Fatigue in FRP strengthen beams - CoMSIRU in FRP strengthen beams Building Codes Recommendations for strengthened structures under fatigue load 1- The ACI 440.2R-08 (2008) recommendation

Fatigue in FRP strengthen beams

9/12/2013


Professor Dr. Björn Täljsten

Luleå University of Technology

Sto Scandinavia AB

and

PhD Student Mohammed Mahal

Luleå University of Technology


Agenda • Type of fatigue

• Assessment of fatigue life (Analysis method)

• Fatigue design of Strengthened Beams

• Building Codes Recommendations for

strengthened structures under fatigue load.

• Ongoing tests at Luleå University of Technology


The fatigue load type can be divided into low and high stress type:

Low Fatigue Stress type (High cycle fatigue) to simulate traffic loads on bridges

,In this type, the maximum effective equivalent stress resulting from maximum

cyclic load in the first cycle is lower than the yield stress of the structure.

High Fatigue Stress type (Low cycle fatigue) for seismic investigation purpose,

in this type, the maximum effective equivalent stress resulting from maximum

cyclic load is above the yield stress of the structure.


Low Fatigue Stress type (High cycle fatigue)

High Fatigue Stress type (Low cycle fatigue)


Assessment fatigue life (Analysis method)

A. Stress versus lifetime (S-N) curves approach

The most common approach to relate the applied stress to the fatigue life of a

material is the stress-life (S-N) approach, in which the number of cycles to failure is

plotted against the applied stress range. S-N Curves are generally plotted on semi-

log or log-log paper where each dot represents the results of a single test specimen.

Fatigue tests tend to be time consuming and expensive; each data point represents

many hours of testing. Fatigue life models do not take into account the actual

degradation mechanisms.


You have to use different models depending on the type of failure that is

expected. Material failure, bond failure, structural failure etc.


Example on regressions equations for CFRP, GFRP and NSM

strengthened beams and are modeled with the following equations,

see also next slide :

Where Sr is the stress range in the reinforcing steel



B- Damage mechanics approach and Fracture mechanics approach

Damage mechanics approach. This approach is most suitable to use in Finite Element

Modeling for simulating high cyclic fatigue where it represents structural behavior at

micro scale.

The model assumes that the damage and plasticity occur at the micro scale and have no

influence on the elastic macroscopic behavior. For that, the fatigue limit stress is taken as

the yield stress. This means that the stress over this value is causing plastic strain, and

no damage happens below this value.

Assessment fatigue life (Analysis method)


The material model for FRP, concrete and steel reinforcement represents

with the following


Double shear joint test of Yun et al. (a) Specimen details (b) FE model of

quarter of the specimen


Comparison between experimental and numerical results


B- Damage mechanics approach and Fracture mechanics approach

Fracture mechanics approach is also most applicable for Finite Element Modeling,

here for simulating low cyclic fatigue especially when we have debonding condition and

crack propagation in concrete



Building Codes Recommendations for strengthened structures

under fatigue load

1- The ACI 440.2R-08 (2008) recommendation to prevent fatigue and creep failure for

GFRP, AFRP and CFRP material is to have a total stress to FRP ultimate strength ratio

below of 0.2, 0.3 and 0.55 respectively.

2- The JSCE recommendations (JSCE 2001) recommended a reduction factor of µ = 0.7

on the interfacial fracture energy relating to the bonding of fiber reinforced polymer sheets

to concrete under fatigue loading.

3- The Italian design guide CNR-DT200 (NRC 2004) recommended a long-term

conversion factor, η = 0.5, multiplied by a property of FRP composites to prevent possible

fatigue failure.

4- ISIS Canada design manual (ISIS Canada 2008) only recommended a reduction

factor to account for the effect of creep on FRP composites without fatigue load.

5- The model code 2010 (draft) indicated different values of stress range for different

types of FRP bar which can be used as reinforcement bars in concrete.


Experimental Work

Beam setup


General view of beam at failure


Crack Propagation


Steel Rupture


Debonding after fatigue failure in the steel reinforcement

Documents

Fatigue in FRP strengthen beams - CoMSIRU in FRP strengthen beams Building Codes Recommendations for strengthened structures under fatigue load 1- The ACI 440.2R-08 (2008) recommendation