L. C. Jensen

Fracto‐emission: The role of charge separation

Fracto‐emission is the emission of particles (e.g., electrons, ions, ground state and excited neutrals, and photons) during and following fracture. We have found that during fracture in vacuum of adhesive bonds and crystalline materials involving large amounts of charge separation on the surface the emission of charged particles, excited neutrals, light, and radio waves occurs with unique and revealing time dependencies. In this paper we report simultaneous fracto‐emission measurements on several systems. We interpret the results in terms of a conceptual model involving the following steps: (1) charge separation due to fracture, (2) desorption of gases from the material into the crack tip, (3) a gas discharge in the crack, (4) energetic bombardment of the freshly created crack walls, and (5) thermally stimulated electron emission, accompanied by electron stimulated desorption of ions and excited neutrals. In addition to evidence from fracture experiments, we present results from studies of electron bombardment of a polymer surface.

Atomic and molecular emission following fracture of alkali halides: A dislocation driven process

During and following fracture of a number of materials, the emission of photons, electrons, ± ions, and neutral species are observed; these emissions are collectively known as fracto-emission . In this work, we present measurements of the neutral particle emission following fracture of two single crystal fcc alkali halides: NaCl and LiF. We observe no measurable emission attributable to release during the fracture event itself. However, after relatively long time intervals of ∼0.5–250 ms, we observe rapid bursts of alkali atoms, as well as molecular species which include NaCl and (LiF) n where n = 1,2,3. Bursts of alkali containing species also occur during loading prior to fracture and for unloaded specimens during heat treatment. We argue that these bursts are due to energetic emergence (“popout”) of dislocations at free surfaces.

The emission of atoms and molecules accompanying fracture of single-crystal MgO

We have been investigating the emission of particles due to deformation and fracture of materials. We observe the emission of electrons (exoelectron emission), ions, neutral species, photons (triboluminescence), as well as long wavelength electromagnetic radiation; collectively we refer to these emissions as fractoemission. In this paper we describe measurements of the neutral emission accompanying the fracture of single-crystal MgO. Masses detected are tentatively assigned to the emission of H2, CH4, H2O, CO, O2, CO2, and atomic Mg. Other hydrocarbons are also observed. The time dependencies of some of these emissions relative to fracture are presented for two different loading conditions.

Fracto-emission from deuterated titanium: Supporting evidence for a fracto-fusion mechanism

Measurements of the emission of charged particles, photons, and radio frequency signals accompanying the deformation and fracture of polycrystalline Ti metal and deuterated Ti are described. Preliminary evidence for charge separation created by crack propagation is presented which supports a proposed fracture mechanism to explain neutron bursts observed during treatment of deuterated metals.

Electrical transients generated by the peel of a pressure sensitive adhesive tape from a copper substrate

Contact electrification during the formation of adhesive bonds can lead to transient electrical signals and even electrical discharges when these bonds are subsequently broken. We present detailed measurements of the electrical current generated by peeling a pressure sensitive adhesive from an electropolished Cu substrate in air and in vacuum. The magnitude of this current depends strongly upon the peel speed. At constant peel speed, transient fluctuations in the rate of interfacial failure along the peel front produce corresponding current fluctuations. Electrical breakdown events during peel produce sharp spikes in the current signal as well as long wavelength electromagnetic radiation and visible/UV emissions with characteristic spectral features. We use these signals to characterize the breakdown events in various atmospheres. Current measurements are a potentially new time-resolved probe of micromechanical and electrical processes accompanying peeling.

Neutral molecule emission from the fracture of crystalline MgO

We present further measurements of the neutral molecule emission accompanying the fracture of single‐crystal MgO. Comparison is made of the intensities and species of this emission for basically two types of MgO: optically clear and so‐called ‘‘cloudy’’ MgO. The latter material contains voids and precipitates of micron and submicron dimensions. In addition to time resolved mass selected measurements of the released neutral species we also examine the fractrography of the fracture surfaces and infrared spectroscopy of the bulk material. These measurements strongly support a correlation between O2, CO, H2O, and CH4 emission intensities and the presence of dissolved H2O, microscopic voids, and precipitates in the MgO. Segregation of gases in zones near precipitates may be the major source of O2 emission.We present further measurements of the neutral molecule emission accompanying the fracture of single‐crystal MgO. Comparison is made of the intensities and species of this emission for basically two types of MgO: optically clear and so‐called ‘‘cloudy’’ MgO. The latter material contains voids and precipitates of micron and submicron dimensions. In addition to time resolved mass selected measurements of the released neutral species we also examine the fractrography of the fracture surfaces and infrared spectroscopy of the bulk material. These measurements strongly support a correlation between O2, CO, H2O, and CH4 emission intensities and the presence of dissolved H2O, microscopic voids, and precipitates in the MgO. Segregation of gases in zones near precipitates may be the major source of O2 emission.

CO2 emission accompanying the fracture of calcite

Time resolved mass spectroscopy of the emissions accompanying the fracture of calcite (rhombohedral CaCO3) show that the principle volatile product, CO2, is released in bursts milliseconds after the fracture event. Similar measurements during the abrasion of calcite and during low temperature thermal decomposition of pulverized calcite show similar CO2 bursts. We argue that the observed bursts reflect localized decomposition of the calcite during the relaxation of reversible plastic deformation created by fracture and abrasion. This implies that mechanical, non-thermal processes play an important role in producing the observed decomposition products.

Positive‐ion emission from the fracture of fused silica

Positive‐ion emission was observed during and after the fracture of fused silica using time‐of‐flight techniques and quadrupole mass spectroscopy. Emissions attributed to Si+, SiO+, and Si2O+ were observed during the fracture event itself. An emission mechanism for the silicon‐containing ions is proposed involving the mechanical scission of at least three of the bonds joining a silica tetrahedron to the rest of the silica network. The production of silicon‐containing ions suggests the activity of nonequilibrium processes which may contribute significantly to the fracture energy of fused silica. A long‐lived emission was attributed to the electron stimulated desorption of O+.

Fracto-Emission from Filled and Unfilled Elastomers

Abstract We have tried to show a variety of EE and PIE results on a number of systems involving fracture of filled and unfilled elastomers and consider some of the parameters that influence this emission. The need for careful studies of the physics and chemistry of these phenomena is obvious. The usefulness of FE as a tool for investigation of failure mechanisms and fracture phenomena in elastomers requires a broad-based attack combining fracture mechanics, materials science, and fundamental fracto-emission studies on well characterized elastomer. Since the field is relatively unexplored, we will conclude by speculating on some potential areas of usefulness of FE, many of which depend critically on further understanding of FE itself.

Recombination on fractal networks: Photon and electron emission following fracture of materials

We report measurements and analysis of fracture-induced photon and electron emissions from several polymeric and inorganic systems on time scales of 10 −2 to 10 3 s following fracture. The dominant mechanism for postfracture emission involves the recombination of mobile free carriers (usually electrons) with immobile recombination centers. The emission decays were modeled as (pseudo)unimolecular and bimolecular recombination on fractal lattices as described by Zumofen, Blumen, and Klafter. 1 Although the decay kinetics shows a great deal of variability from material to material, this random walk description of the recombination process provides an excellent description of the emissions over long time scales. This analysis shows a strong correlation between the local structure at the fracture surface and the resulting decays.