Peak Compressive Stress Research Articles

Effect of pelvis impact angle on stresses at the femoral neck during falls

Improved understanding is required of how the mechanics of the fall affect hip fracture risk. We used a hip impact simulator to determine how peak stresses at the femoral neck were affected by pelvis impact angle, hip abductor muscle force, and use of a wearable hip protector.We simulated falls from standing (2 m/s impact velocity) involving initial hip abductor muscle forces of 700 or 300 N. Trials were acquired for impact to the lateral aspect of the greater trochanter, and impact to the pelvis rotated 5°, 10° and 15° anteriorly (positive) or posteriorly (negative). Measures were acquired with and without a commercially available hip protector. During trials, we measured three-dimensional forces with a load cell at the femoral neck, and derived peak compressive and tensile stresses.Peak compressive stress increased 37% (5.91 versus 4.31 MPa; p < 0.0005) and peak tensile stress increased 209% (2.31 versus 0.75 MPa; p < 0.0005) when the pelvis impact angle changed from 15° anterior to −15° posterior. For lateral impacts, the peak tensile and compressive stresses averaged 73% and 8% lower, respectively, in the 700 N than 300 N muscle force condition, but the effect was reversed for anteriolateral or posteriolateral impacts. The attenuation in peak compressive stress from the hip protector was greatest for posteriolateral impacts (−15 to −5°; 36–41%), and least for anteriolateral (+15°; 10%).These results clarify the effects on hip fracture risk during a fall of pelvis impact angle and muscle forces, and should inform the design of improved hip protectors.

Real-Time Stress Measurement in SiO2 Thin Films during Electrochemical Lithiation/Delithiation Cycling

Oxide coatings have been shown to improve the cyclic performance of high-energy density electrode materials such as Si. However, no study exists on the mechanical characterization of these oxide coatings. Here, thin film SiO2 electrodes are cycled under galvanostatic conditions (at C/9 rate) in a half-cell configuration with lithium metal foil as counter/reference electrode, with 1 M LiPF6 in ethylene carbonate, diethyl carbonate, dimethyl carbonate solution (1:1:1, wt%) as electrolyte. Stress evolution in the SiO2 thin film electrodes during electrochemical lithiation/delithiation is measured in situ by monitoring the substrate curvature using a multi-beam optical sensing method. Upon lithiation SiO2 undergoes extensive inelastic deformation, with a peak compressive stress of 3.1 GPa, and upon delithiation the stress became tensile with a peak stress of 0.7 GPa. A simple plane strain finite element model of Si nanotube coated with SiO2 shell was developed to understand the mechanical response of the core-shell type microstructures under electrochemical cycling; measured stress response was used in the model to represent SiO2 constitutive behavior while Si was treated as an elastic-plastic material with concentration dependent mechanical properties obtained from the literature. The results reported here provide insights and quantitative understanding as to why the highly brittle SiO2 coatings are able to sustain significant volume expansion (300%) of Si core without fracture and enhance cyclic performance of Si reported in the literature. Also, the basic mechanical properties presented here are necessary first step for future design and development of durable Si/SiO2 core shell structures or SiO2-based electrodes.

Stress and Strain Rate Effects on Incipient Spall in Tantalum

Spall fracture is a high-rate tensile damage phenomenon associated with impulsive and shock-load events. Typically, the material undergoes a sequence of compression followed by release into high rate (on the order of 104 s-1 and up) tension, causing voids to nucleate and grow, which can then coalesce into a crack and the material fails. We present a series of experiments on high purity, well characterized tantalum samples subjected to shock-loading via gas-gun plate impact. Through careful selection of the flyer-plate velocity and material we have independent control over the peak compressive stress and the tensile strain rate in the sample. At all times, the spall damage remains incipient, i.e. in the early stages of void formation and the material does not fully fracture. Velocimetry was used on the rear of the sample to record the wave-profiles and determine spall strength. Soft recovery and sectioning of the samples allowed the internal damage to be observed, quantifying the damage amount, distribution, and relationship to microstructural features with both optical and electron based microscopy.

Effect of annealing temperature on the structural, morphological, and mechanical properties of polycrystalline zirconium oxynitride composite films deposited by plasma focus device

In this article, we reveal the post deposition annealing effect on the structural, morphological, and mechanical properties of polycrystalline zirconium oxynitride (P-ZrON) composite films deposited for 40 focus shots using a plasma focus device. The development of Zr(101), ZrN(111), ZrN(200), Zr3N4(320), ZrN0.28(002), and m-ZrO2(200) diffraction peaks confirms the deposition of P-ZrON composite films. The peak intensity, crystallite size, dislocation density, compressive stress, and texture coefficient of the Zr3N4(320) plane and the microstructural features such as the shape, size and distribution of nanoparticles as well as the film compactness are influenced by the annealing temperature. Elemental analysis confirms the presence of Zr, N, and O in the deposited films. The microhardness of the P-ZrON composite film annealed at 500 °C is found to be 11.87 GPa which is 7.8 times that of virgin zirconium.

Finite-element analysis of the tool tilt angle effect on the formation of friction stir welds

The tool tilt angle is one of the key process parameters determining the weld quality during friction stir welding. However, the influence of tilt angle on the weld formation has not been fully understood. Hence, in this paper, a three-dimensional thermal-mechanical coupled finite element model is established, in which the tilt angle is considered in the geometric model. This model is shown to be able to predict the in-process thermal-mechanical state variables (e.g. temperature, flow path and stress) during the welding and the post-welding morphology of the weld. The predicted temperature distribution and weld morphology agree well with the corresponding experimental results. It is shown by the simulation results that wormhole defects appear in the weld with a zero tilt angle, while no such defects are observed in the case that the tilt angle of 2° is employed. In order to get a deeper understanding of the weld formation mechanism, the thermal-mechanical condition around the tool during welding with/without tilt angle are compared and analysed. It is found from our simulation that both the peak temperature and compressive normal stress on material in the rear side of the tool are elevated by including a tilt angle of 2°. This temperature increase induced by the tilt angle will significantly soften the workpiece, meanwhile higher frictional driving force in the rear side will be generated by the increased compressive force. Therefore, the material flow in the rear side of the tool with the title angle is significantly enhanced, which contributes to avoid the generation of wormhole defect on the advancing side.

The mechanical response of a polyetheretherketone femoral knee implant under a deep squatting loading condition.

The current study was designed to investigate the mechanical response of a polyetheretherketone-on-polyethylene total knee replacement device during a deep squat. Application of this high-demand loading condition can identify weaknesses of the polyetheretherketone relative to cobalt-chromium. This study investigated whether the implant is strong enough for this type of loading, whether cement stresses are considerably changed and whether a polyetheretherketone femoral component is likely to lead to reduced periprosthetic bone loss as compared to a cobalt-chromium component. A finite element model of a total knee arthroplasty subjected to a deep squat loading condition, which was previously published, was adapted with an alternative total knee arthroplasty design made of either polyetheretherketone or cobalt-chromium. The maximum tensile and compressive stresses within the implant and cement mantle were analysed against their yield and fatigue stress levels. The amount of stress shielding within the bone was compared between the polyetheretherketone and cobalt-chromium cases. Relative to its material strength, tensile peak stresses were higher in the cobalt-chromium implant; compressive peak stresses were higher in the polyetheretherketone implant. The stress patterns differed substantially between polyetheretherketone and cobalt-chromium. The tensile stresses in the cement mantle supporting the polyetheretherketone implant were up to 33% lower than with the cobalt-chromium component, but twice as high for compression. Stress shielding was reduced to a median of 1% for the polyetheretherketone implant versus 56% for the cobalt-chromium implant. Both the polyetheretherketone implant and the underlying cement mantle should be able to cope with the stress levels present during a deep squat. Relative to the cobalt-chromium component, stress shielding of the periprosthetic femur was substantially less with a polyetheretherketone femoral component.

High Strain Rate Response of Rocks Under Dynamic Loading Using Split Hopkinson Pressure Bar

In the present work, dynamic stress–strain response of five sedimentary and three metamorphic rocks from different regions of India, e.g. Kota sandstone, Dholpur sandstone, Kota limestone, Himalayan limestone, dolomite, quartzite, quartzitic gneiss and phyllite have been investigated through split Hopkinson pressure bar test at different strain rates. The dry density, specific gravity, static compressive strength and tensile strength values of the rocks have also been determined. Petrological studies of the rocks have been carried out through X-ray diffraction test and scanning electron microscope test. It is observed from the stress–strain response of the rocks that the peak stress increases with increasing strain rate. Dynamic increase factors for the strength of these rocks have been determined by comparing the dynamic and the static peak compressive stresses and correlation equations are proposed.

Estimated third metatarsal bending stresses are highly susceptible to variations in bone geometry

Background: Third metatarsal stress fractures are relatively common during Royal Marines recruit training; however, their aetiology is poorly understood. Mathematical modelling of the third metatarsal may aid in understanding risk factors for stress fracture, particularly if the influence of footwear on peak bending stresses can be determined. This study built on previous models of metatarsal bending stress by integrating individual metatarsal geometry and gait data.Methods: Data from five males with size 11 (UK) feet were acquired. MRI images were digitised to determine cross-sectional bone parameters. Gait variables included vertical ground reaction forces, plantar pressure and foot orientation. The magnitude and location of peak bending stresses were calculated for barefoot running, before standard issue combat boots and trainers were compared.Findings: Estimated peak compressive, tensile and torsional stresses were greater in combat assault boots than in trainers (p < 0.05) with medium effect sizes but wide confidence intervals. However, differences in bone geometry between individuals had a much greater influence on estimated peak stresses.Interpretation: Results suggest that bone geometry has a greater influence on third metatarsal stress fracture risk than footwear. Future bone stress simulations should account for bone geometry. Further development of the model in a variety of participants should proceed to verify these suggestions.

Microstructure and mechanical properties of particle reinforced metal matrix composites prepared by gas-solid two-phase atomization and deposition technology

In this paper, a gas-solid two-phase atomization and deposition process for high-quality particle reinforced metal matrix composites was proposed and used in the manufacture of 7055-SiCp composites specimens. The microstructure and mechanical properties of fabricated specimens were tested and then compared with conventional spray formed 7055Al alloy to evaluate the proposed technology. The average grain size of 7055-SiCp composites is about 16μm, which is much smaller than that of 7055Al alloy (about 24μm). The SiC particles uniformly distribute throughout the matrix, and no particle agglomeration can be observed. The compression peak stress of 7055-SiCp composites at room temperature is 687.5MPa, and 4.74% larger than that of spray formed 7055Al alloy. Results indicate the gas-solid two-phase atomization and deposition process proposed in this paper to be a promising high-quality metal matrix composites preparation technology.

Real-Time Monitoring of Piezoresistive Smart Cement To Verify Operations

This article, written by Special Publications Editor Adam Wilson, contains highlights of paper OTC 27060, “Field Test for Real-Time Monitoring of Piezoresistive Smart Cement To Verify Cementing Operations,” by C. Vipulanandan, M. Ali, B. Basirat, A. Reddy, N. Amin, and A. Mohammed, University of Houston; S. Dighe, SPE, Baker Hughes; and H. Farzam, Cemex, prepared for the 2016 Offshore Technology Conference, Houston, 2–5 May. The paper has not been peer reviewed. Copyright 2016 Offshore Technology Conference. Reproduced by permission. A field well was designed, built, and used to demonstrate the concept of real-time monitoring of the flow of drilling mud and smart cement and the hardening of the cement in place. A new method has been developed to measure the electrical resistivity of the materials with the two-probe method. Using the new concept, it has been proved that resistivity dominated the behavior of drilling fluid and smart cement. Introduction With reported failures and growing interest in environmental and economic concerns in the oil and gas industry, integrity of the cement sheath is of major importance. Oilwell cement serves many purposes in cemented oil and gas wells. Foremost among these is to form a sealing layer between the well casing and the geological formation. In order to characterize the cement and cement concrete in various applications, electrical-resistivity measurement has been used by many researchers. Studies have also examined the piezo-resistive behavior of modified cement-based and polymer composites. One conclusion is that the change in resistivity has the potential to be used to determine the integrity of the materials. Smart Oilwell Cement Cement-slurry flow and stability are the major requirements in well cementing. Oil- and gas-well cements are usually made from Portland cement clinker or from blended hydraulic cements. When admixtures are used with cement, tensile and flexural properties are modified. Also, admixtures will have an effect on the rheology, corrosion resistance, shrinkage, thermal conductivity, specific heat, electrical conductivity, and absorption (heat and energy) properties of cement. A smart cement has been developed that can sense any changes going on inside the borehole during cementing and during curing after the cementing job. The smart cement can sense the changes in the water/cement ratio, different additives, and any pressure applied to the cement sheath in terms of piezo resistivity. The failure compressive strain for the smart cement was 0.2% at peak compressive stress, and the resistivity change is on the order of several hundreds, making it more than 500 times more sensitive. Theory and Concept Impedance Model. Equivalent Circuit. It is important to identify the most appropriate equivalent circuit to represent the electrical properties of a material to characterize its performance with time. In this study, different possible equivalent circuits were analyzed to find an appropriate equivalent circuit to represent smart cement and drilling fluid.

Effects of frost-damage on mechanical performance of concrete

The aim of this study is to provide the quantificational change laws of strength, stiffness, and deformation capacity of frost-damaged concrete relating to a united index, the data were obtained by different researchers. Then the index of relative compressive strength (RCS) was introduced as the indicator of frost damage and a large number of mechanical performance testing data of frost-damaged concrete were collected and analyzed. By curve fitting, the correlations between RCS and the initial elastic modulus, the strain at peak compressive stress, and biaxial compressive strength, and tensile strength, and the strain at peak tensile stress were established. Thereafter, the analytical stress-strain response of frost-damaged concrete under monotonic loading was presented using RCS and compared with that of the experimental data. Moreover, an isotropic elastoplastic damage model of frost-damaged concrete subjected to repeated loading was established. Finally, we can systematically estimate the effects of frost-damage on the mechanical performance of concrete, which can be provided for the numerical simulation of frost-damaged concrete structures.

Dynamic Characterization of Himalayan Quartzite using SHPB

In the present work, dynamic stress-strain response of Himalayan quartzite is tested under high loading rates using split Hopkinson pressure bar (SHPB) device for the first time in the literature. The physical and static mechanical properties of quartzite e.g. dry and saturated density, specific gravity, static compressive strength and elastic modulus values are also determined. Petrological studies of quartzite are carried out through X-ray diffraction (XRD) test and scanning electron microscope (SEM) test. In the SHPB tests, it is observed from the stress-strain response that the dynamic peak stress increases with increasing strain rate whereas the elastic modulus does not show any clear trend with increase in strain rate. Dynamic force equilibrium at the incident and transmission bar ends of the rock samples is attained in all tests till the failure of the rock samples. Dynamic increase factor (DIF) for the rock is determined at a particular strain rate by comparing the dynamic to static peak compressive stress. Correlation equation for dynamic strength increase factor with respect to strain rate has been proposed herein.

Application of the discrete element method for the simulation of size effects in concrete samples

This study aims to determine the validity of the discrete element method in predicting the size effect of cylindrical concrete samples under compression. For this purpose, a series of simulations are carried out on a set of particle assemblies with different diameters and aspect ratios. A fixed particle-size distribution and a single set of model parameters are used for all numerical simulations. Using compressive peak stress, peak strain and stress–strain curve as reference, numerical results are compared with experimental observations. The discrete element model is shown to effectively reproduce the effect of aspect ratio on compressive properties. On the other hand, although the method succeeds to capture the so called ‘wall effect’, it fails to show good correspondence with experimental values due to the assumption of spherical particles. The effect of end friction is also addressed. These findings give insight into the scalability of discrete element models when used for the simulation of concrete material.

Allowable stresses in high-frequency mechanical impact (HFMI)-treated joints subjected to variable amplitude loading

The effectiveness of high-frequency mechanical impact (HFMI) is considered to rely on the existence of compressive residual stresses. To determine when residual stress relaxation occurs, and what the resulting influence on fatigue improvement is, local stress-strain response in as-welded and HFMI-treated weld toes was modelled under different peak stress conditions. Then, effective notch stress analysis was used to correlate these results with available experimental observations. The simulations showed that high stress ratios and compressive peak stresses were critical with respect to residual stress relaxation, as expected. A compressive peak stress of 0.6f y (nominal yield strength) resulted in full residual stress relaxation. The relative fatigue damage calculations and the notch stress analysis indicated, however, that fatigue improvement could be expected even after significant residual stress relaxation. Based on this and previously observed benefit for high stress ratios, an increase in maximum allowable stresses for HFMI-treated welded steel joints is suggested. The maximum stress ratio is proposed to be increased from R = 0.52 to R = 0.7, and the maximum stress range to limit compressive stresses is proposed to be increased from ΔS max = 0.9f y to ΔS max = 1.2f y , which corresponds to S min = −0.6f y for stress ratio R = −1.

Accuracy and mechanical properties oforthodontic models printed 3-dimensionally from calcium sulfate before and after various postprinting treatments.

Dental models fabricated with 3-dimensional printing technologies are revolutionizing the practice of orthodontics, but they generally comprise polymeric materials that may not be suitable for certain applications, such as soldering appliances. The objective of this study was to investigate the dimensional accuracy and mechanical properties of 3-dimensional printed ceramic-based models before and after various treatments intended to improve their mechanical properties. Thirty identical models were printed 3-dimensionally from a calcium sulfate-based substrate and divided into 3 groups for treatment: high heat (250°C for 30minutes), low heat (150°C for 30minutes), and Epsom salt treatment. Each model was scanned before and after treatment with a laser scanner, and dimensional stability was analyzed by digital superimpositions using a best-fit algorithm. The models were weighed before and after treatment to evaluate mass changes. Additionally, 3-dimensional printed cylinders treated as described above and an untreated control group were subjected to compressive mechanical testing (n=11 per group). The Epsom salt treatment group had statistically significant increases in both peak compressive stress and modulus of elasticity when compared with the other treatment groups. All treatment groups had statistically significant changes in mass, with the Epsom salt group gaining mass and the 2 heat-treatment groups losing mass. The low-temperature treatment group had a statistically significantly lower mean average for dimensional deviations (0.026±0.010mm) than did the other treatment groups (0.069±0.006 and 0.059±0.010mm for high temperature and Epsom salt, respectively). Dental models printed 3-dimensionally with calcium sulfate and treated with Epsom salt showed significant improvement in compressive mechanical properties and retained clinically acceptable dimensional stability.

Effects of pre-tension on fatigue behavior of rolled magnesium alloy

The effects of pre-tension on the fatigue properties of a rolled Mg-3Al-1Zn (AZ31) alloy were investigated by imposing 2%, 5%, and 10% tensile strains along the rolling direction before low-cycle fatigue tests. The investigation showed that the applied pre-tension increased the dislocation density and introduced a small number of twins into the material. Although the as-rolled sample and all the pre-tensioned samples exhibited asymmetric hysteresis loops owing to the alternation of twinning and detwinning during each cycle, the tensile and compressive peak stresses increased with an increase in the amount of pre-tension during the first cycle. With an increase in the number of cycles, however, the flow stress, mean stress, plastic strain energy density, and hysteresis loop of the samples became similar irrespective of the amount of pre-tension, which is attributed to cyclic strain softening or hardening behavior depending on the initial dislocation density. Despite the relatively large pre-deformation, the pre-tensioned samples exhibited a fatigue life equivalent to that of the as-rolled sample because of an early extinction of the difference in the cyclic stress-strain response caused by pre-tension. This result demonstrates that the pre-tension process can improve the yield strength of rolled Mg alloys without a loss of their low-cycle fatigue resistance.

Microstructure damage evolution associated with cyclic deformation for extruded AZ31B magnesium alloy

Fatigue damage evolution of extruded AZ31B magnesium (Mg) alloy is investigated under strain-controlled tension-compression loading along the extrusion direction at various strain amplitudes, and the different cyclic deformation behaviors are observed. At the strain amplitude of 2%, the tensile peak stress displays significant cyclic softening, whereas the compressive peak stress shows consistent cyclic hardening. At 1%, moderate cyclic hardening is observed at both the tensile peak and compressive peak stresses. At 0.5%, the tensile peak stress presents stable cyclic hardening, whereas the compressive peak stress almost keeps constant. The microstructure morphologies associated with the cyclic deformation are analyzed by scanning electronic microscope (SEM). The degree of deformation twins is evaluated by analyzing X-ray diffraction (XRD) using a normalized parameter λ. The results show the fatigue crack initiation modes and its propagation modes are dependent on the strain amplitude. At 2%, grain boundary (GB) cracking and triple joint cracking are detected after 1st loading cycle. At 1%, fatigue crack initiates at grain boundary (GB cracking), twin boundary (TB cracking) and triple joint of three neighboring grains. Both grain boundary induced (GB-induced) intergranular and persistent slip band induced (PSB-induced) transgranular propagation modes play an important role in the early-stage crack growth. At 0.5%, crack initiation modes are similar to that at 1%, but GB-induced intergranular propagation mode dominates the early-stage crack growth. The effects of the microstructure (texture, grain size and uniformity) on the fatigue damage behavior are discussed.

Strain-hardening behavior and microstructure development in polycrystalline as-cast Mg-Zn-Y alloys with LPSO phase subjected to cyclic loading

The strain-hardening behavior and microstructural development of polycrystalline as-cast Mg-Zn-Y alloys with various volume fractions of the long-period stacking ordered (LPSO) phase subjected to cyclic loading were experimentally evaluated. For all alloys, cyclic loading tests with a constant strain amplitude of 0.5% for up to 100 cycles showed asymmetric cyclic hardening behavior. That is, the absolute value of the compressive peak stress significantly increased during cyclic loading while the tensile peak stress slightly decreased. With increasing volume fraction of the LPSO phase, the stress amplitude significantly increased. Cyclic loading tests after compressive preloading up to 200 or 250MPa resulted in a significant increase in the stress amplitude, while a number of kink bands developed during preloading. For the cyclic hardening behavior, the contribution of the increase in kinematic hardening was significant in the alloys with a higher volume fraction of the LPSO phase. Transmission electron microscopy observation of the cyclically deformed Mg85Zn6Y9 alloy indicated the formation of a deformation-induced band, where the crystal structure was transformed from 18R-LPSO to hcp-Mg with the exclusion of solute elements.

Spalling and brittleness in surface instability failure of rock

Failure of rock close to a free surface is reproduced in laboratory tests on two types of sandstone. Digital image correlation is used to study the in-plane displacement fields prior to and during the successive formation of surface-parallel fractures. The axial and lateral displacements suggest that the displacement gradient is uniform in both directions at early loading stages; as the axial load increases, the free-face effect starts to influence the displacement contours, especially the lateral displacement field. Localised damage forms a spall before reaching peak stress. Using a simple model, a rock brittleness index is proposed as the ratio of uniaxial compressive strength and surface instability peak stress.

Mechanical response of fine grained Ti2AlC under extreme thermo-mechanical loading conditions

An experimental investigation was conducted to evaluate the compressive constitutive behavior and fracture initiation toughness of fine grained (∼4.2μm) Ti2AlC in dynamic and quasi-static loading at different temperatures. A Split Hopkinson Pressure Bar (SHPB) apparatus was used in conjunction with an induction coil heating system for dynamic experiments at elevated temperatures, while a universal mechanical testing machine equipped with an extensometer and furnace was used for quasi-static compressive testing. A series of experiments were conducted at different temperatures from 25°C to 1200°C and strain rates of 10−4s−1 and 500s−1. A single edge notched specimen was used to determine the fracture initiation toughness in dynamic and quasi-static loading from ambient temperature to 900°C. The SHPB apparatus was modified for this purpose and high speed photography was incorporated to determine fracture initiation time, and the load corresponding to this time was used to calculate the dynamic fracture toughness. The results of this study reveal that the peak compressive failure stress in dynamic conditions decreases with increasing temperature, from 1600MPa at room temperature to 850MPa at 1200°C. In the dynamic testing condition, the failure remains predominately brittle even at temperatures as high as 1200°C. Under static loading conditions, the peak compressive stresses were always lower than those in dynamic loading conditions, but they also decreased monotonically with increasing temperature. Also under static conditions, a significant drop in peak compressive failure stress and an increase in ductility was observed above the brittle-to-plastic transition temperature of around 900°C. The fracture experiments reveal that the strength and fracture toughness in dynamic conditions are higher than the corresponding quasi-static values by approximately 35% at room temperature and that they both decrease with increasing temperatures. It was found that high temperature and low strain rate promote plastic behavior of Ti2AlC.