Nondestructive TestingNondestructive TestingKerry HallKerry HallDepartment of Civil and Environmental EngineeringDepartment of Civil and Environmental Engineering

Adapted from L. J. Adapted from L. J. StrubleStruble and J. S. and J. S. PopovicsPopovics

Motivation

� US infrastructure is deteriorating: 2009 ASCE Report card for

American infrastructure gave an overall grade of “D” – estimated $2.2

trillion investment needed for improvements

� Infrastructure agencies are shifting efforts from building new

structures to assessing and rehabilitating existing structures

Minneapolis I-35

bridge collapse

Momentum and CollisionsMomentum and Collisions

�� Elastic collisions, both Elastic collisions, both

momentum and kinetic energy momentum and kinetic energy

are conservedare conserved

�� Inelastic collision, momentum Inelastic collision, momentum

is conserved, but energy is is conserved, but energy is

absorbedabsorbed

�� Object impacts solid, energy Object impacts solid, energy

absorbed, thus absorbed, thus �� V2<V1V2<V1

�� Low strength material absorbs Low strength material absorbs

more energy, thus lower more energy, thus lower

rebound heightrebound height

adapted from J.S. Popovics

Rebound Hammer TestRebound Hammer Test

Rebound Hammer - ASTM C 805

� Measures surface hardness

� Related to Modulus of Elasticity

� Affected by varied conditions� form material and type of finish

� moisture content

� aggregate type and proportion

� surface smoothness

� temperature

� direction of impact

� depth of carbonation of the surface

Rebound Hammer - ASTM C 805� Sometimes called the

Schmidt Hammer or Swiss Hammer

� Light weight, portable, hand operated

� Spring loaded steel hammer impacts plunger – hammer rebound is measured

� A test is the average rebound number of ten determinations made in a small area.

Penetrating Probe – ASTM C 803

� Drives steel probe or pin into concrete

� Measures toughness of the concrete,

the ability to resist fracturing.

� Related to the tensile strength

� Affected by varied conditions

�Nature of formed surface

�Coarse aggregate type, size and hardness

�Moisture content

Penetrating Probe – ASTM C 803

A steel probe driven

into the concrete

surface using a powder

actuated gun

Hazardous – wear

protective equipment

Penetrating Probe – ASTM C 803

� Portable and hand operated

� May require a license to operate

� Three probes shot into the concrete and the average penetration

determined.

Correlation with strength

� A correlation of the nondestructive test parameter to strength for each type of concrete to be tested is necessary to determine in-place

strength.

� A statistical evaluation of the correlated data is

necessary

� ACI 228.1R, In-Place Methods to Estimate

Concrete Strength presents methods for correlation and statistical analysis.

What are waves?Propagation of a disturbance through a medium; mass is not transported in propagation direction

-1.5

-1

-0.5

0

0.5

1

1.5

0 20 40 60 80 100

Time

Dis

pla

ce

me

nt

T

phase delay

The time dependant disturbance is

usually expressed in harmonic form

The period (T) is the time required for

wave motion to complete a round trip (measured in seconds)

The frequency (f) is the inverse of T

(measured in 1/seconds or “Hertz”) In

audible sound, frequency is interpreted as the pitch

Frequency-wavelength relation for all harmonic waves

V = λ f

ω= 2πf is “circular frequency” and k = 1/λ is “wave number”

Wavelength λ in units of distance

Propagation velocity V in units of distance per time

Mechanical body waves in solids: P-waves

Direction of Travel

Excitation

Direction of Particle Motion

also: Longitudinal (L-) Waves, Compression Waves

( )( )( )2ν1ν1ρ

ν1EvP

−+

−=

Wave Velocity: Governing Parameters

Young’s Modulus E

Poisson’s Ratio v

Density ρ

<T. Voigt >

Mechanical body waves in solids: S-waves

Direction of Travel Direction of Particle Motion

Wave Velocity in solids: Governing Parameters

Shear Modulus G

Density ρno propagation in liquids or gases !

also: Transverse (T-) Waves, Shear Waves

Excitation

ρ

GvS =

<T. Voigt >

VP > VS in all known solids

Guided wave modes

<www.lamit.ro/earthquake-early-warning-system.htm>

ρ

EvBAR =

Rayleigh surface wave travels along free surface, slightly slower than S-wave

1-D bar wave travels along long cylinder or prism, slightly slower than P-wave

propagation

a propagating “resonance”, must be set up over distance or time

Guided waves in plates

<N. Ryden>

Lamb wave are set up in large plates

Multiple (infinite) modes of

propagation, with varying motion

character and propagation velocity

Can be visualized as a propagating

resonance

Increasing frequency or plate thickness

Impact echo frequency

Reflection and refraction –normal incidence

<Krautkramer and Krautkramer>

When an incident wave encounters the boundary with another material, part of the incident energy is reflected, and the rest is transmitted (refracted)

pr/po is the reflection coefficient (r); pt/po is the transmission coefficient (t)

r is maximized and t minimized when Z1>>Z2 or Z1<<Z2.

Acoustic impedance : Z = V ρ

Reflection and refraction, mode conversion

<Gibson 2005>

<www.ndt-ed.org>

When an obliquely incident wave encounters the boundary with

another material, reflection and refraction become dependant on ϑi

(Snell’s law). Conversion to other wave modes also occurs.

Beam divergenceThe principles of wave interference and superposition control the

directivity of the generated pressure field. A given transducer may

primarily generate P-wave energy in some directivity field, although

some S-wave and Rayleigh wave energy, may also generated in solid media

<www.ndt-ed.org>

1.2sin

D

λα =

The beam divergence

angle α of a given transducer can be

estimated:α

Beam divergence: point source of waves

<Richard et al. 1970>

Solid material

Snapshot of wave fields (stress) in material owing to transient point load

at some time “t” after wave excitation

Point sources of waves have poor directivity and generate P-waves, S-

waves and Rayleigh waves

ScatteringThe reflection of ultrasonic energy away from the original direction of

propagation; caused by reflection, refraction and mode conversion from internal

inclusions. Causes signal loss, signal dispersion and scattering “noise”

Detected back-

scattered signal

<Oelze 2007>

Absorption and attenuation

<http://www.greerindustries.com>

Wave absorption is the conversion of ultrasonic wave energy to other

forms of energy (heat). A

significant source of wave energy

loss for asphalt concrete

Wave attenuation is the overall loss of wave energy with propagation,

caused by

* beam divergence (geometric) * scattering

* absorption

<www.ndt-ed.org>

Implications: transducer contact (coupling)

<M. Schickert and MSIA Spectrum>

<www.ndt-ed.org>

To eliminate significant wave reflection at the transducer-test material interface, must use a

substance to displace air and ensure good

contact: oil, gel, grease, solid

Problematic for rough or uneven surfaces

Dry point contact transducers obviate the need for couplant

material. Each point transducer

needs vertical pressure to

ensure wave energy transfer

Implications: Detection of defects

General Rule:

Ultrasound can resolve

defects of size x if x is

the same size or larger than the wavelength λ

of wave pulse.

Ultrasonic waves show large reflection at interfaces between high (concrete) and low (air-filled defects) acoustic impedance

time

voltage

Simplified A-scan

Echo height size of defects (but shape dependant!)

<www.ndt-ed.org>

Solution: use small λλλλ (large f)

Implications: lateral defect resolution

time

voltage

αααα

D

1.2sin

D

λα =

The ability to resolve side by side reflectors is improved by reducing α

Solution: use small λλλλ (large f)

Application: Ultrasonic pulse velocity (UPV)

Measurement of very first wave arrival (P-wave) through a specific wave path. Requires good coupling to surface

Standard method in many countries (ASTM C597)

Frequencies between 20kHz to 100 kHz typically used

<Naik, Carino and Popovics 2005>

EffectsParameter on UPV on concrete strength

w/cm ratio

age

moisture content

Agg type and content n/a

Proximity of steeln/a

Presence of defects

useful for relative

measurements within

a single structure

However, UPV cannot be used to measure in placestrength absolutely in most cases!

UPV application: concrete strength?

UPV application: defect detection?

Little to no effectLoss of transmission orapparent lower velocity

Loss of transmission orapparent lower velocity

Defects cause wave path to deviate, thus lowering the apparent velocity

in most cases. However, UPV cannot be used to fully characterize

defects (shape, depth, location, etc.)

crack

void

UPV applications: Modulus determination

0

10

20

30

40

50

60

0 10 20 30 40 50 60

Ed measured from resonant frequency, GPa

Eu m

ea

sure

d fro

m U

PV

, G

Pa

Limestone

River Gravel

Air-Entrained

High Strength

PC, w/c = 0.34

PC, w/c = 0.45

Paste Specimens

υd = 0.25

concrete specimens

υ d = 0.20

Ed is directly related to VP by wave theory. However, measurements obtained

from wave velocity (UPV) do not agree with those obtained by vibration!

Wave propagation over predicts Ed for concrete samples, assuming median

values of Poisson’s ratio

Ultrasonic Pulse Velocity –ASTM C 597

� More sophisticated electronic equipment

� Portable, may require electrical power

� Usually requires two or more people for testing

Ultrasonic pulse velocity Ultrasonic pulse velocity �� Calculate YoungCalculate Young’’s modulus s modulus

from UPV (theoretical):from UPV (theoretical):

�� Calculate strength from Calculate strength from

YoungYoung’’s modulus (empirical): s modulus (empirical):

Serway and Faughn

1.50.043E ρ σ=

where V is velocity (m/s)

E is Young’s modulus (MPa)

ν is Poisson’s Ratio

ρ is bulk density (kg/m3)

σ is compressive strength (MPa)

( )( )1

1 1 2

EV

ν

ν ν ρ

−=

+ −

vsolids > vliquids > vgases

Dynamic (vibration) methods Resonance Frequency AnalysisResonance Frequency Analysis

NDE Technique - shallowImpact-echo (ASTM C 1383)

Phenomena

Propagating waves

generated by impact

event. Multiply-reflected

waves are detected by

surface sensor.

Reflected waves set

up a resonance

condition having a

characteristic frequency

Impact-echoAnalysis

The resonant

frequency (at the

peak) is related to

distance to reflector

(d) and wave velocity

(V):

f = V/(2 d)

Thus,

d = V/(2 f)

Reflection from slab bottom

Reflection from delamination

Impact-echoAnalysis (cont’d)

• Strong wave reflectors

more readily detected.

• Reflections

from embedded rebar

and at the interface of a

slab and a stiff subgrade

are weak.

Impact-echo

Advantages

Disadvantages

Application

Relatively simple test to perform; commercially

available test equipment. Effective for detecting

delamination and slab depth.

Operator experience needed for data interpretation.

Not as effective slabs over very stiff subgrade. Not

effective for rebar detection.

Slab depth and delamination detection for most

slab systems.

NDE Technique - shallowGround Penetrating RADAR (ASTM D 4748)

Phenomena

Wave pulses are reflected

at interfaces having

a difference in

electrical properties (εr )

Reflected pulses (time

and amplitude) are

monitored in the

time domain signal

antennaair: εr = 1

concrete: εr = 6 to 11

soil: εr = 2 to 10

(water: εr = 80; metal εr = infinite)

GPRAnalysis

Many time domain signals stacked together to

form an image

Scan direction

Slab

depth

GPRAnalysis (cont’d)

Large wave reflection from metallic objects and moist areas.

Less reflection from slab-subgrade interfaces and air-filled cracks

Rebar reflections

Slab surface

Scan direction

GPR

• Physical contact

between antenna and

slab not needed

• Allows for rapid

non-contact

scanning

antenna

GPR

Advantages

Disadvantages

Application

Very rapid data collection (non-contact technique).

Sensitive to presence of embedded rebar and moisture.

Very involved data interpretation; operator experience

needed. Testing limited to 750mm depth. Not sensitive to

delaminations. Not effective beyond congested reinforcement.

Rapid scanning of slabs for depth or rebar location.

NDT of Steel

� Liquid Dye Penetrant

� Eddy Currents

� Ultrasound

� X-ray

Defects in SteelDefects in Steel

�� Observe visuallyObserve visually

�� Enhance with penetrating dyeEnhance with penetrating dye

�� Clean surface and apply Clean surface and apply penetrantpenetrant

�� Allow liquid to penetrate Allow liquid to penetrate

then remove excess from surfacethen remove excess from surface

�� Apply developer (draws Apply developer (draws penetrantpenetrant

out of defects)out of defects)

�� No indication of crack depth No indication of crack depth

�� No indication of subsurface No indication of subsurface

defectsdefects

�� Not for porous/rough materialsNot for porous/rough materials

Liquid (Dye) Penetrants

Liquid (Dye) Penetrants Eddy currents

� Magnetic fields setup electrical currents in a conductive material (eddies)

� They in turn generate a secondary magnetic field that counteracts the first

� This change in the field can be detected by original coil or a pick-up coil

Eddy currents

Medium 1

Medium 2

incident

wavereflected

wave

transmitted

wave

θi θr

θt

Medium 1

Medium 2

incident

wavereflected

wave

transmitted

wave

θi θr

θt

incident

wavereflected

wave

transmitted

wave

θi θr

θt

Ultrasonic Wave ReflectionUltrasonic Wave Reflection

�� Reflected angle equals the incident angleReflected angle equals the incident angle

�� Amplitude of reflected wave depends on the properties of the twoAmplitude of reflected wave depends on the properties of the two mediamedia

�� If media have large differences in stiffness and density, If media have large differences in stiffness and density,

most energy is most energy is reflected (flaws!)reflected (flaws!)

�� If media have similar stiffness and density, most energy is If media have similar stiffness and density, most energy is transmittedtransmitted

AngleAngle--Beam Transducer Beam Transducer

Inspection Inspection �� Wave frequency 1Wave frequency 1--10 MHz10 MHz

�� Angle beams allow lateral Angle beams allow lateral

detection of flaws in and detection of flaws in and

around welded areasaround welded areas

�� Vertical cracks are not Vertical cracks are not

detectable by normal beam detectable by normal beam

incidenceincidence

�� Reduce extra echoes with Reduce extra echoes with

angle beamangle beam

Hole Crack

no

detectionCrack

Hole

CrackCrack,

no detection

Ultrasound: Steel vs. ConcreteUltrasound: Steel vs. Concrete

�� Ultrasonic pulse echo not effective in concrete Ultrasonic pulse echo not effective in concrete Why?Why?

�� Aggregate scattering!Aggregate scattering!

�� ff = V/= V/λλ

�� If aggregate size (D) is 1If aggregate size (D) is 1”” and we need and we need λλ > D> D

�� V = 4000m/s V = 4000m/s �� ff < 150 kHz< 150 kHz

�� ff was in MHz for steel!was in MHz for steel!

�� Low frequency pulse echo is problematicLow frequency pulse echo is problematic

�� Difficult to manufacture transducersDifficult to manufacture transducers

�� Low Low ff leads to large beam divergence (poor lateral leads to large beam divergence (poor lateral

resolution)resolution)

�� Transducer face must be very largeTransducer face must be very large

XX--ray Radiographyray Radiography

�� Bright is low xBright is low x--ray ray

intensity due to intensity due to

high absorptionhigh absorption

�� Dark is high xDark is high x--ray ray

intensity due to intensity due to

low densitylow density

NDT Lab TodayNDT Lab Today�� Concrete testsConcrete tests

��Schmidt rebound hammer: Surface hardness, Schmidt rebound hammer: Surface hardness, strengthstrength

��Ultrasonic Pulse Velocity: thickness, strength, Ultrasonic Pulse Velocity: thickness, strength, modulusmodulus

��Ultrasonic Resonance Frequency: modulusUltrasonic Resonance Frequency: modulus

�� Steel TestsSteel Tests��Dye Penetration: surface defectsDye Penetration: surface defects

��Ultrasonic wave reflection: thickness, defectsUltrasonic wave reflection: thickness, defects

��XX--ray: surface/internal defectsray: surface/internal defects

��Eddy Currents: surface defectsEddy Currents: surface defects