Analysis of the extended stress-based forming limit curve considering the effects of strain path and through-thickness normal stress

In this study, an approach based on the modified Marciniak–Kuczynski (M–K) method for computation of an extended stress-based forming limit curve (FLC) is presented. The extended stress-based FLC is built based on equivalent plastic stress versus mean stress. This curve has some advantages in comparison with the conventional FLC. This new criterion is much more strain path independent than the conventional FLC. The effect of strain path on the predicted extended stress-based FLC is reexamined. For this purpose, two types of pre-straining on the sheet metal have been loaded. Moreover, the plane stress state assumption is not adopted in the current study. The influence of a through-thickness compressive normal stress is also investigated theoretically. The verifications of the theoretical FLCs are performed by using some available published experimental data.     

Fatigue strength of steel rollers with failure occurring at the weld root based on the local strain energy values: modelling and fatigue assessment

Weldments geometry with failures occurring at the weld toe or at the weld root cannot, by its nature, be precisely defined. Parameters such as bead shape and toe or root radius vary from joint to joint even in well-controlled manufacturing operations. The worst case configuration can be achieved by modelling as a sharp, zero radius, notch both the toe and the weld root. The intensity of asymptotic stress distributions obeying Williams’ solution is quantified by means of the Notch Stress Intensity Factors (NSIFs). For steel welded joints with failures originated from the weld roots, where the lack of penetration zone is treated as a crack-like notch, units for NSIFs are the same as conventional SIFs used in LEFM. The different dimensionality of NSIFs for different notch opening angles does not allow a direct comparison of failures occurring at the weld toe or at the weld root. In order to overcome the problem related to the variability of the V-notch opening angle, a simple scalar quantity, i.e. the value of the strain energy density (SED) averaged in the structural volume surrounding the notch tip, has been introduced. This energy is given in closed form on the basis of the relevant NSIFs for modes I, II and III. The radius R_c of the averaging zone is carefully identified with reference to conventional arc welding processes being equal to 0.28 mm for welded joints made of steel.The local-energy based criterion is applied here to steel welded rollers produced by Rulmeca and subjected to prevailing mode I (with failures at the weld root). The aim of the paper is firstly to describe the employed methodology for the fatigue assessment and secondly to show the first synthesis of fatigue data by means of local SED for a specific geometry.     

Manual materials handling: a survey of risks,and the selection and training of workers in Singapore

A survey of 83 plants employing safety officers indicated that in 66% of them, workers were involved in manual lifting. The average number of manual lifting tasks was three per plant. Only about 6% of the total workforce were involved in manual lifting. Using the NIOSH Lifting Guidelines, the survey revealed that 30% of the lifting tasks were above the Maximum Permissible Limit, 50% were between the Maximum Permissible Limit and the Action Level, and only 20% were below the Action Level. A majority of the plants used a self-selection method to match a worker to the physical demands of a given manual lifting task. Training of workers in manual materials handling was provided by 76% of the plants. The principal methods of training included a poster campaign and demonstration of lifting techniques. The basic handling skills were taught in 90% of the plants. The content of training was incomplete, however, and further improvement was necessary.     

Characterization of mechanically sheared edges of dual phase steels

In recent years advanced high strength steels (AHSS) received increased interest for light structures with improved performance, but they are often sensitive to edge cracking during sheet metal forming. In this study mechanically sheared edges were characterized for three dual phase steels (DP600, DP780 and DP980), sheared with three die clearances (5%t, 10%t, 15%t) and along rolling and transverse directions. Microstructures of the materials were provided first, and then the sheared edges were examined by optical microscopy and scanning electron microscopy that reveal the morphology and random feature of the sheared edges. A factorial analysis was performed to reveal the general trends of the processing parameters on four edge zones. A new strain measurement method was used for characterizing strain distribution in the sheared region, which shows the peak strain to be higher than 3. The strain quickly decreases from sheared edge to interior, leaving a shear-affected zone of about 500 μm or 31% of the thickness. The fracture processes and involved mechanisms were discussed.     

Experimental study on the dynamic compressive mechanical properties of concrete at elevated temperature

Strain rate effect and temperature effect are two important factors affecting the mechanical behavior of concrete. Each of them has been studied for several years. However, the two factors usually work together in the engineering practice. It is necessary to understand the mechanical responses of concrete under high strain rate and elevated temperature. A self-designed high temperature SHPB apparatus was used to study the dynamic compressive mechanical properties of concrete at elevated temperature. The results show that the dynamic compressive strength and specific energy absorption of concrete increase with strain rate at all temperatures. The elastic modulus decreases obviously with strain rate at room temperature and stabilizes at a level with slightly decrease at elevated temperature. The dynamic compressive strength of concrete at 400 °C increases by nearly 14% compared to the room temperature. However, it decreases at 200 °C, 600 °C and 800 °C with the decrease ratio of 20%, 16% and 48%, respectively. The dynamic elastic modulus decreases largely subjected to elevated temperature. The specific energy absorption at 200 °C, 400 °C and 600 °C is higher than room temperature and decreases to be lower than room temperature at 800 °C. Formulas are established under the consideration of mutual effect of strain rate and temperature.     

Influence of curvature geometry of laminated FRP composite panels on delamination damage in adhesively bonded lap shear joints

Three dimensional non-linear finite element analyses of Lap Shear Joints (LSJs) made with curved laminated FRP composite panels having pre-existing delaminations between the first and second plies of the strap adherend have been carried out using contact and Multi-Point Constraint elements (MPC). Progressive growth of delamination has been simulated by sequential release of the MPC elements. Strain Energy Release Rate (SERR), being an indicative parameter has been computed using Virtual Crack Closure Technique (VCCT) for assessing the growth and propagation of the delamination damage fronts. The inter-laminar stresses and the SERRs at the two fronts of the pre-embedded delamination are found to be significantly influenced by the delamination size. The three individual modes of SERR on the two delamination fronts are found to be much different from each other, indicating dissimilar rates of propagation. The curvature geometry of adherends significantly influences the SERR values. It is seen that decrease of radius of curvature of adherend panels, keeping their widths unchanged, increases the SERR values. Flatter FRP composite adherends have superior resistance to delamination damage propagation as compared to LSJs made with curved composite laminated panels.     

Multi-response optimization of post-fire performance of strain hardening cementitious composite

This paper presents results of an experimental program conducted to optimize the post-fire performance of Strain Hardening Cementititous Composites (SHCC) using Taguchi approach with utility concept. The experiments were first undertaken by determining nine SHCC mixes using a standard L9 (3⁴) orthogonal array of four parameters and each parameter with three levels. The four parameters of SHCC mixes included fly-ash/binder ratio, sand/binder ratio, water/binder ratio and fiber proportions. The responses of SHCC to be optimized were tensile strain capacity, compressive strength and post-fire compressive strength after subjected to 200 °C, 400 °C, 600 °C and 800 °C of isothermal heating. Utility concept was introduced to simplify the multi-response problem into mono-response question together with Taguchi method. The role of different parameters on the composite responses of SHCC was examined. Furthermore, an optimal SHCC mix to maximize multi-responses was determined based on statistical analysis and validated by additional confirmation tests.     

Effects of biaxial strain on bulk 8% yttria-stabilised zirconia ion conduction through molecular dynamics

Tensile strain is thought to give rise to enhanced conduction properties in ion conducting compounds. However, most experimental studies in the field involve simultaneous presence of interface structures and strain, thus complicating separation of the individual effects. Here, we present molecular dynamics calculations that clarify the influence of biaxial strain in bulk yttria-stabilised zirconia. Such a study mimics what may be experimentally observed in epitaxially deposited films. We show that, as expected, tensile strain leads to enhanced ion conduction properties. The maximum enhancement is observed for a 2–3% tensile strain. We show that the increase of bulk diffusion is in part due to an opening of the Zr–Zr and Zr–Y distances induced by tensile strain, leading to a smaller oxygen migration energy. Above a 3% tensile strain, the diffusion coefficient of oxygen is strongly reduced, reaching values even lower than without strain. This decrease is associated with important structural changes of the cation and oxygen network. Also, we show that the diffusion coefficient increases by less than a factor 2 at 833 K for the optimal strain value. This confirms that the great increase of conductivity observed in zirconia/strontium titanate multilayers was due either to an electron contribution from strontium titanate or to the presence of interfaces, but not to the direct influence of strain on the oxygen diffusion coefficient in zirconia.