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.     

Exploring the fracture toughness KIE and KIC of a medium-Strength steel plate using the surface-flaw test

The fracture toughness of a 30 CrMnSiA steel plate of three thicknesses (10,8 and 5 mm) and three widths (110,80 and 56 mm) has been investigated by using surface-flaw method under room temperature. It is not easy to compute the value of K_IE by the maximum applied load. But the values of K_IE and K_IC could be obtained easily, if the computation of the conditional applied load P₁₀ and P₅ based on the relative effective extension Δa/a₀ = 10% and 5% were adopted, together with the conditions of P_max/P₁₀ 1.2 and P_max/P₅ 1.3. The K_R — Δ_a curve, i.e. the resistance-curve described by the parameter K, has been plotted. The values of K_IC and K_IE are then the resistances corresponding to the real extensions of flaws of Δ/a₀ = 2 and 7%, respectively. These values so obtained are in good agreement with the computed values of K_IC and K_IE by using the conditional applied loads. The values of K_IC and K_IE so obtained are also in agreement with the value of K_IC converted from the J-integral and the effective value of K_IE computed by the maximum applied load, respectively.

An approximate relation between K_IC and K_IE has been found to be: K_IC = (0.85˜0.95)K_IE.

The requirements for the dimensions of specimens are: Thickness of plate: B 1.0(K_IC/σ_0.2)² or 1.25(K_ICσ_0.2)²]; Width of plate: 8 W/B 10, 4 W/2c 5; Effective length: l 2W.     

Combined stress hypothesis for mixed-mode fracture toughness criterion

A new hypothesis is presented for determining the mixed-mode fracture toughness criterion of brittle materials such as rocks, cement and ceramics. The obtained criterion gives a reasonable result, which is in good agreement with published data. The data, obtained here for mixed-mode fracture toughness of flyash eement paste, confirm also the use of the hypothesis for the problem of fracture toughness, including negative K_IC.     

Kic Transition-temperature behavior of A517-F Steel

Linear-elastic fracture mechanics has been widely used to obtain K_ic values on very-high-strength steels (yield strengths > 200 ksi) that generally do not exhibit a ductue-to-brittie transition in failure mode as a function of temperature. However, as the use of the K_ic test approach is extended to those steels that do exhibit a ductile-to-brittle transition, information on the K_ic transition-temperature behavior of steels is required. Therefore, to establish general relations between K_ic and Charpy test results, slow-bend K_ic fracture tests and various Charpy tests were conducted on A517-F steel at temperatures between −320 and +80°F.

The results indicated that a plane-strain K_ic température transition does exist for A517-F steel. Furthermore, this transition occurred in the same temperature range (−150 to −50°F) as the transition denned by slow-bend Charpy test results for fatigue-cracked specimens. In both the K_ic tests and the Charpy tests, the transition-temperature behavior appeared to be related to a gradual change in the microscopic fracture mechanism. The upper shelf, as denned by slow-bend Charpy tests, appeared to be a region in which K_ic values cannot be obtained, regardless of specimen geometry, because of general yielding and crack blunting.

A procedure is proposed in which the dynamic K_ic behavior of a material can be predicted from static K_ic test data by shifting the static K_ic values along the temperature axis by the same amount as the static Charpy energy values are shifted by impact testing.

In general, the results of this investigation have demonstrated that a transition in K_ic behavior of A517-F steel does exist as a fution of temperature and that that transition is independent of the K_ic to K_c stress-state transition.     

Principle and method for determining fracture toughness of structural steel by side-cut se (B)

In this paper, the principle and method for determining fracture toughness indexes K_IC, δ_C and J_IC of structural steel by side-cut SE (B) are introduced. This method has many advantages: clear critical point, simple operation, stable date, less cost and will be valuable to use more frequently.     

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.     

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.     

On the influence of microstructure on crack propagation mechanisms and fracture toughness of metallic materials

A survey is given on the effect of microstructure on crack propagation mechanisms and fracture toughness.

The influence of inclusions and of the material's matrix are treated separately. An attempt was made to correlate some simple, but typical microstructures with corresponding crack propagation mechanisms and to establish a qualitative sequence of these microstructures with respect to their effect on K_IC. Because of the lack of sufficient experimental evidence this attempt is necessarily incomplete.

Finally, some K_IC-calculations are compared with measured values.     

The static and dynamic low-temperature crack-toughness performance of seven structural steels

To properly describe the crack-toughness behavior of steels in a quantitative manner, a study was undertaken to establish the effects of strain rate and low temperature on the K_ic values of seven structural steels. Steels having room-temperature yield strengths ranging from 40 to 250 ksi-ABS-C, A302-B, HY-80, A517-F, HY-130(T), 18Ni(180), and 18Ni(250) steels-were evaluated for static and dynamic loading over the range of temperatures for which K_ic values were attainable.

The results indicate that for the ABS-C, A302-B, HY-80, and A517-F steels, an increase in strain rate of approximately six orders of magnitude caused a decrease in the K_ic values measured at the same test temperatures. No significant effect was observed for the HY-130T and 18NI(250) steels. However, the most significant effect of the increased strain rate was the increase in the threshold temperature below which plane-strain behavior occurred.

When all steels — except the 18Ni(180) maraging steel, for which insufficient valid data were obtained were compared on the basis of equivalent critical flaw-size behavior, the crack-toughness performance in terms of _ic/σ_ys for dynamic loading could be separated into three groups. The HY-80 and HY-130(T) steels were best, the ABS-C, A302-B, and A517-F steels were intermediate in performance, and the 18Ni-(250) maraging steel was the poorest. These groupings of performance prevailed over a relatively wide range of test temperatures. As a means of accounting for the differences in strain rate, the K_ic/σ_ys values for all steels investigated were plotted in terms of the rate-temperature parameter, Tln A/ε, which superimposed most of the static crack-toughness performance data into these same levels of performance. In addition, the results of the investigation substantiated the interpretation that the nil-ductility-transition temperature measured in the drop-weight test is the upper limit of dynamic plane-strain crack-toughness behavior for 1-in.-thick plates.

In general, the results of the present investigation provide a quantitative comparison of the plane-strain crack-toughness performance of 1-in.-thick plates of seven structurel steels under both static and dynamic loading conditions. Because of the increase in temperature range over which K_ic behavior occurs with increased strain rate, dynamic loading can be an especially significant factor in the performance of structural steels, particularly those having yield strengths less than approximately 140 ksi.     

The scatter in KIC-results

Scatter in K_IC-results can often be quite extensive, and to make reliable interpretations of the results it is of great importance to understand the nature of it. Cleavage fracture in steels is of a statistical nature and therefore the scatter in K_IC-results will behave similarly.

Two different approaches, one based on a microstructural statistical model and an other based on the Weibull distribution are applied to evaluate the theoretical scatter in K_IC-results. With both methods it is shown that the theoretical value of the relative scatter described through the Weibull slope factor is constant and equal to four.

The reason for the discrepancy between the theoretical value and the experimentally determined values of the slope factor is shown to be caused by inadequate number of experimental K_IC-measurements. The existence of a lower limiting K_min value is verified and a simple procedure for conservative estimation of the K_Ic-mean and lower bound values is presented.     

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.     

Dynamic fracture toughness at crack initiation, propagation and arrest for two pipe-line steels

The influences of temperature and loading rate on fracture toughness of two pipe-line steels at initiation, K_1d, and at arrest, K_1a, and on stretch zone height were measured using specific techniques. Particular attention was given to the mechanism of delamination, typical for the gas internal pressure pipe fracture.     

Influence of specimen size and configuration on the plastic zone size, toughness and crack growth

The main purpose of this investigation is to evaluate the effect of the thickness, width, aspect ratio and geometry of the fracture toughness specimen on the resultant displacement due to growth of plastic zone and crack.

The analytical part evaluates the effect of the width, aspect ratio, geometry and flow properties on the displacement due to the growth of plastic zone as well as the crack. The experimental part evaluates the effect of thickness and width of a compact tension specimen on the displacement and on the thickness direction contraction due to the growth of the plastic zone.

The main result of the investigation is that the plastic zone size decreases and the constraint to yielding increases as the width of a CT specimen increases. Based on this and other analytical and experimental result, a new procedure for the determination of K_IC has been proposed. The procedure is verified by experimental data obtained by other workers. The procedure overcomes the limitation of ASTM E399 for the determination of K_IC.     

Dynamic fracture toughness parameters for HY-80 and HY-130 steels and their weldments

Lower bound dynamic fracture toughness parameters for HY-80 and HY-130 steel and their weld metals are identified. Specific values of the parameters K_Id and K_Im obtained from direct measurements are reported together with estimates inferred from the large body of Charpy energy, nil ductility transition temperature and dynamic tear energy measurements. The emphasis is on reasonable lower bound values at 30°F, the lowest anticipated service temperature, for use in elastodynamic analyses of crack growth initiation, propagation, and arrest in ship structures. For these conditions, it has been found that the ratio K_Id/ σ_Y is approximately equal to 2 in^1/2 for HY-80 steel. For HY-130 steel and the HY-80 and HY-130 weld metals under these same conditions, K_Id/ σ_Y is approximately 1 in^1/2. Consequently, HY-80 plate appears to be substantially more resistant to fracture under dynamic loading than are the other three grades examined.     

Regression-based CVN–KIC Models for hot work tool steels

Dies and tools used in hot metal forming (extrusion, forging, rolling, etc.) are exposed to high pressures, elevated temperatures, and thermo-mechanical fatigue. The most common mode of in-service die failure is fatigue fracture (brittle failure through crack propagation). Reliable determination of fracture toughness of the die material is thus critically important. However, as die steels have a combination of high-hardness and high-strength, and are used at elevated temperatures, standard plane-strain fracture toughness (K_IC) testing methods become impracticable. Alternate testing procedures such as the Charpy impact energy (CVN), together with empirical/semi-empirical correlations of K_IC to other data, are then more viable and economical. Experimental data (values of K_IC, CVN, and HRC) of H13 steels have been collected through an exhaustive literature search. This data set has been augmented through in-house experimentation: samples variously heat treated (different tempering temperatures and times, and both air-cooling and oil-quenching), and tested at different working temperatures. Linear and quadratic models are proposed for determination of fracture toughness, based on experimental (in-house) and published values of Charpy impact energy (CVN) and Rockwell hardness (HRC), both at room and at elevated temperatures.     

Experimental investigation of fracture surface energy of a solid propellant under different loading rates

Fracture tests on solid propellants have been performed under three ranges of loading rate leading to fracture toughness results in terms of J_IC and (Andrews' parameter). Using two different specimen shapes, finite size effects have been pointed out (considering SENT sample) when dealing with Andrews' theory and a modified equation, taking into account the specimen compliance, is proposed. Significant influence of loading rate on J_IC is observed leading to few conclusions concerning fracture process of solid propellants.     

Application of instrumented impact testing for studying dynamic fracture behaviour of rotor steels

The viability of the instrumented Charpy impact testing for studying dynamic fracture behaviour of rotor steels is investigated. This encompasses determination of dynamic fracture toughness (K_Id) and dynamic J-integral (J_Id), establishing correlation between oscilloscope profiles and fracture morphology of the ruptured samples and identifying fracture mechanisms involved. The predicted oscilloscope profiles for common fracture modes, their experimental counterparts, and the inferences drawn from these concerning operating fracture mechanisms are in good accord with the fractographic observations made on broken samples. Thus, the respective oscillographs vividly manifest the observed variations in the fracture processes. Fracture mechanics analysis of load-time and energy-time records of pre-cracked Charpy samples gave dynamic fracture toughness (K_Id) values of 43, 74 and 124MN/m3/2, and dynamic J-integral (J_Id) values of 0.008, 0.03 and 0.06 MJ/m² at −180°, 26° and 96°C respectively. It is possible that the deduced J_Id values correspond to a small but finite amount of crack extension instead of Zero Crack extension, in line with the emerging trends of J_Id estimation. Apart from increasing with temperature, both parameters recorded a true transition around 35°C which is attributed to the combined influence of a change in the fracture mode and relaxation of crack tip constraint. Another significant outcome of this investigation concerns about the existence of a minimum crack depth ratio for valid J_Id determination which, based on a detailed fractographic study, is interpreted in terms of the collective influence of crack tip plasticity and notch constraint.     

Fracture toughness of 80mm thick electroslag welded joints with COD approach application

The paper deals with searching for criteria for the mechanical properties estimation of thick walled electroslag welded joints with an aim to find out whether the COD approach would be reliable for the differentiation of brittle fracture resistance. Three types of C-Mn steels modified by titanium, vanadium and molybdenum respectively with yield point of about 40 kp/mm², applied usually for pressure vessels and bridge constructions were used in the experimental work. The welded joints were tested in as-welded, stress relieved, and normalized and tempered conditions respectively. It was stated that the COD approach distinguishes quite well the above mentioned variants of the heat treatment of welded joints and base materials apart from the necessity of some precision of testing procedure and evaluation of test results.

The adventage not under discussion of COD approach in comparison to classic methods lies in the interpretation of brittle fracture resistance by the parameter of the defect size particularly for the conditions where no valid K_IC values are obtained.     

Subcritical crack growth of trip steels in air under static loads

Pre-fatigued compact tension fracture toughness specimens of TRIP steel were held at constant loads at 25°C in 40 per cent r.h. air. Testing was done using an MTS closed loop universal test machine in the load control mode and the displacement was monitored as a function of time using a clip-on gauge and a strip chart recorder. Subcritical crack growth was observed and the experimental data was used to obtain a correlation between stress intensity and the rate of crack growth. The curves usually exhibited three distinct regions, including a plateau of stress intensity insensitive constant crack growth rates which have been observed by other investigators for titanium alloys and high strength steels. Based on comparisons with other investigators, the mechanism of subcritical flaw growth was tentatively identified as being due to hydrogen embrittlement. Fracture was observed to follow austenite grain boundaries and it was hypothesized that the austenite → martensite transformation sensitizes them to hydrogen by causing a large strain accumulation to be accommodated at the boundaries resulting in a large dilatation. Metallography revealed that the crack growth rate decreased as the strain-induced martensite increased and this was attributed to crack tip blunting by plastic deformation due to the invariant shear of the transformation. There are thus two apparently competing processes in the subcritical flaw growth of TRIP steels. Fractography showed that numerous fracture micromechanisms were operative. The subcritical crack growth characteristics compared favorably with other high strength steels tested under almost equivalent conditions and an apparent threshold for subcritical growth in air was determined to occur at about 56 per cent of K_IC.     

Effects of crack growth on the load-displacement characteristics of precracked specimens under bending

A critical evaluation of the feasibility of obtaining crack growth parameters from quasi-static bend tests is presented. First derived are the governing differential equations which characterize the time-history of bend test parameters for a given elastic material exhibiting power law crack growth behavior v = v_max(K_I/K_IC)^N. A numerical solution scheme is then developed which is capable of solving the initial value problem, thus quantitatively assessing the influence of crack growth on the load-displacement output. The results of this analysis indicate that in order to calculateK_IC accurately based on the peak load data, the loading rate has to be set sufficiently fast compared to v_max (but below dynamic rates), otherwise, it will generally overestimate its true value whenever slow crack growth occurs during the test; and that for high N materials the flexural test method gives a broad error band inN prediction and hence is not a reliable technique. However, it can be used by a designer to quickly screen the new materials with high Nvalues which are potential candidates for structural application.