Load-time Curves Research Articles

Rating of composite suspension insulators related to the long term mechanical strength of rods

The long duration mechanical strength of rods used in composite insulators is a major problem for the line designer. The previous approach to the problem and the requirements of standards give an extrapolation for the long term performance of the rod which is not confirmed by accurate analysis of test results. So, a new model for the load-time curve is determined. Based on the analysis of the behaviour of the rod under tensile stresses with acoustic emission and the determination of the physical limit, a mathematical model of the breakage process is proposed. This model takes into account two damage mechanisms; at high stresses, the slope of the load-time failure curve is governed by the relaxation of the resin matrix, then the curve tends to a horizontal asymptote corresponding to the intrinsic mechanical strength of the glass fibres. The model can solve any problem related to the ultimate behaviour of the fibreglass rod after various load histories. The conditions required to have no reduction of the ultimate strength of the rod in relation with the SML of composite suspension insulators are established. >

Viscoelastic properties of the human lumbodorsal fascia

The purpose of this study is to provide better understanding of the mechanical response of the lumbodorsal fascia to dynamic and static traction loadings. Since the fascia shows a viscoelastic behaviour, tests in which time is a variable were used, namely hysteresis and stress relaxation. Load-strain and load-time curves obtained from the hysteresis and stress-relaxation tests point out three different phenomena. First, an increase in stiffness is noticed when ligaments are successively stretched, i.e. strains produced by successive and identical loads decrease. Second, if a sufficient resting period is allowed between loadings, stiffening is reversed and strains tend to recover initial values. The third phenomenon, observed in stress-relaxation tests as time progresses, is ligament contraction in stretched and isometrically held samples. This third phenomenon may be explained by the possibility that muscle fibres capable of contracting spontaneously could be present in lumbodorsal fascia ligaments.

Application of the instrumented impact test to the study of the fracture of microalloyed steels for forging

One of the reasons why microalloyed steels for forging are not more widely used is that their toughness is lower than that of steel for hardening and tempering, especially when it is measured by the Charpy impact test. In order to achieve a better understanding of the behavior under impact of microalloyed steels for forging, Charpy tests were performed from [minus]100 C to 100 C using an instrumented pendulum. This kind of test offers the advantage that it can also be considered as a dynamic bending test of notched specimens. The type of information obtained is shown in an idealized diagram of a load-time curve representing the results of this work. The value of P[sub GV] corresponds to the general yielding of the section under the notch; P[sub max], is the maximum load applied previous to the start of the fracture, and P[sub f] is the load under which the unstable propagation of the crack occurs. Depending on the brittle behavior of the material, P[sub f] may coincide with P[sub max], P[sub GY] or even be lower than the latter. The results also shows how the total energy absorbed in the impact test, E[sub T], is the result of themore » contribution of the energies corresponding to the different individual mechanisms involved in the fracture of the specimen. The contribution E[sub I] is derived from the elastic and plastic energy (including strain hardening) accumulated by the specimen previous to the start of the fracture.« less

Effect of magnesium, iron (impurity) and solidification rates on the fracture toughness of Al7Si0.3Mg casting alloy

A dynamic fracture toughness study on Al7Si0.3Mg casting alloy with cadmium trace addition and at varying magnesium and iron levels and solidification rates was carried out using an instrumented impact testing machine. With an increase in magnesium and iron contents, the load-time curves showed a change from sinusoidal to triangular shape. Even at a low iron level of 0.2%, with an increase in magnesium content from 0.32 to 0.65% the total energy absorbed to fracture E T dropped significantly by about 50%, demonstrating the enhancement of the embrittlement effect of magnesium on the alloy. At 0.32% Mg, an increase in iron content from 0.2 to 0.8% significantly decreased E T, the dynamic fracture toughness K d and the critical flaw size a cr. The embrittling effect on the alloy containing a higher level of iron (0.8%) is attributed to the presence of β phase (FeSiAl 5). A distinct brittle fracture seen on the fractured specimens and the ratio P max/ P gy of unity both further confirmed this embrittling effect. Generally, an increase in solidification rate produces higher E T, K d and a cr values, and teh effect was more pronounced at lower magnesium and iron levels than at higher levels.

Interfacial Mechanical Properties of Fiber-Reinforced Ceramic Composites

The interfacial properties such as interfacial shear strength, residual clamping stress and coefficient of friction of SiC fiber-reinforced borosilicate glass composites were evaluated using a single-fiber pull-out test. Maximum interfacial shear stress was adopted as an interfacial debond criterion and Poisson's effect of the fiber due to the applied stress was considered in the present evaluation. During the single-fiber pull-out test, AE signal was measured to understand the relationship between load-time curve and interfacial debonding behavior. Interfacial debonding behavior was classified into two types that are with and without stress transfer at the debonded interface. When the shrinkage of the fiber induced by the applied tensile stress overcomes the clamp due to interfacial residual radial stress, abrupt interfacial fracture occurs. Critical condition for these behavior was derived and influence of interfacial debonding behavior on the fiber bridging mechanism was discussed.

Punching Mechanism of Aluminum-Stainless Steel Two-Layer-Clad Material.

Effects have been investigated of punch diameter, clearance and thickness ratio of components in two-layer-clad materials on punching. Two kinds of clad material produced slightly different tendencies in punching load-time curve and specific shearing stress due to mechanical properties and thickness ratio of components. Specific shearing stress measured was compared with those calculated by law of mixture and double punching method of monolithic materials. The stress calculated by the latter is smaller, however not always closer to the measured value than the former.

Instrumented impact testing of an irradiated 20MnMoNi55 PVS weld material

Instrumented impact tests were carried out on a 20MnMoNi55 pressure vessel steel (PVS) weld material before and after fast neutron irradiation to 2.5 × 1019n/cm2 at 160°C and to 3.5 × 1019n/cm2 and 7 × 1019n/cm2 at 290°C. The correlation between the fracture surface appearance and the load-time curve obtained from the tests was investigated. The impact energy was divided into fracture initiation energy and fracture propagation energy. Neutron irradiation decreased the ratio of the initiation energy to the total energy from 15% before irradiation to 10, 9.2, and 6.4% after exposure to 3.5 × 1019n/cm2 (Tirr = 290°C), 7 × 1019n/cm2 (Tirr = 290°C) and 2.5 × 1019n/cm2 (Tirr = 160°C), respectively. The fracture initiation energy was used to establish a correlation with fracture toughness. This correlation can be used to estimate the fracture toughness of the steel after neutron irradiation.

Impact strength of FRC materials

The paper describes toughness measurements carried out on polypropylene and steel fibre reinforced concrete mixes using both quasistatic tests and impact tests. Both types of test were carried out on identical notched fibrous beams subjected to three-point bending. The quasistatic toughness measurements were evaluated by means of two toughness indices (based on ratio of areas under the load-deflection graphs) together with the total energy required to achieve complete failure of the test specimens. The impact toughness was evaluated from the number of blows required to completely fracture the test specimens which was then converted into energy. The impact apparatus was instrumented by two load cells to monitor the impacting loads and the corresponding loads at one of the supports. The load cells were connected to a Signal Memory Recorder which gave load-time curves for the impact events. The load-time graphs showed the greater efficiency of heavier masses in producing impact failure in the specimens. The impact apparatus is versatile and can be used to study a range of variables including impacting mass, drop-height, beam span and notch depth. The overall pattern of results showing the enhanced toughness with increasing fibre concentration was the same in the two methods of testing. A comparison between the quasistatic and impact test results shows that twice as much energy is absorbed in the impact tests as is absorbed in the quasistatic tests. Further work is recommended in this area to determine the relative magnitudes of the various energies lost during the impact event.

A compendium of sources of fracture toughness and fatigue crack growth data for metallic alloys. III: Hudson, C.M. and Seward, S.K. Int. J. Fract. Apr. 1989 39, (4), r43–R63

Accurately realizing the use of Charpy impact test to measure fracture toughness and determine reference temperature of low alloyed steel with ferritic microstructure has important practical engineering significance for the structural integrity assessment of reactor pressure vessels. In this paper, a methodology for measuring fracture toughness and determining reference temperature for RPV steels by Charpy impact test was proposed. The method referred to the ASTM E1820 and E1921 standard. By analyzing the physical meaning of the Charpy impact specimen’s fracture, load-time curve and size parameters, the fracture toughness of the material was directly calculated according to the linear elastic fracture mechanics. The reference temperature was calculated using the multi-temperature procedure formula. The reference temperature and fracture toughness were corrected according to two factors, such as the difference in fracture toughness caused by the difference in the loading rate of the stress intensity factor and the specimen shape and size between Charpy impact test and compact tension test. And the validity of the fracture toughness and reference temperature was evaluated. The corrected two are basically consistent with the data obtained by the compact tension test in accordance with ASTM E1820 and E1921 standards. This method may be used to solve the problem that the nuclear power plant needs to use the impact test data to evaluate the fracture toughness and reference temperature of the reactor pressure vessel in the structural integrity assessment of the reactor pressure vessel operation and life extension process.

Correlation of mechanical properties of vertebral trabecular bone with equivalent mineral density as measured by computed tomography.

We tried to determine whether mineral-equivalent measurements that were obtained using computed tomography could be used to predict the mechanical properties of vertebral trabecular bone. Vertebral bodies that had been obtained during routine autopsy were evaluated by computed tomography. The mechanical properties of the vertebral trabecular bone were determined by subjecting cylindrical specimens to simple compression until failure occurred. The ultimate strength and elastic modulus were determined from load time curves, using constant displacement rate loading. Atomic absorption spectrophotometry was used to determine the weight per cent calcium of each specimen, and quantitative light microscopy was used to determine area fraction bone. Significant positive correlations were found between the observed mechanical properties of the trabecular bone and the equivalent mineral density as measured by computed tomography. Compressive strength (r = 0.720), elastic modulus (r = 0.574), trabecular calcium density (r = 0.780), and area fraction bone (r = 0.579) were all correlated with the equivalent mineral density.

Determination of the crack resistance of a ceramic in bending of beams with a notch

These authors investigate crack resistance in new ceramic materials with particular regard to temperature and to the features of notches. Charts present data on the relationship of experimental values of crack resistance to crack depth for silicon carbide; on the influence of notch depth on crack resistance; on the relationship of stress intensity to notch tip radius; on characteristic load-time curves for various materials; and on the relationship of temperature to crack resistance.

A simple application of the Bailey-Orowan creep model to Fe-39.8 at % AI and γ/γ′-α

A method which allows calculation of recovery rates and work hardening coefficients for creep from simple constant velocity compression experiments is described. Stressing rates and straining rates are computed from the measured load-time curves, and these quantities are then fitted to the universal form of the Bailey-Orowan equation for creep. The techniques were applied to B2 crystal structure Fe-39.8 at % Al intermetallics and the directionally solidified eutectic alloyγ/γ′-α compression tested between 1200 and 1400 K. The recovery rates were found to be functions of nominal strain rate, stress, and temperature; the hardening coefficients were dependent only on temperature. Hardening coefficients for the aluminides are small (<0.002 of the elastic modulus) and are believed to be due to the lack of dislocation barriers. A more typical value for the hardening coefficient (∼ 0.05 of the elastic modulus) was observed inγ/γ′-α. Recovery rates and hardening coefficients can be described in terms of stress and temperature, and the equations used to estimate creep rates.

床のすべりおよびその評価方法に関する研究 : その 3 すべり試験機の設計・試作

This paper presents the process of the development of the new slipperiness tester and the performance of it. Based on the concept of a new slipperiness tester reported in part 2, a tester (DRAG TYPE) of which details remained changeable and could be adjusted by the result of the later experiments was first developed. The result of measurement of many surfaces, namely, the pulling load-time curves were analysed to find the physical value which can indicate the degree of slipperiness, and coefficient of slip resistance (C.S.R.) was finally found. C.S.R is defined as Pmax/W; where Pmax is the maximum pulling load, W is the loading weight. Then, the each specification of the tester was able to determined in a certain way as C.S.R corresponds smoothly enough to the subjective slipperiness value on the sensory scale obtained in part 1. Moreover, even in matters of the test on powdery and oily surfaces which were similar to actual dusty and muddy wet ones, the smooth correspondence was also recognized. These results made clear that the newly developed tester can be regarded as appropriate for measuring the slipperiness of actual floors.

The evaluation of tempered martensite embrittlement in 4130 steel by instrumented charpy V-notch testing

Tempered martensite embrittlement (TME) was studied in vacuum-melted 4130 steel with either 0.002 or 0.02 wt pct P. TME was observed as a severe decrease in Charpy V-notch impact energy, from 46 ft-lb. at 200 °C to 35 ft-lb. at 300 °C in the low P alloy. The impact energy of the high P alloy was consistently lower than that of the low P alloy in all tempered conditions. Fracture was transgranular for all specimens; therefore, segregation of P to the prior austenitic grain boundaries was not a factor in the occurrence of TME. Analysis of load-time curves obtained by instrumented Charpy testing revealed that the embrittlement is associated with a drop in the pre-maximum-load and post-unstable-fracture energies. In specimens tempered at 400 °C the deleterious effect of phosphorus on impact energy became pronounced, a result more consistent with classical temper embrittlement rather than TME. A constant decrease in pre-maximum-load energy due to phosphorus content was observed. The pre-maximum-load energy decreases with increasing tempering temperature in the range of 200 °C to 400 °C, a result explained by the change in work hardening rate. Carbon extraction replicas of polished and etched as-quenched specimens revealed the presence of Fe2MoC and/or Fe3C carbides retained after austenitizing. Ductile crack extension close to the notch root was related to the formation of fine microvoids at the retained carbides.

Verification of Compliance Corrections Used in Determining Load-Line Energies from Dynamic Load-Time Data

Abstract Absorbed energy is readily determined by integrating the load versus load-line displacement function. However, in dynamic testing (strain rate approximately 10/s), load-time data are more conveniently obtained. Conversion of load-time data to the energy absorbed by a test specimen during an instrumented impact test requires that the energy absorbed by the test machine be deleted from the total energy represented by the area under the load-time curve. A technique is described whereby previously reported compliance corrections have been experimentally verified. A test specimen is allowed to deflect to an accurately known value. Integration of the quasi-load-deflection curve yields the energy absorbed by the specimen. This value is shown to compare favorably with that obtained from the compliance-corrected load-time curve.

A strain age study of acicular ferrite X-70 pipeline steels

The effect of plastic straining and subsequent aging on the instrumented impact test (IIT) properties of two acicular ferrite, X-70 pipeline steels was examined. One steel was semi-killed and contained numerous sulphide stringers. The other steel was fully killed and rare earth treated to ensure the presence of small, globular, non-directional sulphides. The data show that the toughness and ductility of the semi-killed steel was reduced by straining and aging whereas both the toughness and ductility of the fully-killed steel increased with straining and aging. The IIT tests revealed that the decrease in toughness of the semi-killed steel was due to a decrease in the propagation energy; the initiation energy, the energy to peak load on the load-time curve, was unaffected by straining and aging. The semi-killed spiral welded pipe was also tested for strain age embrittlement. IIT test specimens were removed from adjacent to the spiral weld, the notch position being chosen such that the structure at that point experienced the necessary combination of time and temperature to cause strain age embrittlement. Strain age embrittlement adjacent to the weld was observed and was manifested as a decrease in toughness and an increase in the dynamic yield strength and RB hardness of the steel.

Consolidation Under Time Dependent Loading

In analyzing consolidation settlements caused by time dependent loading, the engineer typically replaces the actual load-time curve with a series of step loads and applies the theory for instantaneous loading. For the case of one-dimensional vertical flow and simple ramp loading, the time-settlement curve may be approximated using a graphical construction orginiated by Terzaghi or curves presented by Schiffman. However, the method is reviewed and suitable design curves are presented. For one-dimensional vertical flow all of the assumptions of Terzaghi.s basis theory are retained except the assumption of instantaneous loading. For the sand drain case, solutions are obtained for purely radial flow and for combined vertical and radial flow.

Analysis of the Load-Time Curves in Charpy Impact Test of Ductile Cast Iron

Analysis of the Load-Time Curves in Charpy Impact Test of Ductile Cast Iron

Constancy of the Tensile Test Machine Stiffness

Abstract A method is presented for determining the current value of the stiffness of a tensile machine during uniaxial loading tests. The method is based on stress relaxation tests and on the measurement of the slopes of the load-time curve at the moment when the crosshead speed is changed from zero to some finite value. This method has shown that the stiffness can be taken as a constant within fairly wide ranges of the load level, crosshead speed, and specimen dimensions.

Peak load-impulse characterization of critical pulse loads in structural dynamics

The P- I characterization scheme greatly facilitates specification of critical loads for structures subjected to air blast loading and is generally useful for structures subjected to pulse loads. A general feature of the P- I characterization scheme is that critical load curves are represented reasonably well by a rectangular hyperbola [ (P P 0) − 1] [ (I I 0) − 1] = 1 , where P is the peak load, P 0 the step load required to produce the critical displacement, I the impulse, area under the load-time curve, I 0 the ideal (zero duration) impulse required to produce the critical displacement. This representation is useful for common structural elements such as beams, plates and shells. Some increase in accuracy is obtained by replacing the right-hand side with a constant C, determined by fitting to experimental results or to a theoretical critical load curve. A key feature of the P- I characterization scheme is that the critical load curves are not sensitive to pulse shape. The P- I characterization scheme contains the same information as the dynamic load factor (DLF). Although they contain the same information, these two characterizations emphasize different aspects. The DLF characterization displays the relation of dynamic loads to an equivalent static load. The P- I characterization displays equivalent dynamic loads in terms of peak load and impulse, the load parameters that are important at the long-duration and short-duration extremes.