Fatigue Strength Of Metallic Materials Research Articles

Development of a Cavitation Generator Mimicking Pistol Shrimp.

Pistol shrimp generate cavitation bubbles. Cavitation impacts due to bubble collapses are harmful phenomena, as they cause severe damage to hydraulic machinery such as pumps and valves. However, cavitation impacts can be utilized for mechanical surface treatment to improve the fatigue strength of metallic materials, which is called "cavitation peening". Through conventional cavitation peening, cavitation is generated by a submerged water jet, i.e., a cavitating jet or a pulsed laser. The fatigue strength of magnesium alloy when treated by the pulsed laser is larger than that of the jet. In order to drastically increase the processing efficiency of cavitation peening, the mechanism of pistol shrimp (specifically when used to create a cavitation bubble), i.e., Alpheus randalli, was quantitatively investigated. It was found that a pulsed water jet generates a cavitation bubble when a shrimp snaps its claws. Furthermore, two types of cavitation generators were developed, namely, one that uses a pulsed laser and one that uses a piezo actuator, and this was achieved by mimicking a pistol shrimp. The generation of cavitation bubbles was demonstrated by using both types of cavitation generators: the pulsed laser and the piezo actuator.

Laser Cavitation Peening: A Review

During submerged laser peening using a pulsed laser, a bubble that behaves like cavitation is generated after laser ablation (LA). The bubble is referred to as laser cavitation (LC). The amplitude of the shockwave in water generated by LA is larger than that of LC; however, the impact passing through the target metal during LC is larger than that of LA. LC impact can be utilized for peening at optimized conditions. Thus, submerged laser peening is referred to as “laser cavitation peening”, as the peening method using the cavitation impact is known as “cavitation peening”. The impact induced by a hemispherical bubble is more aggressive than that of a spherical bubble with a microjet. Laser cavitation peening can improve the fatigue strength of metallic materials by producing work-hardening and introducing compressive residual stress. Three-dimensional additively manufactured metals (3D metals) such as titanium alloy are attractive materials for aviation components and medical implants; however, the fatigue strength of as-built components is nearly half of that of bulk metals, and this is an obstacle for the applications of 3D metals. In the present study, published research papers are reviewed to identify the key factors of laser cavitation peening, with additional visualization of LC and data. Then, improvements in the fatigue strength of metallic materials, including 3D metals treated by laser cavitation peening, are summarized.

On the evaluation of overload effects on the fatigue strength of metallic materials

In this paper, experimental and numerical studies are performed to quantify the influence of overloads on the fatigue behaviour of the quenched and tempered steel 42CrMoS4 and the wrought aluminium alloy EN AW-6082 T6. The main objective is to incorporate the influence of overloads in the fatigue assessment. Three series of fatigue tests per material were performed, each under load control conditions, at R ═—1:i) constant amplitude loading (CAL) tests with no overloads; ii) CAL tests after five overload cycles of the magnitude OL 1; iii) CAL tests after five overload cycles of the magnitude OL 2. The respective overload levels are selected to achieve 75 % of the static strength of the test samples either estimated according to the FKM Guideline (2020) for OL 1 or determined directly by specimen testing for OL 2. During the overload cycles, the local notch strains were monitored by means of the digital image correlation (DIC) technique. X-ray diffraction was applied to quantify the residual stresses in the specimens due to manufacturing, after applying overload cycles, as well as after completing tests on runouts. Fatigue test results demonstrate that, depending on the material and the overload level, the fatigue strength can either decrease or increase or, in some cases, be only slightly affected by overloads. To explain the experimental findings, residual stresses determined from both experimental measurements and elastic-plastic finite-element analyses are included in the calculations of the notch stresses and strains and a fatigue damage parameter. The latter is then involved in the damage calculations to quantify the effect of overloads in terms of the magnitude and number of cycles to failure.

Neural Network Modeling of the Fatigue Strength of Metal Materials at Low Temperatures

An intelligent system has been developed to predict the fatigue strength of metallic materials over a wide temperature range. A neural network, properly trained, is a model of a dynamic system of fatigue failure of a part and is able to predict the values of the number of loading cycles to failure, as well as the onset of formation and growth rate of fatigue cracks for various test conditions, including at low temperatures.

A test method for manufacturing effect assessment in HCF

Surface conditions produced by manufacturing processes are known to significantly affect the fatigue strength of metallic materials. However, fatigue tests on planar specimens usually present the onset of fatigue crack in a specimen edge, where the superposition of different machining techniques and subsequent machining operations can hide the effects of the studied process. This study deals with the design of a fatigue test able to reliably assess the effects on the high cycle fatigue strength of any machining process used to produce planar surfaces. It was devised a test configuration where the specimen geometry and the loading conditions are aimed to locate the crack nucleation in the middle of the surface machined by the investigated process. This condition is achieved by employing a notched specimen and a three-point bending test configuration, where the position and geometry of the notches are designed to relieve the specimen’s edges. Nevertheless, in the critical region, the stress state is quite uniaxial and uniform in the longitudinal direction. The specimen is manufactured by using the same parameters and processes adopted for full-scale components, thus producing a surface with the same properties as the real component. The designed test configuration was validated in a high cycle fatigue test aimed to compare the effects of broaching and wire electrical discharge machining on the Wöhler curve (S-N curve). The specimen geometry was tailored for a broaching process so that it has sufficient stiffness both in the directions parallel and perpendicular to the broaching direction. Fatigue tests were carried out on a resonant machine, that allowed to detect the crack nucleation by monitoring the test frequency.

Cavitating Jet: A Review

When a high-speed water jet is injected into water through a nozzle, cavitation is generated in the nozzle and/or shear layer around the jet. A jet with cavitation is called a “cavitating jet”. When the cavitating jet is injected into a surface, cavitation is collapsed, producing impacts. Although cavitation impacts are harmful to hydraulic machinery, impacts produced by cavitating jets are utilized for cleaning, drilling and cavitation peening, which is a mechanical surface treatment to improve the fatigue strength of metallic materials in the same way as shot peening. When a cavitating jet is optimized, the peening intensity of the cavitating jet is larger than that of water jet peening, in which water column impacts are used. In order to optimize the cavitating jet, an understanding of the instabilities of the cavitating jet is required. In the present review, the unsteady behavior of vortex cavitation is visualized, and key parameters such as injection pressure, cavitation number and sound velocity in cavitating flow field are discussed, then the estimation methods of the aggressive intensity of the jet are summarized.

Effect of Various Peening Methods on the Fatigue Properties of Titanium Alloy Ti6Al4V Manufactured by Direct Metal Laser Sintering and Electron Beam Melting.

Titanium alloy Ti6Al4V manufactured by additive manufacturing (AM) is an attractive material, but the fatigue strength of AM Ti6Al4V is remarkably weak. Thus, post-processing is very important. Shot peening can improve the fatigue strength of metallic materials, and novel peening methods, such as cavitation peening and laser peening, have been developed. In the present paper, to demonstrate an improvement of the fatigue strength of AM Ti6Al4V, Ti6Al4V manufactured by direct metal laser sintering (DMLS) and electron beam melting (EBM) was treated by cavitation peening, laser peening, and shot peening, then tested by a plane bending fatigue test. To clarify the mechanism of the improvement of the fatigue strength of AM Ti6Al4V, the surface roughness, residual stress, and surface hardness were measured, and the surfaces with and without peening were also observed using a scanning electron microscope. It was revealed that the fatigue strength at N = 107 of Ti6Al4V manufactured by DMLS was slightly better than that of Ti6Al4V manufactured by EBM, and the fatigue strength of both the DMLS and EBM specimens was improved by about two times through cavitation peening, compared with the as-built ones. An experimental formula to estimate fatigue strength from the mechanical properties of a surface was proposed.

Comparison between Shot Peening, Cavitation Peening, and Laser Peening by Observation of Crack Initiation and Crack Growth in Stainless Steel

The traditional technique used to modify the surface of a metallic material is shot peening; however, cavitation peening, a more recent technique in which shot is not used, was developed, and improvements in the fatigue strength of metallic materials were demonstrated. In order to compare the fatigue properties introduced by shot peening with those introduced by cavitation peening, crack initiation and crack growth in specimens of austenitic stainless steel (Japanese Industrial Standards JIS SUS316L) treated using these techniques were investigated. With conventional cavitation peening, cavitation is produced by injecting a high speed water jet into water. In the case of submerged laser peening, bubbles are generated using a pulsed laser after laser ablation, and the impact produced when the bubbles collapse is larger than that due to laser ablation. Thus, in this study, cavitation peening using a water jet and submerged laser peening were investigated. To clarify the mechanisms whereby the fatigue strength is improved by these peening techniques, crack initiation and crack growth in specimens with and without treatment were examined by means of a K-decreasing test, where K is the stress intensity factor, and using a constant applied stress test using a load controlled plane bending fatigue tester. It was found that the improvement in crack initiation and the reduction in crack growth were roughly in a linear relationship, even though the specimens were treated using different peening methods. The results presented here show that the fatigue strength of SUS316L treated by these peening techniques is closely related to the reduction in crack growth, rather than crack initiation.

Improvement of Fatigue Strength of Additive Manufactured Metals by Solid-Liquid-Gas Interfacial Phenomena Induced by Pulse Laser

Although additive manufactured (AM) metals are attractive materials, the fatigue strength of AM metals are considerably weak comparing with that of wrought materials. The mechanical surface treatment such as shot peening can improve the fatigue strength of metallic materials. Recently, a novel mechanical surface treatment using solid-liquid-gas interfacial phenomena induced by pulse laser has been developed. In the present paper, in order to demonstrate the improvement of fatigue strength of AM metals by solid-liquid-gas interfacial phenomena, titanium alloy Ti6Al4V manufactured by electron beam melting EBM was treated by submerged pulse laser and tested by a plane bending fatigue test. The key factors were investigated by evaluating the relation between the fatigue properties and mechanical properties of the surface treated by submerged laser peening, cavitation peening and shot peening.

A Method for Predicting the Effects of Specimen Geometry and Loading Condition on Fatigue Strength

Specimen geometry and loading condition usually have a great influence on the fatigue strength of metallic materials, which is an important issue in evaluating the reliability of component parts. In this paper, a rotating bending fatigue test is performed at first on an hourglass specimen and a notch specimen of a high strength titanium alloy. Experimental results indicate that, in terms of local stress, the notch specimen endures higher fatigue strength in comparison with the hourglass specimen due to its relatively smaller control volume. Then, a probabilistic control volume method is proposed for correlating the effects of specimen geometry and loading condition on the fatigue strength based on Weibull distribution and the concept of control volume. A simple formula is obtained for the fatigue strength in relation to control volumes, in which the parameter is the shape parameter of Weibull distribution of fatigue strength. The predicted results are in good agreement with the present experimental data for high strength titanium alloy and the data for the high strength steel and the full scale EA4T axle in the literature.

An optimization criterion for fatigue strength of metallic materials

In this study, a criterion for optimizing the fatigue strength of metallic materials is proposed, and verified by many experimental results. The strategy for optimizing the fatigue strength is to continuously increase the ultimate tensile strength (UTS) under the condition of keeping certain work-hardening abilities. It is found that the relationship between the true UTS and true uniform elongation can be described by two straight lines, and the optimal fatigue strength can be obtained at the point where the two straight lines intersect with each other. This study has great guiding significance for economizing time and money to optimize the fatigue strength of metallic materials.

Construction of Electronic Factual Database on Very High Cycle Fatigue Properties for Structural Metallic Materials

As a joint project of Committees on Fatigue and Reliability Engineering in the Society of Materials Science, Japan (JSMS), an electronic database on fatigue strength of metallic materials fabricated in Japan had been constructed and published in 1996. Book style of the same data compilation had been published at the same time by the JSMS and Elsevier. About twenty years have passed since the above publications of database and databook. Thus, a lot of new fatigue test data have been obtained during such a long period including many data on the very high cycle fatigue such as gigacycle regime. Based on such a circumstance, the JSMS has organized a new project to construct an electronic database on very high cycle fatigue. A lot of numerical data obtained by fatigue tests would be compiled together with many photographs of fracture surfaces. In this paper, fundamental view and compilation concept of the database are briefly introduced by showing a typical example of proto-type database constructed from domestic data in Japan.

Movement of Dislocations in the Sub-Surface of a Polycrystalline Metal by Cavitation Peening Observed by Transmission Electron Microscopy

The impact produced when cavitation bubbles collapse can be utilized to modify surfaces in the same way as shot peening and it is called cavitation peening (CP). CP is one of a number of surface modification techniques used to improve the fatigue strength of metallic materials by introducing compressive residual stress. Although it has been shown by an X-ray diffraction method that CP decreases the micro-strain related to dislocations in the sub-surface of a polycrystalline material, the mechanism for this decrease is unclear. In this paper, the movement of dislocations by CP was observed using transmission electron microscopy (TEM).

A Study on Estimation of <i>S</i>-<i>N</i> Curves for Structural Steels Based on their Static Mechanical Properties

The aim of this study is to develop a statistical estimation method of S-N curve for iron and structural steels by using their static mechanical properties. In this study, firstly, the S-N data for pure iron and structural steels were extracted from "Database on fatigue strength of Metallic Materials" published by the Society of Materials Science, Japan (JSMS) and S-N curve regression model was applied based on the JSMS standard, "Standard Evaluation Method of Fatigue Reliability for Metallic Materials -Standard Regression Method of S-N Curve-". Secondly, correlations between regression parameters and static mechanical properties were investigated. As a result, the relationship between the regression parameters and static mechanical properties (e.g. fatigue limit E and static tensile strength σB) showed strong correlations, respectively. Using these correlations, it is revealed that S-N curve for iron and structural steels can be predicted easily from the static mechanical properties.

General relation between tensile strength and fatigue strength of metallic materials

With the development of high-strength materials, the existing fatigue strength formulae cannot satisfactorily describe the relation between fatigue strength σw and tensile strength σb of metallic materials with a wide range of strength. For a simple but more precise prediction, the tensile and fatigue properties of SAE 4340 steel with the tensile strengths ranging from 1290MPa to 2130MPa obtained in virtue of different tempering temperatures were studied in this paper. Based on the experimental results of SAE 4340 steel and numerous other data available (conventional and newly developed materials), through introducing a sensitive factor of defects P, a new universal fatigue strength formula σw=σb(C−Pσb) was established for the first time. Combining the variation tendency of fatigue crack initiation sites and the competition of defects, the fatigue damage mechanisms associated with different tensile strengths and cracking sites are explained well. The decrease in the fatigue strength at high-strength level can be explained by fracture mechanics and attributed to the transition of fatigue cracking sites from surface to the inner inclusions, resulting in the maximum fatigue strength σwmax at an appropriate tensile strength level. Therefore, the universal fatigue strength formula cannot only explain why many metallic materials with excessively high strength do not display high fatigue strength, but also provide a new clue for designing the materials or eliminating the processing defects of the materials.

Analytical Results of Fatigue Reliability for Structural Steels Based on “Database on Fatigue Strength of Metallic Materials”

The fatigue properties of the structural steels were investigated based on “Database on Fatigue Strength of Metallic Materials” published by the Society of Material Science, Japan. The rotating bending fatigue test results for pure iron and structural steels JIS S10C-S58C were extracted from the database and hyperbola regression model was applied based on the JSMS standard, “Standard Evaluation Method of Fatigue Reliability for Metallic Materials-Standard Regression Method of S-N Curve-”. The correlations between obtained regression parameters were investigated and it is revealed that there were strong correlation between (1) tensile strength σB and fatigue limit E, (2) slope A and intercept B of inclined part of S-N curve, (3) slope A and the critical number of stress cycles giving fatigue limit Nw. In addition, a number of fatigue data extracted were normalized by tensile strength of σB and pooled altogether. Using these data sets, the P-S-N property was also analyzed and it is confirmed that the fatigue life distribution shows complex pattern and changes at each stress level for evaluation, while fatigue strength distribution shows relatively simple pattern and were approximately identical at different number of cycles.

日本材料学会疲労強度データベース事業の歴史的経緯·到達点と今後の展開

日本材料学会疲労強度データベース事業の歴史的経緯·到達点と今後の展開

Historical Background, Attainment and Future Prospects of JSMS Databases on Fatigue Strength of Metallic Materials

Historical Background, Attainment and Future Prospects of JSMS Databases on Fatigue Strength of Metallic Materials

日本材料学会疲労強度データベース事業の歴史的経緯・到達点と今後の展開

日本材料学会疲労強度データベース事業の歴史的経緯・到達点と今後の展開

Historical Background, Attainment and Future Prospects of JSMS Databases on Fatigue Strength of Metallic Materials

Historical Background, Attainment and Future Prospects of JSMS Databases on Fatigue Strength of Metallic Materials