Measurement Of Stress Intensity Factor Research Articles

MFC 센서를 이용한 응력 확대 계수 측정에 관한 연구

Recently, the structural failures due to fatigue occur frequently with the increase of size of ships and offshore structures. In this respect, the assessment of fatigue life and the residual strength are very important. Currently, the smart materials technology has demonstrated a variety of possibilities for a diagnosis of structural strength and structural health condition for large structures. The benefits and feature of the MFC sensor are more flexible, durable and reliable than conventional smart material. In this study, Micro Fiber Composite (MFC) sensor for the measurement of stress intensity factor (SIF) of two dimensional cracks induced in a structure is developed. Two MFC sensors are placed in the vicinity of the crack tip close to each other with the crack tip in between them. The SIFs of Mode I(<TEX>$K_I$</TEX>) as well as of Mode II(<TEX>$K_{II}$</TEX>) based on the piezoelectric constitutive law and fracture mechanics are calculated. In this study, the SIF values measured by MFC sensors are compared with the theoretical results and measured value.

Contact caustics measurements expanded to anisotropic materials

Regions with high stress gradients represent critical zones of engineering structures, such as crack tip or vicinity of the contact zone. The optical method of caustics is one of the few experimental methods which provide applicable results in these locations. This method was originally developed for crack-tip measurements of stress intensity factors and J-integral for isotropic and then for anisotropic materials. Subsequently, it was extended to the contact problems, where the size and the shape of caustics are related to the amount and the inclination of loading force. Here, the application of the method of caustics is extended to the analysis of contact problems for mechanically anisotropic materials. This makes the caustics method widely applicable to the analysis of any high stress gradient locations in a structure.

313 平板試験片中の分岐切欠の開口変位と応力拡大係数の測定(OS3.計算力学に貢献する実験的アプローチ(4),OS・一般セッション講演)

313 平板試験片中の分岐切欠の開口変位と応力拡大係数の測定(OS3.計算力学に貢献する実験的アプローチ(4),OS・一般セッション講演)

Mode I and Mode II Stress Intensity Factors Based on Crack Opening and Sliding Displacements

It is necessary to study crack opening and sliding discontinuous displacement behavior under mixed-mode conditions because parts or structures of a machine with a crack maybe subject to stress from various directions. In this study ,therefore, using the cracks with different slant angle, which are made in circle stress of modeⅠwith stress ratio of R=0, the opening and sliding discontinuous displacements are measured ,so that modeⅠand mode Ⅱ stress intensity factors (KⅠ)mes and (KⅡ)mes at the crack tip are calculated. As a result, the measured stress intensity factors value of (KⅠ)mes from the fatigue crack with the slant angle β=60 deg. is smaller than the theoretical one (KⅠ). But for mode Ⅱ,(KⅡ)mes is about the same with (KⅡ). On the other hand, for the fatigue cracks with smaller slant angle β=45 deg.,(KⅡ)mes declined because of the crack-surface contact while (KⅠ)mes reduced.

Morphology and Fracture Toughness of Nanostructured Epoxy Resin Containing Amino-Terminated Poly(propylene oxide)

Amino-terminated poly(propylene oxide) (ATPPO) was incorporated into epoxy resin to toughen thermosets. It was found that nanostructured thermosets were obtained; the nanostructures were characterized by means of atomic force microscopy and small-angle X-ray scattering. The formation of the nanostructures is interpreted on the basis of the occurrence of the reaction of terminal groups of ATPPO with diglycidyl ether of bisphenol A; this reaction is suggested to result in the formation of star-shaped block copolymers composed of poly(propylene oxide) (PPO) and epoxy blocks. Due to the presence of the star-shaped block copolymer produced in situ, the phase separation of PPO induced by the reaction was confined to the nanometer scale. The glass-transition behavior and fracture toughness of the nanostructured thermosets were investigated by means of differential scanning calorimetry, dynamic mechanical thermal analysis, and the measurement of critical stress intensity factors. The epoxy thermosets were significantly toughened by the inclusion of a small amount of ATPPO. The thermal and mechanical properties of the nanostructured thermosets are compared to the binary blends of epoxy resin containing hydroxyl-terminated poly(propylene oxide) (HTPPO) with identical molecular weight. With the identical composition, the nanostructured thermosets displayed higher fracture toughness than that of their binary blends. The difference in morphology and properties is interpreted in terms of the formation of the nanostructures.

Influence of finite notch root radius and optically determined crack length on the measured fracture toughness of brittle materials

When measuring the fracture toughness of ceramics, for instance with the Single Edge V-Notched Beam (SEVNB) method, a known problem is given by the finite notch root radius. If this radius is larger than a certain small value of a few micrometers, the measured stress intensity factor is higher than the true intrinsic value. Another problem is that due to technical and physical reasons the optically measured crack length at stable crack growth is smaller than the true crack length, which might lead to an underestimation of the fracture toughness. In the given paper an experimental approach is presented to register these effects quantitatively with stable crack advance and to finally obtain the correct fracture toughness. This is achieved by applying a new computer aided automatic control system for stable crack advance and by analyzing the results in force–displacement and crack length–force diagrams. The used material is zirconia-toughened alumina.

Radial locations of strain gages for accurate measurement of mode I stress intensity factor

Strain gage methods are popular in experimental determination of stress intensity factors (SIFs). Radial location of gages with respect to the crack tip plays an important role in accuracy of strain measurements and thus accurate determination of SIFs. The present work proposes a finite element based simple, accurate and consistent method for determination of the limiting value of the radial distance ( r max) of a strain gage. This parameter is in turn useful in deciding the valid strain gage location for accurate measurement of opening mode SIF. The results obtained from the present investigation agree well with the theoretical predictions and could be used for experimental determination of SIFs for both single ended and double ended cracked specimens. The r max values of center cracked and edge cracked plates with different crack length to width ratio are estimated. The results of the present investigation show that the relative size of the crack length and net ligament length strongly influences the r max value and the effect of Poisson’s ratio is marginal on the r max value.

The caustics method in the contact problems of anisotropic materials

Regions with high stress gradients represent critical zones of engineering structures such as crack tip or vicinity of the contact zone. The optical method of caustics is one of the few experimental methods which provide applicable results in these locations. This method was originally developed for crack-tip measurements of stress intensity factors and J-integral for isotropic and then for anisotropic materials. Subsequently, it was extended to contact problems where the size and shape of caustics on the screen are related to the amount and the inclination of loading force. Here, the method of caustics is extended to the analysis of contact problems for mechanically anisotropic materials. This makes the caustics method widely applicable to the analysis of any high stress gradient locations in a structure.

(3) Measurement of Opening Displacement and Stress Intensity Factor of Bifurcated Notch by Moire Interferometry

(3) Measurement of Opening Displacement and Stress Intensity Factor of Bifurcated Notch by Moire Interferometry

Crack closure and stress intensity factor measurements in nodular graphite cast iron using three-dimensional correlation of laboratory X-ray microtomography images

Three-dimensional (3-D) tomographic images of a nodular graphite cast iron obtained using a laboratory X-ray source were used to analyze the opening of a fatigue crack during in situ mechanical loading. Direct image analysis and digital image correlation are utilized to obtain the 3-D morphology and front location of the crack, as well as the displacement fields in the bulk of the specimen. From digital image correlation results it is possible to extract the crack opening displacement (COD) map in the whole sample cross-section and to compute stress intensity factors (SIFs) all along the crack front, even for COD values that are less than the image resolution. The comparison of COD maps with local values of the SIF enabled for an estimation of the opening SIF ( K op ) equal to 6 MPa m 1/2.

Breakdown of Linear Elastic Fracture Mechanics near the Tip of a Rapid Crack

We present high resolution measurements of the displacement and strain fields near the tip of a dynamic (mode I) crack. The experiments are performed on polyacrylamide gels, brittle elastomers whose fracture dynamics mirror those of typical brittle amorphous materials. Over a wide range of propagation velocities (0.2-0.8c(s)), we compare linear elastic fracture mechanics (LEFM) to the measured near-tip fields. We find that, sufficiently near the tip, the measured stress intensity factor appears to be nonunique, the crack tip significantly deviates from its predicted parabolic form, and the strains ahead of the tip are more singular than the r(-1/2) divergence predicted by LEFM. These results show how LEFM breaks down as the crack tip is approached.

Determination of strain gage locations for the accurate measurement of opening mode stress intensity factors

Determination of strain gage locations for the accurate measurement of opening mode stress intensity factors

Evaluation of Residual Stress Corrections to Fracture Toughness Values

Abstract This paper describes tests carried out to evaluate whether comparable values of fracture toughness may be determined from coupons that contain a range of residual stress. Coupons for fracture toughness testing are taken from a single rolled plate of 7075-T6 aluminum alloy and are fabricated using identical methods. Following fabrication, residual stresses are induced in coupon subsets using laser shock peening. Each coupon subset has a unique laser peening treatment design, such that following fatigue precracking, the stress intensity factor due to residual stress varies over the coupon subsets created from positive to neutral to negative. Subsequent fracture toughness tests show expected trends in the varying toughness values for each coupon subset. Measurements of residual stress on the crack plane, and measurements of the residual stress intensity factor, enable correction of the toughness data through superposition and linear elastic fracture mechanics. The resulting corrected fracture toughness values provide data useful for evaluating residual stress corrections with potential for application to standard fracture test methods. Such corrections would enable measurements of intrinsic fracture toughness in samples that contain significant levels of residual stress.

Measurement of Opening Displacement and Stress Intensity Factor of Bifurcated Notch by Moire Interferometry

Static experiments are performed to confirm the COD method that is the only method to measure the energy release rate of fast propagating cracks just after bifurcation. The crack opening displacement (COD) of the mother cracks of bifurcated cracks is measured in static condition by Moiré interferometry. Thin notches are used instead of cracks. The bifurcation angle is 13.5 degrees that is the same as the angle of bifurcation of fast propagating cracks in PMMA. The measured CODs are proportional to the square root of the distance from the nominal tip of the mother crack. Stress intensity factor of the bifurcated crack is obtained from the measured CODs through the formula of the COD of single crack. The experimental results say that the stress intensity factor of a bifurcated crack has the same value as that of the single crack whose length is the same as that of the bifurcated crack. This is caused by the small bifurcation angle of 13.5 degrees. It is concluded that the COD method is correct to measure the energy release rate of rapidly bifurcating cracks.

Mechanical properties of epoxy resin modified with polycarbonate and reactive polybutadiene

AbstractEpoxy resin Epidian 5 cured with triethylene tetramine was modified with hydroxyl‐terminated polybutadiene (PB) and polycarbonate (PC). Compositions with different amount of modifiers were obtained and tested for their impact strength, flexural strength, as well as resistance to crack propagation. The latter was assessed by evaluating the critical stress intensity factor under three‐point bending mode using single‐edge notched specimens. Scanning electron microscopy was used to analyze the fractured sample surfaces. The obtained results revealed that the mechanical properties of epoxy resin were improved due to the formation of heterogeneous phase with rubber particles, which arrest the propagation of cracks. Moreover, synergism effect was observed with the hybrid composition containing 10% PC and 2.5% of reactive PB. The impact strength was higher by ∼ 15% than the sum of impact strength of compositions containing only one modifier. Another hybrid composition with 2.5% PB and 2.5% PC also exhibited synergism effect with the flexural strain at break, the energy at break under flexure, as well as the brittle fracture energy estimated from the critical stress intensity factor measurements. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2007

Measuring stress intensity factors with a camera: Integrated digital image correlation (I-DIC)

A novel ‘integrated’ approach coupling image correlation and elastic displacement field identification provides a powerful and accurate tool to evaluate mode I and II stress intensity factors. This technique is applied to silicon carbide subjected to a sandwiched three-point bend test, using digital pictures obtained in optical microscopy where the pixel physical scale is about 2 μm. A crack whose maximum opening is 500 nm can be detected and its geometry identified. The toughness is determined well within a 10%uncertainty. To cite this article: F. Hild, S. Roux, C. R. Mecanique 334 (2006).

Stress intensity factor measurements from digital image correlation: post-processing and integrated approaches

Digital image correlation is an appealing technique for studying crack propagation in brittle materials such as ceramics. A case study is discussed where the crack geometry, and the crack opening displacement are evaluated from image correlation by following two different measurement and identification routes. The displacement uncertainty can reach the nanometer range even though optical pictures are dealt with. The stress intensity factor is estimated with a 7% uncertainty in a complex loading set-up without having to resort to a numerical modelling of the experiment.

Roles of acidic functional groups of carbon fiber surfaces in enhancing interfacial adhesion behavior

The gas phase ozone treatment was used as a method to bind acidic oxygen functional groups on carbon fiber surfaces. The ozone treatment on carbon fibers was varied with the ozone concentration and treatment time. Surface analyses of the carbon fibers before and after treatments were performed by FT-IR, X-ray photoelectron spectrometer (XPS), and dynamic contact angle measurements. Mechanical interfacial properties of the fibers/polymer composites were investigated by using critical stress intensity factor ( K IC) and critical energy release rate ( G IC) measurements. From the results of FT-IR and XPS, it was observed that the oxygen functional groups, such as OH, O C O, C O, and C O, were attached on the carbon fiber surfaces after the ozone treatment. The mechanical interfacial properties of the composites also showed higher values than those of untreated composites. Ozone treatment is attributed to the increase of both the acidic functional groups and the degree of adhesion at interfaces between the fibers and polymeric resin in composites.

Measurement of stress intensity factors by digital camera

This paper developed a least-squares method to evaluate the mixed-mode stress intensity factors (SIFs) of the isotropic material using the computer images from the digital-camera experiment. This experiment measures the crack opening displacement (COD) and then evaluates SIFs by using the least-squares method. The attractions of this method include: (1) specimen preparation and experiment procedures are not complicated and (2) the isolation of the micro-vibration is not necessary in the experiment. Both finite element simulations and laboratory experiments were applied to validate the current least-squares method with acceptable accuracy, if the even terms of the Irwin’s equation are removed.

Estimation of mixed-mode stress intensity factors using digital image correlation and an interaction integral

This paper presents a technique for the experimental measurement of stress intensity factors in cracked specimens under mixed-mode loading. This technique is based on full-field measurement using digital image correlation and an interaction integral. Such domain-independent integrals are often used in the finite element method to calculate stress intensity factors. The main advantage of this technique is that the errors made in the estimation of the measured displacement field near the crack’s tip do not affect the measurement of the stress intensity factors. The capabilities of the method are illustrated through fracture measurements on compact tension specimens made of maraging steel. Another test under mixed-mode loading is presented.