Mechanical and fracture properties of aluminium cylinders manufactured by orbital friction stir welding

Butt welding of AA6063 aluminium cylindrical shells was performed using the orbital friction stir welding (FSW) method. Tool rotation speed and orbital speed (i.e., traverse speed of rotating cylinder during welding) were considered as variable, and the strength and the mechanical properties including tensile strength, microhardness, mode I fracture energy and mode I crack growth behaviour of manufactured cylinders were investigated experimentally. A novel and subsized test specimen was designed and manufactured for fracture testing of specimens extracted from both base metal and weld zone region of cylinders. The initial precrack was introduced along (i) the tool penetration through the pipe thickness (i.e., T‐direction) and (ii) along the tool travelling direction (i.e., L‐direction). It was found that the crack growth resistance and fracture energy values of FSW samples are greater than the corresponding values of base aluminium material along both “L”‐ and “T”‐directions. Also, the fracture resistance value in T‐direction was higher than the L‐direction for the whole tested FSW samples with different welding speeds.     

Probabilistic aspects of fracture energy of concrete

A test method to determine fracture energy and strain-softening in direct tension is described. Experimental results on cylinders of equal diameter and varying length are reported. It is found that the tensile strength decreases with increasing volume while the fracture energy remains constant within the observed volume range. By means of numerical simulation, it is shown that in a direct tension test several fracture process zones appear in the initial states of cracking and that final rupture is induced by the development of only one of these fracture zones. This phenomenon has been observed experimentally by other authors. A comparatively large number (44) of identical samples were tested by using the wedge-splitting test. Half the specimens were grooved. The fracture energy of the grooved and ungrooved specimens turned out to be the same within the given range of accuracy. It was observed experimentally and simulated numerically that in grooved specimens the crack is forced to follow a ragged fracture surface which is statistically not the weakest one. In an ungrooved specimen the crack path generally diverts from the centre line and advances through weaker zones. For the formation of these skew cracks, however, more energy is consumed due to aggregate interlock. In addition, the fracture process zone observed in ungrooved specimens is generally wider.     

Structural integrity assurance of high-strength steel gas cylinders using fracture mechanics

This paper describes fracture mechanics criteria employed in the development of fracture-safe, 155–175 ksi strength level steel cylinders. A new steel composition was developed to obtain the toughness required to satisfy leak-before-break and plastic fracture mode criteria. Cylinders were fabricated from the new steel, and tests were conducted to characterize their flaw tolerance capability. Leak-before-break and plastic fracture mode performance was verified. Flawed cylinder tests were also conducted on conventional 3AA 4130X steel cylinders (105–125 ksi strength level) to characterize their flaw tolerance capability. The flaw tolerance capability of the high-strength cylinders was found to be equal to that of the conventional low strength steel cylinders at respective service stresses.     

Compressive fracture of polystyrene

Polystyrene cylinders and blocks were compressed with a variety of flat indentors until fracture occurred. The test cannot be interpreted solely by slip-line field theory, nor solely using Kendall's theory for the splitting of pre-cracked blocks of elastic material. A slip-line field analysis was made of the stress variation within the yielded zone beneath the indentor, then a boundary element analysis was made of the remaining cracked elastic region. This predicted stress intensity factors that are of the correct magnitude for the observed crack velocity.     

Thermophoretic deposition of aerosol particles in laminar mixed-convection flow in a channel with two heated built-in square cylinders

The thermophoretic deposition of aerosol particles in laminar mixed-convection flow in a channel with two heated built-in square cylinders was studied numerically. The objective of this research was to study the effect of free convection and the distance between cylinders, on deposition of particles. Continuity, momentum and energy equations were solved to determine the velocity and temperature profiles in the channel. The particle trajectories were evaluated by solving the Lagrangian equation of motion that included the drag, Brownian diffusion and thermophoresis forces. It was found that the temperature gradient near the channel wall, in mixed flow regime, is higher than the temperature gradient in forced convection regime. Increasing the temperature gradient increased the effect of thermophoresis on deposition of particles. It was observed that the deposition was increased with the Richardson number. The distance between cylinders is a parameter that influences the deposition of particles. Temperature gradient decreases with increasing the cylinders’ distance; on the other hand, the length of the high temperature gradient zone, which is located in the region between the cylinders where the most deposition occurs, will be increased. These two opposite phenomena cause the fact that at a distance which is four times longer than the cylinders’ length, a maximum cumulative deposition fraction occurs. It was eventually concluded that the thermophoresis and the inertial impaction are dominant deposition mechanisms of particles on the channel wall.     

Deformation and fragmentation behaviour of exploded metal cylinders and the effects of wall materials,configuration, explosive energy and initiated locations

Tubular metal specimens are explosively expanded to fragmentation, and the effects of wall materials, thicknesses, notches in walls, explosive driver diameters and the initiated locations are investigated on the deformation and fracture behavior of the cylinders experimentally and numerically. In the standard tests, the driver is a column of low density powder of high explosive PETN, inserted coaxially into the bore of a smooth-walled cylinder and initiated by exploding a bundle of fine copper wires at the column axis using a discharge current from a high-voltage capacitor bank. Notched cylinders with single axial slit, various grooves in the walls, and smooth cylinders with varied wall thicknesses were tested. Low-carbon steels and an aluminum alloy A5052 were provided in addition to the standard smooth-walled 304 stainless steel cylinder, and they were fully or partially charged with varying explosive column diameters. The initiated locations in the explosive column are changed for comparison, placing the bundle of fine copper wires eccentrically from the central axis or replacing the fine wire bundle into a bold wire line except the middle portion at the central axis for central point initiation. Additionally an explosive-filled cylindrical vessel with welded endplate at the one end is initiated at the other end explosive surface exploding wire-rows and expanded by axially propagating explosive detonation to fracture for comparison with the uniform expansion. Deformation and crack initiation of expanding cylinders are observed with high speed camera, and most of the fragments have been recovered successfully. Recovered fragments have been measured and investigated using a fragmentation model. The effects of test parameters on the deformation and fracture behavior of metallic cylinders are discussed with use of numerical simulations, indicating applicability of the fragmentation model and suggesting future necessary studies.     

Examination of the processes of deformation and fracture in a silica-filled epoxy resin

A series of short-term fracture tests have been undertaken on a silica-filled epoxy resin in order to examine the processes of damage initiation, development and fracture in a particulatefilled polymer. Several different types of inelastic deformation and fracture mechanisms were observed within the volume of the material. These included localized shear yielding, particlematrix debonding and micro-cracking. The relative amount of each of these was found to depend upon the test rate and temperature. At low temperatures and high rates of loading, failure was associated with one single debonding event whereas at high temperatures and low rates, debonding and yielding were found to be extensive throughout the volume of the test specimens. A detailed examination of the fractured specimens identified several distinct regions on the fracture surface. Surrounding the defect the particles were often debonded from the matrix suggesting that the crack had propagated in a sub-critical manner. Beyond this zone was a smooth zone corresponding to the region over which the crack was accelerating unstably. The smooth zone then developed into a rough three-dimensional zone in which the crack was propagating at its maximum velocity. The size of each of these zones was found to vary considerably with test temperature and cross-head speed.     

Damage tolerance of composite cylinders

The fracture of pressurized graphite/epoxy cylinders was investigated and their damage tolerance assessed. The cylinders were 610 mm long and 305 mm in diameter and were fabricated from Hercules A370-5H/3501-6 prepreg fabric in quasi-isotropic four-ply configurations: (0,45)_s and (45,0)_s. The cylinders were slit in the longitudinal direction and the critical notch sizes for three pressure levels were determined. Experiments on coupons of similar construction loaded in tension were previously conducted. The critical flaw sizes for the cylinders were well predicted from the flat coupon data corrected for the effects of curvature. In addition, circumferentially wrapped unidirectional plies of Hercules AS1/3501-6 tape of various stacking sequences were used as selective reinforcement on several (0,45)_s cylinders. These reinforcing plies did change the path of damage but did not prevent catastrophic failure.     

Explosive fracture of closed cylinders

We present the results of numerical simulation of the process of fracture of thick-walled elastoplastic cylinders with bottoms caused by the explosion of a blasting charge placed inside the cylinders. We show that the influence of the scheme of explosive loading on spalling effects in the walls of the cylinders is significant. Scientific Research Institute of Applied Mathematics and Mechanics, Tomsk, Russia. Translated from Problemy Prochnosti, No. 4, pp. 44–51, July–August, 1997.     

Investigation of the fracture and fragmentation of explosively driven rings and cylinders

Cylinders and rings fabricated from AerMet^® 100 alloy and AISI 1018 steel have been explosively driven to fragmentation in order to determine the fracture strains for these materials under plane-strain and uniaxial-stress conditions. The phenomena associated with the dynamic expansion and subsequent break up of the cylinders are monitored with high-speed diagnostics. In addition, complementary experiments are performed in which fragments from the explosively driven cylinders are recovered and analyzed to determine the statistical distribution associated with the fragmentation process as well as to determine failure mechanisms. The data are used to determine relevant coefficients for the Hancock–McKenzie (Johnson–Cook) fracture model. Metallurgical analysis of the fragments provides information on damage and failure mechanisms.     

The effect of heat treatment on plane strain fracture of glassy polymers

The effect of annealing on the plane strain fracture of round-notched polycarbonate and poly(methyl methacrylate) bars has been investigated. Morphological observations of thin sections and fracture surfaces revealed that the fracture initiated from internal crazes which were nucleated at the tip of a local plastic zone. The critical hydrostatic stresses for internal craze nucleation were nearly constant regardless of annealing, while the shear yield stress increased with increasing annealing time. The reduction in toughness by annealing can be ascribed to the decrease of the maximum extent of the plastic zone which gives the critical hydrostatic stress for craze nucleation.     

IMPROVEMENT OF THE FRACTURE TOUGHNESS OF AN ALUMINIUM ALLOY

Abstract— The improvement of a material's toughness has significant industrial applications. An example is the manufacture and maintenance of commercial gas containers made in the form of aluminium cylinders; a challenging design problem since they are susceptible to crack growth under a static load (internal pressure). In this investigation a new thermo-mechanical technique was used to improve the apparent fracture toughness. This could increase significantly the safety margin and life of the cylinders. The determination of fracture toughness was carried out using a newly developed small cylindrical specimen which has a considerable cost advantage over the standard compact tension specimen. Experiments on aluminium 6351-T6 cylindrical specimens confirmed the possibility of improving the toughness of this alloy.     

Compression fracture statistics of compacted cement cylinders

A statistical analysis of the compression fracture stress of compacted cement paste cylinders was made by comparison of normal, logarithmic normal and Weibull distributions. The influence of specimen size on the fracture stress was determined and compared with predictions based on crack propagation results. The results of the analysis show that the Weibull modified statistics fit in very well with the behaviour observed.     

Microstructure and Fracture Morphology in the Welding Zone of T91 Heat—resisting Steel Used in Power Station

Microstructure performance in the welding zone of T91 heat-resistant steel under the condition of TIG welding was researched by means of metallography, X-ray diffraction and scanning electron microscope (SEM). Experimental results indicated that microstructure of T91 weld metal was austenite + a little amount of S ferrite when using TGS-9cb filler wire. Substructure inside the austenite grain was crypto-crystal lath martensite, on which some Cr23C6 blocky carbides were distributed. The maximum hardness (HRC44) in the welding zone is near the fusion zone. There existed no obvious softening zone in the heat-affected zone (HAZ). For T91 steel tube of $63 mmx5 mm, when increasing welding heat input (E) from 4.8 kJ/cm to 12 5 kJ/cm, fracture morphology in the fusion zone and the HAZ changed from dimple fracture into quasi-cleavage fracture (QC). Controlling the welding heat input of about 9.8 kJ/cm is suitable in the welding of T91 heat-resistant steel.     

Deformation and fracture of styrene-acrylonitril copolymer - rubber blends: Microscopy studies of deformation zones

A styrene-acrylonitril copolymer (SAN) was toughened by SAN-grafted polybutadiene core-shell rubber particles. Notched tensile specimens were fractured with a tensile speed ranging from 10-4 to 10 m s-1. The deformation processes close to the fracture surface were studied by means of transmission electron microscopy. A marked difference in the structure of the deformation zone was observed between low speed (10-3 m s-1) and high speed (≥1 m s-1) deformed samples. At low tensile speed the structure of the deformation zone correlated closely with fracture mechanics theory. When the tensile speed was increased the deformation zone had a layered structure. In the zone 400–1.5 μm below the fracture surface the deformation structure was similar to that at low speed. In the layer 1.5–0.5 μm from the fracture surface the rubber particles were strongly deformed, but no cavities or crazes could be observed. Directly next to the fracture surface the high speed deformation zone showed a small layer (0.5 μm) where all the deformation had vanished. It is suggested that due to high strain-rate plasticity at the crack tip a temperature rise occurs which is high enough to cause complete relaxation of the deformation in this layer. Therefore, locally the glass transition temperature of the matrix material was reached. The interaction between thermal effects and deformation processes at the crack tip is discussed. This revised version was published online in November 2006 with corrections to the Cover Date.     

Some aspects of the fracture behaviour of Mg65Cu25Y10 bulk metallic glass during room-temperature bending

The fracture behaviour of Mg₆₅Cu₂₅Y₁₀ bulk metallic glass (BMG) during room-temperature three-point bending was investigated. The BMG was initially produced by casting into a wedge-shaped mold which generated an amorphous structure below the ∼4 mm thickness zone of the wedge. Three-point bend testing was then carried out on the BMG with the fracture angles and salient features of the fracture surfaces examined by scanning electron microscopy. Observations indicate that this type of deformation mode results in fracture via crack propagation from both surfaces of the samples where the tensile and compressive stresses are greatest. The direction of crack propagation was also found to deviate considerably from 45° to the length direction of sample. A scanning electron microscopy (SEM) study of the fracture surfaces indicated that deformation banding was a feature of crack propagation within compressive zone whereas the tensile zone generated a featureless surface characteristic of brittle failure. The mechanism of failure of the present alloy is discussed on the basis of the observed features on the fracture surfaces and the direction of propagation of cracks during failure and compared with the failure mechanism of samples fractured under both simple tension and compression.     

Thermal-shock-induced crack arrest of two low-alloy steels

Tests were performed on a C-Mn-Nb steel (E 36) and a C-Mn-Ni-Mo steel (A 50B) to determine the fracture toughness either at crack initiation, K_1c or at crack arrest, K_1a, under a very severe thermal shock. The experimental set-up was designed in such a way that it could provide enough flexibility to investigate various factors, including the specimen size effect in brittle fracture and the variations of K_1c or K_1a with temperature.

The thermal shock experiments were carried out either on small discs (thickness 19 mm) or on larger cylinders (height 220 mm) with an inner diameter and an outer diameter of 46 or 50 mm and 150 mm respectively, containing at their external periphery either a longitudinal sharp notch (0.04 mm) for the cylinders or a fatigue crack for the discs. These specimens are cooled to liquid nitrogen temperature until a homogeneous temperature distribution is reached. Then they are heated up by an induction coil set in the centre of the inner hole. The induction coils were designed to maintain purely radial heating of the specimens in order to induce axisymmetric thermal stresses. Typically, the experimental set-up is able to develop radial temperature gradients as large as 250°C in 20 s in the large cylinders and 500°C in 5 or 10 s in the thinner discs. Under the influence of these thermal gradients, which produce tensile hoop stress at the external periphery of the specimens, a crack is initiated from the notch or the initial fatigue precrack, which propagates very rapidly (-100 j1s) over a distance of a few centimetres and then stops.

The temperature distribution measured continuously during the experiments is used as the input for the numerical calculations. Finite element method calculations were performed to determine the variations of the hoop stress and those of the stress intensity factor across the wall thickness. Results obtained on both materi.als are given. In A50B steel it is shown that the apparent fracture toughness K_1c determmed on these large test pieces is smaller than the toughness measured on smaller convsntional specimens. This size effect is explained in terms of a local approach of brLttle cleavage fracture based on Weibull statistics.     

壁厚对HR2钢柱壳爆轰加载下膨胀断裂行为的影响

通过对6、12 mm两种不同壁厚的HR2钢柱壳进行爆轰加载实验,对其断裂碎片的宏观形貌、断口的微观形貌以及横截面的变形微观结构进行系统表征,研究了金属柱壳在爆轰加载下的膨胀断裂机理。结果表明,在膨胀断裂过程中壳壁厚度的增大导致HR2钢柱壳由纯剪切断裂变为拉剪混合的断裂模式。断裂碎片的微观结构分析结果表明,柱壳的断裂实际上是剪切裂纹从样品内部剪切带形核并扩展、和拉伸裂纹沿柱壳外表面的形核扩展的共同作用及竞争的结果。薄壁柱壳断裂由样品内裂纹沿剪切带的形核和扩展主导发生剪切断裂,而厚壁柱壳中内侧的裂纹沿剪切带的形核和扩展,但是最外侧则为环向拉应力主导发生拉伸断裂,因此表现出拉剪结合的断裂模式。     

Laser-induced fracture in silicon

Utilizing the scanning electron microscope, the laser-impact zone on irradiated P-type silicon wafers has been characterized by a central region of compressive stress which decreases to zero and becomes tensile in character with increasing radius from the impact centre, with symmetrical fracturing occurring around the impact centre at a point of maximum tensile stress. The effect of surface flaws on the fracture of brittle Si wafers and their interaction with the impact zone were observed to be small for scratch widths of 60 μm, and lengths of 1 cm. Direct observations of advancing crack tips were made in the transmission electron microscope, and shown to have associated arrays of partial and total dislocations. The presence of numerous precipitates in the P-type Si wafers possessing long-range strain fields which overlapped along the {111} cleavage planes was observed to have a direct influence on the fracture process. Fracture was observed to occur in wafers containing cleavage-oriented scratches or scratches deviating by ～ 5° from a cleavage-plane orientation, but little influence was noted for randomly oriented scratches or those which deviated from a cleavage-plane orientation by ～ 10° or greater.     

Simulation of dynamic fracture with the Material Point Method using a mixed J-integral and cohesive law approach

A new approach to simulating fracture, in which toughness is partitioned between the crack tip and, optionally, a process zone, is applied to dynamic fracture processes. In this approach, classical fracture mechanics determines crack tip propagation, and cohesive laws characterize process zone response and determine crack root and process zone propagation. The approach is implemented in the Material Point Method, a particle method in which the fracture path is unconstrained by a body-fitted mesh. The approach is found suitable for modeling a range of dynamic fracture processes, from brittle fracture to fracture with crack bridging. A variety of ways of partitioning toughness are explored with the aim of distinguishing model parameters via experimental measurements, particularly R curves. While no unique relationship exists, R curves, or effective R curves, on a suite of materials would provide substantial insight into model parameters. Advantages to the approach are identified, both in versatility and in regards to practical matters associated with implementing numerical fracture algorithms. It is found to perform well in dynamic fracture scenarios.