The effect of loading rate and temperature on fracture initiation in 1020 hot-rolled steel

Fracture initiation experiments were conducted to determine the fracture toughness of an AISI 1020 hot-rolled steel under quasi-static and dynamic loading conditions. The dynamic tests were performed on specimens consisting of notched round bars loaded in tension by means of a stress pulse, thus giving an effective value of $\text{/.K}{}_{\mathrm{I}}\text{= 2 · 10}{}^6\text{ksi √in. s}{}^{\mathrm{?1}}\text{(2.2 · 10}{}^6\text{MPa √m s}{}^{\mathrm{?1}}\text{)}$. The tests were conducted over the temperature range ?157 to 121°C, and results are compared to those obtained in quasi-static tests with like specimens. Parallel tests were conducted to determine static and dynamic flow stresses of the material over the temperature range. Finally, a comparison is made with the behavior of an AISI 1018 cold-rolled steel previously tested in the same apparatus.     

The fracture behaviour of structural adhesives under high rates of testing

Structural adhesive joints were subjected to high loading rates in mode I and their resulting fracture behaviour was studied in detail. Joints were formed between unidirectional carbon-fibre epoxy composites and between aluminium alloy substrates bonded with a tough, single-part automotive adhesive (XD4600) from Dow Automotive. Double cantilever beam (DCB) and tapered double cantilever beam (TDCB) tests were performed, from quasi-static loading rates up to 15 m/s, and a test rig was developed incorporating high-speed video acquisition for the high-speed tests. A detailed data reduction strategy was developed to account for (i) the types of different fracture behaviour regimes encountered, (ii) the dynamic effects in the test data, and (iii) the contribution of kinetic energy in the specimen arms to the energy balance. Using the above data reduction strategy, increasing the test rate over six decades (from 10⁻⁵ to 10¹ m/s) was found to lead to a reduction in the value of the adhesive fracture energy, G_Ic, by about 40% of its quasi-static value, i.e. from 3.5 to about 2.2 kJ/m². Further, at quasi-static loading rates, the measured adhesive fracture energies were independent of substrate material and test geometry (i.e. DCB or TDCB). However, at faster loading rates, the TDCB tests induced higher crack velocities for a given loading rate compared with the DCB test geometry, and neither the test rate nor the crack velocity were found to be the parameter controlling the variation in G_Ic with increased test rate. Thus, an isothermal–adiabatic model was developed and it was demonstrated that such a model could unify the DCB and TDCB test results. Indeed, when the G_Ic values were plotted as a function of 1/√time, where the time was defined to be from the onset of loading the material to that required for the initiation of crack growth, the results collapsed onto a single master curve, in agreement with the isothermal–adiabatic model.     

The fatigue crack growth of a ship steel in seawater under spectrum loading

Fatigue crack growth of ABS EH36 steel under spectrum loading intended to simulate sea loading of offshore structures in the North Sea was studied using fracture mechanics. A digital simulation technique was used to generate samples of load/time histories from a power spectrum characteristic of the North Sea environment. In constant load-amplitude tests, the effects of specimen orientation and stress ratio on fatigue crack growth rates were found to be negligible in the range 2 × 10^?5 to 10^?3 mm/cycle. Fatigue crack growth rates in a 3.5% NaCl solution were two to five times faster than those observed in air in the stress intensity range 25 to 60 MPa √m. The average fatigue crack growth rates under spectrum loading and constant-amplitude loading were in excellent agreement when the fatigue crack growth rate was plotted as a function of the appropriately defined equivalent stress intensity range. This procedure is equivalent to applying Miner's summation rule in fatigue life calculations.     

An investigation of the loading rate dependence of the Weibull stress parameters

This paper examines the dependence of the Weibull stress parameters on loading rate for a 22NiMoCr37 pressure vessel steel. Extensive fracture tests, including both quasi-static and dynamic tests, are conducted using deep- and shallow-cracked SE(B) specimens. The fracture specimens are carefully prepared to ensure the crack fronts are placed at the location where the material is homogeneous. Three dynamic loading rates (in terms of the stress intensity factor rate, in the low-to-moderate range are considered. The load-line velocities for the dynamic tests are chosen so that the resulted values for the deep- and shallow-cracked specimens are the same. Independent calibrations performed at each loading rate (quasi-static and the three dynamic loading rates) using deep- and shallow-cracked fracture toughness data show that the Weibull modulus, m, is invariant of loading rate. The calibrated m-value is 7.1 for this material. Rate dependencies of the scale parameter (σ_u) and the threshold parameter (σ_w-min) are computed using the calibrated m and the results indicate that σ_u decreases and σ_w-min increases with higher loading rates. The demonstrated loading rate invariant of m, when combined with the master curve for dynamic loading, can provide a practical approach which simplifies the process to estimate σ_u as a function of loading rate.     

An experimental investigation into quasi-static and fatigue damage development in bolted-hole specimens

An extensive experimental program has been carried out to investigate and understand the sequence of damage development throughout the life of bolted-hole composite laminates under quasi-static loading and tension–tension fatigue. Quasi-isotropic carbon/epoxy laminates, with stacking sequence [45₂/90₂/-45₂/0₂]_S defined as ply scaled and [45/90/-45/0]_2S defined as sub-laminate scaled, were used. Specimens were cycled at 5 Hz with various amplitudes to 1 × 10⁶ cycles unless failure occurred prior to this limit. For all cases an R ratio of 0.1 was used. Bolt washer pressures of 23 MPa and 70 MPa were investigated. For the ply-level case, the quasi-static test showed both delamination and fibre-dominated pull-out failures for a washer pressure of 23 MPa, and pull-out failure only for 70 MPa. Delamination dominates in fatigue tests. For the sub-laminate case the tests failed by pull-out in both quasi-static and fatigue tests for all washer pressures. It is shown in this paper how the role of delamination is critical in the case of fatigue loading and how this interacts with bolt clamp-up forces. A number of tests were analysed for damage using X-ray CT scanning and comparisons of damage are made with tests from previous open-hole studies.     

Dynamic fracture of bovine bone

True clinical fracture of bones in bovine, race horses or humans occur predominantly during impact loading (e.g. car accidents, falls or physical violence). Although static fracture tests provide an estimate of fracture toughness or R-curve behavior in bones, the static toughness values may be ill suited for predicting failure under dynamic loading conditions due to the visco-elastic response of bone (i.e. strain rate dependent properties). Despite decades of the study on deformation rate dependency of bone properties such as compression and fracture toughness, high-quality dynamic fracture data remain limited. Preliminary tests (compression and fracture toughness) have been conducted on dry and wet bovine bone under both static and dynamic loading conditions. While compression tests have been conducted with loading direction parallel and perpendicular to the bone axis (longitudinal and transverse, respectively), fracture tests were performed only in the transverse direction. The strain rate in compression tests varied between 10^− 3 and 10³ s^− 1, and the stress intensity rate varied between ∼10^− 3 and 10⁵ MPa√m/s. While low strain rate tests were conducted on conventional mechanical testing machines, high strain rate experiments were conducted on a split-Hopkinson bar under compression and a novel three-point bend configuration. The fracture morphology and the extent of damage of bone in each case were characterized using SEM, and an attempt is made to relate these to the rate dependent fracture toughness of the bone. It is believed that such understanding is crucial for mechanistic interpretation of bone fracture phenomenon and eventually for predicting bone failure reliably.     

CRACK NUCLEATION AND PROPAGATION IN BLADE STEEL MATERIAL

Stress corrosion cracking and corrosion fatigue of 12 Cr steel in sodium chloride solution has been investigated. Tests have been performed in air at room temperature and in aqueous solution with 22% NaCl at 80°C. The influence of corrosion pits on crack nucleation has been investigated. On fracture surfaces tested in environment (22% NaCl solution), crack initiation was observed in correspondence with corrosion pits; in this case fatigue life can be described using a fracture mechanics approach. The ΔK value for crack nucleation from a pit in rotating bending fatigue tests is very low in air (about 3 MPa√m). The results of slow strain rate tests on smooth specimens show that there is a threshold stress intensity, K_ISCC, of about 15 MPa√m and a plateau in stress corrosion crack growth rate of about 10^-5mm/s.     

Abnormal da/dN – ΔK curves: New failure mode with Ti – alloys

Fatigue crack propagation (FCP) -rates and -threshold values have been determined on the titanium alloys IMI 834, IMI 685 (turbine disk materials) and Ti-6Al-6V-2Sn (plate material). K_max-constant tests were executed in laboratory air at room temperature and run with 50 Hz on C(T) specimens. It was found that FCP-rates in K_max-constant tests followed the well known FCP behavior up to a certain limiting value K_max, denoted as °K_max. Below °K_max, the FCP-rates da/dN decrease with decreasing ΔK down to the threshold value ΔK_T (ΔK for 10^?7 mm/cycle). For K_max-constant tests with K_max > °K_max, the FCP-rates initially decreased with decreasing ΔK, but reached 10^?7 mm/cycle at smaller ΔK. For K_max ≧ ∧K_max > °K_max, FCP-rates of 10^?7 mm/cycle were never reached as ΔK decreased to and below ΔK_T. Instead, as ΔK approaches or gets smaller than ΔK_T, the FCP-rates stay either constant or increase again. The limit value °K_max for this abnormal FCP-behavior had been determined for IMI 834 to be 22 to 28 MPa√m, for IMI 685 to be 46 MPa√m and for Ti-6A1–6V-2Sn to be 26 MPa√m. The important result from a practical stand-point is the large difference in °K_max for comparable Ti alloys, i.e., IMI 834 and IMI 685.     

FATIGUE BEHAVIOR OF A RAIL STEEL

The fatigue behavior of a hot-rolled, control-cooled, plain carbon eutectoid rail steel has been characterized. The data include monotonic and cyclic stress-strain curves, low cycle fatigue data and near-threshold fatigue crack growth rate behavior in air and in vacuo. The effects of environment and mean stress on the near-threshold fatigue crack growth rates of rail steel are significant. At a low stress ratio (R), ΔK_o is lower in vacuum (7 MPa √m) than in moist air (10 MPa √m). At high R, ΔK_o is higher in vacuum (6 MPa √m) than in air (4 MPa √m). The beneficial effect of moist air on FCGR at low ΔK and low R is attributed to an increase in closure due to fracture surface roughness and oxide film.     

Fracture Toughness Determination for Aluminium Alloy 2011-T6 using Tensile Notched Round Bar (NRB) Test Pieces

While the use of notched round bar (NRB) test pieces to determine planestrain fracture toughness K _Ic is not novel, relatively few authors have so far attempted this. This letter serves to report a series of such tests conducted on Al 2011-T6, a high mechanical strength, free-machining aluminium alloy. A total of 9 specimens was used, comprising 3 specimens each of 3 different notch root radii, ρ. No fatigue precracking was carried out. A graph of apparent fracture toughness K _ρ versus ρ / D was extrapolated to zero, corresponding to a fatigue pre-cracked configuration. The resulting K _Ic for this alloy tested using the NRB specimens was thus found to be 27.9 MPa√m. This is a valid K _Ic value, as there is only a 6.8% difference between it and the median value (26.0 MPa√m) determined previously from 67 tests using the existing ASTM E399 standard. A new geometric correlation based on ρ, D and d is proposed to linearly extrapolate K _ρ values, measured on NRB specimens with arbitrary geometries (but assuming plane-strain conditions hold), back to a single value of K _Ic.     

Stress–strain relationships of polycarbonate over a wide range of strain rate,including a shock wave regime

Stress–strain relationships of polycarbonate (PC) are determined over a very wide range of strain rates, including a shock wave regime. Plate impact tests, drop-weight tests, and quasi-static tests using universal and Instron testing machines are used for the high strain rate (10⁷ s⁻¹), medium strain rate (10² s⁻¹) and low strain rate (10⁻⁴ s⁻¹) tests, respectively. A newly modified unsteady wave sensing system (NM-UWSS) for plate impact tests is developed to determine the stress–strain relationships of PC. The system consists of a powder gun for plate impact tests, three embedded polyvenyliden fluoride (PVDF) gauges, and NM-UWSS. As originally proposed, UWSS is aimed at obtaining experimental inputs for the Lagrangian analysis used in determining the dynamic behavior of materials. We revise this standard system (UWSS) twice to gain a higher time resolution. In the past, the conventional charge mode (Q₂ method) was used. The first modified system (M-UWSS) has been used to study two classes of materials: (1) metallic materials and (2) polymeric materials, where the Q₁ method coupled with a transient differential equation for the equivalent circuit of the measurement circuit for the PVDF stress gauge was used. The latest method (Q_t method) for gaining the highest time resolution of shock wavefront structure by considering the effects of a piezofilm's thickness is proposed for PC at particle velocities of up to 1 km/s. Here we show from basic equations of piezoelectricity that the charge density q, i.e., the charge release per unit area, of the active electrode is proportional to the ratio of the thickness of the shocked region to the total thickness of the piezofilm. It is demonstrated that the rise time of shock charge density q in the piezofilm induced by such shock in the Q₂, Q₁ and Q_t methods, in this order, is becoming much shorter. The latest Q_t method has the highest accuracy among these three methods. Power law relations between stress and strain rate are observed again with PC under conditions of uniaxial strain over a very wide range of strain rates, i.e., 10⁻⁴–10⁷ s⁻¹ including a shock wave regime. For the PC, the effects of strain rate on the stress–strain relationships are estimated using empirical formula.     

Analysis of dynamic conditions during thermal transient events for the structural assessment of a nuclear vessel

The possibility of dynamic loading conditions in a reactor pressure vessel (RPV) has been investigated in this paper. For this purpose, finite element (FE) numerical simulations of several thermal transients were performed including a normal shut down and two accidental thermal shocks, namely a loss of coolant accident (LOCA) and an extreme postulated pressurised thermal shock (PTS). The aim of the present contribution is to evaluate the influence of the loading rate on the fracture properties of the vessel steel of the Santa María de Garoña Spanish nuclear power plant (NPP) in the ductile to brittle transition (DBT) region. To describe the fracture behaviour of the steel in the DBT region, the master curve (MC) reference temperature, T₀, was used. This temperature is normally used for quasi-static conditions; however, it has been recently extended to the determination of dynamic fracture toughness by means of a phenomenological model proposed by Wallin. The dynamic reference temperature, T_0,dyn, was obtained for the loading rates corresponding to the three studied situations numerically simulated and compared with the quasi-static reference temperature, T_0,sta. From these results, conclusions about the importance of loading rates in nuclear vessels were established.     

Einfluss von Gussfehlern auf die Dauerfestigkeit von Aluminium‐ und Magnesiumgusslegierungen

Influence of casting defects on the endurance limit of aluminium and magnesium cast alloys The influence of porosity (voids and shrinkage) on the fatigue properties at very high numbers of cycles is shown for the alloys AZ91 hp, AM60 hp, AE42 hp, AS21 hp and AlSi9Cu3 produced by high pressure die casting. Fatigue tests performed with ultrasonic equipment up to 10⁹ cycles show that these alloys exhibit a fatigue limit. The mean endurance limits (50% failure probability) of the magnesium alloys are 8–50 MPa and of the aluminium alloy 75 MPa. Fatigue cracks initiate at porosity, and whether a specimen fractures or not depends on the stress amplitude and the area and the site of the defect. Regarding the cast defect as an initial crack, a critical stress intensity value (K_cr) may be found to propagate a crack until final failure. K_cr of the magnesium alloys is 0,80–1,05 MPa√m, and 1,80 MPa√m was found for AlSi9Cu3. Using K_cr it is possible to correlate the probability of different defect sizes and the failure probability at different stress amplitudes. Additionally, predictions of the influence of rare large casting defects on the endurance limit are possible.     

Quasi-static and dynamic fracture behavior of composite sandwich beams with a viscoelastic interface crack

Fracture analysis of sandwich beams with a viscoelastic interface crack under quasi-static and dynamic loading has been studied. Firstly, a three-parameter standard solid material model was employed to describe the viscoelasticity of the adhesive layer. And a novel interfacial fracture analysis model called three material media model was established, in which an interface crack was inserted in the viscoelastic layer. Secondly, a finite element procedure based on Rice J-integral and Kishimoto J-integral theories was used to analyze quasi-static and dynamic interface fracture behavior of the sandwich beam, respectively. Finally, the influence of viscoelastic adhesive layer on the quasi-static J-integral was discussed. In addition, comparison of quasi-static Rice J-integral with Kishimoto J-integral under various loading rates was carried out. The numerical results show that the oscillating characteristic of dynamic J-integral is more evident with shorter loading rise time.     

An investigation of the features of development of high-velocity localized shear in metal

An experimental method of shear tests of metals with loading rates up to 10 m/sec has been developed. The results of shear tests of AMg6 alloy with a loading rate of up to 5 m/sec and a deformation rate of up to 10⁵ sec^–1 are presented. Deformation curves and values of critical shear deformations are given. Features of the kinetics of development of high-velocity localized shear in AMg6 alloy are investigated.Translated from Problemy Prochnosti, No. 3, pp. 41–45, March, 1995.     

Strain rate and gage length effects on tensile behavior of Kevlar 49 single yarn

In the present paper, Kevlar® 49 single yarns with different gage lengths were tested under both quasi-static loading at a strain rate of 4.2 × 10^?4 s^?1 using a MTS load frame and dynamic tensile loading over a strain rate range of 20–100 s^?1 using a servo-hydraulic high-rate testing system. The experimental results showed that the material mechanical properties are dependent on gage length and strain rate. Young’s modulus, tensile strength, maximum strain and toughness increase with increasing strain rate under dynamic loading; however the tensile strength decreases with increasing gage length under quasi-static loading. Weibull statistics were used to quantify the degree of variability in yarn strength at different gage lengths and strain rates. This data was then used to build an analytical model simulating the stress–strain response of single yarn under dynamic loading. The model predictions agree reasonably well with the experimental data.     

Optimization of metal heating in plasma-mechanical treatment

Results are presented of an experimental investigation of the process of energy transfer to the anode of an arc in application to plasma-mechanical treatment of components.Notation Q_y energy introduced per unit mass of the layer being cut, W/kg - Q integral energy flux in the anode, W - I arc current, A - L spacing between the plasmotron nozzle exit and the component, m - h spacing between the plasmotron nozzle exist and the cathode, m - d plasmotron nozzle diameter, m - G gas consumption, kg/sec - q energy flux density in the anode spot, W/cm² - r spacing from the center of the anode spot to the point at which the energy flux density is calculated, cm - q_m energy flux density at the center of the anode spot, W/cm² - k_q concentration coefficient, cm^–2 - S_o degree of heat propagation, m - thermal diffusivity factor, m²/sec - time, sec - k spacing between the plasmotron and the cutter, m - v cutting rate, m/sec - coefficient of energy transmission to the anode - N electrical power of the arc, W Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 47, No. 1, pp. 138–143, July, 1984.     

ON THE USE OF QUASI-STATIC TESTING TO ASSESS IMPACT DAMAGE RESISTANCE OF COMPOSITE SHELL STRUCTURES

The response of various composite structures to transverse loading was studied through im-pact and quasi-static testing. The AS4/3501-6 graphtie/epoxy composite structures considered have a [±4.5_n/0_n]_s, layup configuration and include convex and concave shell sections, plates, and full cylinders. The impact tests fall within the so-called large-mass, low-velocity regime, where previous findings for composite plates indicate that quasi-static tests represent the impact response accurately; i.e., impact and quasi-static tests can be considered equivalent. This equivalence includes damage if the same peak force is reached in both the impact and quasi-static tests. The present work extends the impact and quasi-static equivalence from composite plates to various composite (shell) structures, including shells with an instability. Over nearly the entire range of impact events and shell structures considered, impact and quasi-static responses (including damage extent and distribution) are found to be equivalent. A small number of the most flexible (large-span, thin) specimens displayed a large-amplitude oscillatory impact loading response that was not observed for the quasi-static tests. These few specimens indicate one regime where the equivalence is limited The general equivalence demonstrated here for a wide range of composite structures has important implications for testing and design of damage-tolerant aerospace components. The findings also suggest that quasi-static experimentation can often be used to simulate the impact response (particularly damage) of composite shell structures.