TY - RPRT
T1 - Lifetime and residual strength of wood subjected to static and
variable load
AU - Nielsen, Lauge Fuglsang
PY - 1997
Y1 - 1997
N2 - The DVM-theory (Damaged Viscoelastic Material) previously
developed by the author to predict lifetime of wood subjected to
static loads is further developed in this paper such that harmonic
load variations can also be considered. Lifetime (real time or
number of cycles) is predicted as a function of load amplitude,
load average, fractional time under maximum load, and load
frequency. The analysis includes prediction of residual strength
(re-cycle strength) during the process of load cycling. It is
concluded that number of cycles to failure is a very poor design
criterion. The theory is successfully compared with data from
experiments representing different wood products. Master graphs
are developed which can be used in fatigue design of wood products
in general. These graphs are valid for any creep behavior
(relaxation, moisture content) and materials quality (grading,
strength level).It is demonstrated how the theory developed can be
generalised also to consider non-harmonic load variations. An
algorithm is presented for this purpose which might be suggested
as a qualified alternative to the Palmgren-Miner’s method normally
used in fatigue analysis of materials under arbitrary load
variations.It is strongly emphasised throughout the paper that
reliable mechanical durability studies on wood cannot be made if
the influence of wood rheology is disregarded. Lifetime predicted
by Palmgren-Miner methods may cause considerably overestimated
time to failure, especially at low frequencies. Results from
accelerated test can be used in practice only when test conditions
simulate practice. It is, however, not realistic to think of
experiments running under forecasted live conditions.
AB - The DVM-theory (Damaged Viscoelastic Material) previously
developed by the author to predict lifetime of wood subjected to
static loads is further developed in this paper such that harmonic
load variations can also be considered. Lifetime (real time or
number of cycles) is predicted as a function of load amplitude,
load average, fractional time under maximum load, and load
frequency. The analysis includes prediction of residual strength
(re-cycle strength) during the process of load cycling. It is
concluded that number of cycles to failure is a very poor design
criterion. The theory is successfully compared with data from
experiments representing different wood products. Master graphs
are developed which can be used in fatigue design of wood products
in general. These graphs are valid for any creep behavior
(relaxation, moisture content) and materials quality (grading,
strength level).It is demonstrated how the theory developed can be
generalised also to consider non-harmonic load variations. An
algorithm is presented for this purpose which might be suggested
as a qualified alternative to the Palmgren-Miner’s method normally
used in fatigue analysis of materials under arbitrary load
variations.It is strongly emphasised throughout the paper that
reliable mechanical durability studies on wood cannot be made if
the influence of wood rheology is disregarded. Lifetime predicted
by Palmgren-Miner methods may cause considerably overestimated
time to failure, especially at low frequencies. Results from
accelerated test can be used in practice only when test conditions
simulate practice. It is, however, not realistic to think of
experiments running under forecasted live conditions.
M3 - Report
BT - Lifetime and residual strength of wood subjected to static and
variable load
T2 - IUFRO Timber Engineering Conference on Fatigue of wood
Y2 - 1 January 1997
ER -