Numerical and experimental investigation of notch-induced high-speed precise shearing of 42CrMo bar

A notch-induced high-speed precise shearing method was developed for high-strength metal bars, which prefabricated V-shape circumferential notches in batch on the bar surface to make stress concentration, and applied a high-speed load to complete separation on a new type of electric-pneumatic counter hammer. The FE simulation and experimental tests were conducted; the influences of loading speed, notch depth, and axial clearance were analyzed on the fracture behavior and blank quality; the microfracture mechanism was further investigated. The results showed that the circumferential notch inhibited the plastic distortion and obtained high precision chamfered billets, with a roundness error of 1.34%, flatness error of 0.34 mm, and incline angle of 0.87°. Besides, the surface notch effectively reduced Max. impact force and fracture energy. The fractography revealed that: for the notched bar, the cracks initiated from the thin extrusion layers at the bilateral-notch tips, and from micro extrusion and intrusion at the top-notch tip. The predominant microfracture mechanism involves microvoid coalescence and forming of quasi-parabolic dimples along with the shear stress.

Experimental and numerical determination of critical osmotic blister size affecting the strength of aged FRP seawater pipe

Glass fiber-reinforced polymer (GFRP) composite pipelines are used by many industries for fluids transport including seawater for cooling. The durability of these pipes can be affected where the loss of their strength is due to the occurrence of several internal irreversible micro-damages. One of the challenges facing the integrity of these pipelines is the presence of surface defects. The present research aims to determine the critical size of osmotic blister affecting a 30- year-old seawater handling GFRP pipe. Osmotic blisters were simulated through surface notches with two different geometries and sizes. Longitudinal pipe mechanical strength was studied through tensile tests to study the effect of the surface notch size. At a certain surface notch depth, the strength of the pipe wall decreased with increasing the notch surface depth. This represents the critical value of the damage size called maximum undamaged defect size and noted d max. Damage with size below d max does not affect the strength of the pipe wall. To simulate the progressive failure of this aged composite, a 3-D finite element model was employed based on Hashin’s failure criteria.