Dynamic Tensile Strain Research Articles

Dynamic Tensile and Compressive Behaviors of Mild Steel at Wide Range of Strain Rates

The purpose of the present paper is to investigate the mechanical behavior of mild steel at quasi-static (0.001 s−1) and different rates of dynamic tensile (5–750 s−1) and compressive (125–2,350 s−1) strain rates. Quasi-static experiments are conducted on a universal testing machine to study the stress-strain behavior of mild steel. A hydropneumatic machine and a modified Hopkinson bar are used to investigate the dynamic tensile behavior of mild steel specimens at medium and high strain rates, respectively, whereas the specimens are tested on a split Hopkinson pressure bar to acquire understanding of the strain rate sensitivity of mild steel under dynamic compression. The effects of a pulse shaper and gauge length of the specimen in the dynamic compression tests are investigated. High-speed photography has been used to monitor the deformation of the specimen at high strain rate experiments. The applicability of the existing Cowper-Symonds and Johnson-Cook material models to represent the mechanical behavior of mild steel in a plastic zone is examined. Finally, the fractographs of the tested tensile specimens are studied using a scanning electron microscope.

Tensile properties of military chin-strap webbing

Helmet security relies on a retention system that is usually manufactured from webbing. In many helmets (e.g. bicycle, climbing) this is polyester or nylon webbing. In UK military helmets, cotton webbing is currently used for the retention system, including the chin-strap. Cotton is the preferred fiber content rather than a synthetic-polymer fiber due to the potential melt hazard of the latter. Whether the retention system of military helmets is strain rate sensitive at quasi-static and dynamic rates and whether the webbing degrades when exposed to ultraviolet radiation (UVR) are of interest with reference to current conflicts. This paper (i) presents data on quasi-static tensile properties at varying strain rates, (ii) describes a method developed to measure the dynamic tensile strain rate sensitivity and (iii) uses that method to determine the effect of simulated UVR exposure on dynamic tensile properties of cotton webbing typical of that used in UK military helmets.

Mechanical properties of baboon cartilage demonstrate a similar decline with age as human cartilage

Purpose: Osteoarthritis (OA) is the most common form of arthritis and the major cause of activity limitation and physical disability in older people. It is widely believed that OA results from the local mechanical environment of the joint and, particularly, the cartilage, in combination with systemic susceptibility to the disease. Preliminary study of knee OA in the baboon demonstrated that knee OA occurs commonly and naturally in male and female adults and the frequency of OA in older baboons is comparable to that in older humans. The objectives of this study were to evaluate whether the mechanical properties of baboon cartilage follow similar trends with age and sex as humans, and whether the presence of affected cartilage leads to mechanical property degradation. Methods: Sample groups were established based on combinations of age (i.e. young/old), sex (i.e. male/female), and overall femur OA status (i.e. affected/unaffected) (target n 1⁄4 5 in each group combination). Baboons were categorized as young for age> 15 years (developmentally equivalent to 45 human years) and old for age > 22 years (developmentally equivalent to 66 human years). Overall femoral condyle OA status was determined based on established cartilage grading methods. Knee cartilage was obtained from the medial portion of the distal condyle of baboon right femurs collected at routine necropsy at the Southwest National Primate Research Center/Texas Biomedical Research Institute. Dogbone-shaped tensile specimens were prepared with the gage region oriented perpendicularly with the primary direction of collagen orientation. Specimens were kept hydrated and loaded under displacement control to 10% and 20% strain at 0.25%/sec and allowed to stress relax until equilibrium was reached, and finally loaded to failure. Equilibrium tensile modulus, dynamic tensile modulus, peak stress (strength) and strain at the point of peak stress were determined from the normalized load-displacement data. Wilcoxon rank sum tests were used to evaluate statistical significance of group differences. Results: There were significant decreases between the young and old specimens in strength and strain at peak stress (Figure 1). Equilibrium modulus and dynamic modulus demonstrated increased stiffness for cartilage specimens from old animals; however, these differences were not statistically significant (Figure 1). Differences in cartilage properties based on sex and OA status were not statistically significant (p > 0.187 in all cases and p > 0.259 in all cases, respectively). Conclusions: Despite the small sample sizes used in this study, statistical differences were demonstrated between cartilage specimens obtained from young and old baboons in both strength and the strain at peak stress. It is well documented that cartilage mechanical properties demonstrate decline with age in humans and our demonstration that baboon cartilage properties behave in a similar fashion further justifies the use of the baboon as a model for the human osteoarthritic condition, in addition to the physiological similarities between species. We expect that results obtained from larger sample sizes of cartilage specimens obtained from locations throughout the baboon knee joint will further elucidate the effects of age, sex, and OA involvement on the functional behavior of cartilage.

EFFECT OF STRAIN RATE ON MICROSTRUCTURE EVOLUTION AND MECHANICAL BEHAVIOR OF A LOW C HIGH Mn TRIP/TWIP STEELS

The microstructure and mechanical properties of Fe-18Mn low carbon high manganese TRIP/TWIP steels during tensile tests in the range of initial strain rate of 1.67×10 −4 — 10 3 s −1 at room temperature were studied. The inverse effect of strain rate on strength of steel was produced, the strength and ductility of steels decreased with increasing strain rate in the range of quasi-static tensile strain rate of 1.67×10 −4 —1.67×10 −1 s −1 . While inverse effect of strain rate on ductility of steels was produced in the range of dynamic tensile strain rate of 10 1 —10 3 s −1 , the strength and ductility of materials increased significantly with increasing strain rate. The tensile strength of high manganese TRIP/TWIP steels was 957 MPa and their elongation was 55.8%. These results indicated that Fe-18Mn steel had excellent mechanical properties and good fracture resistance. The higher the strain rates applied, the less martensite, the more directions of deformation twins. The microstructure evolution of the specimen was analyzed by SEM, TEM and XRD, martensitic transformation and

CULTIVATION OF CHONDROCYTES AND MENISCUS CELLS IN THERMO-RESPONSIVE HYDROGELS CONTAINING CHITOSAN AND HYALURONIC ACID UNDER MECHANICAL TENSILE STIMULATION

Temperature-responsive hydrogel scaffold containing chitosan and hyaluronic acid was used to entrap primary chondrocytes and meniscus cells. The effect of dynamic tensile strain on the cells/hydrogel constructs was evaluated by measuring cell proliferation, biosynthetic activity, and viability. The results demonstrated that mechanical deformation applied at 15% tensile strain, 0.5 Hz, and 10 min per day for 43 days resulted in substantial increases in glycosaminoglycan (36% for chondrocytes and 31% for meniscus cells) and collagen productions (37% for chondrocytes and 52% for meniscus cells) over static controls while not significantly affecting cell proliferation and viability.

Experimental Study on Impact Compression and Spliting of Granite

In order to systematically study on dynamic mechanics character of granite, impact compression experiments and impact flattened Brazilian disc specimens of granite have been investigated with modified split Hopkinson pressure bar (SHPB) experimental facility, curve about stress versus strain and other parameter at strain rates of 23.9/s～108.4/s were obtained from impact compression. The dynamic tensile strength and critical tensile failure strain at strain rates of 2.3/s～25.6/s were obtained from impact flattened Brazilian. The experimental result show that dynamic compression strength , elastic modulus and failure strain，dynamic tensile strength significantly increase comparing to quasi-static experiment, and the above mechanics parameter include dynamic strength etc will increase with strain rate increasing, granite have the character of impact harding and ductility enhancement. The failure degree of failure will increase with increasing strain rate under impact compression; the failure configurations of the specimens present an center splitting mode under impact flattened Brazilian experiments. Whether impact compression or impact splitting under strain rate including this paper ’s experiments, the relationship between the DIFC or DIFT and the logarithm of strain rate is linear.

Impact Experimental Study for Dynamic Character of Reactive Powder Concrete

In order to systematically study dynamic mechanics character of reactive powder concrete (RPC), impact compression experiments and impact flattened Brazilian disc specimens of RPC have been investigated with modified split Hopkinson pressure bar (SHPB) experimental facility using brass pulse shaper, curves about stress versus strain and other parameters at strain rates of 20.3/s~137.0/s were obtained from impact compression. The dynamic tensile strength and tensile failure strain at strain rates of 3.4/s~26.2/s were obtained from impact flattened Brazilian. For comparison, the quasi-static compress and split tension of RPC were obtained with an MTS 810 materials test system and CSS-88500 electron universal material testing machine.The experimental result show that dynamic compression strength , elastic modulus and failure strain,dynamic tensile strength and failure strain significantly increase comparing to quasi-static experiment, RPC have the character of impact harding and ductility enhancement. RPC exhibit excellent failure patterns at high strain rate. Whether impact compression or impact splitting under strain rate including this paper ’s experiments, the relationship between the DIFC or DIFT and the logarithm of strain rateis linear.

Anti-inflammatory effects of EMD in the presence of biomechanical loading and interleukin-1β in vitro

Enamel matrix derivative (EMD) used to promote periodontal regeneration has been shown to exert anti-inflammatory effects. This in vitro study was performed to investigate if the anti-inflammatory actions of EMD are modulated by the local cellular environment, such as inflammation or occlusal, i.e., biomechanical, loading. Human periodontal ligament cells were seeded on BioFlex plates and incubated with EMD under normal, inflammatory, and biomechanical loading conditions for 1 and 6 days. In order to mimic inflammatory and biomechanical loading conditions in vitro, cells were stimulated with interleukin (IL)-1β and exposed to dynamic tensile strain, respectively. The gene expression of IL-1β, IL-1 receptor antagonist (IL-1RN), IL-6, IL-8, IL-10, and cyclooxygenase (COX)-2 was analyzed by real-time RT-PCR and the IL-6 protein synthesis by enzyme-linked immunoassay. For statistical analysis, Student's t test, ANOVA, and post-hoc comparison tests were applied (p < 0.05). EMD downregulated significantly the expression of IL-1β and COX-2 at 1 day and of IL-6, IL-8, and COX-2 at 6 days in normal condition. In an inflammatory environment, the anti-inflammatory actions of EMD were significantly enhanced at 6 days. In the presence of low biomechanical loading, EMD caused a downregulation of IL-1β and IL-8, whereas high biomechanical loading significantly abrogated the anti-inflammatory effects of EMD at both days. Neither IL-1RN nor IL-10 was upregulated by EMD. These data suggest that high occlusal forces may abrogate anti-inflammatory effects of EMD and should, therefore, be avoided immediately after the application of EMD to achieve best healing results.

Molecular Variations Related to the Regional Differences in Periosteal Growth at the Mandibular Ramus

Periosteal growth at human mandibular ramus is characterized by bone apposition at the posterior border and resorption at the anterior border. Molecular control of this regional variation is unclear. This study examined the expression of several molecules involved in bone apposition/resorption at these regions in vivo and in vitro. By using growing pigs as a model, the periosteal growth was assessed at the mandibular ramus by vital staining and histological observations. In parallel, periosteal tissues were harvested and pulverized for RNA and protein extraction. Periosteal cells were also isolated, expanded in osteogenic media, and subjected to a single dose of dynamic tensile strain (0, 5, or 10% magnitude at 0.5 Hz) to examine their responses to mechanical loading. Real-time RT-PCR and Western blot analyses were used to examine mRNA and protein expression from periosteal tissues and cultured cells. Histological observation confirmed an anterior-resorption/posterior-apposition pattern in the pig mandibular ramus. Both in vivo tissue and in vitro cells demonstrated greater mRNA expression of receptor activator of NF-κB ligand (RANKL)/osteoprotegerin (OPG) ratio and bone morphogenetic protein 2 (BMP2) at the anterior region, while OPG expression at the anterior region was lower than the posterior region. In response to the application of a single dose of dynamic tensile strain, cultured periosteal cells appeared to change the expression profile of osteogenic markers but not that of RANKL/OPG and BMP2. These findings suggest that the unique regional variation of periosteal activity at the mandibular ramus is regulated by a differential expression of RANKL/OPG ratio (likely through differential induction of OPG) and BMP2.

Dynamic small strain measurements of Kevlar ® 129 single fibers with a miniaturized tension Kolsky bar

We adopted a non-contact laser technique to measure axial small strain (<∼5%) of Kevlar ® 129 fibers in a miniaturized tension Kolsky bar. Accurate small strain measurement is necessary to determine the elastic response of the fibers. In this optical arrangement, a laser beam passes through a line generator lens, which is known as a Powell glass, yielding a thin sheet of light with uniform intensity. This laser sheet passes through a gap formed by the fixture holding the fiber specimen. As the specimen is stretched, an increasing amount of light passes through the gap, which is measured by a high-speed, high resolution photo-detector mounted on the other side of the gap. Dynamic tensile experiments performed at a constant strain rate of ∼1500/s on Kevlar ® 129 fibers demonstrate the capability of this non-contact laser technique in the determination of dynamic tensile strain histories with low amplitudes.

Influence of Recycled Asphalt Pavement on Fatigue Performance of Asphalt Concrete Base Courses

This paper presents the physical properties of recycled asphalt pavement RAP and its influence on the mechanical perfor- mance of a binder course asphalt pavement mix. A series of binder course mixes were designed containing varying percentages of RAP. A mix made from only virgin material was selected as the control mix for the investigation. The effect of introducing RAP into the binder course mix was evaluated through a series of laboratory tests including the Marshall test, the indirect tensile stiffness modulus test, the indirect tensile fatigue test and the water sensitivity test. A circular wheel track CWT was developed in order to study the dynamic effects of a rolling wheel traveling over an asphalt pavement. The CWT was commissioned within a temperature controlled room along with a customized data acquisition system. The system involves the testing of rectangular slabs and allows for the investigation of dynamic tensile strain. The laboratory tests have shown that the introduction of RAP to the binder course mix resulted in an improvement in all mechanical properties. In particular, it was found that the mix containing up to 30% RAP, displayed improved fatigue resistance relative to the control mix manufactured from virgin materials.

Effects of cyclic dynamic tensile strain on previously compressed inner annulus fibrosus and nucleus pulposus cells of human intervertebral disc-an in vitro study

Our objective was to investigate whether dynamic tensile strain on previously compressed human intervertebral disc (IVD) cells can restore the biosynthetic effects of collagen and glycosaminoglycan. Inner annulus fibrosus (AF) and nucleus pulposus (NP) tissues of adolescent idiopathic scoliosis cases undergoing thoracoscopic discectomy and fusion were cultured on compressive plates. Compressive stress was applied using 0.4 MPa at 1 Hz, for 2 h twice a day for 7 days, to the inner AF and NP tissues, followed by equibiaxial cyclic tensile strain to deform the released cells onto the plate's flexible bottom. With 10% elongation at a rate of 1 Hz, for 2 h twice a day for 7 days, a significant increase in the level of collagen and glycosaminoglycan of the previously compressed inner AF, as well as the level of glycosaminoglycan of the previously compressed NP cells were found. The DNA content and number of endoplasmic reticulum under transmission electron micrograph of the previously compressed inner AF and NP cell were also significantly increased. The results suggested that equibiaxial cyclic tensile strain at a rate of 1 Hz with 10% tensile strain was capable of increasing collagen and glycosaminoglycan synthesis of previously compressed inner AF cells, and glycosaminoglycan synthesis of previously compressed NP cells.

Particulate ionomer films prepared from dispersions of crosslinked polymer colloids: A structure–property study

Polymers with properties that are governed by ionic interactions in discrete regions are termed ionomers. This work presents a comprehensive study of polymer films prepared from dispersions of crosslinked poly(butadiene/methacrylic acid) (poly(Bd/MAA)) particles in water. We showed recently [O. Pinprayoon, R. Groves, B.R. Saunders, J. Colloid Interface Sci. 321 (2008) 315–322] that poly(Bd/MAA) particles could be considered to have a core-shell structure with a MAA-rich shell. The poly(Bd/MAA) dispersions used here contained added ZnO particles which provided a source of Zn 2+. The films were investigated using AFM, FTIR, small-angle X-ray scattering (SAXS), dynamic mechanical thermal analysis (DMTA) and tensile stress vs. strain measurements. The films showed differences and similarities to conventional ionomers which are discussed. The FTIR data revealed that the Zn 2+ ions formed ionic crosslinks with the carboxylate groups (RCOO −). Mechanical property studies were conducted using DMTA and stress vs. strain measurements. The DMTA data revealed evidence of two main phases in the systems: a poly(Bd)-rich phase and a poly(MAA)-rich phase. A third phase was also found; which was ascribed to an interphase between the particle core and shell. The SAXS data showed that the films were composed of distinct poly(Bd/MAA) particles, implying that only partial coalescence occurred across the particle interfaces during formation of these particulate ionomer films. Ionic crosslinking of the particles was suggested to occur in the interfacial region (knit regions). The SAXS data also showed scattering from Zn 2+(RCOO −) ionic-aggregates. These ionic aggregates had an average separation of 2.9 nm, which is much smaller than that for conventional ionomers. Interestingly, the concentration of these aggregates passed through a maximum and decreased as the degree of neutralisation of the MAA groups approached 100%. A conceptual model for the structure of these unconventional ionomers is discussed.

Regulation of matrix metalloproteinase expression by dynamic tensile strain in rat fibrochondrocytes

We sought to determine the molecular basis for the anticatabolic effects of mechanical signals on fibrocartilage cells by studying the expression of a variety of matrix metalloproteinases (MMPs). Furthermore, we examined whether the effects of biomechanical strain on MMP gene expression are sustained. Fibrochondrocytes from temporomandibular joint (TMJ) discs were exposed to dynamic tensile strain for various time intervals in the presence of interleukin (IL)-1beta. The regulation of the messenger RNA (mRNA) expression and synthesis of MMPs and tissue inhibitors of MMPs (TIMPs) were examined by end-point and real-time reverse transcriptase-polymerase chain reaction (RT-PCR) as well as Western blot analysis. Fibrochondrocytes expressed mRNA for MMP-2, -3, -7, -8, -9, -11, -13, -14, -16, -17, and -19 as well as TIMP-1, -2, and -3, IL-1beta induced a significant (P<0.05) upregulation of mRNA for MMP-3, -7, -8, -9, -13, -16, -17, and -19. The IL-1beta-stimulated upregulation of these MMPs was significantly (P<0.05) abrogated by dynamic tensile strain. However, MMP-2, -11, -14, and TIMPs were not affected by either IL-1beta or tensile strain. Biomechanical strain also inhibited the IL-1beta-stimulated protein synthesis of MMP-3, -7, -8, -9, -13, -16, and -17. Application of mechanical strain for various time intervals during a 24-h incubation with IL-1beta showed that the suppressive effects of mechanical signals are sustained. The data provide evidence that biomechanical signals can downregulate the catabolic activity of fibrocartilage cells in an inflammatory environment by inhibiting the expression of a variety of MMPs. Furthermore, the matrix-protective effects of biomechanical signals are sustained even in an inflammatory environment.

Effects of Shock Vibration on Concrete

This paper reports on the effects of shock vibration on concrete using a newly developed test method that applies hammer blows to prismatic specimens in the longitudinal direction and evaluates the short- and long-term effects by observing crack formation, measuring the immediate change in ultrasonic pulse velocity, and measuring the reductions in 28-day tensile and compressive strengths. A total of 198 prisms cast of typical concrete mixtures with 28-day cube strength ranging from 40 to 60 MPa was tested at ages of 12 hours to 28 days. The tests demonstrated that the major effect of shock vibration is the formation of transverse cracks. Based on the test results, the shock vibration resistances of the concrete mixtures at different ages were ascertained and correlated to their material properties. The study found that the single most important material parameter governing the shock vibration resistance of concrete is the dynamic tensile strain capacity. Additionally, several new sets of vibration control limits, which are less conservative than existing ones, have been established. Unlike previous control limits, the factor of safety is explicitly known, thus the vibration control limits can be adjusted.