Cantilever Beam Technique Research Articles

Crystallographic effects during radiative melting of semitransparentmaterials

Experiments have been performed to illustrate Crystallographic effects during radiative melting of unconfined vertical layers of semitransparent material. Radiative melting of a polycrystalline paraffin was performed and the instantaneous layer weight and transmittance were measured using a cantilever beam technique and thermopile radiation detector, respectively. The effects of radiative flux, initial solid subcooling, spectral distribution of the irradiation, and crystal structure of the solid as determined qualitatively by the sample solidification rate were studied. Experimental results show conclusively the dominant influence of Crystallographic effects in the form of multiple internal scattering of radiation during the melting process. A theoretical model is formulated to predict the melting rate of the material. Radiation transfer is treated by solving the one-dimensional radiative transfer equation for an absorbing-scattering medium using the discrete ordinates method. Melting rate and global layer reflectance as predicted by the model agree well with experimental data. Parametric studies conducted with the model illustrate the sensitivity of the melting behavior to such variables as incident radiative flux, initial layer opacity (material extinction coefficient), and scattering asymmetry factor.

Stresses developed during oxidation of iron thin films

The forces developed by oxidizing thin iron films were measured using the cantilever beam technique, previously used to measure film deposition stress. Iron was evaporated onto a preoxidized stainless‐steel substrate before admission of 7.8×10−2 Pa oxygen to the high vacuum chamber at temperatures from 250 to 310 °C. The measured traction rapidly became large and tensile. A dual quartz crystal microbalance (DQCM) was used to measure oxygen mass uptake versus time curves, which resemble bulk iron oxidation curves from the literature. The plot of tensile traction versus oxygen mass uptake was roughly linear and of similar slope for all experimental temperatures. Auger electron spectroscopy with ion sputtering, Rutherford backscattering spectroscopy, and transmission electron microscopy were used to characterize the oxidized and unoxidized iron films. A sputter ion gun was constructed with the capability of simultaneously sputtering both the DQCM and cantilever beam substrates. The tensile traction loss versus oxide mass loss graph indicated the large tensile force existed in the oxide. The plot of average tensile stress versus oxide thickness gave an approximately constant value for each sample on the order of 100 MPa in the oxide.

Crack front shape effects on propagation stability in thermosetting polyesters

Fracture mechanics testing of the resistance of a polymer to slow crack growth often reveal it to be a unique function of crack speed. However, several thermosetting polyesters, tested using the Double Torsion (DT) and Tapered Double Cantilever Beam (TDCB) techniques, seem unable to sustain stable propagation over particular, isolated ranges of crack velocity with specimen-dependent limits. The meaning of ‘propagation stability’ in this context is discussed, distinguished from the static stability concept normally used, and applied using Liapunov criteria. The corresponding hypothesis that isolated unstable regimes symptomise locally-falling sectors of the resistance versus crack speed crack speed characteristic is supported by an observed crack shape influence, due to which DT tests are inherently more stable than TDCB ones.

Mechanical property changes in sapphire by nickel ion implantation and their dependence on implantation temperature

Sapphire plates, cut parallel to an {0001} plane, have been implanted with 300 keV nickel ions to doses ranging from 5×1012 to 1×1017 Ni cm−2 at specimen temperatures of 100, 300 and 523 K, in order to investigate the effect of implantation temperature on the mechanical property changes in sapphire caused by ion implantation. The measured changes in surface hardness, surface fracture toughness and bulk flexural strength were found to depend strongly on the implantation temperature, and were largely correlated with the residual surface compressive stress measured by using a cantilever beam technique. The surface amorphization that occurred only by the implantation at 100 K and at doses larger than ∼2×10s15 Ni cm−2 reduced the hardness to ∼0.6 relative to the value of the unimplanted sapphire, and considerably increased the surface plasticity. Furthermore, the amorphization was found to involve a large volume expansion of ∼30% and to change drastically the apparent shape and size of a Knoop indentation flaw made prior to implantation. This effect was suggested to reduce stress concentrations at surface flaws and hence to increase the flexural strength.

Tetraglycidyl epoxy resins and graphite fiber composites cured with flexibilized aromatic diamines

AbstractStudies were performed to synthesize new ether modified, flexibilized aromatic diamine hardeners for curing epoxy resins. The effect of moisture absorption on the glass transition temperatures of a tetraglycidyl epoxy, MY 720, cured with flexibilized hardeners and a conventional aromatic diamine was studied. Unidirectional composites, using epoxy‐sized Celion 6000 graphite fiber as the reinforcement, were fabricated. The room temperature and 300°F mechanical properties of the composites, before and after moisture exposure, were determined. The Mode I interlaminar fracture toughness of the composites was characterized, using a double cantilever beam technique to calculate the critical strain energy release rate, GIC.

Stress relaxation in thin aluminium films

Abstract The stress relaxation phenomenon in thin aluminium films deposited onto silicon strips was studied by the cantilever beam technique using X-ray diffraction. At low temperatures the stress decreases linearly as a function of temperature at a rate of −2.17 × 107 dyn cm−2 K−1, in good agreement with the elasticity theory. At high temperatures stress relaxation was observed on heating when the aluminium films could not sustain the compressive thermal stresses created by the difference in thermal expansion between the silicon substrate and the film. Stress relaxation was also found on cooling, for tensile thermal stresses. The activation energy for the compressive stress relaxation was estimated to be 0.43 eV and for the tensile stress relaxation 0.23 eV.

Structural integrity of ion-implanted In0.2Ga0.8As/GaAs strained-layer superlattice

The strain and lattice disorder introduced by room temperature implantation (N, Si, Zn) into strained-layer superlattices (SLS) of In0.2Ga0.8As/ GaAs have been measured by cantilever-beam techniques and by channeled and random Rutherford backscattering spectrometry. These measurements indicate that a maximum lateral compressive stress of ∼5×109 dyn/cm2 is induced for all the ions at fluences corresponding to deposited energy densities of about 1020 keV/cm3 into collisional processes. The compositional modulation of the SLS is maintained at fluences which exceed the yield stress value even in the presence of additional implantation-induced stress and significant numbers of atomic displacements.

On the double cantilever beam technique for studying crack propagation

The problem of a semi-infinite elastic plate loaded along its straight boundary is pursued to develop an analytical and realistic Double Cantilever Beam Model which accounts for the end deflections associated with deformation beyond the crack tip. Deformation of the uncracked portion allows each arm to rotate about the built-in end, producing an end deflection in addition to those caused by bending and shear. Comparisons with the predictions of existing models show reasonable agreement, but comparisons with some available experimental results reveal that the theoretical end deflection is underestimated. This was shown not to be unreasonable when the limitations of the theory were considered.

Crack-size dependence of fracture toughness in transformation-toughened ceramics

Fracture toughness (K IC) has been determined for Y2O3-partially stabilized zirconia, Y2O3-partially stabilized hafnia, CaO-partially stabilized zirconia and Al2O3+ZrO2 composites. It is shown thatK IC determined using the identation technique may not yield a unique number but may depend upon the crack size (C) (on the indent load). The slope ofK IC againstC 1/2 yields the magnitude of the surface stress created by the tetragonal → monoclinic transition on the surface induced by grinding.K IC determined using the double cantilever beam (DCB) technique, on the other hand, is shown to be independent of crack length.

Effects of humidity on stress in thin silicon dioxide films

The stresses of thermally grown as well as chemically vapor deposited (CVD) silicon dioxide were measured by the cantilever beam technique using x-ray diffraction. Thermally grown oxide shows reversible stress changes upon heating or cooling of the films. The linear thermal expansion of such films is similar to that of bulk vitreous silica, 5×10−7 °C−1, the biaxial elastic modulus was found to be 6.3×1011 dyn/cm2. CVD oxides show extensive hysteresis in the stress-temperature curves when tested in ambient air. From stress measurements of such films, deposited on Si and GaAs, it was concluded that their average linear thermal expansion coefficient in the temperature range of −170–115 °C is 4×10−6 °C−1, much higher than that of thermally grown oxide, while their biaxial elastic modulus is only 4.6–5.1×1011 dyn/cm2. The stress in such films was found to increase when the films were exposed to a dry ambient or vacuum. The time constant for this change was found to be several minutes at room temperature.

Measurements of Ion-Implantation Damage in GaP

ABSTRACTWe have applied stress measurements using the cantilever beam technique and Raman spectroscopy to characterize the dose dependence of damage production for He+, C+, or Ar+ implants into GaP. Stress increases monotonically with dose until a speciesdependent critical dose is reached. Above that dose, the material yields at an integrated lateral stress of ∼2×105 dynes/cm2 corresponding to an expansion of ∼1% in the implanted volume. The dose dependence of stress scales well with the volume density of ion energy deposited into atomic collisions. Raman measurements indicate that the material is still crystalline when the yield stress is reached.

Ion bombardment-induced charge state effects CaO single crystals: F+and F centers

Abstract The effects of heavy-and light-ion bombardment on defect formation in CaO have been investigated by UV-absorption spectroscopy and volume measurements. While 500 keV Ar or Ca implantation produces only F+ centers, 240 keVH produces both F+ and F centers at a F+ to F ratio of 5.6 to 1. On the other hand, when an argon implanted sample is subsequently bombarded with hydrogen, about 30% of the F+ centers anneal during 1 ×1014 H/cm2; at higher H fluences, new F+ and F centers are produced. An effect of energy partition between ionization and nuclear/atomic collision processes for the incident ions on the charge state of the resulting defect is thus clearly demonstrated. The formation and annealing of these defects are accompanied by volume changes in the ion implanted surface layer which can be monitored in sltu with a cantilever beam technique. The measurements show volume expansion of the order of 1.5% following 1016 500 keV Ar implantation; subsequent implantation of 1018 240 keV H compacts the previously expanded material by 25 %. These results are in qualitative agreement with the optical data and seem to indicate that volume changes are associated with the formation and annealing of F+ centers.

Epoxy Powder Coating Fracture Analysis by Electronic Speckle Pattern Interferometry

SummaryFollowing an earlier paper, which assessed the fracture properties of epoxy powder coatings with systematically varied crosslink densities using a tapered double cantilever beam (TDCB) technique, further information is presented on electronic speckle pattern interferometry (ESPI). The basic principles of ESPI are presented and a description given of how its use may aid the interpretation of fracture mechanics data. Examples are given of how the dimensions of the deformed zone ahead of a crack tip moving through an epoxy powder coating may be assessed and how this information may be useful in defining the mechanism changeover when fracture toughness or fracture surface energy maxima are observed at critical film thicknesses. Future uses of ESPI as a non-destructive testing (NDT) technique are discussed in the context of the metal finishing industry.

Some measurements of hardness, wear and stresses in ion implanted thin metallic films

Different ions (He, Ne, Ar, B, Al) with energies of 100 or 200 keV and doses up to 3 × 10 17 cm −2 have been implanted in thin metallic films. Layers of gold, copper and aluminium were prepared by evaporation on an oxidized (1000 Å) silicon substrate; thicknesses were in the range of one to two microns. The stress was measured using a cantilever beam technique. The stress is first proportional to the dose, and then, above a fairly well defined dose, turns out to be proportional to the square root of the dose; at higher dose saturation occurs. The Knoop hardness of the implanted layers increases monotonically with the dose and saturates for approximately 10 17 cm −2; 36% improvement in gold hardness and 85% in copper hardness have been obtained. This hardness enhancement is independent of the ion species, hence it depends on the damage but no correlation between the hardness enhancement and the ion induced stress has been found. The hardness enhancement is stable up to at least 200°C. An improved wear resistance was obtained only in copper layers.

An Assessment of the Fracture Properties of Epoxy Powder Coatings

SummaryPrinciples of fracture surface energy measurement previously applied to structural adhesives have been applied to epoxy powder coatings. The fracture behaviour of a range of paints with systematically varied crosslink densities is examined using a tapered double cantilever beam (TDCB) technique. The variation of fracture surface energy with changes in coating thickness are also monitored and interpreted. Irwin-Kies, Berry, Gurney, Mostovoy, Ripling and Bascom's methods of analysis are briefly compared. The practical significance of the results is discussed in terms of the use of epoxy powder paints for pipeline protection.

Thermal strain in thin lead films III: Dependences of the strain on film thickness and on grain size

The effects of film thickness h or of average grain size g on strains ε 33 ' due to the mismatch of the thermal expansion coefficients were studied by an X-ray diffraction technique for thin lead films 0.03–1.0 μm thick evaporated onto silicon substrates. Films with the same average grain size and with different film thicknesses were prepared by a sputter-etch-thinning technique after the film depositions had been completed. Films with small grain sizes were prepared by deposition at liquid nitrogen temperature or by seeding a very thin layer of gold or palladium before the lead deposition. These films were cooled from 300 ro 4.2 K using a cold stage attached to an X-ray diffractometer. For films with h < g/5, the ε 33 ' levels were found to be determined by h. The critical film thickness h c was 0.15 μm. h c was defined so that for h < h c no strain relaxation was observed, independent of grain sizes. For h < h c the ε 33 ' values were found ro be proportional to 1/ h. The h dependence on the ε 33 ' values was analyzed based on an assumption that dislocate glide was the dominant strain relaxation mechanism. It was found that the dislocation pinning distance was about four times smaller than the film thickness, which agrees with the previous result obtained by a cantilever beam technique. For films with h > g/5 the critical grain size g c was determined to be about 1 μm. When g > g c the ε 33 ' levels decreased with increasing g. When g < g c the ε 33 ' values were found to be independent of g and also of h. However, ε 33 ' did not reach the calculated maximum strain value. We propose that the difference between the calculated strain and the measured strain was due to an absorption of the strain at grain boundaries. The fact that g c was about six times larger than h c means that g exerted a stronger effect than h did on the inhibition of strain relaxation during cooling to 4.2 K. It was found that the intermetallic compounds Pb 3Au or Pb 2Pd which were formed in the binary films did not greatly affect the inhibition of strain relaxation. The ε 33 ' levels in these binary films were also determined by the grain sizes.

Volume expansion and annealing compaction of ion-bombarded single-crystal and polycrystalline α-Al2O3

Radiation damage of α-Al2O3 was systematically investigated as a function of incident ion mass and energy partitioning into atomic and electronic processes by measuring the resulting stress in the ion-bombarded layer with a cantilever beam technique. Heavy-ion-bombardment-induced expansion in the implanted surface layer shows strong anisotropy which appears to be related to the higher defect production rate along the c axis. Stress relief is observed at fluences above 1×1015 Ar/cm2 for the [0001] orientation only and is attributed to basal slip. Hydrogen bombardment subsequent to heavy-ion bombardment results in compaction which is also anisotropic. The anisotropy of both the expansion and the compaction point to structure sensitive defects. Approximately 80% of the radiation damage is annealed at 900 °C for the [0001] orientation and 60% for the [011̄0] orientation. A similar expansion-compaction response is found in argon- and hydrogen-bombarded polycrystalline Al2O3.

Fracture Behavior of ZrO2‐Zr Composites

Zirconium oxide‐zirconium 2‐phase alloys with composition 40 at.% Zr‐60 at.% O (23 vol% metal phase) were prepared by hot‐pressing mixtures of the oxide and the metal powders. The microstructure consisted of oxide grains completely surrounded by the metal phase. The effective fracture surface energy, γeff, was determined using the double cantilever beam (DCB) technique and crack lengths were determined by compliance calibration. Rapidly cooled specimens, in which the tetragonal to monoclinic phase transition would cause residual stresses, exhibited γeff values as much as 70% greater than stress‐relieved specimens (65 J/m2 and 38 J/m2, respectively). A shift in the tetragonal ⇋ monoclinic transition temperature in the as‐hot‐pressed and the rapidly cooled specimens, compared to the stress‐relieved specimens, confirmed the existence of residual stresses. The fracture strength, σeff, measured in 3‐point bending, was found to be independent of the thermal treatments. Scanning electron microscopy and optical microscopy revealed that the fracture is transgranular.

Ionization-stimulated annealing effects on displacement damage in magnesium oxide

The effects of ion bombardment into MgO were investigated by measuring resulting volume changes with a cantilever beam technique and by monitoring the F band absorption induced in the uv region of the spectrum. Single crystals of MgO were bombarded near 〈100〉 with 500-keV argon, which resulted in an expansion of the implanted near-surface layer due to the ion-induced lattice damage. Under subsequent 100-keV proton irradiation, however, a large fraction of this expansion is relieved since the material compacts. This seems to indicate that defects with different charge states are produced in MgO by heavy-ion bombardment and that electronic processes account for the volume changes observed during subsequent irradiation with the primarily ionizing radiation from the 100-keV H+ implantation. Identical behavior was found earlier for the highly ionic Al2O3 while no such effect was observed in the predominantly covalent SiO2. The present results thus corroborate our model of the existence of defects with different charge states in ionic materials. This behavior of MgO and Al2O3 is of considerable interest since both materials are candidates for first-wall application in CTR environments.

Physical Properties of Thick Sputter‐Deposited Glass Films

The coefficient of thermal expansion (CTE) of 5μ thick rf sputter‐deposited Corning 1720 glass films was measured using a cantilever beam technique. The films were rf‐bias sputtered in an argon (Ar films) and an argon‐5% oxygen (Ar/O2 films) gas mixture. The Ar films deposited with a low substrate bias (+30, 0, and −50V) exhibited an erratic CTE on initial heating but were well behaved after a 600° air anneal. High bias (−100, −200V) Ar films and Ar/O2 films showed no erratic behavior on initial heating. Chemical analysis indicated no significant change in the minor constituents of the sputtered glass, but neutron activation analysis showed a progressively greater oxygen depleti on with increasing negative bias for both the Ar and Ar/O2 films. Etch rates were shown to increase with increasing negative bias. It is suggested that the structural changes that occur in some glass films at temperatures far below the strain point of the bulk glass (670°C) are due to a combination of intrinsic stress and a high concentration of defects. Surface coverage by the sputterdeposited glass films is discussed.