Fractography, aided by heat tinting, was unable to detect any prior cracks due to the HCF stress transients on the fractured specimens.
The influence of the strengthening material's properties and prior cracking of the member are investigated and discussed by analyzing failure mechanisms, load-deflection curves, and strain measurements for steel bars and CFRP materials observed during loading experiments.
Prior to crack growth test, the specimen was pre-cracked at ambient temperature using a load shedding method which has a sinusoidal waveform, a ratio of 0.1, a frequency of 20Hz and an initial stress intensity factor range (ΔK) of 20 MPa√m.
The dynamic stress intensity factor history prior to crack initiation in each case shows a rapid increase at different rates with the crack on the compliant side of the graded sheet experiencing higher rate of loading relative to the one with the crack on the stiffer side.
Finite element modelling of the stress strain fields in the specimen immediately prior to crack propagation across the ligament was used to extract the flow properties, the damage initiation strain and the crack propagation energy as functions of stress triaxiality and strain rate.
A critical condition exists where excessive out-of-plane displacement may occur upon load reversal prior to crack closure causing the wall to buckle.
The grain refinement might be the result of local plasticity around inclusions in VHCF prior to crack initiation or propagation and thereby causal for the late failure.
During deformation at low and moderate fraction solids, the X-ray radiographs indicate that there is significant feeding of interdendritic liquid in the region of strain localization prior to crack formation.
Analysis of deformation properties of brittle materials revealed (Talonov and Tulinov 1988) that the characteristics of media weakened by fractures with edges interacting under compression stress fields depend on the stress-strain state at the stage prior to crack growth initiation.
The other key finding was that all calculated absolute angles prior to crack initiation were below 5° and hence small, leading to the presumption that at least cracking, but probably also full rupture, takes place in nearly all cases with correction angles above 5°.
