Fracture Mechanics Samples Research Articles

Understanding of fatigue crack growth behavior in welded joint of a new generation Ni-Cr-Mo-V high strength steel

This study was mainly concerned with the long fatigue crack growth rate (FCGR) of a new generation Ni-Cr-Mo-V high strength steel and its welded joints used in modern marine environment. Several major factors, including stress ratio, microstructure, welding residual stresses (WRSs) and specimen thickness were investigated. Results showed that the weld metal (WM) possessed better resistance to crack propagation relative to the base material (BM) under each stress ratio owing to the interlocking acicular ferrite microstructure and the resultant higher crack closure level. WRSs partly retained in the extracted fracture mechanics samples and noticeably affected FCGR of the WM. Post-welding heat treatment could never eliminate the WRSs and might generate new residual stress distribution. Both the crack closure model and the two-parameter model yielded fairly good consolidation for the R-ratio effect of the BM whereas they failed to correlate the data for the WM because of the retained WRSs. Accelerated FCGR with increased specimen thickness was measured for both the BM and the WM. The fracture mechanism was scrutinized by means of fractography and fatigue trajectory map. The results obtained in the present work are helpful in shedding light on the damage tolerance design of the Ni-Cr-Mo-V high strength steel welded joints employed in modern marine structures.

Effect of Joint Scale and Processing on the Fracture of Sn-3Ag-0.5Cu Solder Joints: Application to Micro-bumps in 3D Packages

In 3-dimensional (3D) packages, a stack of dies is vertically connected to each other using through-silicon vias and very thin solder micro-bumps. The thinness of the micro-bumps results in joints with a very high volumetric proportion of intermetallic compounds (IMCs), rendering them much more brittle compared to conventional joints. Because of this, the reliability of micro-bumps, and the dependence thereof on the proportion of IMC in the joint, is of substantial concern. In this paper, the growth kinetics of IMCs in thin Sn-3Ag-0.5Cu joints attached to Cu substrates were analyzed, and empirical kinetic laws for the growth of Cu6Sn5 and Cu3Sn in thin joints were obtained. Modified compact mixed mode fracture mechanics samples, with adhesive solder joints between massive Cu substrates, having similar thickness and IMC content as actual micro-bumps, were produced. The effects of IMC proportion and strain rate on fracture toughness and mechanisms were investigated. It was found that the fracture toughness GC decreased with decreasing joint thickness (hJoint). In addition, the fracture toughness decreased with increasing strain rate. Aging also promoted alternation of the crack path between the two joint–substrate interfaces, possibly proffering a mechanism to enhance fracture toughness.

Effects of fatigue loading and PMMA precoating on the adhesion and subcritical debonding of prosthetic-PMMA interfaces.

Debonding of clinically relevant CoCrMo-polymethylmethacrylate (PMMA) interfaces is shown to occur subcritically under fatigue loading, implying that debonding may occur at loads much lower than those required for catastrophic failure. Interface fracture mechanics samples containing precoated and uncoated grit-blasted CoCrMo substrates and a PMMA layer were constructed and quantitatively evaluated in terms of their critical interface adhesion and subcritical debond behavior. The precoat surfaces had markedly enhanced adhesion and fatigue resistance in both air and simulated physiological environmental conditions compared to the uncoated samples. Constraint of the PMMA layer does not significantly affect the debond process for thickness between 2- and 5-mm. In addition, wear particles were collected and shown to be consistent with particle sizes reported in vivo and are on the scale of the metal surface roughness. Life prediction methods using the subcritical debond-growth data are discussed.

The Effects of Environment and Fatigue on the Adhesion and Subcritical Debonding of Dielectric Polymers

AbstractThe adhesion of thin film polymers will be critical in the integration of low-κ materials into microelectronic processing. This study describes the adhesion of two promising low-κ polymers (polyimide and benzocyclobutene) to a silicon dioxide surface. Critical adhesion values were measured using interface fracture mechanics samples in a double cantilever beam geometry. The effect of subcritical (time-dependent) delamination was also evaluated for these systems. Subcritical debonding data are important in understanding the effect of environment and temperature on interface reliability. To that end, experiments were conducted over a range of humidities to elucidate the effect of moisture on interface delamination. The important effect of the acceleration of debond growth rates due to cyclic loading is also described. In addition, XPS studies are presented to characterize the debond path in these layered systems.

The crack tip strain field of AISI 4340 Part I Measurement technique

This is the first paper in a study on the influence of the environment on the crack tip strain field for AISI 4340. A stressing stage for the environmental scanning electron microscope (ESEM) was constructed which was capable of applying loads up to 60 kN to fracture-mechanics samples. The measurement of the crack tip strain field required preparation (by electron lithography or chemical etching) of a system of reference points spaced at similar to 5 mu m intervals on the sample surface, loading the sample inside an electron microscope, image processing procedures to measure the displacement at each reference point and calculation of the strain field. Two algorithms to calculate strain were evaluated. Possible sources of errors were calculation errors due to the algorithm, errors inherent in the image processing procedure and errors due to the limited precision of the displacement measurements. Estimation of the contribution of each source of error was performed. The technique allows measurement of the crack tip strain field over an area of 50 x 40 mu m with a strain precision better than +/- 0.02 at distances larger than 5 mu m from the crack tip. (C) 1999 Kluwer Academic Publishers.

Adhesion and reliability of interfaces in cemented total joint arthroplasties.

Debonding of the prosthetic/polymethylmethacrylate interface has been implicated in the initial failure process of cemented total hip arthroplasties. However, little quantitative understanding of the debonding process, as well as of the optimum interface morphology for enhanced resistance to debonding, exists. Accordingly, a fracture-mechanics approach has been used in which adhesion at the interface is characterized in terms of the interface fracture energy, G (J/m2), and shown to be a strong function of the morphology, debonding length, and loading mode of the interface. Double-cantilever-beam and four-point-flexure fracture-mechanics samples containing four clinically relevant prosthetic surface preparations were prepared to survey a range of interface roughness and loading modes. Adhesion at the interface could not be characterized with a single-valued material property but was found to exhibit resistance-curve behavior in which resistance to debonding increased with both the initial debond extension and the roughness of the interface. Values of debonding initiation, Go, were relatively insensitive to the roughness of the surface and the loading mode, whereas steady-state fracture resistance of the interface, Gss, increased significantly with the roughness and shear loading of the interface. These quantitative results suggest that debonding of the prosthetic/polymethylmethacrylate interface may be primarily attributed to surface interactions such as interlocking and the pullout of rough asperities that occur behind the debond tip. A simple mechanics analysis of such interactions was performed and revealed increases in the fracture resistance of the interface that were consistent with experimentally measured values.

Validation of experimental and computational fracture assessment methods for flawed pressure components

Abstract Safety and integrity assessments of pressure boundary components require reliable knowledge of the material property values and the validated experimental and computational analysis methods. To improve the accuracy and validity of the experimental and computational fracture assessment methods, a four year Nordic research programme under the auspices of the Nordic Liaison Committee of Atomic Energy was initiated in 1985 and is now under completion. The main technical objective of the programme was to clarify how catastrophic failure can be prevented in pressure vessels and pipings. Experiments with small fracture mechanics specimens and pressure vessels were performed to validate the computational fracture assessment analysis. Two tests were conducted on a decommissioned full-scale chemical reactor pressure vessel from an oil refinery plant, and were extensively instrumented, e.g. by utilizing a 64-channel acoustic emission monitoring system. The scattering of their material property values were determined by numerous fracture mechanics samples. In addition, as a part of the experimental work, the reactor pressure vessel was repaired by welding after the first test. The repair was carried out without postweld heat treatment and welding was done by applying the temper-bead technique. Residual stresses were measured during and after welding. Different fracture assessment methods were developed and subsequently applied to the tested components. Inter-laboratory round robin programmes with the participation of several laboratories were arranged to examine elastic-plastic finite element calculations and fracture mechanics testing.

Testing needs for bcc materials in view of dynamic loadings in fusion reactors

The relevance, the testing needs and the evaluation methods for the dynamic material properties essential for structural design analysis with respect to the abnormal high-rate loading conditions in a fusion reactor are discussed. Emphasis is given to the dynamic fracture toughness and to bcc materials (like ferritic/martensitic steels, Mo or V-alloys) which exhibit brittle/ductile fracture transition temperatures. These increase with n-fluence and loading rate, as well as, in general, also with increasing specimen size and notch acuity. Aiming at realistic transition temperatures it is proposed therefore to systematically investigate the correlation between the results of pre-fatigued impact specimens (of standard and sub-size) and of dynamic fracture mechanics samples of proper dimensions instead of carrying out extensive standard Charpy impact tests.

The influence of static stress on the corrosion behavior of 316L stainless steel in Ringer's solution.

The decrease in corrosion resistance of 316L stainless steel due to static stress was studied in vitro using a 37 degree C Ringer's solution electrolyte. Both potentiodynamic polarization and coulometric techniques were used. Cyclic anodic polarization tests with highly loaded fracture mechanics samples revealed a lowering of breakdown potential and disruption of passive films compared to unstressed controls. Measurements of the time-averaged current density due to a 100 mV anodic overpotential showed that a stress level causing plastic deformation increases the current density by more than an order of magnitude compared to samples stressed to the yield stress or nonloaded controls. The significance of these findings for surgical implant devices in service is discussed.

High-cycle fatigue properties of the ODS-alloy MA 6000 at 850 °C

The high cycle fatigue (HCF) and cyclic crack growth rate (CCGR) properties of the dispersion strengthened ODS-alloy MA 6000 were investigated with smooth bars and with fracture mechanics samples at 850 °C. The material was very coarse grained with the grains elongated in the rolling direction. Fatigue crack initiation and crack propagation were studied parallel and perpendicular to the rolling direction and a pronounced influence of orientation was found. The fatigue limit of sam-ples cut parallel to the grain elongation direction (p-samples) was almost a factor of 2 higher than the one of samples cut transverse to the elongation direction (t-samples). Inclusions were found to be responsible for crack initiation. For p-samples a reasonable agreement between particle size, fatigue limit, and crack growth behavior was found. For t-type samples such an agreement also exists provided differences in the crack growth behavior of short cracks and long cracks are taken into consideration. The low fatigue strength of t-samples could be linked with low Young's modulus in this direction. The crack propagation rate of long cracks is lower in t-samples than in p-samples due to crack branching along the grain boundaries. HCF-strength of MA 6000 is high compared to conventional cast alloys mainly because of reduced size of crack nucleation sites and higher fatigue threshold stress intensity range ΔKth, as a result of higher Young's modulus.

High-cycle fatigue-life of the cast nickel base-superalloys in 738 LC and IN 939

The high cycle fatigue (HCF) properties of two cast nickel base-superalloys, IN 738 LC and IN 939, were investigated using both fracture mechanics samples and smooth specimens. The crack propagation behavior was studied in terms of linear fracture mechanics at RT and at 850 °C. In addition to the influence of temperature, the influences of frequency, mean stress, and environment (vacuum, air, sulfidizing atmosphere) were studied. At 850 °C, the fatigue thresholds were found to be higher in air than in vacuum. This could be explained by crack branching. The high scatter of fatigue crack propagation rates could be related also to this phenomenon. The S/N curves at 850 °C can be predicted treating crack growth from casting pores as the predominant failure mechanism. At RT the same method is not as successful. The reason for this may be that crack growth laws measured on long, branched cracks are not applicable to short, unbranched cracks. At RT, no significant influence of frequency on S/N-curves and fatigue crack growth rates was observed for frequencies up to 20 kHz.

Corrosion of Ticode-12 in A Simulated Waste Isolation Pilot Project (Wipp) Brine

ABSTRACTThe corrosion behavior of TiCode-12 (Ti-0.3 Mo-0.8 Ni) high level nuclear waste container alloy has been studied for a simulated WIPP brine at a temperature of 150°C or below. Crevice corrosion was identified as a potentially important failure mode for this material. Within a mechanical crevice, a thick oxide film was found and shown to be the rutile form of TiO2, with a trace of lower oxide also present. Acidic conditions were found to cause a breakdown of the passive oxide layer. Solution aeration and increased acidity accelerate the corrosion rate. In hydrogen embrittlement studies, it was found that hydrogen causes a significant decrease in the apparent stress intensity level in fracture mechanics samples. Hydride formation is thought to be responsible for crack initiation. Stress corrosion cracking under static loads was not observed. Attention has also been given to methods for extrapolating short term uniform corrosion rate data to extended times.