under dynamic loads. The dynamic response of a material depends on its deformation rate and is broadly categorized into high (i.e., greater than 200/s), intermediate (i.e., 10&#8722;200/s) and low strain rate regimes (i.e., below 10/s). Each of these regimes calls for specific facilities and testing protocols to ensure the reliability of the acquired data. Due to the limited access to high-speed servo-hydraulic facilities and validated testing protocols, there is a noticeable gap in the results at the intermediate strain rate. The current manuscript presents a validated protocol for the characterization of different materials at these intermediate strain rates. Strain gauge instrumentation and digital image correlation protocols are also included as complimentary modules to extract the utmost level of detailed data from every single test. Examples of raw data, obtained from a variety of materials and test setups (e.g., tensile and shear) is presented and the analysis procedure used to process the output data is described. Finally, the challenges of dynamic characterization using the current protocol, along with the limitations of the facility and methods of overcoming potential problems are discussed.

Here we present a methodology for the dynamic characterization of tensile specimens at intermediate strain rates using a high-speed servo-hydraulic load frame. Procedures for strain gauge instrumentation and analysis, as well as for digital image correlation strain measurements on the specimens, are also defined.

The mechanical response of a material under dynamic load is typically different than its behavior under static conditions; therefore, the common ...

Crack Monitoring in Resonance Fatigue Testing of Welded Specimens Using Digital Image Correlation

A procedure using digital image correlation (DIC) to detect cracks on welded specimens during fatigue tests on resonance testing machines is presented. It is intended as a practical and reproducible procedure to identify macroscopic cracks at an early stage and monitor crack propagation during fatigue tests. It consists of strain field measurements at the weld using DIC. Images are taken at fixed load cycle intervals. Cracks become visible in the computed strain field as elevated strains. This way, the whole width of a small-scale specimen can be monitored to detect where and when a crack initiates. Subsequently, it is possible to monitor the development of the crack length. Because the resulting images are saved, the results are verifiable and comparable. The procedure is limited to cracks initiating at the surface and is intended for fatigue tests under laboratory conditions. By visualizing the crack, the presented procedure allows direct observation of macrocracks from their formation until rupture of the specimen.

Digital image correlation is used in fatigue tests on a resonance testing machine to detect macroscopic cracks and monitor crack propagation in welded specimens. Cracks on the specimen surface become visible as increased strains.

A procedure using digital image correlation (DIC) to detect cracks on welded specimens during fatigue tests on resonance testing machines is presented. It ...

A Method to Estimate Cadaveric Femur Cortical Strains During Fracture Testing Using Digital Image Correlation

This protocol describes the method using digital image correlation to estimate cortical strain from high speed video images of the cadaveric femoral surface obtained from mechanical testing. This optical method requires a texture of many contrasting fiduciary marks on a solid white background for accurate tracking of surface deformation as loading is applied to the specimen. Immediately prior to testing, the surface of interest in the camera view is painted with a water-based white primer and allowed to dry for several minutes. Then, a black paint is speckled carefully over the white background with special consideration for the even size and shape of the droplets. Illumination is carefully designed and set such that there is optimal contrast of these marks while minimizing reflections through the use of filters. Images were obtained through high speed video capture at up to 12,000 frames/s. The key images prior to and including the fracture event are extracted and deformations are estimated between successive frames in carefully sized interrogation windows over a specified region of interest. These deformations are then used to compute surface strain temporally during the fracture test. The strain data is very useful for identifying fracture initiation within the femur, and for eventual validation of proximal femur fracture strength models derived from Quantitative Computed Tomography-based Finite Element Analysis (QCT/FEA).

In this protocol, the femur surface strains are estimated during fracture testing using the digital image correlation technique. The novelty of the method involves application of a high-contrast stochastic speckle pattern on the femur surface, carefully specified illumination, high speed video capture, and digital image correlation analysis for strain calculations.

This protocol describes the method using digital image correlation to estimate cortical strain from high speed video images of the cadaveric femoral ...

Bioengineering

Measuring Local Tissue Strains in Tendons via Open-Source Digital Image Correlation

There is considerable scientific interest in understanding the strains that tendon cells experience in situ and how these strains influence tissue remodeling. Based on this interest, several analytical techniques have been developed to measure local tissue strains within tendon explants during loading. However, in several cases, the accuracy and sensitivity of these techniques have not been reported, and none of the algorithms are publicly available. This has made it difficult for the more widespread measurement of local tissue strains in tendon explants. Therefore, the objective of this paper was to create a validated analysis tool for measuring local tissue strains in tendon explants that is readily available and easy to use. Specifically, a publicly available augmented-Lagrangian digital image correlation (ALDIC) algorithm was adapted for measuring 2D strains by tracking the displacements of cell nuclei within mouse Achilles tendons under uniaxial tension. Additionally, the accuracy of the calculated strains was validated by analyzing digitally transformed images, as well as by comparing the strains with values determined from an independent technique (i.e., photobleached lines). Finally, a technique was incorporated into the algorithm to reconstruct the reference image using the calculated displacement field, which can be used to assess the accuracy of the algorithm in the absence of known strain values or a secondary measurement technique. The algorithm is capable of measuring strains up to 0.1 with an accuracy of 0.00015. The technique for comparing a reconstructed reference image with the actual reference image successfully identified samples that had erroneous data and indicated that, in samples with good data, approximately 85% of the displacement field was accurate. Finally, the strains measured in mouse Achilles tendons were consistent with the prior literature. Therefore, this algorithm is a highly useful and adaptable tool for accurately measuring local tissue strains in tendons.

Measuring Local Tissue Strains in Tendons via Open-Source Digital Image Correlation

This paper describes an open-source digital image correlation algorithm for measuring local 2D tissue strains within tendon explants. The accuracy of the technique has been validated using multiple techniques, and it is available for public use.

There is considerable scientific interest in understanding the strains that tendon cells experience in situ and how these strains influence tissue ...

Using Digital Image Correlation to Characterize Local Strains on Vascular Tissue Specimens

Characterization of the mechanical behavior of biological and engineered soft tissues is a central component of fundamental biomedical research and product development. Stress-strain relationships are typically obtained from mechanical testing data to enable comparative assessment among samples and in some cases identification of constitutive mechanical properties. However, errors may be introduced through the use of average strain measures, as significant heterogeneity in the strain field may result from geometrical non-uniformity of the sample and stress concentrations induced by mounting/gripping of soft tissues within the test system. When strain field heterogeneity is significant, accurate assessment of the sample mechanical response requires measurement of local strains. This study demonstrates a novel biomechanical testing protocol for calculating local surface strains using a mechanical testing device coupled with a high resolution camera and a digital image correlation technique. A series of sample surface images are acquired and then analyzed to quantify the local surface strain of a vascular tissue specimen subjected to ramped uniaxial loading. This approach can improve accuracy in experimental vascular biomechanics and has potential for broader use among other native soft tissues, engineered soft tissues, and soft hydrogel/polymeric materials. In the video, we demonstrate how to set up the system components and perform a complete experiment on native vascular tissue.

We describe the use of digital image correlation to characterize the local surface strain field on vascular tissue samples subjected to uniaxial tensile testing. These measurements facilitate precise quantification of the sample mechanical response and the generation of constitutive stress-strain relations.

Characterization of the mechanical behavior of biological and engineered soft tissues is a central component of fundamental biomedical research and ...

Full-field Strain Measurements for Microstructurally Small Fatigue Crack Propagation Using Digital Image Correlation Method

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Concurrent Quantitative Conductivity and Mechanical Properties Measurements of Organic Photovoltaic Materials using AFM

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Ultrasonic Fatigue Testing in the Tension-Compression Mode

Uncoupling Coriolis Force and Rotating Buoyancy Effects on Full-Field Heat Transfer Properties of a Rotating Channel

Intermediate Strain Rate Material Characterization with Digital Image Correlation

Crack Monitoring in Resonance Fatigue Testing of Welded Specimens Using Digital Image Correlation

A Method to Estimate Cadaveric Femur Cortical Strains During Fracture Testing Using Digital Image Correlation

Measuring Local Tissue Strains in Tendons via Open-Source Digital Image Correlation

Using Digital Image Correlation to Characterize Local Strains on Vascular Tissue Specimens