This paper describes an experimental and analytical study on the static fatigue behavior of piezoelectric ceramics under electromechanical loading. Static fatigue tests were carried out in three-point bending with the single-edge precracked-beam specimens. The crack was created perpendicular to the poling direction. Time-to-failure under different mechanical loads and dc electric fields were obtained from the experiment. Microscopic examination of the fracture surface of the piezoelectric ceramics was performed as well. A finite element analysis was also made, and the applied energy release rate for the permeable crack model was calculated. The effect of applied dc electric fields on the energy release rate versus lifetime curve is examined. The most important conclusion we reach is that the lifetimes for the piezoelectric specimens under a positive electric field are much shorter than the failure times of specimens under a negative electric field for the same mechanical load level.
Accurate determination of friction at the die/billet interface in hydrostatic extrusion is a complex issue due to involvement of various operating parameters viz. billet velocity, die geometry, contact pressure, material parameter and the regime of lubrications prevailing at die/billet interface. Therefore, the objective of this paper is to investigate friction and friction stress at die/billet interface in hydrostatic extrusion process of aluminium based alloys. The friction stress at die/billet interface is numerically computed for three lubricants whose rheology is represented by Roelands' viscosity model. Investigations have been carried out for friction stress variations along the work zone for a wide range of extrusion ratios (A = 2 to 10), semi-die angles (θ = 100 to 200) and material parameters (G = 0.67 to 1.86). Moreover, the validation of the proposed model has been done with the published work available in the literature.
Green composites, such as natural fiber reinforced naturally-derived plastics attract much attention because of reducing CO2 emission. Many studies about green composites have enabled the materials substitutes as glass fiber reinforced composites. However, lack of investigations about fatigue properties restricts the actual usage of the composites. In the present study, we investigate fatigue properties of green composites. A hemp fiber yarn reinforced poly(lactic acid) composite was selected as a green composite. Unidirectional (UD) and textile (Textile) composites were fabricated using micro-braiding technique. Fatigue tests results indicated that fatigue damages in UD composites was splitting which occurred just before the final fracture, while matrix crack and debonding between matrix and fiber yarn occurred and accumulated stably in Textile composites. These results were consistent with modulus reduction and acoustic emission measurement during fatigue tests.
The present study aims to clarify the scale dependency of progressive adhesion behavior of work material on micro tools under the dry friction in microforming. Scaled progressive deep drawing test up to 300 times is conducted under dry condition in micro- and milli- scale. The process dimensions of 0.97mm and 5.82mm in drawn cup diameter are produced with the stainless steel foils of 0.05mm and 0.3mm in thickness, respectively. The experimental results show that the transition of maximum punch force has different tendencies in each scale. These tendencies are well corresponded to the transition in surface state of tools and drawn cups. While, the strong adhesive wear is observed for the milli-scale, there is a slight change in the surface state of the tools for micro-scale. To investigate this difference in the adhesion behavior of the work material in each scale, a finite element analysis considering surface asperities is conducted. The distribution of the adhesion volume on the die corner radius is evaluated with a semi-empirical wear model in which is calculated with a function of normal pressure and relative velocity between blank and die. The results show the low adhesion volume in micro-scale, due to the short sliding distance during the process. The progressive adhesion behavior of work material and the advantage of tool life in micro-scale metal forming are demonstrated.
This paper describes about the joint strength and their improvement of a type 5052 aluminum alloy (A5052) autocompleting friction welded joint, that welding method was developed by authors. When the joint was made at a friction pressure of 60 MPa, it had approximately 75% joint efficiency at a groove bottom thickness of 1.1 mm with an overhanging length of the weld faying surface part (overhanging length) for the fixed workpieces of 15 mm. Those joints had the flexural deformation of the fixed workpiece during the welding, although those had the circumferential shear fracture by the increasing insert thickness with reliability. To reduce the flexural deformation of the fixed workpiece, the joint was made with an overhanging length for the fixed workpieces of 5.0 mm. The joint had 100% joint efficiency with the base metal fracture at a groove bottom thickness of 1.4 mm which was made at a friction pressure of 80 MPa, although that did not achieve 100% joint efficiency at a friction pressure of 60 MPa. In addition, the joint with a groove bottom thickness of 1.3 mm or thin did not achieve 100% joint efficiency, and that with 1.5 mm or thick did not have the circumferential shear fracture. To obtain the generating of the circumferential shear fracture with reliability during the friction process and 100% joint efficiency with the base metal fracture, the joint should be made with thick insert piece, opportune groove bottom thickness of the insert piece, short overhanging length for the fixed workpieces, and relatively high friction pressure.
