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UNIVERSITY OF GAZIANTEP FACULTY OF ENGINEERING CIVIL DEPARTMENT

CE-550

NONDESTRUCTIVE TESTING AND EVALUATION IN STRUCTURAL ANALYSIS

Report About :

(Resonance Frequency as anon destructive test method

in structural engineering)

Submitted to:

Doç.Dr.ESSRA GUNAYISI

Prepared by:

Chalak Ahmed Mohammed chalak.mohammed@gmail.com

2014 45056

Date : 13.03. 2015

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Resonant Frequency

Powers originally developed the resonant frequency method in 1938. He

discovered that the resonant frequency of a material can be matched with a

harmonic tone produced by materials when tapped with a hammer (Malhorta &

Carino 1991). Since then, the method has evolved and incorporated the use of

electrical equipment for measurement.

Theory

An important property of any elastic material is its natural frequency of

vibration.

A material’s natural frequency of vibration can be related to its density and

dynamic modulus of elasticity. Durability studies of concrete materials have

been performed indirectly using resonant frequency as an indicator of strength

and static modulus of elasticity. These relationships for resonant frequency were

originally derived for homogenous and elastic materials. However, the method

also applies to concrete.

specimens if the specimens are large in relation to their constituent

materials. (Malhorta & Carino 1991).

The study of physics has determined resonant frequencies for many shapes,

including slender rods, cylinders, cubes, prisms and various other regular three-

dimensional objects. Young’s dynamic modulus of elasticity of a specimen can

be calculated from the fundamental frequency of vibration of a specimen

according to Equation 1 (Malhorta & Carino 1991).

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42 L

4 N

2 d

E ........1

m4 k 2

Where

E = Young’s dynamic modulus of elasticity d = density of

the material

L = length of the specimen

N = fundamental flexural frequency

k = the radius of gyration about the bending axis m = a constant

Testing

ASTM has created a standard test that covers measurement of the

fundamental transverse, longitudinal and torsional resonant frequencies of

concrete specimens for the purpose of calculating dynamic Young’s Modulus of

elasticity. (C-215-97, 2001) This test method calculates the resonant frequencies

using two types of procedures, the forced resonance method or the impact

resonance method.

The forced resonance method is more commonly used than the impact

resonance method due to the ease of testing and interpretation of results. The

forced vibration

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method uses a vibration generator to induce vibration in the test

specimen while the vibration pickup transducer is coupled to the

specimen. The driving frequency is varied until the pickup signal

reaches a peak voltage. The specimen’s maximum response to the

induced vibration occurs at the resonant frequency. Figure 1. illustrates

the typical setup of a resonant frequency device. The vibration

generator is coupled to the right side of the specimen while the pickup

is coupled to the left.

Figure 1.: Typical forced resonant frequency setup

The impact resonance method is similar to the impact-echo and

impulse response methods. The impact resonance method employs a

small impactor to induce a stress wave into the specimen. However, the

forced resonant frequency method uses a lightweight accelerometer to

measure the output signal. The signal is then processed to isolate the

fundamental frequency of vibration.

The standard test method is limited to the testing of laboratory

specimens (i.e. cylinders or prisms), and at present there is no

standardized method applying the use of resonant frequency to larger

specimens or to specimens of irregular shape.

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Limitations

The resonant frequency method has been successfully applied to

the nondestructive testing of laboratory specimens. The test is

somewhat limited by a number of inherent.

problems in the method.

Resonant frequency testing is usually performed on test specimens

to non-destructively calculate cylinder compressive strength. However,

the test actually calculates the dynamic modulus of elasticity.

Extensive laboratory testing has revealed that cylinder compressive

strength and dynamic modulus of elasticity are not an exact correlation.

Thus, when concrete strength is extrapolated from resonant frequency

testing, two sources of error exist. The first source of error is

experimental error, which can be fairly significant when performing the

resonant frequency test. “Limited data are available on the

reproducibility of the dynamic modulus of elasticity based on

resonance tests” (Malhorta & Carino 1991, p155).

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The second source of error is the assumption that has to be made

when converting dynamic modulus to compressive strength. Since the

correlation between the two properties is not absolute, sources of error

will be present in any modulus to strength conversion. Figure 2.

graphically displays the experimental results obtained relating cylinder

compressive strength with dynamic

modulus of elasticity. The experimental data can be predicted

within ±10% assuming the results from a given resonant frequency test

have zero error. In reality, converting dynamic elasticity to

compressive strength would yield an uncertainty greater than ±10%.

Figure 2.: Dynamic modulus of elasticity vs. cylinder compressive strength(Malhorta & Carino 1991)

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Applications

Resonant frequency can be a useful tool for detecting material changes

regardless of whether an actual dynamic modulus or compressive strength can

be calculated.

Resonant frequency can be used to measure qualitative changes in a

material property if used as a monitoring technique. The existence of

structural damage in an engineering system leads to a modification of the

modal parameters, one of which is resonant frequency.

It is possible to monitor a given complex structural element with shape

parameters that prohibit an accurate calculation of geometric parameters such

as radius of gyration or density. Complex structures are often too large or

have immeasurable properties, such as the exact location of internal steel

members, to extract relatively simple material properties that are easily

calculated in the laboratory setting. However, when used as a quantitative

technique resonant frequency can detect material changes between tests. A

review of methods of damage detection using natural frequencies has shown

that the approach is potentially practical for the routine integrity assessment

of concrete structures (Salawu 1997). Using the natural frequency changes of

a structure may not be useful for identifying the location and assessment of

specific cracks and anomalies within a structure. The technique can detect

changes in a structure or structural element, if an acceptable baseline is

established at the time of construction.

Regards...