COMPOSITE CRUSH SIMULATION – EMERGING TECHNOLOGIES … · COMPOSITE CRUSH SIMULATION – EMERGING...

COMPOSITE CRUSH SIMULATION – EMERGING TECHNOLOGIES

AND METHODOLOGIES

COMPOSITE CRUSH SIMULATION – EMERGING

TECHNOLOGIES AND METHODOLOGIES

Graham Barnes

Engenuity Limited, UK

Stuart Nixon

SIMULIA, UK

Marc Schrank

SIMULIA, USA

THEME

Composites

KEYWORDS

Composites, Carbon, Laminate, Layup, Crush, Crashworthiness, Occupant

Safety, Vehicle Crash, Collapse, Energy Absorption, Energy Management,

Composite Impact, Crash Simulation, Lightweight, Minimum Weight, Low

Emissions, Failure, Sled, Aerospace, Automotive

SUMMARY

In the quest to lower environmental impact while maintaining vehicle

performance, automakers and aerospace companies are knocking on the same

door – that is, increasing use of composite materials in order to reduce

structural mass. It can be expected that material costs will drop considerably

over the next few to several years, as the capacity to produce such materials

begins to catch up with the growing demand. The benefits of using these

materials are well-documented, including their substantial capacity to absorb

energy in an impact scenario.

Composite structures generally behave much differently than their metallic

equivalents in a crash event when subjected to large compressive and dynamic

forces. Whereas a metal structure will typically undergo large plastic

deformations to absorb the kinetic energy of the event, a well-engineered

composite member subjected to large axial compression will pulverize in a

progressive manner from one end to the other as the “crush front” moves along

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AND METHODOLOGIES

its length. The crush front is characterized by numerous microscopic

interactions between fiber and matrix, and efficient energy absorption can be

achieved when the moving crush front becomes essentially a continuous event,

traversing steadily along the length of the composite member. However, it is

also this type of crushing failure which can prove difficult to simulate using

conventional finite element methods.

CZone technology has been developed to bridge the gap between experimental

observations regarding a material’s ability to absorb energy in an impact and

the need to understand complex structural interaction and stability in a larger-

scale crash event. CZone technology is being incorporated as an add-on

product for Abaqus/Explicit in order to provide a predictive capability for

composite crush scenarios—the CZone functionality handles the material’s

behavior at the crush front, while the damage and failure models in

Abaqus/Explicit address the integrity of the backup structure.

This paper will demonstrate the technology in action on some automotive scale

structures and will identify the additional material characterization required to

successfully predict their performance in crash.

1: Introduction

Worldwide energy consumption is projected to increase by 50% between 2005

and 2030, and the rate of increase in the transportation sector during this period

is expected to be more rapid than in any other end-use sector [1]. Such

projections focus growing attention on potential energy conservation measures

for transport vehicles of various types, including rail, aircraft and automotive

vehicles.

As in most transport vehicles, the rail industry includes weight as an important

design criterion. A growing emphasis by rolling stock manufacturers on

providing greater operational flexibility has the potential negative consequence

of heavier vehicles. This not only increases energy consumption, but also

reduces track life and quality. Hence, greater usage of composites is an active

area of investigation in the rail industry [2,3]. At the same time, aggressive

targets are being set in Europe to increase railway usage substantially by year

2020, and such targets are accompanied by recommendations to improve

energy efficiency and reduce weight [4].

Composite-intensive aircraft have been common for military purposes for

many years, where for instance, the high stiffness- and strength-to-weight

ratios afforded by composite materials contribute considerably to enabling the

high performance characteristics required in fighter jets. However, more

recently, the wider incorporation of composites for aircraft has moved also to

commercial aviation (Figure 1) [5].


AND METHODOLOGIES

Figure 1: Usage of composite materials in commercial aircraft [5].

Probably the most recognized example of this shift is the Boeing 787

Dreamliner, a new passenger jet that is 50% composite material construction

(by weight), and is projected to provide a 20% improvement in fuel economy

over similar sized planes that are of primarily aluminum construction [6,7].

Airbus is also introducing a similar level of composites content into the A350

aircraft [8].

The automotive industry is confronted with significant challenges in improving

fuel economy and reducing carbon emissions over the coming years. In the

United States, legislation passed in 2007 is mandating a 40% improvement in

corporate average fuel economy (CAFE) standards by 2020, to 35 miles per

gallon. In Europe, stringent requirements on carbon dioxide emissions are

being established which effectively infer substantial improvements in fuel

economy.

Automotive OEMs have recognized the enormous opportunities that composite

materials present. In addition to the high stiffness- and strength-to-weight

ratios for composites already being leveraged in the aerospace industry,

composite materials provide tremendous potential for energy absorption, such

as absorbing the energy of a crash event and mitigating the accompanying

accelerations and forces. Figure 2, from a 2007 report by the National Highway

Traffic Safety Administration (NHTSA), shows a plot of the specific energy

absorption capacity of various materials [9]. Clearly apparent is the appreciably

greater capacity for several types of composite materials to absorb energy on a

per unit mass basis, as compared to more commonly used metals, such as steel

and aluminum. In some cases, the difference is nearly an order of magnitude.


AND METHODOLOGIES

Hence, composites offer the potential to be used as very efficient energy-

absorbing crash structures in automotive vehicles.

Figure 2: Specific energy absorption capacities for various materials [9].

2: Composite Crushing and Crashworthiness

Figure 3 shows a sequence of images of the dynamic crushing of a simple cone

structure made of a laminated composite. A sled mass of 324 kg with an initial

velocity of 10 m/s impacts the left end of the cone while the right end is held

fixed. The cone responds much differently than a metallic structure might be

expected to behave. While a steel or aluminum structure could be expected to

buckle and fold, and perhaps fracture in various locations as it absorbs the

kinetic energy of the sled mass, the composite cone behaves much differently.

It essentially pulverizes into very small fragments as the “crush front”, or

contact interface between the cone and rigid impactor progresses down the

length of the cone structure. The specific energy absorption achieved in this

particular test is 47.5 kJ/kg.

Figure 3: Experimental dynamic crush testing of a simple composite cone structure.


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The micromechanics of what transpires at the moving crush front are quite

complex. There are a number of phenomena that evolve and interact, and

which then lead to the crushing behaviour observed at the global level. These

phenomena can include: localized buckling, delamination, matrix cracking and

fiber breakage. Seeking to account for such phenomena in detail both

accurately and robustly in composite crushing simulations is challenging, with

no generally accepted approach yet established.

This present difficulty in being able to carry out design for composite crash

structures that can be largely based on predictive, robust simulations is in

contrast to the pervasiveness of crashworthiness and occupant safety

simulation in the automotive industry today. Over the past 20 years or so, such

simulation has evolved into having a prominent role in being able to design

modern automobiles to meet increasingly stringent crashworthiness

regulations. However, without reliable capabilities for composite crushing

applications, simulation-based design for composite crash structures is less

feasible, often necessitating an expensive, time-consuming, and often

unacceptable approach of Design-Build-Test-Redesign.

Various efforts have been initiated to support the greater incorporation of

composite crash structures for crashworthiness purposes. For example, the

Composite Materials Handbook (CMH-17) has within the last few years

established a separate Crashworthiness Working Group, to develop “…a new,

self-contained section of the handbook on composite Crashworthiness and

Energy Management for vehicle safety certification”, and also “…address the

needs of the composites and vehicle safety community at large, and to provide

a unique forum of discussion for those working in industry, research

institutions, and government agencies.” [10] Likewise, the U.S. Congress has

funded NHTSA to undertake research and “…continue development of a

program to examine possible safety benefits of Lightweight Plastic and

Composite Intensive Vehicles (PCIV).” [9]

3: CZone Technology

CZone technology has been developed by Engenuity Limited as a way to

enable robust simulation of composite crushing so that simulation-based design

of composite structures for crashworthiness can be carried out with a similar

level of predictability as is presently possible for metallic structures.

The basis of CZone recognizes “crush” or “crush stress” as a distinctive

mechanical property of a composite material. Crush is essentially defined as

the ability of a material to progressively absorb energy through destruction

and disintegration. Like other mechanical properties, it needs to be measured,

just as fiber compression stress needs to be measured in order to be


AND METHODOLOGIES

implemented in a finite element analysis simulation. Crush stress provides the

ongoing resistance when a portion of the composite structure is destroyed

against an impacting body, and it is this resistance that effectively becomes the

input force into the rest of the finite element model.

CZone technology is being made available commercially within the Abaqus

finite element software suite, in particular Abaqus/Explicit, an explicit

dynamics solver used across several industries for various applications,

including crashworthiness and occupant safety. The integration of CZone

within the framework of a commercial finite element software suite has the

noteworthy benefit that the wide range of features and functionality already

available in the commercial software package can be used in conjunction with

the specialized crushing simulation capabilities of CZone.

4: Calibration of Crush Stress

The crush stress mechanical property required for CZone usage can be

obtained through various test methods. The most feasible and cost-effective is

crush testing of coupons extracted from flat plaques or sheets of the candidate

laminate. These coupons can readily be cut from the flat plaque using a water

jet, and they have a saw-tooth shape cut into one end in order to induce the

initiation of crushing (Figure 4). Crushing is carried out with a specially

designed test fixture that can be placed in any suitable loading apparatus, such

as a high-rate Instron machine. The fixture provides lateral restraint against

buckling for the test coupon during crushing, while also providing a means for

the crush debris to freely exit the fixture and not influence further crushing

response.

Figure 4: Crush test coupons cut from flat plaque laminate and crush test fixture.

Crush stress can be dependent on several factors, including temperature,

direction of crush, and crush velocity, as well as a potential dependency on ply

stack sequence. The cost-effectiveness of the flat coupon testing method allows

for these dependencies to be accounted for in the calibration process.


AND METHODOLOGIES

Some recent experimental evidence suggests that crush stress for certain

composite materials exhibits an influence of geometry. In relatively flat regions

of the structure ahead of the crush front (prior to crushing), partial

delamination between plies can occur, influencing the subsequent crush stress.

Whereas in geometries that have sufficient curvature normal to the crushing

direction, this delamination is suppressed, thereby enhancing the measured

crush stress. This difference in measured crush stress can be accommodated for

in crushing simulations by assigning appropriate crush stress values to regions

of the original composite structure that are either considered flat or curved.

Recent work to enhance the flat coupon test fixture by incorporating a pin

restraint device for the coupon seems to reasonably induce the same type of

delamination suppression as due to curved geometry.

Not all composite materials exhibit good crushing behavior, and CZone

technology is not intended for those materials that do not crush well. The flat

coupon test method can be used effectively to not only calibrate crush stress,

but also to screen candidate materials in order to assess their ability to sustain

crush. Two primary characteristics are monitored: the average crush stress over

the length or duration of crush, as well as the stability or consistency of the

crush stress. Three examples of ‘Good’, ‘Fair’, and ‘Poor’ crushability are

shown in Figure 5. For the example showing Poor crush characteristics, the

large variations in crush stress over the length or duration of the test indicate

large fragments of the coupon that break away during the test, with no

continuous crush front ever really being established, similar to the test coupon

also shown in this figure.

Figure 5: Examples of crushing characteristics exhibited by different composite

materials, along with test coupon exhibiting poor crush characteristics.


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5: Examples

The simple cone crush test previously shown in Figure 3 is simulated using

CZone and Abaqus. The experimental cone is fabricated with a layup sequence

that establishes three different thickness regions (thinnest at the left end and

thickest at the right end in Figure 3). Initial impact at the thinnest end causes

local buckling in this region, with some fragments of the cone breaking away

due to bending stresses that develop. Figure 6 shows a comparison of these

fragments produced in the experimental test versus those predicted in the

simulation (dark gray shaded areas plotted on the undeformed cone geometry).

Figure 6: Comparison of fragments from simple cone crush test (left) against those

predicted through simulation (right).

As further impact proceeds, the increasing thickness of the cone leads to

progressive crushing of the cone without further buckling and bending failure,

along with increasing resistance against the moving sled mass, as shown in the

comparison of measured and predicted sled acceleration in Figure 7.

Figure 7: Comparison of measured and predicted sled acceleration for simple cone test.

Figure 8 shows a more complex test cone specimen, similar in size and shape

to an automotive structural rail, and including a through-hole feature also

common in such geometries. The cone has an attached mass of 1150 kg and is

moving at an initial velocity of 9.1 m/s prior to impacting a rigid fixed barrier.

Crushing of the cone progresses to a point where a large fracture develops

suddenly in the transition region between the cone and its backup structure

(denoted by the red oval in the last frame).


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Figure 8: Experimental crushing of a complex cone structure in a sled test, with sudden

fracture subsequently occurring away from the crush front.

Figure 9 shows simulation results for this test, indicating good correlation both

for the prediction of crushing response as well as the sudden fracture in the

transition region between the cone and its backup structure (outside the crush

front).

Figure 9: Simulation results for crushing of complex cone structure, comparing

acceleration history against experimental data, and prediction of fracture away

from the crush front.

This example demonstrates that accounting for the possibility of structural

damage or failure away from the crush front is equally important in the design

of crashworthy composite structures. If this damage or failure occurs, the

energy absorbing capacity of the structure can be substantially reduced or

compromised. In this example, the sudden fracture that develops away from the

crush front causes any further energy absorption potential to be largely lost, as

is evident in the abrupt change in acceleration history shortly after 30 ms when

the fracture occurs.


AND METHODOLOGIES

This example also demonstrates CZone technology working in concert with

existing features available in Abaqus/Explicit. CZone addresses the crushing

response at the interface between the cone structure and rigid wall, while

Abaqus/Explicit addresses all other aspects of the simulation, including the

potential for damage and fracture away from the crush front, in this case

utilizing the Tsai-Wu failure criterion [11].

The last two examples are similar complex cone structures as the previous one;

however, the impact angle against the fixed rigid barrier is 30 degrees rather

than a normal or zero degree impact as in the previous example. The mass

attached to the cone is 1164 kg and the impact velocity is 9.0 m/s. The

sequence of images in Figure 10 shows a direct comparison of physical test and

simulation over the progression of the impact event. In both the physical test

and simulation, virtually no crushing response occurs due to a catastrophic

fracture that develops very early between the cone and its backup structure.

Figure 10: Complex cone sled test at impact angle of 30 degrees; comparison of

experimental and predicted cone deformations.

Figure 11 shows a similar sequence of images, this time for a cone that is

reinforced with additional plies in the backup structure transition region. This

cone also includes additional features, such as indentations, and a bonded rigid

insert, that could be necessary to accommodate when such a structure is

incorporated into a complex vehicle assembly. While the impact angle again is

30 degrees, substantial crushing is evident, both in the physical test as well as

in the simulation.

Figure 11: Sled test at impact angle of 30 degrees for reinforced complex cone;

comparison of experimental and predicted cone deformations.


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6: Summary

Projections of substantial increases in energy consumption by the

transportation sector in the coming years, along with directives to reduce

carbon emissions, are leading to various energy conservation measures for

transport vehicles. Reducing vehicle weight through greater incorporation of

composite materials is one such measure, and commercial aviation is beginning

to adopt composites in a considerable way for certain new passenger airplanes.

From a crashworthiness perspective, composite structures offer the potential

for far greater specific energy absorption capacity, through a crushing

mechanism, than their metallic counterparts. However, the present ability to

accurately and robustly simulate such crushing response with commercial finite

element software is lacking, and this is contributing to the relatively slow

adoption of composites in the automotive industry to date.

CZone technology has been developed as a means to provide more accurate

and robust composite crushing simulation, and has been implemented within

the Abaqus finite element software. CZone utilizes the concept of crush stress

(a measured property) to address the crushing response at the contact interface

between the composite structure and other impacting body. Examples of

different composite cones and load cases show good correlation between

simulation and experimental test data for prediction of the crushing response,

as well as for prediction of gross fracture away from the crush front.

CZone technology provides an effective approach to modeling composite

structures for crash that differs from the more common degradation models

implemented in existing explicit finite element methods. When the appropriate

crushing loads derived from the CZone technology are applied to the

reinforcing structure within the Abaqus finite element software, it is shown to

be proficient at reliably predicting the generation and evolution of damage

away from the crush front. The ability to predict the detailed damage response

to relevant impact events provides increased confidence in the development of

large scale structures to meet their intended requirements, without having to

resort to a protracted and expensive Design-Build-Test-Redesign approach.

Faced with the current challenging economic environment, organizations are

ever more determined to avoid the risk of test failure and associated damages

and costs. Industry is increasingly relying on analysis and simulation tools to

“design right the first time”, and the CZone technology enhancement to

Abaqus now extends these capabilities to include the development of

composite structures for crashworthiness.


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REFERENCES

1. Energy Information Administration, International Energy Outlook 2008,

Report No. DOE/EIA-0484(2008), September 2008

2. INGLETON, S., FOUND, M.S., and ROBINSON, A.M. – Design of

composite vehicle end structures in railway rolling stock. Experimental

Techniques and Design in Composite Materials 4, Ed. Found, M.S.,

Taylor & Francis, 2002

3. NewRail, The Research Requirements of the Transport Sectors to

Facilitate an Increased Usage of Composite Materials. Part III: The

Composite Material Research Requirements of the Rail Industry,

COMPOSIT Thematic Network, June 2004

4. European Rail Research Advisory Council, Strategic Rail Research

Agenda 2020, May 2007

5. EADS Deutschland GmbH, The Research Requirements of the Transport

Sectors to Facilitate an Increased Usage of Composite Materials. Part I:

The Composite Material Research Requirements of the Aerospace

Industry, COMPOSIT Thematic Network, June 2004

6. Boeing Company, http://www.boeing.com

7. SMOCK, D., Boeing 787 Dreamliner Represents Composites Revolution,

Design News, June 4, 2007

8. EADS/Airbus, http://www.airbus.com

9. National Highway Traffic Safety Administration, A Safety Roadmap for

Future Plastics and Composites Intensive Vehicles, Report No. DOT HS

810 863, November 2007

10. Composite Materials Handbook, http://www.cmh17.org/general.aspx

11. TSAI, S.W. and WU, E.M. – A general theory of strength for anisotropic

materials. Journal of Composite Materials. Vol. 5, pp. 58-80, 1971



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