Indentation Fatigue Research Articles

Study of ratcheting by the indentation fatigue method with a flat cylindrical indenter. Part II. Finite element simulation

The finite element method (FEM) was used to study the flat cylindrical indentation fatigue behavior using a kinematic hardening model (A-F model). This study was motivated by the experimental work of the preceding paper [B.X. Xu and Z.F. Yue, J. Mater. Res.21, 1793 (2006)], in which there were obvious similarities in the behavior of conventional fatigue specimens and indentation fatigue specimens. It is proposed that the A-F model can predict the indentation fatigue behavior. Generally, the experimental behavior of the indentation fatigue testing can be explained by the FEM analysis. In addition, the effect of residual stress on the indentation depth per cycle was studied. The effect of friction between the indenter and the specimen and evolution of von Mises stress beneath the indenter was also investigated. Numerical results showed that the effect of friction on the indentation depth propagation can be neglected. Further analysis showed that the steady-state indentation depth per cycle increases with increasing compressive residual stress and decreasing tensile residual stress.

2010 繰返し押し込み試験による遮熱コーティングの界面損傷(G03-2 測定・検出・評価技術(2),G03 材料力学)

Thermal barrier coatings consisting of ZrO_2-8wt%Y_2O_3 and NiCoCrAlY bond coat on a Ni base superalloy are used as test specimens. The indentation fatigue tests are conducted before and after thermal exposure test. The TGO layer increases with increase of thermal exposure time up to 200h. The interfacial damage is observed using scanning electronic microscope and digital microscope, which confirms growth of interfacial crack in TGO layer and spalling of coating layer after thermal exposure of 200h.

Study of the ratcheting by the indentation fatigue method with a flat cylindrical indenter: Part I. Experimental study

Generally, ratcheting is studied on round specimens under tension–compression tests with a nonzero mean load. This work explored the possibility of studying ratcheting by indentation fatigue with a flat cylindrical indenter. In the experiment, emphasis was concentrated on the influence of maximum indentation load (Pmax.), indentation load variance (ΔP = Pmax − Pmin) and frequency of cycling (f) on the indentation depth–cycle curves. The experimental results showed that the indentation depth–cycle curves are analogous to the conventional strain–cycle curve of uniaxial fatigue testing, which has a primary stage of decaying indentation depth per cycle followed by a secondary stage of nearly constant rate of indentation depth per cycle. It was found that the steady-state indentation depth per cycle is an approximate linear function of maximum indentation load (Pmax) and indentation load variance (ΔP = Pmax − Pmin) in the log–log grid. This relationship can be given with a power-law expression as an analogous equation of steady-state ratcheting rate. Further study showed that the influence of frequency of cycling on the steady state indentation depth per cycle can be ignored when the frequency of cycling exceeds a certain value. Finally, comparison was made between the conventional uniaxial fatigue test and indentation fatigue test for the steady-state stage. It was shown that the conventional uniaxial fatigue parameters can be obtained by the indentation fatigue method.

324 繰返し押し込み試験による遮熱コーティングの界面損傷(材料の特性評価)

本研究では、試験片にはNi基合金(インコネル738)基材にボンドコートとして減圧プラズマ溶射によりNiCoCrAlY合金(≈150μm)、トップコートとして大気プラズマ溶射によりジルコニア(ZrO_2-8wt%Y_2O_3:≈250μm)を施した遮熱セラミックスコーティングシステムを大気雰囲気中で熱暴露させたものを使用した。熱暴露試験前後の遮熱セラミックスコーティング試験片を用い、ランプ荷重および繰返し荷重の押し込み試験を行った。試験後の遮熱コーティングの界面損傷を観察し、界面劣化や損傷機構について研究した。また、熱暴露試験を行い熱暴露時間がOhから200hに増加するに従ってTGO層の厚さが増加した。

Nano-hardness testing with ultrasonic excitation

Accelerated wear of the head/disk interface is investigated using nano-indentation measurements of carbon coated magnetic recording sliders in the presence of ultrasonic excitation. The effect of ultrasonic vibrations on the indentation characteristics and fatigue of carbon coated magnetic recording sliders is studied and the effect of vibration amplitude and frequency is investigated. Significant changes are observed in the type of surface damage that occurs if indentation measurements are performed in the presence of ultrasonic excitation. The dependence of film breakthrough on film thickness during ultrasonic excitation is investigated.

Contact Damage of Dental Multilayers: Viscous Deformation and Fatigue Mechanisms

This paper presents the results of recent experimental and finite element studies of contact damage in model dental multilayered systems with equivalent elastic properties to those of crown/join/dentin layers that are found in dental restorations. Subsurface radial cracks are observed to form after Hertzian indentation fatigue loading. In order to explain the possible failure mechanisms, the viscous deformation of the foundation (dentinlike ceramic filled polymer) and epoxy join layers are measured. Finite element and analytical models are then developed in an effort to explain the observed contact-induced deformation of the composite multilayered system. Our results suggest that: viscous deformation of the join and foundation layers can give rise to increased tensile stresses in the top elastic layers (glass or zirconia); defects at the bottom of the top layers (induced by grinding steps before crown attachment) are also shown to promote ratcheting phenomena that can lead to stress build-up in the top layers; and viscous flow of the cement can cause the subcritical crack growth in the dental ceramics.

Residual Compressive Failure Characteristics of Hybrid Composites with Nonwoven Carbon Tissue after Indentation Damage

The purpose of this study is to investigate the relationship between the low-energy impact damages under static indentation loadings and the residual compressive static and fatigue strengths after damage in hybrid composites with nonwoven carbon tissue (NWCT). The hybrid laminates are made by interleaving the NWCT layer between the CFRP layer interfaces. The indentation damages were discussed by investigating the delamination areas and the indentation energies of four kinds of angle-ply laminates ([455/ 455/455] and [453/ 453/453/ 453/453] CFRP laminates, and [455// 455//455] and [453// 453//453// 453//453] hybrid laminates; the symbol “//” means that the NWCT layer is located at an interface between the CFRP layers. The delamination areas of the hybrid laminates were reduced to about half of the CFRP’s by the indentation loading (Lee, S.H., Aono, Y., Noguchi, H. and Cheong, S.K. (2003). Damage Mechanism of Hybrid Composites with Non-Woven Carbon Tissue Subjected to Quasi-Static Indentation Loads, Journal of Composite Materials, 37: 333–349). After the indentation damage, compressive static and fatigue tests are carried out using an anti-buckling guide for the angle-ply laminates. The failure modes of the angle-ply CFRP and hybrid specimens are caused by global buckling under the compressive static loading. However, the failure modes of the angle-ply CFRP and hybrid specimens at Nf <10 6cycles are caused by shearing the damaged part under the compression–compression fatigue loading. Compared with CFRP laminates, compressive fatigue lives of the hybrid laminates were significantly extended in all the stress ranges. The NWCT interleaving effect is examined based on the observations of the damage in the hybrid specimens after the indentation damage. Also, the compressive static and fatigue failure processes are discussed from the observation of the fracture surfaces of damaged parts and failed appearance of specimens when subjected to the compressive loadings after the indentation damage.

Nanoscale fatigue and fracture toughness measurements of multilayered thin film structures for digital micromirror devices

The digital micromirror device (DMD), which lies at the heart of Digital Light Processing™ projection display technology by Texas Instruments, Dallas, Texas, comprises a surface-micromachined array of mirrors on a base CMOS substrate. The nanomechanical properties of the thin-film structures used are important to the performance of the DMD. In this article, the nanoscale fatigue and fracture toughness of the single and multilayered thin film structures which are of interest in DMDs, i.e., TiN/Si, SiO2/Si, Al alloy/Si, TiN/Al alloy/Si, and SiO2/TiN/Al alloy/Si, were measured using nanoindentation techniques. The multilayered thin film structures with hard films on soft interlayer exhibit shorter fatigue life and higher ductility than the corresponding top hard films. The hardness plays an important role in determining the indentation fatigue life. Longer fatigue life is associated with higher hardness. The fracture toughness of TiN/Si and SiO2/Si was measured using the strain energy release method.

Microstructural Design and Evaluation of Porcelain/Mullite/Alumina Layered Structure for Dental Application

AbstractPorcelain (veneer layer)/alumina (core layer) is a typical dental crown structure. Due to its high incidence of failure, a new porcelain/mullite (buffer layer)/alumina trilayer structure is designed, fabricated, and evaluated. Alumina green bodies were prepared by gel-casting process, and then calcined at 900 and 1100°C to infiltrate mullite precursor slurry of silica-rich (Al2O3·2SiO2) composition into the bodies. Porosity in the bodies is not dependent on calcination temperature, resulting in a similar infiltration depth. Porcelain was coated on the alumina sintered at 1600°C with and without mullite buffer layer. There are no delamination or cracks observed after firing the layered materials. Rod type microstructure and continuous composition are indicated at the interface in the case of the layered structure with mullite buffer layer. To investigate the cracking resistance behavior for this new structure, Vickers indentation and Hertzian contact fatigue tests were conducted. Cracks do not penetrate the interface with mullite buffer layer into the porcelain, showing a reversal case for the layered structure without mullite buffer layer. The layered structure with mullite buffer layer shows higher critical load for fracture than that without mullite buffer layer. Fracture mode of the layered structures in cyclic fatigue shows a top layer (porcelain) fracture at relatively low load (P = 250 N) and higher cycles (n = 106), and a bottom layer (alumina) fracture at higher load (P = 300 N) and relatively low cycles (n = 105).

Thermal Shock Resistance of Sintered Alumina/Silicon Carbide Nanocomposites Evaluated by Indentation Techniques

The thermal shock resistance of sintered Al2O3/1, 2.5, and 5 vol% SiC nanocomposites was studied using two indentation techniques. In the first technique, “indentation thermal shock” measurements were made of the extension of median/radial cracks around Vickers indentations after quenching from various temperatures (up to 480°C) into a bath of boiling water. This technique allowed a critical thermal shock temperature, ΔTCInd, to be quantitatively evaluated. In the second technique, “indentation fatigue” tests were conducted on the thermally shocked specimens; repeated indentations were made at the same site, and the number of load cycles needed to initiate lateral fracture was measured. The results showed that nanocomposites with an addition of SiC nanophase as low as 1 vol% had a thermal shock resistance superior to that of pure Al2O3.

Indentation fatigue in silicon nitride, alumina and silicon carbide ceramics

Repeated indentation fatigue (RIF) experiments conducted on the same spot of different structural ceramics viz. a hot pressed silicon nitride (HPSN), sintered alumina of two different grain sizes viz. 1 μm and 25 μm, and a sintered silicon carbide (SSiC) are reported. The RIF experiments were conducted using a Vicker’s microhardness tester at various loads in the range 1–20 N. Subsequently, the gradual evolution of the damage was characterized using an optical microscope in conjunction with the image analysing technique. The materials were classified in the order of the decreasing resistance against repeated indentation fatigue at the highest applied load of 20 N. It was further shown that there was a strong influence of grain size on the development of resistance against repeated indentation fatigue on the same spot. Finally, the poor performance of the sintered silicon carbide was found out to be linked to its previous thermal history.

Damage accumulation caused by cyclic indentation in zirconia ceramics

Indentation fatigue testing is carried out for four kinds of zirconia ceramics; 6Y-FSZ, 3Y-TZP, Mg-PSZ and Ce-TZP; which have different strengthening-toughening mechanisms one another. Each material shows different indentation fatigue behavior. Large sized surface chipping is produced easily in early cycles for non-toughened 6Y-FSZ, while 3Y-TZP toughened by anelasticity has high resistance against chipping. There is a possibility that anelastic characteristic works as resistance against indentation fatigue damage. Stress induced phase transformation also improves fatigue damage, but this effect depends on the extent of transformation and/or occurrence of microcracking.

Indentation Fatigue in Random and Textured Alumina Composites

The effect of grain orientation on contact fatigue behavior has been investigated by using alumina with elongated grains as a model system. Two kinds of composite microstructures, textured and random, were prepared by controlling the processing conditions. The textured material has the platelets aligned parallel to the surface, and the random material has the platelets randomly oriented. The Hertzian indentation results show that, although both materials exhibit damage accumulation with increasing number of cycles due to frictional degradation at the microstructural level, the damage evolution rate is much lower for the textured material. This suppression of fatigue damage in the textured material appears to result from the lower shear stress concentration along the textured weak interfaces between elongated alumina grains. The implication of the present results for structural design in improvement of contact‐fatigue resistance is also addressed.

Comparative Study of Indentation Fatigue in Structural Ceramics

Comparative Study of Indentation Fatigue in Structural Ceramics

Crack Initiation by Indentation Fatigue in Lead Alkali and Soda–Lime Glass

An indentation technique using a conventional Vickers microhardness tester was used to evaluate fatigue properties of lead‐alkali and soda‐lime silica glasses. The specimens were indented repeatedly at the same point with subcritical loads until radial cracks were initiated. The number of cycles to initiate the cracks at different subcritical loads demonstrated typical fatigue curves for both glasses. The uniqueness of the experiment was that the diagonal lengths of the deformed cavity were observed to increase with the number of cycles. This increase of the deformed cavity for a certain number of cycles prior to the visibility of crack initiation was analyzed, correlating the elastic‐plastic phenomenon and the accumulation of the residual stress in each cycle.

Fatigue damage in ceramic materials caused by repeated indentation

Indentation fatigue is the damage accumulation in the surface layer of ceramics caused by local cyclic loading. It has been found that there are two types of indentation fatigue behaviour, Types I and II. Type I is characterized by a slight increase in indentation-induced damaged zone with increased number of indentation cycles, followed by the abrupt chipping of the surface layer. On the other hand, Type II behaviour is characterized by pronounced enlargement of the damaged zone in early cycles. Lateral cracking behaviour during cyclic indentation in various ceramics was also studied by scanning electron microscopy and scanning acoustic microscopy. It was confirmed that a lateral crack zig-zag propagates with repeated indentation in silicon nitride of transgranular fracture type. The relations between lateral cracking, microcracks and the resultant damaged zone in alumina and magnesium oxide were investigated in detail and their indentation fatigue mechanism is discussed.

Microindentation fatigue tests on submicron carbon films

Microindentation fatigue tests have been conducted on submicron coatings using a microindenter. The cyclic indentation was implemented by servo controlling the motion of the indenter so that the loadcell output followed a 0.1 Hz sinusoidal loading pattern. A conical diamond indenter with a nominal 1 μm tip radius was used. A model system composed of two 0.11 μm sputtered carbon films was chosen for investigating the indentation fatigue effects on brittle coatings. Both carbon films were deposited on silicon substrates but with substantially different interfacial adhesion strengths. For the carbon film with poor adhesion tested at the condition of a 4 mN maximum load, an annular coating buckling pattern concentric with the center of the indent appeared during the first loading stage whereas coating spallation occurred at loading cycle numbers ranging from 50 to 140. In contrast to this, under the same test condition, the carbon coating with good adhesion did not show any noticeable failure up to 500 cycles where the experiments were truncated. When the maximum load was increased, fine cracks initiated in the carbon coating and along the circular indent boundaries but the coating still adhered well to the substrate. These fine cracks continued to propagate during subsequent indentations, and eventually spallation of the carbon coating occurred. Because the microindenter has the advantage of continuously monitoring both the applied load and penetration depth during an entire test, the evolution of the fatigue failure of a coating system can be clearly visualized from the corresponding load vs. depth curve. A typical brittle failure event is manifested by several repeatable loading-unloading cycles with a gradual depth increase after the first indentation, then a couple of consecutive abrupt changes in the applied load as well as in the depth followed by stabilized indentation cycles.

Microscratch and load relaxation tests for ultra-thin films

The microindenter has proven to be a powerful device in the characterization of the mechanical properties of thin films. The machine has both high resolution in the applied load and penetration depth measurements, as well as the versatility to perform different types of testing. The former provides the capability to deal with extremely thin films, while the latter allows for other mechanical properties, in addition to hardness, to be acquired. Four types of tests, namely indentation, scratch, load relaxation, and indentation fatigue tests can currently be conducted using the microindenter via different operating procedures. Only the scratch and load relaxation techniques will be covered in this paper. In a microscratch test, the normal load, tangential load, scratch length, and acoustic emission are monitored simultaneously during an entire scratch process for the purposes of measuring the critical load and studying the failure mechanisms of the deposited films. The adhesion strength, scratch hardness, fracture toughness, and friction are the mechanical properties which are possible to obtain by using this technique. Results from aluminum, carbon, and zirconia coatings will be discussed. The load relaxation test provides information on the creep properties of the films and results in an empirical constitutive relation between the applied stress and plastic strain rate. The creep properties of DC sputtered Al films will be used as an illustration of this.

Indentation Fatigue of Silicon Carbide and Silicon Nitride

The fatigue failure mechanism of ceramic material has been hardly made clear under cyclic loading. In this study, we examine the damage of the surface area in ceramics under local cyclic compressive loading by using the cyclic indentation method (indentation fatigue), and discuss its mechanism. Four kinds of materials (transgranular and intergranular fracture type) were used for the experiment. The indentation fatigue test was carried out by indenting repeatedly onto the same point of the specimen surface by using a Vickers hardness tester. The median/radial crack length, the number of cycles to chipping and the size of the chipping area were measured, and the lateral crack growth rate was computed. The fracture surface in the chipping area was also observed by a scanning electron microscope. The median/radial crack growth was slightly accelerated by cyclic loading in any material. During cyclic indentation, the lateral crack propagated, which resulted in the chipping phenomenon. Such damage accumulation remarkably occurred in the transgranular fracture type material rather than in the intergranular fracture type one. Striations were observed in the chipping area and the striation spacing approximately corresponded to the size of the chipping area divided by the cycles to chipping, i.e., the lateral crack propagation rate. The observed zigzag propagation of the lateral crack is considered to be attributable to the difference between the stress fields under loading and unloading. The reason why the lateral crack growth and the resultant chipping behavior are suppressed in intergranular fracture type materials is most likely that crack branching and microcracking easily occur in those materials so that the stress concentration at the crack tip is relieved.

Indentation Fatigue of High‐Purity Alumina in Fluid Environments

The influence of environment on the cyclic fatigue behavior of a high‐purity alumina bioceramic was investigated using the repeated indentation technique. Tests were conducted in the presence of water, a variety of alcohols, toluene, and simulated physiological fluid environments. The results show that these environments do not have any detectable effect on the damage produced by single indentations, but those containing water cause a significant degradation in cyclic fatigue resistance which cannot be quantified in terms of known subcritical crack growth behavior in static fatigue. It is concluded that the effects of fluid environments on the growth of cyclically driven cracks must be an integral part of the mechanism responsible for cyclic fatigue in ceramics.