Fracture behavior of injection-molded short and long glass fiber—polyamide 6.6 composites

Relationships between the fracture toughness, K_Q, and microstructure of chopped short (SGF) and long glass fiber (LGF) reinforced injection-molded polyamide 6.6 composites have been studied. K_Q and elastic modulus, E, of the composites were determined on compact tension specimens as a function of temperature, T, and crosshead speed, v. The microstructure of the composites was characterized by the dimensionless reinforcing effectiveness parameter, R, which was extended in this work for LGF reinforcement. R takes into account not only the processing-induced fiber layer structure, the fiber alignment and the fiber volume fraction but also the aspect ratio and aspect ratio distribution of the reinforcement. The semi-empirical linear relationship between fracture toughness of the composite, K_Q,C, and that of the matrix, K_Q,C, established for SGF-reinforced plastics, i.e. K_Q,C = MK_Q,M = (a + nR)K_Q,M still exists if the newly defined modified R is used. Both the matrix stress condition factor, a, and the energy absorption coefficient, n, have been determined under different testing conditions and tabulated together with K_Q,M. This allows an estimate of K_Q,C for any given R. Normalized fracture maps in form K_Q vs (E,T) have been constructed. Failure mechanisms of both the matrix and the composites which have been revealed by scanning electron microscopy are discussed and summarized in failure maps indicating changes of breakdown processes as a function of T and v.     

Influence of in-plane fibre orientation on mode I interlaminar fracture toughness of stitched glass/polyester composites

The influence of in-plane fibre orientation on the mode I interlaminar fracture toughness, G_Ic of unstitched and stitched glass/polyester composites is investigated in this paper. The G_Ic of planar specimens depends on the fibre orientation, θ in the layers adjacent to the fracture plane, in addition to the property of matrix material. The mode I fracture toughness and fracture behavior of unstitched and stitched 0/0, 30/−30, 45/−45, 60/−60, 90/90 and 0/90 interfaces of unidirectional fibre mats (UD) and 30/−30, 45/−45 and 90/90 interfaces of woven roving mats (WRM) are studied. WRM layer orientation is represented by the direction of warp fibres. Stitching is done by untwisted Kevlar fibre roving of Tex 175 g/km at the stitch densities (number of stitches per unit area) of 10.24 and 20.48 stitches/inch². The specimens having same stitch density, but different stitch distributions are prepared, and the influence of stitch distribution on G_Ic is studied. Double cantilever beam (DCB) tests are carried out and the G_Ic is determined using modified beam theory. The G_Ic of both unstitched and stitched specimens increases with increase in orientation angle, θ upto 45° above which it decreases. The G_Ic values of unstitched 45/−45 delamination interface is around 2.4 times that of the unstitched 0/0 interfaces. The influence of fibre orientation on G_Ic is clearly observed in unstitched specimens, whereas in the stitched specimens, stitching plays an important role in improving the G_Ic and suppresses the influence of fibre orientation; degree of suppression increases with increasing stitch density. When the value of θ is above 45°, transverse cracks are observed in the delamination interface surrounded by UD layers; while in the delamination interface surrounded by WRM layers, transverse cracks are not initiated irrespective of the fibre orientation angle.     

Studies on fracture toughness and fractographic features in Fe---Mn base alloys

Fractographic examinations of fracture surfaces of single edge crack plate tension fracture toughness test specimens of some new Fe---Mn base maraging alloys have been conducted. The interrelations between the fractographic features, fracture toughness and other mechanical properties of these alloys have been studied. It is observed that the width of the stretched zone between fatigue and rapid fracture is related to K/σ_ys of the material where K is either K_IC, K_Q or the stress intensity for onset of microscopic slow crack growth. The stretched zone width is approximately equal to the average dimple size. Also it is of the order of the process zone size (calculated by modified Krafft's model) and the critical crack opening displacement in plane strain condition. Hahn and Rosenfield's model to estimate K_Ic was found to show much higher values in those cases where the fracture mode was predominantly cleavage, quasicleavage or intergranular.     

Fracture toughness of turbine-generator rotor forgings

A comprehensive program is being conducted relative to applying fracture mechanics technology to large turbine-generator rotors. One facet of this program involves the determination of plane-strain fracture toughness (K_Ic) over a range of temperatures for various types of rotor steels. Data have been obtained for ten large production forgings, representing three alloys, using various types of compact K_IC and spin burst test specimens. These results demonstrate that valid K_IC data can be obtained in these types of intermediate-strength, high-toughness steels in the temperature range of practical interest. Data indicate that the plane-strain fracture toughness of these steels increases rapidly with increasing temperature and is rather high (K_tc/σ_YS > 1 in^1/2), in the application range. As a result, the critical defect sizes for catastrophic failure upon a single cycle of loading are relatively large. The plane-strain fracture toughness measurements, as well as the application of these data, are presented and discussed.     

A method to determine the fracture toughness of unidirectional fibre reinforced composites in mode II (forward shear), using a thin tubular specimen

A method has been developed, using thin tubular specimens, to determine the fracture toughness of unidirectional fibre reinforced composites in Mode II. The tubular specimens were loaded under torsion and hence produced pure shear at the crack tips located on the circumference of the tube. The cracks were made parallel to the transverse axis and in the mid-length of the tube. Calibration factors for Mode II were obtained. The stress-intensity factors at instability, K_IIR(INS) were obtained by experiments on thin tubular specimens through a compliance matching procedure. The crack growth resistance at instability and the corresponding initial strain energy release rates were independent of the initial crack in the range of crack length investigated. The stress-intensity factor obtained by the thin torsion tube method is slightly higher than the stress-intensity factor at instability, K_IIR(INS) obtained by the method developed by Giare for end cracked beams [Engng. Fracture Mech. 20, 11–21 (1984)]. This method may be applied to a different geometrical shapes and hence may be useful in determining the fracture toughness of any closed geometrical sections.     

Comparison of fracture toughness test procedures for aluminum alloys

The Dynamic Tear (DT) test permits the measurement of fracture propagation energy across the toughness spectrum for metals which are definable by linear elastic analyses to those requiring gross plastic strains for fracture. The linear elastic fracture mechanics parameter K_ic provides a relationship between critical flaw size and stress level at which crack instability will occur. Unlike the DT test, the K_ic toughness test cannot be utilized for fracture under conditions of elastic-plastic or gross plastic strain.

A correlation has been developed between the DT test and the K_IC parameter for ahuminum alloys. The relationship may also be expressed in terms of β_ic-DT and _ic-DT. The K_ic values were determined with several specimen types and a comparison of the values for different specimens is provided.

The correspondence between K_ic and DT serves several purposes. It provides a frame of reference for DT values obtained from frangible metals that fracture under linear elastic conditions. Accordingly, it permits utilization of the inexpensive DT test to approximate the flaw size-stress instability conditions which otherwise must be determined by the more expensive K_ic test. Furthermore, through extrapolation, it is possible to utilize the DT test to estimate the critical flaw size under an elastic-plastic strain field.     

A theoretical approach for evaluating the plane strain fracture toughness of ductile metals

Fracture Mechanics of Ductile Metals (FMDM) theory is used to obtain the plane strain fracture toughness, K_lc, for different materials. The traditional approach for obtaining the K_lc value is to conduct several standard tests on cracked plates that are costly and time consuming. The fracture toughness value provided by the FMDM theory depends on the stress-strain curve for the material in question, and this is readily available in MIL-HDBK-5 and other reliable sources. The results of the plane strain fracture toughness (K_lc) values provided by the FMDM theory were compared with the experimental data and it was concluded that the two are in excellent agreement. It is proposed that, in the interest of economy and convenience, K_lc testing could be replaced by the FMDM theory.     

Crack tip damage development and crack growth resistance in particulate reinforced metal matrix composites

A study of crack tip damage development and crack growth resistance of aluminium 359/20% V_f silicon carbide and aluminium 6061/20% V_f Micral^Tm particulate reinforced metal matrix composites has been conducted. Observations of crack tip process zone development at the specimen surface have been compared with the results of fractographic examination of the centre of the specimen. Both materials were found to fracture by a process of void nucleation, growth and coalescence. Void nucleation was found to be by fracture or debonding of reinforcement particles, and/or fracture or debonding of secondary matrix particles. The preferred mode of void nucleation was found to vary depending on the constituents of the PR MMC and even the heat treatment state of the material. It was found that in these materials fractured particles identified on the fracture surface fractured during loading rather than being pre-cracked during fabrication. It was further found that observations of damage development from the specimen surface did not necessarily reflect the mechanisms prevailing in the specimen bulk. Under plane strain conditions, both materials were found to exhibit decreasing crack growth resistance as crack extension proceeded, due to the “anti-shielding” effect of damage accumulated in the process zone ahead of the crack tip. In thin specimens of the Comral-85 composite, however, dramatically improved toughness was obtained, and K_R curves have been obtained for such specimens. The method of measuring crack length was found to have a profound effect on the K_R curve; it was concluded that the K_R curve determined using the crack length measured at the specimen surface best reflected the true crack growth resistance of these materials.     

Plane strain fracture toughness of MP35N in aged and unaged conditions measured using modified CT specimens

The multiphase alloy MP35N (35% Ni, 35% Co, 10% Mo, 20% Cr) is a high strength, high toughness alloy of choice for several safety-critical applications in aerospace, oil drilling, and biomedical industries. Several previous attempts in literature to measure the plane strain fracture toughness of commercially drawn MP35N did not produce reliable values since they violated one or more of the criteria stipulated by ASTM standards for a valid measurement of K_Ic. In most cases, the requirements for plane strain and small scale yielding conditions were not met, since the commercially drawn material was available only with limited cross-sectional dimensions. In this investigation specially designed specimens (modified compact tension (CT) specimens) have been used to measure the plane strain fracture toughness of MP35N in both the unaged and the aged conditions. The K_Ic of the commercially drawn (53% reduction level) MP35N was measured to be 126 MPa√m, while that of the commercially drawn and aged MP35N was measured to be 98 MPa√m. Both these measurements satisfied all the required criteria stipulated by ASTM standards for valid measurements of K_Ic. The new procedure used in this study has been verified by the measuring of fracture toughness of Al alloy, using both the modified specimen, as designed here, and the standard one. The results for plane strain fracture toughness of MP35N alloy have been verified by the standard measurement of J_Ic values for both the aged and the unaged alloys. Finally, on a suitably normalized plot, introduced in this paper, the toughness-strength envelop for MP35 is higher than most of structural alloys, but significantly lower than that of the TRIP steels.     

Fatigue crack growth characteristics of rotor steels

Room temperature fatigue crack growth rate data were generated for Ni-Mo-V (ASTM A469, Cl-4), Cr-Mo-V (ASTM A470, Cl-8) and Ni-Cr-Mo-V (ASTM A471, Cl-4 and a 156,000 psi yield strength grade) rotor forging steels. Testing was conducted with WOL type compact toughness specimens and the results presented in terms of fracture mechanics parameters. Data show that the Ni-Cr-Mo-V steels exhibit slower fatigue crack growth rates at a given stress intensity range (ΔK) than do the Ni-Mo-V steels. In addition, the Cr-Mo-V steel was found to exhibit slower growth rates than the other alloys at ΔK levels below 40 ksi √in but somewhat foster rates at ΔK levels in excess of 45 ksi √in. The fatigue crack growth rate properties of the alloys studied conform to the generalized fracture mechanics crack growth rate law where da/dN = C₀ΔK^R. It was noted that the fatigue crack growth rate parameters n and C₀ tend to decrease and increase, respectively, with increasing material toughness, K_ic.     

Evaluation and analysis of interlaminar fracture toughness of bead filled matrix hybrid composite materials using orthotropic fracture mechanics

Evaluation of Mode I interlaminar fracture toughness for unidirectional hybrid composites fabricated with a bead filled epoxies was carried out. The two important fracture toughness parameters, G_IC and K_IC values of hybrid composites, were reviewed in accordance with the orthotropic fracture model. The deviation of measured G_IC and K_IC values from predicted values were explained based on the critical review of the basic assumption of orthotropic fracture model and characteristic material properties of hybrid composites. It can be said that, basically, the orthotropic fracture model can be used for evaluation of hybrid composite materials. However, careful analysis for G_IC and K_IC values which were derived from different source and some correction factor for K_IC values are necessary.     

Microstructural aspects of the fracture process in human cortical bone

Fracture toughness tests were conducted in the transverse and longitudinal directions to the osteonal orientation of human femoral cortical bone tissue to investigate the resulting damage patterns and their interaction with the microstructure. The time history of damage accumulation was monitored with acoustic emission (AE) during testing and was spatially observed histologically following testing. The fracture toughness of the transverse specimens was almost two times greater than the fracture toughness of the longitudinal specimens (3.47 MNm^–3/2 vs. 1.71 MNm^–3/2, respectively). The energy content of the AE waveforms of transverse specimens were greater than those of the longitudinal specimens implying higher fracture resistance in the transverse crack growth direction. The results showed that the propagation of the main crack involved weakening of the tissue by ultrastructural (diffuse) damage at the fracture plane and formation of linear microcracks away from the fracture plane for the transverse specimens. For the longitudinal specimens, the growth of the main crack occurred in the form of separations at lamellar interfaces. The lamellar separations generally arrested at the cement lines. Linear microcracks occurred primarily in the interstitial tissue for both crack growth directions.     

Analysis of mixed-mode interlaminar fracture and damage behavior of GFRP woven laminates at cryogenic temperatures

The objective of this work is to investigate the interlaminar fracture and damage behavior of glass fiber reinforced polymer (GFRP) woven laminates loaded in a mixed-mode bending (MMB) apparatus at cryogenic temperatures. The finite element analysis (FEA) is used to determine the mixed-mode interlaminar fracture toughness of MMB specimen at room temperature (RT), liquid nitrogen temperature (77 K) and liquid helium temperature (4 K). A FEA coupled with damage is also employed to study the damage distributions within the MMB specimen and to examine the effect of damage on the mixed-mode energy release rate. The technique presented can be efficiently used for characterization of mixed-mode interlaminar fracture and damage behavior of woven laminate specimens at cryogenic temperatures.     

The mechanics and mechanisms of fracture in stress corrosion cracking of aluminium alloys

A fracture mechanics approach to stress corrosion cracking is highlighted. The mechanisms of stress corrosion cracking is presented. Experiments on 2024 and 7075 aluminium alloys are carried out to determine their mechanical properties, microstructure and plane strain fracture toughness (K_IC). Stress corrosion cracking tests, namely, cantilever beam tests as well as wedge opening loading tests using sea water as a corrosive medium, are conducted to establish the critical stress intensity factor for stress corrosion cracking (K_ISCC) for each aluminium alloy. It is found that the K_ISCC is in the range of (1/5) to (1/6) of the plane strain fracture toughness, K_IC, depending on the alloy. The scanning electron microscopy of fracture surfaces reveals a great dependence of the cracking and/or pit severity on the applied stress intensity factor. A brief discussion on the dislocation's role in stress corrosion cracking is given.     

Dynamic fracture toughness of X70 pipeline steel and its relationship with arrest toughness and CVN

The dynamic fracture toughness K_1d and J_1d, arrest toughness K_1a and Charpy V-notched impact toughness (CVN) of a pipeline steel, X70, were studied at different temperatures. It was found that fracture toughness was strongly affected by temperature and loading rate. The fracture toughness decreases with decreasing temperature from 213 to193 K and increasing loading rate from to . At constant temperatures, only increasing loading rate can induce the transition from ductile to brittle. There exists a fracture transition caused by loading rate. Through thermal activation analysis, a quantitative relationship has been derived:

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. It can describe the fracture process at different temperatures and loading rates. At a loading rate of , the relationship can predict arrest toughness well. It provides the possibility of measuring arrest toughness with small size specimen. An empirical equation has been derived: CVN=4.84×10⁶T^−2.8K_1d(K_1a), which correlates K_1d and K_1a with CVN in one equation. This means that we can calculate K_1d and K_1a when we get CVN.     

The mixed mode I/II fracture behaviour of lightly tempered HY130 steel at room temperature

The mixed mode I/II fracture behaviour at room temperature of HY130 steel tempered at 350°C has been investigated using edge-cracked bend bar specimens loaded in anti-symmetric and symmetric four point bend configurations. In all cases fracture occurred by a localized shear decohesion mechanism that could not be characterized by the stress intensity factors, K_I and K_II, but for which the crack tip displacements, δ_I, and δ_II, appear to provide a first level of characterization. The results suggest that fracture is described by a maximum shear criterion, and this is consistent with the present understanding of fibrous fracture micro-mechanisms in the material.     

WSM50C钢的裂尖塑性损伤与断裂

本文利用一组紧凑拉伸(CT)试样。测定了WSM50C高炉钢板材料的J_(?)阻力曲线和断裂韧性J_(IC),并采用液氮冲剖试样技术,在扫描电镜下研究了裂纹尖端附近材料塑性损伤与断裂的细观力学行为。观测研讨了内部空穴的演化规律。结果表明,裂尖附近的塑性损伤与断裂可以用细观空穴扩张比参数R/R_0来描述。当J相似文献   

Fatigue and fracture of a 200 ksi grade maraging steel proposed for use in military bridging

Maraging steel containing 18 per cent nickel offers apparent advantages of high strength, weldability, corrosion resistance and toughness. In view of the suitability of the material for bridge construction, a study of the fatigue crack growth and fracture properties was undertaken. A proposed bridge design contained hinged sections of 4 in. and 2 in. Thickness for female and male jaws, and thin welded girders manufactured from plate material 0.18 in thick.

Plane strain fracture toughness tests were carried out on samples taken from the 4 in. And 2 in. Sections using 3 point-bend and compact tension specimens. K_Q values of approximately 75 ksi √in. For the 4 in. samples and 110 ksi √in. For the 2 in. Samples were obtained.

Plane stress fracture toughness tests were conducted on center cracked sheets up to 11 in. in width. Using critical crack lengths determined by compliance measurements, K_C values in excess of 400 ksi √in. have been obtained in specimens of up to 0.180 in. Thickness. Fatigue crack growth rates were determined from these specimens prior to fracture testing.

In both plane strain and plane stress failure modes, laboratory results are in agreement with those estimated from the failure of experimental structures.     

A Weibull stress model to predict cleavage fracture in plates containing surface cracks

This study applies recent advances in probabilistic modelling of cleavage fracture to predict the measured fracture behaviour of surface crack plates fabricated from an A515-70 pressure vessel steel. Modifications of the conventional, two-parameter Weibull stress model introduce a non-zero, threshold parameter (σ_w-min ). The introduction of σ_w-min brings numerical predictions of scatter in toughness data into better agreement with experimental measurements, and calibration of this new parameter requires no additional experimental data. The Weibull modulus ( m ) and scaling parameter (σu ) are calibrated using a new strategy based on the toughness transferability model, which eliminates the non-uniqueness that arises in calibrations using only small-scale yielding toughness data. Here, the Weibull stress model is calibrated using toughness data from deep-notch C(T) and shallow-notch SE(B) specimens, and is then applied to predict the measured response of surface crack plates loaded in different combinations of tension and bending. The model predictions accurately capture the measured distributions of fracture toughness values.     

Effects of microstructure, specimen and loading geometries on KIC of brittle honeycombs

Fracture toughness of brittle honeycombs depends on cell microstructure, specimen and geometries. A microstructure coefficient in the K_IC expression of brittle honeycombs can not be found analytically. In this paper, a finite element program is utilized to numerically determine the coefficient. In practice, fracture toughness can be measured from conducting a three-point bend or uniaxial tension test. Specimen geometry restrictions of three-point bend test for honeycombs are examined and proposed here. Meanwhile, fracture toughness of honeycombs under the two loading geometries is compared; results show that K_IC measured from uniaxial tension test is smaller than that from three-point bend test if the K_IC formulations for solid materials are employed. As a result of that, the K_IC formulation of three-point bend test is modified for honeycomb-like materials.