17 AluReport 02.201416 AluReport 02.2014

AUTOMOBIL

AMAG uses state-of-the-art testing methods to ensure that

only perfect-quality products are supplied to the

aircraft industry.

materials. To control the number and size

of defects and ensure a stable quality lev-

el, aerospace plates for the production of

highly stressed components are subjected

to various tests:

MECHANICAL TESTING (DESTRUCTIVE TESTING):

Crack propagation test:

If such allowable defects are located in

areas of high stress in a component, a

crack can form in that area, which is nor-

mally so small that it cannot be detect-

ed in the completely assembled aircraft

structure using common nondestructive

methods, so it is essential to know the

material behavior of structures or sam-

ples with cracks.

At AMAG’s testing laboratory, a crack is

produced in a sample under a certain cy-

clic load, whose propagation as a function

of the load cycles is measured. !e result-

ing crack propagation curve is divided into

three regions:

Region I – threshold region:

In this region the stress is so low that no

crack propagation occurs even if a crack

has already formed.

Region II – stable crack propagation:

!is region is used to define the inspection

intervals.

Region III – transition to unstable

crack propagation:

!e crack propagation rate increases. !e

loading of the component must not exceed

that limit value, which is to be taken into

account in design and material selection.

At this stage, crack propagation can be

predicted on the basis of the crack prop-

agation curve and knowledge of the stress

and the collective stresses (number and

magnitude of cycles) present in the rele-

vant component in order to define inspec-

tion intervals at which the cracks can be

reliably detected.

Initial situation:

Small defects are assumed to occur in all

technical components and may lead to

crack formation. As opposed to a smooth,

defect-free surface where crack initiation

occurs for the greater part of a compo-

nent‘s lifetime, the lifetime of a real com-

ponent is mainly determined by crack

propagation. Crack propagation must be

monitored using appropriate nondestruc-

tive testing methods and must not result

in unstable crack propagation and finally,

complete component failure.

Science:

Engineering fracture mechanics is an inter-

disciplinary science between engineering

mechanics and material sience that is based

on the assumption that cracks exist in com-

ponents and structures and is used to de-

scribe, in terms of quantity, material dam-

age by crack formation and propagation.

Testing:

It is not possible, or involves huge effort,

to manufacture defect-free engineering

Safe flight with AMAG AluminiumExpertise in testing aerospace plates

The crack propagation rate can be determined

with CCT (center crack tension) samples (Fig.

1). The sample is tested under cyclic load con-

ditions in a testing machine, and the extension

of a defined incipient crack is measured at reg-

ular intervals. The cyclic stress intensity (ΔK)

and crack propagation rate (da/dN) are calcu-

lated from the stress amplitude, crack length

and number of cycles.

The crack propagation curve is a double-loga-

rithmic plot where the two variables are plotted

against each other. Fig. 2 shows a typical result

for plates made of a 7075-T7351 alloy. On the

basis of these curves, we can estimate how

fast a crack growths in the component under

a specific stress and what inspection intervals

should be defined to reliably detect the crack

and prevent component failure as a result of

instable crack propagation.

These measure-

ments are part of

all material quali-

fications required

by every aircraft

manufacturer.

1,0E-06

1,0E-05

1,0E-04

1,0E-03

1,0E-02

0,0010,010,1

K [ MPa m ]

da/d

N [

mm

/cycle

]

2A-1 L-T - F const. - 78 Hz

2A-6 L-T - F const. - 78 Hz

1A-2 L-T - Kmax const. - 78 Hz

1A-3 L-T - Kmax const. - 78 Hz

2A-5 L-T - F const. - 10 Hz

�� � � � �� �

How does a…

CRACK PROPAGATION TEST

work?

Fig.1: CCT-Sample

AluReport 02.2014 17

Fig. 2: Typical crack

propagation curve of

7075-T7351 plates

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Fracture toughness test:

Fracture toughness is defined as the resist-

ance of a precracked material to unstable

crack propagation. In the component, the

stress at the crack tip must be lower than

the material parameter, fracture toughness

KIc, i.e., if the stress and fracture tough-

ness are known, the length of the crack up

to which the component works reliably

can be calculated.

NONDESTRUCTIVE MATERIAL TESTING:

Ultrasonic testing:

If the material is defect-free, the ultra-

sound enters the upper side of the plate,

is reflected at the lower side and exits

through the upper side. Defects in the

material „disturb“ that ideal sound path

in the material and result in additional in-

dications (echoes). At AMAG, ultrasonic

testing is performed using phased-array

technology, where a large single trans-

ducer is subdivided into a large number

of small individual elements that can be

combined and activated as required. In

this way, even extremely small internal

defects (reference defect size approx.

0.8 mm) can be shown. Only plates with

small defect indications (Class A) in ac-

cordance with AMS 2154/ASTM B 594

will be accepted and released for further

processing.

Conductivity testing:

Conductivity testing is an appropriate tool

to draw conclusions about the strength

and toughness conditions resulting from

heat treatment. At AMAG, the entire plate

is subjected to a conductivity test with

continuous measurement to ensure that

uniform material properties are achieved

in the material as a result of heat treatment

and that the high safety requirements of

the aircraft industry can be met.

Additional tests:

Aluminium for aerospace plates is subject-

ed to additional tests as a standard or upon

request:

Spark emission spectroscopy OES = op-

tical emission spectrometry ICO-OES (see report on page 8) ICP-

OES = inductively coupled plasma opti-

cal emission spectrometry

Tensile test (L-longitudinally and/

or LT across the rolling direction and

ST-thickness) Microstructure Stress corrosion cracking Bearing Compression test Smooth bar fatique

Testing of aerospace materials at AMAG

ensures consistent quality within the re-

quired limits. AMAG‘s testing laboratories

determine the material parameters re-

quired to implement the damage-tolerant

design principle in the aircraft component.

!e findings from these tests are reflected

in the optimization of products and of the

process chain.

So, relax and look out of the airplane win-

dow; AMAG aluminium takes you safely

to your destination.

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Ultrasonic testing equipment

The electrical conductivity is tested using eddy-current

testing, which is a comparative method that is to be ad-

justed with a known electrical conductivity using adjusted

reference blocks.

If alternating current flows in a coil in a test probe, an alter-

nating magnetic field will build up. If the probe is put on a

component, that magnetic field will enter the component.

The magnetic field constantly varies in size and direction,

so electric current is induced in the electrically conductive

component (eddy current). The eddy current builds up a

magnetic field.

The magnetic field produced in the component by the eddy

current is in the opposite direction of the exciting magnetic

field from the probe. Both fields act simultaneously on the

coil and generate induced voltage. The voltage induced in

the coil is in the opposite direction of the exciting voltage.

The difference between the voltages is measured to be

able to draw conclusions about the strength and toughness

conditions resulting from heat treatment.

Ultrasonic testing is based on the reflection of sound waves

at the boundaries of different materials, for example, air

and metal.

A transducer, which acts both as transmitter and receiver,

scans the surface of the test specimen in an overlapping

manner, the sound beam being reflected at the boundari-

es of discontinuities, defects (e.g., shrinkage, inclusions,

cracks). The time passed between the transmission and

reception of the sound pulses allows the sound path and

thus the depth of the defect to be calculated.

The reflected portion of the sound beam indicates the size

of the defect. The reflections are plotted against the ult-

rasonic transit time or the sound path in the metal (Fig.

5). This image gives the position and size of the defect (in

technical terminology, this is referred to as discontinuity).

How does a…

… CONDUCTIVITY TEST

work?

Test probe of conductivity testing equipment Ultrasonic testing equipment

Fig. 3: different sizes of CT samples

Coupling medium/water

Probe

Aluminium plate

Surface

Defect

Am

plitu

de [

%]

Rear wall

Transit time [ms]Aluminium plate

Measuring probe High-frequency

alternating

magnetic field

Eddy currents

Fig. 5Fig. 4

18 AluReport 02.2014

How does a …

… ULTRASONIC TEST

work?