陶瓷的断裂韧性与缺口半径 Ⅰ.断裂韧性测试技术

本文用一种新的简单方法制备了不同半径的尖缺口，用单边切口梁试件测试了不同缺口半么氧化铝陶瓷的断裂韧性，地七咱不同测试方法测得的，同一种材料的断裂韧性进行了对比，讨论陶瓷材料断裂韧性的最佳测试方法和测试中对缺口半径的要求。     

陶瓷的断裂韧性与缺口半径 Ⅰ.断裂韧性测试技术

本文用一种新的简单方法制备了不同半径的尖缺口，用单边切口梁试件测试了不同缺口半径氧化铝陶瓷的断裂韧性.对七种不同测试方法测得的、同一种材料的断裂韧性进行了对比，讨论陶瓷材料断裂韧性的最佳测试方法和测试中对缺口半径的要求.     

Experimental research on the compressive fracture toughness of wing fracture of frozen soil

It is commonly found that not only bending fracture but also compressive fracture occur frequently in compression, furthermore, in some specific conditions, compressive fracture sometimes has dominant effect on frozen soil. Therefore, it is extremely necessary to study the mechanical characteristics of the compressive fracture of frozen soil and to investigate the damage and fracture mechanism of frozen soil based on the previous research on frozen soil damage in compression. This study draws on the ideas and methods used in compression fracture research on ice that is very similar to frozen soil, and specific clay in Shenyang region was adopted as the experimental material, to make compressive specimens containing tilted wing crack of different angles, and uniaxial unconfined compression fracture experiments were conducted at different temperatures and loading rates. The fracture toughness K_IC and K_IIC of the main crack tip of the specimens are calculated with obtained experimental results and the law of K_IC and K_IIC changing with tilted angles, temperatures and loading rate is obtained to gain an insight to damage mechanism of frozen soil in compression. This paper presents a meaningful attempt for the research on compressive fracture of frozen soil, so as to better solve practical engineering problems.     

Cyclic fatigue crack growth of SiCw/Y-TZP composites: long- and short-crack behaviour

Cyclic fatigue crack growth behaviour has been investigated for 10 vol % SiC_w/Y-TZP composites with grain sizes varying from 1.50–2.00 m in a residual stress field. To investigate the effect of precracking procedure, cyclic fatigue tests were performed on unannealed specimens using the four-point bending method under two conditions: (a) with a sharp crack and precracking procedure, and (b) with a natural sharp flaw. For all specimens in both conditions, an overall V-shaped da/dN versus K _app relation was obtained. However, for the specimens without precracking, the da/dN had an unusual dependence on the applied stress intensity, giving a zigzag V-shaped curve. Explanations for these different results for the two conditions are discussed in terms of crack-tip shielding effects and residual stress field.     

Influence of non‐proportional bending with torsion on fatigue life of 10HNAP steel within the range of crack initiation and propagation

The paper presents a precise analysis of the influence of non‐proportional loading of specimens on fatigue life during initiation and propagation of fatigue cracks. Simulation of the fatigue life of specimens was based on relations describing propagation rate of the fatigue cracks. The Paris and Forman relations were applied; they were integrated after previous introduction of relationships for the equivalent range of the stress intensity factor ΔK_eq and including the phase shift angle ? between amplitudes of the bending moment and the torsional moment. Under bending with torsion, range of the equivalent stress intensity factor ΔK_eq includes ranges of stress intensity factors for loading modes I and III, i.e. ΔK_I and Δ K_III. The performed tests of 10HNAP constructional steel under cyclic bending with torsion allowed us to determine the influence of the phase shift angle ? on the fatigue life. It has been proved that increase of the phase shift angle from ?= 0° to ?= 60° and the ratio of amplitude of the bending moment M_ag to amplitude of the torsional moment M_as equal to 1.33, 2 and 4 cause increase of the fatigue life of the tested specimens. The maximum increase of the fatigue life of specimens made of 10HNAP steel was 73% (M_ag/M_as= 2, ?= 45°).     

Fracture toughness of sea ice

Linear elastic fracture mechanics, which has been applied to fragile substances and successfully used for studying the brittle fracture of metallic materials, was utilized to gain an understanding of the fracture phenomena of sea ice.The present paper reports the first results of investigations into the fracture-toughness value of sea ice, which was analyzed experimentally as a function of strain rate on the basis of the stress-intensity-factor concept.The fracture toughness, K_IC, of sea ice, which was measured by an in-situ three-point bending test on notched specimens, shows almost constant value if the strain rate is less then 10^?3 s^?1 and decreases with increasing strain rate if the strain rate exceeds 10^?3 s^?1. K_IC data show considerably less scatter than existing data such as the compressive, tensile and flexural strengths.It was confirmed in the present study that the linear elastic-fracture-mechanics concept is effective for analyzing the fracture phenomena of sea ice. Moreover, the K_IC value was shown to be closely related to sea-ice structures (e.g. the size of crack-like flaws such as brine cells).It is also suggested that the fracture-toughness test might prove to be a standard testing method to obtain the sea-ice strength, since once K_1C and crack-like flaw sizes are determined, the less-scattered critical-fracture stress can be calculated.     

Anrißerzeugung in WC-Co-Hartmetall-Proben für die Messung des kritischen Spannungsintensitätsfaktors KIC

Precracking of WC-Co-Hardmetal-Specimens for Fracture-Toughness Testing The determination of a valid critical stress intensity factor K_IC requires an extremely sharp, well defined initial crack. Methods producing such a crack are well known for metallic materials, but they often can not be used with brittle materials, like cemented carbides or ceramics. Their low fracture toughness makes a controlled crack growth under pure tensile stress nearly impossible. More useful are precracking methods, utilizing a stress gradient to stop the crack at defined depth. A very simple methods uses the indentation of a hardness tester to produce a semi-elliptical surface crack, interfered with residual stresses. For different areas of application and specimen geometries, bridge indentation, wedge indentation and composite bending method produce cracks with a straight front. Also under cyclic loading, under tensile as well as under compressive stress, the creation of a sharp precrack, applicable in K_IC measurement, is possible.     

Fatigue of shot peened 7075-T7351 SENB specimen – A 3-D analysis

As‐received or shot peened 7075‐T7351 single‐edged notch bend (SENB) specimens, 8.1‐mm thick, were fatigued at a constant maximum load and at stress ratios of R= 0.1 and 0.8 to predetermined numbers of fatigue cycles or to failure. The SENB specimens were then fractured by overload and the tunnelling crack profiles were recorded. The crack‐growth rate, da/dN, after crack initiation at the notch was determined by crack‐profile measurement and fractography at various fatigue cycles. The shot peened surface topography and roughness was also evaluated by three‐dimensional (3‐D) laser scanning microscopy. Residual stresses in the as‐received specimens and those generated by shot peening at Almen scales of 0.004A, 0.008A, 0.012A and 0.016A, were measured by an X‐ray diffraction stress analyser with an X‐ray target, CrK, every 0.1 mm to a depth of 1 mm. The 3‐D stress intensity factor of the curved crack front was determined by the superposition of the 3‐D finite element solutions of the stress intensity factor of the loaded SENB specimen without the residual stress and the stress intensity factor of the unloaded SENB specimen with a prescribed residual stress distribution. da/dN versus the resultant stress intensity factor amplitude, ΔK_I, plots showed that while the residual stress locally retarded the crack‐growth rate it had no effect on the overall crack‐propagation rate.     

Effects of laser peening on the fatigue strength and defect tolerance of aluminum alloy

The effects of laser peening (LP) on the bending fatigue strength of the 7075‐T651 aluminum alloy were investigated. Accordingly, the defect tolerance of the aluminum alloy subjected to LP is discussed on the basis of fracture mechanics. The results indicate that a deeper compressive residual stress was induced by LP compared with the case of shot peening (SP). The fatigue strengths increased when both peening types were used. Semicircular slits with depths less than 0.4 and 0.1 mm were rendered harmless on the basis of the applications of LP and SP, respectively. The apparent threshold stress intensity factor range ΔK_th,ap increased by approximately five and two times owing to LP and SP, respectively. The increase of the ΔK_th,ap was caused by the compressive residual stress induced by the peening. The Kitagawa‐Takahashi diagram of the laser‐peened specimens shows that the defect tolerance of the aluminum alloy was improved by LP.     

FATIGUE CRACK GROWTH AND CLOSURE BEHAVIOUR THROUGH A COMPRESSIVE RESIDUAL STRESS FIELD

Behaviour of fatigue crack growth and closure through a compressive residual stress field is investigated by performing fatigue crack growth tests on welded SEN specimens of a structural steel (JIS SM50A). Depending on the type of the initial residual stress in the region of crack growth, the growth and closure of the crack show different behaviour. In particular, in the transition region from a compressive residual stress field to a tensile residual stress field, the fatigue crack growth rates cannot be described by the effective stress intensity factor range ΔK_eff, based on the measured crack opening stress intensity factor K_op. Also it is found that the R'-method using the data of da/dN vs ΔK for residual stress-free specimens, with the effective stress ratio R'[=(K_max+K_r)/(K_min+K_r)], gives non-conservative predictions of the growth rates in the transition region. Observations of crack closure behaviour in this study indicates that partial opening of the crack occurs and this plays an important role in crack growth through a compressive residual stress field. Based on the concept of a partial opening point (defined and measured in this work), fatigue crack growth behaviour can be better explained.     

Mode II fracture mechanics properties of wood measured by the asymmetric four-point bending test using a single-edge-notched specimen

Using a single-edge-notched specimen of spruce, an asymmetric four-point bending test was conducted to obtain the mode II fracture toughness G_IIc and critical stress intensity factor K_IIc, and the test method was numerically and experimentally analyzed. A three-point bend end-notched flexure test was also conducted and the results were compared with those of the asymmetric four-point bending tests. The crack length had a small influence on the load/loading-line displacement relationship in the asymmetric four-point bending test, so it was difficult to determine the value of G_IIc, which requires the measurement of loading-line displacement. In contrast, the value of K_IIc obtained by two tests was similar when the initial crack length ranged from 0.7 to 0.85 times the depth of the specimen. These results show that the asymmetric four-point bending test is a promising means of determining K_IIc.     

Effects of machining parameters on fatigue behavior of machined threaded test specimens

The effects of machining parameters on the fatigue strength of fine-machined threaded specimens were investigated by comparing with the endurance limits of circumferentially notched specimens with the same profile. A four-point rotary bending fatigue test machine was used to obtain constant bending moment and pure alternating stress along the thread. All specimens were machined from SAE 4340 steel, the typical material used for deep well oil drilling pipes. Notched specimens were fine-machined according to ASTM standards, while the threaded specimens were machined under the optimized cutting conditions, which maximize tool life and geometric precision, as well as under selected modified conditions. Endurance limits of all specimens after 2 × 10⁶ cycles were experimentally determined and S–N curves were plotted for 90% reliability for all experiments. The effects of cutting force, radial feed, tool wear, and two thread cutting methods on the fatigue strengths of the threaded specimens were determined. Experimental results show that the fatigue strengths of threaded specimens lie within a large range, depending on machining conditions, as compared to circumferentially notched specimens. The most influential factors on the fatigue strength of threaded specimens are tool wear and cutting velocity, while the effects of cutting method and radial feed are less significant.     

A fractographic criterion for subcritical crack-growth boundaries in hot-pressed alumina

The percent intergranular fracture (PIF) was measured along radii extending from fracture origins in hot-pressed alumina specimens, fractured at various loading rates and temperatures, and plotted versus estimates of stress intensity factors (K _I) at the various crack lengths. Minima in PIF occur at values ofK _I that are close to the critical stress intensity factors (K _IC) for cleavage on various crystal lattice planes in sapphire. The subcritical crack-growth boundary (K _I=K _IC of the polycrystalline material) occurs near the primary minimum in PIF suggesting that this minimum can be used as a criterion for locating this boundary. In addition, it was noted that the polycrystallineK _IC (4.2 MPa m^1/2) is very close to theK _IC for fracture on {¯1 ¯1 2 6} planes which is 4.3 MPa m^1/2. These observations suggest that critical crack growth begins when increased fracture energy can no longer be absorbed by cleavage on these planes. There is a secondary minimum atK _I>K _IC that appears to be associated with theK _IC necessary for fracture on combinations of planes selected by the fracture as alternatives to the high fracture-toughness basal plane.     

Fatigue and fracture of a Ni–P amorphous alloy thin film on the micrometer scale

Fracture and fatigue tests have been performed on micro‐sized specimens for microelectromechanical systems (MEMS) or micro system technology (MST) applications. Cantilever beam type specimens with dimensions of 10 × 12 × 50 μm³, approximately 1/1000th the size of ordinary‐sized specimens, were prepared from a Ni–P amorphous thin film by focused ion beam machining. Fatigue crack growth and fracture toughness tests were carried out in air at room temperature, using a mechanical testing machine developed for micro‐sized specimens. In fracture toughness tests, fatigue pre‐cracks were introduced ahead of the notches. Fatigue crack growth resistance curves were obtained from the measurement of striation spacing on the fatigue surface, with closure effects on the fatigue crack growth also being observed for micro‐sized specimens. Once fatigue crack growth occurs, the specimens fail within one thousand cycles. This indicates that the fatigue life of micro‐sized specimens is mainly dominated by a crack initiation process, also suggesting that even a micro‐sized surface flaw may be an initiation site for fatigue cracks which will shorten the fatigue life of micro‐sized specimens. As a result of fracture toughness tests, the values of plane strain fracture toughness, K_IC, were not obtained because the criteria of plane strain were not satisfied by this specimen size. As the plane strain requirements are determined by the stress intensity, K, and by the yield stress of the material, it is difficult for micro‐sized specimens to satisfy these requirements. Plane‐stress‐ and plane‐strain‐dominated regions were clearly observed on the fracture surfaces and their sizes were consistent with those estimated by fracture mechanics calculations. This indicates that fracture mechanics is still valid for such micro‐sized specimens. The results obtained in this investigation should be considered when designing actual MEMS/MST devices.     

Shear mode threshold for a small fatigue crack in a bearing steel

The fatigue behaviour of small, semi‐elliptical surface cracks in a bearing steel was investigated under cyclic shear‐mode loading in ambient air. Fully reversed torsion was combined with a static axial compressive stress to obtain a stable shear‐mode crack growth in the longitudinal direction of cylindrical specimens. Non‐propagating cracks less than 1 mm in size were obtained (i) by decreasing the stress amplitude in tests using notched specimens and (ii) by using smooth specimens in constant stress amplitude tests. The threshold stress intensity factor ranges, ΔK_IIth and ΔK_IIIth, were estimated from the shape and dimensions of non‐propagating cracks. Wear on the crack faces was inferred by debris and also by changes in microstructure in the wake of crack tip. These effects resulted in a significant increase in the threshold value. The threshold value decreased with a decrease in crack size. No significant difference was observed between the values of ΔK_IIth and ΔK_IIIth.     

FATIGUE CRACK GROWTH BEHAVIOUR AND FRACTURE RESISTANCE IN CERAMICS

Fatigue crack growth and the fracture resistance curve (R-curve) were investigated in a polycrystalline alumina (AD90) and a silicon carbide whisker-reinforced alumina composite (Al₂O₃-SiC_w) at room temperature in air using a combined loading technique for stabilizing crack growth, and a surface film technique for monitoring crack length. Fatigue crack growth was evaluated successfully with those experimental techniques. Load shedding tests were performed until the crack became dormant, in order to determine the threshold stress intensity factor K_th. Subsequently, the specimens were used for quasi-static crack growth tests under a monotonic loading condition. The R-curves were determined in this experiment; however, fracture resistance did not increase markedly with crack growth. Detailed observations of the crack growth behaviour revealed that the flat R-curve was attributed to the shielding effect of the fatigue crack tip wake. Thus, the fatigue precrack introduced by the load shedding test was not regarded as an ideal crack for determining the R-curve. Fractographic observations were performed to investigate the mechanistic difference between fatigue and quasi-static crack growth. It was found that the cyclic loading produced fretting damage in the wake region and it reduced the shielding effect of the fatigue cracks. Based on the experimental results, the relationship between the fatigue crack growth and the R-curve is discussed as is the significance of K_th as a material parameter.     

Multiaxial fatigue behaviour of Sintered Steels under combined in and out of phase bending and torsion

This paper discusses the fatigue behaviour of sintered steels under multiaxial loading. These steels are the Fe-1.5% Cu and the Fe-2.0% Cu-2.5% Ni, sintered at low and high temperatures, in the densities 7.1 and 7.4 g/cm³, which are used in the production of several ready to assemble automotive parts. Fully reversed or pulsating combined loading with constant frequency and amplitudes acting in and out of phase, was applied to round notched specimens (K_tb = 1.49, K_tt = 1.24) in the finite fatigue life region (10⁴ ≤ N_f ≤ 2 · 10⁶). The mechanics of crack initiation and propagation as well as rupture were studied using fractography and microfractography. These analyses led to a mechanical model based on local normal stresses for the fatigue life evaluation. The fatigue life evaluation on the base of the local bending stress obtained under uniaxial loading describes the test results for in phase combined bending and torsion satisfactorily. But the increase of fatigue strength and life by out of phase loading is overestimated in the case of fully reversed loading. However for design purposes the out of phase loading can be neglected because of its beneficial effect in increasing fatigue life for this type of material. If the dependence of the different stress concentrations under combined in and out of phase loading on the supportable local bending stress obtained under uniaxial loadings is considered, then the calculation procedure covers all test results.     

Fracture toughness and fatigue crack growth behaviour of an Al2O3-SiC composite

The fracture toughness and fatigue crack growth characteristics of an Al₂O₃-SiC whisker composite were investigated. Quasi static fracture experiments were conducted on double edge-notched tension specimens and on four-point bend specimens containing a through-thickness Mode I crack which was introduced under uniaxial cyclic compression. The toughness results obtained using this procedure are more reproducible than those derived from the indentation technique and the notched bend bar method. The fracture toughness of the composite is about 60% higher than that of the unreinforced matrix material. Crack growth characteristics at room temperature were also investigated in notched plates of Al₂O₃-SiC subjected to fully compressive far-field cyclic loads. In the presence of a stress concentrator, this composite is found to be highly susceptible to fatigue crack growth under cyclic compressive loads.     

CRACK NUCLEATION AND PROPAGATION IN BLADE STEEL MATERIAL

Stress corrosion cracking and corrosion fatigue of 12 Cr steel in sodium chloride solution has been investigated. Tests have been performed in air at room temperature and in aqueous solution with 22% NaCl at 80°C. The influence of corrosion pits on crack nucleation has been investigated. On fracture surfaces tested in environment (22% NaCl solution), crack initiation was observed in correspondence with corrosion pits; in this case fatigue life can be described using a fracture mechanics approach. The ΔK value for crack nucleation from a pit in rotating bending fatigue tests is very low in air (about 3 MPa√m). The results of slow strain rate tests on smooth specimens show that there is a threshold stress intensity, K_ISCC, of about 15 MPa√m and a plateau in stress corrosion crack growth rate of about 10^-5mm/s.     

Simulation of fatigue life of constructional steels within the mixed modes I and III loading

The authors analysed influence of a component of the torsional moment M_as under the complex loading state, that is under bending with torsion, on fatigue life during initiation and propagation of fatigue cracks. Simulation of specimen life was performed according to the relationships describing the crack propagation rate and including the equivalent stress intensity factor range K_eq. Under complex loading, increase of amplitude of the torsional moment M_as for a given initial value of the resultant moment M_aw0 caused a higher fatigue life of specimens made of 10HNAP and 18G2A steels. This fatigue life increase was described by a nonlinear equation, the parameters of which had been determined from the experimental results. The fatigue lives estimated according to the assumed models were compared with those obtained from tests.