scholarly journals Influence of Reversed Fatigue Loading on Damage Evolution of Cross-Ply Carbon Fibre Composites

Materials ◽  
2019 ◽  
Vol 12 (7) ◽  
pp. 1153 ◽  
Author(s):  
Just ◽  
Koch ◽  
Brod ◽  
Jansen ◽  
Gude ◽  
...  
Keyword(s):  
Residual Stresses ◽  
Carbon Fibre ◽  
Crack Formation ◽  
Crack Opening ◽  
Local Stress ◽  
Fatigue Loading ◽  
Stress Concentrations ◽  
Delamination Growth ◽  
Stress Ratios ◽  
Carbon Fibre Composites

Microcrack formation and delamination growth are the main damage mechanisms in thefatigue of composites. They lead to significant stiffness loss, introduce stress concentrations andcan be the origin of subsequent damage events like buckling or fibre breakage, especially in case ofshear and compression stresses during load reversal. Fatigue experiments of carbon fibre reinforcedlaminates were conducted at several stress ratios and analysed in terms of crack and delaminationgrowth. These investigations were accompanied by microscopic imaging, digital image correlationand finite element modelling to take into account the effects of residual stresses and crack closure.It was found that residual stresses significantly change the local stress ratio in off-axis layers andlead to residual crack opening of inter fibre cracks. These cracks remain open and close under highcompression loadings only. Furthermore, crack formation under pulsating compression loadingturned out to be driven by residual stresses leading to perpendicular cracks as observed underpure tension loading. The experimental findings further confirm the severe detrimental effect oftension-compression loading on crack formation and delamination growth compared to pulsatingtension-tension or compression-compression loads.

Computational simulation of the relaxation of local stresses due to foreign object damage under cyclic loading

2010 ◽  
Vol 224 (2) ◽  
pp. 41-50 ◽  
Author(s):  
T Davis ◽  
J Ding ◽  
W Sun ◽  
S B Leen
Keyword(s):  
Stress Relaxation ◽  
Kinematic Hardening ◽  
Computational Simulation ◽  
Fatigue Loading ◽  
Cyclic Fatigue ◽  
Foreign Object ◽  
Stress Concentrations ◽  
Foreign Object Damage ◽  
Stress Ratios ◽  
The Impact

In this study, the phenomenon of residual stress relaxation from foreign object damage (FOD) is numerically simulated using a hybrid explicit—implicit finite-element method. The effects of cycle fatigue loadings on stress relaxation were studied. FOD is first simulated by firing a 3mm cube impacting onto a plate made of titanium alloy Ti-6Al-4V at 200m/s. The FOD impact produces two distinct stress concentrations: one is compressive directly beneath the impact site; the other is tensile around the outer edge of the impact. The plate was then assumed to be subjected to a cyclic fatigue loading. The stress relaxation was investigated under a range of stress ratios and maximum applied stresses. Two different material models were considered for the simulations, namely an elastic—perfectly plastic model and a non-linear kinematic hardening model.

scholarly journals Air-temperature control on diurnal variations in microseismicity at Laohugou Glacier No. 12, Qilian Mountains

2019 ◽  
Vol 60 (79) ◽  
pp. 125-136 ◽  
Author(s):  
Tao Zhang ◽  
Yuqiao Chen ◽  
Min Ding ◽  
Zhongyan Shen ◽  
Yuande Yang ◽  
...  
Keyword(s):  
Short Duration ◽  
Crack Opening ◽  
Low Frequency ◽  
Local Stress ◽  
Stress Concentrations ◽  
Night Time ◽  
Strong Negative Correlation ◽  
Near Surface ◽  
Temperature Change Rate ◽  
Trigger Algorithm

ABSTRACTWe conducted a 9-d seismic experiment in October 2015 at Laohugou Glacier No. 12. We identified microseismic signals using the short-term/long-term average trigger algorithm at four stations and classified them as long and short-duration events based on waveform, frequency, duration and magnitude characteristics. Both categories show systematical diurnal trends. The long-duration events are low-frequency tremor-like events that mainly occurred during the daytime with only several events per day. These events lasted tens of seconds to tens of minutes and are likely related to resonance of daytime meltwater. The dominant short-duration events mostly occurred during the night time with a peak occurrence frequency of ~360 h−1. Their short-duration (<0.2 s), high frequency (20–100 Hz) and dominance of Rayleigh waves are typical of events for near-surface crack opening. A strong negative correlation between the hourly event number and temperature change rate suggests that the occurrence of night-time events is controlled by the rate of night-time cooling. We estimated the near-surface tensile stress due to thermal contraction at night to be tens of kilopascals, which is enough to induce opening of surface cracks with pre-existing local stress concentrations, although we cannot exclude the effect of refreezing of meltwater produced during the day.

Closure and Growth of Fatigue Cracks at Notches

1992 ◽  
Vol 114 (1) ◽  
pp. 1-7 ◽  
Author(s):  
R. C. McClung ◽  
H. Sehitoglu
Keyword(s):  
Finite Element ◽  
Crack Growth ◽  
Fatigue Cracks ◽  
Crack Opening ◽  
Local Stress ◽  
Element Model ◽  
Rate Of Change ◽  
Growth Effect ◽  
Growth Data ◽  
Stress Ratios

The closure behavior of fatigue cracks growing out of notches is studied with an elastic-plastic finite element model. Crack opening stresses are shown to change significantly as the crack extends. Opening stresses are low at first and then gradually rise to stable values as the crack tip moves away from the notch field. These transient changes are not limited to the region of the original inelastic notch field. The rate of change of opening stresses with increasing crack length is a function of both nominal maximum stress and nominal stress ratio. Stable levels are reached more quickly at higher stress ratios and lower maximum stresses. These transient changes in Sopen have been emulated with a simple model which considers only changes in Sopen due to changes in the local stress field. The numerical results are quantitatively consistent with observed trends in experimental crack growth data, which show that accelerated crack growth can occur beyond the original notch plastic boundary. Finite element results and experimental data also both suggest that the accelerated short crack growth effect for cracks near notches is much less pronounced at higher stress ratios.

scholarly journals Deformation processes near a crack initiation site under dwell-fatigue loading of Ti-6Al-4V

2020 ◽  
Vol 321 ◽  
pp. 11074
Author(s):  
C. Lavogiez ◽  
S. Hémery ◽  
P. Villechaise
Keyword(s):  
Crack Initiation ◽  
Local Stress ◽  
Fatigue Loading ◽  
Initiation Site ◽  
Lattice Rotation ◽  
Stress Concentrations ◽  
Slip Bands ◽  
Dwell Fatigue ◽  
Local Lattice ◽  
Deformation Processes

The present article reports an investigation of the mechanism of surface crack initiation of a dwell fatigue tested Ti-6Al-4V alloy with a bi-modal microstructure. Interactions between slip bands and grain boundaries were characterized in order to obtain insights into the crack initiation process and discuss the similarities with models described in the literature. Twinning and local lattice rotation occurred as a result of the slip band blocking at the interface and suggests high local stress concentrations. Nevertheless, crack initiation happened to be intergranular and not transgranular. The crack opened up the basal plane that was located at the interface between two nodules poorly oriented for slip and having a common c axis of the hexagonal unit cell.

A Simple Model for Fatigue Crack Growth Near Stress Concentrations

1991 ◽  
Vol 113 (4) ◽  
pp. 542-548 ◽  
Author(s):  
R. C. McClung
Keyword(s):  
Fatigue Crack ◽  
Simple Model ◽  
Fatigue Crack Growth ◽  
Crack Growth ◽  
Growth Rates ◽  
Fatigue Cracks ◽  
Crack Opening ◽  
Local Stress ◽  
Growth Data ◽  
Stress Concentrations

Fatigue crack growth rates are often difficult to predict for short cracks growing near stress concentrations. This paper presents a simple model to predict those growth rates which incorporates the phenomenon of crack closure. Crack opening stresses are shown to change significantly as cracks grow away from notches, and the simple model is designed to describe those changes. The effective stress range ratio, U, is assumed to be dependent on the local stress at the crack tip location in a corresponding uncracked body. The value of U changes with the normalized maximum stress in unnotched bodies, and this dependence can be quantified with elastic-plastic finite element models or simpler modified-Dugdale crack analyses. The local stress distribution is estimated with a Neuber analysis. A semi-empirical stress intensity factor solution is constructed and calibrated with known exact solutions. The crack growth rate is then calculated with the modified Paris law, taking crack growth constants from long crack data. The model is illustrated with a specific case study, the growth of cracks from center notches in an SAE 1026 steel. Experimental crack growth data for notches of different sizes and shapes compare favorably with the calculations. The scheme is contrasted with previous models for notch fatigue cracks. The implications of the simple model for other fatigue design problems are explored, highlighting the simplicity and generality of the model.

Sign in / Sign up

Export Citation Format

Share Document