spec: add sn-curve-basic specification (#3827)

## New Specification: `sn-curve-basic`

Related to #3826

---

### specification.md

# sn-curve-basic: S-N Curve (Wöhler Curve)

## Description

An S-N curve (also known as a Wöhler curve) visualizes the relationship
between alternating stress amplitude and the number of cycles to failure
for a material under fatigue loading. Both axes typically use
logarithmic scales, with stress on the y-axis and cycle count on the
x-axis. This plot is fundamental for predicting material fatigue life
and identifying key material properties such as ultimate strength, yield
strength, and endurance limit.

## Applications

- Analyzing fatigue test data from metal coupon testing machines to
characterize material behavior
- Predicting the fatigue life of mechanical components subjected to
cyclic loading using Miner's Rule
- Comparing fatigue resistance between different materials or alloys for
engineering design decisions
- Identifying the endurance limit to determine safe operating stress
levels for infinite life design

## Data

- `cycles` (numeric, logarithmic) - Number of cycles to failure (N),
typically ranging from 1 to 10^7 or more
- `stress` (numeric) - Alternating stress amplitude or range in MPa or
ksi
- Size: 10-100 data points from fatigue tests, often with multiple
samples at each stress level
- Example: Fatigue test results showing stress levels vs. cycles to
failure for steel specimens

## Notes

- Both axes should use logarithmic scales for proper visualization of
the wide range of values
- Include horizontal reference lines for key material properties:
Ultimate Strength, Yield Strength, and Endurance Limit
- The curve typically shows three distinct regions: low-cycle fatigue
(plastic), high-cycle fatigue (elastic), and infinite life (below
endurance limit)
- Data points may include scatter from multiple test specimens at the
same stress level
- A power-law or Basquin equation fit line is commonly overlaid on the
data points

---

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---
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plots/sn-curve-basic/specification.md

@@ -0,0 +1,27 @@
+# sn-curve-basic: S-N Curve (Wöhler Curve)
+
+## Description
+
+An S-N curve (also known as a Wöhler curve) visualizes the relationship between alternating stress amplitude and the number of cycles to failure for a material under fatigue loading. Both axes typically use logarithmic scales, with stress on the y-axis and cycle count on the x-axis. This plot is fundamental for predicting material fatigue life and identifying key material properties such as ultimate strength, yield strength, and endurance limit.
+
+## Applications
+
+- Analyzing fatigue test data from metal coupon testing machines to characterize material behavior
+- Predicting the fatigue life of mechanical components subjected to cyclic loading using Miner's Rule
+- Comparing fatigue resistance between different materials or alloys for engineering design decisions
+- Identifying the endurance limit to determine safe operating stress levels for infinite life design
+
+## Data
+
+- `cycles` (numeric, logarithmic) - Number of cycles to failure (N), typically ranging from 1 to 10^7 or more
+- `stress` (numeric) - Alternating stress amplitude or range in MPa or ksi
+- Size: 10-100 data points from fatigue tests, often with multiple samples at each stress level
+- Example: Fatigue test results showing stress levels vs. cycles to failure for steel specimens
+
+## Notes
+
+- Both axes should use logarithmic scales for proper visualization of the wide range of values
+- Include horizontal reference lines for key material properties: Ultimate Strength, Yield Strength, and Endurance Limit
+- The curve typically shows three distinct regions: low-cycle fatigue (plastic), high-cycle fatigue (elastic), and infinite life (below endurance limit)
+- Data points may include scatter from multiple test specimens at the same stress level
+- A power-law or Basquin equation fit line is commonly overlaid on the data points

plots/sn-curve-basic/specification.yaml

@@ -0,0 +1,28 @@
+# Specification-level metadata for sn-curve-basic
+# Auto-synced to PostgreSQL on push to main
+
+spec_id: sn-curve-basic
+title: S-N Curve (Wöhler Curve)
+
+# Specification tracking
+created: 2026-01-14T11:42:24Z
+updated: null
+issue: 3826
+suggested: Eifi1
+
+# Classification tags (applies to all library implementations)
+# See docs/reference/tagging-system.md for detailed guidelines
+tags:
+  plot_type:
+    - line
+    - scatter
+  data_type:
+    - numeric
+    - continuous
+  domain:
+    - engineering
+    - science
+  features:
+    - basic
+    - logarithmic
+    - regression