基于α'组织设计适于激光立体成形的新型高塑性Ti-4.13Al-9.36V合金

为了提高Ti-6Al-4V合金的加工硬化率和塑性,基于其团簇成分式12[Al-Ti₁₂](AlTi₂)+5[Al-Ti₁₄](V₂Ti)设计成分式为4[Al-Ti₁₂](AlTi₂)+12[Al-Ti₁₄](V₂Ti)的(Ti-4.13Al-9.36V, %)合金,采用激光立体成形工艺制备Ti-4.13Al-9.36V和Ti-6.05Al-3.94V(对比合金),研究了沉积态和固溶温度对其显微组织和力学性能的影响。结果表明,沉积态Ti-4.13Al-9.36V和Ti-6.05Al-3.94V合金的显微组织均由基体外延生长的初生β柱状晶和晶内细小的网篮α板条组成。Ti-6.05Al-3.94V合金的初生β柱状晶的宽度约为770 μm,α板条的宽度约为0.71 μm;而Ti-4.13Al-9.36V合金的初生β柱状晶的宽度显著减小到606 μm,α板条的宽度约为0.48 μm。经920℃固溶-淬火处理后Ti-6.05Al-3.94V样品的显微组织为α'+α相,其室温拉伸屈服强度约为893 MPa,抗拉强度约为1071 MPa,延伸率约为3%。经750℃固溶-淬火处理后Ti-4.13Al-9.36V样品的显微组织为α'+α相,与α'马氏体相比,应力诱发的α'马氏体能显著地提高合金的加工硬化能力,其室温拉伸屈服强度约为383 MPa,抗拉强度约为 989 MPa,延伸率达到了17%。这表明,根据团簇理论模型调控α'+α的显微组织能有效提高激光立体成形Ti合金的加工硬化能力和塑性。     

On the effect of sampling volume on the microscopic cleavage fracture stress

Reported observations of an experimental variation in the microscopic fracture stress for transgranular cleavage (σ^*_f) with specimen geometry and size are quantitatively examined in terms of a weakest-link statistical model for brittle fracture, wherein failure coincides with the critical propagation of a particle microcrack into the matrix. By analysing the onset of fracture in the ‘sharp-crack’ (K_Ic) specimen, the ‘rounded-notch’ (Charpy) specimen, and the uniaxial tensile specimen, it is shown that values of σ^*_f are reduced progressively in the ‘sharp-crack’, notched and unnotched geometries and with increasing specimen size, consistent with an increase in statistical sampling volume. Quantitative predictions for the magnitude of this variation are given for a low strength steel.     

Hall—Petch relation in two-Phase TiAl alloys

The Hall—Petch relation in lamellar—gamma duplex structures of TiAl alloys has been evaluated using Fan and Miodownik's model, by assuming that the friction stress varies linearly with the lamellar volume fraction. The results reasonably predict the strength of lamellar—gamma duplex structures at various grain sizes and volume fractions. The individual contributions from three constituting boundaries, lamellar—lamellar (-), gamma—gamma (γ-γ), and lamellar—gamma (-γ) boundaries to the overall strengthening have been calculated as a function of volume fraction: the -γ phase boundaries are not effective obstacles to dislocation motion. The - grain boundaries are rather the strongest obstacles to dislocation motion.     

Application of an elastic-plastic model to the use of small specimen strength ratio for measuring fracture toughness

Data reported by Server and Wullaert correlating specimen strength ratio with fracture toughness were analyzed with the D-BCS-HSW model. This model is based on the Dugdale model, elaborated by Bilby, Cottrell and Swinden, and extended by Heald, Spink and Worthington. The data included instrumented precracked Charpy results on HSST plate 02, as well as static, intermediate, and dynamic tests of 1 — T compact and bend specimens of SA533B-1 steel. The model relates the fracture toughness to the crack length, specimen shape and size, applied failure stress and effective flow strength. The only parameter not provided by the data is the constraint factor, M, the ratio of the effective flow strength to the yield strength. The model was fitted to the data by non-linear least squares methods by which M was determined to be approx. 2.5 for the Charpy data, and from 2.1 to 2.6 for the other specimen data. The fit is considered to be reasonably good throughout the range from linear-elastic fracture mechanics through to plastic collapse. The result for the Charpy data is considered to be as good as that for the other specimens. The determination of only one parameter is needed to establish the relationship between specimen strength ratio and fracture toughness. This relationship then applies to the entire range of fracture regimes.     

Prediction of constant-amplitude fatigue life to failure under pulsating tension by use of the local-strain approach

The aim of this investigation was to compare local-strain approximation (LSA) life predictions with pulsating-tension tests to failure. It was shown that, in the case of pulsating tension, the ratio N_i/N_f does not change significantly and therefore an LSA calculation can be of use for predicting A-M-N lines for R0. The accuracy of the prediction is associated with the choice of the k-value in Neuber's relation K_σK_ε=k². The -values that give the best life predictions were obtained close to those of the corresponding notch factors for pulsating tension (R=S_min/Smax=0). Because of the size and surface effects (which are not taken into account in LSA), and in view of the difficulty in knowing whether the specimen with a given K_F-value has exactly the same cyclic parameters as those used in the LSA calculations, it is then preferable to use a k-value proved to give prediction results in agreement with some pulsating-tension tests. This k-value may be used for all combinations of S_a and S_m (S_aS_m and S_max<S_y). The results are valid for a specific specimen material, geometry, size and surface finish.     

骨料粒径对混凝土动态拉伸强度及尺寸效应影响分析

骨料粒径是影响混凝土力学性能及破坏机理的重要因素。从细观角度出发,将混凝土看作由骨料颗粒、砂浆基质及界面过渡区组成的三相复合材料,考虑细观组分的应变率效应,建立了混凝土动态拉伸破坏行为研究的细观力学分析模型,模拟研究了不同骨料粒径下混凝土动态拉伸破坏行为,并揭示了动态拉伸强度的尺寸效应规律。研究表明:低应变率下骨料不发生破坏,骨料粒径对混凝土动态拉伸破坏模式及拉伸强度影响显著,且拉伸强度的尺寸效应随骨料粒径的减小而削弱;高应变率下裂缝将贯穿骨料,骨料粒径的大小对混凝土动态拉伸强度及尺寸效应影响可忽略。最后,结合应变率效应的影响机制,建立了混凝土拉伸强度的"静动态统一"尺寸效应理论公式,该公式可以较好描述各骨料粒径下混凝土动态拉伸强度与试件尺寸的定量关系。     

Elastic-plastic and failure properties of a unidirectional carbon/PMR-15 composite at room and elevated temperatures

A series of off-axis tensile tests at room and elevated temperatures have been conducted up to 316°C (600°F) to determine the elastic and plastic properties of a unidirectional carbon/PMR15 composite as a function of temperature. The transverse tensile and shear strengths of the composite as a function of temperature have also been determined. The effect of the specimen preparation process (type of machining) on the strength properties of the composite has also been evaluated. It has been shown that elastic (with the exception of Poisson ratios ν₁₂ and ν₂₁), plastic, and strength properties of the composite are significantly affected by elevated temperatures. It has also been demonstrated that the quality of machining can noticeably influence the normal and shear strength data at room and elevated temperatures. Even if the quality of machining is very high, failure of the specimens can occur either in the gage or grip sections. At room temperature, all specimens failed in the grip areas influencing the transverse tensile and shear strength measurements. However, the type of specimen failure does not noticeably affect the strength data at elevated temperatures. The transverse tensile and shear strength properties of the composite at room temperature could only be estimated by extrapolating the normal and shear strength vs temperature curves to room temperature.     

Interaction between plasticity and damage in the behaviour of [+φ, −φ]n fibre reinforced composite pipes in biaxial loading (internal pressure and tension)

In this paper, we propose a new model which describes the behaviour of [+φ, −φ]_n composite laminates. Tests were performed on glass-epoxy pipes subjected to biaxial tensile and internal pressure loading. Experiments showed that [+55, −55]_n pipes exhibit varying types of damaged elastoplastic behaviour depending on the stress ratio σ_zz/σ_θθ (axial stress/hoop stress). A plastic model is based on the definition of a yield criterion and an associated flow rule. Damaging occurs when transverse microcracks appear in the layer. A micromechanical model defines the anisotropy of the damage. Interaction between plasticity and damage was of major importance in the definition of damage kinetics. This effect was observed on proportional loadings as well as on sequential tests: a preliminary loading in pure internal pressure (σ_zz=0) induced large plastic phenomena which blocked crack propagation in additional internal pressure with closed ends effect (IPCEF) tests (R=σ_zz/σ_θθ=1/2), even though IPCEF caused considerable damage on an unloaded specimen.     

ProSINTAP – A probabilistic program implementing the SINTAP assessment procedure

Within SINTAP, a probabilistic procedure has been developed by SAQ Kontroll AB. The procedure can be used to calculate failure probabilities under two conditions: when a defect size is obtained from non-destructive testing (NDT)/non-destructive examination (NDE) or when no defect is detected by NDT/NDE.A computer program based on this procedure, ProSINTAP, has been developed. This program is also able to estimate partial safety factors from the input target failure probability and characteristic values for fracture toughness, yield strength, tensile strength and defect depth. Extensive validation has been carried out, using the computer programs from Nuclear Electric and from SAQ Kontroll AB.     

The tensile failure of brittle matrix composites reinforced with unidirectional continuous fibres

The tensile failure strength of ceramic composites can be measured by tests in bending or in tension, but care must be exercised over the experimental conditions. The strength values obtained are dependent on the test method and specimen size. It is shown that differences between strengths measured in bend and tensile tests can be understood in terms of the statistical distribution of the strengths of individual fibres.     

Reduction in tensile and flexural strength of unidirectional glass fibre-epoxy with increasing specimen size

Size effects in tensile failure were investigated by means of tensile and four-point bending tests. Tapered tensile specimens with plies dropped off internally showed a reduction in strain at failure with increasing gauge length. Scaled bending tests also showed a reduction in strain with increasing specimen size. These two effects and the relationship between the tensile and flexural results could all be fitted satisfactorily with a Weibull strength model.     

Specimen size effect during tensile testing of an unreinforced polymer

Various specimen sizes of an unreinforced polymer, Hercules 3501 -6 thermosetting epoxy, were subjected to a tensile test. The general specimen geometry was a rectangular dog-bone shape with constant gauge length, but with each specimen size having a different crosssectional area. These cross-sectional areas were obtained by varying the thickness of the epoxy during casting, and the gauge section width during grinding. The resulting failure surfaces of the specimens were observed and photographed using scanning electron microscopy. The results indicate that failure stress, dimensions of the critical flaw which caused failure, and a quantity which is proportional to the fracture toughness, are all correlated with specimen size.     

Comments on “Is the cause of size effect on structural strength fractal or energetic-statistical?” by Ba?ant & Yavari [Engng Fract Mech 2005;72:1-31]

Aim of this letter to the Editor is at replying to the criticisms raised by Ba?ant and Yavari [Ba?ant ZP, Yavari A. Is the cause of size effect on structural strength fractal or energetic - statistical? Engng Fract Mech 2005;72:1-31] against the fractal approach to the size-scale effects on the mechanical properties of materials and the concept of the Multi-Fractal Scaling Law presented by Carpinteri [Carpinteri A. Scaling laws and renormalization groups for strength and toughness of disordered materials. Int J Solids Struct 1994;31:291-302]. These criticisms will be analysed thoroughly, showing how they also contain some mistakes and misunderstandings. The presented elucidations should redirect the discussion to a more correct scientific debate.     

Bolted joints of pultruded sandwich composite laminates

This paper describes the tensile behavior of bolted joints of pultruded sandwich composite laminates. The pultruded sandwich laminates have a skin-core-skin structure. Joint strength of longitudinal specimens was independent of specimen width (w), whereas it increased with w in the case of transverse specimens. The joining efficiency of pultruded sandwich laminates was greater in the longitudinal direction than in the transverse direction. The core layer of longitudinal specimens failed by a combination of bearing and shear-out modes, independent of w. The failure mode of skin layers changed from net-tension to bearing mode with increasing w. In transverse specimens, the failure mode of core and skin layers changed from net-tension to bearing with increasing w. Finite element numerical analysis was carried out to predict the failure mode and joint strength. The numerical results were in good correlation with the experimental results.     

Size effect on tensile softening relation

Tensile softening essential to the well-known fictitious crack model should be independent of specimen size and test methods. Commonly observed size effect (SE) on tensile softening relations measured from direct tensile specimens with smooth surface needs to be explained in order to determine the size independent tensile softening behaviour. In this paper, SE on tensile softening from direct tensile tests is elucidated by considering a boundary region where the key tensile softening mechanisms such as aggregate interlocking and frictional pull-out activities are limited in comparison with the inner region where those crack-bridging activities can be fully developed. SE on the tensile softening relation and the closely related specific fracture energy G _f is inevitable if the boundary and inner regions are comparable. The same SE is gradually diminished with the increasing specimen size simply because the relative contribution from the boundary region is reduced in comparison with that from the increasing inner region. In principle, the size independent tensile softening relation and the size independent specific fracture energy G _F in the inner region can be obtained by separating the influence of the boundary region from the test results.     

A stereological analysis of aggregate grading and size effect on concrete tensile strength

One of the most important topics in solid mechanics is the study of the so-called size effects, whose importance has been widely recognised during the last decades. Size effects are particularly strong in quasi-brittle (i.e., concrete-like) materials. In this paper we focus our attention on the tensile strength decrease associated with the size of concrete structures. An original explanation of this well-known size effect was proposed by the first Author based on the assumption of a fractal-like damage localisation at the mesostructural level. This hypothesis leads to a multifractal scaling law (MFSL) for concrete tensile strength. The present contribution provides a scaling law for concrete tensile strength based on its aggregate size distribution. Since the weakest link in normal strength concrete is represented by the interface between the cementitious matrix and the aggregates, it seems reasonable to look for a relationship between the aggregate grading and the material strength. Based on the hypothesis that the strength depends on the largest flaw, we compute the strength of a concrete specimen as a function of its size. Differently from other statistical approaches, we use a truncated distribution (namely the Füller distribution) in order to describe realistically the flaw population inside the specimen. Calculating the distribution of the largest flaw size by means of statistics of extremes, and relating it to the specimen size, we obtain a scaling law for concrete tensile strength whose trend strictly agrees with the MFSL. Finally, we pay particular attention to the computation of the power law exponent characterising the strength scaling at the smallest sizes and present a comparison with available experimental data.     

A stereological analysis of aggregate grading and size effect on concrete tensile strength

One of the most important topics in solid mechanics is the study of the so-called size effects, whose importance has been widely recognised during the last decades. Size effects are particularly strong in quasi-brittle (i.e., concrete-like) materials. In this paper we focus our attention on the tensile strength decrease associated with the size of concrete structures. An original explanation of this well-known size effect was proposed by the first Author based on the assumption of a fractal-like damage localisation at the mesostructural level. This hypothesis leads to a multifractal scaling law (MFSL) for concrete tensile strength. The present contribution provides a scaling law for concrete tensile strength based on its aggregate size distribution. Since the weakest link in normal strength concrete is represented by the interface between the cementitious matrix and the aggregates, it seems reasonable to look for a relationship between the aggregate grading and the material strength. Based on the hypothesis that the strength depends on the largest flaw, we compute the strength of a concrete specimen as a function of its size. Differently from other statistical approaches, we use a truncated distribution (namely the Füller distribution) in order to describe realistically the flaw population inside the specimen. Calculating the distribution of the largest flaw size by means of statistics of extremes, and relating it to the specimen size, we obtain a scaling law for concrete tensile strength whose trend strictly agrees with the MFSL. Finally, we pay particular attention to the computation of the power law exponent characterising the strength scaling at the smallest sizes and present a comparison with available experimental data.     

A CDM-based failure model for predicting strength of notched composite laminates

This paper investigates the ultimate tensile failure strength of laminated composites containing a central circular hole. Based on continuum damage mechanics, a Principal Damage Model is developed by combining the generalized standard material model with the Principal Damage concept of composite materials. Three in-plane failure modes: fiber breakage, matrix cracking, and fiber/matrix interface debonding are included in the present model. After obtaining material constants and damage relations from standard tensile tests, the material constitutive relations with damage model are implemented into commercial finite element code, abaqus. By comparing the predicted results with the experimental data, the proposed model has proven to be capable of predicting failure strength and load–deflection relations of notched laminated composites. The effects of hole size and specimen width are discussed in detail. In addition, the advantage of the present model is demonstrated through comparison with other existing models.     

Size effect in concrete beams

This paper will report on an experimental investigation into the size effect in the strength of hardened cement paste (nominal compressive strength 40 MPa) and high strength concrete (nominal compressive strength 110 MPa) as measured in three point bending. The aim of the investigation is to judge the range of applicability of the various size effect formulae available in the literature. With this aim in mind, the failure loads have been analysed according to the size effect formulae of Bazant and of Karihaloo for the notched beams and according to those of Bazant and of Carpinteri for the unnotched beams. The results of this analysis will be presented. Improvements to Karihaloo’s size effect formula will also be proposed.