Short Fatigue Crack Characterization and Detection Using Confocal Scanning Laser Microscopy (CSLM)

The majority of data on the development of fatigue cracks in tubular joints has come from tests on simple T or Y joints. As part of a joint industry research programme, fatigue tests were carried out on four complex overlapping YT joints. The crack development was monitored using both eddy current and acpd techniques. The crack growth characteristics are presented as a function of the normalised crack growth through the thickness a/af versus the normalised number of cycles to failure N/Nf and as the aspect ratio a/c versus the normalised crack depth a/af. The crack growth rate through the thickness a/al is also determined and the stress intensity factor for the tubular joints inferred from known and accepted crack growth laws. Knowledge of crack development for tubular joints is crucial when planning optimised inspection programmes based on surface crack detection. Knowledge of the crack growth development allows estimates to be made of the associated crack depth and time to failure when a surface crack is detected. Inference of the stress intensity factory for tubular joints allows benchmarking of analytical models to be carried out. The results show that a significant difference in crack development occurs for overlapping joints than for simple joints. Crack initiation occupies more of the total life and through thickness cracking develops much later for overlapping joints compared with simple joints. This variation in crack depth is reflected in the aspect ratio, a much lower value being obtained for the overlapping joints. For the geometries tested, cracking also occurred in the brace wall and not the chord wall. The results allow greater accuracy in predicting time to failure of these joints, once a surface crack is developed. This is of importance in planning optimised inspection strategies and determining how and when to repair detected fatigue cracks. 1.0 INTRODUCTION Fatigue testing of tubular joints has concentrated mainly on the derivation of design life S-N curves for ultimate joint life and ignored, to a certain extent, the study of crack formation and growth. However, development of a reliable system for measuring crack depth during the UKOSRP I programme (1) has enabled greater observations of crack development to be made. Knowledge of the behaviour of damaged components has numerous benefits. It allows analytical models to be modified and benchmarked for accuracy. It also enables understanding of development of damage to final failure for different types of component. This is particularly important in recognising that the way a crack penetrates through a tubular member may vary with joint type. Furthermore, predictions of crack growth rely entirely on growth constants derived from experimental tests under various conditions. In addition to the fatigue crack growth constants, two areas of crack development are of particular interests:the development of cracks through the thickness with respect to type of joint tested, andthe aspect ratio of the crack, defined here as the ratio of crack depth 'a' to the half crack length 'c'.

The experimental analysis of high temperature fracture in Aluminum Alloy 2519-T87 presented in Part I of this paper highlighted the creep-brittle fracture characteristics of the material and showed reasonable correlation of crack growth rates with the stress intensity factor K. Part II continues this investigation numerically using growing crack finite element analyses. Experimentally observed crack growth histories of four aluminum 2519-T87 compact specimens are enforced by controlling the rate of release of finite element nodes along the crack growth path to gain insight into the relation of the crack tip fields to far field fracture parameters and to crack growth rates. A variable time-step, nodal-release algorithm is presented to model the high strain rates that occur during the initial stages of crack growth. The numerical results indicate an initial transient period of crack growth followed by a quasi-steady-state crack growth regime in which the crack tip fields change slowly with increasing crack length. Transition of crack growth to the quasi-steady-state regime, where similitude and small-scale creep conditions roughly exist, is given by a transition time tg that depends on the crack growth history and material properties. Excellent correlation of the stress intensity factor K with the crack growth rates is observed after time tg. Experimental difficulties in measuring the creep component of the load-line deflection rate are also discussed.

This study presents the effects of the geometry of the notches on the fatigue life, crack initiation, and growth in mixed mode and first mode of the fracture. For this purpose, some samples were tested with U-shaped notches with different radii and depths and various orientations with respect to the loading directions under fatigue loads. Using the fatigue crack growth rates, the Paris law coefficients were obtained for the used material in different conditions. It was shown that these coefficients are independent of the geometry of the samples. Fatigue crack growth behaviors in the mixed mode tests were also in good agreement with the result of the numerical simulations. Predictions of the maximum tangential stress criterion and numerical simulation for the position of the crack initiation on the notch root were compared with the tested observations, which showed good agreement. The fatigue life of the test samples was compared with the analytical results provided by the Manson-Coffin law. It was shown that a 25% decrease in notch depth increases the fatigue life by 4 to 6 times, also, an increase of 0.5 mm in the notch radius increases the fatigue life by 40%. Finally, the fracture surface of the samples was checked using an optical microscope. This study showed that the fracture surfaces have one or two lateral shear lips and plane stress conditions were established.

The paper contains the main results of an investigation of an effect of shot peening on residual fatigue life of specimens made of an aircraft V-95 Al-alloy (equivalent to 7075 alloy) containing physically short fatigue cracks. In the first part, an adaptation and verification of direct current potential drop method for detection and measurement of short crack growth is described and discussed. The specific properties of the material and quite large dimensions of flat specimens with side necking with a low stress concentration factor had to be considered when the position of electrodes was specified. The specimen type and dimensions were proposed considering the investigation of shot peening effects. Physically short fatigue cracks of the length from 0.2 mm to more than 3 mm, most of them between 0.8 – 1.5 mm, were prepared under high cycle fatigue loading of the constant nominal stress amplitude ±160 MPa. Specimens with existing short fatigue cracks were shot peened using two different groups of parameters. Crack growth characteristics after shot peening were measured and compared with crack growth in specimens without shot peening. Retardation of crack growth was significant particularly for cracks shorter than 2 mm. Results are also discussed form the viewpoint of fracture mechanics approach. For the specific stress amplitude, evaluated results enable to estimate threshold length of defects, which after the application of shot peening will be reliably arrested.

Fatigue crack growth rate does not depend on mantle thickness: an idealized cemented stem construct under torsional loading

Fatigue crack growth studies were carried out using 7075-T651-aluminum alloy specimens with a semi-elliptical surface crack. Multiple experiments were conducted under constant amplitude loading with a stress ratio R = 0.1, which inherently included the effects of fatigue crack closure, and R = 0.7, which was assumed closure free. Crack length measurements on the free surface were made with a traveling microscope. To allow crack depth measurements, marker bands were used. The aspect ratio evolutions for the two stress ratios tested were compared to determine whether crack closure affects aspect ratio or crack shape evolution. It was found that closure impacts this evolution in a significant manner. In addition, the fatigue crack growth data collected from the multiple specimens was used to perform uncertainty analyses on the crack lengths, crack depths, and the aspect ratio. This was done to determine if crack growth models are best validated using the aspect ratio as the measured quantity or the individual measured crack length and crack depth. The measured aspect ratio was found to exhibit a smaller percentage of experimental uncertainty, and thus should be used for model validations. Finally, the observed crack growth rate-stress intensity factor range relationships for R = 0.7 at the free surface and the deepest point of penetration were compared to existing through-crack data, with a good correlation being observed.

PurposeThis study aims to investigate the reliability issues of microvoid cracks in solder joint packages exposed to thermal cycling fatigue.Design/methodology/approachThe specimens are subjected to JEDEC preconditioning level 1 (85 °C/85%RH/168 h) with five times reflow at 270°C. This is followed by thermal cycling from 0°C to 100°C, per IPC-7351B standards. The specimens' cross-sections are inspected for crack growth and propagation under backscattered scanning electronic microscopy. The decoupled thermomechanical simulation technique is applied to investigate the thermal fatigue behavior. The impacts of crack length on the stress and fatigue behavior of the package are investigated.FindingsCracks are initiated from the ball grid array corner of the solder joint, propagating through the transverse section of the solder ball. The crack growth increases continuously up to 0.25-mm crack length, then slows down afterward. The J-integral and stress intensity factor (SIF) values at the crack tip decrease with increased crack length. Before 0.15-mm crack length, J-integral and SIF reduce slightly with crack length and are comparatively higher, resulting in a rapid increase in crack mouth opening displacement (CMOD). Beyond 0.25-mm crack length, the values significantly decline, that there is not much possibility of crack growth, resulting in a negligible change in CMOD value. This explains the crack growth arrest obtained after 0.25-mm crack length.Practical implicationsThis work's contribution is expected to reduce the additional manufacturing cost and lead time incurred in investigating reliability issues in solder joints.Originality/valueThe work investigates crack propagation mechanisms of microvoid cracks in solder joints exposed to moisture and thermal fatigue, which is still limited in the literature. The parametric variation of the crack length on stress and fatigue characteristics of solder joints, which has never been conducted, is also studied.

Photoelastic study of initial stages of brittle fracture in compression

Diffraction and phase contrast tomography techniques were successfully applied to an austenitic–ferritic duplex stainless steel representing exemplarily a metallic material containing two phases with different crystal structures. The reconstructed volumes of both phases were discretized by finite elements. A crystal plasticity finite-element analysis was executed in order to simulate the development of the experimentally determined first and second order residual stresses, which built up due to the manufacturing process of the material. Cyclic deformation simulations showed the single-grain-resolved evolution of initial residual stresses in both phases and were found to be in good agreement with the experimental results. Solely in ferritic grains, residual stresses built up due to cyclic deformation, which promoted crack nucleation in this phase. Furthermore, phase contrast tomography was applied in order to analyze the mechanisms of fatigue crack nucleation and short fatigue crack propagation three-dimensionally and nondestructively. The results clearly showed the significance of microstructural barriers for short fatigue crack growth at the surface, as well as into the material. The investigation presented aims for a better understanding of the three-dimensional mechanisms governing short fatigue crack propagation and, in particular, the effect of residual stresses on these mechanisms. The final goal was to generate tailored microstructures for improved fatigue resistance and enhanced fatigue life.

Mechanical aspects of corrosion fatigue and stress corrosion cracking

In order to measure the Mode II fatigue crack growth resistance of SS41 steel and a rail steel, a particular double-cantilever type specimen with side grooves was designed. Mode II fatigue crack growth tests by using the newly designed specimens were conducted by a conventional closed-loop type tension-compression fatigue testing machine under a constant load amplitude which corresponds to ΔKII-decreasing test with increase in crack length. The crack length was measured by the AC potential method. Mode II crack growth rate gradually decreased as the crack grew. The typical fracture surface of Mode II fatigue crack growth which was quite different from Mode I fatigue fracture surface was observed. But the trace of local branching of crack growth mode from Mode II to I was observed on the overall fracture surface of Mode II. The relationship between Mode II crack growth rate (da/dN) and ΔKII was obtained.

Prediction of short fatigue crack propagation behaviour by characterization of both plasticity and roughness induced crack closures

Subcellular Localization of Disease-Associated Prion Protein in the Human Brain

Bringing Histology to the Bedside

Confocal fluorescence microscopy is a well‐established imaging technique capable of generating thin optical sections of biological specimens. Optical sectioning in confocal microscopy is mainly determined by the size of the pinhole, a small aperture placed in front of a point detector. In principle, imaging with a closed pinhole provides the highest degree of optical sectioning. In practice, the dramatic reduction of signal‐to‐noise ratio (SNR) at smaller pinhole sizes makes challenging the use of pinhole sizes significantly smaller than 1 Airy Unit (AU). Here, we introduce a simple method to “virtually” perform confocal imaging at smaller pinhole sizes without the dramatic reduction of SNR. The method is based on the sequential acquisition of multiple confocal images acquired at different pinhole aperture sizes and image processing based on a phasor analysis. The implementation is conceptually similar to separation of photons by lifetime tuning (SPLIT), a technique that exploits the phasor analysis to achieve super‐resolution, and for this reason we call this method SPLIT‐pinhole (SPLIT‐PIN). We show with simulated data that the SPLIT‐PIN image can provide improved optical sectioning (i.e., virtually smaller pinhole size) but better SNR with respect to an image obtained with closed pinhole. For instance, two images acquired at 2 and 1 AU can be combined to obtain a SPLIT‐PIN image with a virtual pinhole size of 0.2 AU but with better SNR. As an example of application to biological imaging, we show that SPLIT‐PIN improves confocal imaging of the apical membrane in an in vitro model of the intestinal epithelium.Research HighlightsWe describe a method to boost the optical sectioning power of any confocal microscope. The method is based on the sequential acquisition of multiple confocal images acquired at different pinhole aperture sizes. The resulting image series is analyzed using the phasor‐based separation of photons by lifetime tuning (SPLIT) algorithm. The SPLIT‐pinhole (SPLIT‐PIN) method produces images with improved optical sectioning but preserved SNR. This is the first time that the phasor analysis and SPLIT algorithms are used to exploit the spatial information encoded in a tunable pinhole size and to improve optical sectioning of the confocal microscope.