In this investigation, a combined experimental and computational approach with a Modified Mohr Coulomb (MMC) fracture criterion employing post-initiation element softening is used to simulate stable crack propagation under Mode I, Mode III and combined Mode I/III loading conditions. Results from the studies demonstrate that good correlation exists between the measured load-displacement and the numerically predicted response when the stiffness of the specimen fixture is included in the FE model. The numerical results were able to capture most of the experimentally observed features during crack propagation, such as through-thickness slant fracture, necking, tunneling and local specimen twist, thus confirming that the MMC criterion is suitable for predicting in-plane and out-of-plane tearing of sheets. It was found that in order to predict correctly the load-displacement curve as well as the fracture plane, different amount of softening is needed for Mode I and Mode III loading cases. This observation can be justified on the micro-mechanical level, while there is a competition between the mechanisms of dimple and shear fracture.

Friction stir processing was applied to fabricate ZrO2/2024 surface composite with the expectation for better wear resistance and thermal barrier property.About 200 μm and 700 μm thick ZrO2/2024 surface composites were successfully fabricated at two rotation speeds(1000 r/min and 1600 r/min) with constant travel speed of 20cm/min.The microstructure,the distribution of the reinforcing particle and the microhardness of the obtained surface composites were evaluated.It is shown that the ZrO2 particles in the fabricated surface composites are well-distributed and have very good bonding with the matrix.Microhardness in surface composites fabricated at 1000 r/min and 1600 r/min increases by 83% and 46%,respectively,compared to aluminum alloy matrix.The thickness,the volume fraction of the reinforcing particle and the microhardness of the surface composite can be controlled by different rotation speed at a constant travel speed.

Stable tearing with crack tunneling in ductile materials has been commonly observed, but a quantitative understanding of the 3D complicated crack tunneling phenomena is very limited. In particular, the correlation between the driving force and fracture toughness during stable tearing with crack tunneling has not been well evaluated. In the current study, modeling efforts have been made to simulate stable tearing events with crack tunneling and slanting under remote tension in A2024-T3 plate specimens of two thickness values (2.286mm and 6.35mm). It is observed that values of CTOD, stress constraint and the Lode stress parameter vary in the specimen thickness direction. For the thinner specimen, a higher stress constraint and a lower CTOD in the midsection of the crack front are found, and the critical CTOD decreases approximately linearly with an increasing constraint and is weakly dependent on the Lode stress parameter. The variations of CTOD, the stress constraint and the Lode parameter in the two plate specimens in the thickness direction are very similar, but the magnitudes of these parameters near the midsection of the crack front increase significantly in the thicker specimen. The approximate linear correlations in the two specimens between constraint and CTOD are not identical, probably due to the coupled effects of constraint and the Lode stress parameter in this midsection region of the crack front.

In this study, tensile loading experiments are performed on notched steel bars at an average applied strain rate of 1s−1. Displacement fields are measured across the specimen by coupling digital image correlation (DIC) with imaging using high-speed CCD cameras (4796fps). Results from the experiments indicate the presence of local strain rates ranging from 0.1 to 10s−1 in the notched specimens. The heterogeneity of the strain rate fields provides suitable conditions for determining simultaneously all the elasto-visco-plastic constitutive parameters governing the material behavior. For that, the whole stress fields are reconstructed in the specimen using the full-field deformation measurements. This reconstruction is repeated with different constitutive parameters until the average stress in the specimen matches the one measured with the load cell response. Perzyna’s model is firstly considered for the reconstruction of stresses but it is shown to be unsuited for providing the drop in the average stress that is systematically detected at the onset of plasticity by the load cell. This drop is attributed to the sudden occurrence of plasticity in the material due to Lüders effect. A modified model for elasto-visco-plasticity taking account of Lüders behavior in the material is considered afterwards. It yields a better agreement between the reconstructed stresses and the load cell response, and a more accurate identification of the parameters driving the visco-plastic model. Eventually, it is shown how to use DIC measurements for replacing the load cell measurements when the transient effects in the test reach the resonance frequency of the load cell.

In this study, tensile loading experiments are performed on 2 notched steel bars at an average applied strain rate of 1s -1 . Displacement fields are measured across the specimen by coupling digital image correlation with imaging using high speed CCD cameras. Results from the experiments indicate the presence of local strain rates ranging from 0.1 to 10s -1 in the notched specimens. By coupling the virtual fields method with full-field deformation measurements at selected time intervals during the loading process, it is shown that elasto-visco- plastic constitutive parameters governing the materials behavior can be determined. Specifically, our initial studies have shown that Perzyna's model was unsuited for characterizing the transient effects detected at the onset of plasticity. However, a modified model for elasto-visco-plasticity taking account of Lüders behavior was evaluated and shown to yield promising results.

The experimental identification of parameters governing the elasto-visco-plastic constitutive behavior of materials is a key issue which usually relies on performing simple mechanical tests for which a closed-form solution for the corresponding equivalent mechanical problem is available. These tests, such as tension or compression on prismatic specimens or torsion on thin tubes, usually lead to uniform states of stress and strain and therefore, the identification can be performed from a few strain data obtained through strain gages or extensometers. However, in order to fully characterize the material behavior, multi-axial tests are often necessary, requiring costly testing machines and difficult specimen design to obtain uniform stress states in some area of the specimen. Moreover, the models describing the elasto-plastic or visco-plastic behavior of materials are governed by several parameters which cannot be directly determined from these experiments in all cases.

Fractures in ductile thin-sheet structures, such as a fuselage or automobile panels, often occur under complex loading conditions. In particular, under remote mixed-mode I/III loading conditions, a cracked structure is subjected to a combination of in-plane tension and large out-of-plane tearing deformation, which may lead to crack tip fields consisting of all three fracture modes (modes I, II, and III). Understanding such fracture events in ductile materials is an important component of the structural integrity analysis of load-bearing structures containing ductile, thin sheets. Due to the complex nature of mixed-mode I/III fracture in ductile thin-sheet materials, reports of experimental investigations are very limited in the literature. We configure three-dimensional digital image correlation (3D-DIC) systems to acquire full-field deformations during the loading and stable tearing processes. The full-field deformation measurements are used to characterize the stable crack extension behavior of an aluminum alloy undergoing quasistatic and dynamic mixed-mode I/III loading. Results confirm that 3D-DIC is an excellent methodology for measuring 3-D deformations in the presence of large out-of-plane warping and motion, both dynamically and statically. Data obtained during the fracture process indicate that the introduction of a mode III component into the loading process alters the crack tip displacement and strain fields relative to those measured in the nominally mode I loading. Furthermore, the measured crack-opening displacement (COD) values during quasistatic and impact mixed-mode I/III fracture show that (1) COD is nearly constant for crack extension beyond 2 mm and (2) COD under combined-mode I/III loading is four times larger than observed during mixed-mode I/II or mode I fracture of the same material, indicating that the magnitude of the critical COD is a function of loading mode in highly ductile, thin-sheet materials. (C) 2007 Society of Photo-Optical Instrumentation Engineers.

In order to better match the mechanical properties and physical properties of thermal barrier coatings with matrices,we explore a method for dispersing NiCrAlY particles and NiCrAlY+ZrO2 mixed particulates into aluminum matrix using friction-stir processing(FSP).The exploration results show that the FSP can effectively disperse and distribute the particles and that its temperature control is decisive on the quality of thermal barrier coatings.

For the first time, experiments are conducted on solid-state FSW joints in a pair of aerospace alloys to determine how the process affects notch–strength and notch sensitivity. In this study, AA2524 and AA2024 FSW welds using notched tensile specimens are used to evaluate the notch tensile strength (NTS) of specific regions with different hardness values in the weld process zone. For the FSW process parameters used in this study, the response of the unnotched weld nugget specimen incurs a 4% loss in ultimate tensile strength (UTS) while retaining good ductility in comparison to the base metal. However, when a notch is introduced, the nugget experiences a reduction of ∼13% in the NTS while notch fracture strain (NFE) decreased by 40%. Fractographic results show that the existence of a banded microstructure in the nugget region is a primary factor affecting the reduction in NTS and NFE in the weld nugget. For notched specimens located at both the nugget–HAZ boundary and at the minimum hardness (HAZ), the measured reduction in NTS is directly correlated to a greater amount of inter-granular fracture in these regions due to over-aging in the welding process. Finally, when comparing local mechanical properties for both notched and unnotched specimens, the notch–strength ratio for all FSW zones in the AA2024 and AA2524 FSW joints, NSRFSWzone=(NTS/UTS)FSWzone>1.00, demonstrating that the entire FSW process zone in AA2024 and AA2524 is notch insensitive.

The experimentation and research of AZ31B magnesium alloy friction stir welding were carried out by using plates of 5 mm in thickness.The results showed that the material suitable for the stir welding head was W6Mo5Cr4V2 high-speed steel.Its structure was in shape of concave round platform,the root of which was 5.5 mm in diameter,the butt 2.5 mm,the shoulder 12 mm,and its length was 4.7 mm.The tensile strength of magnesium alloy friction stir welding joint could be 90% of that of the parent metal,and the specific elongation could be 50% of that of the parent metal.The structure of the weld area was dynamic recrystallization.Apparent boundary existed between the weld area of the forward border joint and the parent metal,while trails of little molten showed in the transitional area of the back border.

An important design, construction and maintenance concern for pipelines is the integrity of flaws in the girth welds. Numerous fitness for purpose codes are available to assess weld flaws, many of which were calibrated with reference to wide plate test data. Often, wide plate tests are conducted on girth welded pipe in the as-received condition, i.e. without application of a pipeline coating. The area adjacent to the weld is thus subjected to a thermal cycle due to the heat generated from the welding process. In some pipe materials this thermal cycle might be sufficient to induce strain aging. It is not clear how the welding process changes the behaviour of the area next to the weld. The results of such wide plate experiments are very important in assessing the acceptable flaws in a girth weld under a strain-based design. Therefore, it was important to understand the extent of the aging, specifically the stress-strain behaviour on either side of the girth weld. This paper presents results of cross-weld tensile tests, which utilized a two-dimensional digital image correlation (DIC) technique to determine displacement, and thus infer strain. The local strains were mapped to global stress to obtain local constitutive properties every 12.5mm along the length of the specimen. The DIC test results were very consistent and were also similar to results obtained from standard circumferential tensile tests at corresponding locations. The strength of the specimens, as defined by the relative strength of their stress-strain curves, was found to be highest in the girth weld region, to drop in the HAZ, and then to reach a plateau in the base metal. It was also shown that strain localization in one of the HAZ regions was clearly visible during the loading process and the near-HAZ regions had a stress-strain response with a yield stress value higher than the base metal. This behaviour was observed at 12.5mm away from the girth-weld centerline in both the transverse and longitudinal directions. The reason for this slight change of behaviour can be attributed to the effect of heating supplied to this part during welding (strain aging). The described DIC technique is very promising in obtaining local strain fields within very small areas of the tested specimens.

The friction stir welding of red copper and brass were carried out at a serious of welding parameters. The properties and microstructures of the welded joints were analyzed. Tensile and bending test results indicate that the welded joints can achieve good combination of strength and ductility at rotation speed of 950rpm, welding speed of 60 mm/min and Z-axis force of 2000N. The tensile specimen failed at the base material of red copper. The weld nugget zone has very fine dynamic recrystallization microstructure and its Vickers hardness falls in between those of the two base metals. The sharp interface between the TMAZ and nugget might result in brittle fracture.

Experimental results of the compression fracture strength of ceramic notched elements are summarized and analyzed in the present paper. The fracture model of ceramic notched elements under compression can be put forward based on Griffith's theory of brittle fracture. It is shown that the fracture of Al2O3 ceramics under compression loading is a typical brittle one induced by tensile stress, irrespective of whether the specimens are with or without a notch. The test results show that the product of the compression notch strength and the stress concentration factor is equal to the compression fracture strength, and the validity of the above-mentioned fracture model is thus checked. Finally, the procedures for predicting the fracture criteria with given survivability for Al2O3 ceramic notched elements can be developed based on the test results and the analysis mentioned above.

In the present paper, attempts are made to develop a torsion fracture model for notched elements of brittle materials. Based on the theory of brittle fracture due to Griffith, a formula for the torsion notch strength of a brittle material is thus derived. Observations of the fracture mode show that torsion fracture of both smooth and notched specimens of the inorganic glass tested is typical brittle fracture. Experimental results for torsion fracture strength determined by testing smooth and notched specimens of the brittle inorganic glass are summarised and applied to check the applicability of the abovementioned model. Moreover, it is noted that the torsion strength of a brittle material is not a material constant but dependent on the specimen geometry and fracture toughness. Expressions of torsion notch strength of brittle materials with given survivability are predicted from the probability distribution parameters of torsion fracture strength and the formula for torsion notch strength, and are in good agreement with experimental results. In engineering applications, expressions of torsion notch strength of brittle materials with given survivability could be used as failure criteria in the design and reliability assessment of structural members made from such materials.

The microstructures of the friction-stir welded joints of pure copper to brass and their performance are examined through tension, hardness and bending tests. The results show that the pure copper and brass can be friction stir welded and the welded joints have the same strength as the base material. The nugget zone has dynamic recrystallization microstructure under the thermo-mechanical effect. The microstructure of the heat-affected zone in the joint on the side of brass has some differences between the upside and underside. The latter can be divided into recrystallization zone, incomplete recrystallization zone and dynamic recovery zone, and the former is of apparent segregation. There are some streamline microstructures and a slight grain melting in the heat affected zone on the side of pure copper.

Friction stir welding of T2-H62 was investigated in this paper.Many experiments were carried on the welding machine modified by ourselves.The material,shape and dimension of the nib had been optimized.The significance sequence of processing parameters of friction stir welding had been calculated by signal-noise(SN) ratio experiments.The mechanical performance of T2-H62 welded joints was tested through tensile experiments,hardness tests and bending tests.

Because of the differences of the physical and chemical performance of dissimilar materials, many difficulties exist when they are welded together with age-old welding ways. This paper studies the welding technology for L6-LY12, sets up correct welding technique and takes special measures, and monitors the metallographic organization and mechanics performance, therefore high quality welding joint is got. FSW joint with favorable mechanical property can be obtained by optimizing processing parameters.

The influence of welding parameters on the microstructure and properties of the friction stir welded joints of dissimilar metals L6 LYI2 was investigated. Signal noise ratio orthogonal experiment and analysis of variance were utilized. The parameters in the order of significance are: pressure, welding speed, rotational speed. The experiments conducted on L6-LYl2 with the thickness of 5 mm by friction stir welding suggest that the pressure can range from 2000 N to 3000 N, the welding speed can range from 37.5 mm to 60 ram, and the range of the rotational speed is 950 - 1500 r/min. The optimized parameters are 2500 N, 37.5 mm/min and 950 r/min. The welded joints are of homo-strength or hyper-strength.