Instrumented Charpy Test Research Articles

Identification of neutron irradiation induced strain rate sensitivity change using inverse FEM analysis of Charpy test

A simple methodology how to obtain additional information about the mechanical behaviour of neutron-irradiated WWER 440 reactor pressure vessel steel was developed. Using inverse identification, the instrumented Charpy test data records were compared with the finite element computations in order to estimate the strain rate sensitivity of 15Ch2MFA steel irradiated with different neutron fluences. The results are interpreted in terms of activation volume change.

Impact Toughness of Friction Stir Welded Al-Mg Alloy*

Abstract Al-Mg 5083 alloy was friction stir welded by varying welding variables, i. e., rotational speed, traversal speed and tool tilt angle. Welded specimens have been tested by the instrumented Charpy test to evaluate total absorbed energy, as well as the energy for crack initiation and the energy for crack growth. Subsequently, the fracture surface was examined by optical and scanning electron microscopy (SEM) to evaluate surface type of fracture, and to correlate the microstructure with the impact energy. Based on this, the optimum windows of FSW parameters for Al-Mg 5083 alloy welding have been defined.

Impact Toughness of 0.3 Mass% Carbon Tempered Martensitic Steels Evaluated by Instrumented Charpy Test

The effect of tempering temperature on the impact toughness of 0.3 mass% carbon martensitic steels with prior austenite grain (PAG) size of about 6 μm and 30 μm were investigated. Instrumented Charpy impact test (ICIT) method was used to evaluate the impact toughness. The tempering temperature of 723K gives the largest difference in the Charpy impact energy at room temperature (RT) between the specimens with two different PAG sizes. Investigation of the test temperature dependence of Charpy impact energy in the 723K tempered steels shows a steep transition at around 200 K for the 6 μm PAG specimen, while it shows a continuous slow transition in a wide range of temperature for the 60 μm PAG specimen. ICIT waveform analysis shows that the fracture propagation energy in stead of the fracture initiation energy mainly controls the temperature dependence of the impact energy. The carbide size distribution in these two specimens was investigated by SEM and TEM. The 60 μm PAG specimen shows the distribution of coarser carbides than does the 6 μm PAG specimen, which seems to be the main reason for the observed difference between them in the Charpy impact energy and the other properties of impact fracture.

Microstructural Control of Ductile Crack Propagation in TMCP HSLA Pipeline Steels

Control of ductile fracture propagation is one of the major concerns for pipeline industry, particularly with the increasing demand of new control rolled steel grades required to maintain integrity at high operational pressures. The objective of this research is to understand which microstructural features govern crack propagation, and to analyse the effect of two of them (average grain size, and volume fraction of pearlite). The main disadvantage during classical Charpy test was to discriminate the crack initiation and propagation energy during fracture of a notched sample. The initiation appears to be caused by the stress state in the neighbouring of Ti-containing precipitates or pearlite particles (no presence of M/A constituents or MnS inclusions was detected in the evaluated grades), propagation-arrest of the crack is assumed to play the main role concerning the control of fracture. Our approach to characterize the fracture resistance is to measure the energy absorbed during the crack propagation stage by means of load-displacement curves obtained via instrumented Charpy test. It was observed that the energy absorbed during crack propagation is not influenced by the average grain size but by the fraction and the morphological (banded-not banded) distribution of second pearlitic phase. This suggests that a different approach to characterize the heterogeneities in grain size clustering might be followed to correlate the energy measured during crack propagation and the morphological features of the steel.

Effect of Carbide Size Distribution on the Impact Toughness of Tempered Martensitic Steels with Two Different Prior Austenite Grain Sizes Evaluated by Instrumented Charpy Test

Effect of Carbide Size Distribution on the Impact Toughness of Tempered Martensitic Steels with Two Different Prior Austenite Grain Sizes Evaluated by Instrumented Charpy Test

Experimental Research on New Grade of Steel Protective Material for Light Armored Vehicles

An investigation of new PROTAC 500 armour steel was conducted. Three plates were heat treated to different states. One was quenched, second and third were quenched and low temperature tempered at 220 °C and 280°C for 3 hours. A tensile test, hardness measurements, and an instrumented Charpy test were performed. Metallographic was performed by optical microscopy (OM). Ballistic resistances of all three steel plates were measured. The behaviour of steel was tested using armour piercing projectiles 7.62X39 mm API BZ. The best results were obtained in quenched state

Characterization of radiation induced defects in EUROFER 97 after neutron irradiation

Specimens of EUROFER 97 prepared for impact tests have been irradiated to an average dose of 16.3dpa at irradiation temperatures of 250–450°C. TEM investigations have been performed to study radiation induced changes in the microstructure. The characterization and statistical analysis show the temperature dependant formation of small dislocation loops and He bubbles. The Burgers vector of dislocation loops was ½〈111〉. A novel feature is that within statistical uncertainty the maximum in the dislocation density observed around 300°C decreased with decreasing irradiation temperature down to 250°C. The TEM data are correlated with tensile and instrumented Charpy test results.

Evaluation of hydrogen embrittlement and temper embrittlement by key curve method in instrumented Charpy test

Instrumented Charpy test was conducted on small sized specimen of 21/4Cr-1Mo steel. In the test the single specimen key curve method was applied to determine the value of fracture toughness for the initiation of crack extension with hydrogen free, K IC , and for hydrogen embrittlement cracking, K IH . Also the tearing modulus as a parameter for resistance to crack extension was determined. The role of these parameters was discussed at an upper shelf temperature and at a transition temperature. Then the key curve method combined with instrumented Charpy test was proven to be used to evaluate not only temper embrittlement but also hydrogen embrittlement.

Influence of Loading Rate on the Calibration of Instrumented Charpy Strikers

Abstract One of the key factors for obtaining reliable instrumented Charpy results is the calibration of the instrumented striker. The conventional approach for establishing an analytical relationship between strain gage output and force applied to the transducer is the static calibration, which is preferably performed with the striker installed in the pendulum assembly. However, the response of an instrumented striker under static force application may sometimes differ significantly from its dynamic performance during an actual Charpy test. This is typically reflected in a large difference between absorbed energy returned by the pendulum encoder (KV) and calculated under the instrumented force/displacement test record (Wt). Such difference can be either minimized by optimizing the striker design or analytically removed by adjusting forces and displacements until KV=Wt (the so-called “dynamic force adjustment”). This study investigates the influence of increasing force application rates on the force/voltage characteristics of two instrumented strikers, one at NIST in Boulder, Colorado and one at SCK•CEN in Mol, Belgium. Force was applied to the strikers using servohydraulic testing machines up to maximum loading rates of around 3,000 kN/s, approximately 60 times lower than the typical loading rate during the elastic portion of an instrumented Charpy test (∼180,000 kN/s). At SCK•CEN, force was applied to the striker by cyclic loading at increasing frequencies and using a single ramp function having the desired slope (“single-shot” loading). It is observed that, under cyclic loading, the sensitivity of the strain gages drops significantly above a threshold loading rate, which may depend on the striker design (∼100 kN/s for the NIST tup, ∼1,000 kN/s for the SCK•CEN tup). Conversely, in the case of single-shot loading, the force/voltage relationship does not vary significantly up to ∼3,000 kN/s and remains close to the one obtained from the static calibration.

An experimental study on the dynamic fracture of extruded AA6xxx and AA7xxx aluminium alloys

The dynamic fracture behaviour of extruded AA6xxx and AA7xxx aluminium alloys is investigated using an instrumented Charpy test machine and V-notch specimens. The specimens are made from extruded flat profiles with a rectangular cross-section of 10 mm thickness and 83 mm width. The material is in T6 temper, i.e. the peak hardness condition. The alloys have either recrystallized or fibrous grain structure. For each alloy six different Charpy impact tests are carried out in two series. In Series 1, the notch is parallel to the thickness direction of the profile (i.e. through the thickness), while the notch is perpendicular to the thickness direction in Series 2 (i.e. lying in the plane of the flat profile). In each series, the longitudinal direction of the specimen is parallel, 45° or 90° to the extrusion direction. Comprehensive fractographic investigations are carried out for the different tests and alloys. It is found that the dissipated energy is practically invariant to specimen orientation and notch direction for the recrystallized alloy. For the fibrous alloys the dissipated energy is lower when the longitudinal direction of the specimen is 90° to the extrusion direction, i.e. when the notch is parallel with the fibrous grain structure. Further, the energy dissipation is higher for Series 2 than for Series 1 due to substantial delamination and secondary cracking in Series 2. The precipitate-free zones (PFZs) formed adjacent to the grain boundary are weak areas, preferable for crack initiation and growth. This is seen in the fracture surface as facets with high density of small dimples and is more pronounced for specimens with the notch parallel to the fibre direction.

Diffusion weld study for Test Blanket Module fabrication

According to the current design for the European Helium Cooled Pebble Bed (HCPB) ITER Test Blanket Module (TBM) there are basically six subcomponents which have to be fabricated and assembled: first wall, caps, stiffening grid, breeding units, back plates/manifolds, and attachment system. The main technologies needed for blanket fabrication is joining of parts – particularly production of plates with internal cooling channels – and applying suitable post-weld heat treatments. Both steps together are the key technologies that determine the mechanical strength of the blanket, the ductile-to-brittle transition temperature (DBTT) which is important under neutron irradiation, and the potential for a compact design. While it is certain that the structural material will be EUROFER or another, comparable reduced activation steel, most joining technologies and/or procedures have still to be developed, adapted, or qualified, although substantial advancements have been already reported. The designated fabrication route for plates with inner cooling channels is diffusion or solid phase welding which is either performed in a hot isostatic press (HIP) or in an uniaxial hydraulic press, both after different specific joint preparations. However, for an efficient TBM fabrication the application of different milling processes would be unavoidable. Therefore, the influence of six different common milling procedures on the diffusion weld properties has been studied by instrumented Charpy tests after a one-step and two-step low pressure welding process. The according microstructures have also been examined. Furthermore, the effect of nine typical states of surface contamination on the weld interface properties has been investigated and characterized. It could be demonstrated that two-step HIP diffusion welding can eliminate unfavorable surface fabrication defects and, therefore, might allow for more efficient milling processes.

Influence of Strain Gage Position on the Static and Dynamic Performance of Instrumented Impact Strikers

Abstract The quality and reliability of impact forces obtained from instrumented Charpy tests has been devoted considerable attention at SCK⋅CEN since the 1990’s, in parallel to the development and qualification of advanced methodologies for the surveillance of nuclear reactor pressure vessels (Enhanced Surveillance Strategy). In this framework, careful analysis of the instrumented force/deflection traces from Charpy tests allows the defining of important parameters which can help investigate material characteristics such as flow properties, microcleavage fracture stress, crack arrest behavior and alternative characteristic (index) temperatures. While first efforts at SCK⋅CEN were concentrated on the optimization of the striker calibration procedure, more recently the focus has been put on the in-house development and optimization of reliable instrumented strikers with both 2 mm radius (ISO 148) and 8 mm radius (ASTM E23). Evidence was found that the position of the strain gages affects the static and dynamic performance of the newly developed strikers. Additional investigations in this direction have shown that the linearity of the strikers during static calibration improves as the gages are placed further away from the striking edge. For the ASTM design, moving the strain gages away also reduces the characteristic “tail” that is typically observed just before a ductile specimen is ejected from the anvils. Our research shows that the best striker performance is achieved when the center of the T-rosette is located at about 16 mm from the striking edge, using a top/bottom configuration.

Instrumented Impact Tests: Effects of Machine Variables and Specimen Position

Abstract An investigation has been conducted on the influence of impact machine variables and specimen positioning on characteristic forces and absorbed energies from instrumented Charpy tests. Brittle and ductile fracture behavior has been investigated by testing NIST reference samples of low-, high-, and super-high-energy levels. Test machine variables included tightness of foundation, anvil and striker bolts, and the position of the center of percussion with respect to the center of strike. For specimen positioning, we tested samples that had been moved away or sideways with respect to the anvils. In order to assess the influence of the various factors, we compared mean values in the reference (“unaltered”) and “altered” conditions; for machine variables, t-test analyses were also performed in order to evaluate the statistical significance of the observed differences. Our results indicate that the only circumstance which resulted in variations larger than 5 % for both brittle and ductile specimens is when the sample is not in contact with the anvils. This should be taken into account in future revisions of instrumented Charpy test standards.

The Influence of Instrumented Striker Configuration on the Results of Dynamic Toughness Testing

Abstract Several studies are available on the influence of the edge radius of instrumented strikers (ASTM — 8 mm or ISO — 2 mm) on the results of both non-instrumented and instrumented Charpy tests. This paper investigates the effect of using either a 2 mm or a 8 mm striker when performing dynamic toughness tests at impact loading rates on precracked Charpy specimens. Existing data from tests run in the ductile-to-brittle region (dynamic Master Curve reference temperature) and in the upper shelf regime (ductile initiation fracture toughness and crack resistance curves) have been analyzed. The results show that 2 mm strikers tend to yield lower cleavage fracture toughness in the transition region (although the effect cannot be considered statistically significant), whereas the influence of striker configuration is negligible in the upper shelf regime when data are generated using the low-blow multiple-specimen technique.

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.

Fracture properties of epoxy/poly(styrene‐co‐allylalcohol) blends

AbstractThe epoxy/polystyrene system is characterized by a poor adhesion between the constituent phases, which determines its mechanical properties. The adhesion can be improved via blends based on epoxy resin and random copolymers, poly(styrene‐co‐allylalcohol) (PS‐co‐PA). In this work, the influence of PS‐co‐PA content and the good adhesion between the phases on the tensile properties and the fracture toughness achieved through instrumented Charpy tests have been investigated. The tensile strength and the deformation at break showed an increase in the PS‐co‐PA content while the Young's modulus remained the same. The tensile fracture surfaces revealed that the improvement of these magnitudes was mainly due to a crack deflection mechanism. Also, the fracture toughness of the blends was superior to that of the pure epoxy resin. The main operating toughening mechanism was crack deflection. The fractographic analysis showed that ∼ 80% of the particles were broken, and the crack tended to divert from its original path through the broken PS‐co‐PA particles. The remaining particles were detached from the epoxy resin, and the holes left suffered plastic deformation. Analytical models were used to predict successfully the toughness due to these mechanisms. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 2007

Indirect measuring of crack growth by means of a key-curve-method in pre-cracked Charpy specimens made of nodular cast iron

The determination of dynamic crack resistance curves from single specimen fracture tests requires information about the crack advance during the experiment. Here, attention is focused on crack resistance curves for nodular cast iron based on experimental data from instrumented Charpy tests. In order to estimate the actual crack length a key curve method (KCM) is employed. On the other hand, the Charpy impact tests were realized numerically using finite element calculations in conjunction with a continuum damage model (CDM) to simulate ductile crack growth. The parameters of the CDM model were determined from the experimental data of single specimen fracture tests. Equivalence between the experimental and the numerical realization of a fracture test was ensured by validating the predictions of the numerical simulations by means of low blow fracture tests. Comparison between the crack advance predicted by the numerical simulations and the results obtained using the proposed KCM shows a sufficiently well agreement with the actual crack length. Furthermore, crack resistance curves obtained from single specimen tests using either standard estimation schemes in conjunction with the KCM or numerical simulations are compared with the predictions based on low blow fracture tests.

Toughening with Little Stiffness Loss: Polyamide Filled with ABC Triblock Copolymers

In semicrystalline polymers, important toughening is obtained with dispersions of rubber particles. However, adding rubber most often induces a substantial loss in elastic modulus. In the present study, we investigate how to avoid such softening by using ABC triblock copolymers with low rubber content. A polyamide-12 matrix is toughened with polystyrene-block-polybutadiene-block-poly(methyl methacrylate) (SBM) triblock copolymers containing less than 40 wt % of rubber. We compare the mechanical properties of such systems to those of polyamide-12 toughened with pure reactive rubber. Impact properties are characterized by instrumented Charpy testing and post-mortem observations of crack surfaces. We report that dispersions of 10−20 wt % of SBM improve remarkably the impact strength, even at low temperature, without altering the elastic modulus of the polyamide matrix. Microscopic observations suggest that the large differences in impact strength between pure rubber and SBM-filled polyamide arise from differences in particle size and cavitation stress. Three-point bending measurements and dynamic mechanical analysis show that the high modulus of polyamide toughened with SBM is directly related to the low rubber content of the SBM copolymer and that softening also seems to be attenuated by the presence of hard glassy domains of S and M acting as stiffening agents.

Influence of irradiation on the ductile fracture of a reactor pressure vessel steel

The mechanical properties of 15Ch2MFA steel were characterised by tensile and instrumented Charpy tests. The fracture surfaces of Charpy specimens broken in the ductile-to-brittle transition temperature range contain a certain proportion of ductile fracture correlated to fracture energy. Measured ductile crack lengths show the same dependence on fracture deflection and/or fracture energy for irradiated and non-irradiated specimens. The decrease of upper shelf energy with increasing neutron fluence could be explained by an increasing amount of shear fracture.

Observations about tempered martensite embrittlement

An investigation on the embrittlement of tempered martensite has been carried out with AISI 4340 steel using the static torsion and bending tests together with the instrumented Charpy tests. After the mechanical tests, the specimens were studied by scanning electron microscope. It was observed that in Charpy tests the use of energy was minor when the fracture spread out in the test specimen especially after annealing at tempered martensite embrittlement region. Based on the test results the idea was presented that at the embrittlement region the fracture is not critical, instead the point where dτ/dtgγ becomes negative.