Acoustic Response and Micro-Damage Mechanism of Fiber Composite Materials under Mode-Ⅱ Delamination

Realizing the accurate characterization for the dynamic damage process is a great challenge. Here we carry out testing simultaneously for dynamic monitoring and acoustic emission （AE） statistical analysis towards fiber composites under mode-Ⅱ delamination damage. The load curve, AE relative energy, amplitude distribution, and amplitude spectrum are obtained and the delamination damage mechanism of the composites is investigated by the microscopic observation of a fractured specimen. The results show that the micro-damage accumulation around the crack tip region has a great effect on the evolutionary process of delamination. AE characteristics and amplitude spectrum represent the damage and the physical mechanism originating from the hierarchical microstructure. Our finding provides a novel aud feasible strategy to simultaneously evaluate the dynamic response and micro-damage mechanism for fiber composites.

THEORETICAL ANALYSIS ON THE NUCLEATION OF MICRO-DAMAGE IN CEMENT MORTAR UNDER COMPRESSIVE LOADING AND SULFATE ATTACK

Effect of uniaxial compression on the nucleation of micro-damage in cement mortar under sulfate attack is investigated. Shape and size of micro-voids in cement mortar is detected using Micro Computed Tomography techniques. The formation of delayed ettringite crystal is analyzed using scanning electron microscope and energy disperse spectrum methods. Deformation of micro-voids and the distribution of stress at the surface of a micro-void are calculated. It is found that the nucleation of micro-cracks is caused by the tensile stress at the voids＇ surface, and such damage nucleation will be speeded up by the remote uniaxial compressive load.

Creep Damage Characteristics of Soft Rock under Disturbance Loads

This article focuses on the process of rock creep damage and micro-damage evolution properties of gray green mudstone under impacting disturbance load conditions for the first time using the real time computerized tomography （CT） testing technique. The results indicate that axial load comes into limit strength neighborhood, rock micro-crack links into larger crack, creep rate increases in a short time, larger plastic deformation happens; this is called disturbance accelerating creep stage. When rock is within limit strength neighborhood, there occurs creep micro-damage under smaller disturbance load. When disturbance load is larger, rock directly enters into disturbance accelerating creep stage, failure occurs instantaneously. On the basis of experimental research, the CT scanning method was used to describe the creep micro-damage of soft rock, also helpful in the prediction of roadways＇ service life and evaluation of geotechnical engineering stability.

EXPERIMENTAL STUDY AND NUMERICAL ANALYSIS OF DYNAMIC FRACTURE IN DUCTILE SOLIDS

Spall tests in pure copper and LY12 aluminum alloy are performed. The mechanism of the nucleation, growth and coalescence of micro-voids is observed by means of microscopic observation of the recovered specimens. The model presented in Ref. [4] is incorporated in a hydrodynamic one-dimensional finite-difference computer code to simulate the processes of spallation in pure copper and LY12 aluminum alloy specimens. This model can satisfactorily describe the processes of spallation.

Quantitative characterization of toughening mechanisms of rubber modified polymers

The volume changes of rubber modified polymers under creep at room temperature were successfully used to characterize the toughening mechanisms of blends with brittle polymer matrices such as high impact polystyrene.This approach cannot be applied to pseudo-ductile polymers such as polypropylene and polyamide,because they are ductile when stretched at low speed at room temperature.Based on the time-temperature equivalence princi ple,the volume change at low temperature is proposed to characterize quantitatively the toughening mechanisms of polymer blends with ductile matrices,which is illustrated by applying this approach to rubber modified polypropylene