Machined Notch Research Articles

Fatigue behavior assessment of heavy-duty freight railway axles under different heat treatments

The safety of heavy-duty freight railway axles is threatened by defects, including inclusions, corrosion pits, fretting cracks and foreign object impact damages. Accordingly, surface strengthening techniques are applied to reduce the detrimental effect of defects on the structural integrity of railway axles. This work studied the effectiveness of heat treatment, in which the specimens machined from a heavy-duty freight railway axle were induction hardened under three types of processing parameters and tempered. The fatigue strengths of specimens with electrical discharge machined (EDM) notches and impact craters on their surfaces were tested. The fatigue behavior of induction-hardened (IH) specimens was compared with that of as-received (AR) specimens and fully-quenched (FH) specimens. The fatigue limit of smooth IH specimens is close to that of FH specimens while almost twice as much as that of AR specimens. Inclusions were found on the crack initiation sites on the surface or subsurface of AR and FH specimens. For IH specimens, the crack initiation sites moved to the zone between the case and the core. Fatigue strengths of IH and FH specimens decline with increasing EDM notch size but remain higher than AR specimens’. The influence of impact damage on fatigue properties depends on damage size and heat treatment process.

Concept of “chemical notch” to study stress-corrosion

Abstract In this article, a new partially immersed slow strain rate (SSR) and corrosion-fatigue (CF) test methods are described and discussed. Partial immersion of a specimen in 3.5 % NaCl solution creates an interface between the solution and air. We call this a “chemical notch” stress concentration factor. The outcome of this demonstrates that the SSR test condition on a smooth specimen will fail at the interface if the machined notch stress concentration factor is less than the “chemical notch” stress concentration factor. This “chemical notch” concept can be applied to correlate corrosion-fatigue S–N data for a specific material/environment system.

Sulphide stress corrosion in SAE 1070 steel wire used in the tensile armour of flexible pipelines: Effect of induced superficial defects

This research aims to investigate and quantify potential damage resulting from induced superficial defects on the surface of wires use on tensile armor of flexible pipes. Different values of kt factors and H2S concentration in aggressive gas mixtures with CO2, were analysed to determine the operational conditions that do not result in rupture of the tensile reinforcement during service, under the conditions studied. The tests were based on NACE TM0316 standard, using a 4-point bend test. Two superficial notches were created, which generated known stress concentration factor Kt values of 1.5 and 2.0, and three gaseous concentrations (220, 700, and 1100 ppm of H2S in CO2) were used. After bending tests, the samples were evaluated by metallographic and fractographic analysis, weight loss and liquid penetrant inspection. The results were then analysed using analysis of variance (ANOVA). Only the samples exposed to 220 ppm of H2S in CO2 did not rupture at both Kt values. However, for the Kt value of 2.0, cracks were observed in the longitudinal direction of the wires, originating from the machined notch. Additionally, the samples exposed to 700 ppm and 1100 ppm of H2S had a significantly higher damage score (approximately 15x) than the samples exposed to 220 ppm, regardless of the Kt value.

A new methodology of non-destructive testing in a set of two parallel immersed plates using time reversal process and leaky Lamb waves

Ultrasound has been identified as a suitable method for inspecting complex structures (such as nuclear reactors), as it can propagate through both liquids and steel elements to be inspected. Leaky Lamb waves have been extensively researched to detect damages in plate-shaped structures, which are waves that reemit bulk waves towards the surrounding fluid. An imaging method based on the time reversal of Lamb waves has been applied in preliminary work to detect the edge of a single immersed plate and a machined notch. In this study, the frequency-domain topological energy method is applied to a set of two parallel immersed plates to succeed in accurately localizing defects represented by the edges of the plates. Firstly, the behaviour of leaky Lamb waves is briefly addressed. A 2D-STFT (two-dimensional Short Time Fourier Transform) is then used to separate the different Lamb modes, which is a powerful tool for tracking the modes involved in the propagation over time. The Fast Topological Imaging Method (FTIM), which allows for faster computations with less data storage, is applied to the case of Lamb waves in a set of parallel plates. Lamb modes can be identified then selected to improve the analysis of the obtained images. An original methodology is proposed based on spots (maxima) analysis for interpreting the results of this topological energy and identifying the edges of the two plates. This process could be applied, in the same manner, to other scattering defects in ultrasound non-destructive testing of plates.

Determination of shear strength of additively manufactured poly lactic acid/flax fibre bio-composite via the iosipescu test

The need to reduce CO2 emissions is leading to the search for alternatives to the manufacture of products from petrochemical sources. One of these alternatives is polylactic acid a bio-sourced, and biodegradable material, which is expected to be one of the most widely used materials in the forthcoming years. Recently, the feasibility of enhancing the mechanical characteristics of the PLA, by adding the flax fibre was demonstrated. Flax fibers are becoming an ideal alternative in reinforced polymer composites, thanks to the energy-intensive processing required to elaborate synthetic fibers. Flax fibers are not only recyclable and bio-degradable but also have excellent mechanical characteristics. However, the mechanical properties of flax fibre-reinforced polylactic acid composites, obtained by additive manufacturing techniques, are not well known. In the first part of this study, the quality of the Iosipescu shear experiment is examined with full-field strain measurements. In the second part, the in-plane shear response of manufactured flax fibre reinforced polylactic acid composites notched with the machining process and additive manufacturing technique are compared. This last makes it possible to evaluate the influence of the printing conditions and in particular the shell layers (contour) on the mechanical characteristics. It was perceived that the fully 3D printed specimens had a greater shear strength (+50.47%) and shear modulus (+44.68 %) than machined notch specimens. The fully 3D printed specimens count with the presence of a contour that rounds the vertices of the notches, reducing the stress-concentration factor, whereas in machined notch specimens, such contour was removed.

Surface breaking crack sizing method using pulse-echo Rayleigh waves

Surface cracks are common in various industries. Eddy current testing (ECT) is commonly used for crack sizing but necessitates complex calibration standards and a highly trained inspector. Moreover, for large-area inspections, it requires additional scanning arrangements. In recent years the wedge technique-based Rayleigh wave crack sizing method has attracted significant research interest due to its unidirectional excitability. However, Rayleigh wave features generated at crack tips are often weak and masked under noise, and they mostly attenuate before reaching the receiving probe due to the couplant between the wedge-test specimen interface. Consequently, sizing the crack depth is difficult using a pulse-echo setup. This work presents a wedge-free pulse-echo Rayleigh wave method for surface crack sizing using a conventional phased array transducer. Eliminating the wedge removes a couplant layer leading to lower attenuation, enabling the transducer to capture crack tip features. This allows the sizing of surface cracks in pulse-echo using the time-of-flight (ToF) information. Furthermore, leveraging the phased array system, an averaging technique employed to the time trace signals captured by the transducer elements effectively averages out the other wave modes generated at crack geometries by the scattering of Rayleigh waves. This significantly minimizes sizing errors and enhances the signal-to-noise ratio (SNR). The performance of the proposed method is demonstrated through finite element simulations and experiments. Experiments with electric discharged machined (EDM) notches on test specimen surface at various angles and depths mimicking surface-breaking cracks show accurate sizing within a 5% error. The proposed method offers flexibility in performing inspections using a wide frequency range and can be easily applied to different materials using any conventional phased array transducer. This enhances its adaptability for industrial applications in the characterization of surface cracks.

Damage Detection in a Polymer Matrix Composite from 4D Displacement Field Measurements.

Standard Digital Volume Correlation (DVC) approaches enable quantitative analyses of specimen deformation to be performed by measuring displacement fields between discrete states. Such frameworks are thus limited by the number of scans (due to acquisition duration). Considering only one projection per loading step, Projection-based Digital Volume Correlation (P-DVC) allows 4D (i.e., space and time) full-field measurements to be carried out over entire loading histories. The sought displacement field is decomposed over a basis of separated variables, namely, temporal and spatial modes. In the present work, the spatial modes are constructed via scan-wise DVC, and only the temporal amplitudes are sought via P-DVC. The proposed method is applied to a glass fiber mat reinforced polymer specimen containing a machined notch, subjected to in situ cyclic tension and imaged via X-ray Computed Tomography. The P-DVC enhanced DVC method employed herein enables for the quantification of damage growth over the entire loading history up to failure.

Progress in Evaluation of Deep Artificial Defects from Sweep-Frequency Eddy-Current Testing Signals.

The article discusses the practical application of the method of electromagnetic non-destructive investigation of austenitic materials. To identify and evaluate deep artificial defects, the sweep-frequency eddy current method with harmonic excitation is used. The objects of interest are the surface electric-discharged machined notches, with a defined geometry, fabricated in a plate with a thickness of 30 mm. An innovative eddy current probe with a separate excitation and detection circuit is used for the investigation. The achieved results clearly demonstrate the robustness and potential of the method, especially for deep defects in thick material. By using the fifth probe in connection with the frequency sweeping of eddy currents, it is possible to reliably detect artificial defects up to 24 ± 0.5 mm deep by using low-frequency excitation signals. An important fact is that the measuring probe does not have to be placed directly above the examined defect. The experimental results achieved are presented and discussed in this paper. The conducted study can serve, for example, as an input database of defect signals with a defined geometry to increase the convergence of learning networks and for the prediction of the geometry of real (fatigue and stress-corrosion) defects.

Control of airflow induced vibrations on a cantilever beam by actuating on the exposure of its machined surface notch pattern

Control of airflow induced vibrations on a cantilever beam by actuating on the exposure of its machined surface notch pattern

Characterization of glass fiber reinforced polymer via Digital Volume Correlation: Investigation of notch sensitivity

The results of in-situ cyclic tensile experiments performed on two dogbone specimens made of glass fiber mat reinforced polyester resin are presented (the second specimen contains a machined rectangular notch). The experimental data were obtained by using X-ray Computed Tomography. The reconstructed volumes were analyzed via Digital Volume Correlation. The investigated material was notch-insensitive since both specimens failed at equal stress levels. To further confirm this hypothesis, and to study strain–damage interactions, the major eigen strain and correlation residual maps of both specimens were analyzed. Even in the first loading cycle, an inner strained band extending through the whole ligament area developed within both specimens, already indicating the path to final fracture. It is shown that the final failure of the studied material was primarily driven by the yarn mesostructure, i.e., the influence of the underlying heterogeneities prevailed over the effects due to specimen machining and/or geometric singularity.

Dynamic Inspection of Surface-Breaking Defects using Induction Thermography

In comparison to other active thermography techniques, induction thermography can provide instantaneous results, is not affected by material emissivity/reflectivity and is indifferent to surface geometry. These advantages make it a promising technique to be applied for in-line/dynamic inspections. In this work, the thermal signatures from a titanium sample under different excitation parameters and moving speeds will be studied. To simulate surface-breaking cracks, electrical discharge machining (EDM) notches of varying lengths, widths and depths were made on the titanium sample. As with most applications, only a single side is accessible for inspection. Therefore, induction thermography in the reflection mode will be utilized in this work. After evaluating the signal-to-noise ratio (SNR) under various conditions and inspection speeds, the optimized configuration will be presented. The findings will be beneficial to researchers interested in implementing induction thermography in an in-line/dynamic inspection setting.

The use of circumferential guided waves to monitor axial cracks in pipes

Guided wave testing is in routine use to detect corrosion, particularly in the oil and gas sector, but the detection of axial cracking is difficult using the axially propagating waves commonly used for corrosion detection. This paper presents a novel guided wave monitoring technique for the detection of axial cracking in piping. Circumferential SH0 waves, travelling around the circumference were used in a monitoring configuration to detect axial defects in a 6 inch schedule 80 steel pipe. Finite Element (FE) analysis showed that both the defect reflection and the decline in the transmitted SH0 wave can be used for defect detection. Baseline subtraction was utilised to produce residual signals that can be better analysed for small defect reflections. The Root-Mean-Square (RMS) of the residual signal after baseline subtraction was found to be the most satisfactory means of monitoring defect progression. An amplification effect was identified where the residual signal is compounded by continued interaction with the defect on each revolution of the incident wave. The FE predictions were validated by an experiment in which an Electrical Discharge Machining (EDM) notch was grown in four stages. Temperature compensation using Location Specific Temperature Compensation (LSTC) was applied to the experimental data, allowing residuals to be compared for a ∼10 °C temperature swing over a monitoring period of over 2 months. It was determined that a 10 mm long (∼23% of wavelength), 5 mm deep (∼45% of the wall thickness) defect at an axial offset from the line of the transducers of 1.5 wavelengths (65 mm) would be readily detectable with a very high Probability of Detection (POD) and virtually no chance of a false call. Therefore, this novel guided wave monitoring system for axial crack detection is likely to be attractive for applications in a range of industries for its sensitivity to axial cracking combined with large coverage from a single transducer location.

Effect of flaw orientation on magnetic flux leakage and remote field eddy current inspection of small diameter steel tubes

ABSTRACT This paper presents effect of flaw orientation on magnetic flux leakage (MFL) and remote field eddy current (RFEC) inspection of small diameter (19.05-mm outer diameter, OD and 2.77-mm wall thickness, WT) seamless low carbon steel tubes. Artificial electro-discharge machined (EDM) notches in three carbon steel Tubes A (orientations from 0° to 90° in steps of 10° and 50% WT depth), B (70° orientation and depths from 10% WT to 100% WT in steps of 10% WT) and C (50° orientation and depths from 10% WT to 60% WT in steps of 10% WT) were used to investigate the detectability of notches with different orientations and depths. Studies on different notch orientations reveal that RFEC technique detected all the notches with good signal-to-noise ratio (SNR), while MFL technique detected the notches with orientations from 40° to 90°. The effect of notch orientation is observed more in MFL technique as compared to RFEC technique. Further, the effects of scanning speed and notch depth for the detection of oriented notches were studied. Studies reveal that the effect of scanning speed is also more (40 times) dominant in MFL technique as compared to RFEC technique. Studies also reveal that the scanning effect is again dependent upon the notch depth. For 70° oriented notches, MFL technique is found to be more sensitive to notch depth as compared to RFEC technique. For 50° oriented notches, RFEC technique is found to be more sensitive to notch depth as compared to MFL technique.

Coalescence Behavior of Multiple Cracks in SUS304 Plate with under Creep Rupture Conditions

A creep test was conducted to clarify the coalescence behavior of cracks in a SUS304 flat plate with multiple cracks on different planes. The test temperature is 600 degrees. The shape of the specimen is a plate, and electrical discharge machined (EDM) notches were introduced on both sides of the central smooth part. Then, a fatigue pre-crack was generated at the tip of the notch. The position of the crack was expressed by the distance between the crack surfaces and the distance between the crack tips. As a result of the experiment, the cracks coalesced when the distance between the crack planes was shorter than about 3 mm. On the other hand, when the distance between the crack surfaces was about 7 mm, no crack coalescence was observed, and there was a change point in the crack coalescence behavior. It was found that the change point of the coalescence behavior of the crack obtained in the creep test satisfied codes for nuclear power generation facilities (JSME S NA1-2019). As a result of fracture surface observation, it was found that the crack growth from the fatigue pre-crack due to creep is due to grain boundary rupture regardless of the presence or absence of crack coalescence.

Fracture Strength and Crack Coalescence Behavior of SUS304 Plate with Multiple Cracks

Tensile tests were conducted at room and high temperature to clarify the fracture strength and crack coalescence behavior of multiple cracks in SUS304 flat plates. The shape of the specimen is a plate, and electrical discharge machined (EDM) notches were introduced on both sides of the central smooth part. Then, a fatigue pre-crack was generated at the tip of the notch. The position of the crack was expressed by the distance between the crack surfaces and the distance between the crack tips. As a result of the tensile test at room temperature, when the distance between the crack surfaces was shorter than about 8 mm, the cracks coalesced. On the other hand, when the distance between the crack surfaces was about 10 mm, no crack coalescence was observed, and there was a change point in the crack coalescence behavior. As a result of the high temperature test, crack coalescence was observed when the distance between the crack planes was about 8 mm, but the change point of the crack coalescence behavior is unknown because there is no data. It was found that the change points of the coalescence behavior of the cracks obtained in the experiment satisfy codes for nuclear power generation facilities (JSME S NA1-2019).

POD of NDT Techniques Using High Temperature Oxidized Fatigue Cracks in an Aero Engine Alloy

Establishing probability of detection (POD) or reliability of various nondestructive testing (NDT) techniques is essential for implementing damage tolerant (DT) methodology for aero-engines. This POD is usually established with the help of a large number of service expired aero-engine components containing several fatigue cracks. In the absence of such components, artificial defects such as electrical discharge machining (EDM) notches or starter cracks were explored. However, such artificial defects would not meet the key features such as tightness of the fatigue cracks and the possible oxidation in the crack opening and thus, limiting their usage. Therefore, in the current study, an innovative approach of generating fatigue cracks at 650 °C (~ typical aero-engine service temperatures) with key high temperature service degradation aspects of oxidation and fatigue cracking is demonstrated for the first time using Gleeble® test system. Further, POD is estimated by inspecting these laboratory generated fatigue cracks using fluorescent liquid penetrant technique (FLPT) and eddy current technique (ECT) under HIT (defect detected) vs. MISS (defect not detected) and â (signal response) vs. a (crack size) methodologies. The current study also discusses a statistical approach of random generation of crack sizes for use in NDT reliability analysis. In addition, an attempt has been made to understand the effect of a90/95 values on remnant life calculations. It is concluded that the eddy current response of oxidized fatigue cracks results in better (high sensitive) a90/95 values compared to the eddy current response obtained from non-oxidized fatigue cracks.

Elevated Temperature Notch Sensitivity of Inconel 718 Manufactured by Selective Laser Melting

Notched alloy 718 tensile specimens prepared by selective laser melting (SLM) and wrought bar were tested at 550, 650, and 750 °C. SLM specimens were tested with either an as-printed or a machined notch. For both types there was a decrease in strength with an increase in test temperature; however, the as-built notches are more sensitive to increases in test temperature than the machined notches. At 550 °C, the machined notch exhibits a roughly 20% higher tensile strength over as-built, while at 750 °C there is a 75% increase in strength. The notched strength ratio decreased with temperature for both types of SLM notches samples, while for wrought samples it increased. The temperature dependence of the difference in strength between the two SLM samples indicates that surface roughness—and the corresponding stress intensity factor—are not the only influences on notched strength. Fractography of tested specimens showed a difference in the intergranular crack propagation depth between as-built and machined notches, demonstrating a difference in the environmental susceptibility based on notch preparation that is due to microstructural differences arising in the SLM process that are preserved through heat treatment.

An Algorithm for Automated Feature Detection of Eddy Current Defects Using Sum of Gaussian Method

This article describes an algorithm used to automatically detect differential coil eddy current (EC) defect signal features using a sum of Gaussians approach. Defect signals are decomposed into positive and negative portions then a thresholded binary image is constructed using the Parzen-Rosenblatt window technique. Mathematical morphology operators are then used to denoise the thresholded binary image. Peaks are detected in the denoised binary images using a simplified variant of the <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">K</i> -means algorithm. The resulting parameters obtained are input into a constrained optimization algorithm to obtain an estimate of the denoised EC defect signal using a minimum summed-square error objective function. The algorithm performance is investigated over a range of electrostatically discharged machine (EDM) notch return signal windows that vary in coil diameter, test material, test frequency, and number of defects in the investigated region.

Stir zone stress corrosion cracking behavior of friction stir welded AA7075-T651 aluminum alloy joints

Abstract AA7075 high-strength aluminum alloy, which has many applications in the aircraft, marine and automobile industries, happens to be susceptible to stress corrosion cracking (SCC) when exposed to corrosive environments, resulting in reduced service life of the components. Inappropriate fabrication processes may augment this behavior. The fabrication of AA7075 components using conventional fusion welding processes may produce defects that include hot cracking and porosity. Friction stir welding (FSW) is a solid-state joining process that can avoid these problems and being widely used for components made of aluminum alloys. Because the joining occurs at a temperature that is lower than the melting point of the material, solidification cracking defects can be eliminated. This study investigates the SCC behavior of FSW AA7075-T651 joints. Horizontal-type SCC test was conducted on circumferential-notched tensile (CNT) specimens exposed to 3.5 wt. % NaCl solutions under various axial stress conditions. The different regions of the fractured specimens, such as the machined notch, SCC region and region of ultimate mechanical failure were analyzed by scanning electron microscopy (SEM) to establish the mechanism of SCC. The threshold stress of parent metal (PM) and stir zone (SZ) of the FSW joint were found to be 242 and 175 MPa, respectively.

Effect of Precrack Configuration and Lack-of-Fusion on the Elastic-Plastic Fracture Toughness of Additively Manufactured Ti-6Al-4V parts.

A comparison between fatigue precracked and sharp-notched Charpy-type fracture toughness specimens is presented for characterizing the elastic-plastic fracture toughness of Ti-6Al-4V parts (produced by electron beam melting, a powder bed fusion method). The effects of processing and postprocessing conditions on crystallographic texture, grain morphology, and elastic-plastic fracture toughness of additively manufactured Ti-6Al-4V parts are currently under investigation at the National Institute of Standards and Technology (NIST) in Boulder, Colorado. The specimens tested in this work were subjected to hot isostatic pressing (HIP) for 2 h at 900°C and 100 MPa in Argon environment (sub-β transus HIP), which is a commercial postprocessing step known to seal internal porosity in additively manufactured Ti-6Al-4V parts. Lack-of-fusion (LoF) pores were still visible after HIP treatment. These specific pores were exposed to the external surfaces and were thus immune to HIP treatment. In this work, the following variables and their effects on room temperature fracture toughness (measured by means of three-point-bending unloading compliance tests on Charpy-type specimens) were specifically addressed: notch configuration (fatigue precrack vs. sharp electrodischarge machining notch), specimens directly attached to the build plate (nonsupported) vs. connected to the build plate using standard thin wafer supports (supported), and LoF content. The results of this preliminary investigation will guide the choices for the remaining fracture toughness characterization of Ti-6Al-4V under various processing and postprocessing conditions.