FRACTURE BEHAVIOUR OF UNTREATED AND SILANE-TREATED TALC-FILLED POLYPROPYLENE COMPOSITES

Abstract— Fracture behaviour of injection-moulded polypropylene filled with silane-treated talc was studied as a function of filler volume fraction (0–20%) and compared to that of polypropylene filled with untreated talc. High-rate tests (0.57 m/s) on SENB specimens were carried out using an instrumented Charpy impact pendulum, and linear elastic fracture mechanics (LEFM) was applied to calculate the fracture parameters, K _C and G _C. It was found that moderate fractions of talc which were added to the polypropylene matrix increased the fracture toughness of the composite independent of the talc surface treatment. This general improvement seems to be due to the peculiar orientation of the talc platelets in the injection-moulded specimens. The fracture behaviour of the composites was also studied at low strain rate (1 mm/min) by tests on J -integral type specimens with the same SENB geometry. In this case, the composites with silane-treated talc presented poor J -integral values compared to those of the samples with untreated talc. This was attributed to a reduction of the plastic zone at the crack tip, since the improved coupling between the talc platelets and matrix increased the yield strength of the composite. All the results are explained on a basis of morphological and microstructural details.     

Evaluation of fracture toughness and impact toughness of laser rapid manufactured Inconel-625 structures and their co-relation

The present study involves evaluation of fracture toughness and Charpy impact toughness of Inconel 625 structures fabricated by laser based additive manufacturing. The results of crack tip opening displacement (CTOD) fracture toughness are close to those reported for the Inconel 625 weld metal. The nature of the load–time traces of instrumented Charpy impact tests revealed that the alloy Inconel 625 in laser fabricated condition was associated with fully ductile behavior with Charpy V-Notch impact energy in the range of 48–54 J. Stress relieving heat treatment at 950 °C for 1 h has resulted in marginal improvement in the impact toughness by about 10%, whereas no clear evidence of such improvement is seen in the CTOD fracture toughness. Fractographic examination of the Charpy specimens and the results of the instrumented impact tests imply that the mechanism of crack growth was propagation controlled under dynamic loading conditions. Dynamic fracture parameters were estimated from the instrumented impact test data and compared with the experimentally evaluated fracture toughness results.     

Anwendung des instrumentierten Kerbschlagbiegeversuchs zur Abschätzung der Rissauffangzähigkeit

Application of Charpy V‐notch testing to estimate the crack‐arrest toughness Modern structural integrity assessment relies upon fracture mechanics, thus utilizing fracture mechanical parameters describing the material fracture resistance against crack initiation and crack propagation as well as the material crack‐arrest behaviour. However, crack‐arrest fracture toughness values are usually difficult and expensive to determine. In this paper correlations are proposed for estimating the nil‐ductility temperature (T_NDT) and the crack‐arrest fracture toughness (K_Ia) from a transition temperature, based on instrumented Charpy‐V crack‐arrest load information. The transition criteria used are the 4 kN crack‐arrest force and the mean crack‐arrest fracture toughness of 100 MPa√m according to the master curve approach. Correlations between transition temperatures, T(F_a = 4 kN), T(K_Ia), and T_NDT, which were proposed for various structural steels, work very well for the 18Ch2MFA material.     

Evaluation of dynamic fracture toughness KId by Hopkinson pressure bar loaded instrumented Charpy impact test

A novel method for measuring the dynamic fracture toughness, K_Id, using a Hopkinson pressure bar loaded instrumented Charpy impact test is presented in this paper. The stress intensity factor dynamic response curve (K_I(t)−t) for a fatigue-precracked Charpy specimen is evaluated by means of an approximate formula. The onset time of crack initiation is experimentally detected using the strain gauge method. The value of K_Id is determined from the critical dynamic stress intensity factor at crack initiation. A K_Id value for a high-strength steel is obtained using this method at a stress-intensity-factor rate () greater than 10⁶ MPa .     

Measurement of fracture initiation toughness and crack resistance in instrumented Charpy impact testing

A new method has been developed involving direct measurement of the load-line displacement during instrumented Charpy testing. The method uses a laser interferometer to measure displacement in addition to the load-line displacement derived from the load signal. Tests were conducted using fatigue precracked and V-notched test pieces in the temperature range +23°C to −80°C on a conventional ship grade steel, a pressure vessel steel and two welded joints. Good correlation was found between the J_0.2 initiation fracture toughness determined by the multi-specimen method and the J_i fracture toughness determined from single specimens using the new method to detect ductile fracture initiation.     

Ductile tearing of pipeline-steel wide plates: I. Dynamic and quasi-static experiments

Four low-carbon microalloyed pipeline steel plates were studied with two chemical compositions and different thermo-mechanical treatments, leading to either ferritic–pearlitic or ferritic–bainitic microstructures.Microstructural and mechanical properties were investigated. An original dynamic tensile experiment is used to study crack propagation in full-thickness wide plates under either quasi-static and dynamic conditions. In the latter case, crack speeds up to 20–40 m s⁻¹ were reached and led to ductile shear crack propagation as observed in pipe bursts, while mode I in-plane crack propagation was observed in most quasi-static tests. Shear mode fracture results from strain localization under dynamic conditions and may be detrimental to steel toughness. Steel resistance to crack propagation is evaluated with the use of the energy dissipation rate parameter. The effect of the microstructure as well as material parameters like the anisotropic behavior on fracture toughness were evaluated. It is shown that ferritic–bainitic steels exhibit a better yield stress–toughness compromise than ferritic–pearlitic ones.In a companion paper (Engng. Fract. Mech., submitted for publication), the numerical simulation of crack propagation in wide plates using fully coupled local approach to fracture is presented.     

Threshold toughness of polymers in the ductile to brittle transition region by different approaches

The fracture behavior of polymers in the ductile-to-brittle region is neither completely brittle nor entirely ductile. Besides, scatter in toughness results impairs the situation. Consequently, conventional methods based exclusively either on linear elastic fracture mechanics theory (LEFM) or on non-linear elastic fracture mechanics theory (NLEFM) are not suitable. It was demonstrated previously, that Weibull statistical method could be successfully used to determine the toughness threshold of polymers displaying ductile-to-brittle behavior. The present study compares the threshold toughness value determined by the statistical approach with other critical values calculated following other different suitable approaches: Low temperature plane strain fracture toughness, Plastic zone corrected fracture toughness, Stable and unstable propagation combined model, J extrapolated at zero stable propagation value, and Quasi J-R curve. The analysis was carried out on data points taken from fracture tests performed on polypropylene homopolymer, PPH, and on a blend of PPH and an elastomeric polyolefin, PPH/POes. The results of this analysis indicate that statistical, stable and unstable propagation combined model, and the J extrapolated at zero stable propagation value methods yield to very similar toughness threshold values being practically equivalent. In this case, threshold value was slightly smaller than the minimum J displayed by the experimental replicas, suggesting that it is an actual representative material toughness. Among these methodologies, the Statistical Method is applicable even if stable crack growth is difficult to determine. On the other hand, the methodologies based on LEFM tended to underestimate the fracture toughness, being very conservative while Quasi J-R curve method based on NLEFM overestimated the PPH/POes toughness value.     

A ? method-based numerical simulation of crack growth in linear elastic fracture

This paper presents a method for the automatic simulation of quasi-static crack growth in 2D linear elastic bodies with existing cracks. A finite element algorithm, based on the so-called ? method, provides the load vs. crack extension curves in the case of rectilinear crack propagation. Since the approach is both theoretically general and simple to be performed from a computational point of view, it could be extended for describing the phenomenon of crack growth in different fracture mechanics contexts.     

Crack arrest testing of high strength structural steels for naval applications

The crack arrest fracture toughness of two high strength steel alloys used in naval construction, HSLA-100, Composition 3 and HY-100, was characterized in this investigation. A greatly scaled-down version of the wide-plate crack arrest test was developed to characterize the crack arrest performance of these tough steel alloys in the upper region of the ductile-brittle transition. The specimen is a single edge-notched, 152 mm wide by 19 mm thick by 910 mm long plate subjected to a strong thermal gradient and a tensile loading. The thermal gradient is required to arrest the crack at temperatures high in the transition region, close to the expected service temperature for crack arrest applications in surface ships. Strain gages were placed along the crack path to obtain crack position and crack velocity data, and this data, along with the applied loading is combined in a “generation mode” analysis using finite element analysis to obtain a dynamic analysis of the crack arrest event. Detailed finite element analyses were conducted to understand the effect of various modeling assumptions on the results and to validate the methodology compared with more conventional crack arrest tests.Brittle cracks initiation, significant cleavage crack propagation and subsequent crack arrest was achieved in all 15 of the tests conducted in this investigation. A crack arrest master curve approach was used to characterize and compare the crack arrest fracture toughness. The HSLA-100, Comp. 3 steel alloy had superior performance to the HY-100 steel alloy. The crack arrest reference temperature was T_KIA = −136 °C for the HSLA-100 plate and T_KIA = −64 °C for the HY-100 plate.     

Failure resistance of drilling rig casing pipes with an axial crack

Working conditions of casing pipes in drilling rigs can significantly influence the initiation and development of damage in the material, and therefore also the safe service of the entire system. In this work, an integrity assessment of a pipe with initial defect (machined surface crack) is presented. The position of this defect is on the external surface; unlike transport pipes, where internal surface is often endangered due to the contact with the fluid, casing pipes are also often exposed to damages at the external surface. A pipe segment exposed to internal pressure is examined experimentally and numerically, using the finite element method. Experimental setup included tracking of crack mouth opening displacement (CMOD) values, as well as J integral. Criteria for pipe failure are determined on the finite element (FE) models of the pipe; fracture initiation and plastic collapse are considered as failure mechanisms. Several 3D models with different crack sizes are evaluated. 2D plane strain models are also examined, to determine the applicability limits of this simplified approach. Integrity assessment criteria for the analysed geometries are discussed. Assessment of fracture resistance of the pipeline material is also considered in this work. Besides the standard SENB specimens, Ring specimens cut from the pipe are tested, and the results are compared. Both specimen geometries are modelled using local approach to fracture, by application of the micromechanical Complete Gurson model (CGM), developed by Z.L. Zhang. It is shown that the Ring specimens have similar fracture conditions under bending load as SENB specimens. Since they are much simpler to fabricate from the pipe than standard specimens, it is concluded that they can be used for assessment of fracture of the pipes with axial cracks.     

Electrical failure of piezoelectric ceramics with a conductive crack under electric fields

The failure behavior of piezoelectric ceramics with a conductive crack under purely electric loading is investigated. Electrical fracture tests are conducted to study the influence of the directions of poling and electric loading. Two failure modes of piezoelectric materials are observed: fracture that is accompanied with dielectric discharging and the formation of tubular channels without fracture. The critical J integrals at the onset of both fracture and breakdown are calculated numerically via finite element analysis. The effects of both the direction of the electric field and the poling direction on both fracture and breakdown resistance are discussed.     

Influence of striking edge radius (2 vs. 8?mm) on instrumented Charpy data and absorbed energies

The most commonly used test standards for performing Charpy impact tests (ISO 148 and ASTM E 23) envisage the use of strikers having different radii of the striking edge, i.e. 2 mm (ISO) and 8 mm (ASTM). The effect of striker geometry on Charpy results was extensively studied in the past in terms of absorbed energy measured by the machine encoder, but few investigations are available on the influence of striker configuration on the results of instrumented Charpy tests (characteristic forces, displacements and integrated energy). In this paper, these effects are investigated based on the analysis of published results from three interlaboratory studies and some unpublished Charpy data obtained at SCK·CEN. The t-test was used for establishing the statistical significance of the observed effects. The instrumented variables which are the most sensitive to the radius of the striking edge are the maximum force and its corresponding displacement, with 8 mm-strikers providing systematically higher values. The effect on general yield forces, on the other hand, is less consistent and more difficult to rationalize, although 2 mm-strikers generally tend to deliver higher values. Absorbed energies, obtained both from the instrumented trace and from the pendulum encoder, are almost insensitive to the type of striker up to 200 J. For higher energy levels, the values obtained from 8 mm-strikers become progressively larger. Data scatter is generally higher for 2 mm-strikers.     

Influence of carbides morphology on fracture toughness of cast steel G200CrMoNi4-3-3

We analyze the influence of small modification of chemical composition of G200CrMoNi4-3-3 cast steel on the morphology of carbides and on material crack resistance. Using the Termo-Calc software the volume fraction of carbide phase was determined and the results correlated with microstructure observations. Crack resistance of cast steel was determined using SENB specimens and finding critical values of stress intensity factor K_Q. Metallographic and fractographic observations of fracture surfaces allowed identifying the mechanism of cracking. __________ Translated from Problemy Prochnosti, No. 1, pp. 137–140, January–February, 2008.     

Evaluation of R-curve behavior of ceramic-metal functionally graded materials by stable crack growth

This paper deals with the fracture toughness and R-curve behavior of ceramic-metal functionally graded materials (FGMs). A possibility of stable crack growth in a three-point-bending specimen is examined based on the driving force and resistance for crack growth in FGMs, and the distribution of fracture toughness or R-curve behavior is evaluated on FGMs fabricated by powder metallurgy using partially stabilized zirconia (PSZ) and stainless steel (SUS 304). The materials have a functionally graded surface layer (FGM layer) with a thickness of 1 mm or 2 mm on a SUS 304 substrate. Three-point-bending tests are carried out on a rectangular specimen with a very short crack in the ceramics surface. On the three-point-bending test, a crack is initiated from a short pre-crack in unstable manner, and then it propagates in stable manner through the FGM layer with an increase in the applied load. From the relationship between applied load and crack length during the stable crack growth in the FGM layer, the fracture toughness is evaluated. The fracture toughness increases with an increase in a volume fraction of SUS 304 phase.     

Determination of dynamic crack resistance of ductile cast iron using the compliance ratio key curve method

The compliance ratio method is an analytical approach for instantaneous crack length determination in dynamic single-specimen J-R curve testing of ferritic ductile cast iron (DCI). Comparison testing at room temperature and −40 °C was applied to PCVN and SE(B)15 specimens to examine their performance and suitability for the dynamic key curve method for DCI. An experimental reference database of dynamic crack resistance curves was set up by low-blow multiple-specimen tests and used to validate the results of the CR method. The influence of test temperature, microstructure, loading rate and specimen geometry on fracture behavior of the tested DCI was investigated in great detail and these parameters were linked to fracture mechanical properties. The results obtained show that the CR method is suited to establish valid dynamic crack resistance curves for both types of specimen. Nevertheless, SE(B)15 specimens are preferred for dynamic J-R curve determination of DCI based on their advantages such as higher accuracy.     

Change of fracture mode in Charpy toughness transition temperature range

Abstract

A transition layer of width 5 - 10 μm was found on the boundary between ductile and brittle fracture for Charpy V notch specimens in the transition temperature range of a structural steel having a microstructure of polygonal ferrite -pearlite. The fracture mode in the transition layer was shearing with occasional submicrometre dimples. From tensile tests on notched specimens, the cleavage fracture stress and flow stress by ductile decohesion were determined. Based on the experimental data and the assumption that the volume of metal involved in the plastic deformation during fracture was related to the volume of the dimples, it was deduced that the transition layer width represents the size of the plastic zone immediately before cleavage initiation. The crack opening displacement and the crack tip radius for the change of fracture mode were calculated.     

Monotonic vis-à-vis cyclic fracture behaviour of AISI 304LN stainless steel

This investigation is aimed to examine the monotonic and cyclic fracture behaviour of AISI 304LN stainless steel and its weldments, in order to assess their integrity under seismic loading conditions. The monotonic fracture resistance of the steel has been determined using standard J-integral technique; whereas the cyclic fracture resistance has been evaluated using periodic unloading to different extents fixed by pre-determined R-ratio. Comparison of the fracture toughness values of the steel estimated under monotonic and cyclic loading indicates that the latter could be as low as one-fifth of the former. The observed degradation in cyclic fracture resistance has been attributed to crack tip re-sharpening during cyclic loading.     

Dynamic fracture mechanics analysis of failure mode transitions along weakened interfaces in elastic solids

Dynamic fracture mechanics theory was employed to analyze the crack deflection behavior of dynamic mode-I cracks propagating towards inclined weak planes/interfaces in otherwise homogenous elastic solids. When the incident mode-I crack reached the weak interface, it kinked out of its original plane and continued to propagate along the weak interface. The dynamic stress intensity factors and the non-singular T-stresses of the incident cracks were fitted, and then dynamic fracture mechanics concepts were used to obtain the stress intensity factors of the kinked cracks as functions of kinking angles and crack tip speeds. The T-stress of the incident crack has a small positive value but the crack path was quite stable. In order to validate fracture mechanics predictions, the theoretical photoelasticity fringe patterns of the kinked cracks were compared with the recorded experimental fringes. Moreover, the mode mixity of the kinked crack was found to depend on the kinking angle and the crack tip speed. A weak interface will lead to a high mode-II component and a fast crack tip speed of the kinked mixed-mode crack.     

Fracture mechanics testing on specimens with low constraint--standardisation activities within ISO and ASTM

Fracture mechanics tests are traditionally designed to measure material resistance to stable or unstable crack extension using specimens that are highly constrained to plastic deformation. For a variety of reasons, structural members may be made of thin gage-materials with inherently low constraint to plastic deformation. There is currently little guidance for measuring crack extension resistance under such conditions. The international standards organisations ISO and ASTM are responding to that need, and this paper describes one aspect of their current activity.Two procedures are being developed; one based on the δ₅ crack opening displacement parameter, the other on the constant value of the crack tip opening angle, ψ_c. The measurement of δ₅ is well established and relatively simple, whereas ψ_c is more difficult to determine experimentally. Evaluations of ψ_c from finite-element analyses are currently the most accurate approach, since measurements can only be made on the exterior surfaces similar to δ₅. Questions naturally arise regarding the correspondence of surface indication with full-thickness response in the laboratory experience. Both measures of crack extension resistance are suitable for structural assessment. The δ₅ concept is applied by means of crack driving force formulae from existing assessment procedures and hence relatively easy to use. On the other hand, the CTOA concept is potentially more accurate and can be applied to cases of multiple cracks and complex structures. But its structural application requires numerical methods, which have been successful in predicting the failure of large-scale cracked structural components.     

Control of load/displacement responses of AA6061-T6 and T4 circular extrusions under axial compressive loads

Axial crushing of AA6061-T6 and T4 circular extrusions with variable wall thicknesses was completed under both dynamic and quasi-static loading conditions to investigate the capability of controlling the load/displacement responses of the extrusions. Circular specimens with a nominal original wall thickness of 3.175 mm, an outer diameter of 50.8 mm, and a length of 300 mm were considered. Variations of the wall thickness in the axial direction were completed by material removal from the extrusion sidewall prior to crushing tests. Cutters used in this research had a height of 20 mm and blade tip widths of 1.0 mm. A curved deflector was used to flare the cut petalled sidewalls and facilitate the cutting system. Results from the impact tests illustrated that an initial peak cutting force with a magnitude of 1.08-1.74 times higher than that for the quasi-static loading was needed to initiate the cutting deformation mode. After this transient cutting stage, the load/displacement responses were observed to be similar to that from the quasi-static tests except for some slight fluctuations resulting from a minor amount of material fracture which occurred on the petalled sidewalls. A lesser extend of material fracture was observed on the T4 temper specimens due to the work hardening material property of the T4 temper condition. The mean cutting force from the dynamic tests were determined to be in the range of 0.92-1.09 times the mean force from the corresponding quasi-static test. Control of load versus displacement responses of the extrusions under both impact and quasi-static compressive loading conditions was accomplished through the variation of the wall thickness along the axial direction of extrusions.