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.     

Effect of silicon on the fracture toughness and oxidation behavior of hot pressed NbCr2 alloys

NbCr₂ Laves phase alloyed with 0–7 wt.% Si was fabricated by mechanical alloying followed by hot pressing. The influence of silicon on the mechanical properties and oxidation behavior of NbCr₂ were investigated. It was revealed that Si addition has a beneficial effect on the oxidation resistance and fracture toughness of NbCr₂ alloy. The addition of Si partially occupies the Cr site in the Laves phase and partially forms the hard Nb₅Si₃ phase, which can yield an increase in the hardness of as-HPed NbCr₂ alloys. When alloying with 5 wt.% silicon, the fracture toughness value of NbCr₂ reaches the highest (6.45 MPa √m) which is about 13% more than that of unalloyed NbCr₂ and is 4 times higher than that of cast materials (1.2 MPa √m). Addition of silicon also resulted in a substantial improvement in the oxidation resistance of the NbCr₂ alloys exposed in air at 1373 K and 1473 K.     

An assessment of mechanical behavior and fractography study of glass/epoxy composites at different temperatures and loading speeds

Effect of loading rate on fracture and mechanical behavior of autoclave cured glass fiber/epoxy prepreg composite has been studied at various loading (striking) rates (0.01-10³ mm/min). The maximum load carrying capacity and strain at yield continuously increases with increasing loading speed. The interlaminar shear strength (ILSS) value is high at low loading speed and becomes low at high loading speed with the transition of loading rate at approximately 300 mm/min. The formation of steps, welt interfacial failure and cleavage formation on matrix resin i.e. localized plastic deformation processes were dominating mechanisms for specimens tested at low loading rates, while brittle fracture of fiber, fiber pull-out and impregnation were dominating mechanisms for specimens tested at loading rates of 800 mm/min or higher.     

Effects of Si addition on mechanical properties and superelasticity of Ti–7.5Nb–4Mo–2Sn shape memory alloy

Effects of 0–2.1 at.% Si additions on microstructure and mechanical properties of a Ni-free biomedical superelastic β-Ti alloy, Ti–7.5 at.%Nb–4 at.%Mo–2 at.% Sn (Ti–7.5Nb–4Mo–2Sn), were investigated. The alloys after annealing at 973 K mainly contain β and α″. As the concentration of Si is higher than 1 at.%, Ti₅Si₃ particles can be found in the alloys, and the number density of the particles increases with the increasing of silicon’s concentration. The addition of Si promotes the strength of the Ti–7.5Nb–4Mo–2Sn due to the Si solid solution strengthening effect and fine Ti₅Si₃ precipitates. However, as the Si concentration reaches 2.1%, the alloy exhibits a brittle fracture. The 0.5–1.6 at.% Si additions improve the superelasticity of the Ti–7.5Nb–4Mo–2Sn alloy by increasing the critical stress for inducing martensite (σ_SIM).     

Research into mechanical properties of reaction-bonded SiC composites at cryogenic temperatures

The mechanical properties of reaction-bonded silicon carbide (RBSC) composites at cryogenic temperatures have been reported for the first time. The results show that the flexural strength and fracture toughness increase from 277.93 ± 23.21 MPa to 396.74 ± 52.74 MPa and from 3.69 ± 0.45 MPa·m^1/2 to 4.98 ± 0.53 MPa·m^1/2 as the temperature decreases from 293 K to 77 K, respectively. The XRD analysis of the phase composition reveals that there is no phase transformation in the composites at cryogenic temperatures, indicating cryogenic mechanical properties are independent of phase composition. The enhancement of mechanical properties at 77 K over room temperature could be explained by the transition of fracture mode from predominant transgranular fracture to intergranular fracture and stronger resistance to crack propagation resulting from higher residual stress at 77 K. The above results demonstrate that such composites do not undergo similar deteriorations in the fracture toughness as other materials (some kinds of metals and polymers), so it is believed that such composites could be a potential material applied in cryogenic field.     

Failure analysis of Grade-80 alloy steel towing chain links

An experimental investigation of the causes of failure of chain links that occurred during towing operation of heavy-weight army vehicles is reported in this study. All failures took place at the weld area of the links after a short service life. Tensile tests on both base metal and the weld samples indicated high tensile strength of 800 MPa for both materials. However, the weld exhibited brittle fracture at relatively small strains of ε ≈ 0.05, while the base metal failed at strains ε ≈ 0.20, indicating moderate ductility. Optical metallography and scanning electron microscopy (SEM) analysis revealed that fatigue was initiated due to inherited cracks at the outer circumference of the weld. Fatigue crack propagation was evident by progressive marks and intervening striations. Their distribution was not rotationally symmetric, indicating a possibility of combined cyclic loading on the links. The large area of final rupture indicated a ductile rupture in the weld center and brittle fracture in the outer region of the weld due to overloading. The results suggest that the major causes of chain failure are as follows: high cyclic loading, weld defects, improper post-weld heat treatment, and decrease in material hardness and corrosion resistance due to insufficiency of some alloying elements.     

Failure in a high pressure feeding line of an oil refinery due to hydrogen effect

This work describes failure analysis of a feeding pipeline of an oil refinery. For this analysis visual inspection, dye penetration, optical and electron microscopy, XRD, tensile tests, fracture toughness tests and stress calculations were used. Result of the investigations show that hydrogen embrittlement has played an important role in the failure of the pipeline. For completing the case, hydrogen embrittlement damage of the piping material made from commercial ASTM A105M low alloy manganese steel was studied by using baking and hydrogenation treatments. Baking treatment was carried out at 520 °C for 20 min followed by slow cooling while hydrogenation treatment was carried out in a solution of H₂SO₄ which contained As₂O₃ for 0–6 h under cathodic situation. Then change in the mechanical properties and fracture toughness of the steel after the treatments were measured by tensile and fracture toughness tests. In addition, fractography was carried out using a scanning electron microscope (SEM) and image analyzer. Results show that the baking treatment increased elongation to failure and fracture toughness significantly and reduced yield strength slightly compared to the failed condition of the pipe. Hydrogenation treatment decreased elongation to failure and fracture toughness of the material considerably and increased strength barely. Increase in the hydrogenation time reduced ductility of the steel further more. These results indicate that hydrogen through hydrogen embrittlement mechanism, made the pipe material brittle and susceptible to cracking. Embrittlement with the assistance of an emergency shutdown and stress concentration provides damage nucleation and finally developed brittle fracture.     

Experimental and theoretical investigation on the compressive behavior of aluminum borate whisker reinforced 2024Al composites

The compressive behavior of Al₁₈B₄O₃₃w/2024Al composites fabricated by squeeze casting was investigated under low and elevated temperature. Microstructure shows that the compression exerts a significant effect on whisker fracture and rotation. The theory of synergistic effects caused by different strengthening mechanisms is used to predict the yield strength. Experiments show that compressive yield strength of composites improves by 47% compared with those of 2024Al at 623 K and agrees relatively well theoretical value. The compressive deformation depends on matrix mainly at lower temperature and the main failure mode is shear fracture. Additionally, fracture mechanisms are investigated further through fracture surface analysis. During hot compression, the predominated softening mechanisms also include dynamic recrystallization and strain softening except for dynamic recovery, which corresponds well with the shape of flow curves, microstructural observation and change of activation energy. Lastly, the optimum process parameters are determined to be about 0.1 s^− 1 and 723 K based on Dynamic Material Model and validated by microstructure evolution. Experiments show that the strain rate has a mixed effect on whisker fracture.     

Effect of pore structure on the compressive property of porous Ti produced by powder metallurgy technique

Porous Ti with an average macro-pore size of 200–400 μm and porosity in the range of 10–65% has been manufactured using polymethyl methacrylate (PMMA) powders as spacer particles. The compressive strength and elastic modulus of resultant porous Ti are observed in the range of 32–530 MPa and 0.7–23.3 GPa, respectively. With the increasing of the porosity and macro-pore size, the compressive strength and modulus decrease as described by Gibson–Ashby model. The failure due to cracking (complete fracture) of the struts on porous Ti is controlled primarily by macro-pores. Fractography shows evidence of the brittle cleavage fracture mainly, but containing a few fine shallow dimples and a small amount of transcrystalline fracture of similarly oriented laths. The failure mechanism has been discussed by taking the intrinsic microstructural features into consideration.     

Fatigue crack growth and damage characteristics of high-speed rail at low ambient temperature

Low temperature can be a significant problem affecting safety and maintenance of railway. In this study, the fatigue crack growth rate and rolling contact fatigue damage behaviors of high-speed rail material under different temperature conditions were investigated by a series of experiments. The results indicate that the stress and strength of rail material increase with the decrease of ambient temperature. The crack growth rate at 0 °C and − 20 °C is similar with that at 20 °C. While, when the temperature decreases to − 60 °C, the growth rate of crack increases sharply. The promotion of rail embrittlement at low temperature accompanied with the action of high stress causes the rapid failure and increase of surface crack length and subsurface crack damage. Meanwhile, three crack growth mechanism models at different temperatures can be inferred. The brittle fracture mode is increasingly apparent with the temperature decreasing.     

Translaminar fracture toughness: The critical notch tip radius of 0° plies in CFRP

The relationship between translaminar fracture toughness measured at initiation and specimen initial notch root radius is investigated for the translaminar failure mode of cross-ply IM7/8552 carbon/epoxy laminates. Compact tension specimens with four sizes of notch root radii were tested; the true initiation toughness of the laminate was measured from specimens with notch tip radii of ρ ? 250 μm. Testing of specimens with larger notch root radii, ρ = 750 μm, yielded an apparent toughness that was found to be 30% higher than the true toughness of the laminate. The propagation toughness corresponding to the R-curve plateau was found not to be affected by the initial notch tip radius. Investigation of the fracture surfaces of failed specimens revealed that there is no interaction between the 0° and 90° ply failure mechanisms, and that the critical notch radius is a property intrinsic to the 0° plies of the laminate.     

Simultaneously enhanced cryogenic tensile strength and fracture toughness of epoxy resins by carboxylic nitrile-butadiene nano-rubber

Epoxy resins are important matrices for composites. Carboxylic nitrile-butadiene nano-rubber (NR) particles are employed to improve the tensile strength and fracture toughness at 77 K of diglycidyl ether of bisphenol-F epoxy using diethyl toluene diamine as curing agent. It is shown that the cryogenic tensile strength and fracture toughness are simultaneously enhanced by the addition of NR. Also, the fracture toughness at room temperature (RT) is enhanced by the addition of NR. On the other hand, the tensile strength at RT first increases and then decreases with further increasing the NR content up to 5 phr. 5 phr NR is the proper content, which corresponds to the simultaneous enhancements in the tensile strength and fracture toughness at RT. Moreover, the comparison of mechanical properties between 77 K and room temperature indicates that the tensile strength, Young’s modulus and fracture toughness at 77 K are higher than those at RT but the failure strain shows the opposite results. The results are properly explained by the SEM observation.     

Fatigue properties and fracture analysis of a SiC fiber-reinforced titanium matrix composite

Tension–tension fatigue properties of SiC fiber reinforced Ti–6Al–4V matrix composite (SiC_f/Ti–6Al–4V) at room temperature were investigated. Fatigue tests were conducted under a load-controlled mode with a stress ratio 0.1 and a frequency 10 Hz under a maximum applied stress ranging from 600 to 1200 MPa. The relationship between the applied stress and fatigue life was determined and fracture surfaces were examined to study the fatigue damage and fracture failure mechanisms using SEM. The results show that, the fatigue life of the SiC_f/Ti–6Al–4V composite decreases substantially in proportion to the increase in maximum applied stress. Moreover, in the medium and high life range, the relationship between the maximum applied stress and cycles to failure in the semi-logarithmic system could be fitted as a linear equation: S_max/μ = 1.381 − 0.152 × lgN_f. Fractographic analysis revealed that fatigue fracture surfaces consist of a fatigued region and a fast fracture region. The fraction of the fatigued region with respect to the total fracture surface decreases with the increase of the applied maximum stresses.     

Fabrication of ZK60 magnesium alloy thin sheets with improved ductility by cold rolling and annealing treatment

Thin ZK60 magnesium alloy sheets with ultrafine-grain structure were successfully fabricated by continuous cold rolling with proper intermediate annealing treatments at 503–523 K for 30 min. Meanwhile, microstructure uniformity and planar texture anisotropy were strikingly improved by rolling deformation and static recrystallization, resulting in continuous improvement in the strength anisotropy. Excellent ductility more than 30% in fracture elongation was achieved after further annealing treatment at a lower temperature of 473 K. This was primarily attributed to the significant weakening of the {0 0 0 2} pole intensity and grain refinement during the process. It is shown that mechanical properties of the final sheets could be closely controlled by the present process.     

Stress wave radiation from the cleavage crack extension in 3D bcc iron crystals

We hereby present new results for molecular dynamic (MD) and finite element (FEM) simulations in 3D bcc iron crystals, with embedded central through crack (0 0 1)[1 1 0] of Griffith type, loaded in mode I. The sample geometry and border conditions in MD were chosen in such a way as to invoke a cleavage crack extension. The stress field and acoustic emission sources caused by the crack were analysed on both the atomistic and continuum level.     

Impact of thick PMMA plates by long projectiles at low velocities. Part II: Effect of confinement

A hybrid experimental–numerical investigation of the penetration process in unconfined and confined thick polymethylmethacrylate (PMMA) plates was carried out. The confinement was applied by insertion of the polymeric plate into a conical steel ring. The response of such plates to the impact of long hard steel projectiles having an ogive-head shape in the range of velocities of 165 < V₀ < 260 (m/s), was investigated experimentally. The results show that unconfined targets were perforated and broken due to combined effect of penetration and cracking. By contrast, the confined targets were not perforated and could withstand repeated impacts due to suppression of the brittle damage mechanism by the confinement. The tests were modeled using 3D explicit finite element analyses. A good agreement regarding the trajectory of the projectile and the depths of penetration was obtained. The numerical results show that the confinement introduces a negative triaxiality and even some plasticity within the confined plates prior to impact. The increase of plastic failure strain of the PMMA at negative triaxiality reduces the ductile damage during penetration, while the hydrostatic pressure reduces significantly the brittle fracture mechanism. The resisting force to the penetration depends on the failure strain–triaxality relationship, and does not necessarily increase with higher confinement levels.     

Effects of frequency on fatigue behavior of CVI C/SiC at elevated temperature

Effects of frequency on fatigue behavior of a chemical vapor infiltrated carbon fiber reinforced silicon carbide composite (C/SiC) were investigated at an elevated temperature of 550 °C. Tension–tension fatigue tests were conducted at three frequencies: 0.1, 10 and 375 Hz to establish stress versus cycles to failure (S–N) relationships. There was an increase in cycles to failure at a given stress level as frequency increased from 0.1 to 375 Hz at elevated temperature. This trend was different at room temperature where cycles to failure decreased when frequency changed from 40 to 375 Hz but remained almost same below 40 Hz. There was a reduction in cycles to failure at frequencies less than 40 Hz but cycles to failure remained same at a higher frequency of 375 Hz when test environment changed from room temperature to 550 °C. Analysis of damage mechanisms showed that the oxidation of carbon fibers was the major difference between the room and elevated temperatures, which caused a reduction in cycles to failure with lower frequencies at elevated temperature in comparison to that at room temperature. However, oxidation of carbon fibers was almost absent or negligible at higher frequency at elevated temperature, which caused practically no reduction in cycles to failure at elevated temperature in comparison to their counterparts at room temperature.     

Effect of the thermal cycle on the hot ductility and fracture mechanisms of a C–Mn steel

Transverse cracking on the surface of continuously cast steel products has been one of the main problems of this stage in steelmaking for many years. The incidence of this problem has been found in microalloyed steels as well as in some plain carbon steels containing residual elements. In this work, the hot ductility and fracture mechanisms of a C–Mn steel containing 0.6%Cu and 0.053%Sn as residual elements have been evaluated. To simulate the thermo-mechanical conditions of the straightening operation, tensile tests were carried out at temperatures ranging from 700 to 1100 °C with an initial strain rate of 5 × 10⁻³ s⁻¹. Specimens were subjected to three different reheating temperatures prior to the hot ductility test, including 1100 °C, 1330 °C and melting. After each test, the reduction in area of the samples tested to fracture was used as a measure of the hot ductility. The fracture surfaces were then examined by scanning electron microscopy. The widest and deepest ductility trough was obtained for the specimens tested after melting; for these conditions brittle fractures are interdendritic showing very low ductility. After reheating at 1330 °C, fracture features showed intergranular fracture combined with some plastic deformation corresponding to the test temperature. Reheating at 1100 °C produced a finer microstructure and the fracture features showed a mixture of intergranular with some interdendritic features. Also, ductile behaviours were associated with void coalescence. The different results obtained depending on the thermal cycle can be attributed to the presence of the residual elements in the steel through different segregation and precipitation patterns.     

Short shaped copper fibers in an epoxy matrix: Their role in a multifunctional composite

Previous research indicates that short shaped copper fibers improve the fracture and impact toughness of brittle thermoset polymer matrix composites. This paper investigates the potential multifunctional ability of these same shaped copper fibers by determining their electromagnetic interference (EMI) shielding effectiveness (SE). Fiber shapes were selected based on previous single fiber pullout experiments where they displayed high toughness. The two fiber diameters tested were: 0.325 and 0.162 mm. Fiber shapes used in the experiments were: straight, flat end-impacted, rippled, and acid roughened. A SE of greater than 45 dB at 1.0 GHz was attained in epoxy that contained 15 vol% of 0.162 mm diameter shaped fibers. Composites with 15 vol% of the 0.325 mm diameter shaped fibers showed poor SE, less than 20 dB. Experimental results indicate that besides improving the fracture and impact toughness of a thermoset polymer matrix, short shaped copper fibers can also significantly improve the SE and electrical conductivity of the composite, resulting in a multifunctional material. This increase in SE and electrical conductivity can be attributed to: shape effects that increase the skin volume, surface discontinuities which increase the amount of electromagnetic (EM) wave scattering, and the fiber count which determines the number of conducting paths.     

Taylor impact of polyether ether ketone

Right cylinders of the thermoplastic polyether ether ketone have been impacted onto a hardened steel anvil at velocities from 152 to 408 m s⁻¹. The resultant deformation showed the expected ‘mushrooming’ behaviour up to impact velocities of 303 m s⁻¹, before the impact face began to fracture. No evidence of brittle failure was observed, but rather a ductile process was noted. Discolouration behind the impact face gave evidence of high temperatures in this region, believed to be due to adiabatic heating as the material flowed across the impact face. The concave nature of the impact face after recovery showed that the material had relaxed after the loading had been removed, mostly likely due to the viscoplastic nature of the material.