Search

Thin-film stress changes the shape of nanoindentation loading curve. The change in the indentation load is treated as the effect of the residual stress on the indenting deformation. A residual-stress-induced normal load is defined as a multiple of contact area and plastic deformation-sensitive deviator stress component extracted from the equi-biaxial thin-film stress. A final equation for the residual stress is derived from the residual-stress-induced normal load by considering an integration along a depth-controlled stress-relaxation route. This proposed model is applied to the analyses of the nanoindentation curves for diamond-like carbon thin films. The residual stresses from the nanoindentation analyses were consistent with the values obtained by the conventional curvature method.

Many thermomechanical reliability studies on microelectronics and microsystems have relied upon computational analysis, since experimental work is rather difficult and very time-consuming. For computational analysis, it is essential to use as input accurate material properties; if not, the results of a reliability analysis may be very inaccurate. However, it is still quite difficult to arrive at unified material properties for modeling microelectronic assemblies because of the absence of standards for micro-material characterization, the difference between bulk and in-situ material properties, and so forth. The goal of this study was to determine the uniaxial stress-strain curve of a solder in a flip-chip assembly, using experimental measurements and finite-element analysis (FEA) of the solder's thermal deformation characteristics with increasing temperature. The thermal deformation of flip-chip solder joints was measured by electronic speckle pattern interferometry (ESPI). For the scale of evaluation required, the measurement magnification was modified to allow its application to micromaterials by using a long-working-distance microscope, iris and zoom lens. Local deformation of solder balls could be measured at submicrometer scale, and stress-strain curves could be determined using the measured thermal deformation as input data for finite-element analysis. The procedure was applied to an Sn-36Pb-2Ag flip-chip solder joint.

The small punch (SP) testing method has been used to assess the reliability of industrial facilities such as fusion reactor structures and power-generation systems. Conventional SP tests have evaluated flow properties, transition temperature, fracture strain, and other mechanical properties by analyzing load-deflection curves. However, previous research has not used a fracture-mechanics approach that considers flaws, stress analysis and fracture toughness. In this study, in order to obtain the fracture toughness based on fracture mechanics by SP tests, a sharp notch was machined into the central part of the SP specimens. A stress-intensity factor for sharp-notched SP specimens is proposed from analysis of stress fields near the crack tip. The crack initiation point was determined by analysis of the load-deflection curves combined with acoustic emission signals. The fracture toughness of SA 508 Class 3 steel was successfully evaluated by using the load at the crack initiation point.

Conventional nondestructive techniques for welding residual stress measurement have many disadvantages in the field because of poor repeatability, large scatter in data, complex procedures, inaccurate results, etc. To overcome these difficulties, an instrumented indentation technique was applied to evaluate the welding residual stress in A335 P12 steel welds in electric power-plant facilities before and after stress-relaxation annealing. Comparison of our results with stress values obtained from a destructive saw-cutting test showed that the instrumented indentation technique is very useful for quantitative/nondestructive evaluation of welding residual stresses in industrial facilities such as power-plants.

The Barkhausen noise (BN) method has long been applied to measure the bulk magnetic properties of magnetic materials. Recently, this important nondestructive testing (NDT) method has been applied to evaluate microstructure, stress distribution analysis, fatigue, creep and fracture characteristics. Until now the BN method has been used only qualitatively in evaluating the variation of BN with variations in material properties. For this reason, few NDT methods have been applied in industrial plants and laboratories. The present investigation studied the coercive force and BN while varying the microstructure of ultrafine-grained steels and SA508 cl.3 steels. This variation was carried out according to the second heat-treatment condition with rolling of ultrafine-grained steels and the simulated time-dependent degradation of SA 508 cl.3 steels. An attempt was also made to quantify BN from the relationship between the velocity of magnetic domain walls and the retarding force, using the coercive force of the domain wall movement. The microstructure variation was analyzed according to time-dependent degradation. Fracture toughness was evaluated quantitatively by measuring the BN from two intermediary parameters; grain size and distribution of nonmagnetic particles. From these measurements, the variation of microstructure and fracture toughness can be directly evaluated by the BN method as an accurate in situ NDT method.

This study investigates the correlation between the microstructural change and fracture characteristics in the coarse-grained heat-affected zones (CGHAZs) of the newly developed quenching, lamellarizing and tempering (QLT)-processed 9% Ni steel. The microscopic fracture behaviors of the various sub-zones within the HAZs including local brittle zone (LBZ) were estimated using simulated HAZ specimens. Both results of Charpy impact tests and in situ scanning electron microscopy (SEM) observations on simulated CGHAZ specimens show that the inter-critically reheated coarse-grained HAZ (IC CGHAZ) is a primary LBZ of this steel at cryogenic temperature, but not at room temperature. Microstructural analysis suggests that, unlike in other studies, the cryogenic LBZ phenomenon of the IC CGHAZs cannot be explained simply by the amount of martensite–austenite (M–A) constituents, but is mainly associated with the carbon contents in them. From all results obtained, a mechanism for microscopic toughness change among the CGHAZs is proposed and discussed.

By regarding the tip blunting as a ball indentation at very low depth range (within about 80 nm in our experiments), the flow properties of Au thin films were derived from the indentation load–depth curve obtained by nanoindentation technique. The effects of pile-up or sink-in were considered in determining the real contact between the indenter and the specimen. The representative strain in indentation was defined in various ways and examined by comparing the flow properties derived from indentation load–depth curve with those measured by tensile test. The best definition was found to be the shear strain at contact edge multiplied by 0.1. When we considered the effects of pile-up or sink-in, the representative stress in indentation could also be determined, and was found to be one third of the mean contact pressure for fully plastic regime. As a more intrinsic property than hardness, the yield strengths of Au films with thickness of 0.56 and 0.99 μm were extrapolated from the derived true stress–true strain curve as 261±30 and 154±18 MPa, respectively.

As a part of small specimen testing techniques, the notched specimen technique is studied to overcome the difficulties in introducing fatigue precrack to small fracture toughness specimens. It was noted that stress triaxiality and the plastic constraint ahead of a notch root decreased with increasing notch root radius ( ρ ). Considering that the applied stress at the notch tip is redistributed and relaxed due to the increased plasticity, the fracture toughness obtained from notched specimens was corrected. Fracture toughness testing was conducted using an instrumented impact and a static three-point bending tester. The specimens had notch root radii which ranged from a fatigue precrack to a Charpy notch root radius of 250 μm. Fracture toughness values corrected from the results of notched specimens were very consistent with the plane strain fracture toughness obtained from precracked specimens. In addition, a limiting notch root radius ( ρ 0 ), below which the fracture toughness was independent of ρ , was observed. To investigate this phenomenon, the fracture surface was observed with a scanning electron microscope (SEM).

The present study investigates the influence of local brittle zones (LBZs) on the fracture resistance of the heat-affected zones (HAZs) in quenched, lamellarized, and tempered (QLT) 9 pct Ni steel weld joints. The results of Charpy impact tests using simulated coarse-grained, heat-affected zone (CGHAZ) specimens show that the intercritically reheated (IC) CGHAZ and unaltered (UA) CGHAZ are the primary and secondary LBZs, respectively, of the steel at cryogenic temperature. Compact crack arrest (CCA) tests and crack-tip opening displacement (CTOD) tests were conducted at a liquefied natural gas (LNG) temperature to measure the variations in crack-arrest toughness and crack-initiation toughness along the distance from the fusion line (FL) within the actual HAZ. While CTOD tests show a decrease in toughness when approaching the FL, i.e., the regions containing LBZs, the crack-arrest-toughness values are found to be higher than those in the regions near the base materials. This is due to the fact that the crack-arrest toughness is governed by the fraction of microstructures surrounding LBZs instead of the LBZs themselves. By direct comparison of the brittle-crack-arrest toughness (Ka) with the brittle-crack-initiation toughness (Kc), this investigation has determined that, with regard to crack-arrest behavior, the LBZs of QLT-9 pct Ni steel do not limit the practical safety performance of the weld joints in LNG storage tanks.

Residual stress is a dominant obstacle to efficient production and safe usage of device by deteriorating the mechanical strength and failure properties. Therefore, we proposed a new thin film stress-analyzing technique using a nanoindentation method. For this aim, the shape change in the indentation load-depth curve during the stress-relief in film was theoretically modeled. The change in indentation depth by load-controlled stress relaxation process was related to the increase or decrease in the applied load using the elastic flat punch theory. Finally, the residual stress in thin film was calculated from the changed applied load based on the equivalent stress interaction model. The evaluated stresses for diamond-like carbon films from this nanoindentation analysis were consistent with the results from the conventional curvature method.

The structures used in microelectromechanical system (MEMS) such as sensors and actuators have been miniaturized, and the use of macroinstruments in testing mechanical properties has become limited by the physical size of the materials involved. We propose an electrostatically actuated test device for measuring the elastic and yield properties of Al thin films for MEMS applications. The test device we have designed consists of electrostatically actuated micrometer-sized comb drives for loading a suspended beam. The device was fabricated by surface micromachining. On the basis of the test device, the elastic modulus is determined by analyzing the initial linear load–displacement behavior of the suspended beam, and the yield strength is determined as the deviation from linear elastic behavior.

Residual stress in a thin film was analyzed by the nanoindentation technique. Two dominant effects of residual stress to indentation were summarized as the slope change in loading curve and the invariant value of intrinsic hardness. A stress-sensitive reversibly deformed zone around contact was modeled to explain the indentation behaviors under a residually stressed state. Finally, the residual stress was evaluated from the changes in contact shape and applied load during stress relaxation under the condition of constant indentation depth. The residual stresses in diamond-like carbon and Au films analyzed from this model agreed well with the average values measured by the curvature method.

In this paper, we introduce a floating plasmonic absorber having multiple resonances in the 8 ~ 14 μm spectral range and broadband absorption characteristics by adjusting Drude relaxation rate of metal. This plasmonic broadband resonator capable of capturing light with a large optical cross-section area is able to substantially enhance the performance of micro-bolometer (response time, noise equivalent temperature difference, pixel size and so on) due to the significantly reduced thermal mass and conductance. Firstly, to adjust Drude relaxation rate, the mean crystalline size of metal was optimized by changing the deposition condition and the absorption characteristics of absorber were measured by Fourier transform infrared spectroscopy in the 8 ~ 14 μm spectral range. The measurement results show that 1.62 times of broadening in bandwidth was obtained by decreasing the crystalline size from 5.73 nm to 3.18 nm while maintaining the maximum absorption at resonant wavelength of 10 μm within 93 ~ 95%. Comparisons between measurements and CST microwave studio simulations show similar spectral absorption trends. And then, to integrate plasmonic absorber with micro-bolometer, various kinds of plasmonic absorbers which have combinations of short and long dipole resonators were designed and simulated. Based on these results, 12 μm micro-bolometer pixels integrated with plasmonic broadband Ti absorber are designed and fabricated. The optimized Ti resonators with multiple resonance and small crystalline size absorb 88 % of the unpolarized radiation in the 8 ~ 14 μm spectral range on the average.

The strength characteristics of microphases in ultra-fine-grained steels were analyzed using nanoindentation and AFM. It was found that there were fine ferrite grains ( 1–2 μ m) formed by a strain-induced dynamic transformation in ultra-fine-grained steels. They had equiaxed and polygonal grain shape, and higher hardness and elastic modulus than coarse ferrite transformed statically. Strengthening factors of strain-induced dynamic transformation ferrite were analyzed in terms of cementite particles and dislocation density.

The ball indentation technique has the potential to be an excellent substitute for a standard tensile test, especially in the case of small specimens or property-gradient materials such as welds. In our study, the true stress–true strain relationships of steels with different work-hardening exponents (0.1–0.3) were derived from ball indentations. Four kinds of strain definitions in indentation were attempted: 0.2sinγ, 0.4hc/a, ln[2/(1 + cosγ)], and 0.1tanγ. Here, γ is the contact angle between the indenter and the specimen, hc is the contact depth, and a is the contact radius. Through comparison with the standard data measured by uniaxial tensile testing, the best strain definition was determined to be 0.1tanγ. This new definition of strain, in which tanγ means the shear strain at contact edge, reflected effectively the work-hardening characteristics. In addition, the effects of pileup or sink-in were considered in determining the real contact between the indenter and the specimen from the indentation load–depth curve. The work-hardening exponent was found to be a main factor affecting the pileup/sink-in phenomena of various steels. These phenomena influenced markedly the absolute values of strain and stress in indentation by making the simple traditional relationship Pm/σR ≈ ≈ 3 valid for the fully plastic regime.

The hardness characteristics of thin-film coated composite were analyzed using the plastic-zone volume-law of mixtures theory. In calculating the plastic-zone volumes, the elastic/plastic indentation stress field was analyzed; the substrate was assumed to undergo radial deformation, while the cylindrical deformation mode was applied to the film. In addition, the effect of interface was incorporated. The deformation of the softer material was assumed to be constrained by the harder one under the condition of strain matching at the interface. In experiments, the hardness (H) and elastic modulus (E) of Al2O3 film, glass, and 304 stainless steel were evaluated by analyzing the load–depth curve measured by nanoindentation technique. And, the yield strength (Y) was calculated using the relationships between H, E, and Y. Next, we measured the composite hardness values of Al2O3/glass and Al2O3/304 stainless steel systems with increasing load up to 100 mN, and compared with those predicted. For Al2O3/glass system, the measured trend of composite hardness correlated well with the predicted one. On the other hand, there were some deviations for Al2O3/304 stainless steel system, which was expected to result from the film cracking and/or the change of load distribution for hard coating on very soft substrate.

The elastic properties of diamond-like carbon (DLC ) films were measured by a simple method using DLC bridges which are free from the mechanical constraints of the substrate. The DLC films were deposited on a Si wafer by radio frequency (RF ) glow discharge at a deposition pressure of 1.33 Pa. Because of the high residual compressive stress of the film, the bridge exhibited a sinusoidal displacement on removing the substrate constraint. By measuring the amplitude with a known bridge length, we could determine the strain of the film which occurred by stress relaxation. Combined with independent stress measurement using the laser reflection method, this method allows the calculation of the biaxial elastic modulus, E/(1’n), where E is the elastic modulus and n is Poisson’s ratio of the DLC film. The biaxial elastic modulus increased from 10 to 150 GPa with increasing negative bias voltage from 100 to 550 V. By comparing the biaxial elastic modulus with the plane‐strain modulus, E/(1’n2), measured by nano-indentation, we could further determine the elastic modulus and Poisson’s ratio, independently. The elastic modulus, E, ranged from 16 to 133 GPa in this range of the negative bias voltage. However, large errors were incorporated in the calculation of Poisson’s ratio due to the pile up of errors in the measurements of the elastic properties and the residual compressive stress. © 1999 Elsevier Science S.A. All rights reserved.

Small punch (SP) test has been utilized to analyze the neutron irradiation damage of nuclear vessels. Since this technique is easy, simple, and nondestructive, it can be applied to evaluate the mechanical properties and material degradation of in-service components. Conventional SP test has evaluated the ductile-brittle transition temperature and the equivalent fracture strain by the interpretations of load-deflection curve and the change of specimen thickness, respectively. The assumption that fracture occurs at maximum load is, however, not reasonable because the crack initiates at smaller load. In this study, in order to evaluate quantitatively fracture characteristics based on fracture mechanics, the pre-crack is introduced to SP specimen and acoustic emission is used to determine the crack initiation point. Using the load at crack initiation point, the fracture toughness of thin plate is calculated through bending theory. Therefore, the fracture characteristics of thin plate can be evalualed more reliably by using revised SP test.

A theoretical analysis of the adhesion dependence of film-cracking behavior resulting from the uniaxial tensile loading of a hard film on a ductile substrate is presented. An interface shear stress that develops due to the deformation mismatch of the film and substrate induces a normal tensile stress in the film; these stresses are analyzed as a function of external strain and crack spacing, using the shear lag theory. When the film tensile stress exceeds the film fracture strength σc, film cracking occurs at an early stage of uniaxial loading; but as the loading increases, the interface shear stress increases above the critical value τc and causes the interface failure. After that, no more film cracking occurs, since stress transfer into the film is impossible due to interface damage. Thus, the interface adhesion can be estimated in terms of the shear strength from the external strain esat and crack spacing λsat measured at the time that film cracking stops. The test results for the diamond-like carbon (...

The true hardness of thin film is assessed through micromechanical analysis of the composite hardness measured by a microindentation tester. In partitioning the respective contributions of the film and the substrate to the composite hardness, we apply the modified plastic-zone volume-law-of-mixtures theory by analyzing the indentation stress field of a film/substrate system. At this time, while the substrate is assumed to undergo radial deformation, the film is assumed to be deformed mainly parallel to the surface. On the basis of these stress analyses, the influence of interface is incorporated; the deformation in the softer material is constrained by interface bonding and the plastic-zone radii are modified by considering the respective virtual pressures that the film and the substrate actually support. When an interface has perfect bonding, the hardness value for a film can be calculated analytically from the condition of strain matching at the interface—approximately 2200 kgf/mm2 for diamondlike carbo...

As the first step of the study for the safety performance of LNG storage tank based on the concept of fitness-for-purpose, the change of cryogenic toughness within the X-grooved weld HAZ (heat-affected zone) of SMA (shielded metal arc)-welded QLT (quenching, lamellarizing, and tempering)-processed 9% Ni steels, was investigated qualitatively and quantitatively. In general, CTOD (crack tip opening displacement) test is widely used to determine the fracture toughness of steel weldments. But there is no standard or draft for evaluating the toughness of thick weldment with X-groove such as in this case. Therefore, in this study, modified CTOD testing method for fatigue precracking. calculation of CTOD, examination of fractured specimen was proposed and used. And the results of modified test were compared with those of conventional CTOD test and Charpy V-notch impact test. In addition, the relationship between the fracture toughness and microstructure was analyzed by OM, SEM and XRD. The cryogenic toughness in HAZ decreased as the evaluated region approached the fusion line from base metal. The decrease in toughness was apparently caused by the reduction of the retained austenite content and the absence of grain refinement effect in the coarse-grained zone in HAZ. The austenite reduction resulted from the decrease in nucleation sites for α’γ reverse transformation due to the increase in fraction of coarse-grained zone within HAZ. More complex thermal cycles in the mixed zone of weld metal and base metal caused the poor stability of retained austenite in the zone by the redistribution of alloying element in retained austenite. Due to this reason, the toughness drop with decreasing test temperature in F.L. (fusion line)-F.L.+3 mm was larger than that in F.L.+5 mm and F.L.+7 mm.

In selecting the processing conditions and evaluating the reliability of structural materials, microscopic observations and identification of the fracture mechanisms in local cracking behavior are required. An important instance in the failure of the local brittle zone (LBZ) in the welding zone. The LBZ, which is very brittle, is the coarse-grained heat-affected zone near the fusion line, a zone known to be critical to the fracture toughness of welded parts. Thus, maintaining stable fracture resistance by predicting the microfracture behavior is important when using high-strength low-alloy (HSLA) steels in offshore structural steel welds. Depending on the thermal cycles involved during welding, the ferrite/martensite structure can have various morphologies of martensite particles, for example, fibrous and blocky martensite. In summary, in situ SEM fracture tests reveal that in the L-oriented IQ DCB specimen, a microcrack tends to propagate relatively uniformly throughout the ferrite and well-distributed fine fibrous martensite, yielding good elongation with high strength level. Also, the IQ structure in the T orientation shows similar microfracture behavior. On the other hand, in the SQ structure, where blocky-type martensite is mixed with ferrite, strain is localized into shear bands mostly in the ferrite region, and a local microcrack propagates along the strain-localizedmore » band formed in the ferrite, resulting in the SQ structure in the T orientation, where the ferrite-martensite bands are parallel to the notch direction, the martensite cannot act as an efficient barrier to microcrack advance, and thus the tensile ductility is decreased.« less

A microstructural analysis of local microfracture of cast A356 Al-SiC p composites fabricated by permanent mold re-casting and squeeze-casting methods was made. Notch fracture toughness tests were conducted on these composites to identify critical fracture parameters using a stress-modified critical-strain criterion. The composite microstructure shows continuous networks of densely populated SiC and eutectic Si particles along the intercellular regions. Squeeze casting produces a more homogeneous structure and larger spacing of brittle particles and increases the tensile ductility and fracture toughness, while strength levels are almost identical to the re-casting case. The calculated values of the microstructurally characteristic distancel* for the re-cast and squeeze-cast composites are about 40 µm, which is comparable to the average sizes of the intercellular network. However, the reference critical strain $$\bar \varepsilon _0^* $$ for squeeze casting is larger than that for re-casting, showing a trend to higher ductility and fracture toughness.

Microscopic examination and microstructural analysis of vertical cracking phenomena in deep-drawn cups of hot-rolled SG295 steel strips were carried out in this study. Microvoids initiated preferentially at grain-boundary carbides were observed to form intergranular cracks. These grain-boundary car-bides were identified as (Fe,Mn)3C carbides. The morphology of carbides was varied with processing variables,e.g., killing method and coiling temperature. In the Al-killed steels, the carbide shape tends to change from film type to bulk type, which may be beneficial to elongation and consequently formability. In addition, as the coiling temperature increased, the amount of banded pearlite structures and bulk-type carbides increased. These findings suggest that the deep drawability can be improved by modifying the carbide morphology in the microstructures.

The purpose of this study is to clarify the correlation between microstructural factors and mechanical properties of ultrafine steels processed by thermomechanical controlled treatments. Three steels deformed at high strain rates in a pilot plant rolling mill showed very fine ferritic microstructure, whose grains became more equiaxed and finer with increasing fraction of alloying elements, and had good tensile and fracture properties, although they contained only about 0.01 pct carbon. Especially in the Ni-added steel, tensile properties were greatly improved because of the high dislocation density and the fineness of the ferritic substructure, readily satisfying the requirements for commercial-grade high-strength, high-toughness steels. The formation of ultrafine equiaxed grains in the steels might be explained by a possible strain-induced dynamic transformation mechanism associated with the austenite → ferrite transformation caused by heavy deformation in the austenite range.

Many studies have characterized the residual stress around TSV by using direct experimental measurement, finite element simulation, and microstructural analysis. The experimental approach is very useful in its speed and direct compatibility with processing, but cannot be used under all circumstances. Instrumented indentation testing, on the other hand, has many advantages: easy sample preparation and simple algorithms lead to a simple characterization of the micropartial stress between samples with the load difference at the same indentation depth. However, the current indentation method has the weak point that we cannot measure the residual stress when the stress-free state does not exist. Here we suggest a new algorithm to measure residual stress without knowledge of the stress-free state by analyzing the plastic pile-up mechanism around the contact area due to residual stress and using the invariant indentation properties obtained from the stressed state. We introduced this new algorithm into TSV-stress analysis to obtain the KOZ (keep out zone). In addition, we used in-situ SEM (scanning electron microscope) indentation to characterize more accurately the KOZ in the silicon around TSV, even in very minute areas.

Today, copper is the best material for TSV (through silicon via) filling because it can fill a large structural volume and it has good electrical performance. Nevertheless, copper-filled TSVs experience many reliability problems, the most serious of which are the stress-induced issues that can be large enough to cause interfacial fracture or delamination of complex interfaces. Many studies have been conducted to characterize the residual stress around TSV using direct experimental measurement, finite element simulation, and microstructural analysis. The experimental approach is very useful in its speed and direct compatibility with the process, but they have their own limitations to adapt to every case. Nanoinstrumented indentation testing, on the other hand, has many advantages: easy sample preparation and simple algorithms lead to a simple characterization of the micropartial stress between samples with the load difference at the same indentation depth. Here we introduce an algorithm to measure the micropartial residual stress in the copper area (the TSV itself) using conventional NIT (nanoinstrumented indentation testing). We used in-situ SEM (scanning electron microscope) indentation to characterize more accurately the keep-out zone in the silicon area (around TSV), even in very minute areas. Our study is useful in reliability-based quantitative design by defining keep-out zones below several micrometers between TSVs and other transistor-level devices.

The effects of local microstructure on fracture properties in powder-metallurgy (P/M)-processed 2124/SiC/15w and 2009/SiC/15w composites are analyzed in this study. Ductility and fracture toughness of the 2009/SiC/15w, in which dispersoid-forming elements such as manganese and iron were nearly absent, were greater than in the 2124/SiC/15w, while its tensile and yield strengths were somewhat less. Microstructural examination and fracture parameter analysis revealed that the improved fracture toughness of the 2009/SiC/15w compared to the 2124/SiC/15w was due to the increase in the critical microstructural distance,l* when manganese-containing particles are absent. 2009/SiC/15w was also heat-treated in T4P and overaged (OA) conditions. The OA 2009 composite showed lower fracture toughness than the 2009-T4P composite and the critical fracture strain of the OA condition was much lower, too. Detailed fractographic analyses indicated that interface precipitates facilitate premature SiC whisker failure in the OA condition.

An analytical model for predicting fracture toughness KIC is proposed based on a stress-modified critical strain criterion, that reflects the effect of stress triaxiality on ductile fracture. For KIC modelling, the notch-tip strain and stress state are given by introducing asingular field for the case of power-law-hardening materials. Notch fracture toughness is interpreted in terms of the notch-root radius (ϱ): KIC is predicted to increase with increasing ϱ, but has a minimum at a small ϱ. The microstructuralls characteristics distance and the reference critical strain can be estimated by fitting the KIC vs ϱ data on the model equation. Finally, previous notch fracture-toughness data are re-analyzed with the proposed model: the current analysis explains well the interaction effect between the notch-tip strain field and the local-fracture-controlling microstructure even in the small ϱ range.

Under very high loading rates, the dominant microfracture mechanism may be different from that under standard testing procedures. In dynamic loading conditions, in which material inertia plays a significant role, analysis of the microfracture behavior in materials that respond in either a nominally elastic manner or a fully plastic manner is quite complicated. Recently, Cho et al. performed the dynamic fracture tests to measure fracture toughness (K{sub Id}) values of 2124 Al composites reinforced with SiC whiskers. They found that the dynamic fracture toughness values were greater than those measured under quasi-static loading. Pickard et al. obtained stress-strain curves at a quasi-static strain rate of 0.0025 sec{sup {minus}1} and an impact rate of 1,500 sec{sup {minus}1} using a fully instrumented Hopkinson bar: yield strength, elastic modulus, and fracture strain were observed to increase with strain rate. From the results of fracture surface observations, Cho et al. assumed that the early stage of microvoid initiation under dynamic loading is similar to that found under quasi-static loading, and analyzed the fracture toughness data. However, this simple assumption may not be applicable in the fracture analysis of the 2124 Al-SiC{sub w} composites, since the microfracture mechanism under dynamic loading may be different frommore » that of the quasi-static case. Moreover, the dynamic fracture mechanics must taken into account the time factor in the toughness calculation. Thus, a more detailed study of the microfracture mechanism of the 2124 Al-SiC{sub w} composite under dynamic loading is required for a fundamental understanding of how microfracture process is affected by loading rates. This is the purpose of this paper.« less

As Moore predicted in 1965, the scale of microelectronic devices continues to diminish at tremendous speed, and today the limitations of conventional 2D scaling make such 3D applications as TSV (through-silicon via) and high-stacked thin-die packaging technologies extremely attractive. However, their complicated structures and thermal-cycled processes generate enormous interfacial stresses. In particular, TSV-to-CPB (copper pillar bump)-stacked structures manufactured under various processing conditions have serious stress-induced reliability problems: stresses can be high enough to cause delaminated or open-crack failures. Many technologies have been developed for measuring residual stress, but destructive techniques such as the hole-drilling and cutting methods are too bulky to use at microscales and non-destructive techniques such as XRD (X-ray diffraction), BN (Barkhausen noise) and the curvature method using the Stoney equation yield averaged results that are inappropriate in the local assessment of TSV and CPB interfaces. NIT (nanoinstrumented indentation testing), on the other hand, offers many advantages since it can give a micropartial characterization of stress using the load difference between samples with different residual stresses at the same depth. Here we introduce an algorithm to measure the micropartial residual stress between CPB and TSV through nanoinstrumented indentation testing. To verify our measured outputs, we observe cross-sectioned microstructure of TSV and CPB using an ion miller and ion-beam image by FIB (focused ion beam), and discuss the textures of variously structured and processed TSV and CPB interfaces. In addition, we used finite element analysis (ABSYS) to simulate the stress distribution around them. Our study will, we hope, be useful in reliability-based quantitative design by defining keep-out zones between TSVs.

Mechanical properties must be evaluated at high temperatures to predict high-temperature deformation and fracture behavior, since high-temperature properties differ greatly from those at room temperature. A high-temperature uniaxial tensile test, a representative high-temperature test, is generally used, but it has the limitation of obtaining merely the average material properties. Recently an advanced method for evaluating tensile properties has been developed: the instrumented indentation test (IIT), which simultaneously applies a load and measures displacement. Here we use instrumented indentation testing to evaluate the flow properties (yield strength, ultimate tensile strength, etc.) of heat-resistant steel at high temperature. The contact-area determination algorithm and representative stress-representative strain approach are applied for high temperatures. We compare our experimental results to those of conventional high-temperature uniaxial tensile testing to assess the high-temperature performance of the instumented indentation test.Copyright © 2011 by ASME

The high-temperature fracture behavior of a 2124Al-SiCw composite compared with a 2124A1 alloy was investigated in this study. Axisymmetric tensile tests were carried out over a temperature range from 25 °C to 550 °C and at strain rates from 5 × 10-5 s-1 to 0.3 s-1. Detailed fractographical observations and cross-sectional microstructure analyses were also made to identify local micromechanical processes of cavity initiation at high temperature. One of the important results is that the cavity initiation sites of the composite are strongly influenced by the strain rate at high temperatures: cavities initiate at whisker ends at low strain rates and at whisker sides at high strain rates. Furthermore, the favored direction of cavity growth is also dependent upon the strain rate, being approximately 45 deg at low strain rates and perpendicular to the tensile axis at high strain rates. Such different local fracture processes at different strain rates are interpreted in terms of the role of the SiC whiskers on the load carrier in the composite at high temperatures.

This study is concerned with a correlation of fracture toughness with microstructural factors in heat-affected zones (HAZs) of a normalized high-strength low-alloy (HSLA) steel. In order to explain weld joint performance, tensile and plane strain fracture toughness tests were conducted for the simulated coarse-grained HAZ microstructures. The micromechanisms of fracture processes involved in void and microcrack formation are identified byin situ scanning electron microscopy (SEM) fracture observations and void initiation study. The fracture toughness results are also interpreted using simple fracture initiation models founded on the basic assumption that a crack initiates at a certain critical strain or stress developed over some microstructurally significant distance. The calculated KIc values are found to scale roughly with the spacing of the stringer-type martensite islands associated with voids, confirming that martensite islands play an important role in reducing the toughness of the coarse-grained HAZs. These findings suggest that the formation of martensite islands should be prevented by controlling the chemical compositions and by using the proper welding conditions to enhance fracture toughness of the welded joints of the HSLA steel.

Continuous SiC fiber reinforced aluminum (Al-SiC[sub f]) composites generally have very high elastic modulus and strength combined with light weight, providing potential aerospace applications. In order to develop better composite materials with improved mechanical properties, however, a fundamental understanding of the relationship between microstructure and the mechanisms of deformation and failure is essential. A number of studies have been reported concerning the mechanical properties of Al-SiC[sub f] composites, and the recent studies have emphasized the importance of the interface between matrix and fibers since the presence of brittle reaction products at the interface can seriously degrade mechanical properties. There is, however, only limited information available on the effect of carbon-rich coating layer, which covers the fiber surface, on the fracture process of the Al-SiC[sub f] composites. Thus the principal objective of this study is to elucidate a more fundamental understanding of how the microfracture process is affected by microstructure and processing methods in a 6061 aluminum alloy composite reinforced with SiC fibers. Particular emphasis has been placed on studying in detail a mechanism of microfracture process at a blunt notch tip using an in situ fracture technique.

Most TSVs filled with plated copper offer many reliability problems. When subjected to thermal-cycled plating processes, the very large CTE (coefficient of thermal expansion) mismatch between the copper and the silicon/dielectric generates enormous interfacial thermal stress. In addition, the incoherency of differently grown copper grains plated under various processing conditions produces significant residual stress at grain boundaries that can be high enough to cause delamination or interfacial fracture. Many technologies have been developed for measuring residual stress, but they are too bulky to use at TSV microscales or yield averaged results inappropriate for the local assessment of TSV interfaces. Nanoinstrumented indentation testing, on the other hand, has many advantages in the micropartial characterization of residual stress using the load difference between samples with different residual stresses at the same depth. Here we introduce an algorithm to measure micropartial residual stress of TSV interfaces through nanoinstrumented indentation testing. To verify our measured outputs, we observed cross-sectional TSV morphologies for metallurgical analysis.

Welding residual stresses which are generated in almost all welded structures unavoidably can come to be serious cause of fracture and failure of in-service welded structures. Various techniques have been developed to measure and estimate welding residual stresses such as hole-drilling method, saw-cutting method, X-ray/neutron diffraction method and so on. The instrumented indentation technique (IIT) is being attracted to significant alternative as a measurement method of residual stresses because of it’s nondestructive characteristic and usefulness of measurement on local scales. Basic concept of IIT to evaluate residual stresses is to compare two indentation load-depth curves that are measured experimentally between under stress-free state and under stressed state. In case of using Vickers indenter, average surface residual stress can be measured quantitatively from analyzing measured load diffrerence. Each x, y directional residual stresses can be evaluate by using Knoop indenter. Indenting each directions with Knoop indenter, difference load-depth curves are measured under non-equibiaxial stress state. Residual stress directionality can be expressed as the function of the load-difference ratio calculated from the load-depth curves and the conversion factor ratio that is constant regardless of indentation depth. This function was verified with the experimental data and the results of finite element analyses on various biaxial stress states, Knoop indentation model showed good agreement between the experimental data and the simulation results.Copyright © 2010 by ASME

Understanding the property distribution in the weld zone is very important for structural safety, since deformation and fracture begin at the weakest point. However, conventional tensile tests can measure only average material properties because they require large specimens. Small-scale tests are being extensively researched to remove this limitation, among such tests, instrumented indentation test (IIT) are of great interest because of their simple procedures. Here we describe the evaluation of tensile properties using IIT and a representative stress-strain approach. The representative stressstrain method, introduced in 2008 in ISO/TR29381, directly correlates the stress and strain under the indenter to the true stress and strain of tensile testing by defining representative functions. Using this technique, we successfully estimate the yield strength and tensile strength of structural metallic materials and also obtain profiles of the weld-zone tensile properties.Copyright © 2010 by ASME

This article presents a correlation study between the microstructural parameters and fracture properties in the weld heat-affected zones (HAZs) of high-strength low alloy (HSLA) steels,i.e., a normalized steel and four thermomechanically controlled processed (TMCP) steels. The influence of the local brittle zone (LBZ) on toughness was investigated by means of simulated HAZ tests as well as welded joint tests. The intercritically reheated coarse-grained HAZ ex-hibited the lowest impact energy over the testing temperature range, indicating that this region was the LBZ. By comparing the volume fraction of martensite islands with impact energy val-ues, this LBZ was attributed mainly to the significant increase in the amount of martensite. Niobium was also found to have a deleterious effect on the HAZ fracture toughness because of martensite hardening. This suggests that the formation of martensite islands must be controlled by proper design of chemical compositions to reduce the carbon equivalent and by using the proper welding conditions to limit cooling rates in order to optimize the fracture toughness of welded joints of TMCP steels.

The effect of microstructure on the fracture toughness of 2124-T6 aluminum reinforced with 15 vol pct SiC whiskers was investigated using results of fracture toughness tests and SEM observations. In order to analyze fracture behavior of the composite, a micromechanical fracture model involving ductile fracture initiation was adopted, from which the critical characteristic distance and the critical equivalent strain were calculated to be 14 microns and 0.07, respectively. The computed critical distance agreed well with the average size of secondary roughness on fracture surfaces and with the spacing of large scale microstuctural parameters such as coarse Mn-containing particles, which significantly affect the fracture behavior of the 2124-T6 Al/SiC(w) composite. 14 refs.

We introduce a novel method to correct for imperfect indenter geometry and frame compliance in instrumented indentation testing with a spherical indenter. Effective radii were measured directly from residual indentation marks at various contact depths (ratio of contact depth to indenter radius between 0.1 and 0.9) and were determined as a function of contact depth. Frame compliance was found to depend on contact depth especially at small indentation depths, which is successfully explained using the concept of an extended frame boundary. Improved representative stress-strain values as well as hardness and elastic modulus were obtained over the entire contact depth.

The instrumented indentation test, which measures indentation tensile properties, has attracted interest recently because this test can replace uniaxial tensile test. An international standard for instrumented indentation test has been recently legislated. However, the uncertainty of the indentation tensile properties has never been estimated. The indentation tensile properties cannot be obtained directly from experimental raw data as can the Brinell hardness, which makes estimation of the uncertainty difficult. The simplifying uncertainty estimation model for the indentation tensile properties proposed here overcomes this problem. Though the influence quantities are generally defined by experimental variances when estimating uncertainty, here they are obtained by calculation from indentation load-depth curves. This model was verified by round-robin test with several institutions. The average uncertainties were estimated as 18.9% and 9.8% for the indentation yield strength and indentation tensile strength, respectively. The values were independent of the materials’ mechanical properties but varied with environmental conditions such as experimental instruments and operators. The uncertainties for the indentation yield and tensile strengths were greater than those for the uniaxial tensile test. These larger uncertainties were caused by measuring local properties in the instrumented indentation test. The two tests had the same tendency to have smaller uncertainties for tensile strength than yield strength. These results suggest that the simplified model can be used to estimate the uncertainty in indentation tensile properties.

The instrumented indentation technique (IIT) is a powerful tool for measuring mechanical properties by analyzing the load-penetration depth curve. It differs from conventional test methods such as tensile testing, CTOD, etc., in being applicable to small samples and to localized sections where material properties change rapidly. It also has the significant advantage of simplicity in specimen preparation and experimental procedure. Analytic models and procedures are presented here for evaluating flow properties and stress state using IIT; the flow properties are treated by defining the representative stress and strain underneath a spherical indenter and the residual stress by using a stress-insensitive contact hardness model. Flow properties of 5 steel materials were measured by IIT and compared with those from uniaxial tensile tests. The residual stress states of a welded joint were evaluated and compared with those measured by mechanical saw cutting. Examples of the application of IIT to small-scale materials and localized sections of pressure vessel and piping systems in situ are also presented.Copyright © 2009 by ASME

Instrumented indentation testing (IIT) is a very useful technology for the mechanical characterization of materials. However, existing IIT techniques, which are based on the Hertz model and were developed for hard materials with negligible surface adhesion such as metals and ceramics, are difficult to directly apply to compliant materials such as elastomeric polymers which have viscoelastic hysteresis due to infinitesimal surface/interfacial adhesion. Here we employed some modified model to evaluate the work of adhesion in elastomeric polymer from our previous work, and reinforced our theory and algorithm through empirical approaches so as to consider the time-dependency of viscoelastic material as testing parameter. To do these all, we combined analytic JKR theory and conventional IIT technology to analyze the physical meaning of the theory and then verified our ideas experimentally with elastomeric polymer, PDMS (poly(dimethyl-siloxane)), of various compositions. Our algorithm was developed and verified on a microinstrumented indentation basis and extended even into the nanoinstrumented testing for the micro/nano-scaled applications.

Most small-scale testing techniques are essentially scaled-down versions of conventional testing techniques: they use specimens of similar geometry applied in a similar manner to estimate properties equivalent to those obtained for larger specimens. However, using these techniques for safety assessment of structures and piping systems requires general agreement about the techniques and validation of their results. In addition, these techniques all require destructive testing. In this study we adopt a new nondestructive method to measure the mechanical properties using the instrumented indentation technique. This technique can be applied directly in small-scale and localized sections because of its high spatial resolution. It also has the significant advantage of simplicity of specimen preparation and experimental procedure. During instrumented indentation testing, the load and penetration depth of an indenter tip driven into the sample are monitored, and material properties such as strength, fracture toughness and residual stress are evaluated from this information: the tensile properties by defining a representative stress and strain underneath a spherical indenter; the residual stress values near weldments by using the stress-insensitive contact hardness model.Copyright © 2008 by ASME