Crack Length Measurement Research Articles

Machine learning-based real-time visible fatigue crack growth detection

Many large-scale and complex structural components are applied in the aeronautics and automobile industries. However, the repeated alternating or cyclic loads in service tend to cause unexpected fatigue fractures. Therefore, developing real-time and visible monitoring methods for fatigue crack initiation and propagation is critically important for structural safety. This paper proposes a machine learning-based fatigue crack growth detection method that combines computer vision and machine learning. In our model, computer vision is used for data creation, and the machine learning model is used for crack detection. Then computer vision is used for marking and analyzing the crack growth path and length. We apply seven models for the crack classification and find that the decision tree is the best model in this research. The experimental results prove the effectiveness of our method, and the crack length measurement accuracy achieved is 0.6 ​mm. Furthermore, the slight machine learning models help us realize real-time and visible fatigue crack detection.

Nanoindentation based fracture studies of ITO coating

Nano-scale load (P) dependency of fracture toughness (K1C) of indium tin oxide (ITO) coating on silicon (Si) substrate is evaluated by nanoindentation based direct and energy methods. A Berkovich triangular diamond nanoindenter is used with nine different P values e.g., 10, 15, 20, 30, 40, 50, 100, 150 and 200 mN. Extensive utilization of both optical microscopy (OM) and field emission scanning electron microscopy (FESEM) techniques reveal that up to P ≤ 40 mN only the sharp radial cracks form from three corners of the nanoindent. However, for P > 40 mN, multiple cracks occur over and above the radial cracks. Finally, at P: 150–200 mN coating gets chipped off from the substrate. The K1C values of the ITO coating are calculated on the basis of both the conventional radial crack length measurement based method and the strain energy release based method. The K1C values are strongly sensitive to variations in P. Further, the strain energy method results in higher magnitudes of the K1C values.

The influence of temperature on hydrogen environmentally assisted cracking of AA7449-T7651 in moist air

The 7xxx series of aluminium alloys are sensitive to intergranular hydrogen environmentally assisted cracking (HEAC) in moist air environments. A new generation of 7xxx alloys used in aircraft components (including AA7449) have been found to be more sensitive to HEAC. This investigation aims to quantify the HEAC crack growth rates of AA7449-T7651 in moist air at different temperatures. Double cantilever beam specimens were loaded at a constant displacement and placed in moist air (80–85% relative humidity) at temperatures between 25 and 80 °C. Regular measurements of crack length permitted the determination of the crack growth rate as a function of the stress intensity factor (KI) during stage I (high KI dependence of growth rate) and stage II (low KI dependence of growth rate). Increasing the temperature was found to increase the crack growth rate during stage II cracking, following Arrhenius kinetics. The activation energy for the stage II crack growth rate at a stress intensity factor of 14.5 MPam was estimated as 84.7 kJ/mol. Additionally, the threshold stress intensity factor for HEAC was found to decrease with increasing temperature. This was attributed to a higher hydrogen solubility at higher temperatures. The crack growth rate of AA7449-T7651 at room temperature was similar to that of overaged AA7050 and AA7075-T7651, ranging between 1.3 and 2.1 × 10−7 mm/s. It was inferred that the alloy temper has more influence than the composition during stage II cracking at room temperature.

Crack Detection and Crack Length Measurement with the DC Potential Drop Method–Possibilities, Challenges and New Developments

The direct current potential drop method (DCPDM) is widely used to determine the crack length in fatigue experiments. In practice, some special features of this method must be considered. Aside from general information on the experimental setup and calibration, some special features of the method, such as the influence of the ambient atmosphere and the application of the method to ferromagnetic materials, are presented and discussed. In addition, with the multiple potential drop measurement, a method is presented which improves the resolution of the DCPDM for detection of cracks and allows to determine crack initiation sites. The capabilities provided by this method are demonstrated on the basis of measurements undertaken on notched round bars and single edged notched specimens.

Measurement of crack length in width tapered beam experiments

The width tapered beam method for measuring fracture toughness has been contemplated for use in measuring the adhesion of photovoltaic materials, is promoted by it being a viable method for use directly on modules rather than to engineered test specimens. However, precise determination of crack length is often difficult as failure often involves tendril formation, cavitation voids, and other plastic deformation at the crack front in addition to obfuscation by opaque materials. Because fracture energy varies as the square of crack length, imprecise crack tip length results in large measurement uncertainties. In this work, we develop a method to estimate the crack tip length using the relationship between the applied force and the displacement of the load frame. The crack tip is determined for every data point and can be defined from a calibration curve related to the amount of energy put into the beam, or from a theoretically idealized estimation. Once a calibration curve is set up and the equations programmed, the fracture energy as a function of position on the newly formed crack interface can be accurately estimated. The test additionally provides much easier analysis and insight into the joint failure through more precise knowledge of fracture toughness at each de-bonded area.

Toughening mechanisms responsible for excellent crack resistance in thermoplastic nanofiber reinforced epoxies through in-situ optical and scanning electron microscopy

Epoxy is a material of choice for demanding applications thanks to its high chemical stability, stiffness, and strength. Yet, its brittle fracture behavior is an important downside for many sectors. Here, we show that the addition of electrospun thermoplastic nanofibers is a viable toughening strategy to design nanofiber reinforced epoxy materials with excellent toughness. Moreover, the use of transparent film-like specimens allowed in-situ imaging during mechanical testing. Optical and scanning electron microscopy, digital image correlation and crack length measurements are used to analyze the toughening mechanisms responsible for high toughening efficiency in detail. The addition of polyamide and polycaprolactone nanofibers resulted in an increased plastic energy uptake up to 100%. In-situ observation of the crack tip showed that the main energy-absorbing mechanism was due to bridging nanofibers. There was a profound decrease in toughening efficiency when nanofibers lacked sufficient adhesion with the matrix only when they were oriented parallel with the crack growth direction. The profound understanding of such underlying mechanisms opens up material design in applications where high toughness is required like adhesives, coatings, and fiber-reinforced composite laminates.

Thermocyclic strength of grey cast iron SCh20 at high temperature shifts

To predict the durability of casting molds made of SCh20 cast iron under thermal fatigue during its filling by liquid metal the thermocyclic strength of the casting molds metal (grade SCh20 gray cast iron, analog of A48-30B AISI cast iron) was investigated. The specimens were tested by cyclic heating in the furnace to maximum temperatures from 700°С to 1000°C with following cooling in air and water. The Coffin equation and the dependence of crack growth rate in specimens are obtained on the basis of metallographic measurements of crack length in specimens and strain ranges FEM calculations. The investigations results allow one to predict the durability of casting molds made of SCh20 cast iron under thermal fatigue during its filling by liquid metal.

The Analysis of Resistance to Brittle Cracking of Tungsten Doped TiB2 Coatings Obtained by Magnetron Sputtering

In this work, the authors present the possibility of characterization of the fracture toughness in mode I (KIC) for TiB2 and TiB2 coatings doped with different concentration of W (3%, 6% and 10%). The Young’s modulus, hardness and fracture toughness of this coatings are extracted from nanoindentation experiments. The fracture toughness was evaluated using calculation of crack length measurement. An important observation is that increasing tungsten concentration in the range 0–10% changes the microstructure of the investigated coatings: from columnar structure for TiB2 coating to nano-composite structure for Ti-B-W (10%) coating. It can be concluded that doping with concentration 10 at.% W causes an increase of the fracture toughness for the tested coatings.

Mode I fracture characterisation of rigid and flexible bonded joints using an advanced Wedge-Driven Test

The resistance to fracture of a bonded joint is usually evaluated through the experimental characterisation of its fracture toughness. Different test procedures and data reduction methods are available in the literature, such as the ISO-25217 test standard. The need for crack length measurement has led to inaccurate results when large adhesive thicknesses or flexible adhesives were studied. Advanced test protocols based on the insertion of a wedge, such as the Wedge Test (WT) in ASTM D3762 or the Wedge-Driven Test (WDT), simplify the test procedure, but most such procedures do not allow for the measurement of fracture toughness. Further, experimental results are scarce in the literature, especially when dealing with bonded joints and considerable thicknesses. In the present work, an advanced data reduction method based on the WDT is presented: The Wedge-Driven Test Plus (WDT+). The fracture toughness of specimens made by bonding two aluminium adherends with a rigid and a flexible adhesive was measured, and the effect of wedge thickness and test speed at low displacement rates on the test results was analysed. The results were compared with static tests performed in a double cantilever beam (DCB) following ISO-25217. It has been demonstrated that the proposed data reduction method is less sensitive to test speed and enables different adhesive types, and bondline thickness and wedge thickness configurations, thereby improving the WDT available in the literature.

Measuring the polymerization stress of self-adhesive resin composite cements by crack propagation

ObjectivesTo test the polymerization stress of nine self-adhesive resin composite cements (G-CEM, iCEM, Bifix SE, Maxcem Elite, PANAVIA SA, SoloCem, SmartCem 2, SpeedCEM, RelyX Unicem 2) and one glass ionomer cement (control group; Ketac Cem).Materials and methodsThe crack propagation of a feldspar ceramic (n = 130) was determined by measuring crack lengths that originated from Vickers indentations, prior to and after the application and polymerization of the self-adhesive resin cements. Results for crack propagation were converted to polymerization stress values, and statistical analysis was performed using one-way ANOVA followed by Scheffé post hoc test.ResultsSmartCem 2 presented higher stress values than iCEM, SoloCem, and Ketac Cem, while Ketac Cem showed lower values than Bifix SE, Maxcem Elite, SmartCem 2, SpeedCEM, and RelyX Unicem 2.ConclusionsSelf-adhesive resin composite cements differ in their polymerization stress, which may affect the durability of the restoration. For restorations made from ceramics with lower flexural strength, such as feldspar ceramics, resin composite cement materials with less polymerization stress should be preferred.Clinical RelevanceAs a high polymerization shrinkage may increase crack propagation, the determination of the polymerization stress of self-adhesive resin composite cements employed for fixing all-ceramic restorations is an important factor.

DevTMF – Towards code of practice for thermo-mechanical fatigue crack growth

The current paper presents work on identification and evaluation of a range of factors influencing accuracy and comparability of data generated by three laboratories carrying out stress-controlled thermo-mechanical fatigue crack growth tests. It addresses crack length measurements, heating methods and temperature measurement techniques. It also provides guidance for pre-cracking and use of different specimen geometries as well as Digital Image Correlation imaging for crack monitoring. The majority of the tests have been carried out on a coarse grain polycrystalline nickel-base superalloy using two phase angles, Out-of-Phase and In-Phase cycles with a triangular waveform and a temperature range of 400–750 °C.

Durability study of flexible bonded joints: The effect of sustained loads in mode I fracture tests

Adhesives in bonded structures are exposed both to external loads and environmental conditions; durability studies are currently needed to assess their service lifetime. Conditioning strategies differ in considering external load conditions (such as stressed or not stressed) for the durability analysis of double cantilever beam (DCB) bonded joints. Different test procedures such as ASTM D3762 (wedge testing) or ISO-25217 (DCB testing) exist to characterise the evolution of the fracture strength and toughness found in bonded joints. These methods depend on crack-length measurements, however, and achieving an accurate visual determination may be difficult due to the large fracture process zones (FPZs) that develop in the adhesive layer, especially in flexible or degraded bonded joints. To compensate, crack-length-independent data-reduction methods such as the compliance-based beam method (CBBM) or the J-integral method can be used, but experimental research is lacking on the suitability of these methods in ageing tests. A lack of consensus also exists in testing methodologies to evaluate the durability of bonded joints, especially when examining flexible bonded joints. The present work evaluates the influence of damage on fracture toughness within flexible bonded joints exposed to service conditions. Wedge tests and DCB tests are conducted using DCB specimens bonded with a flexible structural adhesive, proving that the degradation of flexible bonded joints exposed to environmental conditions is significantly accelerated when external loads act on them. The findings show that crack length estimation is affected due to environmental effects and thus, that crack-length-dependent test methods are not applicable in ageing tests.

Prediction of splitting crack growth in prestressed concrete members using fracture toughness and concrete mix design

In this study, the effect of concrete mix design on end-splitting cracks in prestressed concrete members was examined. The concrete material needs to be resistant to splitting-crack growth at the strengths at which the pretensioned member is detensioned. Mixtures with variable coarse aggregate volume, angularity, and water-to-cementitious ratio were designed and Fracture toughness tests on 4 three-point bending concrete specimens for each mix were carried out. Next, pretensioned prisms with three different reinforcement edge distances 0.750, 0.625 and 0.500 in. (19 mm, 16 mm and 13 mm) were fabricated and detensioned when the concrete compressive strength reached 4500 psi (31 MPa). Splitting cracks were quantitatively evaluated by direct measurement. The results indicated a strong correlation between crack length measurements and fracture toughness. Additionally, a linear regression model was developed by which splitting-crack growth in prestressed cross-sections with varied edge-distance thickness can be predicted. The findings from this study could lead to a significant improvement in splitting crack resistance in pretensioned concrete members where the high amount of transverse stress is often introduced at low compressive strengths.

Methods for automated crack length detection in fracture mechanical fatigue tests of unreinforced polymers

The continuous measurement of crack growth during fracture mechanical fatigue tests is of great importance for an accurate lifetime estimation of components. Different methods for an automatic crack length detection exist for linear elastic materials and electrically conductive materials like metals (ASTM International E 647-11, 2011; International Standard ISO 15850, 2002a) or fiber reinforced composites (ASTM International D5528-13, 2013; International Standard ISO 15024, 2001). However, there is no satisfying automatic system for the crack length detection in polymers. For polymers, it is still state of the art to measure the crack length manually by the use of a travelling microscope. Besides other drawbacks, this way of measuring provides only a limited number of data points. Therefore, the aim of this work was to develop methods for the automated crack length detection in these materials. Two measuring techniques were examined regarding their general applicability: (i) infrared thermography (IRT) and (ii) digital image correlation (DIC). For the validation, three types of thermoplastics (Polyvinylchloride (PVC-U), Polymethylmethacrylate (PMMA) and Polyoxymethylene (POM)), which show rather different mechanical behavior, were tested. Satisfactory correlations were obtained for both DIC and IRT method. In general, all tested systems showed good results and the determined average difference in the measured crack lengths was smaller than 5.5 %. Hence, the investigated methods based on DIC and IRT are highly interesting for an automation of fatigue crack growth measurements of polymers.

Resonance track‐and‐dwell testing for crack length measurement on 304L stainless steel

Experimental vibration-fatigue tests were conducted with sine resonance track‐and‐dwell (SRTD) tool by using an electrodynamic shaker on specimens made of 304L stainless steel. Due to cyclic fatigue resulting in stiffness loss, it can be found that the resonant frequency decreases when the specimen experiences substantial crack growth, especially for out-of-plane bending mode. The specimens were equipped by a crack propagation gauge (CPG) to monitor the crack growth. However, the presence of crack was detected late by these gauges. The deviation of the resonance frequency was therefore analysed from the time response measurements, and the results were confronted to the CPG measurements to conclude on the validity of the detection threshold provided by the literature.

A critical evaluation of indentation crack lengths in air

AbstractAn extensive overview is presented of Vickers indentation crack lengths in ceramics in air. Measurement of such crack lengths is one of the most common and powerful assessments of the fracture properties of ceramics and the overview provides a critical evaluation of observed behavior as functions of material type and indentation load, and an extensive basis for comparison of results from new materials and analyses. The overview considers single crystals, polycrystals, transforming materials, glasses, and multiphase materials, including cermets, glass‐ceramics, and tooth enamel. The coverage extends over structural and electronic ceramics, including oxides, carbides, nitrides, and titanates. The data are presented in a single format for ease of interpretation in terms of idealized indentation fracture and for inter‐material comparisons; most data are unique to this work, but the results of selected studies from the published literature are included. The overview considers the precision and accuracy of crack length measurements and demonstrates a simple quantitative evaluation and ranking scheme for ceramic fracture based on load‐adjusted crack length and cracking susceptibility. Indentation hardness and cracking threshold are also determined and related to the susceptibility. Material toughness is related to cracking susceptibility by fracture mechanics analyses: typical crack length measurements in air are shown to provide estimates of inert toughness with a relative uncertainty of ±50%.

Using acoustic emissions (AE) to monitor mode I crack growth in bonded joints

The usual way to evaluate the fracture toughness of bonded joints is via experimental characterization of the critical strain energy release rate. Different test procedures and data reduction methods for mode I fracture characterization can be found in the literature, such as ISO-25217, where crack length measurement is required. However, obtaining an accurate visual determination of crack length is often a challenge due to large fracture process zones (FPZ) and difficulties in reaching the bonded path. To compensate, structural health monitoring (SHM) techniques such as embedded Fiber Bragg Grating (FBG), digital image correlation (DIC), backface strain gauges and ultrasonic inspection are used as crack length monitoring. However, experimental work demonstrates the need for experimentation with non-intrusive methods. In the present work, acoustic emission (AE) testing is proposed to measure mode I crack growth in bonded joints. This is valid both for rigid adhesives with a small FPZ ahead of the crack tip and for flexible adhesives in which a correlation between AE event location and external analytical and numerical models confirm that a large FPZ is behind the crack tip. A correlation between the AE source location, visual location and numerical models determines the nature of AE events during the fracture process.

Location and Length Measurement of Invisible Fatigue Crack in Metal Components Using Wave Mixing Methods

Abstract For the invisible fatigue crack embedded in metal components, wave mixing methods are applied for the fatigue crack location and length measurement. When two incident waves meet within a component, a nonlinear interaction occurs and a third wave is generated, which is related to the damage degree of the meeting position of the two incident waves. When the delay time of incident waves changed in collinear wave mixing experiments, different meeting positions along the length direction of components were detected. The relationship between the sideband amplitude at difference or sum frequencies and delay time is obtained, and the delay time corresponding to the maximum amplitude is applied to locate the fatigue crack along the length direction. Then, adjusting the placing locations of transducers in the noncollinear wave mixing experiment, the meeting position of incident waves is controlled at the fatigue crack decided by the collinear wave mixing experiment. Next, changing the separation distance of two transmitters, the detection positions would be along the vertical direction of the components. The mixing nonlinear parameters of different detection positions are calculated, and the length of the fatigue crack can be measured in a vertical direction based on the distribution of the mixing nonlinear parameter.

Optimization of the DCPD technique for monitoring the crack propagation from notch root in localized plasticity

The Direct Current Potential Drop method is commonly used to monitor the crack growth during fatigue test when optical access to the crack is not possible. The reliability of this method depends on the calibration curve, which correlates the potential drop measurement with the crack length and shape. Using Finite Element Analysis and experimental approaches with heat tints, a procedure for the optimization of the calibration curve has been proposed. The influence of plastic deformation and of the presence of a secondary crack on the calibration curve has been studied. Finally, a test strategy, using two heat tints, has been developed in order to identify the first order parameters for the calculation of calibration curve. This strategy shows higher accuracy of crack length measurement and permit to balance experimental uncertainties out.

Low Velocity Impact of Bistable Laminated CFRP Composites

Laminated carbon fiber reinforced polymer (CFRP) composites have gained popularity in engineering applications due to their lightweight and strong in-plane mechanical properties. This material, however, performs poorly with respect to out-of-plane impact resistance. The objective of this research was to investigate the effect of bistability on the low velocity impact resistance of laminated CFRP composites. Bistable composites have the ability to alternate between two different geometric equilibria through external loading, and when unloaded, do not require external forces to maintain either geometry. Laminated CFRP composite specimens of bistable, flat symmetric, and curved symmetric configurations were subjected to low velocity impacts of 20 J. The acceleration and force of each impact was recorded using an accelerometer and a load cell mounted on the impactor. The sensor data was used to analyze the dynamic response and calculate the energy absorption of each impact. Post-impact crack length measurements and damage characterization were used to conduct a damage resistance analysis. The effects of geometry, stacking sequence, and bistability of the impacted specimens were determined. The results showed that bistability improves the low velocity impact damage resistance of laminated CFRP composites through increased energy absorption and specimen kinetic energy.