ANN Architecture Specifications for Modelling of Open-Cell Aluminum under Compression

-e knowledge on strength properties of porous metals in compression is essential in tailored application design, as well as in elaboration of general material models. In this article, the authors propose specification details of the ANN architecture for adequate modelling of the phenomenon of compressive behaviour of open-cell aluminum. In the presented research, an algorithmwas used to build different structures of artificial neural networks (ANNs), which approximated stress-strain relations of an aluminum sponge subjected to compression. Next, the quality of the built approximations was appraised. -e mean absolute relative error (MARE), coefficient of determination between outputs and targets (R2), root mean square error (RMSE), and mean square error (MSE) were assumed as criterial measures for the assessment of the fitting quality.-e studied neural networks (NNs) were two-layer feedforward networks with different numbers of neurons in the hidden layer. A set of experimental stress-strain data from quasistatic uniaxial compression tests of open-cell aluminum of various apparent densities was used as data for training of neural networks. Analysis was performed in two modes: in the first one, all samples were taken for training, and in the second case, one sample was left out during training in order to play the role of external data for testing the trained network later. -e taken out samples were maximum and minimum density samples (for extrapolation) and one random from within the density interval. -e results showed that good approximation on the engineering level (MARE< 5%)was reached for teaching networks with ≥7 neurons in the hidden layer for the first studied case and with ≥8 neurons for the second. Calculations on external data proved that 8 neurons are enough to actually obtainMARE< 10%.Moreover, it was shown that the quality of approximation can be significantly improved toMARE ≈ 7% (tested on external data) if the initial region of the stress-strain relation is modelled by an additional network.


Introduction
Modern innovations cannot succeed without materials having features that comply with even very high needs. An example of such exceptional materials are porous metals, which due to their structure manifest valuable properties. ese start with the most obvious lower apparent density and thus lower weight compared to solid metals, yet keeping a reasonable level of structural strength at the same time. Another trait beneficial in various application is capacity for dispersion of multiple energy forms (impact energy, sound waves, etc.). Typically for metals, these materials show very good electric conductivity, which combined with large specific surface resulting from the porous structure makes them perfect candidates in catalyst industry applications.
Porous metals can be catalogued, depending on the production method [1], type of porosity [2], skeleton material, certain property level, or application area [3,4]. For the purpose of this article, it is ample to mention shortly that there are three main production routes: liquid metallurgy (direct foaming and casting), sintering metallurgy (powders and semifinished products), and lastly coating. According to the porosity type, we distinguish: closed-cell metallic materials, open-cell metals, sponges, gasars, and others. As for the skeleton, a range of alloys or pure metals can be used, e.g., copper, aluminum, titanium, gold, steel, and others [5][6][7][8][9].
In the present work, the authors investigate computational description of mechanical behaviour of an open-cell metal sponge with an aluminum skeleton, self-produced by investment casting. e article describes one of the stages of broader research, which includes among others study on manufacturing parameter calibration [10], experimental assessment of mechanical behaviour [11,12] and modelling [13]. A preliminary positive verification of the possibility of usage of artificial neural networks in service for modelling the open-cell aluminum compressive behaviour was reported in [14]. e present study is a consequent development of the previous work, consisting in algorithmic creation and comparison and selection of a specific neural network structure to closely represent the material's stressstrain characteristics in compression.
Artificial neural networks constitute an interesting direction in numerical investigations in mechanics of materials and structures and in engineering [15][16][17][18][19][20]. A properly structured network trained on experimental data can become a remarkable asset in either building or verifying mechanical models or surrogate models. However, the following questions arise: How to appropriately choose the network architecture? What criterion to assume as the structure's quality estimator? and, on the contrary, how to pre-and postprocess data in relation to the chosen ANN? e discussion on neural network structural aspects, such as the optimization of weights, the number of inputs, the choice of learning parameters, the number of layers, the number of neurons, activation functions, and inclusion of the statistical approach, has been vigorously ongoing for recent decades, e.g., [21][22][23][24][25]. Many researchers agree, however, that either there exists no universal and explicit method for tuning these parameters or there is rather few guidance in this matter [23,26,27]. With the present study, the authors are aimed at addressing this lack for the field of mechanics of aluminum sponges. For this purpose, a number of ANNs were algorithmically built, trained on real experimental data from uniaxial tests, and evaluated. e evaluation led to formulation of guidelines regarding structural specifications of neural networks. Below in the remaining paragraphs of Introduction, there will be adduced general assumptions, reasoning outline, and contributions of the discussed research.
Our previous, preliminary study [14] had confirmed that the general network structure, which had been a two-layered feedforward network, was successful with regard to the considered phenomenon. Used activation function types were proven adequate: tansig in the first hidden layer and purelin in the second output layer [1]. It was then decided to sustain these general assumptions regarding the network architecture. In addition, they were in agreement with the advised general solution for fitting problems, which is "the multilayer perceptron, with tansig neurons in the hidden layers and linear neurons in the output layer" [28]. e investigated neural networks were trained on the data from previously conducted uniaxial tests [11,12] and included stress, strain, and additionally apparent density of samples. As for data preparation for neural networks, a common way of conduct is to perform one of the following: normalisation, scaling, statistical standardisation, or other mathematical approach [29] in order to transform data into the interval 〈− 1; 1〉 or 〈0; 1〉. However, the fact of performing the preprocessing, as well as its type choice, has impact on the whole process of neural computations. It is necessary to be aware that this impact might be positive, neutral, but even adverse. One of the undeniable benefits is, colloquially speaking, "equalling" of data significance among them, or, in other words, avoidance of favouring any data. On the contrary, data processing could also cause loss of some initial information, which definitely should be counteracted [30]; also, it might lengthen computations [31]. Interestingly, it was proven that, with data sets big enough, networks are able to compute nearly as good results without standardisation as with it [31]. In the present research, it was decided that normalisation would be performed, since the original data ranges were rather different (strain from 0% up to ca. 90%, stress from 0 MPa up to ca. 12.5 MPa, and apparent densities from 0.32 g/cm 3 up to 0.56 g/cm 3 ).
Regarding the description of compressive behaviour, even before preliminary neural network computations, a general choice of variables and relation type between them had to be defined. One of the provisions had been the theoretical formula in the form of the dimensional law given as [32] Cellular material property Skeleton property � C ρ ρ s n .
(1) e above formula links a sought cellular material's property with the respective skeleton's property and both the cellular material's density ρ and the skeleton's density ρ s . e two linking scaling parameters C and n should be determined experimentally for the given material. Using relation (1) allows one to calculate a few selected magnitudes [32], such as the plastic collapse stress (σ pl.c ) and limit stress between densification and plastic collapse (σ * ). Additionally, numerous experiments show that there exists a sort of correlation between stress-strain characteristics of cellular metals in compression and their apparent or relative density, e.g., [11,32,33]. On this basis, the authors decided to choose the following general relationship as the function to be approximated by the neural calculations: where σ is the stress, ε is the strain, and ρ is the apparent density. e approach from the study [32], however, has some significant limitations. Only few characteristic stress values can be determined with its use. Also, it is necessary to calibrate the experimental constants C and n. Additionally, the knowledge on the skeleton's material parameters including its yield point, modulus, and density is necessary for calculations according to the cited approach. Taking this into consideration, we decided to seek for an alternative method to determine compressive behaviour of open-cell aluminum. We decided to employ neural networks for this purpose. is path allows one to transfer the experimental constants' calibration to machine learning. It also frees the user from testing not only sponge samples but also the skeleton material, as it does not require the knowledge on skeleton properties. Finally, with such an approach, it is possible to find stresses for the whole range of strains and densities instead of only single values as in the referred above method. e essence of the study laid in the subsequent stage which was algorithmic creation of 250 different NNs, training them on experimental data to fit the aforementioned relation (2) and assessment of the quality of approximations. For the evaluation, four criterial measures were selected [34]: mean absolute relative error (MARE), coefficient of determination between outputs and targets (R 2 ), root mean square error (RMSE), and mean square error (MSE). For the MARE, there were two assumed threshold values: 10% and 5%; additionally, the minimum was sought. In case of the coefficient of determination, the threshold was assumed as 0.95. e root mean square error had the same unit as targets (MPa) and was used as a measure to complete the assessment done by the unitless MARE. e mean square error was set as the networks' internal performance function with the goal at 0. e outcomes of the presented research met the prior set objective: determination of specifications of the neural network architecture, which could be used as practical guidance in modelling of the phenomenon of compression of open-cell aluminum. Significant contributions of the authors' work are as follows:

Materials and Data from Uniaxial Compression
Experiments. e subject material was an open-cell metal sponge with an aluminum skeleton self-produced by investment casting. e authors limit the present description only to details necessary for clarity and lucidity of communication; more extensive information on production and experimental characteristics of the chosen material is given in separate publications [10][11][12].
Parameters of the manufacturing process were tuned in the course of the production, so two sample groups were obtained: a prototype lot denoted as "P" (exhibiting minor structural imperfections and larger apparent densities) and a regular lot denoted as "R" (with smaller apparent densities and without visible structural mistakes). e structural imperfections of the "P" samples included occasional semiclosed cells, swellings of struts, and form residuals sank inside. Exemplary samples of both types are shown in Figure 1. Samples' dimensions were fit to satisfy the condition of the minimal number of cells for the specimen's volume to be representative [35]. e dimensions were on average (53.0 × 39.5 × 39.0) mm and (62.8 × 39.5 × 38.0) mm, respectively, for the "P" and "R" lots. e pores per inch index was PPI � 5.4/6.2 in both groups. Each sample had different apparent densities (the values are listed in Figure 2); apparent densities' variation was attributed mainly to the impact of structural imperfections within the "P" group but also to the stochastic distribution of cell dimensions in both groups (compare [36]).
Uniaxial compression experiments were performed using a Zwick 1455 20 kN machine and computer programme testXpert II. Assumed testing conditions were as follows: initial force 5 N, data acquisition frequency 100 Hz, and strain speed 0.5% h 0 in mm/s, where h 0 was the initial sample height. Figure 2 shows obtained stress-strain curves. Samples from the group "P" are drawn with solid lines, while samples from the other group are drawn with dashed lines. Along with the plot lines, values of apparent density ρ in g/cm 3 for each sample are given. It can be clearly seen that the compressive behaviour is related to apparent density. e results from the cited compression tests as well as specimens' characteristics (apparent density and indicator of the lot) were used as data for training of neural networks.

Method: e ANN Algorithm.
Computations with neural networks consisted in building a number of neural networks which varied in structure. Each such network will be referred in this article as a unit neural network. e efficiency of unit networks was assessed by comparing the mean absolute relative error (MARE), coefficient of determination between outputs and targets (R 2 ), root mean square error (RMSE), and mean square error (MSE) for each of them. All computations were implemented as an algorithm. e algorithm Mathematical Problems in Engineering 3 was performed in two modes: the first mode for all data and the second mode with one specimen excluded so that it can be used later as testing data. Calculations were performed using Matlab R2019A version. In order to make the method's description more comprehensible, it will be divided into four subsections. First, preparation and processing of data will be discussed (Section 2.2.1); then, the unit network is built (Section 2.2.2); next, the algorithm is built, which compares the unit NNs (Section 2.2.3); and finally, the quality assessment is done (Section 2.2.4).

Data Preparation and Pre-and Postprocessing.
Before entering the network, data obtained in uniaxial compression tests and specimens' characteristics were prepared as follows: (1) 12 open-cell aluminum samples were taken into consideration; for each sample, 1,000 experimental strain and respective stress values were taken. e chosen strain values were equally dispersed among the original strain data range for each sample. Additionally, the sample's apparent density and two parameters: l P and l R , defining to which of the two lots ("P" or "R") the sample belonged were included. e parameters were equal to 1 if the sample belonged to the given lot or 0 in the opposite case.
(2) In coordination with the assumed sought relation form σ � f(ε, ρ) (equation (2)), arguments entering the neural network were set into n vectors: . . , n. e respective value of experimental stress from the i-th data set was assumed as the corresponding target for the network: t i � σ i , with i � 1, 2, . . . , n. (3) Two modes of calculations were performed. In the first mode, all 12 samples were used in the process of network teaching, n � 12000. In the second mode, one aluminum sample was taken out of the input data for teaching the networks (n � 11000); next, this sample was used to test the quality of networks taught in the previous step.
Before the training, data were normalised. Since network implementation was done in the Matlab environment, the inbuilt function mapminmax was used [37][38][39].
is function is a linear transformation into the interval of given boundaries: where V is the original value; V ′ is the transformed value; V max and V min are original interval boundaries; and V max ′ and V min ′ are desired range boundaries, whose values are − 1 and 1 here. Vectors A i were normalised, respectively, into the following input vectors: On the other end, the output of the network which was supposed to correspond to the sought stress was also normalised: y i � σ appr.norm,i , so the reverse action was needed in order to obtain the desired stress approximation: y i � σ appr.norm,i ⟶ post− processing y i � σ appr,i .

Learning Parameters and the Unit Neural Network.
e unit NN's learning consisted of three stages: training, validation, and testing. Data from experiments were divided between these steps in the following proportions: 60% for training, 20% for validation, and 20% for testing. It was specified that the same sets of input vectors were assigned to respective steps for all unit networks. e selection was performed in the manner which is now going to be shortly described. Data from uniaxial experiments were a uniformly dense data point sequence with 12000 elements. Every fourth element was assigned to validation, every fifth to testing, and the rest to training. Such grouping enabled equivalence of data fractions used in all learning steps. Also, since data choice was the same for all unit networks, it did not affect learning, and thus, the quality of the networks themselves could be compared. e assumed training algorithm was the Levenberg-Marquardt algorithm [40] and the mean square error (MSE) was chosen as the performance function. Other assumed training parameters are set in Table 1. Two parameters were changed in comparison with an earlier, preliminary work [14]; namely, the maximum validation failures were risen from 6 to 12 and the maximum number of epochs was from 1000 to 100000. e alternation aimed at reassuring better accuracy and enabling better training. e chosen general network structure type and activation function types were assumed in accordance with what is advised in case of nonlinear function approximation in the literature [28] and with previously investigated examples [14]. e unit neural network was assumed as a feedforward network with two layers: one hidden layer denoted as {1} and one output layer denoted as {2}. A detailed scheme of the unit NN is shown in Figure 3 and will be explained now. In mathematical expressions (4)-(11), the index i � 1, 2, . . . , n, indicating the numbering of a given data set, is going to be omitted, since it is obvious that one cycle of processing by the unit network concerned one and the same set of data and that eventually all data (all i-s) were exhausted and all once, however, not in the sequence of numbering but randomly.
e vector X entered the hidden layer {1} as the input. e number of neurons in the hidden layer was varying and was from the range s 1 { } � (1/50) 1 { } . e activation (transfer) function was tansig-hyperbolic tangent sigmoid, mathematically equivalent to tanh [37]: where arg 1 { } is the argument of the transfer function given by with X being the input column vector, B 1 { } being the column vector of biases for the hidden layer, and W 1 { } being the matrix of weights of input arguments for the hidden layer: As a result of computations in the hidden layer {1}, the column vector of the hidden layer {1} outputs Y 1 { } was produced: Hidden layer {1} outputs entered the second output layer {2}. In this layer, the number of neurons was constant and equal to s 2 { } � 1 2 { } in accordance with a single variable output [29] and the transfer function was purelin [37]: where a � 1 is the directional coefficient and arg 2 { } is the argument of the transfer function given by with Y 1 { } as in (7), b 1 2 { } being the bias for the output layer, and W 2 { } being the row vector of weights of input arguments for the output layer: (10) Finally, after computations in the output layer, the output of the unit network, y, was returned [41]:

e Algorithm for Unit Neural Network
Building. e general purpose for implementation of the algorithm was to execute a number of networks which differed in structure and determine criterial measures-MSE, MARE, RMSE, and R 2 -for each of them (the three measures are explained in the next section). e algorithm was performed in two modes: the first mode for all data and the second mode with 11 samples as the input. Now the structure of the algorithm is going to be described in detail, and an auxiliary scheme is shown in Figure 4. e algorithm consisted of two procedures: the parent procedure P1 and the procedure P2, which was nested in P1. e P1 procedure's overall purpose was to provide varying unit network architecture parameters by attributing a given number of neurons from the range s 1 { } � (1/50) 1 { } to hidden layers of unit NNs. e procedure started with learning data index assignment for training, validation, and testing. Next, two loops for the above-mentioned objectives were repeated until s 1 { } : � 50 1 { } . In the first loop, experimental data were loaded and the P2 procedure was executed. After finishing both loops, the algorithm stopped.
In short, the procedure P2 aimed at building unit networks with structures defined before by the P1 procedure. e procedure P2 started with loading the unit network char- , loading original data, and learning settings. en, computations took place: data assignment to learning steps according to earlier defined indexation and network learning together with preand postprocessing of data. Results were saved. After five reiterations, the procedure P2 ended and exited back to P1.
Several (five) repetitions of calculations with the same number of neurons were implemented in the P2 procedure intentionally, as the initial attribution of weights in training is random [37]. In consequence, a single execution of the process does not guarantee obtaining the best result from the network with the given number of neurons, whereas several reiterations significantly increase the probability of obtaining the best result for the chosen network architecture.

Evaluation Criteria.
As has been said in Section 2.2.2, the mean square error (MSE) was chosen as the internal network quality measure, that is, the performance function. Assessment based on this measure was performed in the validation stage. e criterion for MSE was minimal, which in practice means that its target value was desired as 0. e mean square error was defined in computations as follows: where n � 12000 or n � 11000 is the number of experimental data sets:   Mathematical Problems in Engineering t i � σ i (approximated stress), i � 1, 2, . . . , n; implicitly, the error was also defined as It should be noted that data normalisation and the reverse procedure were performed before calculation of errors and MSEs; hence, the units of errors and MSEs were the unit of stresses and the squared unit of stresses, respectively (MPa and MPa 2 in this case).
Externally, the measure of the mean absolute relative error (MARE) was used as one of the quality indices. is measure was the quality indicator for values calculated in the test stage. Two threshold values were determined as desired engineering accuracy levels: MARE � 5% and MARE � 10%. Apart from these, the minimum was sought. e understanding of the mean absolute relative error in our calculations is explained by the following formula: with n, t i , σ appr.i , and i as mentioned above and with additional definitions of the relative error and absolute relative error as Another measure for the evaluation in the testing stage was the root mean square error (RMSE): with variables, indices, and errors assumed as in (12a) and (12b). e last quality criterion referred to the results of linear regression between the network's outputs and targets for all three stages of training, validation, and testing (t i , σ i , σ appr.i , and i are as above). Initially, the Pearson correlation coefficient R was calculated. en, its square-the coefficient of determination (R 2 )-was computed. e condition assumed for the best approximation was R 2 ⟶ max, which is equivalent to R 2 ⟶ 1, and the condition to regard the approximation as successful was R 2 � 0.95. e coefficient of determination was calculated for the testing stage.

Results and Discussion
e outcome of the study regarding modelling the relationship between stress, strain, and apparent density of cellular aluminum in compression showed that it is possible to determine the general instruction for structural aspects of neural networks such that the approximation attains a specified degree of accuracy.
Calculations with the algorithm were performed in two modes: firstly on data from all 12 specimens and secondly on data from 11 specimens with one sample excluded. e purpose of the first mode was to assess how the quality of approximation changed with respect to the number of neurons in the hidden layer and to roughly determine the required hidden layer size for the assumed accuracy. is is described in Section 3.1. e exclusion mode was performed in order to teach networks similarly as in the first mode, but saving the eliminated sample data for later testing of the networks' quality as on external data. One randomly taken out specimen, no. 8, was excluded with the purpose of checking the approximation within the density interval. e lowest density sample, no. 1, and the greatest density sample, no. 12, were omitted with the purpose of checking the extrapolation capability. Section 3.2 presents results of interpolate and extrapolate testing of chosen networks taught on the 11-sample input.
e results showed that only networks with 7 or 8 neurons produced approximation at the acceptable level.
Since some of the results showed that the initial region of the stress-strain curves was somewhat problematic for the ANNs to approximate, the algorithm was performed once more only for up to 200 initial experimental data for each aluminum sponge. is was done again with exclusion of sample no. 8. Teaching and testing of these networks are described in Section 3.3, and the results show that the used solution allowed to improve the results of the actual MARE in testing to about 7%.
For the purpose of the discussion of results, four additional measures were introduced: (i) MSE [5] : the mean value of mean square errors from five individual tests for networks with the same number of neurons in the hidden layer (ii) MARE [5] : the mean value of mean absolute relative errors from five individual tests for networks with the same number of neurons in the hidden layer (iii) RMSE [5] : the mean value of root mean square errors from five individual tests for networks with the same number of neurons in the hidden layer (iv) R 2 [5] : the mean value of determination coefficients from five individual tests for networks with the same number of neurons in the hidden layer All essential results and figures are presented below in detail. However, for the clarity of the discourse within the main article body, some of the figures are set as Supplementary Materials. Whenever illustrations are transferred to Appendix, the information about it is provided in the text.

Results for Networks with 12-Sample and 11-Sample
Inputs. Figure 5 shows MSE and MSE [5] for validation stages for all taught networks built for sets of 12 samples. It can be seen that, in both cases, MSE of up to 12 neurons for individual networks was slightly scattered around MSE [5] . On the contrary, from 12 neurons on, the mean square error was almost equal to 0 in all instances. Similar characteristics were obtained for networks taught on 11-sample inputs. e boundary number of neurons was 12 for networks taught with excluded sample nos. 1 and 12 and 13 for networks with sample no. 8 eliminated. Analogous graphical representations of MSE and MSE [5] for all networks trained on 11sample inputs are depicted in Figures A.1-A.3 in Supplementary Materials. Figure 6 depicts MARE, MARE [5] , RMSE, and RMSE [5] for networks with the 12-sample input. Characteristic obtained values are summarised in Table 2. As for the networks with the 11-sample input, characteristic obtained values are set in Tables 3-5, while graphs showing MARE, MARE [5] , RMSE, and RMSE [5] are in Figures A.4-A.6 in Supplementary Materials.
For the complete set of samples, the threshold values 10% and 5% of the mean absolute relative error were first obtained for individual networks with 4 and 7 neurons, respectively. is fact implies that, with a larger number of repetitions of tests for such architectures, one could have a chance of achieving a learned network providing satisfactory results. However, the presented research showed that it is more reliable to choose networks with at least 6 and 10 neurons, since for such structures mean values of the criterial measure reached the boundary MARE [5] ≤ 10% and MARE [5] ≤ 5%, respectively. If one would require to be almost sure that the set levels 10% and 5% would not be exceeded, they should select a hidden layer with at least 7 and 11 neurons, respectively. e lowest value of the mean absolute relative error was achieved in the case with 47 neurons and was equal to MARE min � 0.239%. Root mean square errors and coefficients of determination for all above instances are in Table 2. As for the set of samples with a specimen excluded, analogous reasoning can be performed but with respective numbers of neurons, as shown in Tables 3-5. It can be seen that the respective sizes of hidden layers for the given assumed accuracies are the same or different at most by only 1 neuron compared to the 12-sample input mode. is observation leads to a general conclusion that, with 5 neurons, one can reach MARE < 10%, and with 8, MARE < 5%. Also, the aforementioned fact proves that the assumed data set was large enough, since exclusion of about 8% of inputs (with targets) did not affect the results significantly.
In case of the coefficient of determination both for its mean and for consideration of individual instances, it turned out that 2 neurons guarantee the output-totarget relation at the assumed level of accuracy in both modes. Now a detailed insight into network teaching quality will be presented. It will be shown on the example of the network taught on the 12-sample input for which MARE ≤ 5% individually for the first time (7 neurons). Results for the network with the first obtained accuracy below 10% (4 neurons) and for the best accuracy network (47 neurons) are presented in Supplementary Materials. Such a choice was made since the threshold MARE � 10% turned out to produce underfitting, while the high number of neurons leads to overfitting. Also, because of the fact that respective results obtained for 11-sample input networks had very similar general characteristics, the detailed information on them is presented in Supplementary Materials. Figure 7 shows network outputs, i.e., approximate stresses σ appr.i , plotted against targets, i.e., experimental stresses t i � σ i , for all learning stages separately (training, validation, and testing) as well as combined. A linear regression between the two magnitudes was performed. e obtained linear fits are depicted left to each of the graphs, and the Pearson coefficients are above them. e ideal correlation would be σ appr.i � 1 · σ i + 0 and R � 1. Based on Pearson's coefficient, the ideal coefficient of determination would be R 2 � 1 2 � 1. Analogous figures for remaining networks are in Supplementary Materials  (Figures A.7-A.17).
Mathematical Problems in Engineering (12b). In this picture, there are included results for the networks taught on the 12-specimen set with 7 neurons in the hidden layer. Analogous figures for remaining networks are in Supplementary Materials (Figures A.18-A.20).
Results obtained for relative errors are depicted in Figure 9. It is distinct that the initial region of the stressstrain data was problematic; analysis of results showed this region merely corresponds to the linear region before the plateau or the initial phase of the plateau. Mechanically, the beginning of the plateau is related to the compressive strength σ c . One might suspect that, in case of small targets appearing in the denominator (in the aforementioned region, they are at least one order smaller than that for all other data), relative errors could be a misleading measure. Moreover, large values of RE i s from the beginning interval of stresses could significantly

MARE and RMSE for networks with four-argument inputs (for individual tests and mean) Test
Mean absolute relative error MARE {T} MARE [5] (mean for the given number of neurons) {T} Root mean square error RMSE {T} RMSE [5] (mean for the given number of neurons) {T} Figure 6: Values of MARE, MARE [5] , RMSE, and RMSE [5] for networks with the 12-sample input in function of the number of neurons in the hidden layer.      Table 4: Summary results of ANN computations using the 11-sample input in the testing stage; data for sample no. 8 were excluded from inputs.

Condition
Number of neurons in the hidden layer for which the condition is fulfilled RMSE or RMSE [5] (MPa) MARE or MARE [5] (%) R 2 or R 2 [5] MARE [5]  disturb MARE calculated for the whole network, implying the overrated number of required neurons in the hidden layer. is hypothesis seems reasonable, as the problem occurs even for the best networks (Figure 9(b)). Nevertheless, including the problematic results leaves potential designers on the safe side. e solution to this issue was based on the approach proposed in [42] and will be described in

Approximation within and Extrapolation outside the Density Interval: Results of Testing the Taught Networks with
Previously Excluded Aluminum Samples. Characteristic networks, taught before on the 11-sample data sets, were appointed for testing on previously excluded data: (i) ANNs for which MARE ≤ 10% was reached for the first time (ii) ANNs for which MARE ≤ 5% was reached for the first time (iii) ANNs for which the minimum MARE min was reached In order to include a remark on sensitivity of the networks to random weight assumption and to extrapolation, two of the tests were performed twice. Data for sample 1 were also used in another taught network with 8 neurons-the one for which MARE ≤ 5% was individually achieved for the second time in the algorithm. Similar calculations were performed for sample 12. Results for all above-mentioned networks are summarised in Table 6. Similarly as in Section 3.1, detailed description will be provided here for the networks for which MARE ≤ 5%. Generally, it can be stated that at least 8 neurons in the hidden layer are required in order to achieve results with the engineering accuracy of MARE ≤ 10%. One can also observe that linear fits show an overall trend of underestimation of stresses in case of the considered specimen, and this is more visible in the postplateau region.
Judging only from the results in the MARE column in Table 6, one could be misled that approximation of the networks is unsatisfactory; yet, if Figures 12 and A.30-A.32 are also examined, it can be seen that it is the initial region of    Table 6 in the last column; however, their purpose is only illustratory. Again, considerable initial RE i s might be attributed to the fact that, in case of the beginning of the data scope, denominators in relative errors are very small and that prototype samples might have behaved irregularly due to structural imperfections and this region is mechanically more sensitive to such an impact. As has been already mentioned, the solution for the described issue of the initial region was inspired by the study [42] and separate networks were trained for the initial stress-strain curve region. Again, this was performed only for sample 8, as an example. It is described in Section 3.3.
With regard to investigation of networks' extrapolation capabilities, previously taught networks were tested on data external for them. Sample 1 had apparent density below the taught density interval and was regular; on the contrary, sample 12 had apparent density above the upper-domain boundary and was prototype. Figures 13 and 14 present stresses with respect to strains for corresponding samples which were outside the density interval used in teaching of networks. Additionally, these networks were tested twice (first two networks of a given hidden layer size for which MARE ≤ 5%) in order to give insight into sensitivity to random weight assumption and influence of specimens' structure quality (regular vs prototype).
Regular samples turned less problematic for the networks to approximate, yet still the difference in the main criterial measure is visible: MARE 8 � 30.672% and MARE 8′ � 10.871% (Figures 13(a) and 13(b), respectively). is confirms a wellknown fact that internal network parameters (weights) may influence the approximation quality to a significant extent. is   can be interpreted as both adverse and favourable effects. e negative aspect is the considerable sensitivity to finding different local minima in the error hypersurface and lack of the absolute minimum finding algorithm, but the positive is that, first, this can be to some extent countermanded and that, second, weights do not refer to physical material properties (compare [43]), in the mechanical sense, but are constants in mathematical expressions enabling the network to find local minima. e first step to at least partially diminish the disadvantageous randomness influence is to understand the phenomenon and input data possibly well (we are referring here to the general approximation theorem [44]). One of the very purposes of the study presented in this article was to understand better to what relation type the input data comply optimally, that is, what should the structure of the ANN with given learning parameters be. Additionally, in order to gain some control over random processes, one could repeat teaching networks with similar structures, as was performed in our study, and examine obtained approximations, as is being done just now. It is worth mentioning that parameter sensitivity is an affliction present not only in neural networks but also in other attempts on building mechanical models, e.g., [45]. Comparison of plots in Figure 14 leads to a conclusion that, in case of extrapolation over the upper rim of the density interval and for an irregular sample, the two tested networks showed very different results: MARE 7 � 9.0404% and MARE 7 ′ � 57.417% (Figures 14(a) and  14(b), respectively). is fact can be explained by a few hypotheses.
e first interpretation is that we see here nothing more but again just the influence of random weight assumption. However, it might also be that the upper density limit is problematic for networks since it is the prototype samples which are heavier due to structural imperfections. In many cases, production of metal sponges is still at the noncommercial level and technology is being constantly improved [3,10,46]. At this point, a solution to this would be to supplement inputs for neural networks by more information about samples' structural quality [47].

Additional Networks for the Initial Region of the
Stress-Strain Curves. Figure 15 shows MSE and MSE [5] for validation stages for all trained networks. It shows that 6 neurons are enough to obtain MSE for individual networks only slightly scattered. Figure 16 depicts MARE, MARE [5] , RMSE, and RMSE [5] for all networks. Characteristic results are shown in Table 7.  reshold values of MARE: 10% and 5%, were first achieved for individual networks with 4 and 7 neurons, respectively. MARE [5] ≤ 10% and MARE [5] ≤ 5% were reached for 6 and 8 neurons, respectively. All values of MARE were smaller than the boundary values for 6 and 10 neurons, correspondingly. e minimum mean absolute relative error was MARE min � 0.336%, achieved in the case with 48 neurons. Root mean square errors and coefficients of determination for all above instances are in Table 7.
In case of the coefficient of determination both for its mean and for consideration of individual instances, it turned out that 6 neurons guarantee the output-to-target relation at the assumed level of accuracy.  Figure 16: Values of MARE, MARE [5] , RMSE, and RMSE [5] in function of the number of neurons in the hidden layer: case with 200 initial experimental data only.  Figures 17 and 18 present outputs from the networks plotted against targets for all learning stages separately (training, validation, and testing) as well as combined. e obtained equations are shown left to each of the graphs, and the Pearson coefficients are above them. Figure 17 is for the network with 7 neurons in the hidden layer, for which MARE ≤ 5%; Figure 18 is for the network with 48 neurons.
Results for the network for which MARE ≤ 10% was reached for the first time are provided in Supplementary Materials ( Figure A.33).
As for the errors, histograms are depicted in Figure 19. Results obtained for relative errors are presented in Figure 20. Analogously as before, figures in the main text give results for networks with 7 and 48 neurons. e Results are shown in Table 8. Figure 21 presents regression for NNs with 7 and 48 neurons. Figure 22 gives error histograms, and Figure 23     is result is a considerable improvement when compared to the value for the whole length of stress-strain curves (MARE � 9.989%). On the contrary, 4 neurons are not enough (MARE > 20%). Also, 48 neurons give MARE > 21%, which proves that the network used its potential for learning the particular data   instead for searching for the rule reflecting the stress-strain relation.

Conclusions
e ANN algorithm described in the present article allows for building, learning, and comparing results of approximation of experimental stress-strain characteristics of opencell aluminum. Choosing four criterial measures: mean absolute relative error (MARE), coefficient of determination between outputs and targets (R 2 ), root mean square error (RMSE), and mean square error (MSE), and setting their limit values, one can obtain details for the neural network architecture which computes results with desired accuracy. Such structural specifications can be used in building networks for providing designers with models of strength material characteristics of open-cell aluminum with regard to its apparent density. Also, a trained NN could serve as a surrogate model or in metamodelling. e performed analysis of relative errors led to the observation that the presented approach provides satisfactory engineering approximation (MARE < 10%) of the compressive behaviour. Moreover, the results can be improved with the same network architecture, yet modelling an additional ANN for the initial stress-strain region (MARE ≈ 7%). Detailed specifications are presented at the end of this section.
An additional conclusion comprises the comparison between the usage of all samples and exclusion of the testing sample from the input: with 12 specimens, and 1000 experimental data sets each, leaving one sample out does not affect calculations adversely. Such conduct secures an opportunity to test the approximation quality.
Removal of one sample from the input data set was also used for extrapolation quality assessment in reaching outside of the apparent density domain. It was proved that weight parameters are sensitive to the local minima finding intrinsic to artificial intelligence, but repetitions of tries can limit adverse effects. Nevertheless, it should be noted that not only the local minima issue but also belonging of a specimen to a given lot was reflected in the results shown in this study--regular sample approximation did not show as much sensitivity as the prototype one.
As for the further research, authors would like to develop the present ANN study in order to elaborate a new proposition of the mechanical model of compressive behaviour of aluminum sponges in relation to the material's apparent density [13]. Moreover, to make the computations more detailed, it might be beneficial to include also the influence of other material's parameters as inputs, for example, the cell dimensions, such as strut thickness or length or specimen quality [47]. It would also be interesting to investigate efficiency of other NN algorithms, e.g., SVM and ELM. Now the specifications are summarised and given in a conscience list as follows: (i) e general structure of networks was two-layer feedforward networks consisting of one hidden layer with the tansig activation function and one output layer with the linear activation function.
(ii) e teaching algorithm was the Levenberg-Marquardt algorithm with MSE as a performance function. Other learning parameters are given in Table 1.
(iii) Data were divided in proportions of (60 + 20 + 20)% between training, validation, and testing. Processing with the Matlab function mapminmax was used.
(iv) e approximated relation was given by equation (2): σ � f(ε, ρ). For better results, augmentation of inputs by the parameter describing the sample group was done. (v) In case of the assumed criterion MARE ≈ 10%, its fulfillment was possible with 7 and 8 neurons in the hidden layer. (vi) An increase of accuracy assessed with MARE to the level of about 7% was achieved if the same network was used in the plateau and postplateau regions of the stress-strain relation, while the preplateau region was modelled with a separate 7-neuron network.
Data Availability e stress-strain experimental data and apparent density data for the tested samples used to support the findings of this study have not been made available. Disclosure e funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript; or in the decision to publish the results.

Conflicts of Interest e authors declare no conflicts of interest.
Acknowledgments e authors would like to express their gratitude to the following collaborators who helped in production and testing of the material: B. Lipowska, K. Wańczyk, A. Kwiecień, and B. Zając. e authors would also like to indicate with gratitude that precursors for manufacturing of aluminum samples were gifted by Recticel Flexible Foams (Belgium). e funding for the presented research and publication was from the Cracow University of Technology (Kraków, Poland). e production of the aluminum sponge and experimental tests were partially supported by AGH University (Kraków, Poland; grant no. 15.11.130.488).

Supplementary Materials
Supplementary materials for the present article give results for modelling with the use of the network with 4 neurons, both for the case when 12 samples were used as the data source and for the instance with 11 samples constituting the input material. e results have only auxiliary character for the whole research, so they were moved to this section; yet they might be of interest to some readers. e included figures are as follows: Figure A.1: values of MSE and MSE [5] in function of the number of neurons in the hidden layer for networks for the 11-sample input, with sample no. 1 excluded. Figure A.2: values of MSE and MSE [5] in function of the number of neurons in the hidden layer for networks for the 11-sample input, with sample no. 8 excluded. Figure A.3: values of MSE and MSE [5] in function of the number of neurons in the hidden layer for networks for the 11-sample input, with sample no. 12 excluded. Figure A.4: values of MARE, MSE [5] , RMSE, and RMSE [5] for networks with the 11-sample input in function of the number of neurons in the hidden layer, with data for sample no. 1 excluded from inputs. Figure A.5: values of MARE, MARE [5] , RMSE, and RMSE [5] for networks with the 11-sample input in function of the number of neurons in the hidden layer, with data for sample no. 8 excluded from inputs. Figure A.6: values of MARE, MARE [5] , RMSE, and RMSE [5] for networks with the 11-sample input in function of the number of neurons in the hidden layer, with data for sample no. 12 excluded from inputs. Figure A.7: correlation plots between the network's outputs and targets for all learning stages separately and combined for the network taught on the 12-specimen set with 4 neurons in the hidden layer; MARE ≤ 10% individually for the first time. Figure A.8: correlation plots between the network's outputs and targets for all learning stages separately and combined for the network taught on the 12specimen set with 47 neurons in the hidden layer and with best MARE. Figure A.9: correlation plots between the network's outputs and targets for all learning stages separately and combined for the network taught on the 11-specimen set (sample no. 1. excluded) with 5 neurons in the hidden layer; MARE ≤ 10% individually for the first time. Figure A.10: correlation plots between the network's outputs and targets for all learning stages separately and combined for the network taught on the 11-specimen set (sample no. 1. excluded) with 8 neurons in the hidden layer; MARE ≤ 5% individually for the first time. Figure A.11: correlation plots between the network's outputs and targets for all learning stages separately and combined for the network taught on the 11-specimen set (sample no. 1. excluded) with 47 neurons in the hidden layer and with best MARE. Figure  A.12: correlation plots between the network's outputs and targets for all learning stages separately and combined for the network taught on the 11-specimen set (sample no. 8. excluded) with 4 neurons in the hidden layer; MARE ≤ 10% individually for the first time. Figure A.13: correlation plots between the network's outputs and targets for all learning stages separately and combined for the network taught on the 11-specimen set (sample no. 8. excluded) with 8 neurons in the hidden layer; MARE ≤ 5% individually for the first time. Figure A.14: correlation plots between the network's outputs and targets for all learning stages separately and combined for the network taught on the 11-specimen set (sample no. 8. excluded) with 48 neurons in the hidden layer and with best MARE. Figure A.15: correlation plots between the network's outputs and targets for all learning stages separately and combined for the network taught on the 11specimen set (sample no. 12. excluded) with 5 neurons in the hidden layer; MARE ≤ 10% individually for the first time. Figure A.16: correlation plots between the network's outputs and targets for all learning stages separately and combined for the network taught on the 11-specimen set (sample no. 12. excluded) with 7 neurons in the hidden layer; MARE ≤ 5% individually for the first time. Figure A.17: correlation plots between the network's outputs and targets for all learning stages separately and combined for the network taught on the 11-specimen set (sample no. 12. excluded) with 50 neurons in the hidden layer and with best MARE. Figure A