End-shaped copper fibers in an epoxy matrix—predicted versus actual fracture toughening

End-shaped copper fibers are placed in a brittle thermoset epoxy matrix at 10 vol% and tested in four-point bending to determine the fracture toughness of the composite. Results from four-point bend tests agree well with the theoretical predictions of the fracture toughness increment ‘ΔG’ of a metal fiber/brittle thermoset matrix composite based on single fiber pullout (SFP) tests. This close agreement demonstrates that SFP testing, along with the theoretical model, can be used as an effective end-shape screening tool for ductile fibers before full scale composite testing. The model predicts that the composite’s fracture toughness will be 46% higher with flat end-impacted fibers and 4% lower with rippled fibers compared to straight fibers at a 0° orientation. Four-point bend results show the actual composite’s fracture toughness is 49% higher with flat end-impacted fibers and 5% lower with rippled fibers compared to straight fibers. Further, four-point bend results show that end-shaped copper fibers improve both the flexural strength and modulus of the composite, demonstrating that end-shaped ductile fibers provide a good stress transfer to the fibers by anchoring the fibers into the matrix. Lastly, experimental validation of the model also indicates that at low fiber volume fractions, fiber–fiber interaction has only a minor influence on the fracture toughness for the tested ductile fiber/brittle matrix composite.     

Residual fracture energy of cement paste, mortar and concrete subject to high temperature

Quantifying high temperature damage is an issue that can hardly be dealt with experimentally because of the complexity of the loading control, of temperature and of moisture. The experimental investigation was carried out. The measurement of the mechanical characteristics (fracture energy, tensile strength, elastic modulus and thermal damage parameter) of five cementitious materials, cement paste, mortar, ordinary concrete and two HPC concretes were performed by three-point bending tests after heating/cooling cycles at 120, 250 and 400 °C. The tests showed that the cementitious materials behave almost identical when the fracture energy G_f is considered as a function of maximum temperature. The thermal damage due to heating from 120 to 400 °C increases the fracture energy by 50% with the reference tests at room temperature. A more tortuous crack surface is one reasonable explanation for the significant increase in G_f. It is demonstrated that the temperature exposure makes all cementitious materials tested significantly more ductile and less resistant.     

Physical aging and time–temperature behavior concerning fracture performance of polycarbonate

The effects of physical aging on fracture and yielding behavior are polycarbonate are considered. Two groups of Bisphenol A-based polycarbonate, consisted of extruded PC sheets (thickness of 0.25 mm) and injection molded PC bars (thickness of 3.18 mm) are used. These samples were annealed at various temperatures ranging from 60 to 120 °C, for different times varying up to 240 h. For PC sheets the essential work of fracture (EWF) method was used to analyze fracture behavior. The results are compared to the strain energy density with aging time and aging temperature in the ranges investigated. This effect is confirmed by the change in fracture toughness, as measured by three-point bending tests. The concept of fictive temperature (T_f) was used to characterize the degree of aging in the sample. T_f of a glass in an aged state at a time t is defined as the temperature at which the volume would be equal to the equilibrium volume at T_f if the sample were instantaneously removed to that temperature. Differential scanning calorimetry (DSC) was used to determine T_f. The variations of T_f with aging time and aging temperature are in agreement with both the strain energy density measurement and the three-point bending tests. These results contradict the effects of aging on fracture toughness observed by the essential work of fracture approach. The latter showed anomalous regions of increasing fracture toughness with aging, leading to spurious conclusions. The brittle–ductile transition in fracture behavior is analyzed by an activation energy approach. Aging increases the brittle–ductile transition temperature and the effect is more pronounced for the lower molecular-weight sample. Fracture tests also showed a decrease in the entropy with aging, confirming the results observed previously from tension and compression tests.     

Enhancement of fracture toughness of steel with defects by warm-prestressing

The technique of warm-prestressing to improve the resistance of structural steel with defects against low temperature fracture has received considerable attention. It is found that warm-prestressing can improve the fracture toughness and change the COD or δ_c, especially the crack tip plastic opening δ_p.The experimental results obtained from three-point bending tests of 42Mn2 steel specimens at −60°C and −20°C are analyzed. Experiments are also made on the bursting of pressure vessels manufactured from #20 steel. The results indicate that warm-prestressing at room temperature increased the bursting pressure at −40°C for d/t = 0.2 to 0.4, where d is the depth of surface crack and t the vessel thickness.     

Influence of stress triaxiality and temperature on void growth and ductile/brittle transition behavior of 40 Cr steel

Examined experimentally are the influence of stress triaxiality and temperature on the growth of microvoids and the ductile/brittle transition (DBT) macrobehavior of 40 Cr steel subjected to two different heat treatments. This is accomplished by testing more than 300 smooth and notched specimens over a temperature range of 20°C to −196°C. Changes in the microstructure morphology are examined by scanning electron microscopy (SEM) and identified with fracture data on a surface constructed from the uniaxial strain ε_c at fracture, the stress triaxiality R_σ and the temperature T. While stress triaxiality has a significant influence on the DBT temperature T_c, it does not affect the ratio of the average radius of voids R_o to that of inclusions R_i. The ratio R_o/R_i is found to increase with temperature and remains constant in specimens with different notch radii regardless of the temperature. Empirical relations between T_c and R_σ⁻ and R_o/R_i and T are proposed to better understand how macrofracture parameters are influenced by microstructure entities.     

Computational simulation of damage progression of composite thin shells subjected to mechanical loads

Defect-free and defected composite thin shells with ply orientation (90/0/ ± 75) made of graphite/epoxy are simulated for damage progression and fracture due to internal pressure and axial loading. The thin shells have a cylindrical geometry with one end fixed and the other free. The applied load consists of an internal pressure in conjunction with an axial load at the free end, the cure temperature was 177°C (350°F) and the operational temperature was 21°C (70°F). The residual stresses due to the processing are taken into account. Shells with defect and without defects were examined by using CODSTRAN an integrated computer code that couples composite mechanics, finite element and account for all possible failure modes inherent in composites. CODSTRAN traces damage initiation, growth, accumulation, damage propagation and the final fracture of the structure. The results show that damage initiation started with matrix failure while damage/fracture progression occurred due to additional matrix failure and fiber fracture. The burst pressure of the (90/0/ ± 75) defected shell was 0.092% of that of the free defect. Finally the results of the damage progression of the (90/0/ ± 75), defective composite shell was compared with the (90/0/ ± θ), where θ = 45 and 60, layup configurations. It was shown that the examined laminate (90/0/ ± 75) has the least damage tolerant of the two compared defective shells with the (90/0/ ± θ), θ = 45 and 60 laminates.     

定、变载弯曲疲劳钢丝绳失效机理对比研究

为对比揭示定、变载弯曲疲劳钢丝绳断裂机理及磨损演化特性,运用自制钢丝绳弯曲疲劳试验机开展钢丝绳定载、变载弯曲疲劳试验,通过人工拆股统计法和VW-9000系列高速度数码显微系统对比研究钢丝绳断丝分布、断丝数、断口和磨痕形貌等断裂机理,对比分析钢丝绳未断钢丝和断丝的磨痕尺寸演化特性. 结果表明:与钢丝绳定载弯曲疲劳相比,变载弯曲疲劳钢丝绳断丝出现较晚,芯股、螺旋股外层断丝数分别较多、较少,芯股外层钢丝断口挤压变形较大,芯股各层钢丝断口裂纹扩展区占比较低,芯股和螺旋股的各层钢丝磨痕尺寸总体较小,钢丝绳更易达到报废水平.      

Size and temperature effects on the fracture mechanisms of silicon nanowires: Molecular dynamics simulations

We present molecular dynamics simulations of [1 1 0]-oriented Si nanowires (NWs) under a constant strain rate in tension until failure, using the modified embedded-atom-method (MEAM) potential. The fracture behavior of the NWs depends on both temperature and NW diameter. For NWs of diameter larger than 4 nm, cleavage fracture on the transverse (1 1 0) plane are predominantly observed at temperatures below 1000 K. At higher temperatures, the same NWs shear extensively on inclined {1 1 1} planes prior to fracture, analogous to the brittle-to-ductile transition (BDT) in bulk Si. Surprisingly, NWs with diameter less than 4 nm fail by shear regardless of temperature. Detailed analysis reveals that cleavage fracture is initiated by the nucleation of a crack, while shear failure is initiated by the nucleation of a dislocation, both from the surface. While dislocation mobility is believed to be the controlling factor of BDT in bulk Si, our analysis showed that the change of failure mechanism in Si NWs with decreasing diameters is nucleation controlled. Our results are compared with a recent in situ tensile experiment of Si NWs showing ductile failure at room temperature.     

Bending actuation in polyurethanes with a symmetrically graded distribution of one-way shape memory alloy wires

Structures are being actuated by embedding shape memory alloy (SMA) wires into compliant materials, such as polyurethane. To achieve bending actuation, these wires are placed in opposing wire configurations, where multiple wires are often employed to enhance the amplitude of the bending actuation response. In this investigation, a procedure has been developed for fabricating polyurethanes with a symmetrically graded distribution of SMA wires. The effects of grading the distribution of one-way SMA wires have been characterized using full-field displacement deformation measurements obtained with the digital image correlation (DIC) technique. These measurements have been used in a one-dimensional (1D) model of bending actuation to determine the “equivalent two-way shape memory effect (SME)” of the graded wire distribution. To utilize the 1D actuation model, the constitutive properties of the polyurethane structure predicted by rule-of-mixture formulations were reduced to account for the differences in strain between the SMA wires and the polyurethane matrix. The graded wire distribution was also found to significantly stiffen the polyurethane structure. The level of equivalent two-way SME therefore became limited by the maximum recovery stress of the SMA wires, with a maximum level that was approximately 75% less than previously measured levels in an opposing wire configuration. However, the bending actuation behavior was more symmetric, and the actuated bending deflections were similar to those observed when using more compliant materials. It was also predicted that the symmetrically graded wire distribution would exhibit a better balance between actuation amplitude and uniformity, which combined with the more symmetric actuation behavior makes the graded wire distribution potentially more desirable for achieving higher actuation frequencies with distributed actuation concepts in new applications, such as miniaturized double diaphragm pumping devices.     

Fatigue behavior of E-glass fiber reinforced phenolic composites: effect of temperature, mean stress and fiber surface treatment

Phenolic matrix is reinforced by unidirectional E-glass fibers with volume fractions of 0.30 and 0.45. Three different surface treatments are applied to the E-glass fibers. The composite specimens are tested at ambient condition and temperatures of 100°C 150° and 200°C with stress levels of R(σ_min/σ_max) equal to 0 and 0.4 for load frequencies of 1.5, 10 and 25 Hz. Data are presented in terms of S/N curves and assessed by degradation of modulus based on compliance. For a particular fiber glass surface treatment and volume fraction, the composite specimen is notched and tested at room temperature and 200°C. A fatigue strength reduction factor K_f is defined and obtained such that the results could be compared with those of the unnotched specimens. Notch effect is small if the hole diameter is equal to the specimen thickness; it would be important for larger hole sizes. Fractured surfaces are examined by the scanning electron microscope.     

Micro Scale Tensile Behaviour of Thin Bitumen Films

The tensile behaviour of two types of viscoelastic bituminous films confined between mineral aggregates or steel as adherends, was investigated in the brittle and ductile regimes. Uniaxial specimens were fabricated employing a prototype set up allowing construction of micro-scale thin films and visualization of failure phenomena. The effect of key parameters, namely, temperature (23°C and −10°C), binder type (straight run and polymer modified), adherend type (stainless steel and mineral aggregate), and water conditioning were investigated sequentially. The results show that water sensitive aggregate-binder combinations in macro (150 mm diameter) and mega (in service) scales also displayed reduced tensile strength in the micro scale when water conditioned. At 23°C ductile failure and at −10°C brittle fracture were observed. At 23°C phenomena, such as formation of striations during tensile mechanical loading, void nucleation and growth, filamentation and large ductile flow before fracture could be witnessed. When using proper surface preparation procedures, in all types of specimen investigated at 23°C only cohesive failure and at −10°C predominantly adhesive-cohesive failure were found.     

Quasi-static behavior of Mg-alloys with and without short-fiber reinforcement

Magnesium alloys AE42 and AZ91 reinforced with 23 vol.% carbon short fibers (D_f ≈ 7 μm, L_f ≈ 100 μm) were tested under quasi-static loading. The carbon fibers were quasi-isotropically distributed in the horizontal plane (reinforced plane) of the casting. Compression and tensile tests were carried out on both the matrix alloys and the composites at temperatures between 20 °C and 300 °C. Specimens were machined to be loaded either parallel or normal to the reinforced plane. Due to the reinforcement, the compression yield stress of the composite AE42-C increased to a value approximately three-fold greater than the yield strength of the matrix; for composite AZ91-C this parameter was approximately 2.5-fold greater than that of the AZ91 matrix. The improvement in tensile strength was less than that in compression, which could be related to early tensile fracture through decohesion at the matrix–fiber interface, as detected by SEM investigations conducted on failed tensile specimens. Flow curves for the matrix alloys at different temperatures were described by a modified Kocks–Mecking material law. An idealization of a 2-D mesomodel was used for finite-element simulation of the mechanical behavior of the composites. The fibers were first considered as elastic bodies and the behavior of the matrix material was set according to the material law determined from the flow curves for the matrix alloys. Other calculations were carried out by considering elasto-plastic behavior of the fibers for application of a failure initiation technique to simulate the behavior of the composite materials beyond the ultimate stress.     

Time-temperature dependence in fracture behavior of high impact polystyrene

High Impact Polystyrene (HIPS) is one of the first toughened systems in which the brittle polystyrene becomes more ductile with the addition of an elastomer. However, it exhibits a ductile behavior only above a certain temperature and below a certain loading rate. Fracture in this material, like in most toughened systems, can become brittle when the temperature is lowered or the loading rate is increased. The correlation between temperature and loading rate seems to be controlled by the molecular relaxation according to the Arrhenius equation. The objective of this work is to foster the understanding of the effects of time and temperature on the fracture behavior of HIPS. The time and temperature dependence in fracture performance has been found to be governed by the strain energy density criterion. The theory allows prediction of fracture performance at various loading rates and temperatures. The brittle–ductile transition is controlled by an energy activation process. A peak in fracture energy always occurs at the transition region. This is attributed to the relaxation of the polymer macromolecules. The time and temperature dependence of this relaxation can be predicted by the Arrhenius equation. The rise in fracture energy at high loading speeds is not due to the higher frequency oscillations from dynamic effect but is controlled by the critical strain energy density.     

Fracture toughness of CIP-HIP Beryllium at elevated temperatures

The fracture toughness of CIP-HIP Beryllium was determined using the short bar fracture toughness (K_IcSB) method. The K_IcSB value measured was 10.96 MPa√m at room temperature. This falls well within the expected range of 9 to 12 MPa√m as observed from previous fracture toughness measurements of beryllium. Toughness increased rapidly between 400°F and 500°F reaching a value of 16.7 MPa√m at 500°F.     

高温空气下C/SiC复合材料断裂韧性实时测试和微观结构表征分析

为了研究高温空气下C/SiC复合材料断裂韧性和微观结构,采用单边切口梁三点弯曲法实时测试了C/SiC复合材料在高温空气下的断裂韧性,并采用电子扫描显微镜 (scanning electron microscope,SEM)和X 射线衍射分析仪 (X-ray diffraction, XRD)分析了复合材料在不同温度下的破坏断口和失效机制。研究结果表明随测试温度升高,C/SiC复合材料断裂韧性降低,材料的断裂形式由脆性断裂逐渐演变成塑性断裂。从室温升温到1 000 ℃测试温度条件下,C/SiC复合材料的断裂韧性由12.5 MPa·m1/2降低为10.96 MPa·m1/2,降幅仅为12%,C/SiC复合材料高温断裂韧性良好。不同温度下,材料呈现出不同形式的断裂形貌。常温下断口形貌主要可以看到纤维拔出的现象,随着温度的升高,该现象基本消失,断裂截面变得更平整,材料的强度主要取决于基体的强度。     

金属材料脆性断裂机理的实验研究

材料的脆性断裂有许多准则和模型,但对脆断机理和变化规律缺乏合理的描述,给工程应用带来不便。本文对典型脆性材料球墨铸铁、灰铸铁分别进行了拉扭双轴断裂实验和常规拉伸、扭转破坏实验;对韧性金属材料合金钢进行了单轴拉伸颈缩破坏实验。通过上述实验分析了脆性材料和韧性材料发生脆性断裂的机理特征并选择应力三维度作为应力状态参数描述危险点的应力状态,同时考察了脆性材料和韧性材料发生脆性断裂的主导因素。结果表明:韧性材料45#钢和14CrNiMoV合金钢在颈缩断面心部应力三维度值较大时发生脆性拉断,而在颈缩断面边缘处应力三维度值较小时发生剪断;脆性材料球墨铸铁在应力三维度值为0.0～0.33之间变化时均发生脆性断裂;灰铸铁在应力三维度值大于0.0时发生脆性拉断,而在应力三维度值小于0.0时发生剪断。因此可以认为,材料的细观组织结构和危险点应力状态是影响断裂机理及变化规律的主要因素。对于同种材料,随着应力三维度代数值从小向大变化材料的断裂机制由塑性剪切断裂逐渐转变为脆性断裂。本文通过对几种材料的脆性断裂危险点和断裂方向的研究给出了脆断宏观破坏条件。     

The effect of brittle interfacial compounds on deformation and fracture of molybdenum-aluminum fiber composites

The effect of brittle intermetallic compounds at the fiber-matrix interface on the deformation characteristics of molybdenum-aluminum fiber composites was investigated. If the filament is ductile and notch-insensitive, then composite strength degradation is relatively minor and composite strength can be predicted by a modified mixture-rule which neglects the strength contribution of the brittle compound. For the case of notch-sensitive filaments, severe filament degradation occurs upon compound formation. The degradation was shown to result from cracks formed during deformation at the roots of compound nodules. The presence of 10 per cent compound by volume results in a 50 per cent decrease in tensile strength, but larger amounts of compound cause little additional strength reduction. At filament volume fractions of 25 and 34 per cent and compound volume fractions less than 10 per cent, composite fracture occurs by the statistical accumulation of fiber necks or fractures depending on the notch sensitivity of the fiber. At high fiber or compound volume fractions, composite failure occurs upon the first or the second filament fracture.     

Tensile-shear transition in mixed mode I/III fracture

The propensity of the transition of fracture type in either brittle or ductile cracked solid under mixed-mode I and III loading conditions is investigated. A fracture criterion based on the competition of the maximum normal stress and maximum shear stress is utilized. The prediction of the fracture type is determined by comparing τ_max/σ_max at a critical distance from the crack tip to the material strength ratio τ_C/σ_C, i.e., (τ_max/σ_max)<(τ_C/σ_C) for tensile fracture and (τ_max/σ_max)>(τ_C/σ_C) for shear fracture, where σ_C (τ_C) is the fracture strength of materials in tension (shear). Mixed mode I/III fracture tests were performed using circumferentially notched cylindrical bars made of PMMA and 7050 aluminum alloy. Fracture surface morphology of the specimens reveals that: (1) for the brittle material, PMMA, only tensile type of fracture occurs, and (2) for the ductile material, 7050 aluminum alloy, either tensile or shear type of fracture occurs depending on the mode mixity. The transition (in ductile material) or non-transition (in brittle material) of the fracture type and the fracture path observed in experiments were properly predicted by the theory. Additional test data from open literature are also included to validate the proposed theory.     

Fracture mechanisms of CTOD samples of submerged and flux cored arc welding

Three welding procedures commonly used to rebuild worn shafts in sugar cane mills were analyzed: two processes of submerged arc welding and one of flux cored arc welding. Crack tip opening displacement for the welding was determined according to ASTM E 1290 standard. The fracture surface and microstructure of the samples were characterized using scanning electron microscopy and optical microscopy, respectively. The fracture parameter CTOD was correlated with the fracture surface and microstructures. The single deposit of FCAW process and the outer weld deposits of SAW process present acicular and blocky ferrite and non-metallic inclusions with spherical shape distributed randomly in the welding. The inner deposits for SAW process show equiaxed ferrite and pearlite with fine inclusions. Welding material B-MA 1 presented the highest CTOD_c with 0.2115 mm, followed by A-MA 2 with 0.1672 mm and A-MA 1 with 0.1238 mm. Each presented ductile fracture surfaces characterized by spherical dimples, microvoids nucleated in inclusions. Deposits B-MA2 and C-MA 1 presented lower CTOD_c, unstable crack growth and brittle fractures, characterized by intergranular failures due to fine inclusions in the grain boundaries.