1. Introduction
A damping action cancels out external load energy by converting it to internal energy using the inherent properties of the material, and is beneficial in terms of durability as it increases structural safety and reduces response. A damping coefficient is used in a time domain or frequency domain to express a damping action, and accurate identification of its value plays a very important role in understanding mechanical properties of a target system, and various methodologies have been proposed. In particular, in order to express damping in a frequency domain, a frequency damping value corresponding to each resonance point is required, which is referred to as a modal damping coefficient [
1,
2]. In order to obtain the frequency damping value, the value is usually identified through a modal test using an impact hammer, etc., but the physical quantity is vulnerable to external noise, and thus the average value through repeated tests is usually used. The modal test method for measuring the modal damping coefficient has a weakness that it is vulnerable to noise and thus only an approximate value can be found, as mentioned above. In addition, since the method is performed under limited conditions in which the input excitation type is dominated by a specific excitation pattern such as impact, the method is inevitably subject to a large error under various actual input patterns if the modal damping value is affected by the external input pattern. Accordingly, developing a method for identifying modal damping coefficient values considering the effects of input patterns of external loads has a very important meaning in securing the mechanical reliability of materials.
Carbon-fiber-reinforced plastic (CFRP), which has excellent specific strength characteristics, is a next-generation lightweight material that can replace existing steel materials and aluminum materials, and basic research and product development on it are actively being conducted [
3,
4,
5,
6,
7,
8,
9]. Since the product industry that utilizes the product is limited to only some transportation modes such as bicycles and to a small amount thereof, specialized producers are performing production in a custom-made fashion by hand. Accordingly, many technical difficulties, such as low product uniformity for prototypes and the need to develop efficient processes for mass production, have yet to be overcome in order for the product to be applied to mass production industries such as the automobile industry. Mechanical properties of the carbon composite material basically depend on the conditions of the carbon fiber and the polymer resin that make up the product. In addition, what fabrics are combined to form the carbon fiber greatly affects the mechanical properties of the product. In general, there are various candidates for the main fabrics used for carbon composite materials, such as plain weave, twill weave, and unidirectional, and thus there are various laminated structures in which these are combined. Therefore, in order to utilize the carbon composite material in products, it should be noted that the mechanical properties basically vary depending on the laminated structure, and the test results necessarily vary as well [
10,
11,
12].
Modal damping is a very important mechanical property in terms of system stability, as mentioned above, and the conditions equally apply to carbon composite materials. In particular, damping characteristics of carbon composite materials show relatively large values compared to those of other steel materials, and thus they are attracting great attention as materials for parts to be applied for mechanical products, in addition to their superiority in terms of specific strength. Accordingly, when carbon composite materials exhibit different damping characteristics depending on the excitation profile conditions, they inevitably have a large error compared to other materials due to the difference in absolute values. Therefore, in this experiment, we will describe how we derived the damping coefficient by using two half-power points that could be observed near each resonance point of the frequency response function used in existing steel products, etc. In particular, we performed tests while setting boundary conditions of the carbon composite material for measuring the modal coefficients to be the simplest form in which one side was completely fixed, so that the reproducibility of the test results could be high. In a recent study, analysis of the sensitivity of the carbon composite material specimens to different frequency pattern inputs was conducted using the frequency response function, but changes in the modal damping characteristics with respect to the temperature variable were not investigated [
12]. The sensitivity analysis of mechanical system can derive design guideline for engineers under the minimum design modification policy [
13,
14].
In this study, changes in the mechanical properties of the carbon composite material, which is a representative lightweight material, under various temperature conditions were investigated through uniaxial excitation tests. The theoretical frequency response function of CFRP simple material was proposed by assigning two parameters, resonance frequency and modal damping coefficient, as function of temperature condition. Both resonance frequency and modal damping coefficient were obtained through the calculated frequency response function at uniaxial excitation test for five CFRP specimens under different direction of carbon fiber, 0, 30, 45, 60, and 90 degrees. Based on the results of the modal damping measurement test for specimens with different directions, we evaluated the sensitivity of CFRP specimens over a temperature condition on the modal damping results. In addition, the characteristics of modal damping for different direction of carbon fiber were discussed from the experimental consequences. However, the physical reason why the dynamic parameters, resonance frequency, and modal damping coefficient were a function of the temperature condition and have a different variation trend for a different direction of carbon fiber was not considered in this study because this study was focused on the finding the sensitivity of the CFRP material over the temperature condition only.
2. Estimation Method for Modal Damping Coefficient
A typical method of measuring a modal damping value is basically performed through a modal test using an impact hammer to obtain the value. After exciting the measurement target with an impact hammer and obtaining the frequency response function using the acceleration data measured at the position where the response occurs, the damping coefficient at the main frequency is derived using the slope of the cusp near the resonance point of the measured frequency response function. This method is performed under the assumption that the target product shows linearity with respect to the external load because the method uses the frequency response function. In the case of a product with a large nonlinear characteristic, in order to perform a more rigorous experiment, the response characteristics at different frequencies are measured in detail through harmonic excitation in which measurement is performed while raising or lowering the frequency using an exciter, instead of the above-mentioned impact excitation, and then the modal damping coefficient at the resonance point is obtained. The foremost advantage of this method is that it is a method of applying an impact using an impact hammer, and thus it does not affect any dynamic characteristics of the target object because it does not involve any contact with the target object (contact herein refers to a constant connection). However, since the impact excitation method uses an input signal having all frequency components, which are simultaneously inputs, the method has critical disadvantages that distortion may occur in a product having a large nonlinear characteristic, and that the excitation pattern cannot be changed.
In this study, a method of introducing an exciter-based excitation method with limitations of boundary conditions in order to control the excitation source as precisely as possible and to obtain a frequency response function through numerous iterative tests was used. An advantage of the exciter is that desired excitation profiles can be guaranteed under excitation conditions with reliability and little error. In particular, in the excitation conditions of this test method, in addition to the method of simultaneously exciting all frequency bands using an existing impact hammer, the method of sequentially increasing the excitation frequency can be utilized as well. In the former case, the excitation profile is a random excitation in which all frequency values are simultaneously applied within the desired frequency range. In this excitation method, it is possible that some distortion may occur for a target object having nonlinear characteristics. On the other hand, if the starting frequency, the ending frequency, and the slope at which the frequency gradually rises are selected after the entire frequency band is determined, a single frequency can be applied sequentially by automatically raising or lowering the excitation frequency. This test method has a disadvantage that the test is in the form of applying one frequency and receiving a response thereto and thus is performed over a long time. Other than that, it is a method by which the response at the frequency can be received with the highest reliability. Therefore, in this test, a process of measuring the damping characteristic value for the target object, the carbon composite material, was performed using a method of applying harmonic excitation by utilizing a uniaxial exciter.
In order to control the temperature condition, a chamber was used to maintain the intended temperature during the excitation test of CFRP materials. The controller of exciter was operated separately from the controller of temperature chamber and the excitation test was conducted 10 min later after reaching target temperature because the controlled room temperature may be different from the temperature at the responsible CFRP specimen. The configuration of the temperature chamber is illustrated in
Figure 1.
The damping coefficient of a structure was obtained by using a method, which is performed under the assumption that the damping coefficient is a fixed value, and which utilizes the frequency response function. The modal damping value is a very important factor in physically determining the magnitude of the response in the resonance point section and, at the same time, is commonly used in theoretical equations and analysis models through identification of the value. First, the ratio of the response (
R(ω)) to the external excitation input (
F(ω)) in the theoretical equation part is expressed as Equation (1) under the condition of an input frequency of ω (Hz).
Here, the
,
,
and
values represent the numerator component of the frequency response function, mass, damping, and stiffness in the i-th mode, respectively, and usually have one constant value. In addition,
represents the total number of modes and
j denotes imaginary unit. The
value can be measured or obtained using material property information. The modal damping coefficient is generally expressed as a modal damping (%) value suitable for representation in the frequency domain, rather than actually using the physical values. First, when Equation (1) is converted to a modal coordinate system, it can be expressed as Equation (2) below, and the transfer function is represented by
for the
i-th mode.
Here,
,
and
values represent the resonance frequency, modal damping ratio, and normalized residual in the i-th mode, respectively. The corresponding function value is the most common value corresponding to a single input and single output model of a typical linear system. However, the characteristics of the product to be tested in this study depend on the temperature condition and the angle of the carbon composite material. If the variables expressing the conditions are
and
, respectively, the frequency response function of Equation (1) is changed as follows.
The frequency response function condition in Equation (3) is a value that is limited to carbon composite materials, and does not correspond to general material properties. Therefore, the matter to be identified in this study is to perform an analysis after measuring the sensitivity to three variables under the frequency response function condition of Equation (3) through uniaxial excitation tests. Through this, the exact value of Equation (3) can be obtained under different conditions, and it can be seen that the modal damping characteristic is a function value that can be changed by the three variables. Another consideration with respect to the frequency response function is to convert the frequency response function values obtained at different locations into representative frequency response function values. If the frequency response function value measured at the response position
is
using the condition of Equation (3), the representative frequency response function
for calculating the modal damping value is represented by the following Equation (4). Each frequency value is linearly summed and represented as one representative value. This is a possible representative value under the assumption that linearity is guaranteed for the observed CFRP specimen.
We used the measured frequency response function to obtain the representative frequency response function as shown in Equation (4), then we utilized the relationship between the resonance points and the half-power points as a method to obtain modal damping corresponding to each mode in the frequency domain. If the frequency response function is given, the damping value corresponding to the
i-th resonance point (
,) is as shown in Equation (4). Here,
and
correspond to two half-power points whose energy value is halved with respect to the center frequency corresponding to the resonance point.
By using the modal damping measurement method of Equation (4), the damping value for each mode of the carbon composite material can be measured, and the modal damping value varies depending on the resonance point. Assuming that the resonance point of the carbon composite material is affected by the directionality and that the value varies depending on the temperature condition, the modal damping value is inevitably dependent on temperature according to Equation (4), and Equation (3), which is the frequency response function related to the carbon composite material, is valid.