Curve squeal — the loud tonal noise of railway wheels on tight curves — is classically attributed to negative damping from a falling friction–creepage curve, yet squeal persists when friction modifiers flatten that curve to zero or positive slope. Liu and Xiao (2021) proposed a vertical-dynamics mechanism, in which dynamic variation of the normal contact force sustains lateral self-excited vibration even under non-negative damping, but left it unconfirmed: the interplay with structural mode coupling is unaddressed, a possible contribution of transient 'instantaneous' creepage cannot be measured with current techniques, the model reduces the wheel to one degree of freedom without rail dynamics, and the dynamic normal-force amplitude is an assumed 200 N rather than a quantity derived from contact mechanics. To date, no published work has experimentally confirmed or refuted the vertical-dynamics mechanism under non-negative damping, resolved its interplay with mode coupling, or physically determined the dynamic normal-force amplitude; a 2023 two-disc-rig study has experimentally demonstrated mode-coupled squeal under dry conditions, but not under friction-modified non-negative damping and not in interaction with the vertical-dynamics mechanism. The problem — to establish and experimentally validate the generation mechanism(s) of squeal under non-negative damping — remains open.

Background

Curve squeal is the intense, narrow-band tonal noise generated when a railway wheel rolls around a tight curve with its wheel–rail contact in full sliding. In the classical explanation, due to Rudd (1976), the lateral self-excited vibration is caused by negative damping: the friction coefficient measured as a function of creepage (creepage is the small relative sliding velocity at the contact, normalized by the rolling speed; the friction–creepage curve is the measured relation between friction coefficient and creepage) typically rises to a peak and then falls, and the negative slope of this falling branch acts as a negative damping that feeds energy into a wheel resonance. Application of friction modifiers — thin layers of friction-control products on the rail — can flatten or reverse this negative slope, creating so-called non-negative damping conditions under which the classical theory predicts no self-excitation. Experiments on two-disc test rigs nevertheless show that squeal still occurs under such conditions (Liu and Meehan, 2016; Meehan and Liu, 2019). Two candidate explanations coexist. First, structural mode coupling: the coupling of two close structural modes of the wheel–rail system through the friction contact, which relies on including both the lateral and the vertical (normal to the contact) dynamics, with energy exchanged between the normal and lateral directions; simplified-model assessments (Ding et al., 2018) show it can cause squeal without falling friction, and a two-disc-rig study has since reproduced mode-coupled squeal experimentally under dry conditions, at frequencies above those typical of falling-friction tests and in agreement with an analytical mode-coupling model (Liu, De Silva, Chen, Leslie and Meehan, 2023); nevertheless, as a 2018 state-of-the-art review notes (Thompson et al., 2018), the generation mechanism remains controversial, and the role of mode coupling specifically under friction-modified, non-negative damping conditions remains untested. Second, the vertical-dynamics mechanism of Liu and Xiao (2021): a time-varying normal force at the contact modulates the friction force in phase with the lateral motion so that net power is injected into the lateral wheel vibration even when the quasistatic creep curve has zero or positive slope; a companion idea, the instantaneous creepage hypothesis (Meehan and Liu, 2019), is that the transient creep–friction relation during squeal differs from the quasistatically measured curve, so the effective slope may still be negative at the oscillation timescale. Liu and Xiao supported their model with test-rig sound pressure level (SPL) measurements, but the wheel was represented by a single degree of freedom (1-DOF, i.e., one generalized coordinate describing one vibration mode), rail dynamics was omitted, the dynamic normal-force amplitude AA was set to 200 N for illustration rather than derived from contact mechanics, and predicted SPL disagreed with measurements below the critical creepage (the creepage value at which the friction curve saturates), where rolling noise from the wheel and rail roughness dominates.

Problem Statement

Establish, by coupled modeling and experimental validation, the generation mechanism of railway curve squeal under non-negative damping conditions — that is, when the quasistatically measured friction–creepage curve has zero or positive slope and the classical negative-damping theory predicts no self-excitation. Concretely: (1) determine whether, and under what contact conditions (rolling speed, angle of attack — the yaw misalignment between the wheel and the rail at the contact — friction modifier type and dose, and normal load), each of the candidate mechanisms — vertical (normal-direction) contact dynamics of the kind proposed by Liu and Xiao (2021), structural mode coupling, and transient 'instantaneous' creepage behaviour differing from quasistatic curves — contributes to the self-excited lateral wheel vibration, and quantify their interplay and relative power inputs rather than treating them as mutually exclusive; (2) determine the amplitude of the dynamic normal contact force from contact mechanics instead of assuming an illustrative value such as 200 N, and quantify the sensitivity of squeal occurrence, frequency, and level to this amplitude; (3) demonstrate the explanation against measured experimental data (existing or newly acquired) using a model that includes the wheel's modal dynamics (beyond a single degree of freedom) and rail dynamics, reproducing measured squeal occurrence, dominant frequencies, and sound pressure levels in regimes where the negative-damping mechanism is excluded; and (4) characterize the transition regime near and below the critical creepage where rolling noise dominates and squeal-model predictions currently disagree with measurements. All four components are jointly required for a complete resolution.

References

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