The relation between compressive strength of carbon/epoxy fabrics and micro-tow geometry with various bias angles

In this paper compressive tests of carbon/epoxy (plain weave, 3k) fabric composites were performed to investigate the relation between compressive strength and various bias angles and shear angles. Compressive properties such as chord modulus and maximum strength of the fabric composite materials are essential to analyze the drape behaviour and estimate the quality of the final products. Various specimens with different bias and shear angles which were fabricated by using autoclave de-gassing moulding process were prepared to estimate the strength and chord modulus with respect to the bias and shear angle variations. The stacking sequences of the compressive test specimens are [0]_10T, [15]_10T, [30]_10T and [45]_10T for bias specimens and [±37]_10T, [±32]_10T, [±28]_10T, [±22]_10T for sheared specimens. Micro-tow structures were observed to correlate the fabric compressive strength with crimp angle. Anti-buckling rig was involved to prevent specimens from buckling during the compressive tests. The compressive test was performed with 1.3 mm/min strain rates. Compressive test results were compared with calculation results. Facture modes which were classified in two different modes were analyzed using microscopic observation.     

Fracture mechanics of air-entrained concrete subjected to compression

Air voids are entrained in concrete for protection of constructed elements, especially highway pavements, against freeze-thaw damage. Entrained air void systems inadvertently reduce the compressive strength of the concrete. The present study describes development of an analytical model for evaluation of the effects of entrained air void system on the compressive strength of concrete. The model developed here will assist in predicting the compressive strength of concrete for specified mix designs. The constitutive relationships for air-entrained concrete were established by considering a micro cracked porous material with randomly distributed circular air voids and uniformly oriented cracks from the air voids. Linear elastic fracture mechanics was employed to explain the evolution of damage due to the individual voids and cracks that emanate from such voids. The damage model considers the interactions among the voids and cracks during various stages of loading. The analytical results from this study were evaluated through an experimental program for comparison of the computed and measured compressive strengths. A wide range of samples were examined that included concretes with air contents ranging from 2% to 13% air by volume of concrete. The experiments involved microscopic determination of air content and spacing factors as well as compressive strength tests for all the concrete samples.     

Effect of pore structure on the compressive property of porous Ti produced by powder metallurgy technique

Porous Ti with an average macro-pore size of 200–400 μm and porosity in the range of 10–65% has been manufactured using polymethyl methacrylate (PMMA) powders as spacer particles. The compressive strength and elastic modulus of resultant porous Ti are observed in the range of 32–530 MPa and 0.7–23.3 GPa, respectively. With the increasing of the porosity and macro-pore size, the compressive strength and modulus decrease as described by Gibson–Ashby model. The failure due to cracking (complete fracture) of the struts on porous Ti is controlled primarily by macro-pores. Fractography shows evidence of the brittle cleavage fracture mainly, but containing a few fine shallow dimples and a small amount of transcrystalline fracture of similarly oriented laths. The failure mechanism has been discussed by taking the intrinsic microstructural features into consideration.     

Compressive sampling-based strategy for enhancing ADCs resolution

The paper deals with the problem of simultaneously enhancing both horizontal and vertical resolution of analog-to-digital converters, with specific regard to low-cost conversion systems. To this aim, the authors propose the combined exploitation of a suitable Compressive Sampling (CS) approach and a proper digital signal processing stage. In particular, starting from a reduced number of digitized samples, the proposed CS-based sampling approach allows to recover an oversampled version of the input signal, whose spectral content is properly shaped to reject the most of in-band noise. The successive processing stage, implementing a low-pass filter, is mandated to drastically attenuate out-of-band noise components.Tests carried out on an actual microcontroller (namely, PIC32MX360L512 by Microchip) evidence the promising performance of the proposed sampling strategy. Results obtained either on single tone or multisine signals highlight a gain up to 3.5 bits in vertical resolution, while the sample rate increases 50 times with respect to the actual one adopted to randomly sample the input signal of interest.     

Study of lime hemp concrete (LHC) – Mix design,casting process and mechanical behaviour

This paper deals with the mechanical behaviour of lime hemp composites. LHC blocks have been produced by compression in a rigid die at a relatively high compression pressure. This process allows the production of LHC with a high proportion of hemp shiv. New mechanical parameters are proposed to compare experimental results of this study with those found in published literature. This paper shows that a high compaction pressure enhances the compressive strength and can offset a reduction of binder. Consequently, a new formula is proposed to predict the strength of LHC which depends on both the binder content and the compaction state of the shiv particles. The study leads to recommendations for the mix design of such composites.     

Reducing back stress to nursing personnel: an ergonomic intervention in a nursing home

A prospective epidemiologic study was conducted in two units (140 beds and 57 nursing assistants) of a nursing home to demonstrate the efficacy of an ergonomic intervention strategy to reduce back stress to nursing personnel. The total programme involved the following: determining patient handling tasks perceived to be most stressful by the nursing assistants (NAs); performing an ergonomic evaluation of these tasks; and conducting a laboratory study to select patient transferring devices perceived to produce less physical stress than existing manual patient-handling methods. The intervention phase included training NAs in the use of these devices, modifying toilets and shower rooms, and applying techniques to patient care. Immediately after completing the intervention programme, a post-intervention analysis (which lasted eight months in unit 1 and four months in unit 2) was performed.

A biomechanical evaluation of the physical demands required to perform stressful patient-handling tasks showed that the mean compressive force on the L5/S1 disc, the mean hand force required to make a transfer, and the strength requirements (expressed as percentage female population capable) were 1964?N, 122?N, and 83% after intervention as compared to 4751?N, 312?N, and 41% before intervention. Subjectively, the mean rating of perceived exertion was less than ‘very light’ after intervention as compared to between ‘somewhat hard’ and ‘hard’ before intervention. Overall, the mean acceptability rates for the walking belt and the mechanical hoist were 81% and 87% for patient transfers. The incidence rate for back injuries prior to the intervention, 83 per 200 000 work-hours, decreased to 47 per 200 000 work-hours after the intervention. There were no injuries resulting in lost or restricted work days during the last four months of the post-intervention. It is concluded that an appropriate ergonomic intervention programme offers great promise in reducing physical stress and risk of low-back pain to nursing personnel. However, large-scale studies in different nursing homes are needed to confirm the above findings.     

Influence of nucleation seeding on the performance of carbonated MgO formulations

The continuation of the hydration and carbonation reactions within reactive MgO cement formulations is inhibited by the formation of hydrate and carbonate phases around MgO particles, resulting in a low MgO utility and limited mechanical performance. This study introduces carbonate seeds into the pore space of MgO-based concrete mixes to enable the nucleation and growth of carbonates on the seed surfaces. The influence of seeds on the hydration and carbonation capability, mechanical performance and microstructural development was evaluated through isothermal calorimetry, water absorption and compressive strength measurements, along with TGA, XRD and SEM analyses. The introduction of ≤1% seed within the initial mix design increased the carbonate phase content and improved carbonation degree by up to 96% by increasing the availability of Mg(OH)₂ for carbonation. The dense formation of carbonates in seeded samples enabled improved microstructures and 28-day strengths of 64 MPa, which were 33% higher than unseeded samples.     

Effects of compressive stresses on torsional fatigue

Rolling contact fatigue (RCF) is the dominant failure mode in properly installed and maintained ball and roller element bearings. Lundberg and Palmgren in their seminal publication indicated that this failure is due to the alternating component of shear stress. Thus, torsional fatigue experiments have been used to predict the RCF behavior of bearing materials. In non-conformal contacts, due to Hertzian pressure the contact experiences large compressive stresses. Hence, it is critical to take into account the effect of these large compressive stresses in torsional fatigue to better simulate RCF conditions. This paper presents an investigation of torsional fatigue of bearing steels, while the effects of combined compressive stress and its relevance to material behavior in rolling contact fatigue is examined. An MTS test rig was used to investigate the fatigue life of several bearing steels and their failure mechanisms were evaluated through fractography. Then the effects of compressive stresses on torsional fatigue were investigated. A set of custom designed clamp fixtures were designed, developed and used to apply Hertzian pressures of up to 2.5 GPa on the torsion specimens. The experimental results indicate that at high cycle fatigue, a combination of shear and biaxial compression, by application of Hertzian contact, is more detrimental to fatigue life than shear alone; however, as expected it has little to negligible effects in the low cycle fatigue regime. Also the failure mode changes such that fracture planes form a cup and cone pair with multiple internal cracks as opposed to helical planes observed in pure torsion which are formed by a single crack. A 3D finite element model (using ABAQUS) was developed to investigate the fatigue damage accumulation, crack initiation, and propagation in the material. The topology of steel microstructure is modeled employing a randomly generated Voronoi tessellation wherein each Voronoi cell represents a material grain and the boundaries between the cells are assumed to represent the weak plane in the steel matrix. Continuum damage mechanics (CDM) was used to model material degradation during the fatigue process. A comprehensive damage evolution equation is developed to account for the effect of mean stress on fatigue. The model predicts the fatigue lives and crack patterns successfully both in presence and absence of compressive stresses.     

Gas-diffusion layer's structural anisotropy induced localized instability of nafion membrane in polymer electrolyte fuel cell

The design of robust polymer electrolyte fuel cell requires a thorough understanding of the materials' response of the cell components to the operational conditions such as temperature and hydration. As the electrolyte membrane's mechanical properties are temperature, hydration and rate dependent, its response under cyclic loading is of significant importance to predict the damage onset and thus the membrane lifetime. This article reports on the variation in stress levels in the membrane induced due to the gas-diffusion layer's (GDL) anisotropic mechanical properties while accurately capturing the membrane's mechanical response under time dependent hygrothermomechanical conditions. An observation is made on the evolution of negative strain in the membrane under the bipolar plate channel area, which is an indication of membrane thinning, and the magnitude of this strain found to depend upon the GDL's in-plane mechanical properties. In order to come up with a strategy that reduces the magnitude of tensile stresses evolved in the membrane during the hygrothermal unloading and to increase the membrane's lifetime, we numerically show that by employing a fast hygrothermal loading rate and unloading rate strategy, significant reduction (in this study, nearly 100%) in the magnitude of tensile stresses is achievable. The present study assists in understanding the relation between materials compatibility and durability of fuel cell components.     

GUIDANCE,RESTRICTION AND KNOWLEDGE OF RESULTS

The offsets of five kinds of training wore compared with the performance of a control group in a manual positioning task. The training conditions were two schedules of mechanical guidance, a form of restrictive guidance, and two levels of knowledge of results. Performance deteriorated after the removal of detailed knowledge of results. All forms of training appeared to improve accuracy, the restriction condition being at least as effective as knowledge of results.