CATCH, Characterization of Automated Tape Composites at High strain rate (ROLI) : open access to JRC research infrastructures : Call 2018-1-RD-ELSA-HopLab : user access report HopLab, Hopkinson Bar facility.
Date: 2022-12-19
Institutions: Joint Research Centre
Subjects: composite materials, database, materials technology, research infrastructure, research report, scientific research

ISSN 1831-9424 

CATCH – Characterization of Automated 
Tape Composites at High strain rate 

Open Access to JRC Research 
Infrastructures 
Call 2018-1-RD-ELSA-HopLab: User 
Access Report 
HopLab, Hopkinson Bar facility 

Peroni, M; Tsionis, G; Pellegrino, A; Martinez 
Hergueta, F; Kok, R; Ramakrishnan, K R; Petrinic, N; 
Zumpano, G; Tagarielli, V; Thomson, D 

2022 

EUR 31352 EN 

 
 
 
 
This publication is a Technical report by the Joint Research Centre (JRC), the European Commission’s science and knowledge service. It aims 
to  provide  evidence-based  scientific  support  to  the  European  policymaking  process.  The  contents  of  this  publication  do  not  necessarily 
reflect  the  position  or  opinion  of  the  European  Commission.  Neither  the  European  Commission  nor  any  person  acting  on  behalf  of  the 
Commission is responsible for the use that might be made of this publication. For information on the methodology and quality underlying 
the  data  used  in  this  publication  for  which  the  source  is  neither  Eurostat  nor  other  Commission  services,  users  should  contact  the 
referenced source. The designations employed and the presentation of material on the maps do not imply the expression of any opinion 
whatsoever on the part of the European Union concerning the  legal status of any country,  territory, city or  area or  of  its authorities,  or 
concerning the delimitation of its frontiers or boundaries. 

Contact information 
Name: Marco Peroni 
Address: Via E. Fermi, 2749, 21027 Ispra (VA), Italy 
Email: marco.peroni@ec.europa.eu 
Tel.: 00390332789775 

EU Science Hub 
https://joint-research-centre.ec.europa.eu 

JRC131809 

EUR 31352 EN 

PDF 

ISBN 978-92-76-60489-1 

ISSN 1831-9424 

doi:10.2760/807683 

KJ-NA-31-352-EN-N 

Luxembourg: Publications Office of the European Union, 2022 

© European Union, 2022 

The reuse policy of the European Commission documents is implemented by the Commission Decision 2011/833/EU of 12 December 2011 
on the reuse of Commission documents (OJ L 330, 14.12.2011, p. 39). Unless otherwise noted, the reuse of this document is authorised 
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permission must be sought directly from the copyright holders. 

How  to  cite  this  report:  Peroni,  M;  Tsionis,  G;  Pellegrino,  A;  Martinez  Hergueta,  F;  Kok,  R;  Ramakrishnan,  K  R;  Petrinic,  N;  Zumpano,  G; 
Tagarielli, V; Thomson, D, CATCH – Characterization of Automated Tape Composites at High strain rate; Call 2018-1-RD-ELSA-HopLab: User 
Access Report, Publications Office of the European Union, Luxembourg, 2022, doi: 10.2760/807683, JRC131809. 

 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Contents 

Abstract ...................................................................................................................................................................1 

Foreword .................................................................................................................................................................2 

1 

Introduction .......................................................................................................................................................3 

2  Specimen manufacturing...................................................................................................................................5 

3  Test equipment and execution ..........................................................................................................................8 

3.1  FRANK Servo-hydraulic testing machine (static tests) ..............................................................................8 

3.2  Hopkinson Pressure Bar (high strain-rate tests) .......................................................................................9 

4  Data elaboration ..............................................................................................................................................12 

4.1  Frank Servo-hydraulic testing machine ...................................................................................................12 

4.2  Split Hopkinson Pressure Bar ..................................................................................................................13 

5  Database structure and data results ...............................................................................................................16 

6  Results overview ..............................................................................................................................................17 

7  Conclusions ......................................................................................................................................................19 

References ............................................................................................................................................................20 

List of abbreviations and definitions .....................................................................................................................21 

List of figures .........................................................................................................................................................22 

List of tables ..........................................................................................................................................................23 

i 

Abstract 

This report presents the technical aspects of the experimental test campaign relevant to the project “CATCH  - 
Characterisation  of  Automated  Tape  Composites  at  High  strain  rate”,  whose  proposal  was  successfully 
reviewed  and  selected  in  the  framework  of  the  programme  for  open  access  to  JRC  Research  Infrastructures. 
The research conducted concerns the investigation of the tensile mechanical behaviour, at high strain-rates, of 
a series of advanced composite materials with a tape interlaced laminates structure. Specimens were partially 
provided  by  the  team  of  Users  (the  composite  materials)  and  assembled  with  proper  metallic  parts  at  JRC. 
Static  tests  have  been  performed  on  a  servo-hydraulic  standard  testing  machine.  High  strain-rate  tests  have 
been  carried  out  using  the  HopLab  (Hopkinson  bar)  facility  for  ductile  materials,  modified  for  this  peculiar 
application. The report also presents the elaboration of the raw experimental data in order to obtain the stress-
strain  curves  of  the  tested  specimens.  The  project  has  reached  its  objectives  and  the  activity  is  deemed  a 
success by both parties. The obtained experimental data are of high quality and of significant scientific interest 
for the advanced manufacturing processes and materials investigated. 

1 

Foreword 

The European Commission’s Joint Research Centre (JRC) opens its scientific laboratories and facilities to people 
working in academia and research organisations, industry, small and medium enterprises (SMEs), and more in 
general to the public and private sector. 

Offering access to visiting researchers is part of JRC's strategy to: 

• 

• 

• 

• 

enhance dissemination of scientific knowledge; 

boost competiveness; 

bridge the gap between research and industry; 

provide training and capacity building. 

Scientists have the opportunity to work in the following fields: 

• 

• 

• 

• 

• 

• 

nuclear safety and security (Euratom Laboratories); 

chemistry; 

biosciences/life sciences; 

physical sciences; 

ICT; 

Foresight. 

The  present  document  constitutes  the  "User  Access  Report",  which  summarises  the  results  from  the  access 
given to the Hopkinson Bar facility (HopLab). 

Authors 

JRC Research Infrastructure (HopLab, Hopkinson Bar) 

- Peroni, M 

- Tsionis, G 

User Access Project (CATCH – Characterization of Automated Tape Composites at High strain rate) 

- Pellegrino, A (University of Oxford) 

- Martinez Hergueta, F (University of Edinburgh) 

- Kok, R (University of Edinburgh) 

- Ramakrishnan, K R (University of Oxford) 

- Petrinic, N (University of Oxford) 

- Zumpano, G (Rolls Royce) 

- Tagarielli, V (Imperial College) 

- Thomson, D (University of Oxford) 

2 

1 

Introduction 

Composite  materials  have  been  used  in  aerospace  applications  such  as  engine  blades,  brackets,  interiors, 
nacelles, propellers/rotors, single aisle and wide body wings. The potential for very high-stiffness and excellent 
thermal stability over a wide range of temperatures make composite materials ideal in aerospace and defence 
applications  [1-2].  However,  composite  structures  might  be  subjected  to  dynamic  loads,  e.g.  from  blast  and 
impact,  during  their service  life  and  it  is  essential  to  undertake  dynamic  characterization  of  their  mechanical 
properties in tension, shear and compression, using various testing techniques, such as servo-hydraulic system, 
drop  weight,  pendulum,  high  rate  servo-hydraulic  machine  and  Split  Hopkinson  Pressure/Tension  Bars 
(SHPB/SHTB) to properly satisfy critical design requirements. 

A  growing  trend  in  composite  fabrication  is  replacing  traditional  woven  reinforcements  and  hand-layup 
processes  with  tape  interlaced  laminates  [3].  These  have  a  pseudo  woven  structure  that  improves  impact 
damage tolerance while retaining the strength and stiffness of conventional angle ply laminates. Reliable  data 
on the dynamic properties of this new generation of composite materials are sparse because of experimental 
difficulties.  Complete  data  of  mechanical  characterization  is  a  pre-requisite  for  the  implementation  of 
constitutive model of these composites.  The quasi static behaviour of the tape composites has been recently 
studied  [4][5] but, due to the dimension of the Representative Volume Element  of the tape composites, it  is 
practically  difficult  to  study  the  dynamic  behaviour.  Typical  laboratories  specialized  in  the  mechanical 
characterization are not able to test specimens with such large dimensions. The use of the HopLab large testing 
infrastructure 
information  on  samples  representing  the  realistic 
microstructure of the composite. 

is  the  unique  possibility  to  obtain 

The proposed testing campaign will open the possibility to enlarge the analysis of strain rate effect carried out 
at  smaller  scale  by  the  User  team  in  a  project  aiming  at  designing  and  manufacturing  the  world’s  largest 
composite  fan  case  in  support  of  the  UltraFan™  demonstrator  programme  [5].  The  proposed  project  on  the 
study of the high strain rate tension behaviour of tape composites is a topic of critical interest to ensure the 
safety of critical aerospace structures in both civil and defence applications. 

More specifically, the aim of this experimental campaign is to determine the influence of the curvature aspect 
ratio  on  the  dynamic  strength  of  interlaced  composites.  Interlaced  composites  present  an  improved  damage 
tolerance against low velocity impact and similar stiffness compared to their cross-ply counterparts. However, 
the  micromechanical  stress  concentrations  in  the  weaved  regions  decreases  the  final  strength  of  the  panels. 
The  onset  of  damage  during  tensile  quasi-static  characterisation  tests  is  associated  to  matrix-related  failure 
modes such as tape debonding, delamination and matrix cracking, which might present a significant strain-rate 
dependency and increase the final dynamic strength of the composite material. In this experimental campaign, 
we  aimed  to  determine,  for  the  first  time,  the  dynamic  response  of  interlaced  panels  manufactured  with 
different  curvature  aspect  ratios  and  compare  it  against  the  mechanical  response  of  conventional  cross-ply 
unidirectional  panels.  This  will  provide  valuable  material  parameters  to  assist  the  future  design  of  complex 
interlaced composite structures. 

The  experimental  campaign  included  baseline  cross-ply  and  quasi-isotropic  panels  for  comparison  purposes 
and  two  interlaced  configurations:  quasi-isotropic  fully  interlaced  (stacking  sequence  at  0,  90,  45,  and  -45 
degrees, [0/45/90/-45]4S) and cross-ply (0 and 90 degrees fully interlaced, [0, 90]16S). The subscripts 4S and 16S 
indicate that the stacking sequences enclosed by the square brackets are repeated 4 and 16 times respectively, 
and that the laminates are symmetric about the midplane (such that both laminates contain 32 plies through 
the thickness). Static and dynamic tests have been performed at the ELSA HopLab laboratory. Static tests have 
been  carried  out  on  a  servo-hydraulic  testing  machine  at  a  velocity  of  0.01  mm/s.  Dynamic  tests  have  been 
performed using the HopLab facility, after slightly modifying the test procedure and instrumentation to cope 
with the particular material. In fact, the Hopkinson technique was developed to test ductile materials and it is 
hardly  applicable  to  test  brittle  materials.  In  addition,  a  particular  grips  system  has  been  designed  and 
manufactured for both static and dynamic tests, as will be described in the next sections. 

Moreover,  the  adoption  of  optical  measurement  techniques  substantially  increased  the  amount  of  data 
available. On the other hand, the complexity of the tests increased due the needed additional instrumentation 
and  procedures:  tests  require  a  suitable  illumination  with  cold  lamps,  and  proper  speckle  on  the  specimen 
surface has to be created according to the spatial resolution of the camera adopted for the different tests. 

Concerning the logistics of the project, the activity took place in December 2020 during the COVID pandemic 
emergency and observing all the safety rules in force at the JRC Ispra site. On the other hand, a complex design 

3 

phase (antecedent to test execution) was necessary to manufacture the innovative clamping system adopted in 
both  static  and  dynamic  tests.  The  experimental  tests  were  carried  out  exclusively  by  the  ELSA-HopLab  staff 
under  the  scientific  supervision  of  Marco  Peroni.  The  User  team  has  been  informed  about  the  test  progress 
with daily online meetings and documenting the tests with a series of photo-sequences. In practice, ELSA staff 
performed preliminary tests and elaborated the data  for each adopted setup, and then deeply discussed the 
results with the User team before completing the tests. 

4 

2  Specimen manufacturing 

All  specimens  were  manufactured  using  SHD  Composites  VTC401  unidirectional  prepreg.  Interlaced  and 
baseline  laminates  were  laid  up  by  hand  in  a  process  emulating  automated  fibre  placement.  Prepreg  sheets 
were  slit  into  10  mm  wide  tapes,  then  laid  onto  moulds  in  the  aforementioned  quasi-isotropic  and  cross-ply 
configurations (Figure 1). Printed guides were used to ensure the tapes were placed in the correct order and at 
the correct orientation. 

Figure 1. Layup configurations of the cross-ply (left) and quasi-isotropic (right) interlaced laminates. 

40 mm 

40 mm 

Source: JRC 

Each panel was cured in a heated hydraulic press according to the curing cycle illustrated in Figure 2. 

Figure 2. Curing cycle for the CATCH test campaign specimens. 

)
C
°
(
e
r
u
t
a
r
e
p
m
e
T

120

100

80

60

40

20

0

4.5

4

3.5

3

2.5

2

1.5

1

0.5

)
r
a
b
(
e
r
u
s
s
e
r
P

0

5000

10000

Time (s)

Source: JRC 

15000

0
20000

The final specimen geometry was obtained by means of computerised numerical control machining according 
to the design reported in Figure 3. 

5 

 
 
 
 
Figure 3. Specimen for the CATCH test campaign (dimensions in mm). 

Source: JRC 

R  and  W  parameters  have  been  changed  for  the  four  tested  materials,  trying  to  avoid  fracture  outside  the 
specimen gauge length (the portion with constant length 40 mm). To properly clamp the specimens into the 
grips and to strengthen the clamping portion of the specimen (the portion with the  four holes and width 65 
mm), two aluminium tabs per side have been glued to the specimen with three different  adhesives (Araldite 
Standard,  Araldite  2021,  and  Permabond  ET5428).  Figure  4  (left)  presents  the  “tabbing”  procedure  with  the 
pliers  adopted  to  compress  the  tabs  during  the  initial  curing  of  the  adhesive  and  to  maintain  the  correct 
position of all parts. After this phase, the specimens were oven cured according to the adhesive datasheet and 
then grinded/machined to remove the adhesive surplus and achieve the correct specimen tolerances. The final 
specimen configuration is reported in Figure 4 (right). The tabbing procedure and specimen refining have been 
performed by JRC staff. 

Figure 4. “Tabbing” procedure (left) and final specimen configuration (right). 

Source: JRC 

As  reported  before,  probably  the  most  challenging  part  of  the  test  campaign  involved  the  design  of  the 
specimen gripping tools suitable for both static and dynamic tests. After a series of attempts, the final design is 
reported  in  the  3D  model  of  Figure  5  (left).  Both  grips  are  provided  with  a  final  tread  compatible  with  the 
HopLab facility and the conventional servo-hydraulic testing machine adopted. The clamping force is provided 
by  four  M10  bolts  (class  12.9)  and  the  friction  with  the  specimen  is  improved  by  using  a  proper  surface 
treatment (knurling) on the internal surfaces of the grips. In addition, four pins have been adopted to reduce 
bending  and  constrain  the  movement  of  the  grip  upper  plate.  The  manufactured  grips,  reported  in  Figure  5 
(right), were adopted to perform the static and dynamic tests and, in both configurations, reasonably limited 
the number of specimen fractures outside the gauge length. 

6 

 
   
 
Figure 5. 3D model (left) and final solution for grips (right). 

Source: JRC 

Tabbed specimens needed another preparation step before testing: the application of a speckle suitable for the 
digital  image  correlation  (DIC)  measurement  methodology.  The  speckle  size  has  to  be  related  to  the  camera 
resolution adopted during the test, with a  dimension larger than 3 pixels to avoid aliasing phenomena. With 
reference to the camera adopted for static test, the speckle has been obtained using a paint  spray can with an 
average speckle size smaller than 0.3 mm (about 4.7 pixels) as reported in Figure 6 (left). In the high strain rate 
tests,  the  camera  resolution  is  definitively  smaller  compared  to  the  static  ones  and,  in  addition,  the  spatial 
resolution is normally reduced to reach higher acquisition rate. Maintaining a similar speckle size in terms of 
pixels (about 5) and considering the adopted resolution for high-speed photo sequence (512x208 pixels), the 
physical speckle dimension for the dynamic tests was about  1 mm. This dimension cannot  be obtained using 
classical  paint  spray-can  techniques  but  only  with  proper  stamps  or  using  marker  pens.  Since  white  inks  for 
stamps were not suitable for the specimen surface, the speckle was created by hand using white markers. 

Figure 6. Speckle pattern for the static (left) and dynamic (right) tests. 

m
m
0
4

40 mm 

30 mm 

Source: JRC 

7 

   
 
   
 
 
 
3  Test equipment and execution 

As  requested  by  the  User  team,  all  materials  have  been  tested  at  two  velocities  of  deformation  in  order  to 
assess  the  effect  of  the  strain-rate  on  the  material  behaviour.  Due  to  the  large  range  of  strain-rates 
investigated, the tests have been performed with two different experimental apparatuses. 

Static tests (0.01 mm/s of actuation speed) have been performed on a universal testing machine (ELSA FRANK) 
and adopting a DIC measuring system. 

High strain-rate tests have been performed using the Hopkinson Bar technique and adopting a high speed DIC 
measuring system on the HopLab facility. 

In the next sections a short description of the equipment is given, including the detailed list of instruments and 
sensors employed. 

3.1  FRANK Servo-hydraulic testing machine (static tests) 

As  mentioned  before,  the  static  tests  have  been  performed  with  a  servo-hydraulic  universal  testing  machine 
(ELSA  FRANK  with  a  maximum  force  capability  of  200  kN)  controlled  with  the  ElsaREC  control  system  (the 
servo-hydraulic  control  system  developed  at  ELSA  laboratory  based  on  EtherCAT  hardware  and  software). 
During  the  tests,  a  camera  was  used  to  acquire  a  photo  sequence  for  Digital  Image  Correlation  purposes 
synchronized with the tensile machine data acquisition sensors (i.e. the force derived by the load cell and the 
piston displacement recorded with a MTS Temposonics sensor). 

As mentioned before, the machine was managed by a ElsaREC controller system that imposed the user-defined 
displacement  history  (with  a constant  velocity  of  0.01  mm/s)  to  the  specimen  and  acquired  the  data  (with  a 
sample rate of 10 Hz). In addition, the ElsaREC system generated the synchronization signal that triggered the 
high-resolution  camera  (5  megapixel  camera  PCO  edge  5.5)  with  a  frame  rate  of  0.5  Hz.  Illumination  was 
provided by cold neon lights. The final test setup is presented in Figure 7. 

Figure 7. Testing setup for tensile tests (left) and closer view of the specimen and grips equipment (right). 

Source: JRC 

The  same  grips  developed  for  the  SHPB  tests  were  adopted  to  clamp  the  specimens  and  to  have  the  same 
boundary conditions in static and high strain-rates tests as reported in  Figure 7 (right). As mentioned before, 
the  specimen  clamping  is  guaranteed  by  four  M10  12.9  class  bolts  for  each  side  and  the  friction  between 
specimen and grip is improved adopting a rough finishing on the internal grip side. 

Table  1  summarizes  the  static  tensile  tests  performed  on  the  specimens,  as  required  by  the  User  team.  QI 
stands  for  quasi-isotropic  (i.e.  a  laminate  containing  0,  90,  45,  and  -45  degree  plies),  CAL  for  calibration 

8 

    
 
(baseline  non-interlaced  laminates),  XP  for  cross  ply  (laminates  containing  0  and  90  degree  plies),  the  first 
number for the panel the specimen was extracted from and the second number identifies each specimen. The 
rows in red concern specimens that failed outside the gage-length as already mentioned in paragraph 2. In any 
case,  although  it  is  not  possible  to  evaluate  the  specimen  strength,  these  tests  are  useful  to  investigate  the 
Young modulus of the tested material. 

Table 1. Static tensile tests 

Test Label 

Test Type 

Velocity (mm/s) 

Date 

b01 
b02 
g01 
g02 
h01 
h02 

Monotonic MTS 
Monotonic MTS 
Monotonic MTS 
Monotonic MTS 
Monotonic MTS 
Monotonic MTS 

0.01 
0.01 
0.01 
0.01 
0.01 
0.01 
Source: JRC 

16-12-2020 
16-12-2020 
16-12-2020 
16-12-2020 
16-12-2020 
16-12-2020 

Specimen 

QI-1-1 
QI-1-2 
QI-CAL-2-2 
QI-CAL-2-4 
XP-CAL-2-3 
XP-CAL-2-2 

3.2  Hopkinson Pressure Bar (high strain-rate tests) 

Due  to  the  large  size  of  the  adopted  specimens,  high  strain-rate  tests  have  been  performed  at  the  large 
Hopkinson bar for ductile materials of the HopLab laboratory1. HopLab, with its unique features, is the largest 
existing Hopkinson bar apparatuses with a length of more than 200 m and a bar diameter of 72 mm  made of 
high  strength  steel  (Figure  8).  Different  from  a  classical  split  Hopkinson  bar,  where  a  projectile  is  used  to 
generate the loading pulse, a high strength steel cable, which is the physical continuation of the input bar, is 
used. Through statically pre-tensioning and suddenly releasing this cable (using an explosive bolt), rectangular 
force pulses of up to 1 MN, 250 μs rise-time and 40 ms duration can be generated and applied to the specimen 
tested. 

Figure 8. HopLab facility: large Hopkinson bar for dynamic mechanical testing. 

Source: JRC 

Both  input  and  output  bars are  equipped  with  ohmic  and  semiconductor  strain-gages  connected  with  a  full-
bridge  configuration.  The  Wheatstone  bridge  signal  is  amplified  using  a  strain-gage  amplifier  with  a  cut-off 
frequency  of  500  kHz  (EFS  SGA02  high-speed)  and  then  recorded  at  a  sample-rate  of  5  MHz  with  a  fast 
transient recorder (National Instruments PXIe-1071 and acquisition boards PXIe-6366). The classical Hopkinson 
instrumentation  has  been  slightly  adapted  to  the  particular  test  campaign  to  increase  the  sensitivity  of  the 

1  https://joint-research-centre.ec.europa.eu/laboratories-and-facilities/european-laboratory-structural-assessment-large-hopkinson-bar-

facility-elsa-hoplab_en 

9 

 
 
 
output  bar  due  to  the  relatively  slow  strength  of  the  tested  specimens.  In  detail,  both  semiconductors  and 
ohmic strain-gages gains have been increased to improve resolution. 

In addition to the classical Hopkinson bar instrumentation, a high speed camera (Photron SA1.1) has been used 
to  capture  a  photo  sequence  of  each  test  (with  a  frame-rate  of  50000  fps)  synchronized  with  the  transient 
recorder.  A  proper  speckle  has  been  applied  to  the  specimens  on  one  side  to  apply  DIC  algorithms  and 
accurately evaluate the local strain field. Cold lights (Veritas Constellation 120) have been used to ensure  an 
optimal illumination during the test. Figure 9 shows the instrumentation adopted during the test and a detail of 
the specimen before the test. The high-speed camera is mounted perpendicularly to the specimen face as in 
standard 2D DIC setup. As mentioned before, the grips adopted for high strain-rate tests are the same used for 
static tests: in this way the only parameter that changes in the two configuration is the test strain-rate. 

Figure 9. View of testing area with the instrumentation adopted (left) and detail of the specimen positioning (right). 

Source: JRC 

Tests  performed  are  summarized  in  Table  2,  and  similarly  to  Table  1,  red  rows  highlight  tests  where  the 
fracture occurred outside the specimen gage-length. All tests have been carried out with the same cable pre-
loading of 900 kN in order to generate inside the specimen the requested strain-rates (of about 25 1/s). 

Table 2. High strain-rate tensile tests 

Test Label 

Test Type 

Velocity (m/s) 

Date 

c01 
c02 
c03 
c04 
c05 
c06 
d01 
d02 
d03 
d04 
d05 
d06 
d07 
e01 
e02 
e03 
e04 
e05 
f01 
f02 
f03 
f04 
f05 

Monotonic SHPB 
Monotonic SHPB 
Monotonic SHPB 
Monotonic SHPB 
Monotonic SHPB 
Monotonic SHPB 
Monotonic SHPB 
Monotonic SHPB 
Monotonic SHPB 
Monotonic SHPB 
Monotonic SHPB 
Monotonic SHPB 
Monotonic SHPB 
Monotonic SHPB 
Monotonic SHPB 
Monotonic SHPB 
Monotonic SHPB 
Monotonic SHPB 
Monotonic SHPB 
Monotonic SHPB 
Monotonic SHPB 
Monotonic SHPB 
Monotonic SHPB 

18-12-2020 
18-12-2020 
18-12-2020 
18-12-2020 
18-12-2020 
18-12-2020 
21-12-2020 
21-12-2020 
21-12-2020 
21-12-2020 
21-12-2020 
21-12-2020 
22-12-2020 
22-12-2020 
22-12-2020 
22-12-2020 
22-12-2020 
22-12-2020 
23-12-2020 
23-12-2020 
23-12-2020 
23-12-2020 
23-12-2020 

3 
3 
3 
3 
3 
3 
3 
3 
3 
3 
3 
3 
3 
3 
3 
3 
3 
3 
3 
3 
3 
3 
3 

Source: JRC 

10 

Specimen 

QI-1-3 
QI-1-4 
QI-2-1 
QI-2-2 
QI-2-3 
QI-2-4 
XP-1-1 
XP-1-2 
XP-1-3 
XP-2-1 
XP-2-2 
XP-1-4 
XP-2-3 
QI-CAL-1-1 
QI-CAL-1-2 
QI-CAL-1-3 
QI-CAL-2-1 
QI-CAL-2-3 
XP-CAL-1-1 
XP-CAL-1-2 
XP-CAL-1-3 
XP-CAL-2-1 
XP-CAL-2-4 

   
 
The large number of tests with fractures outside the central portion of the specimens is a common feature in 
tests  on  composite  materials,  especially  with  large-cell  structures  as  the  ones  of  the  CATCH  project.  The 
majority of fractures outside the gage-length is related to “d” specimens that correspond to interlaced cross-
ply configuration. This material seemed the more influenced by the holes present  in the gripping part  of the 
specimen (fractures typically occurred between the two holes near the gage section). 

11 

4  Data elaboration 

All recorded data in the above described tests are not directly useful to derive the force-displacement curve of 
the tested specimen. Some signals have to be simply scaled (because they are recorded by an AD converter in 
Volt)  but  there  are  some  additional  elaboration  techniques  that  improve  substantially  the  accuracy  of  the 
results  derived  from  the  acquired  data.  In  addition,  especially  for  the  Hopkinson  bar  apparatus,  the  signal 
elaboration is not trivial and requires particular procedures due to the wave propagation phenomena that are 
exploited  in  this  experimental  technique.  Sections  4.1  and  4.2  explain  the  principles  of  the  data  elaboration 
adopted to derive the stress-strain curves of each specimen in the static and Hopkinson bar tests respectively. 

In  addition  to  conventional  measurements  (derived  by  classical  mechanical  transducers  as  for  example  load 
cells, displacement sensors, etc.), in this test campaign an intensive use of DIC techniques have been used to 
correctly evaluate specimen strains [7]. In this context, DIC measurements are fundamental mainly because, for 
brittle  materials,  the  global  specimen  displacement  is  not  representative  of  the  strain  field.  Since  the  final 
elaboration  of  photo-sequences  is  under  responsibility  of  the  User  team,  a  series  of  preliminary  DIC  analysis 
have  been  performed  by  ELSA  staff  using  MatchID  software  to  ensure  the  quality  of  test  photo  sequence 
concerning  DIC  application.  For  this  reason,  the  optimal  speckle  size  has  been  determined  for  each  test  to 
guarantee a satisfactory resolution for DIC algorithms. 

4.1  Frank Servo-hydraulic testing machine 

For  what concerns the experimental data  obtained in a  test  performed with a  standard testing machine, the 
raw  data  are  converted  in  mechanical  units  (Forces  [kN]  and  displacement  [mm])  by  the  machine  control 
software. Due to the load applied by the testing machine to the specimen, also its mechanical frame deforms 
affecting the precision of measured displacement (the total displacement is the sum of the net displacement 
applied to the specimen and the one related to the frame elongation). Knowing the testing frame stiffness, it is 
possible to compensate for this systematic error (the specimen and the frame can be considered as two springs 
in  series  and  thus  subjected  to  the  same  load)  as  reported  in  the  elaboration  block  diagram  summarized  in 
Figure 8. 

Figure 10. Elaboration block diagram for static tests. 

Source: JRC 

The  previously  mentioned  procedure  could  be  avoided  by  using  extensometers  (to  directly  measure  the 
deformation of the specimen gage-section) but in the present case, due to the large-scale specimen adopted 
and  the  brittle  behaviour  of  investigated  materials,  they  cannot  be  adopted  because  they  will  be  damaged 
when the load  is instantaneously released after specimen  fracture. In addition, (once again due to specimen 
brittle behaviour) results obtained with the procedure described by the flow diagram in Figure 8 is not totally 
satisfying in small strain regime. 

Concerning  the  last  observation,  Figure  11  (left)  presents  the  raw  data  recorded  by  the  testing  machine  and 
Figure  11  (right)  the  stress-strain  curve  obtained  adopting  only  testing  machine  data  and  the  procedure 
explained  in  Figure  8  (blue  curve).  As  easily  predictable,  elaborated  data  over-estimate  strains  due  to  the 
elongation of the grips and specimen portion outside the gage-length. This is highlighted by the comparison of 
the  stress-strain  curve  with  the  expected  Young  modulus  (dashed  black  line):  elaborated  data  over-estimate 
strains by about ten times and are not satisfactory. 

12 

 
Figure 11. Static raw data (left) and stress-strain curve (right) for test b02. 

Source: JRC 

For  this  reason,  the  standard  elaboration  procedure  has  been  slightly  modified  incorporating  data  obtained 
using DIC method as reported in the elaboration block diagram presented in  Figure 12. In practice, specimen 
stress  is  still  derived  using  force  data  directly  recorded  by  testing  machine.  On  the  other  hand,  strains  are 
computed by averaging data obtained by the DIC methodology on the specimen gage section and neglecting at 
all what occurs outside this zone. For the acquisition settings adopted, DIC data have a frequency twenty times 
smaller compared  with the testing machine data  (force).  For  this reason, to compute  the stress-strain curve, 
the  force  signal  has  to  be  interpolated  with  the  camera  frequency  (in  practice  only  1  point  every  20  is 
computed). 

Figure 12. Modified elaboration block diagram for static tests with DIC measurements. 

Source: JRC 

By  combining  testing  machine  data  for  stresses  and  DIC  data  for  strains  as  diagrammed  in  Figure  12,  it  is 
possible to obtain a stress-strain curve substantially improved and close to the actual materials properties. As 
reported in Figure 11, the stress strain curve with incorporated DIC data (red curve) presents an initial linear 
trend that practically overlaps with the expected Young modulus of the tested material. 

4.2  Split Hopkinson Pressure Bar 

The elaboration of data recorded in a Hopkinson bar test is substantially different from the techniques adopted 
in standard mechanical testing methods. Since this machine exploits the propagation of elastic waves in elastic 
rods,  the  elaboration  procedure  requires  some  typical  algorithms  of  signal  processing  like  Fast  Fourier 
transforming, deconvolution, phase shifting, etc. 

For  the relatively low- strength specimen (HopLab works normally with specimens with a  strength five times 
larger  and  in  the  order  of  500  kN)  all  the  data  concerning  Hopkinson  tests  presented  later  on  have  been 
derived by semiconductor measurements. This choice was motivated by the higher sensitivity of these sensors 
compared with ohmic strain-gages. Since semiconductors are not perfectly linear in their measurement range, 

13 

   
 
 
they  can  be  linearized  in  a  small  application  range  (as  in  this  case)  and  they  provide  substantially  improved 
results in terms of resolution. 

Figure 13 presents the elaboration block diagram of the recorded signals in a Hopkinson bar test. 

Figure 13. Elaboration block diagram for Hopkinson bar tests. 

Source: JRC 

Specifically, the signals, which are proportional to the input/output bar strains (recorded in three locations for 
each  bar),  are  converted  into  forces  and  then  with  a  deconvolution  algorithm  the  two  travelling  waves 
(ascending and descending) in each bar are distinguished and the incident, reflected and transmitted waves are 
determined  [8][1].  In  addition,  this  algorithm  compensates  for  distortions  due  to  dispersion  phenomena 
because a wave pulse, when propagating through an elastic confined medium, changes its shape (the different 
frequency wave components propagate with slightly different velocities). 

Knowing the ascending and descending waves in each bar, the forces and displacements at the specimen ends 
can  be  accurately  reconstructed  and  then  the  equilibrium  of  the  specimen  checked  [8].  If  the  specimen  is  in 
equilibrium (input force equal to the output force), the reconstructed forces and displacements are appropriate 
for the evaluation of the stress-strain curve of the tested material (the specimen reaches uniform stress and 
strain  fields).  Due  to  the  small  test  duration  and  the  non-negligible  mass  of  the  fixtures,  it  is  not  easy  to 
evaluate equilibrium conditions using conventional Hopkinson bar signals as reported in Figure 14 (left). 

Figure 14. Equilibrium check: force deduced by Hopkinson theory (left) and strain from DIC measurements (right) for test 
f01. 

Source: JRC 

14 

 
   
 
Considering Figure 14 (left), that reports specimen forces applied to input and output sides, equilibrium is not 
evident. The low amplitude and duration of the tests compared with the machine resolution (already improved 
with  semiconductors),  oscillations  (limited  but  not  cancelled  by  equipment  supports)  and  additional  masses 
due  to  the  grips  make  this  check  difficult.  Considering  in  addition  that  the  input  curve  is  computed  by 
subtracting two large amplitude waves (incident and reflected waves), it is quite obvious that the equilibrium 
check is not satisfactory using force signals. An alternative and efficient approach to evaluate equilibrium is to 
apply  DIC  directly  to  the  specimen  and  use  the  specimen  ends  as  load  cell.  Comparing  strains  measured  at 
these  locations  as  reported  in  Figure  14  (right),  it  is  possible  to  draw  the  conclusion  that  equilibrium  occurs 
during  the  test  and  the  output  force  (typically  evaluated  with  greater  precision)  can  be  considered 
representative of the force applied in all specimen sections. 

As for the static tests, also for the Hopkinson bar ones, a mixed standard/DIC data elaboration is proposed to 
improve  the  results  especially  for  what  concerns  estimation  of  the  Young  modulus.  In  this  context,  a  small 
hurdle  arose  because  forces  are  evaluated  at  the  specimen  ends,  while  strains  are  computed  at  the  central 
portion of the specimen gage-length. This very small delay can be computed and compensated by producing an 
accurate  synchronisation  as  reported  in  Figure  15  (left)  (consider  the  slope  start  of  the  two  curves).  At  this 
point, it is possible to combine Hopkinson stresses and DIC strains as reported in Figure 15 (right). As happened 
for  static  curves,  also  in  dynamic  tests  the  stress-strain  curve  with  DIC  correction  allows  a  more  accurate 
evaluation of the material Young modulus. 

Figure 15. Fine synchronization of Hopkinson and DIC data (left) and stress-strain curve (right) for test f01. 

Source: JRC 

15 

   
 
5  Database structure and data results 

For  the  tests  of  CATCH  project,  a  considerable  amount  of  data  has  been  stored  and  managed.  This  section 
provides a basic description of the database structure of the user data (raw and elaborated), which is similar to 
the  database  adopted  since  2000  in  the  ELSA  laboratory.  Diagrammatically  the  database  is  organized  as 
depicted in Figure 16. 

Figure 16. Database structure. 

Source: JRC 

The main Project folder contains one excel file (specimen.xlsx), which contains the table with all experiments 
performed (test  label, test  type and other details about  the  test  configurations).  The  main folder contains in 
addition: 

• 

• 

• 

• 

The Experiments folder. This folder contains several subfolders named with the test labels described in the 
specimen.xlsx  file.  In  each  experiment  folder,  there  are  four  subfolders  that  contain:  the  raw  data 
acquired  during  the  experiments  (Results  Raw),  the  MATLAB®  routines  used  to  elaborate  the  data  and 
save them in the format required by the users (Treatment Programs), the treated results (Results Treated) 
and the photo sequence acquired during the tests (Video). 

The Photo folder contains all the photos shot during the tests campaign. The original photos are stored in 
the raw folders while the elaborated ones (in general subsampled at 300 dpi) are saved in the elaborated 
folders (with a label that identifies the test campaign phase). 

The  Utility  folder  contains  some  applications  useful  for  the  elaboration  of  the  data  results.  In  this  case, 
this folder contains two software applications useful to open the test photo sequences. 

The Data_Summary folder contains some MATLAB files that show an overview of the experimental data 
produced in the project. 

In the next paragraphs, a small description of the data in the Experiments folders will be given to help the users 
in the elaboration phases of all experimental data. 

Raw results consist of two .tdms files that include the strain-gage measurements from six points along the bars 
(one for ohmic and one for semiconductor strain-gages sensors). Each file contains all the details concerning 
the data acquired (number of channels, sample-rate, etc.). The conversion factors of each strain sensor can be 
found in the MATLAB elaboration files of each test. 

The elaborated data contains three columns without headers: test time, specimen strain and stress (MPa) and 
have been saved as txt and MATLAB file. Concerning Hopkinson tests, the elaborated data do not start at time 
zero (that correspond to the detonation time of the explosive bolt) because the incident wave takes a certain 
time to arrive to the specimen location and initial data are simply zero. Anyway, data are synchronized with the 
photo sequence that on the contrary start at t=0 (in this case the acquiring frequency is substantially lower and 
only few photos are not interesting for the test). 

16 

 
6  Results overview 

This section presents an overview of the main results obtained in the CATCH test campaign and some specific 
considerations  concerning  the  elaboration  of  Hopkinson  tests  data.  Anyway,  the  results  presented  in  this 
section are purely demonstrative and a detailed data elaboration, in order to draw conclusions on the material 
behaviour, is under the responsibility of the User team. Especially DIC data presented here are the outcome of 
a preliminary analysis that could be improved by the User team applying state-of-the-art DIC algorithms to the 
available photo-sequences. 

Figure  17  and  Figure  18  present  all  the  data  obtained  in  the  CATCH  project  for  quasi-isotropic  and  cross-ply 
composites both for baseline and interlaced configurations. In the graphs are reported all the tests repetitions 
including the ones that failed within the grips or, in any case, outside the gage-length part. 

Figure 17. Stress-strain curves for baseline quasi-isotropic (left) and cross-ply (right) configurations. 

Source: JRC 

Figure 18. Stress-strain curves for interlaced quasi-isotropic (left) and cross-ply (right) configurations. 

Source: JRC 

As  can  be  noticed,  the  Young  modulus  is  always  repeatable  also  for  specimens  with  “problematic”  fractures 
(inside the grips) for which stress-strain curves became more complex due to cyclic loading and loading due to 
progressive damage and slipping phenomena (see for example specimens e05, f05 or c04). 

When the fracture is in the gage-length portion, the stress presents a  limited  scattering probably due to the 
large  volume  of  the  specimen,  compared  with  other  experimental  results  available  in  literature  that  used 
substantially  smaller specimens [9]. It must  be underlined that also for the “positive” tests (fracture into the 
gage  length)  the  strain  evaluation  is  no  more  accurate  after  peak  stress  due  to  paint  flaking.  In  fact,  the 

17 

   
 
    
 
unloading  wave  that  propagates  into  the  specimen  after  the  fracture  prevents  accurate  tracking  of  DIC 
algorithms. 

Figure 19 reports a photo sequence acquired with the high-speed camera (the input side is on the right). After 
the fracture, the specimen unloading is so fast that the paint flakes away and it is no longer possible to track 
displacements near the fracture region as previously reported. 

Figure 19. Stress-strain curves for interlaced quasi-isotropic (left) and cross-ply (right) configurations. 

Source: JRC 

18 

 
7  Conclusions 

This report has presented the main technical aspects of the CATCH test campaign carried out in the context of 
the  JRC  OPEN  ACCESS  activity  at  the  HopLab  laboratory.  A  description  of  the  test  machines  and  elaboration 
procedures has been reported to help the user effectively use the results. The large amount of data, in terms of 
sensor signal time histories and image acquisitions, has been organized following the established structure of 
the ELSA laboratory database, and stored on the JRC BOX for a fast data exchange with the potential users. 

The  project  has  reached  its  objectives  and  the  activity  is  deemed  to  be  a  success  by  both  parties.  The 
experimental  data  obtained  are  considered  to  be  of  high  quality  and  of  significant  scientific  interest  for  the 
advanced manufacturing processes and materials investigated, and they could constitute a solid starting point 
for future collaborative research. 

19 

References 

[1] 

[2] 

https://www.hintsteiner-group.com/en/blog/carbon-defence 

https://www.globenewswire.com/news-release/2020/01/30/1977323/0/en/Carbon-Fiber-in-the-
Aerospace-and-Defense-Market-2020-2025-Promising-Opportunities-in-Commercial-Aircraft-Regional-
Aircraft-General-Aviation-Helicopter-and-UAV-Industries.html 

[3]  M. H. Nagelsmit, “Fibre Placement  Architectures  for Improved Damage Tolerance,”  Delft  University of 

Technology, 2013. 

[4] 

[5] 

[6] 

[7] 

[8] 

[9] 

C. V. Rad, F. D. Thomas, B. Seay, and M. J. L. Van Tooren, “Manufacturing and characterization of novel 
clutch non-conventional fiber- reinforced composite laminates,” Compos. Struct., vol. 215, pp. 454–470, 
2019. 

Rutger  Kok,  Amos  Lim,  Francisca  Martinez  Hergueta,  Filipe  Teixeira-Dias:  Multiscale  Virtual  Testing 
Framework  To  Optimize  Interlaced  Composites  For  Impact  Applications,  7th  Eccomas  Thematic 
Conference on the Mechanical Response of Composites. 18-20 September 2019, Girona, Spain. 

https://www.rolls-royce.com/media/our-stories/innovation/2016/advance-and-ultrafan.aspx 

http://www.ncorr.com/index.php/dic-algorithms 

Bussac  M.,  Collet  P.,  Gary  G.,  Othman  R.  An  optimisation  method  for  separating  and  rebuild  one-
dimensional dispersive waves from multi-point measurement. Application to elastic or viscoelastic bars. 
J. Mech. Phys. Solid. 2002; 50:321–349. doi: 10.1016/S0022-5096(01)00057-6. 

G.C.  Ganzemuller,  Pklappert  D.,  Trippel  A.,  Hiermaier  S.  A  Split-Hopkinson  Tension  Bar  study  on  the 
dynamic strength of basalt-fibre composites. Composites Part B 2019; 171:310-319. 

20 

List of abbreviations and definitions 

DIC 

Digital image correlation 

SHPB 

Split Hopkinson pressure bar 

SHTB 

Split Hopkinson tension bar 

21 

List of figures 

Figure 1. Layup configurations of the cross-ply (left) and quasi-isotropic (right) interlaced laminates. ................ 5 

Figure 2. Curing cycle for the CATCH test campaign specimens. ............................................................................ 5 

Figure 3. Specimen for the CATCH test campaign (dimensions in mm).................................................................. 6 

Figure 4. “Tabbing” procedure (left) and final specimen configuration (right). ..................................................... 6 

Figure 5. 3D model (left) and final solution for grips (right). .................................................................................. 7 

Figure 6. Speckle pattern for the static (left) and dynamic (right) tests. ................................................................ 7 

Figure 7. Testing setup for tensile tests (left) and closer view of the specimen and grips equipment (right). ...... 8 

Figure 8. HopLab facility: large Hopkinson bar for dynamic mechanical testing. ................................................... 9 

Figure 9. View of testing area with the instrumentation adopted (left) and detail of the specimen positioning 
(right). ................................................................................................................................................................... 10 

Figure 10. Elaboration block diagram for static tests. .......................................................................................... 12 

Figure 11. Static raw data (left) and stress-strain curve (right) for test b02. ........................................................ 13 

Figure 12. Modified elaboration block diagram for static tests with DIC measurements. ................................... 13 

Figure 13. Elaboration block diagram for Hopkinson bar tests............................................................................. 14 

Figure 14. Equilibrium check: force deduced by Hopkinson theory (left) and strain from DIC measurements 
(right) for test f01. ................................................................................................................................................. 14 

Figure 15. Fine synchronization of Hopkinson and DIC data (left) and stress-strain curve (right) for test f01. ... 15 

Figure 16. Database structure. .............................................................................................................................. 16 

Figure 17. Stress-strain curves for baseline quasi-isotropic (left) and cross-ply (right) configurations. ............... 17 

Figure 18. Stress-strain curves for interlaced quasi-isotropic (left) and cross-ply (right) configurations. ............ 17 

Figure 19. Stress-strain curves for interlaced quasi-isotropic (left) and cross-ply (right) configurations. ............ 18 

22 

 
List of tables 

Table 1. Static tensile tests ..................................................................................................................................... 9 

Table 2. High strain-rate tensile tests ................................................................................................................... 10 

23 

 
 
 
 
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