In order to suppress the formation of intermetallic reaction layer during laser welding of steel and aluminum alloy dissimilar materials, effect of backing block (heat sink) was investigated. FEM ...analysis as well as welding experiments with various backing blocks for controlling heat flow were performed. It was found that thickness of intermetallic reaction layer was reduced by increasing welding speed and by applying a backing block with higher thermal conductivity. FEM analysis also showed that when increasing welding speed or applying a backing block with higher thermal conductivity, molten time of aluminum alloy became shorter and predicted thickness of intermetallic reaction layer became thinner. It could be concluded that the thickness of intermetallic reaction layer could be suppressed by using a backing block (as a heat sink) for controlling heat flow in welding region. The joining strength was strongly dependent on the thickness of intermetallic reaction layer. Therefore the backing block was significantly effective for controlling heat flow and consequently for attaining good joining strength.
•Combined approach of in situ fatigue crack growth observation and EBSD analysis.•Fatigue crack growth mechanism under mechanically large grain condition.•Clarifying fatigue crack deflection and ...crack branching mechanisms.•Effect of crystal orientation on crack deflection and branching.•Significance of crack tip stress shielding on fatigue crack growth resistance.
In situ SEM observation of fatigue crack growth behavior of low carbon steel under mechanically large grain condition, and EBSD analysis were carried out to investigate mechanisms of crack deflection and branching and effects of crystal orientation on them. It is found that the crack propagates along the activated slip bands in the present material and the crack deflection can be formed when a new activated slip band is created in a different direction from the growing crack. The crack branching can be formed by the following process: (1) the slip band along the crack cannot be activated further due to the restraint by such as grain boundary or strain hardening, (2) the crack cannot propagate further in the same direction, and (3) the secondary slip plane is activated to form a new slip band behind the crack tip. Not the stress intensity factors Kdef for deflected crack and ΔKeff for crack closure but rather the ΔKeff,tip is the intrinsic fracture mechanics parameter for controlling fatigue crack growth behavior of the material with crack deflection and branching.
•TiAlN coatings with thicknesses of 1 µm, 5 µm and 15 µm were deposited on Ti-alloys.•Effect of coating thickness on fatigue strength was investigated.•Optimum thickness of 5 µm for the longest ...fatigue life was obtained.
Fatigue strength tests of TiAlN coated Ti-alloys with coating thicknesses of 1 µm, 5 µm and 15 µm were conducted to investigate the effect of coating thickness on fatigue strength. Under same applied stress amplitude, optimum thickness with longest fatigue life was found to be around the intermediate coating thickness of 5 µm. It is found from detailed observations and discussion that lower fatigue life of coating thickness of 1 µm results from fracture of coating layer under strong influence of deformation of substrate crystal. On the other hand, lower fatigue life of 15 µm coating thickness results from higher tensile residual stress induced in substrate near coating layer and substrate interface.
The use of waste or natural resources is an interesting approach to preparing adsorbent materials. Most adsorption materials are powder-based, making them impractical for a variety of applications. ...In this work, the natural kaolin clay and hydroxyapatite synthesized from biogenic waste were studied as defluoridation materials. The point of zero charge (pHPZC), the fluoride adsorption capability and the adsorption isotherm of calcined kaolin and mixed calcined kaolin/hydroxyapatite in both powdered and moulded forms were investigated. The hardness of the moulded (post-formed) samples was tested before and after in immersion in a fluoride solution. The maximum hardness was 15.8 kilo-pounds for the post-formed calcined kaolin sample. Sample hardness values slightly decreased after immersion in a fluoride solution due to the formation of micro-cracks. Most samples presented high pHPZC values, implying that these materials are suitable for the capture of fluoride anions. The adsorption properties varied with the ratio of calcined kaolin to hydroxyapatite. These properties for post-formed samples were different from those in powdered form. Post-formed samples showed higher fluoride adsorption. The maximum fluoride adsorption capacity and efficiency of the post-formed samples (calcined kaolin) at pH 3 were 1.74 F− mg/g and 87%, respectively. The sorption of fluoride of hydroxyapatite and mixed calcined kaolin/hydroxyapatite powders was found to have the form of the Langmuir isotherm, which indicates a monolayer adsorption on the adsorbent surface. Isotherms of calcined kaolin powder, post-formed calcined kaolin and mixed calcined kaolin/hydroxyapatite samples followed the Freundlich isotherm, which indicates multilayer adsorption on a heterogenous adsorbent surface.
Stress distribution at the contact edge plays a dominant role in fretting fatigue strength. In the previous studies, based on the stress distribution at the contact edge, a generalized tangential ...stress range–compressive stress range (TSR–CSR) diagram has been proposed as a fretting fatigue fracture criterion. It has been also confirmed that the proposed diagram would be very useful to predict the fretting fatigue strength regardless of contact geometry, loading condition, material strength, environment, etc. for laboratory-type specimens. In the present study, fretting fatigue strengths of actual components, such as a dovetail joint and a bolted joint, have been predicted based on the generalized TSR–CSR diagram. To verify the effectiveness of the prediction based on the generalized TSR–CSR diagram, the fretting fatigue tests of dovetail joints and bolted joints were carried out. The fretting fatigue strengths of dovetail joints and bolted joints predicted based on the generalized TSR–CSR diagram were in good agreement with the experimental results.
•FF strength of actual components has been predicted based on FF design curve.•Predicted FF strength was verified by conducting FF test.•Predicted FF strength was well accepted with the experimental result.
In-situ observation of fatigue crack growth behavior under ΔK-constant cyclic loading in Paris regime for two Thermo-Mechanical Control Process (TMCP) steels with different grain size was carried ...out. It was found from the results that crack closure dominantly contributed to fatigue crack growth resistance in the fine grain material, while crack tip stress shielding induced by crack deflection and crack branching dominantly contributed to fatigue crack growth resistance in the coarse grain material. The electron back scatter diffraction (EBSD) analysis revealed that crack deflection and crack branching in the coarse grain material were induced in the grain with (101) crystal orientation intensively under co-existence of the grain with (001) crystal orientation and also along the grain boundary between the grains with (101) and (001) crystal orientations. Hence, higher content ratio of the grains with (001) crystal orientation could contribute to higher crack growth resistance.
Overall fatigue strengths and hardness distributions of the aluminum alloy similar and dissimilar friction stir welding (FSW) joints were determined. The local fatigue strengths as well as local ...tensile strengths were also obtained by using small round bar specimens extracted from specific locations, such as the stir zone, heat affected zone, and base metal. It was found from the results that fatigue fracture of the FSW joint plate specimen occurred at the location of the lowest local fatigue strength as well as the lowest hardness, regardless of microstructural evolution. To estimate the fatigue strengths of aluminum alloy FSW joints from the hardness measurements, the relationship between fatigue strength and hardness for aluminum alloys was investigated based on the present experimental results and the available wide range of data from the references. It was found as:
(
= -1) = 1.68 HV (
is in MPa and HV has no unit). It was also confirmed that the estimated fatigue strengths were in good agreement with the experimental results for aluminum alloy FSW joints.
In the present work, fatigue crack growth tests of epoxy resin composite reinforced with silica particle under various
R-ratios were carried out to investigate the effect of
R-ratio on crack growth ...behavior and to discuss fatigue crack growth mechanism. Crack growth curves arranged by Δ
K showed clear
R-ratio dependence even under no crack closure, where the values of Δ
K
th
were 0.82 and 0.33
MPa
√m for
R
=
0.1 and 0.7 respectively. However, crack growth curves arranged by
K
max merged into almost one curve regardless of
R-ratio, which indicated that crack growth behavior of the present composite was time-dependent. The value of
K
max,
th
were in the range from 0.78 to 1.12
MPa
√m. In situ crack growth observation revealed the crack growth mechanism: micro-cracking near the interface between silica particle and resin matrix occurs ahead of a main crack and then micro-cracks coalesce with a main crack to grow. The crack path was in the epoxy matrix, which was consistent with the time-dependent crack growth.
This study aims at observing the effect of Mg concentration on the interfacial strength and corrosion fatigue behavior of Al-Mg coating layers on structural steel (SS400) substrates. Al-Mg coating ...has been applied to components of bridges as sacrifice coating layers. Although Al or Al-Mg coating layers are typically applied to the components located in severely corrosive environment, a mechanism behind the effective protection provided by increasing the concentration of Mg has not yet been clarified. The interfacial strength test using four-point bending revealed that a Al-Mg coating layer of higher Mg concentration showed a higher interfacial strength only before immersion in 3.5-wt% NaCl aq.. After immersion in 3.5-wt% NaCl aq. for 30 days, such the difference in interfacial strength was lost due to rapid dissolution of Mg in the coating layers. As regards the fatigue crack growth behavior, the Al-Mg coating with higher Mg concentration exhibited a lower resistance to vertical crack propagation and interfacial delamination. A fracture mechanics model, which includes both effects of corrosion and delamination/cracking was proposed. Numerical simulation based on the fracture mechanics model successfully predicted exposure lives of substrates due to fatigue failure of the Al-Mg coating layers. These results could provide a selection policy for Al-Mg coating layers in which a loading level and the severity of corrosive environment were considered.
•Higher concentration of Mg in Al-Mg coating induced higher interfacial strength.•NaCl immersion lost the enhancement effect of Mg in interfacial strength.•Higher concentration of Mg in Al-Mg coating layers reduced corrosion resistance.•Higher concentration of Mg in Al-Mg coating layers decreased crack growth resistances.•Fracture mechanics model can predict exposure lives of substrates.
The residual stress distributions in hydroxyapatite (HAp) coating with and without mixed hydroxyapatite/titanium (HAp/Ti) bond coating on commercially pure Titanium substrate (cp-Ti) were evaluated ...by Raman piezo-spectroscopy analysis. The Raman shifted position 962cm−1, which is the symmetrical stretching of surrounded oxygen atoms with phosphorous atom (PO43−), was referred to analyses of stress dependency. The piezo-spectroscopic coefficient, which is a Raman shift value per stress (cm−1/GPa), was fitted from the result of four-points bending test of rectangular HAp bar and as-sprayed HAp on Zn plate. The calculated values were 3.89cm−1/GPa for the former and 7.11cm−1/GPa for the latter. By using these calibrations, the compressive residual stress in HAp coating with HAp/Ti bond coating (HA-B) has been found to be distributed in the range of −137MPa to −75MPa. For the heat-treated HAp coating (HA-B-HT) specimen, the compressive residual stresses placed in the range of −40–−22MPa. The changes in the values of residual stress of the HAp coating after immersion in SBF were also evaluated. The residual stress in HA-WB specimens tend to change from compressive to tensile after 30 days immersion. The HA-B-HT specimens exhibited similar behavior and reached to zero stress after the immersion. The mechanism of the changes in residual stress would be the effect of stress redistribution around melted calcium phosphate particles to remained HAp splats.
PDF
