Showing papers on "Fractography published in 1972"

Some contributions of electron fractography to the theory of fracture

Abstract: A survey is given of the results of five years electron fractography as conducted at the National Aerospace Laboratory N.L.R. and of the contribution of this work to the comprehension of fatigue cracking, ductile fracture, cleavage and stress corrosion.

Fractography of a phenol—formaldehyde polymer

Abstract: Samples of a phenol-formaldehyde polymer with a deliberately introduced flaw were fractured in tension. The appearance of the fracture surface near the flaw suggested the disruption of particles pre-existing in the polymer. At a greater distance from the flaw, a featureless surface was observed which was succeeded by one showing interference colors. At still greater distances, linear features were observed to be lying in the direction of crack propagation which, in most areas, were regular and evenly spaced. There were indications that these features were formed by the curling of a surface film. The above observations are interpreted as providing evidence that tensile fracture is accompanied by plastic deformation at the fracture surface.

Fatigue Behavior of Epoxy and Polyester Resins

Abstract: The behavior of an epoxy (Epon 828) and polyester resin (Paraplex P43) have been studied when subjected to cyclic fatigue stresses. A rotating beam fatigue machine was used to apply the stresses which varied from compression to tension (mean stress = 0). The surface temperature of the specimens was recorded by using an infrared temperature detector and the temperature rise was studied as a function of cyclic frequency and stress level. Fatigue life of the materials was studied as a function of cyclic frequency and the influence of moisture on fatigue life was also studied. It has been determined that the temperature rise during fatigue is critical in determining the life of the material. Fractography studies are also discussed and characteristic fracture surface features will be indicated.

High temperature porosity in tungsten

Abstract: A study of the development of porosity in the temperature range of 3000° to 3350°C in several grades of tungsten wire has been performed. In addition to measurements of the kinetics of pore growth and pore number density by light microscopy, examinations on heat-treated wire by means of scanning electron microscopy and electron fractography were accomplished and revealed submicron-sized pores principally arranged in linear arrays. The observations were evaluated in terms of various theories of pore growth. While the phenomena encountered were complex and overlapping, it seems probable that initial pore growth is by plastic deformation due to volatilization of dopant. The constancy of the number of growing microscopic-sized pores with time suggests that coalescence processes are not rate controlling. Since an activation energy of 110 kcal per mole was measured for pore growth in doped tungsten, vacancy diffusion from the surrounding matrix to the pore under internal pressure was considered. A simplistic analytic treatment yielded a 1/5 power-time dependence in comparison to the linear relation observed.

The measurement of fracture speeds in silicon and germanium

Abstract: Fracture speeds in small specimens of silicon and germanium have been measured by the method of ultrasonic fractography, in which an ultrasonic shear wave is used to produce a sinusoidal ripple on the fracture surface. This can be achieved wherever the fracture surface is non-crystallographic. Crack speeds have been measured and related to the various regions of the surface and the passage of reflected waves. The relation between crystallographic and noncrystallographic fracture in silicon and germanium is discussed.

The effect of cleaning an experimental spherical Ag3Sn alloy on the tensile strength and fracture of dental amalgam

Abstract: Experiments have been conducted with the aim of improving the tensile strength of dental amalgam. Anticipating a surface contamination of Ag3Sn, the powder was washed with 5% HCl before amalgamation. The resulting amalgam showed a considerable decrease in the scatter of strength data and an increase in fracture stress. At 40 w/o mercury content the strength increase was found to be 40%. The reason for this substantial improvement in the strength of dental amalgam was revaled by electron fractography. It was found that the bond between unconsumed Ag3Sn particles and the amalgam matrix had been strengthened to such an extent that fracture took place through the silver-tin particles, which allowed Ag3Sn to perform its role as the strongest component in a dispersion hardened system.

Scanning electron microscope fractography of continuously cast high purity copper after high temperature creep

Abstract: Fracture surfaces produced by high temperature creep were studied using the scanning electron microscope. The material investigated was continuously cast high purity copper containing a nodal impurity segregation structure at which grain boundary voids are formed during creep. The observed void shape suggests that vacancies are supplied mainly via grain boundaries, and also by enhanced diffusion via segregation nodes; the vacancies seem to originate mainly at internal sources. The known distribution of potential nucleation sites was used to study the efficiency of the segregation structure in nucleating voids under various test conditions. Within the range of conditions employed, three different fracture modes were observed in separate regions of the stress-temperature plane. The regions are sequentially denotedA, B, and C as the temperature is increased at a given stress; they shift to lower temperatures as the stress is increased. In regionA fracture is initiated by extensive cavitation along grain edges (line of junction of three grains); cavitation at the segregation structure seems to be of secondary importance. In regionB formation and growth to coalescence of voids at segregation nodes governs fracture; the change of growth mechanisms with test conditions is discussed. In region C fracture is controlled by plastic instability.

Off-Axis and Transverse Tensile Properties of Boron Reinforced Aluminum Alloys

Abstract: Abstract : Tension specimens of boron-reinforced 7075 aluminum and 6061 aluminum were tested at angles of 0, 30 and 90 degrees to the unidirectionally aligned filaments. Metallurgical structures in the matrix and at interfaces were correlated with the stress-strain curves, with acoustic emission from boron-filament breakage during the tests, and with fractographic studies. The effects of aging heat treatments and subzero cooling on tensile properties were related primarily to the changes in metallurgical structures produced in the 7075 aluminum matrix. The filament-matrix interfacial bond strengths exceeded the strength of the matrix alloys. Mildly degraded boron filaments had no apparent effect on the ultimate tensile strength parallel to the filaments but drastically altered the fracture mode in off-axis tensile specimens of 7075 aluminum-boron. In the 0 degree samples, fracture appeared to initiate in the core of boron filaments or at the core-boron interface. Diametral splitting of boron filaments was concluded to occur as a result of the stress state developed in the filaments during their manufacture, and limited the ultimate stengths of off-axis and transverse tension specimens. (Author)

On Creep-Fatigue Interaction During Crack Growth,

Abstract: : The interaction between creep and fatigue processes was investigated during crack gorwth in sheet specimens of the aluminum alloy 5052 at 400F. Fractographic studies indicate that at high frequencies (1/30 sec. per cycle) the fatigue process of crack advance is dominant, whereas at low frequencies (e.g., 60 sec. per cycle) the creep process prevails. It was found that at low frequencies and at a low peak stress level (10 ksi) the cyclic process could lead to an increase in the total time to failure as compared to the creep process alone. (Author)

An investigation of plastic fracture in aluminum alloys

Abstract: The brittle fracture of many high strength alloys such as steel, titanium, and aluminum was shown to occur by a process called plastic fracture. According to this process microscopic voids form at impurity particles, then grow and coalesce to cause the final rupture. To further understand the role of impurities, four aluminum alloys were investigated: 2024-T851, 2124-T851, 7075-T7351 and 7079-T651. Fractography, quantitative metallography, and microprobe studies assessed the roles of various impurity particles relative to these alloys.

Fractography of Low-Carbon Steel in Fatigue Fracture

Abstract: Notched specimens of annealed 0.03%C steel were fatigued under completely reversed in-plane bending. The topography on the fatigue fractured surface was examined by electron and optical microscopys, and the change in topography with the crack propagation rate was investigated. The range of the propagation rate obtained in this study was 10-8∼5×10-3mm/cycle. The results are summarized as follows:(1) The three features, the facets of grain boundary fracture, the imbricate pattern and the parallel pattern, were observed on the fractured surface when the propagation rate was lower than 10-5mm/cycle.(2) Striations were observed on the surface fractured with the rate higher than 2×10-5mm/cycle. The spacing between striations was equal to the propagation rate within the range of 10-4∼2×10-3mm/cycle. The spacing was larger than the rate at dl/dN=2×10-5∼10-4mm/cycle, while it was smaller at dl/dN>2×10-3mm/cycle.(3) Cleavage facets were observed on the fractured surface when the propagation rate was higher than 3×10-4mm/cycle. The summed area of cleavage facets on the fractured surface increased with the propagation rate to about 10% of the total area of the fractured surface at dl/dN=5×10-3mm/cycle.(4) The tip profile of fatigue cracks that ran within the specimen in accordance with its thickness bent in convexwise in the direction of growth when the propagation rate was higher, while it bent in concavewise when the rate was lower. The change in the tip profile was explained by the difference in growth mechanism with the propagation rate.