Cyclic Indentation Tests Research Articles

Characterization of the precipitation behavior and resulting mechanical properties of copper-alloyed ferritic steel

From literature, it is well known that Cu precipitates in steels lead to an increase of hardness and quasi-static strength. However, the relation between Cu precipitates and cyclic mechanical properties has been investigated rather limited. Consequently, in the present work the evolution of Cu precipitates in a copper-alloyed low carbon, ferritic steel, aged at 550 °C, 600 °C and 650 °C for various times, and their influence on the resulting static and cyclic material properties were analyzed. Three-dimensional atom probe tomography (3DAPT) provided evidence that the ageing treatment led to the formation of Cu precipitates with different sizes and distribution. The existence of copper precipitates resulted in an increase of the ultimate tensile strength and yield strength of the investigated ferritic steel. Moreover, cyclic indentation tests were performed to investigate the influence of the ageing treatment and, hence, Cu precipitates, on hardness and especially cyclic properties of the material. The present results show a maximum of hardness and cyclic hardening potential at sizes of the precipitates of around 2.2 nm. Additionally, synchrotron X-ray diffraction (SYXRD) measurements were in a first approach proved to be viable for investigating the crystallographic structure of Cu precipitates within ferritic steels. Considering the evolution of Cu precipitates, determined with synchrotron as well as atom probe, a high dependency of quasi-static and cyclic properties on Cu precipitates could be demonstrated.

Influence of the Chemical Composition of the Used Powder on the Fatigue Behavior of Additively Manufactured Materials

To exploit the whole potential of Additive Manufacturing (AM), a sound knowledge about the mechanical and especially cyclic properties of AM materials as well as their dependency on the process parameters is indispensable. In the presented work, the influence of chemical composition of the used powder on the fatigue behavior of Selectively Laser Melted (SLM) and Laser Deposition Welded (LDW) specimens made of austenitic stainless steel AISI 316L was investigated. Therefore, in each manufacturing process two variations of chemical composition of the used powder were utilized. For qualitative characterization of the materials cyclic deformation behavior, load increase tests (LITs) were performed and further used for the physically based lifetime calculation method (PhyBaLLIT), enabling an efficient determination of stress (S)–number of cycles to failure (Nf) curves (S–Nf), which show excellent correlation to additionally performed constant amplitude tests (CATs). Moreover, instrumented cyclic indentation tests (PhyBaLCHT) were utilized to characterize the materials’ defect tolerance in a comparably short time. All material variants exhibit a high influence of microstructural defects on the fatigue properties. Consequently, for the SLM process a higher fatigue lifetime at lower stress amplitudes could be observed for the batch with a higher defect tolerance, resulting from a more pronounced deformation induced austenite–α’-martensite transformation. In correspondence to that, the batch of LDW material with an increased defect tolerance exhibit a higher fatigue strength. However, the differences in defect tolerance between the LDW batches is only slightly influenced by phase transformation and seems to be mainly governed by differences in hardening potential of the austenitic microstructure. Furthermore, a significantly higher fatigue strength could be observed for SLM material in relation to LDW specimens, because of a refined microstructure and smaller microstructural defects of SLM specimens.

Determination of the influence of a stress-relief heat treatment and additively manufactured surface on the fatigue behavior of selectively laser melted AISI 316L by using efficient short-time procedures

To use Additive Manufacturing (AM) technology for production of safety critical structural components, it is indispensable to investigate thoroughly the materials AM specific microstructure as well as the resulting mechanical and especially cyclic properties. Besides their special microstructure, AM materials show a highly inhomogeneous distribution of residual stresses as well as numerous process induced microstructural notches, e.g. pores in the materials volume and high roughness of additively manufactured surfaces. Therefore, in production of AM components stress-relief heat treatments as well as surface post-processing are commonly used. In the present work, the influence of stress-relief heat treatment as well as the additively manufactured surface condition on the cyclic properties was investigated at selectively laser melted (SLM) specimens made of AISI 316L stainless steel. Specimens were manufactured in horizontal as well as vertical building direction, leading to layer planes oriented parallel and perpendicular to the loading direction, respectively. To reduce the high material and time effort in fatigue investigations, the short-time methods PhyBaLCHT, which is based on cyclic indentation tests and enables a characterization of the materials defect tolerance, as well as load increase tests (LITs) were used for qualitative analyzes of the cyclic deformation behavior. Moreover, the physically based lifetime calculation approach (PhyBaLLIT) was applied for quantitative determination of the fatigue behavior of heat treated specimens, showing excellent correlation to additional constant amplitude tests (CATs), performed for validation. While the heat treatment does not significantly influence the fatigue behavior of vertically built specimens, an increase of fatigue strength could be observed for horizontal building direction, which is mainly caused by an improved defect tolerance. The higher defect tolerance was determined with PhyBaLCHT and could be proved by fracture mechanics considerations using the area-concept. Furthermore, the results show a significantly lower fatigue lifetime for additively manufactured surface condition compared with polished samples, which is more pronounced for vertically built specimens, because of the higher surface roughness of specimens manufactured in this building direction. Moreover, a double-staged S-Nf curve could be observed for specimens with additively manufactured surface, which correlates to the cyclic deformation behavior observed in LITs and CATs.

Investigation of the anisotropic fatigue behavior of additively manufactured structures made of AISI 316L with short-time procedures PhyBaLLIT and PhyBaLCHT

To exploit the high potential of additive manufacturing (AM) technology for production of safety relevant structural components, it is indispensable to get a sound knowledge about the fatigue behavior of additively manufactured materials. However, fatigue investigations are time and material consuming, leading to high costs for fatigue validation of AM materials. To reduce this effort, in this research work the short-time procedures PhyBaLCHT (CHT: cyclic hardness test), based on cyclic indentation testing, and PhyBaLLIT (physically based lifetime calculation; LIT: load increase test), based on load increase tests (LITs) are used to comprehensively characterize the fatigue behavior of selectively laser melted specimens made of austenitic stainless steel AISI 316L in three different building directions. The different building directions lead to layer planes perpendicular (SLM-V), in 45° direction (SLM-45) and parallel (SLM-H) to the loading direction. In addition to the short-time procedures, several constant amplitude tests (CATs) were performed to validate the results. The results of CATs showed only slight differences in the fatigue behavior of SLM specimens, which correlates well with the results of LITs. Furthermore, the results of CATs demonstrate a high influence of the microstructural defects i.e. pores on the fatigue behavior and different defect tolerances depending on the building direction. The vertical building direction lead to the highest defect tolerance, which is proved by the results of PhyBaLCHT. The S-Nf curves, calculated by the PhyBaLLIT approach, are in good accordance to the CATs and hence, can describe the characteristics of the fatigue properties of the different orientations with good accuracy. Consequently, the influence of different building directions on the fatigue behavior of the SLM specimens could be determined efficiently with the presented short-time procedures.

Failure of DLC Films Caused by Cyclic Sphere Indentation and Effects of Film Structures on Failure Mode

As a new strength evaluation method for hard coatings such as diamond-like carbon (DLC), the cyclic indentation test was proposed. Then, a test rig was fabricated and used for the evaluation of DLC. DLC and a Cr/C inclined interlayer for the improvement of the adhesion strength of DLC were prepared by the unbalanced magnetron (UBM) sputtering method on heat-treated and polished JIS SCM415 substrates. Si3N4 spheres having diameters of 10 mm were used for the cyclic indentation test. 10000 cycle indentations brought about DLC fracture and detachment in the indentation impression. Also ring cracks were observed around the impression. I(D)/I(G), which is the ratio of the D to G peak intensities, decreases towards the center of the impression. It was considered to indicate decreasing sp2 cluster size in DLC. On the other hand, compressive stress increased near the surface of DLC in the contact area, as indicated by the change in its G peak position. It seemed to be the cause of the fracture and detachment of DLC in the impression.

PhyBaLCHT – Influence of indentation force on the results of cyclic hardness tests and investigations of comparability to uniaxial fatigue loading

Short-time procedures for determining the fatigue properties of materials can provide significant improvements in cost efficiency and are therefore of great scientific and industrial interest (Jost et al., 2017). A promising short-time procedure is PhyBaLCHT, which is based on cyclic indentation tests and can be used to determine the cyclic hardening potential of different materials at different conditions (Kramer et al., 2014; Bambach et al., 2016). In the present research work, this procedure was used to explore the cyclic hardening potential of 42CrMo4 steel (SAE 4140) in various heat treatment states as well as of 18CrNiMo7-6 steel variants with different chemical composition and heat treatments. A special focus of the investigation was the influence of maximum indentation force on the results determined with PhyBaLCHT. As expected, a higher dependency on local effects is seen at lower indentation forces. However, consistent values of cyclic hardening potential were determined down to low indentation forces, and indent diagonals below 5 µm. This gives perspective to describe e.g. gradients in material cyclic deformation behavior with high local resolution. Furthermore, the results of cyclic indentation tests were compared to the results of uniaxial cyclic compression tests, showing transferability of the cyclic indentation tests to the cyclic deformation behavior under uniaxial cyclic loading.

Fatigue response evaluation of stainless steel SS 304 L(N) and SS 316 L(N) through cyclic ball indentation studies

This paper presents the results of an experimental investigation of fatigue response of stainless steel SS 304 L(N) and SS 316 L(N) using cyclic ball indentation test method. A Tungsten Carbide (WC) spherical ball of 1.57 mm diameter is used for applying compression-compression fatigue cycling on the test specimen having a nominal thickness of 5 mm; the displacement response is monitored as a function of every cycle of loading. The study focused on cases where the stainless steel specimens were welded by two different welding processes – Activated flux TIG welding and conventional multi-pass TIG welding. Fatigue response was monitored at locations of weld zone, heat affected zone (HAZ) and base metal to identify the effect of microstructure variation on fatigue response. It is observed that there is a steady increase in depth of penetration of the spherical indenter due to fatigue cycling; however, after a number of cycles, there is a sudden increase in depth of penetration which indicates the failure of the material beneath the indenter. The specimens after cyclic ball indentation were examined using a scanning electron microscope and one could observe the presence of secondary cracking in the penetrated region of the specimen.

Acoustic emission characterization of local bending behavior for adhesively bonded hybrid external patch repaired glass/epoxy composite laminates

This article investigates the influence of homogeneous and hybrid external patches based on glass and Kevlar plain weave woven fabrics on local bending response of adhesively bonded external patch repairs in damaged glass/epoxy composite laminates. The intent of using hybrid external patches was to combine the excellent high displacement to failure property of Kevlar fiber as a ductile reinforcement with the superior mechanical property of glass fiber as a brittle reinforcement. The undamaged normal specimens were taken as the standard specimen for evaluation of residual mechanical properties. In all hybrid patches, the proportion of Kevlar and glass fibers was equal (i.e. 50% of Kevlar and 50% of glass by volume fraction), while lay-up configurations were different. This further allowed studying the associated effects of hybridization and lay-up configuration on local bending response of the repaired laminates. All the specimens were subjected to cyclic quasi-static indentation tests with a step loading method. The indentation tests have also been monitored in real time by acoustic emission system. The acoustic emission results illustrated various damage profiles and correlates with the mechanical test results to point out the load to a transformation in damage mechanisms during indentation loading with respect to the effect of each patch material on the performance of the repaired glass/epoxy specimens. Results showed that hybridization and lay-up configurations of the external patches played a significant role on local bending response (i.e. ultimate load, stiffness, residual deformation, displacement to failure, and damage pattern) of the repaired glass/epoxy specimens. Specimens repaired using intra-ply hybrid patches showed the best local bending response.

Characterization of fatigue crack growth of concrete mortar under cyclic indentation loading

Abstract This study establishes a computational model for contact fractures in concrete mortar subjected to both monotonic and cyclic contact loadings. We perform experiments using monotonic and cyclic indentation tests with a ball indenter (i.e., cyclic spherical indentation). During the tests, the acoustic emission (AE) method is utilized to detect micro-scale damage in contact fractures. For monotonic contact loading, we determine the critical contact load when the radial crack initiates. It is found that radial cracks occur outside the contact zone (outside of the impression crater). Next, cyclic contact loading is performed, and it is found that the crack propagates with the increasing number of cycles. This fatigue crack growth can be captured by the AE method. In addition, a finite element method (FEM) is used to elucidate the stress distribution of crack initiation and crack propagation during both indentation loadings. FEM computation with a damage-based cohesive zone model (CZM) is carried out to simulate crack growth during cyclic indentation tests. The cohesive model follows a linear damage-dependent traction–separation relationship coupled with a damage evolution equation. It is found that our computation successfully simulates fatigue crack growth. By using the comprehensive experimental/computational framework, the nucleation process (mechanism) of such a complicated crack system is clarified. This methodology may be useful for examining crack propagation of other brittle materials under both monotonic and cyclic contact loadings.

Internal stress on adhesion of hard coatings synthesized by multi-arc ion plating

The performance of PVD hard coatings is strongly affected by the level of residual stress as it may cause the coatings to detach from the substrate and thus shorten the tool's life. In this study, the residual stress on adhesion of hard coating synthesized by multi-arc ion plating (MAIP) has been investigated. The results indicate that the residual stress of hard coatings has a strong dependence on processing conditions, which is shown by increasing bias voltage and/or decreasing working pressure. Low critical load is assessed for thick coating indicating high interfacial stress, yet the average residual stress obtained from substrate curvature is decreased with increasing thickness. By using finite element modeling (FEM), it is found that high compressive residual stress induces large shear stress at the coating/substrate interface. This implies that the accumulated compressive residual stress is more appropriate than the average one in characterizing the effect of residual stress on coating's adhesion strength. In addition, the effects of residual stress on coating's bonding strength are clearly revealed by using cyclic spherical indentation (CSI) test.

Deformation of asphalt surfacings under stationary and slow-moving traffic

The effect of traffic speed and type of loading on the deformation of asphalt surfacings was investigated. An asphalt indentation test was used to simulate the effect of stationary and slow-moving traffic on asphalt deformation. Indentation tests were carried out under both static and cyclic loading conditions. Static indentation tests were carried out by applying a constant load for a period of time (creep test) to the surface of an asphalt specimen. Cyclic indentation tests were carried out at different loading frequencies to simulate various traffic speeds. Standard tests, including wheel tracking and cyclic compression tests, were also performed. It was found that the deformation of asphalt surfacings under static loading depended on the duration and magnitude of the contact pressure. In addition, deformation increased when the frequency of loading was reduced – that is, when traffic speed decreased. Indentation tests were found to be more suitable than standard tests to characterise the resistance of asphalt surfacings to deformation due to static and slow-moving loads. These tests can be used to better select asphalt mixtures for the type of loading and traffic conditions expected on site.

Study of Fatigue Properties of Materials through Cyclic Automated Ball Indentation and Cyclic Small Punch Test Methods

One of the important inputs while estimating the remaining life of critical components is the fatigue property of materials. Fatigue data, in the form of stress vs. cycles to failure (or) strain vs. cycles to failure (or) fatigue crack growth rate data is used to predict the residual life. Material’s fatigue property degrades with time and usage; hence, it is appropriate to use the current properties for remaining life assessment. Often the quantity of material available for generating fatigue data is limited, especially, if the material is scooped out of existing component of a power plant. Further, fatigue response being probabilistic in nature, requires multiple specimens to be tested at any given stress/strain levels. This has prompted us to develop test procedures to determine the fatigue data of materials from a limited volume of material. This paper presents the results of cyclic ball indentation test method as well as cyclic small punch test method that is used to generate the fatigue data at different stress levels. There are several fine details relating to these test techniques – viz., establishing a equivalent damage criteria for failure life with standard LCF/HCF test specimens. The influence of one of the variables, viz., friction at the specimen-tool interface of a small punch test is investigated through numerical simulation and the results are presented here.

Depth-sensing cyclic nanoindentation of tantalum

In the present paper, depth-sensing multi-cycling instrumented indentation testing of tantalum is studied to assess depth-dependent plastic deformation of the metal at room temperature. Loading/unloading/reloading scheme in cyclic indentation testing is a reliable indentation-based approach in which data collection is not necessarily adversely affected by lateral in-homogeneities of the sample leading to more accurate measurements. The experimental results show indentation size effect (ISE) and depth-dependent strain rate sensitivity in Ta. To interpret these phenomena, the density of geometrically necessary dislocations (GNDs) and statistically stored dislocations (SSDs) constructed by the Nix-Gao model were used. The observed dependence of hardness and activation volume upon indentation depth indicated that the average flow stress, and hence the average plastic strain, is higher around the small indentations than around the deep indentations.

Quantitative evaluation of bonding strength for hard coatings by interfacial fatigue strength under cyclic indentation

The adhesion of coating to substrate is an important factor governing the performance and durability of coated engineering components. Existing evaluation methods cannot produce quantitative measurement of bonding strength for strong adherent coating. This paper proposes a novel evaluation method based on the cyclic spherical indentation (CSI) test, which uses successive indentations of a spherical ball onto a coated specimen to induce the detachment of the coatings along the interface. Compared with earlier methods, this method creates only adhesive failures, which only occur during elastic deformation of the substrate, yet it is applicable to coating with strong bonding. The evaluation method creates a curve of shear stress range versus failure cycles for each coating/substrate system. These curves can be used directly to compare the bonding strength of hard coatings.

CYCLIC INDENTATION TEST FOR EVALUATING STRENGTH OF DLC FILMS

The authors proposed a new evaluation method “Cyclic indentation tests”. In this method, 118.6N of cyclic force of 118.6 N (1.46 GPa of Hertzian contact pressure) acts on DLC films without sliding. The present paper describes evaluation of DLC films strength and failure mode using the cyclic indentation test. DLC films, which consisted of Cr/C gradient and a-C:H layers, were prepared using unbalanced magnetron sputtering. Thickness of DLC films was approximately 2μm. Substrates of DLC films are flat plates as lapped, and material of them was SCM415. Three kinds of DLC films, which have different hardness by changing substrate bias voltages were prepared. As a results, DLC films were damaged around indentation tracks, and failure modes were thought to be so called “buckling pattern”. In addition, protrusions were observed around indentation tracks. Height of these protrusions were smaller as increasing hardness of DLC films.

AE法を併用した静的および繰返しインデンテーション試験による高温環境下でのDLC膜の強度評価

This study aims to evaluate strength of a diamond-like carbon (DLC) film on a metal for glass press molding at high temperature. Static and cyclic indentation tests at high temperature up to 300℃ with AE monitoring technique were performed. AE monitoring reveled crack generation load or cyclic number during the indentation tests. The static strength of the films at each temperature was determined from maximum stress in the film in the radial direction induced by sink-in deformation due to static indentation. The maximum stress at each temperature was estimated with indentation loads at first AE generation and FEM analysis. Thermal stress in DLC film was at each temperature also calculated. The film strength estimated by taking the thermal stresses into account was decreased with an increase of temperature. In cyclic indentation test, AE due to cracks in film was detected after 1.0×104 cycles at the load where no crack generated under one loading cycle. The cyclic number to crack initiation for the sample in 300℃ was 1/50 smaller than that in room temperature.

繰返し球圧子押込み試験におけるセラミックス薄膜のき裂発生に及ぼす球径の影響

The objective of this study is to investigate the effects of the diameter of spherical indenter 2R on the ring crack initiation strength of ceramic thin film evaluated by cyclic sphere indentation test. Cyclic sphere indentation tests were carried out for 3 type ceramic thin films; TiAlN, “DLC by PVD” and “DLC by CVD”, by using Si3N4 balls of 4 types of diameter 2R, 5.96 mm, 7.93 mm, 9.53 mm and 12.6 mm. For all films, the ring crack initiation strength decreased with the increase of 2R. The decrease in “DLC by PVD” was smaller than the other films. The influence of the substrate on the ring crack initiation strength of TiAlN film was also investigated by using two type substrates, WC-Co and high-speed tool steel.The ring crack initiation strength obtained by small indenter changed with the substrate, however, the strength obtained by the indenter 2R = 12.6 mm, did not depend on the substrate material.

Failure assessment of a hard brittle coating on a ductile substrate subjected to cyclic contact loading

Hard brittle films and coatings are often employed as a protective coating for metallic ductile substrates. In use, such coatings are generally subjected to cyclic/repeated contact loading and sliding over long periods of time. This study investigated the monotonic and cyclic contact fracture mechanism of hard coatings on ductile substrates (an electroplated Ni–P coating on a stainless steel substrate, SUS304) in order to evaluate their mechanical durability. In the experiment, both monotonic and cyclic indentation tests using a ball indenter with large contact force were performed. The fracture nucleation process was identified using the acoustic emission method. For monotonic contact loading (single indentation), coating cracks are produced by the excessive plastic deformation of the substrate, itself caused by contact loading, which makes the bending curvature of a coating a critical moment. Subsequently, cyclic contact loading (cyclic indentation) was applied to the coating in order to investigate the cycle number of film cracking. It was found that the critical contact force for coating fracture decreases, compared with that of monotonic loading. This critical force is dependent on the number of loading cycles. This may be due to the fact that cyclic contact loading encourages large plastic deformation of the SUS304 substrate owing to cyclic plasticity. Therefore, the cyclic plastic deformation behavior of the substrate was investigated using cyclic microindentation tests and the finite element method. In the computation, the Chaboche model was employed to compute the cyclic plastic deformation of the substrate, since it simulates cyclic plasticity. We clarified the cyclic contact fracture mechanism of electroplated Ni–P coating on an SUS304 substrate. Based on this, we finally predicted the coating lifetime (i.e., mechanical durability) under cyclic contact loading. Therefore, the present study is useful for obtaining information about film/coating fracture properties under both monotonic and cyclic contact loadings.

J0410305 繰返し球圧子押込み荷重下のセラミック被覆超合金における薄膜の強度特性評価([J041-03]工業材料の変形特性・強度およびそのモデル化 クリープ・変態・被膜強度・締結,機械材料・材料加工部門)

The purpose of this study is to clarify the possibility of the sphere indentation test to evaluate properties of crack initiation and wear progress in ceramic thin films deposited on cemented carbide. Three kinds of ceramic thin films were deposited onto cemented carbide WC-Co substrates. Cyclic sphere indentation tests were carried out by using spherical indenter to investigate the aspects of crack initiation, wear progress in ceramic film and to obtain the influence of the applied maximum load P_<max> and load ratio L_s on the cyclic number of crack initiation N_i and wear value in thin films. The main conclusions are the following. (1) The loading cycle of ring crack initiation N_i increase with increasing L_s. (2) The wear value of S=S/(d_1-d_2) linearly increase accordance with P_<max> and loading cycles N, where S is obtained by measuring the cross-sectional shape of contact area and d_1, d_2 are diameter of wear region in the contact area. (3) The values of S^^-/(P_<max>×N) is almost constant independent of L_s, P_<max> and N.

Experimental Analysis on Deformation Evolution and Crack Propagation of Rock Under Cyclic Indentation

Indentation-induced cracking may have many implications in rock drilling and cutting processes. In the present work, cyclic indentation tests have been conducted on Yunnan sandstones to investigate the deformation and failure process. Deformation fields on the sample surface have been obtained via the digital image correlation (DIC) technique. It has been found that the macroscopic force–indentation curve exhibits hysteresis and the residual displacements increase during the loading–unloading cycles. Furthermore, a median crack has been observed to nucleate, running almost parallel to the loading axis when the indentation force amounts to a critical value. Such a crack always results in the final failure of the sandstone samples. Experimental analysis reveals the deformation and failure mechanism of sandstone during cyclic indentation. By the DIC technique, the displacement distribution along the crack surface can be easily calculated and, hence, the fracture parameters, such as crack opening displacement and fracture toughness.