REVIEW 4 major objections 6 minor 39 references
Recovering Mullins damage hyperelastic behaviour with physics augmented neural networks
T0 review · 4 major / 6 minor · reviewed 2026-08-12 · deepseek-v4-flash
Pith's one-line read The paper claims that a small feed-forward neural network trained only on uniaxial, equibiaxial, and planar tension data can reproduce the full three-dimensional hyperelastic response with Mullins-type damage.
desk verdict A capable, honest NN constitutive model for Mullins damage that trains on three stress curves and validates in 3D FEA at roughly one percent error; the strongest claim needs an invariant-coverage check, but the paper deserves serious peer review. read the letter →
The pith
A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.
The reading
What carries the argument
The load-bearing object is an invariant-based feed-forward network with two shared-weight blocks and no biases. The hidden layer uses the linear-exponential activation $g(x)=e^{\alpha x}-1$, so the energy starts at zero in the undeformed state and, with suitable non-negative weights, is automatically non-negative and polyconvex; objectivity and material symmetry enter by construction because the inputs are the invariants $I_1,I_2$ of the right Cauchy-Green tensor. The same block is evaluated at the current invariants and at the invariants of the state where the undamaged energy was maximal, giving $\psi_0$ and $\psi_{0,\max}$, and the Mullins damage factor $1-\zeta$ multiplies the output. Stress is obtained by differentiating the network output with respect to the invariants, which is what enforces thermodynamic consistency, and the incompressibility pressure is fixed from the plane-stress condition during training and from the finite-element formulation in the 3D examples.
What would settle it
Apply the trained network to a 3D boundary value problem whose local deformation states fall outside the invariant envelope covered by the training tests — for example, combined torsion and inflation of a thick cylinder where the invariant pair exceeds $I_1 > 50$ or $I_2 > 625$ — and compare the resulting stress, energy, and damage fields against the Ogden-with-Mullins reference solution. If the relative errors grow substantially beyond the ~1% seen inside the envelope, the claim of full 3D recovery would be restricted to interpolation of the training data.
Extended reading notes
Core claim
The central claim is that the damaged strain energy $\psi = (1-\zeta)\psi_0$ can be learned by a single feed-forward network whose undamaged block computes $\psi_0(I_1,I_2)$ from exponential activations $g(x)=e^{\alpha x}-1$ acting on weighted combinations of $I_1-3$ and $I_2-3$, while the same block, with shared weights, computes the historical maximum $\psi_{0,\max}$ that drives the damage parameter $\zeta = \zeta_\infty(1-e^{-\psi_{0,\max}/\iota})$. The network is trained on stresses only, obtained by automatic differentiation of the energy and the plane-stress Lagrange multiplier, so it never sees energy values during training and yet recovers energy and damage evolution. In the authors' tests the unconstrained variant (no polyconvexity constraints) reproduced the reference Ogden-with-Mullins model with median relative errors around 0.4% in the simple verification tests, about 0.9–1% in the rolling-disc boundary value problem, and about 0.4% in the diabolo tension–torsion problem.
Load-bearing premise
The weakest link is the assumption that stress data from three plane-stress tests determine the two-invariant strain energy and the damage law over every deformation state the network will later see, including the 3D states in the validation examples; the paper checks accuracy inside its training envelope but gives no proof that states outside it are recovered rather than extrapolated.
Editorial extensions
If this is right
- If the claim holds, a constitutive model for a new rubber-like material with Mullins damage could be obtained from three standard laboratory tests, without postulating or calibrating a closed-form strain energy function.
- The same network can be dropped into existing finite-element software through a UHYPER-style subroutine, since only energy derivatives with respect to invariants are needed.
- The damage variable and the undamaged energy are recovered even though the network never sees energy or damage data during training, so the method extracts more from stress curves than a direct stress fit.
- Dropping polyconvexity constraints gave the most accurate model in these tests, implying that strict convexity is not required for stable, accurate recovery on the tested domain.
- The approach extends naturally to damage laws of unknown form by replacing the exponential damage expression with a small subnetwork whose output is bounded between 0 and 1.
Reading between the lines
- Editorial inference: the three training curves trace a two-dimensional surface in the $(I_1,I_2)$ plane, so the method should be understood as strong interpolation inside that envelope; its success on the disc and diabolo examples suggests the tested 3D states stayed within or near the trained region.
- Editorial inference: a direct test of the method's boundary would be to run a 3D example whose deformation states clearly fall outside the $I_1 \le 50$, $I_2 \le 625$ envelope used for validation; if errors stay small there, the network is genuinely recovering the energy surface rather than interpolating it.
- Editorial inference: the same architecture could be adapted to anisotropic damage by adding pseudo-invariants, which the authors note as a future direction; the subnetwork version of the damage law is the piece that makes this straightforward.
- Editorial inference: because the network is trained on stress derivatives, its energy accuracy is only pinned at the undeformed state; a user who needs accurate total energies in large deformations should verify the recovered $\psi_0$ against calorimetric or independent data.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The manuscript presents a physics-augmented feed-forward neural network for incompressible hyperelasticity with isotropic Mullins-type damage. The network outputs strain energy as a function of the invariants I1, I2 and of a maximum-history undamaged energy, with stress obtained by automatic differentiation; a damage branch multiplies the undamaged energy by (1−ζ), using either a specialised exponential saturation law or a trainable tanh subnetwork. Training data are synthetic uniaxial, equibiaxial, and planar tension stress-stretch curves with loading/unloading cycles, generated from a three-term Ogden model with damage (ζ∞=0.8, ι=1). Six constrained and unconstrained architecture variants are compared; the unconstrained network (case 4) gives the lowest loss and the best agreement in subsequent tests. The model is implemented in an Abaqus UHYPER subroutine and tested on homogeneous verification tests, a rolling solid rubber disc, and a diabolo under tension-torsion, with reported median errors mostly below 1.5% for stresses and reactions, and good qualitative agreement for damage evolution.
Significance. The practical aim — a compact, few-parameter neural-network constitutive model for incompressible rubber with Mullins damage, trained only on three plane-stress tests and usable in a commercial finite-element package — is attractive, and the paper provides a useful engineering recipe. Strengths include the complete UHYPER implementation details (Algorithms 1 and 2), a systematic comparison of six architecture/constraint cases, and quantitative error reporting on stresses, energies, reaction forces, and damage fields. The claim of 'full 3D recovery' is plausible but not fully established: the paper does not show that the 3D deformation states lie inside the trained invariant envelope, and the specialised damage branch partly assumes the generator's functional form. The paper's own Fig. 14 example demonstrates that stress data alone can admit models with wrong energy and damage, so the identifiability gap is real and should be addressed before the strongest conclusions are drawn.
major comments (4)
- [Sec. 2.1, Sec. 4.2, Sec. 4.3] The central claim that three one-parameter training curves determine the full two-dimensional energy and damage evolution is not verified for the 3D examples. Training data are uniaxial, equibiaxial, and planar tension curves (Sec. 2.1); Fig. 13 validates the energy on a random invariant domain with I1≤50 and I2≤625, but the paper never reports the (I1,I2) states actually visited at integration points in the rolling-disc (Sec. 4.2) and diabolo (Sec. 4.3) simulations. If those states fall within the trained envelope, the examples are interpolation; if they fall outside, the conclusion that the network 'correctly captures the full 3D behaviour' rests on extrapolation. Please add invariant-trajectory plots for the 3D examples overlaid on Fig. 12, and report the maximum energy error along those trajectories.
- [Sec. 3.1, Eq. (10)] The specialised damage branch uses the same exponential saturation law as the data generator (Eq. (10)), with ζmax and ι_NN trained. Consequently the agreement of ζ and energy in Figs. 11c, 22, and 27 confirms parameter identification within a known functional form rather than discovery of the damage law from data. Report the trained values of ζmax and ι_NN for the case-4 network and compare them with the generator values (0.8 and 1.0); this will clarify the epistemological status of the 'recovery' claimed in the abstract.
- [Abstract, Sec. 3.2.1, Sec. 5] The abstract states that the architecture fulfils polyconvexity and non-negativity of the energy, but the recommended model is case 4 (Sec. 3.2.1), for which no weights are constrained and neither property is guaranteed. Sec. 3.2 explicitly relaxes the polyconvexity criteria, and the conclusion says overconstraining the network to be polyconvex is too restrictive. The abstract should be rephrased to distinguish the constraints enforced a priori (objectivity, thermodynamic consistency, normalisation) from properties that are only numerically observed (non-negativity) or not enforced (polyconvexity).
- [Sec. 4.1.1, Fig. 14] The trainable-β subnetwork experiment is a load-bearing counterexample that deserves more discussion than a brief remark. It fits the stress curves with 4.3% error but mispredicts energy and damage for λ>5, with β=0.86 exceeding the generator's ζ∞=0.8. This demonstrates non-uniqueness of energy and damage given stress-only training data. The paper should either supply an identifiability argument for the specialised damage branch (e.g., that the exponential form plus shared weights removes the degeneracy), or add information (energy measurements, dissipation data, or additional loading paths) to justify the claimed recovery of energy and damage.
minor comments (6)
- [Sec. 4.3.1] The paragraph begins with the typo 'In tIn this section'; please correct it.
- [Sec. 1] The word 'refferred' should be 'referred'.
- [Sec. 3.2.1] The word 'futlfilled' should be 'fulfilled'.
- [Eq. (A.3)] The notation C_i is introduced in the brace in Eq. (A.3) but is not used elsewhere; the derivative expression is correct, but the brace annotation is unnecessary and could be removed.
- [Fig. 13] The color map for relative error and the number of random samples used to create the invariant domain are not specified; please add a color bar and state the sample count.
- [Algorithms 1 and 2] Algorithm 2 is referenced in the text but appears without a formal caption; label it consistently with Algorithm 1.
Circularity Check
Damage evolution is reported as a prediction but is produced by the same exponential saturation law used to generate the training data; the energy/3D recovery remains an independent fit.
-
fitted input called prediction
[Sec. 2 Eq. (10); Sec. 2.1; Sec. 3.1 Fig. 5; Sec. 4.1 Fig. 11(c)]
"The damage parameter ζ is described with the expression ζ(γ) = ζ∞ ( 1−exp(−γ/ι) ) ... All of the data is generated using the Ogden material model with the constants from Table 1, ζ∞ = 0.8 and ι = 1. ... The calculation of the maximum strain energy and damage variable from Eq. (10) was added to the NN ... the trainable parameters in this NN are ... ζmax and ιNN when calculating the damage parameter ζ."
The training data are produced with Eq. (10) using ζ∞=0.8 and ι=1. The specialised damage branch in Fig. 5 implements exactly Eq. (10), with only ζmax and ιNN as trainable parameters. Consequently, the 'predicted' damage evolution shown in Fig. 11(c) (and used in the 3D examples) is not recovered independently from stress data; it is the same exponential saturation law as the generator, evaluated with fitted constants and with ψ0,max from the fitted energy subnetwork. The paper even states the form is 'taken from the literature' and offers the tanh subnetwork as a more general alternative, confirming that the specialised branch is an assumed ansatz.
full rationale
The central energy-recovery claim is not circular: the network is trained only on 2nd Piola-Kirchhoff stress data via Sobolev-style derivative loss, and the undamaged energy ψ0 is validated over an invariant domain (Fig. 13) and in 3D boundary-value problems against the same Ogden reference model. That is interpolation/verification, not reduction to the training data by construction. The one genuine circularity-like step is the damage branch: the specialised Mullins function used in the network is identical in form to the generator's damage law (Eq. (10)), so the reported damage evolution is a fitted version of the input assumption rather than an independent recovery. The paper partially mitigates this by testing a more general tanh subnetwork (Sec. 3.1.1 and Sec. 4.1.1), which does not assume the exponential saturation form and still captures the damage when β is omitted; this shows the method can work without the exact ansatz and keeps the central claim from collapsing entirely. The missing invariant-trajectory check for the 3D examples is a correctness/coverage concern, not circularity, and is excluded from the score. Self-citations (e.g., [11]) are descriptive and not load-bearing.
Assumptions & free parameters
free parameters (5)
- NN weights w1,i, w2,i, w3,i, α_i (base energy block) =
not reported
- damage parameters ζ_max and ι_NN (specialised damage function) =
not reported
- subnetwork weights and β (alternative damage model) =
β = 0.86 when trainable
- activation exponent initialisation α=0; ζ_max=1; ι=1; Glorot initialisation =
initialisation values
- number of hidden neurons n (base NN) =
unspecified
assumptions (6)
- domain assumption Incompressibility J=det F=1, with I1≥3 and I2≥3; a Lagrange multiplier p handles the constraint.
- domain assumption Mullins damage is isotropic and described by ζ(γ)=ζ∞(1-exp(-γ/ι)) with γ=max ψ0.
- ad hoc to paper The three plane-stress deformation modes sample enough of the (I1,I2) domain to determine ψ0 and the damage evolution for the 3D cases.
- ad hoc to paper The unconstrained NN (case 4) remains material-stable in the FE examples even without polyconvexity.
- standard math The exponential activation g(x)=e^{αx}-1 with non-negative weights yields a polyconvex strain energy for the fully constrained case.
- domain assumption TensorFlow automatic differentiation correctly computes the derivatives of the NN output for Sobolev training.
Cite this review
Pith. "Pith review of Recovering Mullins damage hyperelastic behaviour with physics augmented neural networks." pith.science (2026). https://pith.science/paper/5ARNRA7A
@misc{pith2026241113185,
author = {Pith},
title = {Pith review of: Recovering Mullins damage hyperelastic behaviour with physics augmented neural networks},
year = {2026},
howpublished = {\url{https://pith.science/paper/5ARNRA7A}},
note = {Machine review of arXiv:2411.13185}
}
read the original abstract
The aim of this work is to develop a neural network for modelling incompressible hyperelastic behaviour with isotropic damage, the so-called Mullins effect. This is obtained through the use of feed-forward neural networks with special attention to the architecture of the network in order to fulfil several physical restrictions such as objectivity, polyconvexity, non-negativity, material symmetry and thermodynamic consistency. The result is a compact neural network with few parameters that is able to reconstruct the hyperelastic behaviour with Mullinstype damage. The network is trained with artificially generated plane stress data and even correctly captures the full 3D behaviour with much more complex loading conditions. The energy and stress responses are correctly captured, as well as the evolution of the damage. The resulting neural network can be seamlessly implemented in widely used simulation software. Implementation details are provided and all numerical examples are performed in Abaqus.
Figures
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