Instrumented Charpy Impact Test Research Articles

Dynamic mechanical behavior of CNT-reinforced epoxy under medium-strain rate: A comparative study

Carbon nanotubes (CNTs) are generally recognized as one of the most effective fillers to improve the properties of polymeric materials. Subjected to dynamic loading, polymer-based materials often exhibit high strain rate sensitivity. This research investigates the effectiveness of CNTs fillers on the mechanical properties of an epoxy resin at low to medium strain rates through conducting a comparative experimental study. Performing tensile and shear tests, the elastic modulus and strength properties of CNT/epoxy and pure epoxy resin are measured at different strain rates. By proposing an empirical logarithmic relationship, the prediction of dynamic elastic modulus and strength properties at any desired low to medium strain rate is facilitated. For measuring the dynamic fracture toughness of CNT/epoxy and pure epoxy resin at medium strain rate, the instrumented charpy impact test based on the Dynamic Key Curves (DKC) method is used. Experimental measurements still imply on significant enhancements in axial and shear properties and also dynamic fracture toughness up to medium strain rates of 150 s−1 by adding small portion of CNTs.

Investigation of Impact Properties under Instrumented Charpy Test

The Instrumented Charpy impact test is a promising method for determining a material’s impact response. Stainless steel has higher impact energy absorption capacity, high tensile, and yield strength compared to aluminum. Performance varies among grades; for instance, Aluminum 7075-T6 exceeds Aluminum 6061-T6 in tensile and yield strength. However, information regarding their energy capacity and impact signal pattern is lacking. This study investigated the impact properties using a Charpy machine, a data acquisition system, and a sensing element. Strain gauges were used to record the impact strain signal, enabling the analysis of impact duration, maximum strain, and the area under the curve. Specimens experimented include Stainless Steel 304, Aluminum Alloy 6061-T6, and Aluminum Alloy 7075-T6. The Charpy machine measures absorbed energy, while the theoretical impact energy is computed from software data. The area under the strain-time curve reflects the material energy absorption capacity. Stainless Steel 304 demonstrates superior energy absorbed, impact duration, and area under the curve, followed by Aluminum 6061-T6 and Aluminum 7075-T6. Despite higher tensile and yield strength, the inferior impact response of Aluminum 7075-T6 highlights the importance of factors like ductility, elongation, and alloy composition. Consequently, Aluminum 6061 is commonly used in the automotive industry, while Aluminum 7075 is preferred in aerospace applications.

A Study on Crack Initiation and Propagation of Welded Joints under Explosive Load

Welded joints in naval ship hull structures are weak areas under explosive load, but there are relatively few studies investigating the failure characteristics of welded joints through dynamic fracture and explosion tests. In order to explore and predict the failure characteristics of welded joints under explosive load, instrumented Charpy impact tests, explosion tests, and numerical simulations were carried out. The dynamic fracture toughness of ultra-high strength ship hull structural steel welded joints was obtained, and the dynamic stress intensity factors, together with the correlation between stress wave and crack propagation at different positions, were acquired. The results showed that the stress state at the crack tip of a Charpy impact specimen was consistent with that of a welded joint under explosive loads, and the crack initiated when the dynamic stress intensity factor exceeded the dynamic fracture toughness. The results indicated that the dynamic fracture toughness obtained by instrumented Charpy impact tests could be used to predict the crack initiation characteristics of welded structures under explosive load, and the stress wave at the crack tip was basically perpendicular to the crack propagation surface and promoted the rapid propagation of cracks.

Synergistic toughening by a layered microstructure and texture of Si-enriched ferritic/martensitic(F/M) heat resistant steel

In order to achieve favorable heat corrosion resistance, it is effective to add high Si contents to F/M heat resistant steel. Unfortunately, the high Si content results in a significant deterioration of impact toughness. Determining how to balance heat corrosion resistance and impact toughness has been a challenge. In this paper, we optimize austenite isothermal hot-rolling (AIHR) technique to the 10Cr1SiY F/M dual-phase heat resistant steel with 1% Si(wt.), to obtain layered microstructure with thread-like δ-ferrite. This decreases ductile-brittle transition temperature (DBTT) from 149 °C to 15 °C, and increases impact energy from 8 J to 107 J, compared to the quenched and tempered steel with the same chemical composition. According to the instrumented Charpy impact tests, higher impact energy derives from the absorbing energy during crack propagation, accounting for 94.9% of the total absorbing energy. The weak interfaces in the layered microstructure effectively deflect cracks, while the {110} slip plane induced by strong textures from multi-pass rolling promoted dislocation slip, thus rendering plastic deformation and weakening the crack tip stress; synergistic interaction is responsible for the improved toughness, resulting in the delamination fracture of the steel. The reduced grain size and M23C6 phases also contributes to the improved toughness.

Comparison of the Mechanical Properties of Hardfacings Made by Standard Coated Stick Electrodes and a Newly Developed Rectangular Stick Electrode.

Cladding with a stick electrode is one of the oldest arc processes for adding a deposit on a base material. The process is suitable for outdoor working, but the disadvantages are low productivity and large dilution rates. In this work, a simple solution is proposed, which would enable cladding of a larger area with one pass and decrease the dilution rate at the same time-a new type of electrode was developed, exhibiting a rectangular cross-section instead of a round one. Hardfacings, welded with E Fe8 electrodes according to EN 14 700 Standard were welded on mild steel S355 J2 base material with three different coated stick electrodes. The first one was a commercially available, standard, round hardfacing electrode, the second was the same, but with a thinner coating, and the third one was a newly developed rectangular electrode. All three types had equal cross-sections of the metallic core and the same type of coating. Manufacturing of the rectangular electrodes in the laboratory is explained briefly. One- and multi-layer deposits were welded with all three types. Differences were observed in the arc behavior between the round and rectangular electrodes. With the rectangular electrode, the microstructure of the deposit was finer, penetration was shallower, and dilution rates were lower, while the hardness was higher, residual stresses predominantly compressive, and the results of instrumented Charpy impact tests and fracture mechanics tests were better.

Toughness behavior and deformation mechanisms in FCC-based Fe45Co30Cr10V10Ni5-xMnx high-entropy alloys: Insights from instrumented Charpy impact tests

This study investigated toughness properties of FCC-based Fe45Co30Cr10V10Ni5-xMnx high entropy alloys (HEAs). Ductile-dimpled rupture mode in all alloys and test temperatures complicated precise explanations on Charpy impact energy, but the instrumented Charpy test provided a breakdown of total energy (ET) into initiation energy (EI) and propagation energy (EP), offering critical insights into Mn-content- and temperature-related energy variations. Pmax and TP, reflecting maximum flow stress and time, respectively, until initiation of crack propagation from the notch tip, involved a transition of deformation mechanisms from slip to TWIP, then to BCC-TRIP, thereby inducing increased strain hardening and delaying plastic instability at the notch tip. At 25 °C, an increase in Mn content prompted a transition in deformation behavior from slip to TWIP and subsequently to BCC-TRIP, correlating well with increased Pmax and TP and consequently ET due to enhanced strain hardenability. At −196 °C, predominant BCC-TRIP activity in all alloys led to increased formation of BCC martensite, intensifying strain hardening and requiring higher Pmax for crack initiation than at 25 °C. Due to minimal slip line field formation associated with reduced plastic deformation, however, cracks initiated directly from the notch-tip center, representing fast initiation of crack propagation and consequently reduction in EI and ET. Thus, utilizing parameters from instrumented Charpy tests, including Pmax, TP, EI, EP, and ET, provided insights into fracture phenomena and their interrelations in the present HEAs.

Effect of a Gradient Temperature Rolling Process on the Microstructure and Mechanical Properties of the Center of Ultra-Heavy Plates

As there is a small amount of deformation in the center during the rolling process of ultra-heavy plates, it is extremely easy to cause poor mechanical properties in the center. Increasing the deformation in the center is the most feasible method to eliminate the deformation effects in the cross-section of ultra-heavy plates. In this study, the gradient temperature rolling (GTR) process is compared with the traditional uniform temperature rolling (UTR) process. It is found that the GTR process can significantly increase the deformation in the center and thereby refine the grains.. The room temperature tensile test and instrumented Charpy impact test are used to test the strength at room temperature and impact energy at low temperature. Combined with the obtained impact load/energy displacement curve, the deformation and damage process under impact load are analyzed. The microstructure morphology and impact fracture obtained by different rolling processes in the center are analyzed by experimental methods such as OM, SEM, EBSD, etc. The prior austenite grain (PAG) boundary morphology is analyzed and the densities of grain boundaries are statistically quantified. The results showed that the strength, plasticity, and low-temperature toughness of the GTR process are improved compared to the UTR process, with increased dislocation density in the center microstructure, the density of PAG boundaries, and the density of packet boundaries. The size of the PAG in the center is refined by ~49%, the density of PAG boundaries increased by ~140%, the density of high-angle packet boundaries increased by ~39%, and the density of low-angle packet boundaries increased by ~49%. The crack propagation in the instrumented Charpy impact test of the GTR process showed stable expansion, indicating a ductile fracture compared to the semi-brittle fracture of the UTR process. The densities of PAG boundaries and high-angle packet boundaries are the most important factors affecting the strength and low-temperature toughness.

Assessing the master curve reference temperature T0 estimated from the Charpy impact transition curve and consequences on the correlation between Charpy DBTT- and fracture toughness T0-shifts

Integrity assessment of reactor pressure vessels of light water reactors follows well established procedures relying on the surveillance program and defined by codes, standards and nuclear regulatory requirements. These procedures were developed in times where scientific knowledge was limited increasing therefore conservative safety margins to overcome the scientific shortcomings.Taking profit from the development of a number of test techniques and evaluation, it is possible nowadays to propose an alternative interpretation of the test results that provides a complementary information to the standard regulatory approach. In this work, the focus is on the Charpy impact test as it plays a critical role in surveillance programs monitoring reactor vessel degradation induced by neutron irradiation. The development of methodologies allowing experimental determination of fracture toughness from small size specimens triggered alternative approaches of extracting fracture toughness from the Charpy impact test. One of those approaches was recently successfully developed using the instrumented Charpy impact test from which the dynamic and static fracture toughness can be estimated with a reasonably good accuracy. This paper aims to provide an equally performant but simplified procedure allowing estimation of the master curve reference temperature, T0, from the Charpy impact transition curve applied to a large variety of reactor materials and irradiated conditions.

Testing of Post‐fire Brittleness for Certain Standard Duplex and Lean Duplex Stainless Steel Grades

AbstractThe results of post‐fire brittle cracking susceptibility tests conducted on samples made of selected duplex type stainless steel grades exhibiting austenitic‐ferritic structure, in particular on standard duplex X2CrNiMoN22‐5‐3 steel and on lean duplex X2CrMnNiN21‐5‐1 steel, are presented and widely discussed. General conclusions are based on the force‐displacement graphs obtained by the instrumented Charpy impact test. Prior to the test all samples have been subjected to simulated fire action following the steady‐state heating regime and then cooled down to room temperature. Two heating levels have been considered, namely 600°C and 800°C. The first of these levels has been considered as to low, while the second as sufficiently high, to induce in the considered steel structural changes of permanent character. For comparison, two groups of samples have been tested. Samples belonging to the first group have been heated for one hour (simulation of a short fire), while the ones belonging to the second group have been heated for ten hours (simulation of a long fire). In addition, two alternative cooling modes have been considered, i.e. slow cooling in the muffle furnace and rapid cooling in water mist. The toughness tests on samples made of both steel grades have been conducted at two different temperature values, that is at +20°C to simulate the continued service in summer conditions and at ‐20°C to simulate this future service in winter conditions.

On the impact toughness of amorphous/crystalline metallic laminates

The creation of amorphous/crystalline (A/C) laminated structures offers a promising approach towards the production of damage-tolerant A/C metallic materials. However, constraints in fabrication techniques have resulted in limited explorations into the toughening behavior of bulk A/C laminates. This study scrutinizes the impact toughness of macroscale A/C Ni–P/Ni laminates through instrumented Charpy impact testing. The findings reveal that the thickness of the amorphous Ni–P layer significantly influences the impact toughness of A/C Ni–P/Ni laminates. A/C Ni–P/Ni laminates possessing thin Ni–P layers (1 or 4 μm) demonstrated high impact energy, which was comparable to the monolithic coarse-grained (CG) structure; however, the impact energy decreased with the increasing Ni–P layers thickness. Microstructural characterization of the crack-path profile disclosed that the desired impact toughness in the A/C Ni–P/Ni laminate with a thin Ni–P layer is primarily due to intrinsic toughening induced by plastic deformation ahead of the crack tip, and extrinsic toughening afforded by interface delamination. As the Ni–P layer thickens, brittle interface delamination emerges as the dominant extrinsic toughening source to resist fracture, thereby rendering the A/C Ni–P/Ni laminate vulnerable to impact loading. This work not only elucidates the dependence of toughening mechanism on the Ni–P layer thickness in A/C Ni–P/Ni laminates but also proposes an effective strategy for designing damage-tolerant metallic materials subject to impact loading by incorporating thin, brittle amorphous and thick, ductile crystalline materials to create laminated structures.

Comparison of RSW technologies on DP steels with modified instrumented Charpy impact test

In recent decades the toughness of welded structures becomes more and more important. This trend is particularly valid for the advanced high-strength steels (AHSS), which have reduced toughness comparing to the low-strength structural steels. Dynamic characteristics of the welded joints of the newly developed high-strength steel sheets required by the automotive industry have been neglected, with welding procedures being optimized on the static joint properties, mainly on the tensile-shear force. Up to the present time, testing of dynamic properties of the spot-welded joints has been performed by increased testing speed during tensile testing. The authors have developed a new dynamic testing method and designed new testing equipment for impact bending, which can give a numeric result to characterize the resistance of spot-welded joints against dynamic load. In this paper, this method will be used to evaluate resistance spot-welded joints made on DP600 and DP800 steels with three different technological parameters, comparing long-time welding, short-time welding, and two-pulse welding. The different parameters cause different weld nugget sizes, failure modes, impact energies, and force–time diagrams. All test results show that bigger weld nugget diameters cause better impact energies and impact forces, but the differences are not perfectly correlated.

The effect of gradient order on impact toughness of carbon S38C axle steels with induction hardening

Induction hardened S38C axles exhibit the balance between good crack growth resistance at the body and wear resistance at the wheel seat, which can be physically attributed to the gradient structure with fine grains (FG) near the surface and coarse grains inside the base metal (BM). However, the impact toughness of such bulk gradient engineering structure is less reported particularlly when experiencing high-speed impacts. Therefore, the impact behaviors of the S38C steel are thoroughly examined by using the V-notch instrumented Charpy impact testing in this work. It is clearly found that the gradient order has a significant effect on the impact response. For the gradient structure termed as FG → BM, the absorbed energy is significantly lower than the BM structure but the higher strength and hardness, which indicates that the FG → BM structure is difficult to tailor the fatigue crack growth and the resulting fatigue life of railway axles. By contrast, the BM → FG structure exhibits more excellent impact resistance primarily owing to the micropore coalescence. To acquire the exceptional damage tolerance, the inner bore and external surface of the S38C axles can be sequentially strengthened to acquire an integrated gradient structure.

Effective grain size dependence of crack propagation resistance in low carbon steel

Unlike numerous studies focused on the crack initiation occurred in steel materials, the influence of effective grain size on crack propagation resistance does not receive enough attention and the intrinsic mechanism is not elucidated. In this work, comparable matrix microstructures with varying effective grain sizes were designed using different weld thermal simulations. Instrumented Charpy impact tests were conducted on these specimens to document the variation of fracture behaviors. The results showed that the impact toughness changes notably regardless of their similar matrix microstructure. By sub-dividing the total impact absorbed energy into two parts based on the maximum load, we find a new inverse proportional relation between the effective grain size and energy dissipation at the crack propagation stage. This new function, which is akin in form to the classical RKR model, may have board applications for optimizing the fracture behavior because it explicitly displays the effect of metallurgical factors on crack propagation resistance. This relation further implies that the crack propagation resistance is more strongly dependent on the grain size compared to the crack initiation resistance. With focus on the brittle crack propagation stage, a sharp decrease in brittle cracking speed for fine-grained specimens is attributed to the mechanism that numerous tear-ridges and side ligaments formed by local plastic deformation exert a closure stress at the wake of crack tip.

Insights into the Impact Behavior and Deformation Substructure Evolution of the N‐Bearing QN1803 and 304 Stainless Steels

The impact behaviors of N‐bearing QN1803 and 304 stainless steels are studied using instrumented Charpy impact test in the range of −192–100 °C. The fracture and deformation substructures are systematically investigated by scanning electron microscopy and transmission electron microscopy. The results show that QN1803 presents ductile–brittle transition temperature (DBTT) and 304 shows ductile fracture. With increasing temperature, the stacking fault energy (SFE) of QN1803 and 304 increases, but the former is always lower than the latter. QN1803 presents low impact toughness with low‐density dislocation at −192 °C. At high temperature, QN1803 and 304 show good impact toughness and the deformation substructures are dominated by high‐density dislocations and deformation twinning.

Remarkable strength-impact toughness conflict in high-strength Al-Mg-Sc-Zr alloy fabricated via laser powder bed fusion additive manufacturing

Laser powder bed fusion (LPBF) of AlMgScZr alloys have a combination of high strength and good ductility, which may apply to damage-tolerant structures. However, there have been only limited studies on the toughness performance of this promising alloy system. In the present work, the impact toughness of an LPBF-processed AlMgScZr alloy, which is critical for the application that involves high-strain-rate loading, was systematically investigated using instrumented Charpy impact testing. The results demonstrated that the AB(as-built)-LPBF-processed AlMgScZr alloy had a good combination of yield strength and impact toughness (338 ± 3 MPa, 27.47 ± 1.04 J), which were about 2 and 4 times those of the AC(as-cast)-processed AlMgScZr alloy (160 ± 1 MPa, 6.21 ± 0.66 J). However, post-aging treatment greatly deteriorated the impact toughness. On one hand, the strain-hardening capability of the LPBF-processed AlMgScZr alloy was reduced after post-aging; on the other hand, the coplanar slip associated with the L12 superlattice structure of the secondary Al3(Sc,Zr) phase promoted the intergranular brittle fracture. This resulted in the remarkable strength-impact toughness conflict in the AA(as-aged)-LPBF-processed AlMgScZr alloy (512 ± 2 MPa, 4.27 ± 0.09 J). This study not only provides valuable insights into the dynamic fracture resistance of the LPBF-processed AlMgScZr alloys but also sheds light on the importance of choosing whether to post-age the LPBF-processed AlMgScZr alloy based on service loading conditions.

Revealing impact failure behavior of tungsten composite using instrumented Charpy impact testing

The impact toughness of W-Ni-Fe alloys with different W content (90, 93 and 95 wt%) were studied by means of instrumented Charpy impact testing, and the relationship between impact toughness and microstructural parameters was discussed, with an emphasis on impact failure behavior. With increasing of W content, the elongation decreases from 31% for 90 W to 21% for 95 W, but the tensile strength slightly increases from 940 to 974 MPa and the yield strength increases from 655 to 688 MPa. The impact load-displacement curves reveal that the impact toughness is predominantly controlled by the crack initiation energy and decreased with increasing of W content, whereas the crack propagation energy is relatively low and remains constant. The decrease in the crack initiation energy attributed to a decrease in matrix volume fraction and an increase in W contiguity. 90 W shows the highest crack initiation energy due to the severe plastic deformation during crack initiation process, which contains multiple deformation characteristics such as dislocation tangles, dislocation forest, stacking faults, nano-scaled twins and subgrains. The formation of those features is discussed, and it is found that they are expected to provide a toughening effect. From the perspective of crack nucleation and growth, it is also found that the high crack initiation energy of 90 W profited from the high energy-consuming processes of W cleavage events, micro-crack arrest, blunting and bridging mechanisms. Although these three alloys display different failure features of crack propagation paths, they present similar crack propagation energies. The current study provides a deep insight into the impact failure behavior of WHAs.

Damage‐Tolerant Woven Glass Fiber Composites Developed Using Polyvinyl Butyral (PVB) Unsaturated Polyester (UP) Blends

The thermoset matrix is brittle and shows low damage characteristics, and their impact and damage performance can be improved significantly by blending with the thermoplastic matrix. In this way, the properties of both the matrices can be gathered in one composite. This study is focused on the development and optimization of novel blends of unsaturated polyester (UP) resin with polyvinyl butyral (PVB), a thermoplastic polymer, to improve the mechanical properties, especially delamination and impact behavior of associated glass fiber composites. The five blends of UP and PVB were prepared in different concentrations by the solution mixing method. Composite samples of woven glass fabric were fabricated using prepared blends and pure resins as matrices on compression molding. Tensile, flexural, T-peel tests, and the instrumented Charpy impact tests were conducted on the developed samples. A significant improvement in the impact energy absorption (102%) and delamination resistance (110%) was observed for a blend ratio of 40 : 60 and 50 : 50 of PVB : UP, respectively, as compared to pure UP composite samples.

Impact properties of carbon fiber/cellulose nanofiber reinforced epoxy composite using asymmetric stacking sequence of non-hydrophobic cellulose nanofiber dispersion layer

Cellulose nanofibers (CNFs) are sustainable materials with high specific strengths. To enhance the impact properties of carbon fiber reinforced plastics (CFRPs), the authors have developed an efficient process for incorporating a non-hydrophobized CNF dispersion layer into CFRPs. In this study, the impact properties of CFRP with asymmetrically stacked CNF dispersion layer was investigated by conducting the instrumented Charpy impact tests. Two types of CNFs were used for comparison. Consequently, by inserting the CNF dispersion layer into the opposite side of the striking surface, the Charpy impact value increased by 26% compared to CFRP without CNF dispersion layer.

The effects of post weld heat treatment for welded high-Mn austenitic steels using the submerged arc welding method

The high-Mn austenitic steels exhibit fully austenitic structure at room temperatures while showing twinning mechanisms during deformations. In the SAW welding process, the microstructure and mechanical properties of heat affected zone (HAZ) and weld metal (WM) are affected by the welding heat causing potential degradation of mechanical properties due to the segregation and depletion of Mn in HAZ and WM regions. In this study, post weld heat treatment (PWHT) was performed at 800 °C, 900 °C and 1000 °C to improve the mechanical properties of welded high-Mn austenitic steels. We examined electron backscattered diffraction (EBSD) and energy dispersive X-ray spectroscopy (EDS) analysis to investigate the relationship between microstructure and mechanical properties after heat treatment. In particular, mechanisms of impact toughness improvement was identified by analyzing the microstructure and deformation behavior such as mechanical twin and grain size through the instrumented Charpy impact test and EBSD analysis.

Analysis of Fracture and Toughness of a Denture Base with Polyvinyl Fibers

Fracture resistance of polymer reinforced with short fibers consists of a sum of contributions from matrix and fiber fracture, fiber de-bonding and pull-out. The existing models for predicting dependence of fracture toughness on structural variables were derived for the commercially important fiber volume fractions, i.e., for vf≥0.1. In this contribution, modification of the existing model for the dependence of the critical strain energy release rate, GIC, on the fiber type, length and aspect ratio, interfacial adhesion and volume fraction has been attempted to allow predictions at low vf < 0.10. The predictions based on the modified model were compared with experimental data on fracture toughness of lightly x-linked PMMA used to manufacture base of removable dentures toughened with short randomly oriented deformable fibers. The composite toughness was measured under impact loading to simulate typical mode of fracture of removable dentures. The GIC for composites containing short Kevlar 29, S2-glass and poly(vinyl alcohol) (PVOH) fibers were obtained using instrumented Charpy impact tests at room temperature and impact speed of 1.0 m/s. Theoretical prediction based on the proposed model and experimental results agreed reasonably well.