rolling stock crashworthiness in the modern age/media/files/s/snc-lavalin... · 2020. 12. 8. ·...

Rolling Stock Crashworthiness

in the Modern Age

Dr Jademond KiangSenior Consultant, Specialist Engineering, Australia

Rolling stock Crashworthiness in the Modern AgeRail

In the context of passenger rolling stock body shell

structures, Australian crashworthiness design

requirements and standards (and standards from

around the world) provide a good design basis for

achieving inherently crashworthy designs. However,

they do not necessarily cover some commonly

occurring scenarios applicable to the Australian rail

environment. For the scenarios that they do cover,

the design criteria do not necessarily reflect the

severity seen in practice.

In the Australian rail environment, which has level

crossings in regional areas along with increasing

popularity of mixed traffic light rail systems in urban

areas, the risk of impact by road vehicles onto the

side of rolling stock is elevating in recent times. This

raises the question of whether designing rolling stock

to meet the requirements of crashworthiness

standards alone can be considered sufficient due

diligence. Or would further due diligence be

warranted?

What is crashworthiness?

Crashworthiness can be described as how well a

piece of rolling stock performs in terms of its ability to

mitigate severity of consequences in a collision in a

controlled manner, and how well the passengers are

protected during such an event.

During a train collision, an enormous amount of

energy is involved. The forces involved would be in

the order or mega-Newtons and the energy levels

involved would be in the order of mega-Joules. Such

high levels of collision energy and forces need to be

managed appropriately in order to provide

Rolling Stock Crashworthiness

in the Modern Age

Rollingstock collisions are few and far in between. In the modern age, train protection systems in the rail

network are deployed to prevent trains from colliding. However, when a train crash does occur, the

consequences can potentially be catastrophic due to the magnitudes of energy involved, says Dr Jademond

Kiang, Senior Consultant – Specialist Engineering, SNC-Lavalin Atkins Australia.

passengers with a better chance of survival should

a collision is to occur.

Crashworthiness standards

There are various local and international standards

in relation to whole-consist crashworthiness

requirements. One of the most notable and

prevalent standard today is the European Standard

EN15227.

Examples of other international crashworthiness

standards from various regions around the world

are listed in Table 1 below:

Table 1 –Crashworthiness Standards from various

regions

The RISSB standard AS7520.3 applies to the

Australian rail industry. However, this standard

references EN15227 for crashworthiness

requirements for frontal crash requirements.

One notable requirement of EN15227 is that full

train-on-train head-on collision is required to be

assessed, which can involve impact speeds of up

to 36km/h depending on the type of rolling stock.

However for light rail vehicles, the impact speed for

crashworthiness assessment is only 15km/h.

Country of Origin Standard

UK, Europe, parts of Asia EN15227 [2]

Australia AS/RISSB7520.3 [3]

USA FRA 2010 [5]

NZ M3000 [4]

These standards almost exclusively address

frontal collision scenarios without regard to other

collision scenarios such as side-on impacts on

rolling stock.

Side impacts are not uncommon in the Australian

rail environment due to popularity prevalence of

level crossings in regional areas and mixed traffic

conditions in urban light rail systems.

Lateral inward strength of the side of the carbody

at sole bar and waist rail levels are considered by

AS7520.3, which are 180kN and 30kN static

loads respectively. However these loads are

intended to address rollover rather than side-on

impact.

Australian train crash statistics

A CRC research report [1] has summarised train

incident statistics over the period 1997-2010

published by the Australian Transport Safety

Bureau (ATSB). Some pertinent statistics

relevant for this discussion are:

1. On average, head-on collision speeds are

higher than defined in EN15227;

2. 62% of incidents involved kinetic energy of

less than 212MJ;

3. Majority of incidents were less than 40MJ ;

4. Kinetic energy of 220MJ was involved (on

average) for impacts between 0° and 45° on

the structure.


These statistics infer that the majority of incidents

(or on average) in Australia for the period 1997-

2010 involve speeds higher than crashworthiness

design speeds. Statistics for side-on impacts are not

available for discussion in this paper. However side-

on impacts are generally expected to be detrimental

to passenger survivability as the side walls of rail

vehicles inherently do not have sufficient space to

house energy-absorbing crumple zones.

According to these statistics, the majority of rail

incidents involve less than 40MJ of energy. For a

typical double decker passenger train, this

magnitude of energy translates to an incidentce

speed of approximately 48km/h. For a metro

train, this translates to 76km/h.

The maximum head-on impact scenario for

crashworthiness design in EN15227 is 36km/h.

With metro systems and light rail networks,

impact speed for crashworthiness design

reduces down to 25km/h and 15km/h

respectively.

In addition, average non head-on frontal impacts

(between 0° to 45°) involve a kinetic energy

magnitude of 220MJ. This translates to over

100km/h for typical heavy rail passenger vehicle

in Australia.

Figure 1 – Waterfall train disaster


As discussed previously, side-on impacts on rolling

stock are not directly addressed in EN15227 or

AS7520.3, not even for mixed traffic environments

such urban light rail vehicles.

It is up to the individual owners, purchasers or

operators to exercise voluntary due diligence to

mitigate risks associated with side-on impacts or

any other collision scenarios not covered by the

design standards. This often involves undertaking

engineering investigation by first-principles involving

FEA or physical testing.

Engineering assessment of crash

scenarios

Regardless of mandatory or voluntary nature of the

crashworthiness requirements, they need to be

assessed by sound engineering methodologies.

One way to address modern crashworthiness

requirements is by physical testing of various

designs and/or modifications in an effort to assess

or determine the most appropriate design.

Another way to address crashworthiness

considerations is by virtual crash testing. In other

words, crashing trains in a computer by undertaking

FEA to assess structural performance.

Method 1 - Physical Crash Testing

The most direct method of verifying crashworthiness

performance is physical testing of the car body

structure or sub-components of the structure. Whilst

physical testing can theoretically be done, it has a

lot of undesirable attributes in practice. These

include:

• Monetary cost associated with the preparation of

the full-scale test article;

• Monetary cost in relation to test facility;

• Long timeframe between test cycles leading to

an inefficient design process;

• Variance between as-built test specimen and

actual production article leading to test results

not reflective of actual performance;

• Safety risks associated with large collision

energy magnitudes (in the order of mega-

Joules);

• OH&S and insurance complications associated

with full-scale physical testing.

Method 2 - Virtual Crash Testing

A lot of the undesirable attributes associated with

full-scale physical crash tests would no longer be

applicable if the process is performed virtually:

• Cost of preparing test articles and facility (FEA

models) will be significantly reduced;

• Timeframe between ‘test’ cycles will be

significantly compressed;

• No OH&S and insurance complications;

• And most important of all, there would be no

safety risks.

Mathematical Theory of Virtual Crash Testing

As described earlier, virtual crash testing involves

the use of FEA technology. This section provides a

high level summary of the mathematics behind the

finite element method.

Crash scenarios are dynamic events. Therefore, the

effects of mass, stiffness, inertia, acceleration/

deceleration, velocity and damping all need to be

taken into consideration.

In a dynamic system, the system of differential

equation is as follows:

[m]{a}+[c]{v}+[k]{x}={F} (1)

where:

[m] is the mass matrix;

{a} is the acceleration vector;

[c] is the damping matrix;

{v} is the velocity vector.

For crashworthiness work, the differential equation

is solved by re-arranging it into the following format

and solving for acceleration directly (damping is

removed for simplicity of discussion):

{a}=[m]-1({F}-[k]{x}) (2)

This is often referred to as ‘explicit’ FEA because all

quantities on the right-hand-side of the equation are

explicitly known.


Assessing impact scenarios not covered by

design standards

As aforementioned, one pertinent crash scenario not

covered by design standards is side-on impact.

To demonstrate the consequence of a side-on

impact, the scenario is simulated for a typical metro

rail car using explicit FEA software LS-Dyna. The

model setup is shown in Figure 2.

The objective of this assessment is to investigate the

difference in response from the various impact

masses and speeds that are conceivable in the

Australian rail environment.

Structural response of the system (such as velocity,

acceleration, stress and strain distributions) can

then be calculated from the acceleration vector at

each time step.

The type of effects the explicit scheme can capture

efficiently include (but not limited to) the following:

• Large plastic strains and geometry changes;

• Post-yield and material fracture;

• Stress wave propagation;

• Pre and post buckling response;

• Surface-to-surface contact and associated

friction.

Further technical details relating to the finite element

method can readily be found in FEA literature such

as [6-10].

The explicit FEA scheme is a widely accepted

method of virtually assessing train crash scenarios

reliably and accurately. This method is widely used

to design and optimise rolling stock fleets in detail

before production of the first physical prototype. For

this reason, this process is sometimes referred to as

virtual prototyping.

Assessing impact speeds higher than

stipulated in design standards

It may be impractical to mandate head-on crash

scenarios at higher impacts speeds than currently

stipulated in the standards.

Crash energy (kinetic energy) is proportional to the

square of the impact speed a shown in Equation (3)

below.

Kinetic energy (KE) is calculated as follows:

KE=mv2/2 (3)

In the rolling stock consulting environment, there are

occasional requests from clients to assess head-on

collisions at higher speeds (approximately 30km/h

compared to 25km/h stipulated in EN15227). The

impact energy increases by a sizeable 44% from

the design requirement for a 5km/h increase in

impact speed (refer to Equation (3)).

Therefore, designing a structural system to

accommodate higher impact energies associated

with small increases in impact speed may render

the structure uneconomical to manufacture and

operate.

Figure 2 – Impact of truck analogue into side

of a typical metro rail car

In this assessment, 20t and 40t truck analogues

impact the side of a typical metro rail car. The truck

has a frontal footprint of 2.6m by 3.5m.

ScenarioMass

(t)

Impact

speed

(km/h)

Kinetic

Energy

(MJ)

1 (semi) 20 25 0.48

2 (road train) 40 25 0.96

3 (road train) 40 50 3.80

Table 2 – Truck impact speeds and

associated kinetic energy

The impact energy is linearly proportional to the

mass of the truck and proportional to the square of

the impact speed (refer Equation (3)). Therefore,

increasing the impacting mass from 20t to 40t

doubles the kinetic energy of the impact whilst

increasing the impact speed from 25km/h to 50km/h

(same mass) results in a four-fold increase in

energy.

The results of the various collision scenarios are

shown in Figure 3 through Figure 5.


Under Scenario 1 (which can be considered as a

moderately sized truck found in regional areas

impacting at a moderate speed), where the kinetic

energy is halved further, the intrusion of occupancy

space is significantly reduced, albeit not

insignificant.

As a comparison, the peak dynamic impact force

acting on the sidewall in Scenario 1 (obtained from

FEA) is over 1MN, which is well in excess of the

static loads stipulated by AS7520.3. The static

lateral inward loads defined in AS7520.3 are 180kN

and 30kN for sole bar and waistrail, respectively.

However, these lateral inward loads are intended for

rollover strength assessment rather than side-on

impact (there are no other load cases in AS7520.3

that address side wall lateral inward loads).

The dynamic impact forces are greater still for the

higher impact mass and speeds such as in Scenario

2 and 3.

It should be noted that these scenarios are not

meant to be photorealistic. Trucks, in reality, would

have their own crumple zones thus some of the

impact energy would be absorbed by plastic

deformation at the front of the trucks. The

simulations presented in this paper serve to

demonstrate the susceptibility of typical rail cars in a

conceivable side-on impact in the Australian rail

environment.

The simulation results presented also demonstrate

the discrepancy between the impact forces obtained

from dynamic simulations compared to the static

design loads stipulated by AS7520.3. The

discrepancy between simulation and design

standard widens further for more onerous impacts

such as that in Scenario 3.

Therefore, it may be necessary for rolling stock

designers to consider the side-on impact scenario

as an out-of-standard requirement. Indeed, some

Australian light rail procurement projects in modern

times include the side-on impact scenario to be

assessed by dynamic simulation (explicit FEA).

In these out-of-standard assessments, the

acceptance criteria are usually set and agreed

between the supplier and purchaser rather than by

design standards.

Figure 3 – deformation from 20t truck

impacting at 25km/h


impacting at 25km/h

The results illustrate that a typical metro rail car

would be deformed significantly under Scenario 3. It

is however unreasonable to expect a rail car to

accommodate the impact of a 40t truck (analogous

to a road train) travelling at 50km/h without significant

deformation.

If the same truck us travelling at half the speed

(Scenario 2), the kinetic energy is reduced by 75%.

Under this scenario, there is still significant intrusion

of occupancy space.


impacting at 50km/h


Is the Australian rolling stock industry industry

enhance its awareness aware of to the risks

associated with rollingstock collisions by simply

following the design standards? Should these risks

be mitigated by considering the performance of

rollingstock in collision scenarios that are outside of

standard?

Dr Jademond Kiang is the Senior Consultant – Specialist

Engineering at SNC-Lavalin’s Atkins business, now one of

the world’s largest consultancies with more than 55,000

people employed globally of which 5,000 are specialist rail

consultants. We offer clients a complete end-to-end

range of services, unique breadth of capability, and a

resource base that enables us to deliver all scales of rail

projects globally. We combine traditional engineering with

the latest techniques to deliver safe, reliable and

sustainable outcomes, transforming rail networks and

shaping the future of transport.

Conclusions

The passenger rolling stock crashworthiness design

standard used in the Australian rail industry

(AS7520.3) is similar in scope and stringency to

those from other parts of the world. In fact the

crashworthiness requirements in AS7520.3

stipulates the use of European Standard EN15227,

which is a widely used and accepted international

standard.

However, these standards do not necessarily cover

all conceivable impact scenarios found in the

Australian rail environment. Some scenarios may be

indirectly addressed by simplified methods of

assessment such as static structural analysis.

As demonstrated in this paper, this simplification

may not necessarily reflect the realities of a dynamic

event that is a collision scenario.

Meeting the requirements of the design standards

alone do not guarantee a train that is crashworthy in

all conceivable circumstances.

However, following and meeting the requirements of

the standards would likely provide a solid foundation

and good design practice that will be engineered

into the rolling stock, thus increasing inherent

crashworthiness in out-of-standard crash scenarios.

rolling stock crashworthiness in the modern age/media/files/s/snc-lavalin... · 2020. 12. 8. ·...

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