1. Introduction
Thermal manufacturing processes of high strength steels such as welding or cutting lead to the local deterioration of the mechanical properties [
1]. By knowing the local mechanical properties, not only can the full potential of lightweight constructions be exploited, but also important and hardly accessible local stress–strain relations of welded joints can be obtained for the prediction of the strength of the welded components [
2].
The instrumented indentation test (IIT) is a well-known semi-destructive method that enables the determination of the mechanical properties [
3] of small areas such as welded zones [
4]. In general, the indentation response depends on the material mechanical properties such as the stress–strain curve. By correctly analyzing the indentation response, it is possible to predict the causing parameters that cannot be directly calculated [
5].
The indentation force and penetration depth are constantly measured during the execution of the IIT to create the force–indentation depth curve. Three main methods have been introduced to evaluate the mechanical tensile properties from the experimentally measured force–indentation depth curve [
6]. The first two methods are the representative stress–strain approach based on Tabor’s work [
7] and the inverse simulation using the finite element analysis [
8]. In the representative stress–strain method, the true stress–strain diagram can be calculated by determining the contact angle, the pile up or sink in [
9] height, and the contact area. Numerous experiments [
10,
11] have proven the robustness of this method for a wide range of materials. In the inverse simulation method, the quality of the numerical simulation results of the indentation test depends on the modeling of the pile up and sink in effect as well as considering the friction between the indenter and substrate [
12]. Although it is possible to implement any material model or use a different type of indenter [
13], this method is numerically expensive and needs long-standing experience in the field of numerical simulation. Moreover, it is necessary to determine the starting parameters properly and use an additional optimization algorithm to find acceptable material parameters. There is enough experimental evidence [
14,
15,
16,
17] to prove the applicability of this method for metals and ceramics.
The other method implemented in this paper is the evaluation of the data by means of an artificial neural network (ANN). The ANN has demonstrated great potential to predict the mechanical properties of the indented surface. Huber et al. successfully determined the Poisson ratio [
18,
19], the parameters of a viscoplastic material model [
20,
21], and the strain hardening properties [
22,
23] from the indentation test. Moreover, there is good agreement between the experimental data and the ANN prediction of the local stress–strain properties of the resistance spot [
24] and friction stir welded joints [
25]. In addition, the application of an ANN has been widespread in computational mechanics to solve various inverse problems such as the crack growth analysis of welded specimens [
26]. Moreover, a method was proposed by Li et al. [
27] to characterize the mechanical properties of heterogeneous materials through images taken from a mesoscale structure. In another example, Ye et al. [
28] correlated the stress–strain diagrams with the images taken from the complex microstructures of the composites. Xu et al. [
29] applied the convolution neural network on images obtained from the chemical composition of hot rolled steel to predict their mechanical properties. Furthermore, Chun et al. [
30] predicated the residual strength of the structural steels by visual inspection of the damage and analyzing the taken images with a convolutional neural network (CNN). In another work, Psuj [
31] introduced a novel approach for the characterization of defected areas in steel elements. He implemented the material state evaluation model by using a deep CNN.
The works described above demonstrate the application of machine learning in the field of material characterization. In the current research, we would like to present a novel approach to determine the mechanical properties in a more practical way by using less complex equipment and focusing on the macroscale, which can be used easily in the industry.
ANN is one of the machine learning (ML) algorithms inspired by the biological neuronal network in the brain. ML has gained much popularity these days due to its capability to perform specific tasks without modeling the specific problem, instead relying on patterns or trends of the examples [
32]. The learning process of ML can be mainly distinguished between the supervised and unsupervised methods. The supervised learning algorithm learns from the training data containing the inputs and desired outputs. The goal of supervised learning is to approximate functional dependency between the input and the output. However, the training data that do not have outputs can be identified based on its pattern by the so-called unsupervised learning approach [
33].
ANN is one of the supervised learning algorithms since the output is required in the training procedure. During the training, the ANN adjusts its weights or hidden layers in order to minimize the error between the desired and the calculated output. The hidden layer is located between the input and output layer where artificial neurons receive a series of weighted inputs and produce an output. Furthermore, the weight shows the influence of the neurons on each other. If an input has a greater weight, this means that this input is more important when compared to other inputs. The trained ANN has this ability to predict the output of an unknown input within the training data space, despite the non-linearity and noise in the data. This capability of the ANN is called generalization [
34].
Increasing the number of neurons or hidden layers enhances the complexity of the network and has been found to greatly affect the performance of the ANN [
35]. Therefore, variables of the training datasets have to be dimensionally reduced without missing any important information. To train an ANN with training data in the form of a curve, the data points in a curve must be given to ANN for the training. The other alternative is to use parameters that define a curve through a function such as material model parameters that define the stress–strain curve.
Furthermore, to identify objects with a large number of variables such as images, deep learning algorithms with CNN can be applied. Within CNN, the images are passed through multiple hidden layers to extract the local features of the image before they are finally identified. A typical CNN consists of convolution layers, pooling layers, and fully connected layers [
36]. For instance, by using the images of a steel surface as the training data, a CNN can be trained to classify the steel [
37] or detect a surface defect such as cavities on a rail [
38]. However, feature extraction with CNN layers requires a handful of labeled images for the training procedure [
39]. Another alternative is to use image segmentation as one of the computer vision algorithms. With segmentation, an image is decomposed into multiple parts (segments), which can be more efficiently used for further analysis [
40]. Clustering is one of the most common unsupervised methods to perform image segmentation. Within clustering, one can recognize the pattern in large amounts of data and partition them based on its similarity into groups called clusters. Besides image segmentation, clustering is used in many fields of application such as text information classification [
41] and mobile data analysis [
42]. This approach has been shown to be effective to perform clustering by employing a square error criterion [
43].
The basic procedure for training an ANN to determine the mechanical properties of materials is described in [
44] and was implemented in this paper. Initially, numerous correlations between the indentation path and the mechanical behavior were required to train the ANN. Therefore, in order to generate data, it is necessary to start with the numerical simulation model of IIT and validate the model through a comparison with the experiment. The validated model was used to generate the force–indentation depth curves in large quantities to perform the training. Therefore, further imaginary stress–strain curves were generated randomly at certain intervals. By defining the material model parameters as input to the simulation model, the corresponding force–indentation depth curve was calculated numerically. In addition to the generation of these curves, the deformation of the indented surface by the indentation test was also obtained from the simulation.
From the generated data, training datasets for the training of ANN were extracted. A training dataset contains material parameters that describe a stress–strain diagram as the output and a corresponding force–indentation depth curve or indented surface profile as the input. The ANN then learns from these training datasets. Finally, the performance of the trained ANN was tested with the parameters of real materials.
After establishing the relationship between the indented surface and material behavior, the images of the indented surface were used for the training of the ANN. The images were captured with a three-dimensional (3D) measuring optics sensor and additionally with a simple light microscope. In this step, an image segmentation was performed before training the ANN. The training images were divided into several clusters due to the similarity of the color with the k-means algorithm. After segmentation, the image contains fewer variables, but are representative of the original image. This information was then used as input for the training of the ANN. The k-means algorithm identifies k clusters and then assigns each data point of the dataset to the nearest cluster.
Figure 1 shows the general workflow of the ANN training with the four different types of database presented in this paper.
The aim of training the ANN with four different datasets was to show the development of the methodology and also to make the procedure of material characterization less complicated. In the first method (force–indentation depth), it is necessary to perform the indentation test with the instrumented indentation machine. In the second method (profile of the indented surface), we do not need this machine, but the indentation must be performed in the same way for all samples. The third method (3D measurement image) is easier to use for the user, because only a 3D image from the sample surface has to be taken. The fourth method, on the other hand, presents an approach that can be performed without a 3D measuring sensor. For this method, we only need a simple light microscope, which is available in many research institutes or companies. In summary, an attempt was made to simplify the process of quantifying the mechanical properties for the user by using less complex equipment.