Showing papers on "Fractography published in 1975"

The fracture of epoxy- and elastomer-modified epoxy polymers in bulk and as adhesives

Abstract: The fracture behavior of a piperidine/bisphenol A diglycidyl ether (A) resin has been determined in bulk and as an adhesive using the linear elastic fracture methods developed by Mostovoy1. The effect of adding carboxy-terminated butadiene–acrylonitrile (CTBN) elastomer to resin A was investigated. The opening-mode fracture energy () of resin A was 120 to 150 J/m2, and largely attributable to plastic deformation. Fractographic evidence was obtained for plastic flow at the crack tip during crack initiation. Propagation was unstable due to the rate dependence of the plasticity. There were no significant differences in the bulk and adhesive fracture behavior. Addition of 5–15% CTBN to resin A produced minute elastomer particles which increased to ∼4000J/m2 (at 15%). Further CTBN addition resulted in an elastomer–epoxy blend and a decrease in fracture energy. Fractography again indicated that crack initiation involved plastic deformation but that the elastomer had greatly increased the volume in which the deformation occurred. The adhesive fracture of the elastomer–epoxy was found to be strongly dependent on the crack-tip deformation zone size (ryc) in that was a maximum when bond thickness was equal to 2 ryc. At bond thicknesses less than 2 ryc, there was a restraint on the development of the plastic zone resulting in lower values.

Low cycle fatigue, fatigue crack propagation and substructures in a series of polycrystalline Cu-Al alloys

Abstract: Low Cycle Fatigue (LCF) on smooth hour glass specimens and Fatigue Crack Propagation (FCP) studies on Single Edge Notch (SEN) specimens were carried out at room temperature on four Cu-Al polycrystalline alloys to investigate the effects of Stacking Fault Energy (SFE) and mechanical property variations on fatigue characteristics. Significant improvements in fatigue properties were observed for alloys of low SFE. A microhardness technique was used to delineate the fatigue plastic zone ahead of stopped cracks at several stress intensity ranges for all the alloys. Planar slip was associated with a less than a second power dependence of plastic zone size on the stress intensity range. Transmission Electron Microscopy (TEM) was used to observe the substructures that developed both in LCF at different strain ranges and also ahead of fatigue cracks at different stress intensity ranges. Fractography was carried out to study the micromechanisms of crack propagation using a two stage replication technique. The experimental results were in good agreement with a theoretical model for FCP developed previously by the authors which incorporates mechanical and microstructural variables.

Strength-Size Relationships in Ceramic Materials: 1. Investigation of a Commercial Alumina. 2. Effect of Flaw Shape on Strength of Ceramics.

Abstract: : Bend strengths of a commercial sintered alumina, 3M Company's Alsimag 614, are reported. Fracture stresses in specimens differing in each linear dimension by a factor of five were measured at room temperature under environmental conditions which either minimized or enhanced subcritical crack growth prior to catastrophic fracture. Strength was found to be dependent on specimen size under both test conditions. Fractographic identification of strength-controlling flaws, coupled with the analysis of strength-size data, indicated a limited applicability of Weibull's statistical approach.

Fractography of fatigue crack propagation in 2024-T3 and 7075-16 aluminum alloys in air and vacuum

Abstract: Electron fractography and transmission electron microscopy were used to study fatigue crack propagation processes in 2024-T3 and 7075-T6 aluminum alloys in vacuum and air. There was evidence that crack growth occurs cycle-by-cycle in vacuum, but only for fa tigue in air was it possible to relate dislocation substructure band spacings, immediately below the fracture surface, to cyclic crack growth. A discussion of crack propagation mechanisms suggested that the Tomkins and Biggs model of striation formation comes closest to explaining the fractographic features.

Time-temperature dependent fracture toughness of PMMA

Abstract: Some observations are made on the fractography of surfaces obtained by cracking “compact tension” profile testpieces of PMMA over a range of temperatures and crack speeds, both stably and unstably. To a first approximation, it was possible to group and “shift” (as in visco-elastic transformations) characteristic surface markings at various fracture toughness/temperature/crack velocity combinations, particularly in the range where a toughness-biased Ree-Eyring relationship described the experimental toughness data.

On the Fractography of Transgranular Stress Corrosion Failures in a Mg-Al Alloy

Abstract: A fractographic study has been carried out on transgranular stress-corrosion cracks produced in both tensile and bend specimens of a Mg-7.5 Al alloy tested in an aqueous NaCl-K2CrO4 solution. It is concluded that the complex fractography reported in earlier papers is fully consistent with the view that failure occurs basically by cleavage on planes. Thus the coarse markings reported previously are shown to correspond to a stepped or pfeated topography consisting of alternate faces produced by cleavage and ductile rupture parallel to (0001), the latter exhibiting flutings of the type observed in other HCP alloys. The reasons for the occurrence of the ductile component are discussed.

Stress corrosion cracking of alpha brass in a tarnishing ammoniacal environment: Fractography and chemical analysis

Abstract: Alloy 260 brass specimens under stress were exposed at room temperature to 15 N aqueous ammonia solution with 8 g/1 of cupric copper predissolved. This environment causes tarnishing of the brass surface and intergranular stress corrosion cracking. Scanning electron microscopy, energy dispersive X-ray analysis, and Auger electron spectroscopy were employed to study fractography, corrosion product composition and distribution within the stress corrosion crack, and fracture surface chemistry characteristic of stress corrosion cracking in this system. A thin oxidized film was detected by Auger spectroscopy at the leading edge of the propagating crack. With continued exposure to the corrosive environment, deposits form on the fracture surface, then coalesce to form a continuous tarnish film that is depleted of zinc. No bulk depletion of zinc was detected in the alloy at the stress corrosion crack leading edge. No evidence of noncrystallographic crack arrest marks was found on the intergranular fracture surface.

Fractography of polyester resins

Abstract: The effect of resin flexibility on the fracture surface characteristics of a series of polyester resins has been investigated by scanning electron microscopy. Fracture surfaces were obtained from fracture toughness specimens, un-notched tensile specimens and notched specimens cycled over a constant range of stress intensity factor. Characteristic surface markings have been identified with regions of stable crack growth, slow unstable crack growth and fast unstable crack growth. The three regions were only present on the surfaces of failed fracture toughness specimens. The extent of each region is a function of the resin flexibility. The absence of a given region on the fracture surfaces of other specimens is a function of the loading conditions.

Fractography of poly(methyl methacrylates).

Abstract: For convenience in clinical manipulation, it is the practice to fabricate PMMA prostheses from mixtures of powder and monomer. When the monomer is subsequently polymerized an unusual 2-phase polymeric material results in which grains of PMMA are dispersed in a matrix of the same polymer. The mechanical properties of the 2-phase materials are inferior in certain respects relative to 1-phase polymers. The purpose of the present work is to evaluate the failure of 2-phase materials by microscopical examination of their fracture surfaces. A granular microstructure was clearly distinguishable and a distinction made between materials which fail exclusively by transgranular fracture and others which additionally exhibit intergranular fracture. In order to interpret markings observed on the fracture surfaces of the complex 2-phase systems a study was made of the influence of molecular weight on the fractography of 1-phase PMMA. Molecular weight was reduced by degradation of samples by exposure to γ-rays. The spacing of periodic rib markings on fracture surfaces was found to decrease with molecular weight and this relationship used to provide an estimate of the molecular weight of polymer in the matrix of 2-phase materials.

SEM/TEM Fractography Handbook

Abstract: : A SEM/TEM Electron Fractography Handbook was prepared with the primary intent of assisting in service failure analyses. The handbook consists of three separate sections. Section 1 describes the basic techniques used in handling and preparing specimens for electron fractography. Only those techniques that are routinely used in fracture surface analyses are discussed. In Seciont II of the handbook, the fracture surface characteristics and mechanisms associated with the different fracture modes are examined and illustrated. A discussion is given on the four principal fracture modes: dimple rupture, cleavage, fatigue, and decohesive rupture. Representative SEM and TEM fractographs depicting these fracture modes are also included in this section. Section III contains SEM and TEM electron fractographs of numerous alloys that were fractured under controlled conditions. This atlas of electron fractographs is organized by alloy type, and within each alloy subsection, by the specific test conditions that produced the fracture.

Fatigue crack propagation behavior of the Ni-Ni3Cb eutectic composite

Abstract: The experimental procedures used in the investigation are considered, taking into account solidification and heat treatment, fatigue testing, questions of test specimen and loading fixture design verification, metallographic techniques, and electron microscopy techniques. Attention is given to the characteristics of the microstructure, aspects of fatigue crack propagation (FCP) in the Ni-Ni3Nb composite, the effect of test variables on FCP, the effect of metallurgical variables on FCP, and fatigue fracture mechanisms. Fractographic and metallographic observations are discussed.

Stress Corrosion Cracking of 60/40 Cupro-Nickel Alloy: Fractography and Chemical Analysis

Abstract: The copper-nickel alloys have found extensive application in steam generation systems. In general, these materials have excellent resistance to corrosion-erosion and stress corrosion cracking (SCC). However, service failures, characterized by intergranular cracking and believed to be the consequence of stress corrosion, have occurred. The use of harder tempers to meet the demands of higher operating temperatures and pressures has increased susceptibility to SCC. Further, the treatment of feedwater with organic amines or hydrazine, which produce ammonia upon breakdown, enhances corrosion and stress corrosion of these materials.1,2 Accelerated stress corrosion cracking (SCC) was induced in copper alloy 720 (40–43 Ni, 1.5–2.5 Fe, 0.5–1.7 Mn, balance Cu) by exposing stressed c-ring specimens to the vapor over ammoniacal solutions in autoclaves at 500 F (260 C). The scanning electron microscope (SEM) was employed for fractographic analysis of the resultant intergranular cracking and for micro-chemical...

Correlation of fractography, microstructure and fracture toughness behavior of high strength alloys.

Abstract: Thesis. 1975. Ph.D.--Massachusetts Institute of Technology. Dept. of Materials Science and Engineering.

Vacuum fatigue fracture of Ti--6Al-4V

Abstract: The fractography of structure-sensitive and structure-insensitive vacuum fatigue in fine grained Ti--6Al--4V was investigated using both scanning electron microscopy and transmission electron microscopy of replicas. It is shown that structure-sensitive fatigue of fine grained Ti--6Al--4V in vacuum takes place principally by basal or near-basal cleavage. It is likely that the cleavage is essentially noncyclic, and mechanically rather than environmentally induced. The structure-sensitive to structure-insensitive transition correspond with a cyclic plastic zone size intermediate between the minor and major dimensions of the elongated alpha grains. Cyclic crack growth was detached for structure-insensitive fatigue, as reported by others. (auth)

Fractography of fatigue failures

Abstract: 1. Three fields of fractography were examined: microfractography, fatigue crack propagation, and macrofractography. A list of characteristic features, quantities, and parameters was given and an attempt was made to synthesize concepts for particular fields which have often been studied separately up to the present. A characteristic feature of contemporary fractographic analysis is the trend away from a subjective qualitative evaluation to an objective quantitative assessment of fracture characteristics. 2. A contemporary approach was proposed to such a procedure, which is based on black box concepts using simple methods from mathematical statistics.