Load-controlled Tests Research Articles

Design of an apparatus for measuring tool/fabric and fabric/fabric friction of woven-fabric composites during the thermostamping process

The thermostamping of prepreg woven fabrics shows promise as a low-cost high-volume manufacturing process for composite parts. One concern associated with the process is the unwanted formation of defects in the form of fabric wrinkling. This wrinkling can be prevented during the thermostamping process by inducing in-plane forces through the use of one or more metal binder rings. However, if the in-plane forces are too low, then the fabric may wrinkle as the fabric conforms to the shape of the punch, and conversely, if the in-plane forces are too high, then the yarns in the fabric can separate and the fabric may tear and yarns can break. The in-plane forces are a result of the friction between the fabric and the metal binder rings. As the fabric slides over the surfaces of the punch and die, further friction is induced between the metal tooling and the fabric part. In addition, most composite parts consist of multiple layers, and therefore as the fabric is drawn into the die adjacent layers of fabric may slide relative to one another. Thus, the friction at the tool/fabric interface and the interlaminar friction must be understood and quantified to predict part quality as a function of the processing parameters. In this paper, the design and implementation of a load-control test apparatus used to measure the friction between the tool and the fabric and between adjacent layers of fabric during a composite forming process is presented.

Stability of ultrafine-grained structure of copper under fatigue loading

Stability of microstructure of ultrafine-grained materials under cyclic loading is a crucial condition for their good fatigue performance. Changes of microstructure due to fatigue in bulk, localization of cyclic plasticity into cyclic slip bands and related development of microstructure were experimentally studied on ultrafine-grained copper prepared by equal channel angular pressing. Different reaction of ultrafine-grained structure to plastic strain-controlled and load-controlled tests was found. The different susceptibility to dynamic grain coarsening under load and plastic strain-controlled tests reported in literature cannot be explained by differences in purity or details of equal channel angular pressing. The localization of the cyclic plasticity and the development of cyclic slip bands resulting in fatigue crack initiation take place in material volumes which can be characterized as “near by oriented” regions. They correspond to the shear bands, which are characteristic for ultrafine-grained structure after equal channel angular pressing.

Derivation of shakedown factors (Ks) for Type 316 and Type 321 stainless steel at high temperature

The shakedown factor, Ks, is defined in R5 Volume 2/3 as the factor applied to the minimum 0.2% proof stress, Sy, in order to obtain the material ratchet limit, in terms of the maximum stress level at which the materials exhibits cyclically stable behaviour. This experimentally derived factor is used in the R5 assessment procedure to perform simple checks for shakedown and also when a more detailed shakedown analysis is required. In addition, it is also used to determine the start-of-dwell stress as part of a creep-fatigue damage assessment.This paper presents the derivation of this shakedown factor, Ks, from a number of cyclic load-controlled tests, which were carried out on several casts of Types 316H and 321 steel between room temperature and 650°C. These tests involved subjecting uniaxial specimens to blocks of continuous load controlled cycles, the stress range remaining constant during each block, but then being increased until material cyclic instability was observed. Since the type of cycle was expected to be related to reactor trip or fault conditions, and only a few hundred were expected in the lifetime of the plant, the tests were based on blocks of 500 repeated cycles. Tensile tests were also carried out on the same materials in order to provide values of the 0.2% and 1% proof stress which are used to derive the Ks parameter. The Ks factor is sensitive to the strain rate used in the tensile tests.Finally, the technique used to evaluate the shakedown factor is discussed, along with possible modifications to the R5 methodology to improve the representation of stress-strain cycles at elevated temperatures.

Static Fatigue Controls Particle Crushing and Time Effects in Granular Materials

Based on observations from constant strain rate experiments and from creep and stress relaxation experiments initiated at different stress levels it is found that sand exhibits patterns of time effects different from those observed in clays. It appears that time effects in sand may be associated with crushing of particles, and a mechanistic picture of time effects in granular materials is constructed in which time effects depend on interparticle friction, grain crushing and grain rearrangement. This mechanistic picture is based on measured behavior in drained triaxial compression tests on three different sands in which strain rate effects are observed as small to negligible. While creep and relaxation are caused by the same underlying phenomenon, it appears that results of creep tests cannot be obtained from results of relaxations tests, and vice versa. The phenomenon of static fatigue of individual particles seems to be at the root of time effects in sand. A review of previous studies of static fatigue is presented. Triaxial tests on a beach sand incorporating creep and stress relaxation are followed by grain size analysis to prove that grain crushing relate to the observed time effects. Additional triaxial tests are presented in which the effect of water is demonstrated in support of the static fatigue mechanism. Load-controlled tests on individual sand particles in the form of spherical glass beads (quartz) were performed by maintaining constant loads lower than the short term crushing loads. As do rock and concrete specimens in triaxial compression, the glass beads show effects of time to crushing.

In vitro response of the natural cadaver knee to the loading profiles specified in a standard for knee implant wear testing

The purpose of this study was to examine how a natural knee responds to the inputs of a total knee replacement testing standard developed by the International Organization for Standardization (ISO). This load control standard prescribes forces to be used for wear testing of knee replacements independent of implant size or design. A parallel ISO standard provides wear testing inputs that are displacement based instead of force based. Eight fresh frozen cadaveric knees were potted and tested in a 6 degree of freedom knee simulator using the load-control standard. The resulting displacements during load-control testing were compared to the prescribed displacements of the ISO displacement standard. At half the tibial torque prescribed by the load standard there was three times more average internal tibial rotation (20.3°) than is prescribed by the displacement standard (5.7°). The AP motion resulting from load testing was much different than is specified by the displacement standard. All eight knees had anterior tibial translation with respect to the femur during swing phase while the displacement standard specifies posterior tibial displacement. The variation in these motions among knees and their difference from the ISO displacement standard may be one factor that explains why wear results of total knee replacements based on ISO load or displacement testing frequently do not agree with each other or with clinical retrievals.

X線法によるポリイミド基板スパッタ銅薄膜の変形挙動の評価

Copper thin films were fabricated by RF-magnetron sputtering under target power densities of 0.5 and 5W/cm^2. The polyimide film was used as a substrate to apply a plastic deformation to the copper thin film. Average grain sizes of the copper films obtained at 5W/cm^2 were 125nm and 230nm. On the other hand, the sizes at 0.5W/cm^2 were about 50nm. Load-controlled and strain controlled cyclic deformation tests were conducted on the polyimide films where the copper films deposited. Stress, full width at half maximum (FWHM) of diffraction profile, crystallite size, and microstrain of the copper films were measured during the deformation tests using X-ray method. In the load-controlled tests, the crystallite size decreased with applied strain during loading period, although the applied strain hardly affect crystallite size during unloading. On the other hand, the microstrain during unloading period was sensitive to the applied strain: the microstrain increased after initial rapid decrease. In the strain-controlled tests, the X-ray stress averaged over a strain cycle decreased with strain cycles. This is because the copper film was subjected to large plastic deformation which can cause internal compressive stress during unloading. The averaged value of the FWHM decreased also with strain cycles. This decrease in the FWHM was consistent with texture development during cyclic loading. On the other hand, the changes in the crystallite size and the microstrain during cyclic loading were insignificant.

A modified model to determine limiting values of coating toughness by nanoindentation

The bound model to determine coating toughness limits by nanoindentation tests was originally proposed by Toonder et al. However, the assumptions in this method conflict with each other. It provides lower and upper toughness limits for load control tests; however, it is unable to give the lower toughness limit for displacement control tests. In this paper, a modified model is presented by more comprehensive analysis of the unloading curves at the points where the crack starts and stops respectively. This modified method equally provides the lower and upper limits of fracture dissipated energy for both load control and displacement control. The model developed here has been successfully applied to fullerene like CNx coatings deposited on various substrates such as Si, SiC and Al2O3. It is also applicable to other coated systems provided that the through thickness fracture induced excursion is observed in the load–displacement curve.

Relating viscoelastic nanoindentation creep and load relaxation experiments

Abstract Nanoindentation characterization of polycarbonate is considered for load-controlled creep testing and displacement-controlled load-relaxation testing using a pyramidal Berkovich tip. An elastic – viscoelastic correspondence approach is used to obtain closed-form analytical solutions for fitting experimental data to deconvolute spring coefficients and viscosity values independently from creep and from relaxation tests. The a priori assumption of linear viscoelasticity is tested by varying the peak load and displacement levels by a factor of twenty. Results demonstrate surprisingly good agreement between parameters computed from relaxation and creep tests and little variation with load or depth. This result supports a linearly viscoelastic analysis under the conditions utilized in the current experiment, namely a Berkovich indenter tip and a glassy polymer. Plastic deformation is not explicitly considered in the current analysis, as all time-independent elastic and plastic deformation was lumped into a single effective deformation resistance term with a quadratic dependence of load on displacement. As such, obtained numerical values of the shear modulus are not representative of the material's elastic modulus. However, the technique demonstrates predictive capability across the interface between load-control and displacement-control, motivating further research on displacement-controlled nanoindentation relaxation for polymeric and biological material characterization.

Innovative Approaches to Verifying Demand Response of Water Heater Load Control

This study describes a pilot effort to measure load reductions from a residential electric water heater (EWH) load control program using low-cost statistically based measurement and verification (M&V) approaches. This field experiment is described within the larger framework of overcoming barriers to participation of noninterval metered customers in Demand Response (DR) Programs. We worked with PJM Interconnection and a Curtailment Service Provider (CSP) to collect hourly load data for two substations and several hundred households over six weeks of load control testing. The experimental design reflected constraints imposed by limited funding, manpower, equipment, and the routine operation of the load control system by the CSP. We analyzed substation- and premise-level data from these tests in an attempt to verify several point estimates taken from the hourly diversified demand curves used by the CSP to establish aggregate load reductions from their control program. Analysis of premise-level data allowed for provisional verification that the actual electric water heater load control impacts were within a -60 to +10% band of the estimated values. For sub-station level data, measured values of per-unit load impacts were generally lower than the CSP estimated values for Electric Cooperative #2, after accounting for confounding influences and operational test problems. Based on this experience we offer recommendations to ISO and utility DR program managers to consider before undertaking further development of alternatives to the conventional but costly program-wide load research approach.

Measurement, analysis and prediction of fretting wear damage in a representative aeroengine spline coupling

A methodology to predict fretting wear in complex couplings is described and validated against results obtained from a reduced scale aeroengine-type spline coupling subjected to complex cyclic load cases. The methodology uses three-dimensional finite element analysis, together with coefficient of friction data obtained from stroke controlled round-against-flat fretting tests, to determine spline tooth contact pressure and slip distributions; the latter as a function of number of loading cycles. A modified Archard equation is used to calculate wear depths from the contact pressure and slip distributions using wear coefficients obtained from the round-against-flat fretting tests. The slip distributions, and, concomitantly, the wear distributions, are found to depend strongly on the coefficient of friction, which, in turn, depends on the state of lubrication and number of loading cycles. For constant coefficient of friction, the slip distributions stabilise quickly with increasing numbers of loading cycles. The methodology predicts greater wear under lightly lubricated conditions than without added lubrication for the essentially load-controlled tests on the reduced scale aeroengine-type coupling. The wear depth trends are predicted correctly, both axially along the spline teeth and around the tooth flank, and the predicted wear depths bracket the measured values, dependent on the lubrication conditions considered; the latter attributable to the sensitivity of spline wear to the evolving coefficient of friction during testing. The methodology provides a basis for further development.

Constitutive Modelling of Geomaterials

Constitutive Modelling of Geomaterials

Corrosion Fatigue of Refractory Materials in Boiling Nitric Acid

Refractory materials such as zirconium, niobium and titanium alloys with excellent corrosion resistance in boiling nitric acid have been selected for use as structural materials of spent fuel reprocessing equipment. In this study, the fatigue crack growth rates of these materials were investigated by load control tests as a function of the stress intensity factor range in boiling 3 kmol/m 3 nitric acid and in air at room temperature. The fracture surfaces were observed by scanning electron microscopy. The fatigue crack growth rates of zirconium and niobium were enhanced in boiling nitric acid compared with those in air at room temperature. Acceleration effect due to corrosion fatigue was not observed in the crack growth of Ti-5Ta alloy. The fracture surfaces of Ti-5Ta alloy showed the ductile striation in both environments. On the other hand, the fracture surfaces of niobium represented the fatigue striation in air and the brittle striation due to corrosion in nitric acid. The fracture surfaces of zirconium in nitric acid showed brittle fracture and the ductile fracture related to stress corrosion cracking.

Influence of load and deformation-controlled multiaxial tests on fatigue life to crack initiation

Generally, areas of components from ductile steels with notches or geometrical transitions are critical because of the resulting stress/strain concentrations. In these areas due to the stress-gradients and constraint local deformations are displacement controlled even if the material's yield stress is exceeded, as long as the deformations are below the structural yield point. Therefore, load controlled tests in the elasto—plastic region with unnotched specimens from ductile materials under combined axial loading and torsion are not suitable for the interpretation of a component's behaviour because of uncontrolled local deformations. Thus, the influence of multiaxial stress/strain states on the fatigue behaviour of a compoment under elasto—plastic deformations can be determined reliably with unnotched specimens only by deformation controlled tests, if plastic racheting is not expected in critical areas. The modified effective equivalent strain hypothesis (MEES) for ductile materials renders a satisfactory evaluation of the in- and out-of-phase test results.

Microthermomechanical Analysis of Lead-Free SN-3.9AG-0.6CU Alloys; Part I : Viscoplastic Constitutive Properties

ABSTRACTThis is part I of a two-part paper on the mechanical behavior of lead-free solders. The constitutive properties of Sn3.9Ag0.6Cu lead-free alloy are presented and compared against baseline data from eutectic Sn63Pb37 solder. Monotonic, displacement-controlled and load-controlled tests are performed over various temperatures, strain rates and stresses using the thermo-mechanical-microstructural (TMM) test system. It is shown that the lead-free alloy exhibits creep strain rates that are from one to five orders of magnitude lower than the eutectic SnPb alloy, depending on the stress level and the homologous temperature.

Simulation of elbow and forearm motion in vitro using a load controlled testing apparatus

The purpose of this study was to compare passive to active testing on the kinematics of the elbow and forearm using a load-controlled testing apparatus that simulates muscle loading. Ten fresh-frozen upper extremities were tested. Active control was achieved by employing computer-controlled pneumatic actuators attached to the tendons of the brachialis, biceps, triceps, brachioradialis and pronator teres. Motion of the radius and ulna relative to the humerus was measured with an electromagnetic tracking system. Active elbow flexion produced more repeatable motion of the radius and ulna than when tested passively ( p<0.05). The decrease in variability, as determined from the standard deviation of five successive trials in each specimen, was 76.5 and 58.0% for the varus–valgus and internal–external motions respectively (of the ulna relative to the humerus). The variability in flexion during simulated active forearm supination was 30.6% less than during passive testing. Thus under passive control, in the absence of stability provided by muscular loading across the joint, these uncontrolled motions produce increased variability amongst trials. The smooth and repeatable motions resulting from active control, that probably model more closely the physiologic state, appear to be beneficial in the evaluation of unconstrained kinematics of the intact elbow and forearm.

Flow Constraint and Loading Rate Effects on Prosthetic Liner Material and Human Tissue Mechanical Response

The compressive mechanical properties of four prosthetic liner materials (urethane, two silicones, and a thermoplastic elastomer) were compared to human muscle as a function of geometric flow constraint and loading rate. Loading rate effects on calcaneal heel pad were also measured. The mechanical properties evaluated were from force-displacement data obtained during material tests and included stiffness, the percentage of energy absorbed, and residual displacement (or thinning) 8 seconds after unloading. The geometric flow constraint involved a 25.4-mm diameter piston pushed into an 11.5-mm deep cylindrical cavity with radial piston/cylinder clearances of 0.8 mm, 1.55 mm, and 3.2 mm. The haversine wave-form load-controlled tests increased the load from 50 N to 550 N in 10.0, 1.0, and 0.2 seconds; loading rates of 0.1 Hz, 1.0Hz, and 5.0 Hz. The variation of test parameters had a large and mixed effect on the different materials. The average stiffnesses for each material over all geometries and loading rates were as follows: urethane, 537 N/mm; silicone A, 442 N/mm; human muscle, 394 N/mm; thermoplastic elastomer, 220 N/mm; heel pad, 134 N/mm; and silicone B, 99 N/mm. The average percentages of energy absorbed were as follows: human muscle, 90%; silicone A, 74%; urethane, 71%; heel pad, 59%; thermoplastic elastomer, 56%; and silicone B, 41%. The average residual displacements were as follows: urethane, 0.23 mm; silicone A one layer, 0.43 mm; silicone B, 0.47 mm; thermoplastic elastomer, 0.66 mm; silicone A two layers, 0.74 mm; human muscle, 1.09 mm; and heel pad, 1.6 mm. Impact tests were also performed that involved dropping a 4.45 N mass 151 mm onto the liner materials placed on a piezoelectric sensor. The 820 N impact force without liner materials was decreased to 186 N with muscle, 258 ˙N with urethane or two layers of silicone A, 268 N with a thin layer of heel pad, and 309 N for the thermoplastic elastomer. A single layer of silicone A and the silicone B material both ripped repeatedly during impact testing. Using standard material selection methods, urethane appears to be the optimal prosthetic liner material by offering the unique combination of best response (via highest stiffness), best impact protection (via lowest impact forces), and least thinning (via least residual displacement 8 seconds after unloading). The preliminary data reported here suggests that the differences in comfort reported by amputees who have used multiple liner material types are justified.

Residual Strength Degradation of Cross-Ply Titanium Matrix Composite Due to Elevated Temperature

This study investigates the residual strength degradation of cross-ply, SCS-6/Ti-15-3 titanium matrix composite (TMC) due to elevated temperature fatigue. To accomplish this, several specimens were cycled at 427°C to certain fractions of their expected fatigue lives, then monotonically loaded to failure. Fatigue tests were conducted in both load-control mode with a load ratio (Rσ) of 0.05 and strain-control mode with a strain ratio (Re) of − 1. Maximum stresses in the load-controlled tests were 300 and 450 MPa with frequencies of 5 and 10 Hz, respectively. The strain amplitudes ranged from 0.25 to 0.4% in the strain-controlled tests. Rather than being conducted at a constant frequency, these tests were performed at a strain rate of 0.1% per s. Various mechanical responses during cycling are discussed and compared for the load-controlled and strain-controlled tests. After failure, specimens were sectioned and studied using both optical and scanning electron microscopy. Residual strength data were then correlated to the amount and type of each damage mechanism. It was found that the residual strength degradation in cross-ply laminate, when exposed to different fatigue conditions, correlates together with the fraction of the cyclic life left in the composite.

The effects of testing methods on the flexural fatigue life of human cortical bone

A flexural model of four-point bending fatigue that has been experimentally validated for human cortical bone under load control was used to determine how load and displacement control testing affects the fatigue behavior of human cortical bone in three-point and symmetric four-point bending. Under load control, it was predicted that three-point bending produced no significant differences in fatigue life when compared to four-point bending. However, three-point bending produced less stiffness loss with increasing cycles than four-point bending. In four-point bending, displacement control was predicted to produce about one and a half orders of magnitude greater fatigue life when compared to load control. This prediction agrees with experimental observations of equine cannon bone tested in load and displacement control (Gibson et al., 1998). Displacement controlled three-point bending was found to produce approximately a 25% greater fatigue life when compared to load control. The prediction of longer fatigue life under displacement control may have clinical relevance for the repair of damaged bone. The model can also be adapted to other geometric configurations, including modeling of whole long bones, and with appropriate fatigue data, other cortical bone types.

Steady-State Concepts and Static Liquefaction of Silty Sands

The results from an experimental study on silty sands are presented and evaluated in view of the framework of critical-state or steady-state soil mechanics. Drained and undrained compression tests were performed on Nevada sand containing nonplastic silt. The drained and undrained steady-state lines diverged. This divergence was caused by the tendency for static liquefaction at low pressures. Results from undrained tests with different initial void ratios produced different steady-state lines. Unique steady-state lines may therefore not always exist for silty sands. Denser specimens liquefied at low confining pressures, while looser specimens at higher confining pressures showed stable behavior. Thus, a unique relation between void ratio and confining pressure, namely the steady-state line, may not represent the behavior of loose silty sands. This occurs because silty sands exhibit “reverse” behavior characteristics; more dilative behavior is observed with increasing confining pressure under undrained shearing. Load and deformation control testing was performed to evaluate the effect of strain rate. Compressibility was examined as an alternative measure of liquefaction potential.

Load-strain-displacement response of geosynthetics in monotonic and cyclic pullout

Mobilization of the pullout resistance of geosynthetics in monotonic and cyclic modes is described from both displacement- and load-controlled tests performed at normal stresses in the range 4-17 kPa. The tests were performed on three geogrids and two geomembranes embedded nearly 1.0 m in a uniformly graded sand. Results for load-controlled tests at a constant rate of 0.25 kN/(m ·min-1), followed by several series of load cycles of increasing amplitude, are compared with displacement-controlled tests at a constant rate of 0.5 mm/min. In general the geogrids behave as an equivalent textured sheet. Pullout behaviour, and especially the incremental displacement mobilized at cyclic loads close to the maximum resistance, is found to vary with type of geogrid. In only one case was cyclic pullout resistance of a grid found to exceed the monotonic resistance. A comparison of the cyclic and monotonic response yields a constant ratio of pullout resistance at large displacement, but one which is not unique to a particular specimen. Cyclic strains of decreasing amplitude are mobilized along a test specimen, with most of the necessary relative displacement occurring close to the loaded end and the embedded end showing little response.Key words: pullout testing, monotonic, cyclic, dynamic, geosynthetics, reinforced walls.