ASSESSMENT OF HYGROTHERMAL AGEING AND DAMAGE IN SANDWICH
FOAM USING TSA.
E. Lembessis, J.M.Dulieu-Barton, R.A. Shenoi.
School of Engineering Sciences, University of Southampton, Southampton, SO17 1BJ, UK
ABSTRACT
The present paper is an introduction to the use of Thermoelastic Stress Analysis (TSA) to assess the response to closed-cell
PVC structural foams from damage sustained during service in a marine environment. The effects of damage were
accelerated by hygrothermal ageing at a temperature of 60ºC in distilled water. The CTS-type foam specimens contained a
scalpel induced ‘edge-crack’ which were loaded at 0º, 10º, 30º, 45º, 60º,80º and 90º to obtain fracture toughness properties. A
correlation was observed between increasing foam density and resistance to moisture uptake, with moisture acting as a
plasticizer replacing that which has been leached away during immersion. After exposure to moisture specimens exhibited
increased brittleness with significant reductions in energy to fracture. Linear foams were found to absorb almost twice the
moisture of a cross-linked equivalent, and experience the greatest change in energy to fracture, most notably in shear, the
most vulnerable loading mode for a sandwich structure. A CEDIP ‘Silver’ infrared system comprising an array of 320 x 240
detectors was used to perform TSA and evaluate the effects of fracture damage. The system recording capability allows crack
propagation to be captured in real-time. The thermoelastic signal data at the crack-tip was related to stress using a calibration
constant obtained from a specimen in its aged or unaged states when loaded with a known uniform stress. In addition, a highresolution lens attachment allowed the cellular response to a crack-tip to be magnified, revealing the complex stress
distribution on the thin struts around individual foam cells.
Introduction
Foam cored sandwich composites commonly employed in yacht and boat hulls are subjected to punishing conditions during
their service life, exposing the structures to extremes of loading and the environment. Structural safety factors are largely
based upon the tensile strength of the skin while the vulnerability of the core in shear is recognised with an extra safety factor
of 0.5.This may be reduced for cores with good shear fatigue performance such as linear foams which are commonly used in
regions within the hull that are subjected to slamming [1]. In the rest of the hull structure a common structural core is a PVC
cross-linked, closed-cell foam of density 130 kgm-3, i.e. of the minimum density recommended in design guidelines. Lower
values may be permitted but are not encouraged [2 ,3]. Core failure in a shear or mixed mode has been an ongoing concern
aggravated by the destructive influence of moisture which can access the core not only through gross damage, but via
diffusion mechanisms through the skin (Figure 1). An issue which has hindered the understanding of sandwich core behaviour
has been the difficulty in obtaining stress information direct from the material as strain gauges are better suited to bonding on
skins. This work seeks to utilise the full-field technique of Thermoelastic Stress Analysis (TSA) to assess damage sustained by
the core due to ageing and its response to an event such as a crack. The intention is that this new approach will provide
improved understanding of core failure and may assist designers in developing safer designs for the marine industry.
Diffusion through gel coat leading to
residual stress, swelling, capillary action
along fibres, matrix cracking, and
delamination
SKIN
CORE
Hydrolysis of coupling agents forming
molecules larger than diffused moisture
entrapped in acidic pockets leading to void
erosion small molecules of plasticizers &
additives leached away & act as electrolyte
enhancing osmotic pressure attracting
moisture. Therefore higher density
seawater less aggressive
than freshwater
Surface damage exposing
fibres or core directly from
collision or grounding
Voids, freeze-thaw of trapped
moisture may exploit minute
defects and microcracks
Cell outgassing (commonly CO2)
accelerated by temperature rise,
eg. warm seas, sun
Scoring, breather holes, saw
blade cuts, exposed cut cells
readily absorb moisture
Figure 1. Illustration of moisture damage and access routes in a marine sandwich
Test Specimens
Two representative closed-cell PVC foam materials were tested, a cross-linked (C70.130) and a linear blend (R63.140) from
Airex, both of the type commonly used in marine structures. The specimens were cut using a fine toothed band saw from a
single 20 mm thick sheet and in the same direction with the dimensions of 150 mm long and 100 mm wide. Loose particles
were removed with vacuum suction prior to testing and/or immersion. The material composition and processing are
unavailable in open literature, but the bulk material properties provided by the manufacturer are presented in Table 1. Cells are
commonly considered as tetrakaidecahedron forms (14 faces, 36 edges, 24 vertices) though size appears to vary for the two
foam types [4]. Micrographs in Figure 2 (a) and (b) depict a finer structure for the cross-linked cells with a mean diameter of
0.345 mm supported by thinner cell walls and struts 10 µm thick and edges approximately 0.19 mm long. After ageing,
specimens set aside for mechanical testing had starter cracks inserted using a no.11 scalpel so that a crack-tip was
approximately the same size as a single cell. The loading holes were machined at either end of the specimen and were
protected by softwood end-tabs bonded to the specimens with Araldite adhesive.
Density
(kgm-3)
Compressive
Strength
(N/mm2)
Compressive
Modulus
(N/mm2)
Tensile
Strength
(N/mm2)
Tensile
Modulus
(N/mm2)
Shear
Strength
(N/mm2)
Shear
Modulus
(N/mm2)
Shear
Elongation
%
Thermal
Conductivity
W/mK
130
2.6
155
3.8
115
2.3
50
30
0.039
140
1.6
110
2.4
90
1.85
37
80
0.039
C70.130
(X-linked)
R63.140
(Linear)
Table 1 Material properties for primary cross-linked and linear foams tested
R63.140 (linear)
0.6mm Molded surface
(a)
0.6mm
C70.130 (X-linked)
Molded surface
(b)
x 10
R63.140 (linear)
(c)
x 10
C70.130 (X-linked)
(d)
x 50 Thin cell wall membrane
(e)
Figure 2 Micrographs of cellular features in experimental specimens
Hygrothermal Ageing
Methods for age acceleration are application rather than material orientated, so the approach to testing is based on ‘the
suitability’ of a material for service in a set environment, e.g. a laminate for an aeronautics application, rather than determining
the environments suitable for the material to be exposed to. Consequently, few methods link the material glass transition
temperature (Tg) to the ageing temperature, which may effect thermal damage of the material rather than acceleration of
damage due to moisture uptake. PVC materials and the hygrothermal diffusion of water across thin membranes such as foam
cell walls are more relevant to medical applications where standards scrutinize durability and hygrothermal degradation
closely, and in which the maximum recommended test temperature for PVC is restricted to 60ºC [5]. Despite generally
excellent resistance to water, all polymers are permeable to some extent due to their long chain molecular structure, allowing
water molecules to diffuse into interstitial sites and bond to the polar groups of the polymer causing swelling. The increased
distance between polymer molecules weakens intermolecular forces, reducing the glass transition temperature (Tg) to produce
a plasticizing effect and freeing molecules to rotate and translate, making Tg dependant on chain flexibility. Cross-linking can
improve the strength of the polymer and hinder moisture ingress by the physical constraint of ‘blocking’ pathways and
molecular chains [6].
Specimens were immersed in a Grant temperature regulated immersion tank of distilled water at 60ºC and the rate of
absorption monitored gravimetrically. Five of the test specimens for each foam type, were periodically removed from the
immersion tanks, ‘pat’ dried to remove excess free-water on the surface [7], weighed on a Mettler moisture balance accurate to
0.0001 g, and re-immersed within 3 mins from initial extraction. Specimens allocated for further characterisation by mechanical
testing were removed at four stages during immersion 500 hrs, 1000 hrs, 10000 hrs, and 15000 hrs in addition to the final
maximum exposure specimen. Once the desired ageing cycle was complete, the specimens were re-dried in air at 50ºC until
no further weight loss was recorded. Specimens were then left at room temperature and humidity for a further 72 hrs, before
being reweighed and prepared for mechanical testing.
A progression of the gravimetric results for both foam types is shown in Figure 3. In Figure 4 the maximum percentage weight
increase for 150√hrs immersion is shown for a range of material types. The materials had different densities and were
-3
indicated by the suffix in kgm ;‘C’ and ‘R’ denoted cross-linked and linear respectively. Of primary interest is not the amount
absorbed by each specimen, which is specific to the specimen surface preparation and exposure conditions of an elevated
temperature environment, but in the relationship between the cross-linked and the linear foams. Both C70.130 and R.63.140
share a similar initial rate as water molecules enter cells cut open at the surface and through diffusion and capillary action, are
transported across the thin cellular membranes to swell the cells while leaching out plasticizers and additives. Micrographs
showed apparent swollen cells and pitting-like damage to cell struts and material loss may explain the fluctuations recorded in
weight despite the trend of net increase.
300
C70.200
Linear (R.63.140)
C70.130
250
C70.90
200
18ºC (Room Temp)
40ºC
60ºC
C70.75
Material
% Weight Increase
R.63.140
Cross-linked (C70.130)
150
R.63.80
C51.60
100
C70.55
C70.40
50
R.63.50
R82.80
0
0
5000
10000
15000
20000
25000
0
100
200
Figure 3. Gravimetric results for cross-linked and linear
foam specimens (C70.130 and R63.140) at 60ºC.
(Each data point is averaged from five specimens).
300
400
500
600
700
Maximum % Weight Increase
Immersion Time (hrs)
Figure 4. Final gravimetric results for range of foam materials,
Predictable relationships emerged for immersion temperature and foam density with respect to moisture absorption, though
the linear foam showed nearly twice the weight gain than the cross-linked material. Weight increase trends of the linear foam
resemble that of a lower density cross-linked material while thicker cell walls appear to be less influential in resistance to
moisture, as the lesser degree of cross-linking in the polymer chains permits a greater degree of mobility and rotation, allowing
greater freedom of movement to water molecules than the thinner walled cross-linked cells. Interpretation of the trends in
Figure 4 suggests that the practical implication if a linear R63.140 foam were required to match the moisture resistance of its
-3
cross-linked counterpart, would require its density to be upgraded to 200kgm . Additionally, at 60ºC the increase in weight as
a percentage of the initial virgin weight was approximately 266% and 136% for R63.140 (linear) and C70.130 (cross-linked)
foams respectively. In Figure 4, data for the R63.140 and C70.130 at 40ºC and 18 ºC, is shown allowing an ageing
approximation based upon an Arrhenius relationship by estimating a doubling of the absorption rate for every 10ºC increase in
immersion temperature. In this manner 18500 hrs immersion at 60ºC may be conservatively related to a 30 year service life if
the average exposure temperature is 18ºC. Thereby indicating that specimens aged beyond 18500 hrs may potentially be
over-aged. Consequently, mechanical tests on specimens denoted as ‘aged’ were conducted on those extracted at the 15000
hrs interval.
Fracture Toughness and Energy Absorption
As a measure of the effects of ageing, tensile fracture tests were conducted using a CTS type rig shown in the experimental
arrangement in Figure 5. Initial fracture tests were conducted at 1mm/min. The rig allowed the same specimen configuration to
be used for testing at radial increments between mode I and mode II, providing information on the response of an age
damaged specimen to the propagation of a crack. In this case the starter notch was half the specimen width. Fracture
toughness values obtained for the virgin condition are given in Table 2. There is less confidence in the mode II data as during
testing large bulk deformation of the foam was observed prior to failure. In general the fracture toughness of the linear foam
appeared less than that of the cross-linked material, though resistance to crack propagation of virgin specimens was excellent.
Linear foam was observed to suffer the greatest change with an almost 70% reduction in energy to fracture (Gc). The brittle
behaviour due to hygrothermal ageing contrasted greatly to their excellent resistance to crack propagation in the virgin state,
which generated lower confidence in KII results due to the extent of deformation exhibited and reflected in the ductile behaviour
exhibited in the load-extension plots.
Cross-linked
X-linked
(C70.130)
Linear
(R.63.140)
Crack/tensile
axis
orientation
KIC KI
MPam0.5
KIICKII
MPam0.5
KKICI
MPam0.5
KK
IICII
MPam0.5
0º
10º
30º
45º
60º
80º
90º
0.281
0.255
0.227
0.167
0.102
0.058
0
0
0.031
0.063
0.074
0.086
0.116
0.144
0.177
0.162
0.143
0.105
0.064
0.033
0
0
0.015
0.04
0.047
0.055
0.088
0.116
Table 2: Initial virgin K values
Figure 5 Experimental arrangement showing a C70.130 specimen in the mixed-mode loading rig within a servo-hydraulic test
machine with the Cedip system.
In the virgin condition, linear foams (more than cross-linked), allow a greater degree of rotation of the cell struts towards the
tensile axis thereby alleviating the bending moment. The fracture surface may then display signs of local plastic tearing of cell
struts and a degree of kinking in the pathway of the crack (see Figure 6a). Following ageing, increased brittleness is apparent
with the fracture surface showing limited tearing and kinking (see Figure 6b) and fracturing in brittle fashion across cells at the
crack-tip in contrast to the virgin crack-tip (see Figure6c). While in the wet condition, the water molecules absorbed into the
cells, become positioned interstitially between polymer chains act as plasticizers and swelling the cells and material (see
Figure 6d), but once specimens are re-dried, the extent of ageing damage is more readily observed. Leaching of polymer
additives caused pitting-like damage to the cell struts indicated by Figure 6(e), reducing the ability of the strut to rotate and
causing local weakening, which results in increased brittle fracture behaviour. Fracture toughness tests on aged specimens
were conducted in mode I and II only, pending completion of TSA tests on the mixed mode samples. Figure 7 shows the
calculated critical fracture energy (Gc), values derived from experimental KC values.
(a)
Virgin C70.130 (x-linked)
Mode I propagating cracktip from scalpel insert
(b)
(c)
(d)
(e)
15000hrs aged C70.130
Mode I propagating crack tip from scalpel insert
Virgin R63.140 (Linear)
Mode I propagating crack tip from scalpel insert
In-situ R63.140 (Linear)
Cells swollen with moisture
15000 hrs aged R63.140
Pitted damage to struts
Figure 6 Micrographs of cells before, during and after accelerated ageing (15000hrs) in the re-dried condition (mag. x10).
Response of the aged cells to impact was of additional interest as linear foams are frequently incorporated into boat-hull
structures for their resilience and energy absorption properties in areas subject to high impact loads. Cell response was
determined by Vernier calliper depth and diameter measurements of impacts made by a 4.5mm steel ball from a 0.5 m
-1
distance with velocity 90 ms for a high strain rate test, and by an ASTM D3029-84(F) drop test [8], with a 4.5 mm ball-tipped
tup with a total weight 0.725 kg from a low height of 0.25 m for a low strain rate test. Five specimens per data point with the
standard configuration (150 x 100 x 20 mm) were used. The values in Figure 7 are not absolutes for the material but subject to
specific test conditions (e.g. impacting diameter, strain rate), and associated calculations such as those that give geometrical
factors to solve for K [9], and some variation is expected as more specimens become available for testing. Consequently, it is
the comparative changes in virgin/aged foams which are of significance. Overall, linear foam specimens were seen to be the
most sensitive to hygrothermal ageing, with the greatest vulnerability exposed by loading in shear. However, K values, and
consequently any values based upon them did not convey the impressive resistance offered by linear specimens to the
propagation of a crack. R63.140 in its virgin state would deform extensively under shear loads, but frustrating crack growth and
leading to eventual plastic tearing of the cells rather than fracture. Similar yet subdued behavior was observed in the aged
specimens, and with microscopic examination of the fracture surface showing tearing of cells at the core centre, but brittle-like
fracture at the edges. This would reflect the uneven depth penetration of moisture and subsequent variation of Young’s
modulus through the specimen thickness. Impact testing indicated linear foam to be most sensitive to strain rate in both virgin
and aged conditions, and also most vulnerable to the effects of hygrothermal ageing for both strain rates. At a low strain rate,
virgin linear cells were observed to effect a recovery in the indentation depth which was absent in the aged specimen. Under
visual examination indentation edges appeared smoothed with cells plastically compressed, however, cross-linked cells
showed signs of brittle fracture which were emphasized by ageing. While at a high strain rate the change caused by ageing in
linear foams was only 10 %, it was six times greater than the change observed for cross-linked cores. At a low strain rate the
aged linear cells greatly resisted indentation by 76%, while cross-linked cell resistance decreased by 66%.
Impact Depth
Fracture Energy (Gc)
Virgin
R63.140
0.97
-2
3.16
2.689x10
1.81
4.14
4.1
(-23.2%)
3.18 (-23%)
5.212
(-24.02%)
5.2
(-24%)
Aged
4.5x10-3 (-68%)
2.41
6.86
6.7
Virgin
C70.130
1.406x10-2
2.75 (+1.31) mm
1.07
(-70.6%)
1.1 (-70%)
1.33 (-61.7%)
1.3(-62%)
Aged
Thermoelastic Coefficient
2.69mm
3.48
3.4
3.64
3.6
3.01
3.1
Mode I
Mode II
1.853x10-2
High strain rate
Low strain rate
Figure 7 Preliminary experimental values for virgin C70.130 and R.63.140, with property changes after 15000 hrs ageing
Damage sustained by the cells during hygrothermal ageing appears to have affected their energy absorption response
considerably, with the implication that a hull cored with a cross-linked foam may be at greater risk to low velocity collision or
grounding, while linear cored hulls designed for wave energy impact absorption may risk a reduced fatigue life with increased
vulnerability to shear crack development.
Thermoelastic Stress Analysis
The basis of TSA is that a material will experience a small change in temperature if the volume is fractionally altered by an
applied force thereby altering the energy within. Adiabatic conditions are met by rapid cycling of the load to promote energy
conservation between the loading and unloading stages. The relationship between temperature and load is linear, which for an
isotropic material, where the small temperature change (∆T) is directly proportional to the change in the sum of the principal
stresses in the material ∆(σx+ σy). In the current work, the small temperature is detected by the 320 x 240 InSb infra-red
detector array of a CEDIP ‘Silver’ system. This detected signal is processed and correlated to a loading cycle ‘reference’ signal
from an Instron servo-hydraulic test machine. The thermoelastic signal produced by the Cedip system is measured in ‘DL‘
signal units and the phase angle difference between reference and the thermoelastic signal is also provided. The relationship
between the thermoelastic signal (S), and stress has been described in detail in the literature. The working equation is as
follows [10]:
S=
(
∆ σx + σy
)
(1)
A
where ‘A’ is a constant dependant on the system and material parameters.
lin
e
‘A’ was obtained for each foam type by loading a specimen under a known cyclic tensile stress to obtain an average signal
value from an area on the specimen that can be related to the applied stress, σapp = σy. This is repeated for aged specimens to
obtain a new ‘A’ to calibrate for material changes caused by ageing. In this work, a 10Hz loading frequency was applied and
testing was conducted in position control to accommodate any changes in the foam material properties. Specimens could not
be coated with matt black paint as the irregularity of the surface resulted in non-uniform coating. Though fracture data is
incomplete pending finalisation of non-destructive TSA examinations, the magnitude of the change was supported by a similar
change in the TSA calibration coefficients for virgin and aged specimens, and by supplementary high and low strain rate
impact tests. Stress data from TSA may be exported and mixed-mode stress intensity factors KI , KII values be derived using
relationships based on the well established Westergaard equations [11,12] using known principles of cardioid geometry as
shown in Figure 8, to form a familiar TSA relationship as follows [13]:
cr
ac
k
y
Tangent 2
2 K I2 + K II2
θ⎞
K
⎛
cos ⎜ φ + ⎟ where φ = tan−1 I
2⎠
KII
⎝
2 πr
(2)
Pr
Tangent 1
oj
ec
t
ed
AS =
θmax
x
Tangent 3
Figure 8 Crack cardioid geometry and notation
From the particulars of cardioid geometry, rmax is the length of any full
chord through the cusp of the curve, the enclosed area of this curve (Σ)
being related to rmax by the following:
Σ=
(
2
3 πrmax
3 ⎡ K I2 + K II2
=
⎢
8
2 π ⎢⎣ A 2S 2
)⎤⎥
⎥⎦
2
where rmax =
(
2 K I2 + K II2
πA 2S 2
)
(3)
Stanley and Dulieu-Smith [13] solved for the tangents of a curve defined by a signal value (S), which are positioned at the
points on the curve that are separated by the maximum angle from the projected crack-line (θmax) and subsequently formed
relationships for KI and KII :
[
(
K I = C 1C 22 1 + C 22
)]
1
2
[ (
and K II = C 1 1 + C 22
)]
1
2
, where
C1 =
2πΣ
KI
⎛ 3θ
⎞
= tan ⎜ max + β ⎟ and C2 = KI2 + KII2 = A 2S2
3
K II
⎝ 2
⎠
(4)
Initial manipulation of the thermoelastic data collected from specimens with growing cracks using Eq (4) shows that the
cardioid form expected is present only in some data sets with the most promising results obtained from aged linear foams. In
previous work a routine was established that allowed the relationships in Eq (4) to be applied automatically [11]. Difficulties in
applying this method to foams arise from the distribution of high signal values present from cell alignment and localized high
signal values due to intact cell membranes. In Figure 9a this is illustrated by a zoomed plot of data close to the crack-tip.
Interestingly, in pure shear, damage can be most clearly detected by changes in phase of the signal with respect to the lock-in
reference signal from Instron. In Figure 9(b) the damage extent is clearly displayed in the phase plots but there is no indication
of the cardioid form in the TSA data.
(a)
TSA
Figure 9 (a) Aged C70.130. Area zoom of high signal distribution
(b)
Phase
(b) Virgin R63.140 Mode II. Damage site is out of phase
Figure 10 shows data from a G1 high resolution lens. This clarifies the source of localized high signal values. The individual
cells are apparent in the TSA images. Zooms of the data at these locations is also shown with a uniform distribution of the
signal. This demonstrats the potential of TSA in micromechanics and assisting in the validation of theoretical cell structure
models.
(a)
(b)
Figure 10 Aged linear (a) and cross-linked (b) showing uncalibrated signal stress distribution near crack-tip, across the surface
of 10 µm thick cell membranes.( G1 lens, 285 Hz sampling, 10 Hz loading).
The energy absorption mechanisms used by a polymer foam that can result in strut realignment and redistribution of stress at
the crack-tip are degraded by hygrothermal ageing, with the linear foams the most at risk. TSA is uniquely equipped with the
potential to provide quantitative information on these changes and clarify the behavior of cells at a growing crack tip. Figures
11 and 12 shows excerpts of the TSA video option in Cedip Altair software, obtained using the highest sampling rate available
of 269 Hz with a 27 mm lens showing a difference in behavior for a virgin and aged linear foam. Specimens were cyclically
loaded at 10 Hz in ‘position control’ until a critical crack was induced. The number of cycles at each interval seen by the
specimen is indicated in each frame in Figures 11 and 12. Over this period the crack advanced only a small amount (a<1mm)
It should be noted that while oversampling ensures the highest quality data necessary for work on foams, it is computationally
expensive with 700 Mb per data set so each capture was restricted to 18.59 seconds (or 5000 frames). Differences in the
signal pattern in Figures 11 and 12 may be attributed to apparent increase in brittle behavior in aged specimens. It is
reasonable to suggest that the maximum signal value for the virgin linear specimen in Figure 11, and subsequent distribution
of this maximum relates to the accumulation of damage in cell walls leading to rupture followed by flexural deformation of the
surrounding cells, as observed in kinked pathways during fracture toughness tests (see Figure6a), distributing stress at the
crack-tip until damage accumulation forces a new rupture. The drop in signal level in the final frames of Figure 11, contrast
with the apparent continued signal increase for the aged specimen in Figure 12. This implies that cells at the crack-tip in Figure
12 are restricted from the flexural deformation that inhibits growth of the crack, supporting observations of cell damage in
microscopy and increased brittle behavior in fracture. Figures 11 and 12 indicate that crack growth is largely governed by local
plastic deformations and local damage accumulation process, the extent of damage being influenced by the local distribution
of material and topography of cells, which Figure 10 shows to be irregular. The increase in the signal values at the crack tip for
both materials is clearly too great to be accounted for by an increase in stress intensity caused by the small change in
geometry. It is likely that the ‘damage’ is causing localized heating and assessment based on this would be more appropriate.
critical cycles
+5 cycles
+10
+15
+20
+25
+30
+ 35
+40
+45
+50
+55
Figure 11. Mode I signal redistribution at crack tip by a virgin linear foam
critical cycles
+30
+5
+35
+10
+15
20
25
+40
+45
+50
+55
Figure 12 Mode I signal at crack-tip in aged linear foam showing localised concentration of signal
Conclusions and Future Work
The current work has used gravimetric testing, fracture toughness assessment, impact testing and TSA to assess the
behaviour of two types of foam commonly used in the marine industry. It has been found that:
1 Linear foam is sensitive to temperature and when hygrothermally aged at 60ºC will increase in weight almost twice as
much a cross-linked foam. Ageing caused loss of material from the foam, and seen as pitting like defects in cell
struts.
2 In the virgin state linear foams exhibit excellent resistance to crack propagation most notable in shear where test
specimens deformed considerably without fracture. In the aged state preliminary tests showed a significant drop in
fracture toughness caused by embrittlement of the cells.
3 Impact testing also showed an apparent reduction in energy absorption characteristics
4 TSA can be applied to foam materials and the advance of a crack can be monitored in real time but cell morphology
can be a source of scatter in results. Mode I specimens produced expected cardioid forms, but the extent of damage
in shear specimens could be more readily detected by phase angle plots.
5 TSA was able to detect differences in behaviour and energy distribution at a crack-tip due to hygrothermal ageing.
Under conditions for crack growth, the linear foam in its virgin state appeared to inhibit crack growth by a
‘redistribution’ of stress at the crack-tip. After ageing, restricted in cell deformation resulted in the accumulation of
local damage at the tip leading to propagation of the crack.
Work for the near future includes growing a crack to obtain propagation rate comparisons for virgin and aged states, and
improvement in the application of Eq 4 to obtain K values, with special consideration for higher mixed-modes. Further
investigation is required as initial work has shown that there may be localised heating the crack-tip which will affect TSA
data.
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